Genome-edited birds

Abstract

The present disclosure provides exogenous polynucleotide cassettes for generating chimeric avian cells and chimeric birds. The polynucleotide cassettes can be used to generate conditionally lethal phenotypes in male avian embryos. In one embodiment, the present disclosure provides methods for destroying male chick embryos in ovo.

Description

[Technical field]

[0001] (Presentation of sequence listing) This application contains a Sequence Listing that has been submitted electronically in ASCII format and is incorporated by reference in its entirety. The ASCII copy created on February 24, 2020 is named P-585110-PC_ST25.txt and is 355.8KB in size.

[0002] The present disclosure relates to DNA editing agents and their use in preparing DNA edited cells and birds. The present disclosure further relates to a method for conferring a conditionally lethal phenotype on male embryos in DNA edited avian eggs. [Background technology]

[0003] The ZW sex determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans, some insects, and some reptiles. The letters Z and W are used to distinguish this system from the XY sex determination system. In a ZW system, the egg determines the sex of the offspring. Males are homogametic (ZZ) and females are heterogametic (ZW). The Z chromosome, like the X chromosome in the XY system, is larger and has more genes.

[0004] Early sex identification and separation is an important aspect of all commercial bird applications, especially in the table egg industry. In the case of broilers and turkeys, sex separation allows for better management and feeding schemes that cater to the different needs of the two sexes. Essentially all commercial hatcheries use sex separation of flocks. Male chickens, which have less commercial value, are culled at the hatchery and the female chickens are used for egg production.

[0005] Currently, there are three methods available for sexing poultry. The sex of day-old chicks can be determined either by anal / cloacal identification or by plumage sorting. Alternatively, male and female chicks can be reared together until secondary sex characteristics are evident, after which the chicks are separated based on sex. Anal / cloacal sorting relies on the appearance of anatomical structures associated with sex. Plumage sorting is based on plumage characteristics that differ between male and female chicks, e.g., down color patterns, and fast / slow growth rates of wing feathers. The third method relies on the manifestation of natural secondary sex characteristics, e.g., the comb and wattles are larger in males than in females.

[0006] Anal / cloacal sexing of day-old chicks is difficult and expensive. Highly skilled personnel are required to identify the sex of the birds. Although easier to perform, plumage sorting has the disadvantage of being limited to a particular genetic cross of birds. Sexing by secondary sex characters is the easiest method to perform but has the disadvantage that birds of both sexes must be kept together for the first few weeks after hatching, which can be more costly for the hatchery compared to the cost of anal / cloacal sorting due to feeding costs and feed switching.

[0007] Most importantly, in the United States and Europe alone, almost one billion male chicks are killed each year by various methods, making this not only an economic but also, increasingly, an ethical issue.

[0008] In the commercial hatchery sector, there is a need for high throughput methods to prevent the production of male chicks, preferably even at the egg stage, thus avoiding the problems associated with viable male chicks. Summary of the Invention [Means for solving the problem]

[0009] Provided herein is technology, including compositions and methods, for the in-ovo destruction of male embryonic chicks. Such compositions and methods are beneficial because they allow those skilled in the art of bird breeding, such as farmers, to skew the natural 1:1 ratio between male and female offspring in favor of more commercially useful female offspring.

[0010] In one embodiment, provided herein is a DNA editing agent comprising a polynucleotide cassette having the formula 5'-LHA(left homology arm)-OIE(optogenetic inducible element)-LIE(lethality inducible element)-RHA(right homology arm)-3' or the formula 5'-LHA-LIE-OIE-RHA-3', wherein (i) the LHA comprises a first nucleotide sequence substantially homologous to a first corresponding nucleotide sequence on avian chromosome Z, (ii) the OIE comprises a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, (iii) the LIE comprises a third nucleotide sequence encoding a lethality-promoting protein operably linked to activity of an inducer-activated site-specific recombinase enzyme, and (iv) the RHA comprises a fourth nucleotide sequence substantially homologous to the second corresponding nucleotide sequence on avian chromosome Z.

[0011] In certain embodiments, one or both of the LHA and RHA are substantially homologous to a corresponding nucleotide sequence located in an open transcribed region on avian chromosome Z. For example, the open transcribed region can be located at or downstream of the histidine triad nucleotide-binding protein 1-Z (HINT1Z) locus on avian chromosome Z.

[0012] In certain embodiments, the inducer-activated site-specific recombinase enzyme can be Cre recombinase (Cre) (SEQ ID NO: 113), or Mag (SEQ ID NO: 114 and SEQ ID NO: 65). In certain embodiments, the expression of the inducer-activated site-specific recombinase enzyme is induced by an inducer. In certain embodiments, the inducer is electromagnetic energy. For example, the inducer can be blue light having a wavelength of 450 to 485 nm.

[0013] In certain embodiments, the inducer-activated site-specific recombinase enzyme comprises a non-functional peptide fragment of an inducer-activated site-specific recombinase that binds in the presence of an inducer to form an active inducer-activated site-specific recombinase enzyme.

[0014] In certain embodiments, the death-inducing protein may be a toxin, a pro-apoptotic protein, an inhibitor of the Wingless / integrated (Wnt) signaling pathway, a bone morphogenetic protein (BMP) antagonist, a fibroblast growth factor (FGF) antagonist, wild-type caspase 3, constitutively active caspase 3, Noggin, or a death-inducing fragment of any of the above proteins.

[0015] In one embodiment, the DNA editing agent disclosed herein comprises (i) an LHA comprising a sequence of SEQ ID NO: 105; and (ii) an OIE comprising a sequence of SEQ ID NO: 116, a sequence of SEQ ID NO: 104, a sequence of SEQ ID NO: 102, a sequence of SEQ ID NO: 103, a sequence of SEQ ID NO: 101, a sequence of SEQ ID NO: 116, a sequence of SEQ ID NO: 100, or a sequence of SEQ ID NO: 116, a sequence of SEQ ID NO: 104, a sequence of SEQ ID NO: 101, a sequence of SEQ ID NO: 103, a sequence of SEQ ID NO: 102 .... an OIE comprising a sequence of SEQ ID NO:100, connected to a sequence of SEQ ID NO:104, connected to a sequence of SEQ ID NO:108, connected to a sequence of SEQ ID NO:103, connected to a sequence of SEQ ID NO:107, connected to a sequence of SEQ ID NO:116, or an OIE comprising a sequence of SEQ ID NO:100, connected to a sequence of SEQ ID NO:116, connected to a sequence of SEQ ID NO:104, connected to a sequence of SEQ ID NO:107, connected to a sequence of SEQ ID NO:103, connected to a sequence of SEQ ID NO:108, connected to a sequence of SEQ ID NO:116; (iii) a LIE comprising a sequence of SEQ ID NO:92, or SEQ ID NO:94, or SEQ ID NO:96, or SEQ ID NO:98; and (iv) a RHA comprising a sequence of SEQ ID NO:106.

[0016] In certain embodiments, the polynucleotide cassettes disclosed herein may be applied to birds such as chickens, turkeys, ducks, geese, quail, pheasants, or ostriches.

[0017] In another embodiment, the present disclosure further provides an avian cell comprising the polynucleotide cassette disclosed herein. In another embodiment, a chimeric bird is provided comprising an avian cell comprising the polynucleotide cassette disclosed herein.

[0018] In another aspect, a method is further provided for using the DNA editing agent disclosed herein to generate chimeric birds.In certain embodiments, the method comprises contacting bird cells with the exogenous polynucleotide cassette disclosed herein, thereby generating genome-edited bird cells, and then transferring these genome-edited bird cells into a recipient bird embryo.In another aspect, a chimeric bird generated from the above method is further provided.

[0019] In another aspect, there is further provided a method of inducing lethality in avian male embryos, comprising the steps of administering a DNA editing agent disclosed herein to a population of avian cells, thereby generating genome-edited avian cells; transferring these genome-edited avian cells into a recipient avian embryo; and exposing the embryo to an inducer that induces expression of a lethality-promoting protein encoded by the DNA editing agent, thereby inducing lethality in the avian male embryos.

[0020] In another embodiment, the DNA editing agent disclosed herein further comprises a safelock element inserted downstream of the promoter in the OIE but upstream of the sequence encoding the inducer-activated site-specific recombinase. The safelock element comprises a nucleotide sequence (STOP element) that prevents transcription of the inducer-activated site-specific recombinase encoded by the OIE. In one embodiment, the STOP element is flanked by two FRT sites. In one embodiment, each of the DNA editing agents having one of the sequences of SEQ ID NOs: 120-127 comprises a safelock element. In another aspect, a method of generating a chimeric bird using a DNA editing agent comprising a safelock element is further provided.

[0021] In another embodiment, a method of using a DNA editing agent containing a safe-lock element to induce lethality in avian male embryos is provided, the method comprising the steps of administering such a DNA editing agent to a population of avian cells, thereby generating genome-edited avian cells; transferring these genome-edited avian cells to a recipient avian embryo; and exposing the embryo to an agent that removes the STOP element from the DNA editing agent, thereby inducing expression of a lethality-promoting protein encoded by the DNA editing agent and inducing lethality in the avian male embryo.

[0022] Unless otherwise defined, all technical and / or scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the aspect or embodiment pertains.Although methods and materials similar or equivalent to those described herein can be used to carry out or test the embodiment of the aspect, exemplary methods and / or materials are described below.In case of conflict, the patent specification, including definitions, shall prevail.Furthermore, the materials, methods, and examples are merely illustrative and are not necessarily intended to be limiting. [Brief description of the drawings]

[0023] Certain embodiments are described herein, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the details shown are by way of example and for purposes of illustrative discussion of particular embodiments. In this regard, the description taken together with the drawings will make apparent to those skilled in the art how the embodiments may be practiced.

[0024] The subject matter which is deemed to be provided herein is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, the embodiments, both as to organization and method of operation, together with its objects, features, and advantages, may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

[0025] [Figure 1]FIG. 1 illustrates an embodiment of the generation of an optogenetically inducible chicken line in which only female laying chicks hatch. By mating a wild type rooster (ZZ) with a genetically modified hen (ZW), all female fertilized eggs carry a wild type ZW chromosome. All male fertilized eggs carry a ZZ chromosome, where the genetic modification Z comes from the genome of the genetically modified hen. Upon blue light illumination of the fertilized eggs, the optogenetic system of this genetically modified chromosome becomes active and activates a death mechanism that causes early male embryo death shortly after egg laying. Females that are unaffected by blue light illumination hatch, grow into adults, and lay edible infertile eggs. [Diagram 2] An embodiment of a strategy to control gene expression by blue light illumination is shown. Two fusion proteins are created: Cry2 with the inactive N-terminus of Cre (Cry2-CreN-term) and CIBN fused to the inactive C-terminus of Cre (CIBN-Cre-C-term). In the absence of blue light illumination, Cre is inactive. Upon blue light illumination, Cry2 and CIBN form a complex, and the two parts of Cre come together to form the active Cre enzyme. [Diagram 3] FIG. 3 shows an embodiment of a homology arm on the Z chromosome. The downstream genomic region of the HINT1Z locus is depicted. The 5' and 3' arms are HA-1 (left homology arm, LHA) and HA-2 (right homology arm, RHA), respectively. The primers for amplifying the arms are indicated by open arrows (FWD HA5' P1 and REV HA3' P2). On both DNA strands between the homology arms are CRISPR-Cas9 sequences (open boxes, CRISPR guides 1 and 3). The lower part of FIG. 3 details the region between the two homology arms. The sequence set forth in SEQ ID NO: 1 is shown, including the LHA-CRISPR-RHA fragment. [Figure 4A]Different embodiments of targeting vectors or DNA editing agents are shown, showing a targeting vector that includes three main elements. The first element is the 5' and 3' homology arms (HA) for homologous recombination (HR), which flank the entire exogenous insertion cassette. The second element is the light induction system, in this case Cry2-CreN and CIBN-Cre-C. The third element is the lethal gene cassette. In one embodiment of the single targeting vector strategy, the 5'HA is followed by a pGK promoter that drives the expression of the CRY2-CreN and CIBN-Cre-C genes, separated by the self-cleaving peptide P2A. This element is followed by the exogenous lethal gene cassette containing the pGK promoter, followed by a LoxP-STOP-LoxP site (LSL), followed by the lethal induction gene. This exogenous cassette is followed by the 3'HA. Upon light induction, CRY2-CreN and CIBN-Cre-C dimerize to form the active form of Cre, which then excises the LSL element, thus allowing expression of the lethal-inducing gene, leading to the death of all embryos carrying this vector. [Figure 4B] The alternative approach in different embodiments of targeting vector or DNA editing agent is shown. Instead of using LSL element, Dio-lox inversion strategy is used. Between Dio-lox sites, GFP is followed by polyadenylation site #1 (PA1), and lethal gene is followed by different polyadenylation site #2 (PA2) in reverse orientation. In this case, pGK promoter drives the expression of GFP before light activation. Upon light activation, the cassette between Dio-lox sites is inverted, and lethal gene is now in the forward orientation to be expressed, while GFP is now in the reverse orientation and is no longer active. [Figure 4C] Another embodiment of targeting vector or DNA editing agent is shown.After Cre activation, LSL is removed, and Cas9 and single guide RNA (sgRNA) are expressed.This leads to the introduction of missense mutation in the coding region of essential gene (targeted by sgRNA), thus inducing embryonic lethality. [Figure 5A]PGC line induction and characterization. Culture of PGCs. [Figure 5B] PGC line derivation and characterization. Left, mRNA expression of different pluripotency and germ cell markers. Right, representative characterization of sex identification of female PGCs (left, two PCR products for ribosomal S18 and W chromosome) and male PGCs (right, only ribosomal S18). [Figure 5C] PGC line induction and characterization. PGCs stained with anti-SSEA1 antibody. [Figure 5D] PGC line induction and characterization. Transfection of PGCs with pCAGG-GFP plasmid using Lipofectamine 2000 reagent. [Figure 5E] PGC line induction and characterization. Transfection of PGCs with pCAGG-GFP plasmid using electroporation. [Figure 5F] PGC line induction and characterization. Embryonic gonad (testis), 10 days after transplantation of GFP-expressing cultured PGCs. [Figure 6A] Designing sgRNA sites for CRISPR-mediated targeting. Examples of genomic regions on the Z chromosome for potential CRISPR targeting sites are shown. [Figure 6B] Design of sgRNA sites for CRISPR-mediated targeting. The top 12 sgRNA sequences are shown (guide #1-#12). Guide #1 and #3, which partially overlap in opposite orientations, were selected for further experiments. The 3-nucleotide PAM sequence is not part of the guide sequence, and the PAM sequence is not included in SEQ ID NOs: 66-77. [Figure 6C] Designing sgRNA sites for CRISPR-mediated targeting. Shown are the top 10 results of a search for potential off-targets for guide #1. SEQ ID NOs: 78-87 do not contain the 3 nucleotide PAM sequence. [Figure 7A]Validation of CRISPR activity using an endonuclease assay. Positive control for the endonuclease assay was the annealed WT 320bp PCR product and the mutated product at the predicted cleavage site of CRISPR1 in the indicated ratios. [Figure 7B] Validation of CRISPR activity using endonuclease assay. Endonuclease assay of 12 colonies transfected with CRISPR1 plasmid. [Figure 7C] Validation of CRISPR activity using endonuclease assay. Endonuclease assay of 12 colonies transfected with CRISPR3 plasmid. There is a 12 bp distance between the two predicted cleavage sites of CRISPR1 and CRISPR3. [Figure 8A] Validation of CRISPR activity using DNA sequencing. A DNA chromatogram of the wild-type (WT) genomic region of the predicted cleavage site of CRISPR1 shows normal sequence as a negative control. [Figure 8B] Validation of CRISPR activity using DNA sequencing. As a positive control, the sequence of a mixture of WT and mutated PCR products shows the appearance of a double peak (arrowhead) after the predicted cleavage site. [Figure 8C] Validation of CRISPR activity using DNA sequencing. Sequencing of negative colonies showing normal sequences. [Figure 8D] Validation of CRISPR activity using DNA sequencing. Sequence of positive colonies showing the appearance of a double peak following the CRISPR1 cleavage site (arrowhead). [Figure 9]Construction of a targeting vector for genome integration into the Z chromosome, consisting of (A)-(F). (A) Genomic DNA was used as a template for PCR reactions using primers P1 and P2 in the 5'HA and 3'HA regions (delimited by dashed lines) flanking the region containing the CRISPR site. (B) The approximately 3 kb product located downstream of the HINT1Z locus was ligated into the shuttle vector pJet1.2. This plasmid was used as a template for PCR using primers P3 and P4. These primers have extended overhang sequences (delimited by curly brackets) that correspond to the equivalent region of the pCAGG-Neo-IRES-GFP fragment. (C) The linearized product (vector) containing two homology arms excluding the CRISPR site-containing region, flanked by sequences that will bind to the ends of the pCAGG-Neo-IRES-GFP cassette during the Gibson reaction. (D) The pCAGG-Neo-IRES-GFP plasmid was used as a template for PCR reactions using primers P5 and P6. These primers contain extended overhang sequences (demarcated by curly brackets) that correspond to the equivalent regions at the ends of the homology arms. (E) The linearized insertion cassette is flanked by sequences that bind to the ends of the homology arms. The vector and insert were joined by a Gibson assembly reaction to generate the final targeting vector plasmid shown in (F). [Figure 10A] Co-transfection of targeting vector and CRISPR plasmid into PGCs. Lipofection-mediated co-transfection of PGCs with CRISPR1 and HR targeting vector plasmids. [Figure 10B] Co-transfection of targeting vector and CRISPR plasmid into PGCs. After 2 weeks of G-418 selection, >99% of resistant PGCs were positive for GFP. [Figure 10C] Co-transfection of targeting vector and CRISPR plasmid into PGCs. Ten days after injection of targeted PGCs into the host embryos, a large number of cells were found to be localized in the gonads (testes). [Figure 10D] Cotransfection of targeting vector and CRISPR plasmid into PGCs. Gonads were dissected, immunostained with anti-GFP antibody, and scanned using a confocal microscope (GFP antibody staining in green, nuclei counterstained with 4',6-diamidino-2-phenylindole (DAPI) in blue). [Figure 11A] Validation of HR integration in FACS sorted PGCs. FACS sorting of G-418 resistant PGCs. FACS gating was designed to sort sin GFP positive cells sorted as pools or individual cells in 96 well plates. [Figure 11B] Validation of HR integration in FACS-sorted PGCs. For PCR analysis, two sets of primers were designed for the 5' integration site (P7 and P8) and the 3' integration site (P9 and P10). [Figure 11C] Validation of HR integration in FACS-sorted PGCs. Genomic DNA extracted from pooled cells was used as template for PCR and WT DNA served as a negative control. The predicted 1.6 kb and 1.8 kb bands were evident for proper HR integration at the 5' and 3' regions, respectively. [Figure 11D] Validation of HR integration in FACS-sorted PGCs. Genomic DNA extracted from male and female cell colonies derived from single-cell FACS-sorted PGCs was used as template for PCR and WT DNA served as a negative control. The predicted 1.6 kb and 1.8 kb bands were evident for proper HR integration at the 5' and 3' regions, respectively. [Figure 12A] Southern blot analysis of HR integration. Schematic of the expected BglII cleavage products in Southern blot analysis for WT alleles and alleles that have undergone HR integration. The probes used for 5', 3' integration sites and neo are marked as short bars. The expected product size after BglII digestion is shown for each DNA probe. [Figure 12B]Southern blot analysis of HR incorporation. Preparation of Dig-labeled probes by PCR. Labeled (+) or unlabeled (-) probes were analyzed on an agarose gel. The dig-labeled products were shifted higher than their actual size, confirming the incorporation of the dig-labeled nucleotide. The primer sets used to amplify the probes are indicated. [Figure 12C] Southern blot analysis of HR integration. Southern blot analysis using 5' and 3' probes of DNA extracted from pooled pure colonies of male-derived PGCs. WT DNA extracted from the original line before HR served as a negative control. [Figure 12D] Southern blot analysis of HR integration. Southern blot analysis using 5' and Neo probes on female-derived PGCs. A single 7.5 kb band was evident in both cases, indicating that proper HR had occurred and that only a single copy of the targeting vector had been integrated. [Figure 13] Validation of the optogenetic system in HEK293 cells in vitro. Triple transfection with pmCherry-Cry2-CreN, pmCherry-CIBN-Cre-C and PB-RAGE-GFP plasmids. 24 hours after transfection, cells in the experimental group were exposed to blue light illumination for 15 seconds while control cells were kept in the dark (top row). After illumination (bottom row), cells were incubated for another 24 hours. GFP expression was evident in these cells, confirming the activation of Cre enzyme upon blue light illumination. [Figure 14]Validation of the optogenetic system in-ovo in chick embryos incubated for 54-60 h prior to electroporation. Triple electroporation into chick embryos with pmCherry-Cry2-CreN, pmCherry-CIBN-CreC and PB-RAGE-GFP plasmids. 12 h after electroporation, embryos from the experimental group were exposed to blue light illumination in-ovo for 15 s while the control embryos were kept in the dark (top row). After illumination (bottom row), embryos from both groups were incubated for an additional 12 h. After incubation, GFP-expressing cells clearly appeared in the illuminated group, thus confirming the activation of the optogenetic system and Cre enzyme upon blue light illumination in chick embryos in-ovo. [Figure 15A] Construction of a single optogen expression vector under the CAGG promoter. The optogen plasmids pmCherry-CIBN-CreC and pmCherry-Cry2-CreN were used as templates to amplify the optogen fusion proteins using primers P40–P41 and P42–P43, respectively. [Figure 15B] Construction of a single light gene expression vector under the CAGG promoter. The two products share an overlapping sequence at the P2A site introduced in primers P41 and P42. [Figure 15C] Construction of a single light gene expression vector under the CAGG promoter. One cycle of overhang extension PCR was performed to combine the two fragments (see Figure 15B) into one piece, which could be ligated into the pJet1.2 shuttle vector. [Figure 15D] Construction of a single light gene expression vector under the CAGG promoter. Primers P44 and P45, which contain tails with SmaI and NheI restriction sites, respectively, were used to generate the product. [Figure 15E] Construction of a single light gene expression vector under the CAGG promoter. The product shown in Figure 15D was digested with appropriate restriction enzymes and ligated into pCAGG-IRES-GFP digested with the same enzymes. [Figure 15F]Construction of a single light gene expression vector under the CAGG promoter. The resulting vector is shown. [Figure 16] Verification of pCAGG-Optogene plasmid activity in HEK293 cells. Co-transfection with pCAGG-Optogene and pB-RAGE-mCherry plasmids. 24 hours after transfection, cells in the experimental group were exposed to blue light illumination for 15 seconds (lower row) while the negative control group was kept in the dark (upper row). After illumination (lower row), cells were incubated for another 24 hours. In these cells, expression of mCherry was evident (white arrow), confirming activation of Cre enzyme by pCAGG-Optogene plasmid upon blue light illumination. [Figure 17] Validation of the single vector strategy using pCAGG-Optogene plasmid in-ovo. Chick embryos at stages 14-16H&H were co-electroporated with pCAGG-Optogene and pB-RAGE-mCherry plasmids. The latter plasmid serves as a reporter gene for the activity of the optogenetic system. 12 hours after electroporation, experimental group embryos (bottom row) were exposed to blue light illumination in-ovo for 15 seconds while control embryos were kept in the dark (top row). The embryos were incubated for another 12 hours. After incubation, GFP-expressing cells were evident in both groups, indicating successful electroporation, whereas mCherry-expressing cells were evident only in the illuminated group, confirming activation of the optogenetic system and Cre enzyme upon blue light illumination in chick embryos in-ovo. [Figure 18]Expression of DTA under the pGK promoter inhibits protein synthesis in-ovo. Stage 14-16 H&H embryos were electroporated with either pGK-IRES-GFP (top row) or pGK-DTA-IRES-GFP (bottom row) expression vectors. Negative control embryos broadly express GFP (top row, arrows), indicating normal protein synthesis. DTA-expressing cells show no GFP expression (bottom row), indicating that protein synthesis in these embryos is inhibited. Images of GFP only, bright field only, and GFP overlaid on bright field are shown. [Figure 19A] An embodiment of a targeting vector. In these vectors, the activating enzyme (such as Cre) is separated from the lethal gene cassette. The activating enzyme is inserted into the genome of the mother hen, and the inactive lethal cassette is inserted into the Z chromosome of the rooster, where the Z chromosome is homozygous for this allele. In this case, the activation of lethality in male embryos is performed by mating the two transgenic parents without the need for light induction. Cre in all males removes the LSL in the maternal Z chromosome, thereby allowing the lethal gene to be expressed, while the female embryos retain the inactive lethal cassette and are therefore unaffected. [Figure 19B] An embodiment of a targeting vector. In these vectors, the activating enzyme (such as Cre) is separated from the lethal gene cassette. The mother hen's Z chromosome is targeted with a Dio-Lox flipping cassette containing the FLP recombinase in the correct orientation followed by a reversed lethal gene driven by the CAGG promoter. The rooster is homozygous for the Z chromosome, again targeted with a CAGG-Cre cassette flanked by FRT sites. Upon mating of the two, the male embryo expresses Cre located on the paternal Z chromosome, the Dio-Lox cassette is flipped and the lethal gene becomes active, leading to embryonic lethality of the male embryo. The female embryo zygote from this mating contains a maternal contribution of the FLP recombinase enzyme produced during oogenesis. This maternal protein removes the CAGG-Cre cassette from the Z chromosome, leaving the female embryo viable only with a FRT "scar" on the Z chromosome. [Figure 20] RT-PCR of cDNA from total RNA extracts from freshly laid whole blastoderms (Bl) and PGCs using primers for HINT1Z and GAPDH as a positive control (GAPDH primers: forward-(SEQ ID NO:90), reverse-(SEQ ID NO:91), 93 bp). A band of the predicted size of 153 bp indicates that HINT1Z, located on the Z chromosome, is transcribed in both samples. [Figure 21] Schematic diagram of the two plasmids (pCAGG-CIBN-Cre-C-P2A-Cry2-Cre-N and pCAGG-Cry2-Cre-N-P2A-CIBN-Cre-C) containing the modified optogenetic system. [Figure 22] Validation of the optogenetic system in cultured HEK293 cells. The optogenetic plasmid pCAGG-CIBN-CreC-IRES-Cry2-CreN-IRES-GFP was cotransfected with pB-RAGE-mCherry. Similar to the PB-RAGE-GFP vector described above, pB-RAGE-mCherry contains multiple stop codon sequences flanked by LoxP sites upstream of the mCherry coding region. Upon Cre activation, the stop codons are removed, thus allowing mCherry to be expressed. In cotransfected HEK293 cells kept in the dark, there were no mCherry-positive cells, whereas in cotransfected HEK293 cells exposed to blue light illumination, many cells expressed mCherry, confirming that the single vector strategy of pCAGG-Optogene preserves the optogenetic properties of the system. [Diagram 23] Validation of the optogenetic system by electroporation in chick embryos. pCAGG-CIBN-IRES-Cry2, which also encodes GFP, was electroporated together with PB-RAGE-mCherry, which fluoresces red in the presence of active Cre recombinase. Upon induction with blue light, Cry2-CreN and CIBN-CreC dimerize, thus enabling Cre activity. White circles indicate areas of overlapping fluorescence. [Figure 24A]Induction of cell death in PGCs using DTA. The effect of DTA on cell death in PGCs is shown. PGCs were transfected with control1 PGK-IRES-GFP, control2 pCAGG-GFP, or PGK-DTA-IRES-GFP and pCAGG-GFP plasmids and incubated for 24, 48, and 72 hours. Cell death was assessed using flow cytometry for GFP and PI. Results show the ratio between GFP+PI cells and GFP only cells. [Figure 24B] Induction of cell death in PGCs with Caspase 3. The effect of Casp on cell death of PGCs is shown. PGCs were transfected with control PGK-IRES-GFP, PGK-WT Caspase 3-IRES-GFP or PGK-CA Caspase 3-IRES-GFP plasmids and incubated for 24, 48 and 72 hours before analysis as shown in FIG. 24A. [Diagram 25] Schematic diagram of the targeting vector containing all elements and the lethality-inducing cassette activated by blue light illumination. [Figure 26A] Schematic diagram of a targeting vector containing the "safe-lock" mechanism described in Example 3. [Figure 26B] Schematic diagram of a targeting vector containing the "safe-lock" mechanism described in Example 3. [Figure 27A] Validation of the targeting vector in cultured HEK293 cells. For in-vitro validation, HEK293 cells were transfected with TV4 alone. [Figure 27B] Validation of the targeting vector in cultured HEK293 cells. For in-vitro validation, HEK293 cells were transfected with pCAGG-Cre. [Figure 27C] Validation of the targeting vector in cultured HEK293 cells. For in-vitro validation, HEK293 cells were transfected with pCAGG-FlpO plasmid. Cells were kept in the dark. [Figure 27D]Validation of the targeting vector in cultured HEK293 cells. For in-vitro validation, HEK293 cells were transfected with pCAGG-FlpO plasmid. Cells were exposed to blue light for 15 seconds 24 hours after transfection. After illumination, cells were incubated for an additional 24 hours. [Figure 28A] Validation of targeting vectors by electroporation in chick embryos. Chick embryos were injected with plasmids into the neural tube and electroporated as described herein. The white lines indicate the dorsal midline of the neural tube and limb bud for orientation purposes. Four treatment groups were tested, and Figure 28A shows expression of TV4 only. [Figure 28B] Validation of targeting vectors by electroporation in chick embryos. Chick embryos were injected with plasmids into the neural tube and electroporated as described herein. The white lines indicate the dorsal midline of the neural tube and limb bud for orientation purposes. Four treatment groups were tested, and Figure 28B shows co-electroporation of TV4 and pCAGG-Cre plasmids as a positive control. [Figure 28C] Validation of targeting vectors by electroporation in chick embryos. Chick embryos were injected with plasmids into the neural tube and electroporated as described herein. The white lines indicate the dorsal midline of the neural tube and limb bud for orientation purposes. Four treatment groups were tested, and Figure 28C shows co-electroporation of TV4 and pCAGG-FlpO plasmids. Cells were kept in the dark. [Figure 28D] Validation of targeting vectors by electroporation in chick embryos. Chick embryos were injected with plasmids into the neural tube and electroporated as described herein. The white lines indicate the dorsal midline of the neural tube and limb bud for orientation purposes. Four treatment groups were tested, Figure 28D, after co-electroporation of TV4 and pCAGG-FlpO plasmids, exposed to blue light for 15 seconds and incubated for an additional 12 hours. [Figure 29A] Light-dependent activity of the lethality-inducing gene Noggin. Chick embryos were electroporated into the neural tube with the targeting vector TV1, pCAGG-FlpO, and pCAGG-IRES-GFP plasmids. The targeting vector TV1 contains the coding sequence of Noggin as a lethality-inducing element. Results without light induction; top, dorsal view; bottom, right lateral view. [Figure 29B] Light-dependent activity of the lethality-inducing gene Noggin. Chick embryos were electroporated into the neural tube with the targeting vector TV1, pCAGG-FlpO and pCAGG-IRES-GFP plasmids. The targeting vector TV1 contains the coding sequence of Noggin as a lethality-inducing element. Results are shown for blue light induction. [Figure 30A] Noggin can arrest embryo development at the blastoderm stage. Blastoderms were treated with an exogenous source of Noggin. Plasmids pCAGG-Noggin-IRES-GFP or pCAGG-IRES-GFP (as a negative control) were transfected into HEK293 cells. Total proteins extracted from transfected cells were analyzed by Western blot with anti-Noggin and anti-α-tubulin-HRP antibodies. [Figure 30B] Noggin can arrest embryo development at the blastoderm stage. Blastoderms were treated with an exogenous source of Noggin. Plasmids pCAGG-Noggin-IRES-GFP or pCAGG-IRES-GFP (as a negative control) were transfected into HEK293 cells. Conditioned media from control and Noggin-expressing cells were injected into freshly laid fertilized eggs and then incubated for 24 hours. [Figure 30C]Noggin can arrest embryo development at the blastoderm stage. Blastoderms were treated with an exogenous source of Noggin. Plasmids pCAGG-Noggin-IRES-GFP or pCAGG-IRES-GFP (as a negative control) were transfected into HEK293 cells. Conditioned media from control and Noggin-expressing cells were injected into freshly laid fertilized eggs and then incubated for 24 hours. [Figure 30D] Noggin can arrest embryo development at the blastoderm stage. Blastoderms were treated with an exogenous source of Noggin. Plasmids pCAGG-Noggin-IRES-GFP or pCAGG-IRES-GFP (as a negative control) were transfected into HEK293 cells. Conditioned media from control and Noggin-expressing cells were injected into freshly laid fertilized eggs and then incubated for 54 hours. [Figure 30E] Noggin can arrest embryo development at the blastoderm stage. Blastoderms were treated with an exogenous source of Noggin. Plasmids pCAGG-Noggin-IRES-GFP or pCAGG-IRES-GFP (as a negative control) were transfected into HEK293 cells. Conditioned media from control and Noggin-expressing cells were injected into freshly laid fertilized eggs and then incubated for 54 hours. [Figure 31A] PGCs that underwent HR on the Z chromosome at TV1 successfully colonized the gonads in chick embryos. Figure 31A shows a pure female PGC line that underwent HR at TV1 and expressed GFP. [Figure 31B] PGCs that underwent HR on the Z chromosome at TV1 successfully colonized the gonads in chick embryos. Figure 31B shows a ventral view of an embryo 5 days after PGC injection. PGCs colonized the genital ridge, the anus of the gonad (Figure 31B, arrowhead). Female chicks were sacrificed 10 days after hatching to analyze the ovaries. [Figure 31C]PGCs that underwent HR on the Z chromosome at TV1 successfully colonized the gonads in chick embryos, Figure 31C shows an ovary (indicated by lines) containing numerous GFP-positive PGCs.

[0026] It will be understood that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, dimensions of some elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] In the following detailed description, numerous specific details are described to provide a thorough understanding of the compositions and methods provided herein. However, it will be understood by those skilled in the art that the present disclosure can be practiced without these specific details. In other instances, well-known methods, procedures, and components are not described in detail so as not to obscure the compositions and methods provided herein.

[0028] Before describing at least one embodiment of the present disclosure in detail, it is to be understood that the disclosure is not necessarily limited in its application to the details set forth in the following description or illustrated by way of examples The disclosure may include other embodiments or may be practiced or carried out in various ways.

[0029] The terms "comprises," "comprising," "includes," "including," "having" and their conjugations encompass "including but not limited to."

[0030] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. For example, the term "a compound" or "at least one compound" may include a plurality of compounds, including mixtures thereof.

[0031] In some embodiments, the term "about" refers to a deviation of 0.0001-5% from a given number or range of numbers. In some embodiments, the term "about" refers to a deviation of 1-10% from a given number or range of numbers. In some embodiments, the term "about" refers to a deviation of up to 25% from a given number or range of numbers. In some embodiments, the term "about" refers to ±10%.

[0032] Throughout this application, various embodiments may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity, and should not be construed as an inflexible limitation on the scope of an embodiment. Thus, the description of a range should be considered to specifically disclose all possible subranges and individual numerical values ​​within that range. For example, the description of a range such as 1-6 should be considered to specifically disclose subranges such as 1-3, 1-4, 1-5, 2-4, 2-6, 3-6, and individual numerical values ​​within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0033] Whenever a numerical range is given herein, it is meant to include any recited numbers (fractional or integer) within the given range. The expressions "range between" a first recited number and a second recited number, and "range from" a first recited number to a second recited number, are used interchangeably herein and are meant to include the first recited number and the second recited number and all fractional and integer numbers therebetween.

[0034] In one embodiment, the techniques, products and methods provided herein produce a breed of chicken in which only female laying hens hatch and male embryos stop developing soon after fertilization. Thus, eliminating the need to dispose of male chicks and saving 50% of valuable incubation space in hatcheries. Importantly, both the females and eggs obtained from the methods disclosed herein are in all respects identical to the laying hens and table eggs commonly consumed today.

[0035] As used herein, the term "bird" or "avian species" refers to any bird, including, but not limited to, chickens, turkeys, ducks, geese, quail, pheasants, and ostriches. In certain embodiments, the bird is poultry. In certain embodiments, the bird is Gallus gallus. In certain embodiments, the bird is domesticated Gallus gallus. In certain embodiments, the bird is Gallus gallus domesticus.

[0036] In certain embodiments, the bird is female. In certain embodiments, the bird is male. In certain embodiments, the bird is a broiler. In certain embodiments, the bird is a hen. In certain embodiments, the bird is a layer hen. In certain embodiments, the bird is a poultry. In certain embodiments, the bird is a domesticated layer hen of Gallus gallus domesticus.

[0037] As used herein, the term "egg" refers to an avian egg, including a viable or live embryonic bird. In one embodiment, the term "egg" is intended to refer to a fertilized avian egg. In one embodiment, an egg is a fertilized avian egg, an egg that contains an avian embryo capable of undergoing normal embryogenesis.

[0038] Genome editing

[0039] Genome editing using engineered endonucleases refers to a genetic technique in which nucleases are used to cleave and generate specific double-stranded breaks at desired locations in the genome (e.g., on the Z chromosome in birds), which are then repaired by cell-intrinsic processes such as homology-directed repair (HDR) and non-homologous end joining (NHEJ). NHEJ directly joins DNA ends at the double-stranded break, while HDR utilizes a homologous sequence as a template to regenerate the missing DNA sequence at the break. To introduce specific nucleotide modifications into genomic DNA, a DNA repair template containing the desired sequence needs to be present during HDR. Genome editing cannot be performed using conventional restriction endonucleases because most restriction enzymes recognize only a few base pairs on DNA as their targets, and it is highly likely that the recognized base pair combinations will be found in many locations throughout the genome, resulting in multiple breaks that are not restricted to the desired location. To overcome this challenge and generate site-specific single- or double-stranded breaks, several distinct classes of nucleases have been discovered and bioengineered to date. These include meganucleases, zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and CRISPR / Cas systems.

[0040] Meganucleases-They are generally classified into four families: the LAGLIDADG family, the GIY-YIG family, the His-Cys box family, and the HNH family. These families are characterized by structural motifs that affect catalytic activity and recognition sequences. For example, members of the LAGLIDADG family are characterized by having one or two copies of the conserved LAGLIDADG motif. The four families of meganucleases are largely separated from each other in terms of conserved structural elements and thus DNA recognition sequence specificity and catalytic activity. Meganucleases are generally found in microbial species and have the unique property of having fairly long recognition sequences (>14bp), thus making them naturally quite specific for cleavage at desired locations. This can be exploited to perform site-specific double-strand breaks in genome editing. Although the skilled artisan can use these naturally occurring meganucleases, the number of such naturally occurring meganucleases is limited. To overcome this challenge, mutagenesis and high-throughput screening methods were used to generate meganuclease variants that recognize unique sequences. For example, various meganucleases are fused to create hybrid enzymes that recognize new sequences. Also, the DNA that interacts with the amino acids of meganucleases can be modified to design sequence-specific meganucleases (e.g., U.S. Patent No. 8,021,867). Meganucleases can be designed using methods described, for example, in Certo, MT et al. Nature Methods (2012) 9:073-975, U.S. Patent Nos. 8,304,222, 8,021,867, 8,119,381, 8,124,369, 8,129,134, 8,133,697, 8,143,015, 8,143,016, 8,148,098, or 8,163,514.

[0041] ZFNs and TALENs - Two distinct classes of engineered nucleases, zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), have both proven effective in generating targeted double-stranded breaks. Essentially, ZFN and TALEN restriction endonuclease technologies utilize a non-specific DNA cleaving enzyme linked to a specific DNA binding domain (a series of zinc finger domains or TALE repeats, respectively). Typically, a restriction enzyme is selected whose DNA recognition and cleavage sites are separated from each other. The cleavage portion is separated and then linked to a DNA binding domain, thereby obtaining an endonuclease with fairly high specificity for the desired sequence. An exemplary restriction enzyme with such properties is Fokl. Furthermore, Fokl has the advantage that it requires dimerization to have nuclease activity, which means that specificity is exponentially increased because each nuclease partner recognizes a unique DNA sequence. To enhance this effect, Fokl nucleases were engineered that function only as heterodimers and have enhanced catalytic activity. Heterodimeric functional nucleases avoid the possibility of undesired homodimer activity and therefore enhance the specificity of double-strand breaks.

[0042] Thus, for example, ZFNs and TALENs can be constructed as nuclease pairs, where each member of the pair is designed to bind adjacent sequences at the target site. Upon transient expression in cells, the nucleases bind to their target sites and heterodimerize with the FokI domain to generate double-stranded breaks. Repair of these double-stranded breaks via the non-homologous end joining (NHEJ) pathway most often results in small deletions or indels, which are small sequence insertions. Because each repair performed by NHEJ is unique, the use of a single nuclease pair generates a series of alleles with different extents of deletion at the target site. Deletions typically range from a few base pairs to several hundred base pairs in length, although larger deletions have been successfully generated in cell culture by using two pairs of nucleases simultaneously (see, e.g., Carlson et al., 2012, Proc Natl Acad Sci USA.; 109(43):17382-7; Lee et al., 2010, Trends Biotechnol.; 28(9):445-6). Furthermore, when a fragment of DNA with homology to the targeted region is introduced in combination with a nuclease pair, the double-stranded break can be repaired via homology-directed repair to generate specific modifications (see, e.g., Li et al., 2011, Nucleic Acids Res. 39(1):359-72; Miller et al., 2010, Nat Struct Mol Biol. 17(9):1144-51; Urnov et al., 2005, Nature 435(7042):646-51).

[0043] Although the nuclease moieties of both ZFNs and TALENs have similar properties, the difference between these engineered nucleases lies in their DNA recognition peptides. ZFNs rely on Cys2-His2 zinc fingers and TALENs on TALEs. Both of these DNA recognizing peptide domains have the characteristic that they are naturally found in protein combinations. Cys2-His2 zinc fingers are usually found in repeat sequences separated by 3 bp and are found in various combinations of various nucleic acids that interact with proteins. On the other hand, TALEs are found in repeat sequences with a one-to-one recognition rate between amino acids and recognized nucleotide pairs. Since both zinc fingers and TALEs occur in a repeat pattern, various combinations can be tried to generate a variety of sequence specificities. Approaches for generating site-specific zinc finger endonucleases include, for example, modular assembly (zinc fingers correlating with triplet sequences are attached in a row to cover the required sequence), OPEN (low stringency selection of peptide domains versus triplet nucleotides followed by high stringency selection of combinations of peptides versus the final target in a bacterial system), and bacterial one-hybrid screening of zinc finger libraries, among others.

[0044] CRISPR-Cas systems - Many bacteria and archaea contain endogenous RNA-based adaptive immune systems that can degrade the nucleic acids of invading phages and plasmids. These systems consist of clustered regularly interspaced short palindromic repeats (CRISPR) genes that produce an RNA component, and CRISPR-associated (Cas) genes that code for a protein component. CRISPR (crRNA) contains short stretches of homology to specific viruses and plasmids and acts as a guide for Cas nucleases to degrade complementary nucleic acids of the corresponding pathogen. Studies of the type II CRISPR / Cas system of Streptococcus pyogenes showed that three components form an RNA / protein complex that together are sufficient for sequence-specific nuclease activity, namely, the Cas9 nuclease, the crRNA that contains 20 base pairs of homology to the target sequence, and the trans-activating crRNA (tracrRNA) (Jinek et al. Science (2012) 337:816-821). It was further shown that a synthetic chimeric guide RNA (gRNA) consisting of a fusion of crRNA and tracrRNA could direct Cas9 to cleave a DNA target complementary to the crRNA in vitro. It has also been shown that transient expression of Cas9 in combination with synthetic gRNAs can be used to generate targeted double-stranded brakes in a variety of different species (e.g., Cho et al., 2013, Nat Biotechnol. 31(3):230-2; Cong et al., 2013, Science 339(6121):819-23; DiCarlo et al., 2013, Nucleic Acids Res. 41(7):4336-43; Hwang et al., 2013, Nat Biotechnol. 31(3):227-9; Jinek et al., 2013, Elife. 2013 Jan 29;2:e00471; Mali et al., 2013, Nat Methods. 10(10):957-63).

[0045] The CRIPSR / Cas system for genome editing is known to include two distinct components: a guide RNA (gRNA) and an endonuclease, e.g., Cas9. The gRNA is typically a 20 nucleotide sequence that encodes a combination of a target homologous sequence (crRNA) and an endogenous bacterial RNA that links the crRNA to the Cas9 nuclease (tracrRNA) in a single chimeric transcript. The gRNA / Cas9 complex is recruited to the target sequence by base pairing between the gRNA sequence and complementary genomic DNA. For successful binding of Cas9, the genomic target sequence must also contain a suitable protospacer adjacent motif (PAM) sequence immediately following the target sequence. Binding of the gRNA / Cas9 complex localizes Cas9 to the genomic target sequence such that Cas9 can cleave both strands of DNA causing a double-stranded break. Similar to ZFNs and TALENs, the double-stranded breaks generated by CRISPR / Cas can undergo homologous recombination or NHEJ. In certain embodiments, the CRISPR / Cas system comprises a single guide RNA (sgRNA) and a Cas protein. In certain embodiments, the CRISPR / Cas system comprises a complex of a single guide RNA (sgRNA) and a Cas protein. In certain embodiments, the Cas of the CRISPR / Cas system comprises a single polypeptide. In certain embodiments, the Cas of the CRISPR / Cas system is an endonuclease. In certain embodiments, the CRISPR / Cas is CRISPR / Cas9.

[0046] The Cas9 nuclease has two functional domains, RuvC and HNH, each of which cleaves a different DNA strand. When both domains are active, Cas9 causes a double-stranded break in genomic DNA. A major advantage of CRISPR / Cas is that the high efficiency of this system coupled with the ability to easily create synthetic gRNAs allows multiple genes to be targeted simultaneously. The apparent flexibility of the base-pairing interactions between the gRNA sequence and the genomic DNA target sequence allows imperfect matches to the target sequence to be cleaved by Cas9.

[0047] Modified versions of the Cas9 enzyme that contain a single inactive catalytic domain, i.e., either RuvC- or HNH-, are called "nickases." With only one active nuclease domain, Cas9 nickases cleave only one strand of the target DNA, creating a single-strand break or "nick." Single-strand breaks or nicks are typically rapidly repaired via the HDR pathway, using the complete complementary DNA strand as a template. However, two proximal opposite-strand nicks introduced by Cas9 nickases are treated as a double-strand break, often referred to as a "double-nick" CRISPR system. The double nick can be repaired by either NHEJ or HDR, depending on the desired effect on the gene target. Thus, when specificity and reduced off-target effects are important, using Cas9 nickases to create a double nick by designing two gRNAs with target sequences in close proximity and on opposite strands of the genomic DNA results in a nick that has reduced off-target effects compared to either gRNA alone and does not alter the genomic DNA.

[0048] The modified version of Cas9 enzyme, which contains two inactive catalytic domains (deadCas9 or dCas9), has no nuclease activity but can still bind to DNA based on gRNA specificity. dCas9 can be utilized as a platform for DNA transcription regulators to activate or suppress gene expression by fusing the inactive enzyme to a known regulatory domain. For example, the binding of dCas9 alone to the target sequence of genomic DNA can prevent gene transcription. In certain embodiments, the CRISPR / Cas is CRISPR / dCas9.

[0049] To use the CRISPR system, both gRNA and Cas9 need to be expressed in target cells. The insertion vector can contain both cassettes on a single plasmid, or the cassettes are expressed from two separate plasmids. CRISPR plasmids, such as Addgene's px330 plasmid, are publicly available. In addition, the mRNA encoding Cas9 and gRNA can be introduced into target cells, and the recombinant Cas9 protein complexed with gRNA (i.e., can insert RNP complex into cells).

[0050] In certain embodiments, the CRISPR / Cas system is a class 1 CRISPR / Cas system. In certain embodiments, the class 1 CRISPR / Cas system comprises a multi-subunit crRNA-effector complex. In certain embodiments, the CRISPR / Cas system is a type I CRISPR-Cas system. In certain embodiments, the CRISPR / Cas system is a type III CRISPR / Cas system. In certain embodiments, the CRISPR / Cas system is a type IV CRISPR-Cas system.

[0051] In certain embodiments, the CRISPR / Cas system is a class 2 CRISPR / Cas system. In certain embodiments, the class 2 CRISPR / Cas system comprises a single subunit crRNA effector module. In certain embodiments, the CRISPR / Cas system is a type II CRISPR-Cas system. In certain embodiments, the CRISPR / Cas system is a type V CRISPR / Cas system.

[0052] In certain embodiments, the Cas in the class 2 CRISPR / Cas system can be Cas9, Cpf1, C2c1, C2c2 or C2c3. Those skilled in the art will understand the classification of CRISPR / Cas systems as it is well known in the art (e.g., Nat Rev Microbiol. 2017 March, 15(3): 169-182, Nat Rev Microbiol. 2015 November, 13(11): 722-736), and this classification has evolved over time (Mol Cell. 2015 November 5, 60(3): 385-397). In some embodiments, the CRISPR / Cas is any CRISPR-associated protein (CAS) endonuclease known in the art.

[0053] Genome editing using recombinant adeno-associated virus (rAAV) is based on rAAV vectors, which allow the insertion, deletion or replacement of DNA sequences in the genome of living mammalian cells. The rAAV genome is a single-stranded deoxyribonucleic acid (ssDNA) molecule, either positive or negative, and is approximately 4.7 kb in length. These single-stranded DNA viral vectors have the unique property of having a high transduction rate and promoting endogenous homologous recombination in the absence of double-stranded DNA breaks in the genome. Those skilled in the art can design rAAV vectors to target desired genomic loci and perform both gross and / or fine endogenous genetic changes in cells. rAAV genome editing has the advantage of targeting one allele and not resulting in off-target genomic changes.

[0054] DNA editing agents

[0055] The technology described herein provides, in certain aspects and embodiments, DNA editing agents. The DNA editing agents may be constructed using recombinant DNA techniques well known to those of skill in the art.

[0056] In one embodiment, the DNA editing agent disclosed herein may be included in a single nucleic acid construct or may be included in a combination of nucleic acid constructs. In one embodiment, the DNA editing agent comprises at least two key elements, as described below.

[0057] The first element is a nucleotide sequence cassette that is destined to be stably integrated into a specific location in the bird's genome. When integrated into the bird's genome, the first element alters the bird's genotype, but in certain embodiments and under certain conditions, also alters the bird's phenotype. The altered bird phenotype, compared to other bird phenotypes, is the object of the technology provided herein. As briefly and in detail explained by the embodiments provided herein, the altered phenotype is useful for preventing the development of viable male chicks from male embryos. This prevention can prevent the need to sacrifice viable male chicks, as well as reduce significant economic burdens on farmers and hatcheries.

[0058] The second element is a first and a second nucleotide target sequence that flank the first element. The second element serves to determine the location in the bird's genome where the first element is stably integrated. Random integration of foreign DNA into the genome of any organism would be harmful if it interferes with genes responsible for basic cellular functions. Alternatively, random integration may not be important if the foreign DNA is integrated into an inactive segment of DNA. The second element performs the important function of directing integration of the first element into a defined, predetermined region of the bird's DNA. In one embodiment of the present disclosure, the second element directs integration of the first element into an openly transcribed region of the bird's chromosome Z without adversely affecting basic cellular functions.

[0059] Those skilled in the art will understand that the term "DNA editing agent" generally refers to any molecule, such as a nucleotide sequence or enzyme, that facilitates an alteration of the genome of an organism, such as a bird. The alteration can be, for example, an addition to the DNA by an agent that is incorporated into the DNA, a replacement of a sequence of the DNA, for example by homologous recombination, or a deletion of the DNA.

[0060] In one embodiment, DNA editing agent can be constructed in viral vector (e.g., using a single vector or multiple vectors). Such vectors are generally used in gene transfer and gene therapy applications. Different viral vector systems have their own advantages and disadvantages. The viral vector that can be used to integrate the first nucleotide sequence of certain embodiments into bird Z chromosome includes, but is not limited to, adenovirus vector, adeno-associated virus vector, alphavirus vector, herpes simplex virus vector, retrovirus vector or lentivirus vector.

[0061] Viral constructs, such as retroviral constructs, include at least one transcriptional promoter / enhancer or locus-defining element, or other elements that control gene expression by means such as alternative splicing, nuclear RNA export, or post-translational modification of messengers. Such vector constructs also include packaging signals, long terminal repeats (LTRs) or portions thereof, and plus and minus strand primer binding sites appropriate for the virus used, unless already present in the viral construct. In addition, such constructs typically include a signal sequence for secretion of the peptide from the host cell in which it is placed. In certain embodiments, the signal sequence may be a mammalian signal sequence. Optionally, the construct may also include a signal directing polyadenylation, as well as one or more restriction sites and a translation termination sequence. By way of example, such constructs typically include a 5'LTR, a tRNA binding site, a packaging signal, an origin of second strand DNA synthesis, and a 3'LTR or portions thereof. Other vectors that are non-viral, such as cationic lipids, polylysine, or dendrimers, can be used.

[0062] In one embodiment, provided herein is a DNA editing agent comprising a polynucleotide cassette having the formula 5'-LHA(left homology arm)-OIE(optogenetic inducible element)-LIE(lethal inducible element)-RHA(right homology arm)-3' or the formula 5'-LHA-LIE-OIE-RHA-3', wherein (i) the LHA comprises a first nucleotide sequence substantially homologous to a first corresponding nucleotide sequence on avian chromosome Z, (ii) the OIE comprises a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, (iii) the LIE comprises a third nucleotide sequence encoding a lethal-promoting protein operably linked to activity of an inducer-activated site-specific recombinase enzyme, and (iv) the RHA comprises a fourth nucleotide sequence substantially homologous to the second corresponding nucleotide sequence on avian chromosome Z.

[0063] As will be appreciated by those skilled in the art, the formula 5'-LHA-OIE-LIE-RHA-3' or the formula 5'-LHA-LIE-OIE-RHA-3' refers to the respective position of any element (e.g., LHA or OIE or LIE or RHA) relative to all other elements within the same DNA editing agent molecule. The terminology of 5' and 3' is widely accepted and well known to those skilled in the art.

[0064] In certain embodiments, the polynucleotide cassette comprises the formula 5'-LHA-OIE-LIE-RHA-3'. In certain embodiments, the OIE is upstream of the LIE. In certain embodiments, the first promoter in the OIE is operably linked to a second nucleotide sequence encoding an inducer-activating site-specific recombinase enzyme and is not further operably linked to a third nucleotide sequence encoding a death-promoting protein.

[0065] In certain embodiments, the polynucleotide cassette comprises the formula 5'-LHA-LIE-OIE-RHA-3'. In certain embodiments, the LIE is upstream of the OIE. In certain embodiments, the first promoter in the OIE is operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme and is not further operably linked to a third nucleotide sequence encoding a death-promoting protein.

[0066] In certain embodiments, the DNA editing agent comprises the formula 5'-LHA-OIE-LIE-RHA-3', wherein (i) LHA comprises the sequence set forth in SEQ ID NO: 105; (ii) OIE comprises the sequence set forth in SEQ ID NOs: 101-103, or the sequences set forth in SEQ ID NOs: 107, 103, and 108; (iii) LIE comprises the sequence set forth in SEQ ID NO: 92, or the sequence set forth in SEQ ID NO: 94, or the sequence set forth in SEQ ID NO: 96, or the sequence set forth in SEQ ID NO: 98; (iv) RHA comprises the sequence set forth in SEQ ID NO: 106; or (v) any combination of (i), (ii), (iii), and (iv).

[0067] In certain embodiments, the OIE comprises a sequence set forth in SEQ ID NO: 101, which is connected to a sequence set forth in SEQ ID NO: 102, which is connected to a sequence set forth in SEQ ID NO: 103. In certain embodiments, the OIE comprises a sequence set forth in SEQ ID NO: 102, which is connected to a sequence set forth in SEQ ID NO: 101, which is connected to a sequence set forth in SEQ ID NO: 103.

[0068] In certain embodiments, the OIE comprises a sequence set forth in SEQ ID NO: 107, which is connected to a sequence set forth in SEQ ID NO: 103, which is connected to a sequence set forth in SEQ ID NO: 108. In certain embodiments, the OIE comprises a sequence set forth in SEQ ID NO: 108, which is connected to a sequence set forth in SEQ ID NO: 103, which is connected to a sequence set forth in SEQ ID NO: 107.

[0069] In certain embodiments, (i) the LHA comprises a sequence set forth in SEQ ID NO: 105, and (ii) the OIE comprises a sequence set forth in SEQ ID NO: 116, a sequence set forth in SEQ ID NO: 104, a sequence set forth in SEQ ID NO: 102, a sequence set forth in SEQ ID NO: 103, a sequence set forth in SEQ ID NO: 101, a sequence set forth in SEQ ID NO: 116, a sequence set forth in SEQ ID NO: 100, or the OIE comprises a sequence set forth in column number 116, a sequence set forth in SEQ ID NO: 104, a sequence set forth in SEQ ID NO: 101, a sequence set forth in SEQ ID NO: 103, a sequence set forth in SEQ ID NO: 102 .... or the OIE is connected to the sequence set forth in SEQ ID NO: 116, connected to the sequence set forth in SEQ ID NO: 104, connected to the sequence set forth in SEQ ID NO: 107, connected to the sequence set forth in SEQ ID NO: 116, or comprises the sequence set forth in SEQ ID NO: 100; (iii) the LIE comprises the sequence set forth in SEQ ID NO: 92, or comprises the sequence set forth in SEQ ID NO: 104, connected to the sequence set forth in SEQ ID NO: 94, or the sequence set forth in SEQ ID NO: 96, or the sequence set forth in SEQ ID NO: 98; and (iv) the RHA comprises the sequence set forth in SEQ ID NO: 106.

[0070] In certain embodiments, the DNA editing agent comprises one of the sequences set forth in SEQ ID NOs: 120-127.

[0071] The DNA editing agent may also code for a reporter protein that is easily detectable by either its presence or activity, including, but not limited to, luciferase, fluorescent proteins (e.g., green fluorescent protein), chloramphenicol acetyltransferase, β-galactosidase, secreted placental alkaline phosphatase, beta-lactamase, human growth hormone, and other secreted enzyme reporters. In general, reporter genes code for polypeptides that are not produced by the host cell and are detectable by cell analysis, for example, direct fluorescent analysis, radioisotope analysis, or spectrophotometric analysis of the cell, usually without the need to kill the cell for signal analysis. In certain embodiments, reporter genes code for enzymes that generate changes in the fluorescent properties of the host cell that are detectable by qualitative, quantitative, or semi-quantitative function or transcriptional activation. Exemplary enzymes include esterases, β-lactamases, phosphatases, peroxidases, proteases (tissue plasminogen activator or urokinase), and other enzymes whose function can be detected by appropriate chromogenic or fluorogenic substrates known to those of skill in the art or developed in the future.Reporter genes may report on successful integration of the construct into the Z chromosome.

[0072] In certain embodiments, the DNA editing agent may comprise a nucleotide sequence encoding a reporter polypeptide. In certain embodiments, the reporter polypeptide may be green fluorescent protein (GFP) (SEQ ID NO: 115) or mCherry / RFP (SEQ ID NO: 119).

[0073] In certain embodiments, the DNA editing agent further comprises a positive and / or negative selection marker for efficient selection of transformed cells that have undergone a homologous recombination event with the construct. Positive selection provides a means to enrich the population of clones that have incorporated foreign DNA. Non-limiting examples of such positive markers include markers that confer antibiotic resistance, such as glutamine synthetase, dihydrofolate reductase (DHFR), and neomycin, hygromycin, puromycin, and blasticidin S resistance cassettes. Negative selection markers must be selected against random integration and / or excision of marker sequences (e.g., positive markers). Non-limiting examples of such negative markers include herpes simplex thymidine kinase (HSV-TK), which converts ganciclovir (GCV) into a cytotoxic nucleoside analog, hypoxanthine phosphoribosyltransferase (HPRT), diphtheria toxin (DT), and adenine phosphoribosyltransferase (ARPT).

[0074] In certain embodiments, the codons encoding the protein of the DNA editing agent are "optimized" codons, i.e., the codons are those that frequently occur in highly expressed genes in, for example, birds, instead of those frequently used in, for example, influenza viruses. Such codon usage provides efficient expression of the protein in avian cells. Codon usage patterns are known in the literature for highly expressed genes of many species (e.g., Nakamura et al., 1996, Nucleic Acids Res. 24(1):214-5; McEwan et al., 1998, Biotechniques. 24(1):131-6, 138).

[0075] In certain embodiments, the DNA editing agent may further comprise a self-cleaving peptide, such as, but not limited to, P2A, T2A, E2A (Wang et al., Scientific Report 5, Article 16273 (2015)), or an internal ribosome entry site (IRES) sequence.

[0076] Left and right homology arms

[0077] It is generally accepted that the size of the homology arms for HR should be proportional to the size of the insert between the arms. Those skilled in the art will easily determine and construct homology arms of appropriate length. In one embodiment, the homology arms can be as short as 50 bases. In certain embodiments, (i) the length of the LHA is about 0.5 to about 5 kilobases (kb), (ii) the length of the RHA is about 0.5 to about 5 kb, or (iii) any combination of (i) and (ii). In certain embodiments, (i) the length of the LHA is about 1.5 kb, (ii) the length of the RHA is about 1.5 kb, or (iii) any combination of (i) and (ii). In certain embodiments, the LHR and / or RHA can be as short as 50 bases.

[0078] In certain embodiments, the length of the LHA is about 0.5 to about 5 kilobases (kb). In certain embodiments, the length of the LHA is about 0.5 kb. In certain embodiments, the length of the LHA is about 1 kb. In certain embodiments, the length of the LHA is about 1.5 kb. In certain embodiments, the length of the LHA is about 2 kb. In certain embodiments, the length of the LHA is about 2.5 kb. In certain embodiments, the length of the LHA is about 3 kb. In certain embodiments, the length of the LHA is about 3.5 kb. In certain embodiments, the length of the LHA is about 4 kb. In certain embodiments, the length of the LHA is about 4.5 kb. In certain embodiments, the length of the LHA is about 5 kb. In certain embodiments, the LHR may be shortened by up to 50 bases.

[0079] In certain embodiments, the length of the RHA is about 0.5 to about 5 kilobases (kb). In certain embodiments, the length of the RHA is about 0.5 kb. In certain embodiments, the length of the RHA is about 1 kb. In certain embodiments, the length of the RHA is about 1.5 kb. In certain embodiments, the length of the RHA is about 2 kb. In certain embodiments, the length of the RHA is about 2.5 kb. In certain embodiments, the length of the RHA is about 3 kb. In certain embodiments, the length of the RHA is about 3.5 kb. In certain embodiments, the length of the RHA is about 4 kb. In certain embodiments, the length of the RHA is about 4.5 kb. In certain embodiments, the length of the RHA is about 5 kb. In certain embodiments, the RHA may be shortened to as little as 50 bases.

[0080] In certain embodiments, the length of each of the LHA and RHA is about 0.5 kb. In certain embodiments, the length of each of the LHA and RHA is about 1 kb. In certain embodiments, the length of each of the LHA and RHA is about 1.5 kb. In certain embodiments, the length of each of the LHA and RHA is about 2 kb. In certain embodiments, the length of each of the LHA and RHA is about 2.5 kb. In certain embodiments, the length of each of the LHA and RHA is about 3 kb. In certain embodiments, the length of each of the LHA and RHA is about 3.5 kb. In certain embodiments, the length of each of the LHA and RHA is about 4 kb. In certain embodiments, the length of each of the LHA and RHA is about 4.5 kb. In certain embodiments, the length of each of the LHA and RHA is about 5 kb. In certain embodiments, the LHR and RHA can be shortened to 50 bases.

[0081] In certain embodiments, the length of each of the left and right homology arms is sufficient to allow specific recombination into the bird's chromosomal DNA. In one embodiment, the LHA and / or RHA are at least 500 nucleotides long, for example, 500-3000 nucleotides long. Typically, the required size of the LHA and / or RHA homology arms depends on the length of the cassettes flanking these arms. The smaller the cassette, the shorter the arms required, and vice versa. In one embodiment, the homology arms can be as short as 50 bases.

[0082] In certain embodiments, (i) the LHA is substantially homologous to a corresponding first nucleotide sequence located in an open transcribed region on avian chromosome Z, (ii) the RHA is substantially homologous to a corresponding second nucleotide sequence located in an open transcribed region on avian chromosome Z, or (iii) both (i) and (ii).

[0083] As will be apparent to those skilled in the art, a first sequence is "substantially homologous" to a second sequence if the first and second sequences are similar or identical in sequence, so long as they can be replaced by each other by homologous recombination. Methods for testing and identifying homologous recombination are well known in the art.

[0084] In certain embodiments, substantially homologous is at least 50% identical. In certain embodiments, substantially homologous is at least 60% identical. In certain embodiments, substantially homologous is at least 70% identical. In certain embodiments, substantially homologous is at least 80% identical. In certain embodiments, substantially homologous is at least 90% identical. In certain embodiments, substantially homologous is at least 95% identical. In certain embodiments, substantially homologous is at least 99% identical.

[0085] In a particular embodiment, the first nucleotide sequence in the LHA is 50% to 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the first nucleotide sequence in the LHA is 80% to 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the first nucleotide sequence in the LHA is 85% to 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the first nucleotide sequence in the LHA is 90% to 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the first nucleotide sequence in the LHA is 95% to 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the first nucleotide sequence in the LHA is 99% to 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z. In certain embodiments, the first nucleotide sequence in the LHA is 100% identical in sequence to the first corresponding nucleotide sequence on chromosome Z.

[0086] In a particular embodiment, the fourth nucleotide sequence in RHA is 50% to 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the fourth nucleotide sequence in RHA is 80% to 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the fourth nucleotide sequence in RHA is 85% to 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the fourth nucleotide sequence in RHA is 90% to 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the fourth nucleotide sequence in RHA is 95% to 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z. In a particular embodiment, the fourth nucleotide sequence in RHA is 99% to 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z. In certain embodiments, the fourth nucleotide sequence in RHA is 100% identical in sequence to the second corresponding nucleotide sequence on chromosome Z.

[0087] In certain embodiments, the LHA and / or RHA are homologous to or exhibit about 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% homology or identity to at least one nucleotide sequence within a target locus in avian chromosome Z that serves as an integration site.

[0088] Those skilled in the art will understand that the term "openly transcribed region in a chromosome" generally refers to a region of a chromosome that contains genes that are transcribed at a sufficient level to allow other genes to be transcribed easily enough. A non-limiting example of an openly transcribed region is a region adjacent to a housekeeping gene that is highly transcribed during the life of a cell or organism. Other non-limiting examples of openly transcribed regions are regions between gene loci (e.g., chromatin regulatory elements, non-coding DNA, "junk DNA", etc.). A non-limiting example of an under-transcribed region is the region at the end of each chromosome called a telomere, which is not transcribed during the life of a cell or organism. In a particular embodiment, the openly transcribed region is located at or downstream of the Hint1Z gene on avian chromosome Z.

[0089] In one embodiment, the LHA and / or RHA correspond to genomic sequences present on the bird's Z chromosome. Preferably, the genomic sequences are located at or downstream of a transcriptionally active gene, such as at or downstream of the Hint1Z gene (gene ID: 395424). Another possible target is Isl1 (gene ID 369383), which is also on chromosome Z, expressed starting from early stages of embryo development. Figure 3 shows an embodiment of a homology arm on chromosome Z downstream of the Hint1Z gene.

[0090] The LHA and / or RHA target sequence may be selected such that the LHA and / or RHA target sequence is specifically integrated into the Z chromosome and not into any other chromosome of the cell, for example, by spontaneous homologous recombination or homology-directed repair (HDR). Homologous recombination may occur naturally. Furthermore, the LHA and / or RHA target sequence may be selected depending on which method is relied upon to integrate the first target sequence into the chromosome. Methods of integrating nucleotide sequences into chromosomes include targeted homologous recombination, site-specific recombinases, and genome editing with engineered nucleases, and are well known in the art (e.g., Menke D. Genesis (2013) 51:-618; Capecchi, Science (1989) 244:1288-1292; Santiago et al., Proc Natl Acad Sci (2013) 244:1288-1292). (See USA (2008) 105: 5809-5814, International Patent Application No. WO2014 / 085593, WO2009 / 071334 and WO2011 / 146121, U.S. Patent No. 8771945, U.S. Patent No. 8586526, U.S. Patent No. 6774279 and U.S. Patent Application Publication No. 2003 / 0232410, U.S. Patent No. 2005 / 0026157, U.S. Patent No. 2006 / 0014264). PB transposase is also contemplated. Agents for introducing nucleic acid changes into genes of interest can be designed by publicly available resources.

[0091] In certain embodiments, the first 5' nucleotide of the LHA corresponds to chromosome Z, position 44,914,961. In certain embodiments, the first corresponding nucleotide sequence of the LHA is located on chromosome Z, position 44,914,961 to position 44,916,456.

[0092] In certain embodiments, the first 5' nucleotide of the LHA corresponds to Gallus gallus chromosome Z, assembly GRCg6a, NC_006127.5, position 44,914,961. In certain embodiments, the first corresponding nucleotide sequence of the LHA is located in Gallus gallus chromosome Z, assembly GRCg6a, NC_006127.5, position 44,914,961 to position 44,916,456.

[0093] In certain embodiments, the first 5' nucleotide of RHA corresponds to chromosome Z, position 44,916,480. In certain embodiments, the second corresponding nucleotide sequence of RHA is located on chromosome Z, from position 44,916,480 to position 44,918,043.

[0094] In certain embodiments, the first 5' nucleotide of RHA corresponds to Gallus gallus chromosome Z, assembly GRCg6a, NC_006127.5, position 44,916,480. In certain embodiments, the second corresponding nucleotide sequence of RHA is located in Gallus gallus chromosome Z, assembly GRCg6a, NC_006127.5, position 44,916,480 to position 44,918,043.

[0095] In certain embodiments, (i) the LHA comprises the nucleotide sequence set forth in SEQ ID NO: 105, or a fragment thereof, such as at least 50 or at least 500 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 105, (ii) the RHA comprises the nucleotide sequence set forth in SEQ ID NO: 106, or a fragment thereof, such as at least 50 or at least 500 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 106, or (iii) both (i) and (ii). In certain embodiments, (i) the LHA comprises the nucleotide sequence set forth in SEQ ID NO: 105, or a fragment of at least 1000 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 105, (ii) the RHA comprises the nucleotide sequence set forth in SEQ ID NO: 106, or a fragment of at least 1000 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 106, or (iii) both (i) and (ii).

[0096] In certain embodiments, the LHA comprises the nucleotide sequence set forth in SEQ ID NO: 105. In certain embodiments, the LHA comprises at least 50, or at least 500, contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 105. In certain embodiments, the LHA comprises at least 1000 contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 105. In certain embodiments, the LHA comprises 500 contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 105. In certain embodiments, the LHA comprises 1000 contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 105.

[0097] In certain embodiments, the RHA comprises the nucleotide sequence set forth in SEQ ID NO: 106. In certain embodiments, the RHA comprises at least 50, or at least 500, contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 106. In certain embodiments, the RHA comprises at least 1000 contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 106. In certain embodiments, the RHA comprises 500 contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 106. In certain embodiments, the RHA comprises 1000 contiguous nucleotides from the nucleotide sequence set forth in SEQ ID NO: 106.

[0098] In certain embodiments, the LHA comprises the nucleotide sequence set forth in SEQ ID NO: 105 or a fragment of at least 50 or at least 500 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 105, and the RHA comprises the nucleotide sequence set forth in SEQ ID NO: 106 or a fragment of at least 50 or at least 500 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 106. In certain embodiments, the LHA comprises the nucleotide sequence set forth in SEQ ID NO: 105 or a fragment of at least 1000 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 105, and the RHA comprises the nucleotide sequence set forth in SEQ ID NO: 106 or a fragment of at least 1000 contiguous nucleotides of the nucleotide sequence set forth in SEQ ID NO: 106. In certain embodiments, the LHA comprises the nucleotide sequence set forth in SEQ ID NO: 105 and the RHA comprises the nucleotide sequence set forth in SEQ ID NO: 106.

[0099] Optogenetically inducible elements

[0100] promoter

[0101] As will be apparent to one of skill in the art, a "promoter operably linked to a nucleotide sequence" includes a promoter located upstream and participating in cis in the transcription of the nucleotide sequence. A non-limiting example of a promoter operably linked to a nucleotide sequence is a first promoter in an OIE that drives the transcription of a nucleotide sequence in the OIE that encodes an inducer-activated site-specific recombinase enzyme.

[0102] In certain embodiments, the first promoter is a constitutive promoter in the bird. As will be appreciated by those skilled in the art, a "constitutive promoter" encompasses a promoter that allows for continuous transcription of its associated nucleotide sequence or gene.

[0103] In certain embodiments, the first promoter is an inducible promoter in the bird. As will be apparent to those skilled in the art, "inducible promoter" encompasses promoters that allow non-continuous transcription of their associated nucleotide sequence or gene. In certain embodiments, the non-continuous transcription is regulated by an inducer. In certain embodiments, the inducer is exogenous to the bird cell.

[0104] In certain embodiments, the first promoter may be pCAGG (SEQ ID NO: 100), pGK (SEQ ID NO: 109), pCMV (SEQ ID NO: 110), phSyn (SEQ ID NO: 111), or pEF1-a (SEQ ID NO: 112).

[0105] Inducer-activated site-specific recombinase

[0106] In certain embodiments, the inducer-activated site-specific recombinase enzyme can be Cre recombinase (Cre) (SEQ ID NO: 113), or Mag (SEQ ID NO: 114 and SEQ ID NO: 65).

[0107] According to certain embodiments, the inducer initiates or increases transcription of a nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme. In other embodiments, the inducer initiates or increases translation of an mRNA encoding an inducer-activated site-specific recombinase. In yet other embodiments, the inducer initiates or increases activity of an inducer-activated site-specific recombinase enzyme. According to certain embodiments, the inducer initiates or increases the formation of a functional inducer-activated site-specific recombinase enzyme by complexing non-functional fragments of the site-specific recombinase with each other. As will be apparent to one of skill in the art, multiple non-functional fragments (peptides) of an enzyme in which the enzyme is found as a polypeptide in nature can interact to form a functional enzyme even though each fragment (peptide) is not covalently linked to the other fragments (peptides), as shown in FIG. 2. In certain embodiments, the inducer-activated site-specific recombinase comprises non-functional peptide fragments of the inducer-activated site-specific recombinase that combine in the presence of the inducer to form an active inducer-activated site-specific recombinase enzyme.

[0108] The term "inducible" as used herein may encompass all aspects of the switch, regardless of the molecular mechanism involved. Thus, the switch may include, but is not limited to, an antibiotic-based induction system, an electromagnetic energy-based induction system, a small molecule-based induction system, a nuclear receptor-based induction system, and a hormone-based induction system. In some embodiments, the switch is a light-inducible system, a tetracycline (Tet) / DOX-inducible system, an abscisic acid (ABA)-inducible system, a cumate repressor / operator system, a 40HT / estrogen-inducible system, an ecdysone-based induction system, or an FKBP12 / FRAP (FKBP12-rapamycin complex) induction system. In certain instances where the inducer is administered to an unhatched egg, the inducer can penetrate the egg shell. In certain instances, the inducer is not toxic to the female embryo in the egg and does not alter the development of the female embryo in the egg.

[0109] As used herein, the term "switch" refers to a single component or set of components that act in concert to affect a change and encompasses all aspects of a biological function, such as activation, repression, promotion or termination of that function. In one embodiment, the switch relates to an inducible and / or repressible system used in gene regulation. In general, an inducible system may be turned off unless some molecule or form of energy (called an inducer) is present that allows gene expression. This molecule is said to "induce expression". How this happens depends on the control mechanism and different cell types. Repressive systems are "off" except in the presence of some molecule or form of energy (called a suppressor) that suppresses gene expression. How this happens depends on the control mechanism and different cell types.

[0110] Exemplary optogenetic switches are shown in Figures 4A-4C, each of which utilizes the light-sensitive dimerization protein domains cryptochrome 2 (CRY2) and CIB1 from Arabidopsis thaliana, and a site-specific recombinase as the effector molecule. CRY2 is fused in-frame to one half of the Cre recombinase, while CIB1 is fused in-frame to the other half of the Cre recombinase, i.e., the split recombinase enzyme. Thus, when an inducer (blue light) is provided, CRY2 and CIB1 heterodimerize to produce a functional Cre recombinase capable of carrying out site-specific recombination.

[0111] In certain embodiments, expression of the inducer-activated site-specific recombinase enzyme is induced by an inducer. In certain embodiments, the inducer is electromagnetic energy. In certain embodiments, the electromagnetic energy is visible light or a component of visible light with a wavelength of 380-740 nm. In certain embodiments, the component of visible light is blue light with a wavelength of 450-485 nm.

[0112] In one embodiment, the inducer-activated site-specific recombinase is induced using electromagnetic energy. The visible light component may have a wavelength in the range of 450 nm to 700 nm or 450 nm to 500 nm, i.e., blue light. The blue light may have an intensity of at least 0.2 mW / cm2 or at least 4 mW / cm2. The visible light component may have a wavelength in the range of 620 nm to 700 nm, i.e., red light. Single or multiple applications of visible light in any order and in any combination are contemplated. Visible light may be delivered as single or multiple sequential applications or as pulses (pulsed delivery).

[0113] Examples of such optogenetic switches are described in Muller et al., Biol Chem. 2015 Feb, 396(2):145-52. doi:10.1515 / hsz-2014-0199, Motta Mena et al., Nat Chem Biol. 2014 Mar, 10(3):196-202, and WO2014 / 018423.

[0114] Cre recombinase from P1 bacteriophage and Flp recombinase from yeast Saccharomyces cerevisiae are site-specific DNA recombinases that each recognize a unique 34 base pair DNA sequence (termed "Lox" and "FRT", respectively). Sequences adjacent to either Lox or FRT sites can be readily removed via site-specific recombination upon expression of Cre or Flp recombinase, respectively.

[0115] In certain embodiments, the recombinase recognition site can be Lox511, Lox5171, Lox2272, m2, Lox71, Lox66, FRT, F1, F2, F3, F4, F5, FRT(LE), FRT(RE), attB, attP, attL, or attR.

[0116] For example, Lox sequences consist of an asymmetric 8-base pair spacer region flanked by 13-base pair inverted repeat sequences. Cre rejoins the 34-base pair Lox DNA sequence by binding to the 13-base pair inverted repeats and catalyzing strand breaks and religation within the spacer region. The staggered DNA breaks made by Cre in the spacer region are separated by 6 base pairs to give overlapping regions that act as homology sensors to ensure that only recombination sites with the same overlapping region rejoin.

[0117] In certain embodiments, a site-specific recombinase system is used to remove DNA, such as a selection cassette, from bird chromosomes after homologous recombination. Notably, Cre and Flp recombinases leave a 34-base pair Lox or FRT "scar". The remaining Lox or FRT sites are usually left behind in introns or 3'UTRs of the modified locus, and these sites typically do not significantly disrupt gene function. This system also allows the generation of conditional alleles that can be inactivated or activated temporarily or tissue-specifically.

[0118] Thus, Cre / Lox and Flp / FRT recombination may involve the introduction of a targeting vector with 3' and 5' homology arms containing the mutation of interest, two Lox or FRT sequences, and a selectable cassette that is typically located between the two Lox or FRT sequences. Positive selection is applied to identify homologous recombinants containing the targeted mutation. Transient expression of Cre or Flp combined with negative selection results in excision of the selection cassette, selecting for cells lacking the cassette. The final targeted allele contains a Lox or FRT scar of the exogenous sequence.

[0119] In certain embodiments, the inducer is heat, ultrasound, electromagnetic energy, or a chemical, hi certain embodiments, the inducer is delivered to the egg during the process of egg production inside the pre-laying bird.

[0120] While unwanted male embryos can be freely exposed to sunlight (or blue light, for example), transgenic cells and organisms need to be kept under special conditions to prevent unwanted activation of the optogenetic system provided herein. Also, females are known to be more productive when reared under green / red light conditions.

[0121] In certain embodiments, the methods provided herein are carried out under green light. In certain embodiments, the cells and organisms provided herein are kept under green light. In certain embodiments, the green light is of a wavelength of 500-565 nm.

[0122] In certain embodiments, the methods provided herein are carried out under red light. In certain embodiments, the cells and organisms provided herein are kept under red light. In certain embodiments, the red light is of a wavelength between 625 and 740 nm.

[0123] Several methods of energy activation are contemplated, such as electric field energy and / or ultrasound, which have similar effects. If necessary, the protein pairing of the switch is altered and / or modified for maximum effect with another energy source.

[0124] The electric field energy may be administered substantially as described in the art using one or more electric pulses of about 1 volt / cm to about 10 kilovolts / cm under in vivo conditions. Instead of or in addition to pulses, the electric field may be delivered continuously. The electric pulses may be applied for 1 to 500 milliseconds, preferably 1 to 100 milliseconds. The electric field may be applied continuously or in pulses for about 5 minutes. As used herein, "electric field energy" is the electric energy to which the cells are exposed. In certain embodiments, the electric field has a strength of about 1 volt / cm to about 10 kilovolts / cm or more under in vivo conditions (see, e.g., WO97 / 49450).

[0125] As used herein, the term "electric field" includes one or more pulses with variable capacitance and voltage, including exponential and / or square wave, and / or modulated and / or modulated square wave forms. References to electric field and electricity should be taken to include reference to the presence of a potential difference in the environment of the cells. Such environment may be established by electrostatics, alternating current (AC), direct current (DC), etc., as known in the art. The electric field may be uniform, non-uniform, or not, and may vary in strength and / or direction in a time-dependent manner.

[0126] Single or multiple applications of electric fields and single or multiple applications of ultrasound are also possible in any order and in any combination. The ultrasound and / or electric fields may be delivered as single or multiple sequential applications or as pulses (pulsed delivery).

[0127] lethal-inducing factors

[0128] As will be apparent to those skilled in the art, a "first protein operably linked to the activity of a second protein" encompasses the second protein controlling the activity of the first protein in trans. A non-limiting example of a first protein operably linked to the activity of a second protein is a death-promoting protein encoded in the LIE that is activated by the activity of an inducer-activated site-specific recombinase enzyme encoded in the OIE. The terms "in cis" and "in trans" are widely accepted and understood by those skilled in the art.

[0129] Since the inducer-activated site-specific recombinase enzyme activates the death-promoting protein in trans, the respective locations of the OIE and LIE in the same molecule or different molecules may differ in different embodiments. In certain embodiments, the OIE and LIE are in the same molecule. In certain embodiments, the OIE and LIE are in different molecules.

[0130] In certain embodiments, the activating enzyme (e.g., a recombinase enzyme such as Cre) is separated from the lethal gene cassette. In this case, the activating enzyme is inserted into the genome of either a male or female bird, and the inactive lethal cassette is inserted into the Z chromosome of the corresponding sex of the bird. In this case, activation of the lethality of the male embryo is performed simply by mating the two transgenic parents. Figures 19A-19B are embodiments of targeting vectors in which the activating enzyme (e.g., Cre) is separated from the lethal gene cassette.

[0131] In certain embodiments, the LIE further comprises a second promoter operably linked to a third nucleotide sequence encoding a death-promoting protein. In certain embodiments, the second promoter is a constitutive promoter in birds. In certain embodiments, the second promoter is an inducible promoter in birds. In certain embodiments, the second promoter can be pCAGG (SEQ ID NO: 100), pGK (SEQ ID NO: 109), pCMV (SEQ ID NO: 110), phSyn (SEQ ID NO: 111), or pEF1-a (SEQ ID NO: 112).

[0132] Death-promoting proteins

[0133] As used herein, the term "lethal protein" refers to a protein that is lethal to an avian embryo (eg, a male embryo), thus preventing the hatching of a live male bird from an egg.

[0134] In certain embodiments, lethal-inducing proteins that can disrupt fundamental steps of early embryonic development, such as N-cadherin, and proteins that disrupt essential signaling pathways, such as those mediated by bone morphogenetic proteins (BMPs) or fibroblast growth factors (FGFs).

[0135] In certain embodiments, the death-inducing protein may be a toxin, a pro-apoptotic protein, an inhibitor of the Wingless / integrated (Wnt) signaling pathway, a bone morphogenetic protein (BMP) antagonist, a fibroblast growth factor (FGF) antagonist, or a death-inducing fragment thereof. In certain embodiments, the death-inducing protein is a toxin or a death-inducing fragment thereof. In certain embodiments, the death-inducing protein is a pro-apoptotic protein or a death-inducing fragment thereof. In certain embodiments, the death-inducing protein is an inhibitor of the Wnt signaling pathway or a death-inducing fragment thereof. In certain embodiments, the death-inducing protein is a BMP antagonist or a death-inducing fragment thereof. In certain embodiments, the death-inducing protein is an FGF antagonist or a death-inducing fragment thereof. As will be apparent to one skilled in the art, a "death-inducing fragment" of a molecule is any fragment of a molecule sufficient to induce death.

[0136] In certain embodiments, the death-inducing protein may be diphtheria toxin A (DTA) (SEQ ID NO: 93), wild-type caspase 3 (SEQ ID NO: 95), constitutively active caspase 3 (SEQ ID NO: 97), or Noggin (SEQ ID NO: 99). In certain embodiments, the death-inducing protein may be Pseudomonas exotoxin (GenBank Accession No. ABU63124), diphtheria toxin (GenBank Accession No. AAV70486), or ricin toxin (GenBank Accession No. EEF27734). In certain embodiments, the death-inducing protein may be interleukin 2 (GenBank Accession No. CAA00227), CD3 (GenBank Accession No. P07766), CD16 (GenBank Accession No. NP_000560.5), interleukin 4 (GenBank Accession No. NP_000580.1), or interleukin 10 (GenBank Accession No. P22301).

[0137] In certain embodiments, lethality is mediated by RNA-guided DNA endonuclease enzyme. In certain embodiments, the DNA editing agent further comprises a nucleotide sequence encoding a guide RNA that targets an essential gene of the bird, and the nucleotide sequence is operably linked to the activity of an inducer-activated site-specific recombinase enzyme. In certain embodiments, the essential gene can be bone morphogenetic protein receptor type IA (BMPR1A, gene ID: 396308), bone morphogenetic protein 2 (BMP2, gene ID: 378779), bone morphogenetic protein 4 (BMP4, gene ID: 396165), or fibroblast growth factor receptor 1 (FGFR1, gene ID: 396516).

[0138] As will be apparent to those skilled in the art, the term "RNA-guided DNA endonuclease enzyme" encompasses a DNA endonuclease enzyme that unwinds DNA and searches for a site complementary to the guide RNA molecule. In certain embodiments, the guide RNA molecule comprises a nucleotide sequence set forth in one of SEQ ID NOs: 66-77.

[0139] Safe Lock Element

[0140] In one embodiment, the DNA editing agent disclosed herein may further comprise a "safe-lock" element that ensures that the optogenetic killing mechanism is inactive until the safe-lock element is removed. This element essentially renders the optogenetic killing system inactive by default. The optogenetic killing system becomes active only when the genome-edited cells are exposed to an agent that can remove the safe-lock element derived from the DNA editing agent. This "safe-lock" mechanism ensures that the HR-treated cells do not need to be protected from light throughout the manufacturing process, since the optogenetic system is essentially inactive.

[0141] In one embodiment, the safelock element is inserted downstream of the promoter in the OIE but upstream of the sequence encoding the inducer-activated site-specific recombinase. The safelock element comprises a nucleotide sequence (STOP element) that prevents transcription of the inducer-activated site-specific recombinase encoded by the OIE (see FIG. 26A, inactive "locked" state). In one embodiment, the safelock element comprises a coding sequence for a protein followed by a polyadenylation site. In other embodiments, any other sequence that prevents transcription of downstream coding sequences can also be used as a safelock element. In one embodiment, the safelock element is flanked by two FRT sites. Thus, the safelock element can be removed upon expression of the Flp recombinase. When the safelock element is removed, the light gene encoded in the OIE can be transcribed and become active in a light-dependent manner (FIG. 26A, active "unlocked" state).

[0142] In another embodiment, the sequences encoding the safelock element and the inducer-activated site-specific recombinase are flanked by Lox sequences (Figure 26B, inactive "locked" state). Upon expression of Cre recombinase, the sequences encoding the safelock element and the inducer-activated site-specific recombinase can be removed, thus allowing expression of the lethality-promoting protein encoded in the LIE (Figure 26B, lethality-activated state).

[0143] Chimera Chick

[0144] As used herein, the term "chimeric", "chimera" or "chimeric chick" refers to bird cells containing the DNA editing agent disclosed herein or birds having cells containing the DNA editing agent disclosed herein. It should also be noted that chimeric embryos or chimeric adults can be referred to as "surrogates", and thus these terms can be used interchangeably. Representative examples of chimeric bird cells include, but are not limited to, avian primordial germ cells (PGCs), such as gonadal PGCs, blood PGCs, germinal crescent PGCs, or gametes, containing the DNA editing agent disclosed herein. Representative examples of chimeric birds include, but are not limited to, chickens, turkeys, ducks, geese, quails, pheasants, or ostriches, having cells containing the DNA editing agent disclosed herein.

[0145] As used herein, the term "method" refers to methods, means, techniques, and procedures for accomplishing a given task, including, but not limited to, methods, means, techniques, and procedures known by or readily developed from known methods, means, techniques, and procedures by a practitioner of chemistry, pharmacology, biology, biochemistry, or medicine.

[0146] In one embodiment, an avian cell is provided that comprises an exogenous polynucleotide cassette as disclosed herein comprising the formula 5'-OIE-LIE-3' or the formula 5'-LIE-OIE-3', where (i) the OIE is an optogenetically inducible element comprising a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, and (ii) the LIE is a lethal-inducible element comprising a third nucleotide sequence encoding a death-promoting protein operably linked to activity of the inducer-activated site-specific recombinase enzyme. The exogenous polynucleotide cassette is stably integrated into the Z chromosome of the cell.

[0147] In another aspect, a population of avian cells is further provided, comprising an avian cell comprising an exogenous polynucleotide cassette as disclosed herein. The polynucleotide cassette comprises the formula 5'-OIE-LIE-3' or the formula 5'-LIE-OIE-3', (i) the OIE is an optogenetically inducible element comprising a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, and (ii) the LIE is a lethal-inducible element comprising a third nucleotide sequence encoding a death-promoting protein operably linked to the activity of the inducer-activated site-specific recombinase enzyme. The exogenous polynucleotide cassette is stably integrated into the Z chromosome of the cell.

[0148] In certain embodiments, the avian cell comprises an exogenous polynucleotide cassette comprising the formula 5'-LHA-OIE-LIE-RHA-3' or the formula 5'-LHA-LIE-OIE-RHA-3', where (i) LHA is a left homology arm comprising a first nucleotide sequence substantially homologous to a first corresponding nucleotide sequence on the avian chromosome Z, (ii) OIE is an optogenetic inducible element comprising a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, (iii) LIE is a lethal-inducing element comprising a third nucleotide sequence encoding a lethal-promoting protein operably linked to activity of an inducer-activated site-specific recombinase enzyme, and (iv) RHA is a right homology arm comprising a fourth nucleotide sequence substantially homologous to a second corresponding nucleotide sequence on the avian chromosome Z.

[0149] In certain embodiments, the avian cells comprising the exogenous polynucleotide cassette comprise avian primordial germ cells (PGCs). In certain embodiments, the avian PGCs can be gonadal PGCs, blood PGCs, or germinal crescent PGCs.

[0150] As used herein, the terms "primordial germ cells" and "PGCs" refer to diploid cells that are present in the early embryo and can differentiate / develop into haploid gametes (i.e., sperm and eggs) in the adult bird. PGCs can also be obtained from the blastoderm at early stages of development.

[0151] As known to those skilled in the art, primordial germ cells can be isolated from various developmental stages and various sites in the developing avian embryo, including, but not limited to, the genital ridge, developing gonads, blood, and genital crescent (Chang et al., Cell Biol Int 21:495-9, 1997; Chang et al., Cell Biol Int 19:143-9, 1995; Allioli et al., Dev Biol 165:30-7, 1994; Swift, Am J Physiol 15:483-516; PCT International Publication No. WO99 / 06533). The genital ridge is a part of the developing embryo that is well known to those skilled in the art (Strelchenko, Theriogenology 45:130-141, 1996; Lavoir, J Reprod Dev 37:413-424, 1994). Typically, PGCs can be positively stained by the Periodic Acid Schiff (PAS) technique. In some species, PGCs can be identified using anti-SSEA antibodies (one notable exception of PGCs that do not display SSEA antigen is the turkey). Various techniques for isolating and purifying PGCs are known in the art, including enrichment of PGCs from blood using Ficoll density gradient centrifugation (Yasuda et al., J Reprod Fertil 96:521-528, 1992).

[0152] In vitro culture of PGCs is possible using media containing chicken and bovine serum, conditioned medium, feeder cells, and growth factors such as FGF2 (van de Lavoir et al. 2006, Nature 441:766-769. doi:10.1038 / nature04831; Choi et al. 2010, PLoS ONE 5:e12968. doi:10.1371 / journal.pone.0012968; MacDonald et al., 2010, PLoS ONE 5:e15518. doi:10.1371 / journal.pone.0015518). It has been shown that PGCs can be expanded using feeder replacement medium containing growth factors that activate FGF, insulin and TGF-β signaling pathways (Whyte et al. 2015, Stem Cell Rep 5:1171-1182. doi:10.1016 / j.stemcr.2015.10.008).

[0153] Primordial germ cells (PGCs) can be provided and formulated to carry out the subject matter of the present disclosure by any suitable technique, and can be stored, frozen, cultured, etc., prior to use as desired. For example, primordial germ cells can be collected from donor embryos at appropriate embryonic stages. The stages of avian development are described herein according to one of two art-recognized staging systems: the Eyal-Giladi & Kochav system (EG&K: Eyal-Giladi & Kochav, Dev Biol 49:321-327, 1976), which uses Roman numerals to refer to preprimitive streak stages of development, and the Hamburger & Hamilton staging system (H&H: Hamburger & Hamilton, J Morphol 88:49-92, 1951), which uses Arabic numerals to refer to post-laying stages. Unless otherwise specified, the stages described herein are according to the H&H staging system. In certain embodiments, the PGCs are derived from blood isolated from stage 14 (H&H) embryos. In certain embodiments, the PGCs are derived from blood isolated from stage 15 (H&H) embryos. In certain embodiments, the PGCs are derived from blood isolated from stage 16 (H&H) embryos.

[0154] In one embodiment, PGCs can be isolated at the germinal crescent stage, at stage 4, or through stage 30 with cells collected from the blood, genital ridges, or gonads at later stages. Primordial germ cells are generally twice the size of somatic cells and can be easily distinguished and separated from somatic cells based on size. Male (or homogametic) primordial germ cells (ZZ) can be distinguished from heterogametic primordial germ cells (Zw) by any suitable technique, such as collecting germ cells from a particular donor and typing other cells from that donor, with the collected cells being of the same chromosome type as the typed cells.

[0155] An alternative to the use of PGCs is direct transfection of sperm using the DNA editing agents disclosed herein (Cooper et al., 2016 Transgenic Res 26:331-347, doi:10.1007 / s11248-016-0003-0).

[0156] In one embodiment, to generate chimeric birds from in vitro edited PGCs, exogenous edited cells are injected intravenously into surrogate host embryos at the stage when their endogenous PGCs migrate to the genital ridges. The "donor" PGCs may be of the same species as the surrogate host embryo or a different species. The edited "donor" PGCs must remain viable and, in one embodiment, outcompete endogenous PGCs when they colonize the gonads they form and transmit edited chromosomes through the germline. To provide an advantage to the donor PGCs, the number of endogenous PGCs can be reduced by chemical or genetic ablation (Smith et al., 2015, Andrology 3:1035-1049. doi:10.1111 / andr.12107). Exposure of surrogate embryo blastoderm to emulsified busulfan has been shown to increase germline transmission of donor PGCs to over 90%, but this rate is significantly reduced when PGCs are cultured or cryopreserved (Nakamura et al., 2008, Reprod Fertil Dev 20:900-907. doi:10.1071 / RD08138; Naito et al., 2015, Anim Reprod Sci. 153:50-61. doi:10.1016 / j.anireprosci.2014.12.003). Other methods to skew the ratio of edited to native PGCs are described in U.S. Patent Application No. 2006 / 0095980.

[0157] In certain embodiments, generally genetically modified PGCs can be transplanted into adult gonads as known in the art (Trefil et al., 2017 Sci Rep, Oct 27;7(1) / :14246 doi:10.1038 / s41598-017-14475-w).

[0158] Genetically modified cells (e.g., PGCs) may be formulated for administration to other birds by dissociating the cells (e.g., by mechanical dissociation) and intimately mixing the cells with a pharma- ceutically acceptable carrier (such as phosphate-buffered saline). In one embodiment, the primordial germ cells are gonadal primordial germ cells, or blood primordial germ cells ("gonad" or "blood" refer to the tissue of origin of the original embryo donor). The primordial germ cells administered may be heterogametic (Zw) or homogametic (ZZ). In one embodiment, the PGCs may be administered in a physiologically acceptable carrier, at a pH of about 6 to about 8 or 8.5, in an amount appropriate to achieve the desired effect (e.g., 100 to 30,000 PGCs per embryo). The PGCs may be administered without other components or cells, or other cells and components may be administered with the PGCs.

[0159] Administration of primordial germ cells to a recipient animal in-ovo can be performed at any suitable time when the PGCs can still migrate to the developing gonad. In one embodiment, administration is performed from about stage IX according to the Eyal-Giladi & Kochav (EG&K) staging system to about stage 30 according to the Hamburger & Hamilton staging system of embryonic development, or in another embodiment, at stage 15. Thus, for chickens, the time of administration is days 1, 2, 3, or 4 of embryonic development, e.g., days 2-2.5. Administration is typically performed by injection into any suitable target site, such as the area defined by the amnion (containing the embryo), the yolk sac, etc. In one embodiment, the cells are injected into the embryo itself (including the embryonic body wall). In alternative embodiments, intravascular or intracoelastic injection into the embryo may be used. In other embodiments, injection is performed into the heart. The subject method disclosed herein can be carried out in-ovo with prior sterilization of the recipient bird (e.g., by chemical treatment using busulfan with gamma or X-ray irradiation). As used herein, the term "sterilization" refers to partial or complete inability to produce gametes derived from endogenous PGCs. When donor gametes are collected from such recipients, they can be collected as a mixture of donor and recipient gametes. This mixture can be used directly or can be further processed to increase the proportion of donor gametes therein.

[0160] In-ovo administration of primordial germ cells may be performed by any suitable technique, either manually or in an automated manner. In one embodiment, the in-ovo administration is by injection. The mechanism of in-ovo administration is not critical, but the mechanism must not unduly damage the tissues and organs of the embryo or the extraembryonic membranes surrounding it, so that the process does not unduly reduce hatchability. A hypodermic syringe fitted with an approximately 18-26 gauge needle is suitable for the purpose. A glass sharp-pointed puller pipette with an opening of approximately 20-50 microns in diameter may be used. Depending on the exact stage of development and the location of the embryo, the one-inch needle terminates either in the fluid above the chick or in the chick itself. To prevent damage or blunting of the needle, a pilot hole can be punched or drilled into the shell prior to inserting the needle. If necessary, the egg can be sealed with a substantially bacteria-impermeable sealing material, such as wax, to prevent subsequent ingress of unwanted bacteria. It is envisioned that high-speed injection systems for avian embryos are suitable for carrying out the subject matter disclosed herein. All such devices adapted to carry out the methods disclosed herein include a syringe containing a formulation of primordial germ cells as described herein, which is positioned to inject an egg carried by the device. Additionally, a sealing device operably connected to the injection device may be provided to seal the hole in the egg after injection. In another embodiment, a pulled glass micropipette may be used to introduce the PGCs into the appropriate location within the egg, for example, directly into the bloodstream, either into a vein or artery, or directly into the heart.

[0161] Once the eggs are injected with the modified PGCs, the chimeric embryos are incubated and allowed to hatch, in one embodiment, the chicks are raised to sexual maturity and the chimeric birds produce gametes derived from the donor PGCs.

[0162] In certain embodiments, the avian cells include avian gametes. Gametes (eggs or sperm) from the chimeras (or from directly genetically engineered material as described herein above) are then used to breed founder chickens (F1). Germline transmission may be confirmed using molecular biology techniques known in the art (e.g., PCR and / or Southern blot). The F1 chickens may be backcrossed to generate homozygous ZZ carrier males and carrier females (F2). The founder chicken-derived gametes F2 may then be used to expand breeding colonies. Colonies are typically grown to sexual maturity. Fertilized eggs obtained from these flocks may be tested for early male embryonic lethality by exposure to an inducer (e.g., blue light) that induces a lethal phenotype. After induction (e.g., by blue light illumination), the eggs are incubated (e.g., for 8 days) and screened (e.g., by light candling) to detect early embryonic lethality.

[0163] In one embodiment, a method for generating a chimeric bird is provided, comprising administering to a population of bird cells an exogenous polynucleotide cassette having the formula 5'-OIE-LIE-3' or the formula 5'-LIE-OIE-3', where (i) the OIE is an optogenetically inducible element comprising a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, and (ii) the LIE is a lethal-inducing element comprising a third nucleotide sequence encoding a lethal-promoting protein operably linked to the activity of the inducer-activated site-specific recombinase enzyme. The exogenous polynucleotide cassette is stably integrated into the Z chromosome of the cells. These genome-edited cells are then injected into a recipient bird embryo.

[0164] In another embodiment, a method of generating a chimeric bird is provided, comprising administering to a population of avian cells an exogenous polynucleotide cassette having the formula 5'-LHA-OIE-LIE-RHA-3' or the formula 5'-LHA-LIE-OIE-RHA-3', wherein (i) LHA is a left homology arm comprising a first nucleotide sequence substantially homologous to a first corresponding nucleotide sequence on chromosome Z of the bird, (ii) OIE is an optogenetically inducible element comprising a first promoter operably linked to a second nucleotide sequence encoding an inducer-activated site-specific recombinase enzyme, (iii) LIE is a lethal-inducing element comprising a third nucleotide sequence encoding a lethal-promoting protein operably linked to activity of an inducer-activated site-specific recombinase enzyme, and (iv) RHA is a right homology arm comprising a fourth nucleotide sequence substantially homologous to a second corresponding nucleotide sequence on chromosome Z of the bird. The exogenous polynucleotide cassette is stably integrated into the Z chromosome of the cells. These genome-edited cells are then injected into recipient bird embryos.

[0165] In certain embodiments, the method further comprises incubating the chimeric bird embryo in-ovo until it hatches, hi certain embodiments, the method further comprises raising the chimeric bird to sexual maturity, wherein the chimeric bird produces gametes derived from the administered cells.

[0166] In certain embodiments, the genome-edited cells are administered by in-ovo injection. In another embodiment, the embryo can be removed from the eggshell, injected "ex-ovo", and then placed back into the surrogate eggshell. In certain embodiments, the administered cell population is from the same avian species as the recipient avian embryo. In certain embodiments, the administered cell population is from a different avian species than the recipient avian embryo.

[0167] In another embodiment, chimeric birds can be generated by injecting genome-edited PGCs into blastoderm.Generally, genome-edited PGCs can be returned or injected back into the location where endogenous PGCs are located.In one embodiment, genome-edited PGCs can be returned to blastoderm.Alternatively, genome-edited PGCs can be returned to reproductive crescent, blood, embryonic gonad, or even adult gonad.

[0168] In certain embodiments, the genome-edited avian cell population is administered when the recipient embryo is about stage IX according to the Eyal-Giladi & Kochav staging system. In certain embodiments, the avian cell population is administered when the recipient embryo is about stage 30 according to the Hamburger & Hamilton staging system. In certain embodiments, the cell population is administered when the recipient embryo is about stage IX according to the Eyal-Giladi & Kochav staging system and about stage 30 according to the Hamburger & Hamilton staging system. In certain embodiments, the cell population is administered when the recipient embryo is after stage 14 according to the Hamburger & Hamilton staging system.

[0169] In certain embodiments, the genome-edited avian cell population is administered after irradiation of the embryo. In certain embodiments, the irradiation comprises gamma irradiation or X-ray irradiation. In certain embodiments, the irradiation comprises gamma irradiation of 600-800 radians. In certain embodiments, the irradiation comprises irradiation of 600-800 radians. In certain embodiments, the irradiation comprises irradiation of 400-1000 radians. In certain embodiments, the irradiation comprises irradiation of 200-1200 radians.

[0170] In another aspect, there is further provided a chimeric bird obtained from the above method.

[0171] How to use

[0172] In one embodiment, a method of generating an avian cell is provided, comprising contacting an avian cell with an exogenous polynucleotide cassette having the formula 5'-OIE-LIE-3' or the formula 5'-LIE-OIE-3' described herein. The exogenous polynucleotide cassette is stably integrated into the Z chromosome of the cell. In another embodiment, the method comprises contacting an avian cell with an exogenous polynucleotide cassette having the formula 5'-LHA-OIE-LIE-RHA-3' or the formula 5'-LHA-LIE-OIE-RHA-3'. The exogenous polynucleotide cassette is stably integrated into the Z chromosome of the cell.

[0173] In another embodiment, a method of inducing lethality in male embryos in fertilized avian eggs is provided, comprising administering a DNA editing agent disclosed herein to a population of avian cells to generate genome-edited avian cells; transferring these genome-edited avian cells to a recipient avian embryo; and exposing the embryo to an inducer that induces expression of a lethality-promoting protein encoded by the DNA editing agent, thereby inducing lethality in the avian male embryo. Various elements of the DNA editing agent, such as optogenetic inducible elements, inducer-activated site-specific recombinases, inducers, and lethality-promoting proteins, have been discussed above.

[0174] In one embodiment, the DNA editing agent used in the above method of inducing lethality in a male embryo comprises: (i) an LHA comprising the sequence of SEQ ID NO: 105; (ii) an OIE comprising the sequence of SEQ ID NO: 116, the sequence of SEQ ID NO: 104, the sequence of SEQ ID NO: 102, the sequence of SEQ ID NO: 103, the sequence of SEQ ID NO: 101, the sequence of SEQ ID NO: 116, the sequence of SEQ ID NO: 100; or an OIE comprising the sequence of SEQ ID NO: 116, the sequence of SEQ ID NO: 104, the sequence of SEQ ID NO: 108, the sequence of SEQ ID NO: 103, the sequence of SEQ ID NO: 107, the sequence of SEQ ID NO: 116, the sequence of SEQ ID NO: 100; (iii) an LIE comprising the sequence of SEQ ID NO: 92, or SEQ ID NO: 94, or SEQ ID NO: 96, or SEQ ID NO: 98; and (iv) an RHA comprising the sequence of SEQ ID NO: 106.

[0175] In another embodiment, a method for inducing male embryo lethality in a fertilized avian egg is provided, comprising administering to a population of avian cells a DNA editing agent comprising a safe-lock element as disclosed herein, thereby generating genome-edited avian cells; transferring these genome-edited avian cells to a recipient avian embryo; and exposing the embryo to an agent that removes the STOP element from the STOP element, thereby inducing expression of a lethality-promoting protein encoded by the DNA editing agent and inducing male embryo lethality in the fertilized avian egg. Various elements of the DNA editing agent have been discussed above, such as optogenetic inducible elements, inducer-activated site-specific recombinase, inducers, safe-lock elements, lethality-promoting proteins, etc. In one embodiment, the DNA editing agent used in the above method for inducing male embryo lethality comprises one of the sequences of SEQ ID NOs: 120-127.

[0176] In one embodiment, removal of the STOP element from the DNA editing agent requires expression of Flp when the STOP element is flanked by two FRT sites (see FIG. 26A, inactive "locked" state). In one embodiment, expression of Flp can be achieved by contacting genome-edited avian cells with a nucleotide sequence encoding a Flp protein (e.g., SEQ ID NO: 129 or SEQ ID NO: 131). Upon expression of Flp recombinase, the STOP element inserted between the promoter and the sequence encoding the inducer-activated site-specific recombinase in the OLE is removed, thus allowing expression of the inducer-activated site-specific recombinase (FIG. 26A, active "unlocked" state). Further exposure to an inducer (e.g., blue light) activates the inducer-activated site-specific recombinase, resulting in removal of the OLE and expression of the lethality-promoting protein encoded in the LIE (FIG. 26A, lethality-activated state).

[0177] In another embodiment, removal of the STOP element can be achieved by contacting the genome-edited avian cell with a nucleotide sequence encoding a Cre protein (e.g., SEQ ID NO: 128 or SEQ ID NO: 132). In one embodiment, the safe lock element and the sequence encoding the inducer-activated site-specific recombinase are flanked by Lox sequences (Figure 26B, inactive "lock" state). Upon expression of the Cre recombinase, the safe lock element and the sequence encoding the inducer-activated site-specific recombinase are removed, thus allowing expression of the lethality-promoting protein encoded in the LIE (Figure 26B, lethality-activated state).

[0178] According to the principles of the technology provided herein, the step of inducing lethality can be performed at any developmental stage, for example, immediately after egg laying. The sooner death is induced, the sooner the embryo will die. Thus, embryonic lethality is achieved thereafter. If toxins or pro-apoptotic agents are used to induce embryonic death, the embryo will die soon after induction. If LIE is based on genes that disrupt essential signaling pathways, such as BMP, the pathway is activated to induce cell death at the required developmental time point, for example by expressing the BMP4 antagonist Noggin. For example, BMP is active during stages including blastula formation, gastrulation, neurulation, and organogenesis. In certain embodiments, lethality is induced during egg laying, and the embryo dies within about 36 hours after exposure to Noggin. As an example, if the optogenetic system is activated immediately after egg laying (stages X-XIII EG&K), the embryo dies at that stage. BMP4 knockout mouse embryos die in utero between 6.5 days post-coitum (dpc) and about 8.5-9 dpc, which corresponds to the first 30-36 hours of incubation in chick embryo development. In certain embodiments, lethality is induced during the 21 days from fertilization to hatching. In certain embodiments, lethality is induced more than once during the 21 days from fertilization to hatching. In certain embodiments, lethality is induced in the egg prior to the early blastula stage, known as stage X-XIII EG&K (Eyal-Giladi and Kochav, 1976).

[0179] In certain embodiments, the method is performed in-vivo. In certain embodiments, the method is performed ex-vivo. In certain embodiments, the method is performed in-ovo. In certain embodiments, the method is performed in-vitro.

[0180] It is understood that certain features of certain embodiments that are described in the context of separate embodiments for clarity may also be provided in combination in a single embodiment. Conversely, various features of certain embodiments that are described in the context of a single embodiment for brevity may also be provided separately or in any suitable subcombination or in any other described embodiment as appropriate. Certain features that are described in the context of various embodiments should not be considered essential features of those embodiments, unless the embodiment is inoperable without those elements.

[0181] The various embodiments and aspects as delineated hereinabove and as claimed in the claims section below find experimental support in the following examples. EXAMPLES

[0182] Reference is now made to the following examples which, together with the above description, illustrate some embodiments in a non-limiting manner.

[0183] Generally, the nomenclature used and the laboratory procedures utilized herein include molecular, biochemical, microbiological and recombinant DNA techniques. Such techniques are fully explained in the literature, see, for example, "Molecular Cloning: A laboratory Manual" Sambrook et al., (1989); "Current Protocols in Molecular Biology" Volumes I-III Ausubel, RMed. (1994); Ausubel et al., "Current Protocols in Molecular Biology", John Wiley and Sons, Baltimore, Maryland (1989); Perbal, "A Practical Guide to Molecular Cloning", John Wiley & Sons, New York (1988); Watson et al., "Recombinant DNA", Scientific American Books, New York; Birren et al. (eds) "Genome Analysis: A Laboratory Manual Series", Vols. 1-4, Cold Spring Harbor Laboratory Press, New York. New York (1998), U.S. Pat. Nos. 4,666,828, 4,683,202, 4,801,531, 5,192,659, and 5,272,057; "Cell Biology: A Laboratory Handbook", Volumes I-III Cellis, JE, ed. (1994); "Culture of Animal Cells-A Manual of Basic Technique" by Freshney, Wiley-Liss, NY (1994), Third Edition; "Current Protocols in Immunology" Volumes I-III Coligan JE, ed. (1994); Stites et al.(eds), "Basic and Clinical Immunology" (8th Edition), Appleton & Lange, Norwalk, CT (1994), Mishell and Shiigi (eds), "Selected Methods in Cellular Immunology", WHFreeman and Co., New See, for example, U.S. Pat. Nos. 3,791,932; 3,839,153; 3,850,752; 3,850,578; 3,853,987; 3,867,517; 3,879,262; 3,901,654; 3,935,074; 3,984,533; 3,996,345; 4,034,074; 4,098,876; 4,879,219; 5,011,771; and 5,281,521; all of which are incorporated herein by reference in their entireties. Synthesis" Gait, MJ, ed. (1984), "Nucleic Acid Hybridization" Hames, BD, and Higgins SJ, eds. (1985), "Transcription and Translation" Hames, BD, and Higgins SJ, eds. (1984), "Animal Cell Culture" Freshney, RI, ed. (1986), "Immobilized Cells and Enzymes" IRL Press, (1986), "A Practical Guide to Molecular Cloning" Perbal, B., (1984) and "Methods in Enzymology" Vol. 1-317, Academic Press, "PCR Protocols: A Guide To Methods And Applications", Academic Press, San Diego, CA (1990), Marshak et al.See, "Strategies for Protein Purification and Characterization--A Laboratory Course Manual" CSHL Press (1996), all of which are incorporated by reference herein as if fully set forth herein. Other general references are provided throughout the specification. The procedures therein are believed to be well known in the art and are provided for the convenience of the reader. All of the information contained therein is incorporated herein by reference.

[0184] Example 1

[0185] The generation of genome-modified chicken lines is a multi-step process. The end product is a female egg-laying hen line that is completely identical in terms of genome content to the laying hens used today in industry (see Figure 1).

[0186] In one embodiment, the workflow includes five major steps: (1) generating and culturing chicken primordial germ cell (PGC) lines; (2) genomic modification in cultured PGCs; (3) implantation of modified PGCs into embryos and producing chimeric chickens screened for germline transmission to identify potential founder carriers; (4) breeding founder chickens from genetic material obtained from the chimeras; and (5) expanding the founder chicken colonies into founder flocks and re-verifying germline transmission.

[0187] material and method

[0188] PGC medium: Avian PGC medium consists of DMEM (Gibco) calcium-free medium diluted with water to 250 mOsmol / L, containing 12.0 mM glucose, 2.0 mM GlutaMax (Gibco), 1.2 mM pyruvate (Gibco), 1x MEM vitamins (Gibco), 1x B-27 supplement (Gibco), 1x NEAA (Gibco), 0.1 mM β-mercaptoethanol (Gibco), 1x nucleosides (Biological Industries), 0.2% ovalbumin (Sigma), 0.1 mg / ml sodium heparin (Sigma), 0.15 mM CaCl2 (Sigma), 1x MEM vitamins (Gibco), 1x Pen / Strep (Biological Industries), and 0.2% chicken serum (Sigma) in avian DMEM. The following growth factors were added before use: human activin A at 25 ng / mL (Peprotech), human FGF2 at 4 ng / mL (R&D Biosystems), and ovotransferrin at 5 μg / ml (Sigma). AkoDMEM refers to a dilution medium containing glucose, pyruvate, and vitamins.

[0189] Induction of PGC lines: PGC lines were induced by placing approximately 1.0–3.0 μL of blood isolated from stage 14–16 (H&H) embryos into 300 μL of medium in a 48-well plate. The medium was changed every 2 days. A total cell number of 1 × 10 5 When the total volume of medium reached 100, replace it every 2 days and culture 2–4 × 10 cells. 5 Cells were grown at 1000 cells / ml medium. Cells were frozen in PGC medium containing 10% DMSO, the temperature was gradually lowered to −80°C, stored for 1–3 days, and transferred to liquid nitrogen.

[0190] Sex determination and characterization of PGC lines: Each PGC line was characterized for sex, mRNA expression of PGC markers, and protein expression of the known PGC marker SSEA14. DNA from donor embryos was isolated and stored for future reference. For sex, 2–4 × 10 5DNA from PGC cells was collected, resuspended in tail buffer (102-T, Viagen) containing 100 μg / ml proteinase K (Sigma) and incubated at 55°C for 3 h. Proteinase K was inactivated at 85°C for 45 min. PCR for sex determination was performed using primers derived from the female chromosome (P17, P18) and W chromosome targeting ribosomal S18 (P19, P20) as controls. For gene expression analysis, RNA was purified using TRIZOL reagent and 1 μg of RNA was used for cDNA library generation by reverse transcription PCR reaction (GoScript Reverse transcriptase, Promega). The cDNA served as template for PCR by using Dazl, Sox2, cPouV, Nanog, Klf4, and cVH primers, P21-P22, P23-P24, P25-P26, P27-P28, P29-P30, and P31-P32, respectively.

[0191] Immunohistochemical staining with anti-SSEA1 antibody: Cells were harvested, fixed with 4% PFA, blocked with 5% normal goat serum in PBS 0.1% Triton, and stained overnight with a 1:100 dilution of anti-SSEA1 antibody (DSHB, Hybridoma bank) in blocking buffer. Secondary antibody was added for 1 h (Alexa Fluor 488, Molecular Probes). After washing cells for 30 min with PBS 0.1% Triton, cells were counterstained with DAPI (Sigma), mounted and covered with mounting medium (Histomount, electron microscopy sciences).

[0192] For transfection, selection and FACS sorting of PGCs, plasmid transfection of PGCs was performed using lipofection or electroporation. For lipofection, Lipofectamine 2000 was used according to the manufacturer's protocol. 3–5 × 10 5Cells were seeded in 96-well plates in AkoDMEM containing NEAA, pyruvate, vitamins, CaCl2, and growth factors (activin A, hFGF, and ovotransferrin). 100 ng of plasmid and 0.25 μl of Lipofectamine 2000 (invitrogen) were separately diluted in 20 μl of OPTI-MEM mix, incubated for 20 min, and pipetted onto the cells. For electroporation, 3 × 10 5 pieces~1.5×10 6 Cells were washed with AkoDMEM, electroporated at 1000V, 12ms, 3 pulses in a neon electroporator (Invitrogen), and immediately plated in 96-well or 48-well plates, respectively, in antibiotic-free PGC medium. Medium was changed after 1–3 hours. Selection with 25–100μg / ml G418 was initiated 72 hours later for 2–4 weeks. Following selection, cells were isolated either manually or by FACS sorting individually. For FACS sorting, gentle cell pipetting was performed and cells were sorted in PGC medium. Positive GFP cells were sorted or pooled as one cell per well into a new 96-well plate using a FACS Aria II. (FACS analysis was performed using a BD FACS Aria II flow cytometer (BD, USA).)

[0193] Plasmid preparation:

[0194] Cloning of CRISPR plasmids: CRISPR sequences were designed using the CRISPR design tool, Zhang lab, MIT. px330-GFP plasmid (modified from Addgene plasmid #42230) was cut using BbsI restriction enzyme to serve as a backbone for CRISPR site insertion to form sgRNA. Oligos-CRISPR1, CRISPR3 (oligos P34-P35 and P36-P37, respectively) for sgRNA CRISPR sites were denatured at 95°C for 30 seconds, slowly annealed, ligated to BbsI-cut plasmid, transformed into E.coli, purified, and sequence verified as described (Cong L, et al., Science. 2013 Jan 3.10.1126 / science.1231143 PubMed 23287718).

[0195] Cloning of pJet-HAs plasmid: The genomic region downstream to the HINT1Z locus on the Z chromosome, including both the 5'HA and 3'HA, was amplified from PGC DNA with primers P1 and P2 using PCR (Kapa, ​​Roche). The PCR product was purified and ligated into the pJet1.2 plasmid (Invitrogen) according to the manufacturer's protocol.

[0196] Construction of targeting vector: pCAGG-IRES-Neo-GFP plasmid was used as template for PCR, and P5-P6 primers were used to amplify the insert pCAGG-IRES-Neo-GFP. pJet-HAs plasmid was used as template for PCR, and P3-P4 primers were used to amplify the vector containing 5'HA and 3'HA. Gibson assembly reaction was performed on the purified vector and insert PCR products taking linearized products of 0.03 pm and 0.06 pm, respectively. Gibson assembly reaction products were transformed into E. coli for plasmid preparation and sequence verified.

[0197] Construction of pCAGG-Optogene vector: To generate the pCAGG-Optogene vector, the optogen plasmids pmCherry-CIBN-CreC and pmCherry-Cry2-CreN11 were used as templates to amplify the optogen using primers P40-P41 and P42-P43, resulting in 1.3 kb and 2.1 kb products, respectively. The two products share overlapping sequences at the P2A site introduced in primers P41 and P42. Single-cycle overhang extension PCR was used to join the two fragments into a single 3.5 kb product that was cleaned from the agarose gel. This product was ligated into the pJet1.2 shuttle vector, which was used as a template for PCR, using primers P44 and P45, which contain tails with SmaI and NheI restriction sites, respectively. This product was digested using the appropriate restriction enzymes and used as an insert for ligation to ligate into the SmaI- and NheI-digested pCAGG-IRES-GFP plasmid, which served as a vector. The ligation products were transformed into E. coli and the propagated plasmids were sequence verified.

[0198] Construction of pGK-DTA-IRES-GFP vector: To generate pGK-DTA-IRES-GFP, the expression vector pSKBS-PGK-DTA was used as a template for PCR with primers P46 and P47, which contain extended sequences for XmaI and NheI restriction sites, respectively. The 0.65 kb product was digested with the respective enzymes and used as an insert for ligation into the XmaI-NheI complementary sites in the pGK-IRES-GFP plasmid, which served as a vector for ligation. The ligation product was transformed into E. coli and the grown plasmid was sequence verified.

[0199] In-ovo electroporation: In-ovo electroporation was essentially performed as previously described. Fertilized eggs were incubated at 37.8 °C for 56–60 h, a window was opened in the eggshell, and plasmid DNA at a concentration of approximately 2 μg / μl was injected into the neural tube using a sharp micropipette with an opening of 10–15 μm in diameter. Three pulses of 25 V, 30 ms were delivered using an ECM830 square wave electroporation system (BTX). After electroporation, the eggshell was sealed with parafilm and the embryos were further incubated until analysis.

[0200] Endonuclease assay: PGCs were transfected with CRISPR1 or CRISPR3 plasmids using Lipofectamine 2000 reagent. After 48 h, individual GFP-positive cells were dissociated into 96-well plates and cultured to form pure colonies. DNA was harvested and a 350 bp region flanking the CRISPR site was PCR amplified with P38-P39 primers. PCR products were denatured at 95°C, slowly annealed, and incubated with T7 endonuclease at 37°C for 1 h. For calibration purposes and as a positive control, the 350 bp PCR product was subcloned into pJet1.2 and the CRISPR site was mutated using site-directed mutagenesis. The mutation introduced replaced the WT sequence ATACCAGATAACGTgCCTTATTTGGCCGTT (SEQ ID NO:2) with ATACCAGATAACGTaatCCTTATTTGGCCGTT (SEQ ID NO:3). This artificial mutation served as a positive control for both the endonuclease assay (FIG. 7A) and control sequencing (FIG. 8B).

[0201] Southern blot assay: Dig labeling of 5'HA, 3'HA and Neo gene probes were prepared by PCR amplification (Longamp, NEB) with primers P13-P14, P15-P16 and P11-P12, respectively, using DIG DNA labeling mix (Roche). 15 μg of genomic DNA was digested with BglII restriction enzyme overnight at 37 °C. DNA fragments were separated by electrophoresis in 0.8% (w / v) agarose gel (20 V, 12 h) and transferred to a positively charged nylon membrane (GE Healthcare). After transfer, the wet membrane was crosslinked using UV light set at 254 nm for 3 min on each side and then rinsed with 2XSSC. The membrane was prehybridized for 2 h at 42 °C using DIG Easy-Hyb hybridization solution (Roche). The probe (50ng / ml) was denatured by heating to 95°C for 5 min and immediately immersed in ice. The denatured probe was added to 10ml of warm DIG Easy-Hyb solution and hybridized at 42°C for 12 h. The membrane was washed twice for 10 min in 2XSSC with 0.1% SDS at room temperature with agitation, then washed three times for 30 min at 65°C in 0.2XSSC with 0.1% SDS with agitation. Further washing and blocking were performed with DIG wash and block buffer set (Roche) and according to their protocol. DIG labeling was detected using anti-digoxigenin-AP antibody 1:10000 (Roche) followed by chemiluminescence reaction using CDP-Star reagent (Roche). Images were taken using a G:BOX gel imaging system (Syngene).

[0202] PGC injection into embryos and whole-mount staining: Freshly laid eggs were incubated tip-up for 58–62 h at 37.8 °C and 55% humidity. After incubation, a 4–8 mm window was opened in the eggshell and 3000–8000 PGCs were injected into the bloodstream using a sharp micropipette with an opening of approximately 30–40 μm. The window was covered with white egg membrane and further sealed with Parafilm or Leukoplast (BSN medical GmbH) tape. Embryos were incubated until they hatched. Several gonads of injected embryos were isolated and taken for whole-mount GFP staining. The gonads were fixed in 4% PFA, washed in PBS for 2 h, blocked with 5% normal donkey serum in PBS 1% Triton, and stained overnight with a 1:20 dilution of mouse anti-SSEA1 antibody or rabbit anti-GFP antibody 1:500 (Abcam) in blocking buffer. After washing with PBS 1% triton for 2 h, secondary donkey anti-mouse cy3 antibody 1:500 (Jackson Immunoresearch Laboratories) or secondary alexa488 anti-rabbit antibody 1:500 (Molecular Probes) was added in blocking buffer for 3 h. Tissues were counterstained with DAPI (Sigma), mounted in glycerol, and imaged under a confocal microscope (Leica, TCS SPE, Wetzlar, Germany).

[0203] The sequences of primers P1 to P32 are set forth in SEQ ID NOs: 4 to 35, and the sequences of primers P34 to P47 are set forth in SEQ ID NOs: 36 to 49.

[0204] The sequence of the plasmid is given below.

[0205] 1. pX330-GFP (SEQ ID NO: 50), 2. CRISPR1 (SEQ ID NO: 51), 3. CRISPR3 (SEQ ID NO: 52), 4. pJet-Has (SEQ ID NO: 53), 5. pCAGG-Neo-IRES-GFP (SEQ ID NO: 54), 6. Targeting Vector (SEQ ID NO: 55), 7. pmCherry-Cry2-CreN (SEQ ID NO: 56), 8. pmCherry-CIBN-CreC (SEQ ID NO: 57), 9. pB-RAGE-GFP (SEQ ID NO: 58), 10. pCAGG-IRES-GFP (SEQ ID NO: 59), 11. pCAGG-Optogenes (SEQ ID NO: 60), 12. pB-RAGE-mCherry (SEQ ID NO: 61), 13. pSK BS-PGK-DTA (SEQ ID NO: 62), 14. pGK-IRES-GFP (SEQ ID NO: 63), 15. pGK-DTA-IRES-GFP (SEQ ID NO: 64).

[0206] result

[0207] Derivation and characterization of PGC lines

[0208] During early stages of embryo development, immediately after egg laying and before the onset of gastrulation, PGCs migrate rostrally to the germinal crescent region of the embryo in front of the extraembryonic mesoderm layer. This migration is thought to "protect" the PGCs from undergoing the differentiation process, as somatic cells do. After approximately 2.5 days of incubation (stages 14-17H&H), it is not until the formation of the Area Opaca Vasculosa, blood and heartbeat that the cells return to the embryo via the bloodstream and colonize the genital ridges inducing the gonads. At these stages, using a micropipette with an opening of approximately 40-60 μm in diameter, 1-3 μl of blood was collected from the embryo's vasculature and transferred to PGC medium in a 48-well plate. PGC medium allows for fast division of PGCs (cell cycle of 20-24 h) while maintaining their undifferentiated state under feeder-free conditions. After 2-3 weeks of culture, the blood cells degraded and disappeared. Within another 1-2 weeks, the cultured PGCs became confluent (Figure 5A). These cells could be further cultured for genetic modification or successfully frozen and thawed for the latter modification. Chicken PGCs in culture have been extensively characterized in the literature using morphological features, protein and mRNA expression patterns, and finally by their ability of gonadal migration when injected into the vasculature of stage-matched recipient embryos. These properties were examined in the generated PGC cell cultures to show that they retained well-established PGC functions. Morphologically, PGCs are large, slightly granulated cells of about 15-20 μm in diameter containing a large nucleus. PGCs are totipotent cells and therefore express pluripotency markers such as cPouV, SOX2, KLF4 and Nanog, as well as two unique germ cell markers, namely, cVH and DAZL. For each PGC line, DNA was extracted for sex determination using primers for ribosomal S18 (P19-P20, product size of 256 bp) as a positive control, and primers for the W chromosome (P17-P18, product size of 415 bp) to specify females (Figure 5B). Additionally, PGCs express membrane SSEA-1 antigen 4 (Figure 5C).

[0209] Ten lines of PGCs were established from laying hens and broiler chickens, both male and female lines. Transfection of plasmids was performed using the cationic lipid transfection reagent Lipofectamine 2000, which interacts with negatively charged DNA, allowing its penetration into the cells. Transfection of a plasmid encoding GFP (pCAGG-GFP) resulted in a transfection efficiency of approximately 15-20% (Figure 5D). Furthermore, transfection of PGCs using electroporation resulted in a higher efficiency of up to 90% (Figure 5E). To demonstrate that cultured PGCs successfully colonize the gonads, GFP-expressing PGCs were injected into the bloodstream at stages 14-16 H&H and embryos were incubated for 10 days. Embryos were dissected and GFP-positive cells were identified in the gonads (Figure 5F).

[0210] Design of CRISPR-Cas9 targets on the Z chromosome

[0211] In one embodiment, DNA editing into the Z chromosome was performed using CRISPR-Cas9 and a homologous recombination process. While the CRISPR-Cas9 system directly cuts DNA at a specific site on the Z chromosome, an endogenous repair system using a homologous recombination process allows targeted insertion of the desired DNA into a precise location. For this purpose, construction of a targeting vector plasmid containing homology arms corresponding to the insertion site on the Z chromosome is required. A site for DNA insertion in the Z chromosome downstream of the coding gene HINT1Z was selected. The use of the CRISPR system has been shown in many studies to improve direct DNA insertion events. The px330 plasmid, which is widely used for that purpose, contains an sgRNA site and a Cas9 enzyme. The sgRNA site contains a unique sequence that directs the Cas9 enzyme to the target site and leads to a specific genome-targeted DSDB. Using the CRISPR design engine tool, unique sequences were identified for the sgRNA as shown in Figure 6A. The top 12 guides according to score are shown in Figure 6B. The sequences of guides 1-12 are set forth in SEQ ID NOs: 66-77.

[0212] Guides #1 and #3 were used to check for possible off-target sites in the chicken genome, selected by conventional secondary structure similarity and scored by the degree of mismatch. The top 10 results of the search for potential off-targets for guide #1 are shown in Figure 6C and SEQ ID NOs: 78-87. Notably, there were 4 mismatches among the top 6 off-targets, highlighting the specificity of this guide.

[0213] DNA sequence insertion was performed by cleaving a modified px330 plasmid containing an in-frame GFP fused to the c-terminus of Cas-9. The annealed primer containing the sgRNA sequence was ligated with BbsI restriction enzyme as described above (Figure 6B). The ligation product was transformed into E. coli, the plasmid was purified, and the insertion of the sgRNA was verified by sequencing.

[0214] Validation of CRISPR-Cas9 system activity

[0215] Growing PGCs in feeder-free medium allows the obtaining of pure colonies derived from single cells, thereby allowing the characterization of the efficiency of the CRISPR-Cas9 system. For this, PGCs were transfected with either pX330-GFP-CRISPR1 and pX330-GFP-CRISPR3 plasmids, and clonal colonies were grown. Total genomic DNA was extracted from colonies derived from single cells expressing GFP. DNA was analyzed by endonuclease assay and sequenced. For the endonuclease assay, a positive control was designed. This control was a 320 bp PCR product with a mutation inserted at the predicted site of CRISPR-Cas9 activity. This product was mixed with a WT product of similar length in different mutation:WT ratios of 1:15, 1:7, and 1:1, respectively, and the annealed mixture was subjected to endonuclease activity (Figure 7A). Two short bands at the predicted sizes of 136 bp and 184 bp were clearly visible at 1:7 and 1:1 ratios, indicating that the assay was working properly. Similarly, the same assay was performed on genomic DNA obtained from 12 colonies transfected with either the CRISPR1 and CRISPR3 plasmids (Figure 7B, 7C). A clear doublet of the predicted size was observed in 9 of the 12 colonies. This indicates that both the CRISPR1 and CRISPR3 plasmids efficiently generate DSDBs at the predicted sites.

[0216] For sequence analysis, the PCR products used for the endonuclease assay (Figures 7A-7C) were also sequenced (Figures 8A-8D). Sequencing of the WT negative control revealed the predicted cleavage site of CRISPR1 (Figure 8A). As a positive control, sequencing a mixture of WT and artificially mutated products revealed the appearance of a double peak in the DNA chromatogram immediately after the predicted cleavage site (arrow, Figure 8B). Similar sequencing of the same genomic region of transfected colonies revealed both negative (Figure 8C) and positive colonies (arrow in Figure 8D), the latter in >70% of cases.

[0217] Construction of targeting vector for genome integration

[0218] To demonstrate targeted genomic integration into the Z chromosome using HR, a targeting vector was designed (Figure 9). The vector contains the pCAGG promoter followed by a neomycin selection gene, an internal ribosome entry site (IRES), GFP and a rabbit beta globin polyadenylation site. The cassette was flanked by approximately 1.5 kb homology arms at the 5' and 3' ends, respectively. To generate this vector, a DNA fragment of approximately 3 kb containing both homology arms was amplified using primers P1 and P2 and ligated into the shuttle vector pjet1.2. Complete sequencing of this fragment was found to be identical to the chicken genomic sequence. This plasmid pJet-HAs was used as a template to generate a linearized PCR product containing two separate homology arms except for the 23 bp sequence containing the CRISPR sgRNA site between them. Amplification was performed using primers P3 and P4 containing sequences at the 5' ends corresponding to the ends of the pCAGG-Neo-IRES-GFP cassette. This linearized PCR product is referred to as the "vector." The pCAGG-Neo-IRES-GFP plasmid was used as a template to generate a linearized PCR product. This fragment was amplified using primers P5 and P6, which contain sequences corresponding to the 3' and 5' ends of the 5'HA and 3'HA ends, respectively. This product is referred to as the "insert." The vector and insert were joined together using a Gibson assembly reaction to create the final targeting vector.

[0219] Homologous recombination into the Z chromosome using targeting vectors and CRISPR plasmids

[0220] The ability to obtain pure PGC colonies from single cells allows the identification of positive colonies that have undergone properly inserted HR using methods such as PCR and Southern blot. For PGC transfection, we used lipofection with a transfection efficiency of 5-10% (Figure 10A) or electroporation with an efficiency of >40%. Transfection was performed with two plasmids, a targeting vector and one of the two CRISPRs mentioned above (CRISPR1 or CRISPR3). After transfection, cells were left for 24 h to recover and transferred to G-418-containing medium for selection. After 2 weeks of selection, only G-418-resistant cells survived, of which >99% were GFP positive (Figure 10B). To confirm that the cells retained the ability to colonize the gonads, they were injected into host embryos as previously described in Figure 1F (Figure 10C). The gonads were immunostained with anti-GFP antibody, and colony formation of GFP-positive PGC cells in the gonads was verified using confocal microscopy (FIG. 10D).

[0221] G-418-resistant GFP-positive cells consist of a potentially heterogeneous population. Therefore, to verify HR integration and obtain a pure homogenous population, single GFP-positive cells were separated into 96-well plates using FACS sorting (Figure 11A). Pure colonies were bred and genomic DNA was extracted for PCR and Southern blot analysis. In parallel, pooled GFP-positive cells were FACS sorted. For PCR analysis, two sets of primers were designed. First, forward P7 upstream to 5' HA and reverse P8 from CAGG promoter (1.6 kb product size), second, forward P9 from rabbit beta globin polyadenylation site and reverse P10 downstream to 3' HA (1.8 kb product length, Figure 11B). The expected products for 5' and 3' were detected in both pooled cells (Figure 11C) and pure colonies (Figure 11D), indicating that proper HR integration occurred in these cells.

[0222] To further verify proper HR integration and ensure that only a single copy of the targeting vector was integrated into the genome, Southern blot analysis was performed. Two PGC cell lines from male and female donors were analyzed. Of note, the female line has only a single copy of the Z chromosome. Three digoxigenin-labeled (dig-labeled) DNA probes were designed (see Figure 12A and Figure 12B). The first two probes, amplified using primers P11-P12 and P13-P14, each 500 bp long, are located upstream and downstream of the 5' and 3' HA, respectively. The third probe, amplified using primers P15-P16, 704 bp long, is designed to detect the Neo gene inside the targeting vector, thus ensuring that only a single copy of the vector was integrated. BglII restriction enzyme was used to cut genomic DNA for analysis. Two restriction sites, approximately 6.5 kb apart from each other, are located on the WT chromosome upstream and downstream of the 5' and 3' probes, respectively. An additional BglII site is located within the targeting vector, yielding the predicted 7.5 kb and 3.3 kb fragments, identifying the correct HR integration. Results of Southern blot analysis of genomic DNA extracted from male PGC lines revealed two bands of the predicted size, i.e., 6.5 kb for the WT allele and 7.5 kb and 3.3 kb for the allele that underwent correct HR integration, for the 5' and 3' sites, respectively. This was confirmed for both DNA from pooled cells and for pure colonies (Figure 12C). A similar analysis was performed for female PGC cell lines. In this case, a single band was found at 7.5 kb, the predicted size of the 5' integration site. The WT allele (6.5 kb) was not detected, since the female genome contains only a single copy of the Z chromosome. Critical examination of the Neo gene revealed a single band of the predicted size of 7.5 kb, confirming that only a single copy of the targeting vector had been integrated into the genome (Figure 12D).

[0223] Validation of the optogenetic system in HEK293 cells in vitro and in chick embryos in ovo.

[0224] To verify the activity of the inducible system in vitro and in chick embryos in-ovo, three plasmids, pmCherry-Cry2-CreN, pmCherry-CIBN-CreC and the reporter PB-RAGE-GFP, were transfected into HEK293 cells (Figure 13) and chick embryos (Figure 14). The first two optogenetic plasmids encode the reporter gene mCherry, which confirms successful transfection. The PB-RAGE-GFP suppression vector contains multiple stop codon sequences flanked by LoxP sites upstream of the GFP coding region. Upon Cre activation, the STOP codons are removed, thus allowing GFP to be expressed. Meanwhile, in negative control HEK293 cells that were triple transfected and kept in the dark, there were no GFP-positive cells (Figure 13, top row). In cells exposed to blue light illumination 24 hours after transfection, many cells expressed GFP (Figure 13, bottom row), confirming the activation of the optogenetic system in these cells.

[0225] To verify the activity of the optogenetic system in-ovo, triple transfection with pmCherry-Cry2-CreN, pmCherry-CIBN-CreC and PB-RAGE-GFP plasmids was performed by electroporation into chick embryo neural tubes at stage 16H&H. 12 hours after electroporation, experimental embryos were exposed to blue light for 15 seconds while negative control embryos were kept in the dark. The embryos were incubated for another 12 hours and GFP expression was confirmed by fluorescent stereoscope (Figure 14). In the embryos kept in the dark (Figure 14, top), only mCherry was expressed, confirming successful electroporation, whereas in the experimental embryos, GFP positive cells clearly appeared (Figure 9, bottom), confirming the activation of light-induced Cre.

[0226] The light gene plasmids pmCherry-Cry2-CreN and pmCherry-CIBN-CreC use the CMV promoter, which is not preferred in chicken cells, to drive gene expression. To overcome this and combine the two into a single vector, we designed a plasmid vector that drives the expression of CIBN-CreC and Cry2-CreN linked by a P2A self-cleaving peptide, followed by IREG-GFP, under the CAGG promoter, which is highly active in chicken cells. The synthesis of pCAGG-CIBN-CreC-P2A-Cry2-CreN-IRES-GFP was based on the modification of the original light gene plasmid described in Kennedy et al., 2010 (Nat Methods. 2010 December;7(12):973-975). Each of these plasmids encodes an IRES sequence with either CIBN-CreC (a truncated form of CIB1 fused to the C-terminus of the Cre enzyme) or CRY2-CreN (cryptochrome 2 fused to the N-terminus of the Cre enzyme, Figure 15A) followed by mCherry. The goal of the next cloning was to combine the two fusion light genes with the self-cleaving peptide P2A followed by IRES-GFP under the CAGG promoter. For this purpose, the CIBN-CreC plasmid was used as a template for PCR with primers P40 and P41, and the CRY2-CreN plasmid was used as a template for PCR with primers P42 and P43 (Figure 15A). Notably, primers P41 and P42, which contain the P2A cleavage site, share an overlapping sequence that allows the two products to be merged by one cycle of overhang extension PCR (Figure 15B). The product, containing CIBN-CreC-P2A-CRY2-CreN, was ligated into shuttle vector pJet1.2 and sequence verified (Figure 15C). This plasmid served as a template for PCR using primers P44 and P45, which added SmaI and NheI restriction sites to the product at the 5' and 3' ends, respectively (Figure 15D). The product was digested with restriction enzymes and ligated into pCAGG-IRES-GFP plasmid, which was also cut using the same enzymes (Figure 15E).The ligation product contains the CAGG promoter followed by CIBN-CreC, P2A self-cleaving peptide, CRY2-CreN, IRES, GFP and rabbit beta globin polyadenylation site (referred to herein as pCAGG-Optogene) and was sequence verified (Figure 15F).

[0227] To verify the activity of the pCAGG-Optogene vector in vitro, a plasmid expressing GFP as a reporter for successful transfection was co-transfected with pB-RAGE-mCherry into HEK293. Similar to the PB-RAGE-GFP vector described above (Figure 13), pB-RAGE-mCherry contains multiple stop codon sequences flanked by LoxP sites upstream of the mCherry coding region. Upon Cre activation, the stop codons are removed, thus allowing mCherry to be expressed (Figure 16). Meanwhile, in the co-transfected and maintained HEK293 cells kept in the dark, there were no mCherry-positive cells (Figure 16, top row). In contrast, in the cells exposed to blue light illumination, many cells expressed mCherry (Figure 16, bottom row), confirming that the single vector strategy of pCAGG-Optogenes maintained the optogenetic properties of the system.

[0228] To verify the activity of the pCAGG-Optogene vector in in-ovo live chick embryos, the plasmid was co-transfected with pB-RAGE-mCherry into stage 14-16 H&H chick embryos by electroporation. 12 h after electroporation, the eggs in the negative control group were kept in the dark, while the embryos in the experimental group were exposed to blue light for 15 s (Figure 17). Both groups were incubated for another 12 h and examined under a fluorescent stereoscope. After incubation, high levels of GFP expression were evident in both groups, indicating successful electroporation. However, only in the light-exposed group (Figure 17, bottom row), mCherry-positive expressing cells were identified, indicating that the optogenetic system using the pCAGG-Optogene single-vector strategy was activated in a light-inducible manner.

[0229] Induction of chick embryo lethality

[0230] To demonstrate the feasibility of using a toxin to cause death, the coding region of DTA12, commonly used as a negative selection marker, was cloned into an expression vector containing the pGK promoter followed by IRES-GFP (pGK-IRES-GFP). This plasmid also served as a negative control. The DTA coding region was cloned upstream of an IRES sequence that induces pGK-DTA-IRES-GFP, which upon expression in cells inhibits protein synthesis leading to cell death.

[0231] To test the effect of DTA expression in chicken embryos, stage 14-16 H&H embryos were electroporated with pGK-IRES-GFP or pGK-DTA-IRES-GFP as a negative control. 12 hours after electroporation, embryos were analyzed for GFP expression under a fluorescent microscope (Figure 18). GFP was expressed broadly in the neural tube in control embryos (Figure 18), whereas no GFP expression was detected in embryos expressing DTA, indicating that protein synthesis was blocked in these cells.

[0232] Example 2

[0233] The aim of the experiments described herein was to introduce a lethal induction cassette into the Z chromosome of either male or female derived PGCs. The ultimate goal was to obtain hens containing a genomic insertion in the Z chromosome. This chromosome would segregate exclusively into the next generation of male embryos, and upon blue light induction, the lethal induction cassette would be activated, thus causing the male embryos to die at an early stage of embryo development.

[0234] The cassette, also known as targeting vector (TV), has three elements as described herein. The first element is the "homology arm". The second element is the "optogenetic inducible element". The third element is the "lethality induction cassette". In one embodiment, two approximately 1.5 kb homology arms are placed at both the 5' and 3' ends of the TV. It is designed to direct homologous recombination (HR) downstream of the HINT1Z locus located on the Z chromosome. This site was chosen for HR because the HINT1Z gene is transcribed in PGCs and throughout the blastoderm, which are freshly laid chick embryos. All other openly transcribed regions on the Z chromosome are also potentially good candidates for this purpose.

[0235] Figure 20 shows RT-PCR using FWD primer (SEQ ID NO: 88) and REV primer (SEQ ID NO: 89) on cDNA from PGC and whole blastoderm. Expected product size 153 bp.

[0236] The second element is the "optogenetic inducible element". In one embodiment, the optogenetic system is based on proteins that are excited under a specific wavelength of light and change into a conformation that allows them to dimerize. It is possible to fuse these proteins with additional proteins so that transcription of the target gene occurs only when the two optogenetic proteins dimerize. For example, if the first protein is fused to a DNA targeting domain such as Gal4-BD (Gal4 binding domain) and the second protein is fused to a transcription activator such as Gal4-AD (Gal4 activation domain), it is possible to activate downstream genes. Similarly, repression domains may be used to repress gene expression. However, the problem with optogenes is that they have a certain degree of basal dimerization and thus activation of genes occurs regardless of light induction. Finding the right balance between sensitivity to induction and basal dimerization is key to their use. A modified version of the system developed in Kennedy, MJ et al. (Rapid blue-light mediated induction of protein interactions in living cells. Nat. Methods 7, 973-975, 2010) was used. The original light gene plasmids are pmCherry-Cry2-CreN and pmCherry-CIBN-CreC, which use the CMV promoter, which is not preferred in chicken cells, to drive the expression of the genes. To overcome this and combine the two into a single vector, a plasmid vector was designed in which the CAGG promoter, which is highly active in chicken cells, drives the expression of CIBN-CreC and Cry2-CreN linked by an IRES sequence, followed by IREG-GFP. The synthesis of pCAGG-CIBN-CreC-IRES-Cry2-CreN-IRES-GFP was based on the modification of the original light gene plasmid described in Kennedy et al., 2010 (Nat Methods. 2010, 7(12):973-975).The goal of the cloning was to link two fusion light genes with an IRES followed by IRES-GFP under the CAGG promoter. The construction of a similar plasmid with the order of light genes, i.e. Cry2-CreN followed by CIBN-CreC, was also successfully performed with similar results. Figure 21 shows a schematic diagram of the above plasmids.

[0237] To verify the activity of the pCAGG-Optogene vector in vitro, a plasmid expressing GFP as a reporter for successful transfection was co-transfected with pB-RAGE-mCherry into HEK293. This plasmid contains a Lox-STOP-Lox (LSL, RAGE) element between the pCAGG promoter and the mCherry reporter gene. Thus, when co-transfected with the optogen plasmid, upon blue light irradiation and activation of Cre, LSL is removed and mCherry is expressed. In the dark, where the system is inactive, only the reporter GFP from the optogen plasmid is expressed, as shown in Figure 22.

[0238] To verify the activity of the optogenetic system in living chick embryos in-ovo, the optogenetic plasmid and the reporter pB-RAGE-mCherry plasmid were co-electroporated into the neural tube of chick embryos incubated for 56–60 h. The plasmid DNA mixture was injected into the lumen of the neural tube using a sharp micropipette, and electroporation was applied using two tungsten electrodes (electroporator BTX830) spaced 3–5 mm apart. Four pulses of 30 V, 45 ms each were delivered with an interval of 745 ms between each pulse. After electroporation, the eggs were sealed and incubated for 12–18 h. Then, the eggs of the control group were kept in the dark, while the eggs of the experimental group were exposed to 1 min of blue light illumination through the eggshell. Both groups were placed back inside the incubator for another 12–18 h before analysis.

[0239] As shown in Figure 23, in all experimental groups, the reporter GFP was highly and widely expressed in all embryos, indicating successful electroporation and expression of the plasmid. There was no expression of mCherry in eggs kept in the dark. In contrast, mCherry was expressed in the neural tube in eggs illuminated with blue light.

[0240] An improved system of CIBN-Cry2 can be created by modifying Cry2, including the insertion of the L348F mutation in a truncated Cry2 gene -Cry2-Δ535-L348F, which contains only the first 535 amino acids, as published in Nature Chemical Biology volume 12, pages 425-430 (2016).

[0241] As an alternative approach to the CIBN-Cry2 optogenetic system, another conceptually similar system can be used to achieve the same goal. This system is based on the MAGNET system, in which two optogenetic proteins - a positively charged P-Mag and a negatively charged N-Mag - dimerize upon blue light irradiation. Each of these proteins is fused to an inactive part of the Cre recombinase enzyme, and dimerization of the two optogenes generates an active form of Cre (Nature Chemical Biology volume12,1059-1064 2016).

[0242] The third element is a "lethality-inducing cassette". In one embodiment, the lethality-inducing cassette contains a gene (e.g., a toxin) that drives early embryonic death by severely interfering with cell death or molecular signaling pathways (e.g., BMP4) required for early stages of embryonic development. For example, this can be achieved by inducing programmed cell death (apoptosis) with a toxin such as diphtheria toxin A (DTA), expressing a caspase gene such as caspase 3 or a mutated constitutively active mutated form of caspase 3, or expressing an inhibitor protein of BMP4-Noggin. The nucleotide sequence for DTA is set forth in SEQ ID NO:92. The amino acid sequence of DTA is set forth in SEQ ID NO:93.

[0243] In one embodiment, the nucleotide sequence for chicken Casp3 CDS(WT) is set forth in SEQ ID NO: 94. The amino acid sequence for chicken Casp3 CDS(WT) is set forth in SEQ ID NO: 95.

[0244] In one embodiment, the nucleotide sequence for a constitutively active (mutated) form of chicken Casp3 is set forth in SEQ ID NO: 96. The amino acid sequence for a constitutively active (mutated) form of chicken Casp3 protein is set forth in SEQ ID NO: 97.

[0245] In one embodiment, the nucleotide sequence for Noggin is set forth in SEQ ID NO: 98. The amino acid sequence for Noggin is set forth in SEQ ID NO:99.

[0246] In certain embodiments, the "lethality-inducing cassette" is based on a gene that disrupts an essential signaling pathway. The sooner death is induced, the sooner the embryo will die. In certain embodiments, the essential signaling pathway comprises bone morphogenetic proteins (BMPs). In certain embodiments, the BMP is BMP4. In certain embodiments, the death-promoting protein is an antagonist of BMP4. In certain embodiments, the BMP4 antagonist is Noggin. In certain embodiments, expression of the death-promoting protein Noggin is induced at a developmental stage where the BMP pathway is active and required. In certain embodiments, the BMP pathway is active or induced during blastula formation, gastrulation, neurulation, or organogenesis. In certain embodiments, the optogenetic system is activated immediately after egg-laying (stages X-XIII EG&K). In certain embodiments, the optogenetic system is activated at stage X to induce the death-promoting protein. In certain embodiments, the optogenetic system is activated at stage XI. In certain embodiments, the optogenetic system is activated at stage XII. In certain embodiments, the optogenetic system is activated at stage XIII. In certain embodiments, the optogenetic system is activated at any time during the initiation of embryo development from fertilization to hatching. In certain embodiments, lethality is induced during the 21 days from fertilization to hatching. In certain embodiments, lethality is induced 1 day after fertilization. In certain embodiments, lethality is induced 2 days after fertilization. In certain embodiments, lethality is induced 3 days after fertilization. In certain embodiments, lethality is induced 4 days after fertilization. In certain embodiments, lethality is induced 5 days after fertilization. In certain embodiments, lethality is induced 6 days after fertilization. In certain embodiments, lethality is induced 7 days after fertilization. In certain embodiments, lethality is induced 8 days after fertilization. In certain embodiments, lethality is induced 9 days after fertilization. In certain embodiments, lethality is induced 10 days after fertilization. In certain embodiments, lethality is induced 11 days after fertilization. In certain embodiments, lethality is induced 12 days after fertilization.In certain embodiments, lethality is induced 13 days after fertilization. In certain embodiments, lethality is induced 14 days after fertilization. In certain embodiments, lethality is induced 15 days after fertilization. In certain embodiments, lethality is induced 16 days after fertilization. In certain embodiments, lethality is induced 17 days after fertilization. In certain embodiments, lethality is induced 18 days after fertilization. In certain embodiments, lethality is induced 19 days after fertilization. In certain embodiments, lethality is induced 20 days after fertilization. In certain embodiments, lethality is induced 21 days after fertilization. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 30 hours of incubation. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 31 hours of incubation. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 32 hours of incubation. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 33 hours of incubation. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 34 hours of incubation. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 35 hours of incubation. In certain embodiments, the optogenetic system is activated during chicken embryo development after up to 36 hours of incubation. In certain embodiments, the optogenetic system is activated in a bird. In certain embodiments, the optogenetic system is activated prior to the formation of an eggshell surrounding the embryo. In certain embodiments, the optogenetic system is activated in a bird prior to the formation of an eggshell surrounding the embryo. In certain embodiments, the optogenetic system is activated by directly contacting the embryo with an inducer. In certain embodiments, the inducer is inserted into the egg. In certain embodiments, the eggshell is first at least partially opened and then the inducer is administered directly to the embryo.

[0247] To verify induction of cell death in chicken cells, DTA and the two forms of caspase 3 were cloned into expression vectors with the PGK (phosphoglycerate kinase) or CAGG promoter followed by a functioning IRES GFP to confirm transfection efficiency. The PGK promoter is weaker and slower than pCAGG in chicken embryonic cells.

[0248] In one embodiment, the vectors constructed were PGK-DTA-IRES-GFP, pCAGG-DTA-IRES-GFP, PGK-CASP3-IRES-GFP, pCAGG-CASP3-IRES-GFP, PGK-mCASP3-IRES-GFP, and pCAGG-mCASP3-IRES-GFP. Expression vectors without lethal induction genes were used as controls (PGK-IRES-GFP and pCAGG-IRES-GFP).

[0249] The expression vector was transfected into PGCs by electroporation, and the cells were incubated for 24, 48, and 72 hours, and then analyzed using flow cytometry to detect positive expressing cells by green fluorescent protein (GFP) and to identify dead cells using propidium iodide (PI) staining. The nucleotide sequence encoding GFP is set forth in SEQ ID NO:115.

[0250] The results shown in Figures 24A-24B show the percentage of dead cells among all GFP-positive cells.

[0251] In one embodiment, a targeting vector was constructed that integrates the above three elements into a single active unit that can be integrated into the Z chromosome. As shown in FIG. 25, the unit is flanked by 5' and 3' homology arms. A promoter (pCAGG) drives the expression of the two units of the optogenetic system, separated by an IRES sequence. The optogenetic system is followed by a polyadenylation sequence (PA). The optogenetic cassette is flanked by two LoxP sites. The optogenetic system can be based on CIBN-Cry2, MAGNET, or other systems mentioned above. Following the second LoxP site is a lethal induction coding sequence, followed by a second polyadenylation sequence. This lethal induction coding sequence does not have a promoter, so it is inactive until blue light irradiation is applied, which results in dimerization of the optogen and activation of the Cre enzyme, which removes the optogen cassette flanked between the LoxP sites. This excision places the lethal coding sequence immediately downstream of the pCAGG promoter that drives its expression. This strategy benefits from a shorter design, as it contains a single promoter and does not require a "STOP" sequence.

[0252] In one embodiment, the nucleotide sequence for the pCAGG promoter is set forth in SEQ ID NO: 100. The nucleotide sequence for the pGK promoter is set forth in SEQ ID NO: 109. The nucleotide sequence for the pCMV promoter is set forth in SEQ ID NO: 110. The nucleotide sequence for the phSyn promoter is set forth in SEQ ID NO: 111. The nucleotide sequence for the pEF1-a promoter is set forth in SEQ ID NO: 112.

[0253] In one embodiment, the nucleotide sequence for Optogen1-NLS-Cry2-Δ535-L348F-CreN(AA19-104) is set forth in SEQ ID NO: 101. The nucleotide sequence for Optogen2-CIBN(aa1-170)-NLS-Cre-C(aa106-343) is set forth in SEQ ID NO: 102. The nucleotide sequence for Cre recombinase is set forth in SEQ ID NO: 113. The nucleotide sequence for CreN(AA19-104) is set forth in SEQ ID NO: 117. The nucleotide sequence for CreC(AA106-343) is set forth in SEQ ID NO: 118.

[0254] In one embodiment, the nucleotide sequence for the IRES sequence is set forth in SEQ ID NO: 103. The nucleotide sequence for the polyadenylation site sequence (rabbit beta globin) is set forth in SEQ ID NO: 104. The nucleotide sequence for the 5' homology arm (LHA) is set forth in SEQ ID NO: 105. The nucleotide sequence for the 3' homology arm (RHA) is set forth in SEQ ID NO: 106.

[0255] In one embodiment, with respect to the MAGNET system, the following sequences are alternative optogens that may be used in place of the Cry2-CIBN system. For example, the nucleotide sequence for CreN(aa18-59)_N-Mag_NLS is set forth in SEQ ID NO: 107. The nucleotide sequence for NLS_P-Mag_Cre-C(aa60-237) is set forth in SEQ ID NO: 108.

[0256] In one embodiment, implantation of modified PGCs into embryos and generation of chimeric chickens screened for germline transmission and potential founder carriers was performed as follows. Following generation of the targeting vector, PGC lines derived from both female and male embryos were co-electroporated with a plasmid encoding px330-all-in-one CRISPR-Cas9-GFP and the targeting vector. The CRISPR plasmid creates a DNA double-strand break at a designed site in chromosome Z, thus facilitating homologous recombination of the targeting vector. The CRISPR plasmid encodes Cas9 fused in-frame with GFP, thus allowing confirmation of successful transfection. 24-72 hours after transfection, positive single cells were FACS sorted in single cells into 96-well plates, expanded to form pure PGC colonies, and screened by PCR and Southern blot to identify colonies that had undergone proper homologous recombination.

[0257] Chimeric chickens were generated as follows. Chimeric chickens are a means of converting genetically modified PGCs into functional gametes. To generate chimeras, stage 14-16 H&H embryos were injected into the bloodstream (e.g., the heart) with >3000 PGCs (usually up to 8000-10000). These cells colonize the embryonic gonads alongside the endogenous PGCs, and thus, they are called chimeras. The efficiency of germline transmission, i.e., the ability to convert genetically modified PGCs into functional PGCs, depends on several factors, including the ratio of endogenous and injected PGCs in the gonad. To reduce the amount of endogenous PGCs, freshly laid fertilized eggs are irradiated with 600-800 radians of gamma radiation. In certain embodiments, the irradiation comprises irradiation with 600-800 radians. In certain embodiments, the irradiation comprises irradiation with 400-1000 radians. In certain embodiments, the irradiation comprises an irradiation of 200 to 1200 radians.

[0258] It is believed that PGCs are more susceptible to gamma irradiation than somatic cells, or that after irradiation PGCs have a lower ability to regenerate like somatic cells. Thus, after irradiation, the total amount of endogenous PGCs is reduced. After irradiation, eggs were usually cultured until the embryos reached stage 14-16 H&H, which is suitable for PGC injection. As an indication of irradiation efficiency, the incubation time required for the embryos to reach stage 14-16 H&H increases by about 10 hours, and 70% of the irradiated embryos do not develop normally, and therefore are not used for injection of PGCs. For various chicken strains, eggshell types, thicknesses, and colors, different irradiation conditions may be required, and therefore the amount of energy requires calibration. After injection, the eggs are sealed and incubated until the chimeric chicks hatch. The chicks are then bred until sexual maturity, and in males, sperm are collected to analyze germline transmission using semi-quantitative or real-time PCR.

[0259] Example 3

[0260] This example presents a targeting vector that contains an additional element, referred to herein as a "safe-lock" element. This element essentially locks the optogenetic lethal system, such that by default the system is inactive. Only by mating with a "safe-unlock" strain does the optogenetic lethal system become active. This element benefits the entire system by better protecting the breed, since an additional "unlock" strain must be used to activate it. This also ensures that the cells that have undergone HR do not need to be protected from light throughout the entire production process, since the optogenetic system is essentially inactive.

[0261] This example discloses eight "all-in-one" targeting vectors and shows that they function as designed, both in-vitro and in-vivo. The data also show that the MAGNET optogenetic system is functional in chicken embryos and can activate lethality-inducing mechanisms. The MAGNET system is described above. Furthermore, the data show that Noggin, as an embryonic lethality-inducing gene, arrests embryo development from oviposition (from freshly laid eggs).

[0262] The eight targeting vectors disclosed herein cover eight combination options of two optogenetic systems (MAGNET and CIBN-Cry2) and four lethality-inducing genes (DTA, Noggin, caCASP3, and mCherry, used for the control and validation process). To be clear, mCherry itself does not induce lethality but was used to confirm the action of other elements.

[0263] As discussed above, the purpose of the experiments described herein is to introduce the lethality-inducing cassette into the Z chromosome of either male or female-derived PGCs. The ultimate goal is to obtain hens that contain a genomic insertion in the Z chromosome. This chromosome segregates only into the next generation of male embryos, and upon blue light induction, the lethality-inducing cassette is activated, and thus the male embryo dies at an early stage of embryonic development. Thus, due to sex chromosome segregation, female laying hens do not acquire modified genetic material, since they acquire a WT Z chromosome from the rooster side and a WT W chromosome from the mother hen side (see FIG. 1). In this example, the targeting vector contains four elements, as described below (see example in FIG. 26).

[0264] As shown in Figure 26, the first element is the "homology arm". In one embodiment, two approximately 1.5 kb homology arms are located at both the 5' and 3' ends of the vector. It is designed to direct homologous recombination (HR) downstream of the HINT1Z locus on the Z chromosome. This site was chosen for HR because the HINT1Z gene is transcribed in PGCs and the entire blastoderm, which are freshly laid chick embryos. All other openly transcribed regions on the Z chromosome are also potentially good candidates for this purpose.

[0265] The second element is a "safe-lock" mechanism that ensures that the optogenetic killing mechanism is inactive until the STOP cassette is removed. The STOP cassette is flanked by two FRT sites and is located between the pCAGG promoter and the optogen, thus preventing expression of the optogen. Upon mating with an Flp-expressing strain as described below, the STOP element is removed, allowing the optogen to be transcribed and become active in a light-dependent manner (Figure 26A, active "unlocked" state). In this example, the "safe-lock" element contains the coding sequence for GFP, followed by a polyadenylation site. Other sequences that prevent transcription of downstream elements may also be used as "safe-lock" elements.

[0266] The third element is the "optogenetic inducible element." As discussed above, optogenetic systems are based on a protein that, when excited under a specific wavelength of light, changes conformation and dimerizes with a second specific protein. Examples of optogenetic systems are discussed above.

[0267] In this example, two alternative optogenetic systems were tested and used. The first is the MAGNET system, in which two optogenetic proteins - a positively charged P-Mag and a negatively charged N-Mag - dimerize upon blue light irradiation. The sequences encoding the site-specific recombinase enzyme Mag are set forth in SEQ ID NO: 114 and SEQ ID NO: 65. Each of these proteins is fused to an inactive portion of the Cre recombinase enzyme, and when the two optogenes dimerize, an active form of Cre is generated (see, for example, Nature Chemical Biology volume 12, 1059-1064 2016). In this case, the sequence of the self-cleaving peptide P2A ("Link" in Figure 26) is located between the two optogenes.

[0268] The second light gene system used is an improved system of CIBN (CreC)-Cry2 (CreN). In this case, the modification involves the insertion of the L348F mutation into a truncated Cry2 gene containing only the first 535 amino acids - Cry2-Δ535-L348F (see Nature Chemical Biology 25 volume 12, pages 425-430 (2016)). In one embodiment, the linker between the two light genes is an IRES sequence ("Link" in Figure 26).

[0269] The fourth element is a "lethality-inducing cassette". In one embodiment, the lethality-inducing cassette comprises a gene that promotes early embryonic death by cell death or by severely interfering with a molecular signaling pathway required for the early stages of embryonic development, such as BMP4. As discussed above, examples of lethality-inducing genes include, but are not limited to, toxins such as diphtheria toxin A (DTA), caspase genes such as caspase 3 or a mutated constitutively active mutated form of caspase 3, or the inhibitor protein of BMP4-Noggin.

[0270] The following table shows the functionality of these eight targeting vectors: In one embodiment, these vectors were cloned into the pJet1.2 shuttle vector plasmid.

[0271] [Table 1]

[0272] The activity of the elements in the targeting vector (TV) was evaluated in-vitro in HEK293 cells (Figure 27) and in-ovo in chicken embryos (Figures 28-29). For in-vitro validation, HEK293 cells were transfected with TV4 alone (Figure 27A), pCAGG-Cre (SEQ ID NO: 128) (Figure 27B), or pCAGG-FlpO (SEQ ID NO: 129) plasmids. The latter were performed under two treatments, one kept in the dark (Figure 27C) and the other exposed to blue light for 15 seconds 24 hours after transfection. After illumination, the cells were incubated for another 24 hours (Figure 27D).

[0273] When expressed alone, TV4 expressed GFP as did all other TVs, thus indicating activity of the "safe-lock" state (Figure 27A). When co-expressed with the pCAGG-Cre plasmid, the "safe-lock" and optogenetic cassettes were completely removed (Figure 26B, Figure 27B), as indicated by the loss of expression of GFP and the onset of mCherry expression (as a surrogate for the lethal-inducing gene). This result confirms that excision of the "safe-lock" and optogenetic elements activates the lethal-inducing elements. When TV4 was co-expressed with the pCAGG-FlpO plasmid, only the "safe-lock" element was removed, as indicated by the loss of expression of GFP (Figure 26A, Figure 27C-D), and the optogenetic system was activated in a light-dependent manner. When cells were kept in the dark, there was no activation of the optogenetic system and no expression of mCherry, indicating that the lethal cassette was not expressed (Figure 27C). However, under the same conditions, when cells were illuminated, the optogenetic system became active and was excised to express the lethal elements (see Figure 26A). This is indicated by the expression of mCherry as shown in Figure 27D.

[0274] To demonstrate the activity of TV elements in-vivo, chick embryos were injected with the plasmid into the neural tube and electroporated as described above. In Figure 28, the white lines indicate the dorsal midline of the neural tube and limb bud for orientation purposes. Four treatment groups were tested: 1. Expression of TV4 alone (Figure 28A), 2. Co-electroporation of TV4 and pCAGG-Cre plasmid as a positive control (Figure 28B), 3. Co-electroporation of TV4 and pCAGG-FlpO plasmid kept in the dark (Figure 28C), and 4. Exposure to blue light for 15 seconds after co-electroporation of TV4 and pCAGG-FlpO plasmid and further incubation for 12 hours (Figure 28D).

[0275] When electroporated alone, TV4 expressed GFP, indicating a default inactive "safe-lock" state, and mCherry was not expressed (Figure 28A). When co-expressed with the pCAGG-Cre plasmid, the "safe-lock" and optogenetic cassettes were removed (Figure 26B, Figure 28B), and thus, there was no GFP expression, but mCherry was expressed. This serves as a positive control for the activation of the lethal-inducing element. By co-expressing TV4 with the pCAGG-FlpO plasmid, only the "safe-lock" elements adjacent to the FRT were removed, and thus, there was no GFP expression (Figure 28C-28D), but the optogenetic system was activated in a light-dependent manner. When the cells were kept in the dark, there was no GFP expression, indicating that the optogenetic system was in its active state, but mCherry was not expressed, i.e., the lethal cassette was inactive (Figure 28C). However, when the embryos were illuminated 12 hours after electroporation, the optogenetic system became active and was excised to express the lethal element. This is demonstrated by the expression of mCherry as shown in Figure 28D.

[0276] To demonstrate the activity of the lethality-inducing gene Noggin in a light-dependent manner, TV1, which contains the coding sequence of Noggin as a lethality-inducing element, was co-electroporated with pCAGG-FlpO plasmid into the rostral neural tube (at the midbrain and hindbrain axis level) of chick embryos incubated for 36 h. At this stage, neural crest cells delaminate from the dorsal neural tube in a BMP4-dependent manner. Therefore, inhibiting the BMP4 signaling pathway by ectopic expression of Noggin in the dorsal neural tube is predicted to inhibit the delamination of neural crest cells. To visualize neural crest cells, we used an anti-HNK-1 antibody to stain the neural crest marker HNK-1.

[0277] Figure 29 shows embryos electroporated into the neural tube with TV1 (SEQ ID NO: 120), pCAGG-FlpO (SEQ ID NO: 129) and pCAGG-IRES-GFP (SEQ ID NO: 59) plasmids. The latter was added as a positive control to allow monitoring of transfected cells. 24 hours after transfection, in embryos kept in the dark throughout the experiment (Figure 29A, top, dorsal view; bottom, right lateral view), neural crest cells delaminated and migrated normally from the dorsal neural tube. HNK-1 staining revealed a bilateral expression pattern in both the left and right neural folds (the most dorsal aspect of the neural tube that gives rise to neural crest cell migration), and in the dorsal tube in migrating neural crest cells (Figure 29A, arrowheads). However, in embryos exposed to light 12 hours after electroporation, neural crest cells failed to detach and migrate out of the tube, and HNK-1 staining revealed a marked reduction in expression in the right neural fold, electroporated side (Figure 29B, arrow). Thus, these results indicate that deletion of the "safe-lock" element from TV1 allows the expression and activity of Noggin to be regulated in a light-dependent manner by the MAGNET optogenetic system.

[0278] To demonstrate that Noggin can arrest embryo development as soon as eggs are laid, blastoderms were treated in-ovo with an exogenous source of Noggin at the stage of the blastoderm embryo (Figure 30). For this purpose, pCAGG-Noggin-IRES-GFP (SEQ ID NO: 130) or, as a negative control, pCAGG-IRES-GFP (SEQ ID NO: 59) plasmids were transfected into HEK293 cells. To verify the expression of Noggin in transfected cells, total proteins extracted from transfected cells were analyzed by Western blot using anti-Noggin antibody (Abcam, ab16054, expected size approx. 24 kDa) and anti-α-tubulin-HRP antibody (Abcam, ab40742, expected size approx. 55 kDa) as a loading control. Figure 30A shows that there was no expression of Noggin in negative control transfected cells (CON), whereas pCAGG-Noggin-IRES-GFP transfected cells (NOG) produced Noggin. Prestained protein ladder is displayed in the left lane (Thermo Scientific, PageRuler#26617). Conditioned media from control and Noggin expressing cells were injected into freshly laid fertilized eggs, which were then incubated for 24 or 54 hours, as shown in Figures 30B-C and 30D-E, respectively. Figures 30B-C show high resolution episcopic microscopy (HREM) 3D reconstruction models of treated embryos. While control treated embryos (Figure 30B) continued to develop normally and underwent normal gastrulation, as indicated by the formation of a primitive streak (demarcated by arrows), Noggin treated embryos failed to gastrulate and had no obvious primitive streak, indicating that the process of embryonic development was essentially arrested (Figure 30C). When the embryos were allowed to develop further by 54 hours of incubation (Figure 30D-E), control-treated embryos developed normally and underwent the normal neurulation process to form a beating heart (Figure 30D), whereas Noggin-treated embryos formed a mass of cells without any obvious features (Figure 30E, arrow).

[0279] Taken together, the above results indicate that the molecular mode of action strategy, including the "safe-lock" element and the MAGNET optogenetic system that activates the lethal-inducing element in a light-dependent manner, works as predicted, as shown in Figure 26. Furthermore, inhibition of the BMP signaling pathway by treating the blastoderm with Noggin in freshly laid eggs halted the progression of embryo development.

[0280] Generation of pure lines of PGCs containing the targeting vector integrated on the Z chromosome

[0281] In one embodiment, a ribonucleoprotein (RNP) system was used to deliver CRISPR / Cas9 to generate PGCs with targeted integration into the Z chromosome. The use of the RNP system benefits, among other things, from high efficiency, rapid DNA cleavage, and rapid removal of the RNP complex from transfected cells. The RNP complex is composed of a mixture of recombinant Cas9 or high-fidelity Cas9 nuclease and a crRNA:tracrRNA complex. tracrRNA is commercially available (IDT) and crRNA can be custom-made with a specific 20-nucleotide sequence corresponding to the desired cleavage site on the target genomic DNA. In one embodiment, two crRNA oligos were synthesized using SEQ ID NO:66 and SEQ ID NO:68 disclosed herein (see FIG. 8).

[0282] PGC medium and line induction are described above. To electroporate PGCs, 5 × 10 5Cells were washed with AkoDMEM and transferred to buffer "R" (Neon buffer, Invitrogen) containing 0.7 μg of targeting vector plasmid (TV1 in this example), 1 μM Alt-R® cas9 electroporation enhancer (IDT), and recombinant Cas9 or high fidelity Cas9 nuclease (Alt-R® SpCas9 nuclease or SpHiFi Cas9 nuclease V3, IDT, respectively) at a final concentration of 1.5 μM, and RNP complex with 1.8 μM crRNA:tracrRNA (sgRNA complex, custom Alt-R® CRISPR-Cas9 crRNA and Alt-R® tracrRNA, IDT).

[0283] The crRNA:tracrRNA complex was prepared by heating the RNA oligos at 95 °C for 1 min and cooling to room temperature. The RNP complex (Cas9 protein + crRNA:tracrRNA) mixture was prepared by adding recombinant Cas9 or high-fidelity Cas9 nuclease protein to the crRNA:tracrRNA complex and incubating at room temperature for 10–20 min. It is generally believed that high-fidelity Cas9 should cause less off-targeting.

[0284] For electroporation, 10 μl of buffer "R" containing the RNP complex, electroporation enhancer reagent, and targeting vector plasmid was added to the PGC pellet, and the mixture was immediately electroporated with three pulses of 1000 V, each with a duration of 13 ms, using a neon electroporator (Invitrogen). Subsequently, the cells were immediately plated in 48-well plates in PGC medium containing 1 μM SCR7-pyrazine (SML1546 sigma). The medium was replaced after 1–4 h, and the transfected cells were allowed to recover for 7–10 days. SCR7-pyrazine was added 48 h after electroporation. The transfected cells were individually isolated by FACS sorting. For FACS sorting, gentle cell pipetting was performed, and the cells were sorted in PGC medium. Single GFP-positive cells were sorted in a FACS Sony sorter (Sony) into U-shaped 96-well plates. The sorted cells were grown for 2–3 weeks to form pure colonies. From these colonies, total genomic DNA was extracted and cryopreserved for analysis, and positive colonies were injected into surrogate host embryos. The PCR and Southern blot analyses used to verify correct integration of the targeting vector have been described above (see, for example, Figures 15-16). Methods for generating surrogate chimeric chicks have also been described above.

[0285] Figure 31A shows a pure female PGC line that underwent HR on TV1 and expressed GFP. Cells from this colony were injected into a surrogate host embryo that was incubated for 5 days after implantation. Figure 31B shows a ventral view of the embryo 5 days after PGC injection. At this stage, PGCs colonized the genital ridge, the anus of the gonad (Figure 31B, arrowhead). Other injected embryos were cultured until hatching, and female chicks were sacrificed 10 days after hatching to analyze the ovaries. Figure 31C shows an ovary (indicated by a line) that contained numerous GFP-positive PGCs. These results indicate that PGCs that underwent HR on the Z chromosome on TV1 successfully colonize the gonad.

[0286] Cre or FlpO expressing “unlocked” strains.

[0287] Transgenic chicken strains expressing targeting vectors TV1-8 have a "safe lock" element in their genome, and therefore the light gene-lethal system is inactive. To remove the "safe lock" element flanking the FRT site (see Figure 26A), these strains must be crossed with a chicken strain expressing FlpO. To generate this strain, a targeting vector was created that contains the FlpO enzyme followed by IRES-GFP (TV-FlpO-IRES-GFP, SEQ ID NO: 131). As a positive control and as a means to activate the lethal induction cassette in a light-independent manner, a Cre-expressing variety may be used. In this case, the "safe lock" and light gene elements flanking the LoxP sites are removed to activate the lethal induction element (see Figure 26B). To generate this strain, a targeting vector was created that contains the Cre enzyme followed by P2A-GFP (TV-Cre-P2A-GFP, SEQ ID NO: 132). These two targeting vectors share the same 5' and 3' homology arms and the reporter gene GFP, and are designed to integrate into the Z chromosome as previously described for TV1-8.

[0288] The above targeting vectors (TV-FlpO-IRES-GFP and TV-Cre-P2A-GFP) were used to generate PGCs that were injected into surrogate chimeric embryos. Methods for transfection of PGCs, FACS analysis, verification of HR integration, embryo injection, etc. have been described above.

[0289] In another embodiment, there is the option of using recombinant CRE or FlpO proteins to remove the "safe-lock" element. In one embodiment, the sequence encoding the recombinant protein may be applied to cells or embryos, including a sequence for the cell-penetrating peptide TAT peptide, followed by a nuclear localization sequence (NLS), followed by a sequence encoding either the Cre or FlpO enzyme. These recombinant proteins are effective in cultured cells and are commercially available. It is believed that these recombinant proteins can be injected into embryos or adults, potentially activating the system without the need to generate and breed Cre / Flp transgenic chickens. Access to the chick embryo is available through an opening in the eggshell, allowing for direct injection into the bloodstream or near the embryo. Thus, in one embodiment, the recombinant proteins described above can be injected directly into the embryo to remove the "safe-lock" element.

[0290] While a particular embodiment has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims.

[0291] All publications, patents, and patent applications mentioned in this specification are incorporated herein by reference in their entirety to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated herein by reference. Furthermore, citation or identification of any reference in this application should not be construed as an admission that such reference is available as prior art to the present disclosure. To the extent section headings are used, they should not necessarily be construed as limiting.

[0292] While certain features of aspects or embodiments have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will immediately occur to those skilled in the art, and it is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the aspects or embodiments provided herein.

Claims

1. A DNA editing agent comprising: A polynucleotide cassette having the formula 5'-LHA-OIE-LIE-RHA-3' or the formula 5'-LHA-LIE-OIE-RHA-3' is included in the DNA editing agent; (i) the LHA comprises a first nucleotide sequence that is at least 90% identical to a first corresponding nucleotide sequence on avian chromosome Z; (ii) the OIE comprises a first promoter operably linked to a second nucleotide sequence encoding a photoactivated site-specific recombinase enzyme; (iii) the LIE comprises a third nucleotide sequence encoding a death-inducing protein operably linked to the activity of the photoactivated site-specific recombinase enzyme, the death-inducing protein being selected from the group consisting of a toxin, a pro-apoptotic protein, an inhibitor of the Wnt signaling pathway, a BMP antagonist, an FGF antagonist, wild-type caspase 3, constitutively active caspase 3, Noggin, and a death-inducing fragment thereof; and (iv) the RHA comprises a fourth nucleotide sequence that is at least 90% identical to a second corresponding nucleotide sequence on avian chromosome Z, and the first corresponding nucleotide sequence or the second corresponding nucleotide sequence is located in an open transcribed region on chromosome Z, and the open transcribed region on chromosome Z is selected from any one of the histidine triad nucleotide binding protein 1-Z (HINT1Z) locus or a region downstream thereof, and the Isl1 locus.

2. (i) the LHA comprises a nucleotide sequence set forth in SEQ ID NO: 105 or a fragment thereof; (ii) the RHA comprises a nucleotide sequence set forth in SEQ ID NO: 106 or a fragment thereof; (iii) both (i) and (ii); or (iv) The DNA editing agent of claim 1, wherein the first promoter is pCAGG (SEQ ID NO: 100), pGK (SEQ ID NO: 109), pCMV (SEQ ID NO: 110), phSyn (SEQ ID NO: 111), or pEF1-a (SEQ ID NO: 112).

3. the photoactivated site-specific recombinase enzyme (i) comprising Cre recombinase or Mag recombinase; (ii) encoded by a nucleotide sequence comprising the sequence of SEQ ID NO: 113, SEQ ID NO: 114, or SEQ ID NO: 65; (iii) induced by electromagnetic energy; or (iv) a non-functional peptide fragment of the photoactivated site-specific recombinase enzyme that forms the active photoactivated site-specific recombinase enzyme upon binding in the presence of light.

4. the polynucleotide cassette further comprises a fifth nucleotide sequence encoding a guide RNA targeting an essential gene of the bird, the fifth nucleotide sequence being operably linked to the activity of the light-activated site-specific recombinase enzyme; The DNA editing agent of any one of claims 1 to 3, wherein the essential gene is bone morphogenetic protein receptor type IA (BMPR1A), bone morphogenetic protein 2 (BMP2), bone morphogenetic protein 4 (BMP4), or fibroblast growth factor receptor 1 (FGFR1).

5. (i) the death-inducing protein is a toxin, a pro-apoptotic protein, an inhibitor of the Wnt signaling pathway, a BMP antagonist, an FGF antagonist, wild-type caspase 3, constitutively active caspase 3, Noggin, or a death-inducing fragment thereof; or (ii) The DNA editing agent of any one of claims 1 to 4, wherein the death-inducing protein has the amino acid sequence of SEQ ID NO:93, SEQ ID NO:95, SEQ ID NO:97, or SEQ ID NO:

99.

6. (i) the LHA comprises the sequence of SEQ ID NO: 105; (ii) the OIE is connected to the sequence of SEQ ID NO:116, connected to the sequence of SEQ ID NO:101, connected to the sequence of SEQ ID NO:103, connected to the sequence of SEQ ID NO:102, connected to the sequence of SEQ ID NO:104, connected to the sequence of SEQ ID NO:116, or comprises a sequence of SEQ ID NO:100; or the OIE is connected to the sequence of SEQ ID NO:116, connected to the sequence of SEQ ID NO:107, connected to the sequence of SEQ ID NO:103, connected to the sequence of SEQ ID NO:108, connected to the sequence of SEQ ID NO:104, connected to the sequence of SEQ ID NO:116, or comprises a sequence of SEQ ID NO:100; (iii) the LIE comprises the sequence of SEQ ID NO:92, or SEQ ID NO:94, or SEQ ID NO:96, or SEQ ID NO:98; and (iv) The DNA editing agent of any one of claims 1 to 5, wherein the RHA comprises the sequence of SEQ ID NO:

106.

7. further comprising a safelock element inserted downstream of the first promoter in the OIE and upstream of the second nucleotide sequence encoding the photoactivated site-specific recombinase enzyme; the safelock element comprises a STOP element that prevents transcription of the photoactivated site-specific recombinase enzyme encoded by the OIE; The DNA editing agent of any one of claims 1 to 6, wherein the STOP element is flanked by two FRT sites.

8. The DNA editing agent of claim 7, wherein the DNA editing agent comprises a polynucleotide having one of the sequences of SEQ ID NOs: 120-122 or 124-126.

9. A population of avian cells, the avian cells comprising a DNA editing agent according to any one of claims 1 to 8.

10. A chimeric bird comprising a population of bird cells according to claim 9.

11. Administering a DNA editing agent according to any one of claims 1 to 8 to a population of avian cells, thereby generating genome-edited avian cells; transferring the genome-edited avian cell into a recipient bird embryo, thereby producing a chimeric bird.

12. 1. A method for inducing lethality in avian male embryos, comprising the steps of: Administering a DNA editing agent according to any one of claims 1 to 8 to a population of avian cells, thereby generating genome-edited avian cells; transferring the genome-edited avian cells into a recipient avian embryo; exposing the embryo to an inducer that induces expression of the lethality-inducing protein encoded by the DNA editing agent, thereby inducing lethality in a male embryo of the avian.

13. 1. A method for inducing lethality in avian male embryos, comprising the steps of: Administering the DNA editing agent of claim 7 or 8 to a population of avian cells, thereby generating genome-edited avian cells; transferring the genome-edited avian cells into a recipient avian embryo; exposing the embryo to an agent that removes the STOP element from the DNA editing agent, thereby inducing expression of the lethality-inducing protein encoded by the DNA editing agent and inducing lethality in a male embryo of the bird.

14. (i) the agent that removes the STOP element is a polynucleotide sequence encoding a FlpO protein or a Cre protein; or 14. The method of claim 13, wherein (ii) inducing lethality in male embryos further comprises exposing the embryos to blue light having a wavelength of 450-485 nm.

Images