Agent for improving quality of iPS cells, method for producing iPS cells, iPS cells, and composition for producing iPS cells

Abstract

A quality improving agent for iPS cells that comprises a polynucleotide, said polynucleotide containing H1foo gene and a regulatory sequence. When the H1foo gene is introduced into cells, the regulatory sequence can regulate the amount and / or timing of the presence of H1foo protein expressed by the H1foo gene in the cells.

Description

[Technical Field]

[0001] The present invention relates to an agent for improving the quality of iPS cells, a method for producing iPS cells, iPS cells, and a composition for producing iPS cells. This application claims priority based on Japanese Patent Application No. 2020-149447, filed on September 4, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] iPS cells (induced pluripotent stem cells; also known as "artificial pluripotent stem cells" or "induced pluripotent stem cells") can be generated from somatic cells by introducing Oct3 / 4, Sox2, Klf4, and c-Myc (Non-Patent Document 1, Patent Document 1). This can be achieved by reprogramming the transcriptional network and epigenetic signature of the parent somatic cell. iPS cells offer various benefits for basic research, pharmaceutical innovation, and regenerative medicine. However, the heterogeneous quality of generated iPS cell populations compared to embryonic stem (ES) cell populations remains a serious problem. For example, ES cells exhibit little variation in cell properties, and almost all cells can be differentiated into the desired cell type. In contrast, iPS cells exhibit significant variation in cell properties, and some cells often fail to differentiate into the desired cell type. Obtaining a population of iPS cells that are consistent in cell-to-cell quality and exhibit high quality is important for basic research and clinical purposes.

[0003] Many attempts have been made to solve the problem of heterogeneous quality of iPS cell populations. For example, Patent Document 2 describes that introducing predetermined amounts of Oct3 / 4 gene, Klf4 gene, c-Myc gene, and Sox2 gene into somatic cells a predetermined number of times can improve the production efficiency and stability of iPS cells. Patent Document 3 describes that introducing the Prdm14 gene or its gene product, Esrrb gene or its gene product, and Sall4a gene or its gene product into somatic cells in addition to the Oct3 / 4 gene or its gene product, Sox2 gene or its gene product, Klf4 gene or its gene product, and c-Myc gene or its gene product, can efficiently produce iPS cells of excellent quality in a short period of time. Patent Document 4 describes that high-quality iPS cells can be produced efficiently and in a short period of time by introducing the Jarid2 mutant gene or its gene product into somatic cells in addition to the Oct3 / 4 gene or its gene product, the Sox2 gene or its gene product, the Klf4 gene or its gene product, and the c-Myc gene or its gene product. However, there is still room for improvement in the quality of iPS cells. Therefore, there is a need for the development of a method for producing iPS cells of higher quality and with less variation in quality.

[0004] The linker histone H1 family binds to linker DNA and generates higher-order chromatin structures to regulate gene expression. Members of the linker histone H1 family include histones H1a, H1b, H1c, H1d, H1e, H1foo, H1x, H1.0, H1t, H1T2, and HILS1. Most members of the linker histone family consist of somatic linker histones that condense chromatin. Therefore, such structures generally suppress global gene transcription activity (Non-Patent Documents 2 and 3). The present inventors have found that when inducing iPS cells from somatic cells, introducing the H1foo gene, a member of the linker histone H1 family, in addition to the above genes enables the production of high-quality iPS cells with minimal quality variation (Patent Document 5). [Preliminary Technology Documents] [License]

[0005] [License 1] Patent No. 4183742 [License 2] Special Announcement No. 2011-004674 [License 3] Special Announcement No. 2014-217344 [License 4] Special Announcement No. 2014-217345 [Patent Document 5] International Publication No. 2017 / 010080 [Non-licensed literature]

[0006] [Non-licensed Document 1] Takahashi, K. & Yamanaka, S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell126, 663-676 (2006) [Non-licensed Document 2] Steinbach, OC, Wolffe, AP & Rupp, RA Somatic linker histones cause loss of mesodermal competence in Xenopus. Nature 389, 395-399 (1997) [Non-licensed Document 3] Hebbar, PB & Archer, TK Altered histone H1 stoichiometry and an absence of nucleosome positioning on transfected DNA. The Journal of biological chemistry 283, 4595-4601 (2008) Summary of the Invention [Problem to be solved by the invention]

[0007] The iPS cell production method described in Patent Document 5 can produce iPS cells of higher quality and with less variation in quality than conventional methods, but the number of colonies obtained after iPS cell induction is similar to that of conventional methods. Therefore, an objective of the present invention is to provide an agent for improving the quality of iPS cells, which is capable of increasing the number of colonies obtained after iPS cell induction, a method for producing iPS cells, iPS cells produced by such a production method, and a composition for producing iPS cells. [Means for solving the problem]

[0008] As a result of extensive research to solve the above-mentioned problems, the inventors discovered that the number of colonies after iPS cell induction can be significantly increased by controlling either the timing or amount of H1foo protein expressed from the H1foo gene introduced into somatic cells, and thus completed the present invention.

[0009] The present invention includes the following aspects. [1] An agent for improving the quality of iPS cells, comprising a polynucleotide having an H1foo gene and a control sequence that, when the H1foo gene is introduced into a cell, can control at least one of the amount and timing of presence in the cell of the H1foo protein expressed from the H1foo gene. [2] The agent for improving the quality of iPS cells described in [1], wherein the polynucleotide is inserted into an expression vector in a state in which the H1foo gene can be expressed in the cells into which it is introduced. [3] The agent for improving the quality of iPS cells according to [2], wherein the expression vector is a Sendai virus vector. [4] An iPS cell quality improvement agent described in any one of [1] to [3], wherein the control sequence includes a nucleotide sequence encoding a destabilization domain, the destabilization domain is a domain that promotes the degradation of a fusion protein containing the destabilization domain by the proteasome, and the control sequence is linked to the H1foo gene so as to be able to express a fusion protein of the destabilization domain and the H1foo protein. [5] The agent for improving the quality of iPS cells according to any one of [1] to [4], wherein the control sequence includes a promoter sequence that controls the transcription of the H1foo gene in response to a chemical stimulus. [6] An agent for improving the quality of iPS cells, comprising a fusion protein of an H1foo protein and a destabilization domain, wherein the destabilization domain is a domain that promotes degradation of the fusion protein by the proteasome. [7] Nuclear reprogramming materials and A method for producing iPS cells, comprising the step of introducing the agent for improving the quality of iPS cells according to any one of [1] to [6] into somatic cells. [8] The method for producing iPS cells according to [7], wherein the iPS cells are primed or naive iPS cells. [9] The method for producing iPS cells according to [7] or [8], wherein the nuclear reprogramming substance comprises at least one selected from the group consisting of genes of the Oct gene family, genes of the Sox gene family, genes of the Klf gene family, genes of the Myc gene family, genes of the Lin gene family, and Nanog genes, and their gene products.

[10] The method for producing iPS cells according to any one of [7] to [9], wherein the nuclear reprogramming substance is the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, L-Myc or c-Myc, or a gene product thereof.

[11] The method for producing iPS cells according to any one of [7] to [9], wherein the nuclear reprogramming substance is at least one gene selected from the group consisting of genes of the Oct gene family, genes of the Sox gene family, genes of the Klf gene family, genes of the Myc gene family, genes of the Lin gene family, and Nanog genes, and the at least one gene is inserted into an expression vector in a state in which the at least one gene can be expressed in a cell into which the at least one gene is introduced.

[12] The method for producing iPS cells according to

[11] , wherein the expression vector is a Sendai virus vector.

[13] A method comprising the step of introducing a nuclear reprogramming substance and an H1foo gene into a somatic cell, A method for producing iPS cells, wherein at least one of the timing and amount of presence of the H1foo protein expressed from the H1foo gene introduced into the somatic cells is controlled within the somatic cells.

[14] A method for producing iPS cells described in

[13] , wherein the H1foo gene is inserted into an expression vector in a state in which the H1foo gene can be expressed in the cells into which the H1foo gene is introduced.

[15] The method for producing iPS cells according to

[13] or

[14] , wherein the nuclear reprogramming substance is at least one gene selected from the group consisting of genes of the Oct gene family, genes of the Sox gene family, genes of the Klf gene family, genes of the Myc gene family, genes of the Lin gene family, and Nanog genes, and the at least one gene is encoded by an expression vector capable of expressing the at least one gene.

[16] The method for producing iPS cells according to

[14] or

[15] , wherein the expression vector is a Sendai virus vector.

[17] iPS cells produced by the method for producing iPS cells according to any one of [7] to

[16] .

[18] A composition for producing iPS cells, comprising a nuclear reprogramming substance and the iPS cell quality improver according to any one of [1] to [6].

[19] A method for producing iPS cells, comprising the step of introducing into the somatic cells a nuclear reprogramming substance and a substance that induces the reprogramming of the somatic cells and has the function of suppressing natural immunity, any time between 2 and 15 days after the introduction of the nuclear reprogramming substance into the somatic cells. [Effects of the Invention]

[0010] The present invention provides an agent for improving the quality of iPS cells, a method for producing iPS cells, iPS cells produced by the method, and a composition for producing iPS cells. Furthermore, the iPS cell populations produced in this way include many cells with high quality as iPS cells, such as high pluripotency. [Brief explanation of the drawings]

[0011] [Figure 1] 1 shows the structures of three types of Sendai virus vectors containing the H1FOO gene (SeV18+H1FOO / TS15ΔF, SeV18+H1FOO-DD / TS15ΔF, SeV18+DD-H1FOO / TS15ΔF) used in the Examples. [Figure 2] Human skin fibroblasts were transfected with one of the three Sendai virus vectors shown in Figure 1, and the expression levels of H1FOO (2), H1FOO-DD fusion protein (3), and DD-H1FOO fusion protein (4) were compared by Western blotting. In Figure 2, "(1) Control" represents the result of using a Sendai virus vector containing the Azami-Green gene instead of the H1FOO gene in "(a) SeV18+H1FOO / TS15ΔF" shown in Figure 1 (the same applies below). [Figure 3]The figures show the number of alkaline phosphatase (ALP)-positive colonies in primed iPS cells generated from human skin fibroblasts. Primed iPS cells were generated using one of the Sendai virus vectors (a) to (c) shown in Figure 1, together with a Sendai virus vector containing the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, and the L-Myc gene. [Figure 4A] The figure shows the number of ALP-positive colonies in primed iPS cells generated from human peripheral blood mononuclear cells (PBMCs). Primed iPS cells were generated using SeV18+H1FOO-DD together with a Sendai virus vector containing the Oct3 / 4, Sox2, Klf4, and L-Myc genes. [Figure 4B] The number of ALP-positive colonies is shown for naive iPS cells generated from human skin fibroblasts. Naive iPS cells were generated using SeV18+H1FOO-DD together with a Sendai virus vector containing the Oct3 / 4, Sox2, Klf4, and L-Myc genes. [Figure 4C] The figures show the number of ALP-positive colonies in naive iPS cells generated from human peripheral blood mononuclear cells (PBMCs). Naive iPS cells were generated using SeV18+H1FOO-DD together with a Sendai virus vector containing the Oct3 / 4, Sox2, Klf4, and L-Myc genes. [Figure 5A] The results of a comparative evaluation of gene expression variability in primed iPS cells generated from human skin fibroblasts are shown below for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 5B] The results of a comparative evaluation of the variation in gene expression levels in naive iPS cells generated from human skin fibroblasts are shown below for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 6A]The results of a comparative evaluation of DNA methylation variation in primed iPS cells generated from human skin fibroblasts are shown below for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 6B] The results of a comparative evaluation of DNA methylation variation in naive iPS cells generated from human skin fibroblasts are shown below. These cells are (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 7A] The figure shows an example of flow cytometry measurement of the percentage of TNNT2-positive cells in cells induced to differentiate from primed iPS cells using cardiomyocyte differentiation-inducing medium. The left figure shows an example of measurement for (1) Control-iPS, and the right figure shows an example of measurement for (3) H1FOO-DD-iPS. [Figure 7B] The results of a comparative evaluation of the percentage of TNNT2-positive cells in cells induced to differentiate from primed iPS cells using cardiac differentiation-inducing medium were shown for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 8] The results of semi-comprehensive qPCR evaluation of the expression of endoderm-related markers in cells induced to differentiate into endoderm from primed iPS cells are shown below for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 9] The figure shows the results of comparative evaluation of ASGR1 expression by qPCR in cells induced to differentiate into hepatocytes from primed iPS cells: (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 10] The figure shows the comparative results of albumin secretion evaluated by ELISA in cells induced to differentiate into hepatocytes from primed iPS cells: (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 11A]The left panel shows an example of a measurement of (1) Control-iPS cells, and the right panel shows (3) H1FOO-DD-iPS cells. [Figure 11B] The results of a comparative evaluation of the percentage of cells positive for both PDGFRA and ANPEP in cells induced to differentiate from naive iPS cells using a primitive endoderm differentiation-inducing medium were shown for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 12A] The results of a comparative evaluation of spare respiratory capacity in naive iPS cells were shown for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 12B] The oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) of naive iPS cells were compared between (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 13A] These are examples of fluorescent microscopy images of mRNA-FISH using UTX, HUWE1, and XIST probes in naive iPS cells. The left image shows an example of HUWEI+ / +XIST+ / +, and the right image shows an example of HUWEI+ / -XIST- / -. [Figure 13B] The results of a comparative evaluation of the expression patterns of HUWE1 and XIST in naive iPS cells are shown below for (1) Control-iPS and (3) H1FOO-DD-iPS. [Figure 14]The results of measuring FKBP1A expression levels by qRT-PCR are shown. HDF: human dermal fibroblasts (TIG)120; H9 ESC: H9 ES cells; H1FOO OE HDF: human dermal fibroblasts transfected with H1FOO-DD on day 2; OSKL day2: human dermal fibroblasts transfected with nuclear reprogramming factors (OCT4 / SOX2 / KLF4 / LMYC) on day 2; OSKLH day2: human dermal fibroblasts transfected with nuclear reprogramming factors (OCT4 / SOX2 / KLF4 / LMYC) and H1FOO-DD on day 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] The term "comprise" means that it may contain components other than the target component. The term "consist of" means that it does not contain components other than the target component. The term "consist essentially of" means that it does not contain components other than the target component in a manner that performs a special function (such as a manner that completely loses the effect of the invention). In this specification, when "comprise" is used, it includes an embodiment that "consists of" and an embodiment that "essentially consists of."

[0013] Proteins (polypeptides), peptides, polynucleotides (DNA, RNA), vectors, and cells may be isolated. "Isolated" means a state separated from its natural state. The proteins (polypeptides), peptides, polynucleotides (DNA, RNA), vectors, and cells described herein may be isolated proteins (isolated polypeptides), isolated peptides, isolated polynucleotides (isolated DNA, isolated RNA), isolated vectors, and isolated cells.

[0014] The term "gene" refers to a polynucleotide containing at least one open reading frame that encodes a specific protein. A gene may contain only exons, or may contain exons and one or more of introns, 5'UTRs, and 3'UTRs.

[0015] The term "operably linked" when used in reference to a polynucleotide means that a first base sequence is located sufficiently close to a second base sequence that the first base sequence can affect the second base sequence or a region under the control of the second base sequence. For example, when a polynucleotide is operably linked to a promoter, it means that the polynucleotide is linked so that it is expressed under the control of the promoter. When a promoter is located upstream of a gene, the gene is usually operably linked to the promoter.

[0016] The term "expressible state" refers to a state in which a polynucleotide can be transcribed in a cell into which the polynucleotide has been introduced. An "expression vector" refers to a vector containing a target polynucleotide and equipped with a system that makes the target polynucleotide expressible in a cell into which the vector is introduced.

[0017] [iPS cell quality improvement agent] First Embodiment In one embodiment, the present invention provides an agent for improving iPS cell quality, which comprises a polynucleotide having an H1foo gene and a nucleotide sequence (hereinafter referred to as a "control sequence") that, when the H1foo gene is introduced into a cell, can control at least one of the amount and timing of presence in the cell of the H1foo protein expressed by the H1foo gene.

[0018] The iPS cell quality improver of this embodiment can improve the quality of iPS cells derived from somatic cells by introducing it into somatic cells together with a nuclear reprogramming substance (described below). Examples of "iPS cell quality" include various properties in iPS cells, such as high expression of undifferentiation markers (e.g., Nanog, Tra-1-60, ALP), high embryoid body formation ability, uniform size of embryoid bodies formed from iPS cells, low aberrant methylation, high chimera formation ability in mouse iPS cells, and high differentiation ability into differentiated cells (e.g., cardiomyocytes), as well as uniformity of these various properties among iPS cells. When the iPS cell quality improver of this embodiment is introduced into somatic cells together with a nuclear reprogramming substance, it can improve the various iPS cell properties or the uniformity of these various properties among iPS cells, compared to when only a nuclear reprogramming substance is introduced. In particular, the iPS cell quality improver of this embodiment can improve the ability to generate primed iPS cells that express ALP and the ability to generate naive iPS cells that express ALP. Furthermore, when the iPS cell quality improving agent of this embodiment is introduced into somatic cells together with a nuclear reprogramming substance, it produces effects such as improved uniformity of gene expression, improved uniformity of DNA methylation, improved differentiation ability into target cells and the uniformity of their properties, improved ability to differentiate into a highly uniform differentiated cell population, and improved expression of naive phenotypes (metabolic function, HUWE1 / XIST expression pattern, etc.) in the generated iPS cells. The uniformity of differentiated cells can be confirmed, for example, by examining the variation in the expression levels of differentiated cell markers. Examples of differentiated cells include, but are not limited to, endodermal cells, mesodermal cells, ectodermal cells, cardiomyocytes, hepatocytes, kidney cells, muscle cells, fibroblasts, neurons, immune cells (e.g., lymphocytes), vascular cells, ocular cells (e.g., retinal pigment epithelial cells), blood cells (e.g., megakaryocytes, erythrocytes), and other tissue cells, as well as their progenitor cells.

[0019] (H1foo gene) As used herein, "H1foo gene" refers to a polynucleotide encoding an H1foo protein. The biological species from which the H1foo gene is derived is not particularly limited and can be appropriately selected depending on the purpose, and examples include any mammalian species, such as humans, mice, rats, cattle, sheep, horses, and monkeys. Sequence information for the H1foo gene can be obtained from publicly known databases. For example, it can be obtained from GenBank under accession numbers BC047943 (human) or BC137916 (mouse). The nucleotide sequence of the human H1foo gene corresponding to the above accession number is shown in SEQ ID NO: 1, and the amino acid sequence of the human H1foo protein encoded by the nucleotide sequence is shown in SEQ ID NO: 2. The nucleotide sequence of the mouse H1foo gene corresponding to the above accession number is shown in SEQ ID NO: 3, and the amino acid sequence of the mouse H1foo protein is shown in SEQ ID NO: 4. As used herein, "H1FOO" written in all capital letters refers to the human H1foo gene or H1foo protein. When "H1foo" is mentioned, it encompasses the H1foo gene or H1foo protein of all living species, including humans.

[0020] The H1foo gene is not limited to the wild-type H1foo gene, but may also contain mutations (deletions, substitutions, insertions, and additions, or a combination thereof). The number of mutations is not particularly limited as long as it has a nucleotide sequence that encodes a protein with H1foo activity. In this specification, "H1foo activity" refers to at least one of the functions possessed by the wild-type H1foo protein. Examples of H1foo activity include the activity of binding to linker DNA that connects nucleosomes. More preferably, H1foo activity is the activity of binding to linker DNA that connects nucleosomes and maintaining the linker DNA region in a relaxed state.

[0021] Examples of the H1foo gene include the following (a) to (g): (a) a wild-type H1foo gene (e.g., a polynucleotide consisting of the nucleotide sequence represented by SEQ ID NO: 1 or 3) (b) a polynucleotide consisting of a nucleotide sequence encoding a wild-type H1foo protein (e.g., a protein consisting of the amino acid sequence represented by SEQ ID NO: 2 or 4); (c) a polynucleotide encoding a protein having an amino acid sequence in which one or more amino acids are mutated in the amino acid sequence of the wild-type H1foo protein (e.g., the amino acid sequence represented by SEQ ID NO: 2 or 4) and having H1foo activity. (d) a polynucleotide encoding a protein having an amino acid sequence having 70% or more sequence identity with the amino acid sequence of a wild-type H1foo protein (e.g., the amino acid sequence represented by SEQ ID NO: 2 or 4) and having H1foo activity; (e) a polynucleotide encoding a protein having H1foo activity, the polynucleotide consisting of a nucleotide sequence in which one or more nucleotides are mutated in the nucleotide sequence of the wild-type H1foo gene (e.g., the nucleotide sequence represented by SEQ ID NO: 1 or SEQ ID NO: 3); (f) A polynucleotide consisting of a nucleotide sequence having 70% or more sequence identity with the nucleotide sequence of the wild-type H1foo gene (e.g., the nucleotide sequence represented by SEQ ID NO: 1 or SEQ ID NO: 3) and encoding a protein having H1foo activity. (g) a polynucleotide that hybridizes to a wild-type H1foo gene (e.g., a polynucleotide comprising the nucleotide sequence represented by SEQ ID NO: 1 or SEQ ID NO: 3) under stringent conditions and encodes a protein having H1foo activity;

[0022] In the above (c) and (e), the "mutation" may be any of deletion, substitution, addition, and insertion, or a combination thereof. In (c) above, the term "multiple" is not particularly limited as long as the resulting protein has H1foo activity, but examples include 2 to 30, with 2 to 20 being preferred, 2 to 10 being more preferred, 2 to 5 being even more preferred, and 2 or 3 being particularly preferred. In (e) above, the term "multiple" is not particularly limited as long as the resulting polynucleotide encodes a protein having H1foo activity, but examples include 2 to 60, with 2 to 50 being preferred, 2 to 40 or 2 to 30 being more preferred, 2 to 20 or 2 to 10 being even more preferred, and 2 to 5 or 2 or 3 being particularly preferred. In (d) and (f) above, the sequence identity is not particularly limited as long as it is 70% or more, but is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more. The sequence identity between amino acid sequences or nucleotide sequences is determined by aligning two amino acid sequences or nucleotide sequences, with gaps at the positions corresponding to insertions and deletions, so that the number of corresponding amino acids or nucleotides is maximized, and then calculating the percentage of matching amino acids or nucleotides relative to the entire amino acid sequence or the entire nucleotide sequence excluding gaps in the resulting alignment. The sequence identity between amino acid sequences or nucleotide sequences can be determined using various homology search software known in the art. For example, the sequence identity of amino acid sequences or nucleotide sequences can be calculated based on the alignment obtained using the known homology search software BLASTP or BLASTN. In (g) above, "stringent conditions" include, for example, those described in "Molecular Cloning—A Laboratory Manual, Third Edition" (Sambrook et al., Cold Spring Harbor Laboratory Press). For example, hybridization conditions include incubation for several hours to overnight at 42 to 70°C in a hybridization buffer consisting of 6xSSC (20xSSC: 3M sodium chloride, 0.3M citric acid solution, pH 7.0), 5xDenhardt's solution (100xDenhardt's solution: 2% by weight bovine serum albumin, 2% by weight Ficoll, 2% by weight polyvinylpyrrolidone), 0.5% by weight SDS, 0.1 mg / mL salmon sperm DNA, and 50% by volume formamide. The washing buffer used for washing after incubation is preferably 1xSSC solution containing 0.1% by weight SDS, more preferably 0.1xSSC solution containing 0.1% by weight SDS.

[0023] In the above (b) to (e), it is preferable that the degenerate codons used are those that are frequently used in the somatic cells in which the iPS cell quality improving agent of this embodiment is used. For example, when used in human somatic cells, it is preferable that the codons used are those that are frequently used in human cells. That is, it is preferable that the codons are optimized for human codons. Furthermore, when used in mouse somatic cells, it is preferable that the codons used are those that are frequently used in mouse cells. That is, it is preferable that the codons are optimized for mouse codons.

[0024] As used herein, the term "H1foo protein" may refer to a fusion protein of the H1foo protein with a destabilization domain, as described below, or the H1foo protein region within the fusion protein.

[0025] (control array) The control sequence is a nucleotide sequence that can control at least one of the amount and timing of the H1foo protein present in a cell when the H1foo gene is introduced into the cell. The control sequence is not particularly limited as long as it can control either the amount or timing of the H1foo protein present in a cell. Examples of the control sequence include a nucleotide sequence encoding a destabilization domain. Alternatively, examples of the control sequence include a promoter sequence that controls the transcription of the H1foo gene in response to a chemical stimulus.

[0026] As used herein, a "destabilized domain" (DD) refers to a domain that promotes the proteasomal degradation of a fusion protein containing the domain. That is, by linking a destabilized domain to the N-terminus or C-terminus of a desired protein to form a fusion protein containing the destabilized domain, the proteasomal degradation of the fusion protein containing the desired protein is promoted. The destabilization domain is not particularly limited, and any known destabilization domain can be used as long as it promotes the degradation of a fusion protein containing the domain. Examples of destabilization domains include the destabilization domain derived from FKBP12 (Banaszynski et al., Cell. 2006 Sep 8;126(5):995-1004.) and the destabilization domain derived from ecDHFR (Iwamoto et al., Chem Biol. 2010 Sep 24;17(9):981-8.). An example of an FKBP12-derived destabilization domain is one having the amino acid sequence set forth in SEQ ID NO: 14, and an example of a nucleotide sequence encoding the amino acid sequence is the nucleotide sequence set forth in SEQ ID NO: 13. Note that a fusion protein of a desired protein and a destabilization domain may contain a polypeptide between the desired protein and the destabilization domain. Degradation of fusion proteins containing these destabilizing domains can be suppressed by membrane-permeable low-molecular-weight compounds (hereinafter referred to as "stabilizing compounds") called Shield1 (in the case of the FKBP12-derived destabilizing domain) or Guard (in the case of the ecDHFR-derived destabilizing domain). Polynucleotides encoding these destabilizing domains (hereinafter referred to as "destabilizing domain genes") and the stabilizing compounds can be obtained from the PreteoTuner TM Shield System (Clontech), PreteoTuner TM It can be used with Guard System (Clontech) etc. The destabilization domain can also be referred to as a degron sequence. Examples of degrons include, but are not limited to, mTOR degron (US Patent Application Publication No. 2009 / 0215169), dihydrofolate reductase (DHFR) degron (US Patent Application Publication No. 2012 / 0178168), PEST (WO 99 / 54348), TetR degron (WO 2007 / 032555), and auxin-inducible degron (AID) (WO 2010 / 125620).

[0027] The destabilization domain gene may be linked to the H1foo gene so as to express a fusion protein of the destabilization domain and the H1foo protein. Thus, when the iPS cell quality improvement agent of this embodiment is introduced into cells, the H1foo protein is expressed as a fusion protein with the destabilization domain. Therefore, the H1foo protein is rapidly degraded by the proteasome after expression. That is, by expressing the H1foo protein as a fusion protein with the destabilization domain, it is possible to control the amount of H1foo protein present in the cell to be low. Furthermore, by using a vector that expresses the gene in the cytoplasm without integrating it into the host chromosome and that rapidly disappears from the cell after gene expression as the expression vector for the fusion protein, it is possible to control the timing of the presence of the H1foo protein within a certain period of time after introduction of the expression vector. In this way, by controlling the expression of the H1foo protein to be expressed for a certain period of time from the introduction of the nuclear reprogramming substance to the completion of reprogramming and to be present at a relatively low level thereafter, high-quality iPS cells can be produced with high efficiency.

[0028] Furthermore, the amount and timing of the presence of a fusion protein containing a destabilizing domain in a cell can be controlled by adding a stabilizing compound. For example, after introducing the iPS cell quality improving agent of this embodiment into a somatic cell together with a nuclear reprogramming substance described below, the amount and timing of the presence of H1foo protein can be controlled by adding a stabilizing compound for a desired period of time and removing the stabilizing compound after the desired period has elapsed.

[0029] Furthermore, the polynucleotide contained in the iPS cell quality improving agent of this embodiment may be in a form in which the H1foo gene is inserted into an expression vector in a state in which it can be expressed in the cells into which it is introduced. The destabilization domain gene may be linked to the replication-associated protein gene so as to express a fusion protein of the destabilization domain and a protein required for replication of the expression vector (hereinafter referred to as "replication-associated protein"). "Replication-associated protein gene" refers to a polynucleotide encoding a replication-associated protein. A replication-associated protein expressed as a fusion protein with a destabilization domain is promptly degraded by the proteasome after expression. Therefore, a decrease in the amount of the replication-associated protein inhibits replication of the expression vector. This indirectly controls the timing and amount of H1foo protein present in cells. The replication-related protein to be fused with the destabilization domain is not particularly limited and may be appropriately selected depending on the type of expression vector. For example, when the expression vector is a Sendai virus vector, examples of the replication-related protein include nucleocapsid protein (N), phosphorylated protein (P), matrix protein (M), and large protein (L). Among these, the P protein is preferred. When an expression vector contains multiple replication-associated protein genes, a destabilization domain gene may be ligated to any two or more of the multiple replication-associated protein genes.

[0030] When the regulatory sequence includes a destabilization domain gene sequence, the regulatory sequence may be linked to at least one of the 5' and 3' ends of the H1foo gene or replication-associated protein gene. The regulatory sequence may be linked to either the 5' or 3' end of the protein gene, or to both the 5' and 3' ends. The regulatory sequence is linked to the H1foo gene or replication-associated protein gene so that frameshift does not occur between the destabilization domain gene and the H1foo gene or replication-associated protein gene (i.e., in-frame). As an example, SEQ ID NO: 15 shows the nucleotide sequence when the FKBP12-derived destabilization domain gene is linked to the 3' end of the H1foo gene. The nucleotide sequence of SEQ ID NO: 15 encodes a fusion protein (SEQ ID NO: 16) in which the FKBP12-derived destabilization domain is added to the C-terminus of the H1foo protein. As an example, SEQ ID NO: 17 shows the nucleotide sequence when the FKBP12-derived destabilization domain gene is linked to the 5' end of the H1foo gene. The nucleotide sequence set forth in SEQ ID NO: 17 encodes a fusion protein (SEQ ID NO: 18) in which a destabilization domain derived from FKBP12 is added to the N-terminus of the H1foo protein. The regulatory sequence may be linked to both the H1foo gene and the replication-related protein gene.

[0031] The control sequence may include a promoter sequence that controls transcription of the H1foo gene in response to an external stimulus. Examples of external stimuli include chemicals, heat, light, pH, osmotic pressure, etc. A promoter that controls transcription of the H1foo gene in response to an external stimulus (hereinafter referred to as an "external stimulus-responsive promoter") is positioned upstream of the H1foo gene so as to control transcription of the H1foo gene. The external stimulus-responsive promoter is not particularly limited, and any known promoter can be used. Examples of external stimulus-responsive promoters include tetracycline-responsive promoters (e.g., tetracycline response element: TRE).

[0032] (Polynucleotide) The agent for improving iPS cell quality of this embodiment comprises a polynucleotide having the above-mentioned H1foo gene and the above-mentioned regulatory sequence, wherein the polynucleotide is inserted into an expression vector in a state in which the H1foo gene can be expressed under the control of the regulatory sequence. In addition, the "expression vector capable of expressing the H1foo gene" may be one that expresses the H1foo protein, or one that expresses a fusion protein of the H1foo protein and a destabilization domain.

[0033] If necessary, the expression vector preferably contains, in addition to the H1foo gene and control sequence, a promoter that controls the expression of the H1foo gene or the H1foo gene and the destabilization domain gene (collectively referred to as "H1foo gene, etc."). In the expression vector, the promoter is positioned upstream of the H1foo gene, etc. so as to control the expression of the H1foo gene, etc. However, if the control sequence is an external stimulus-responsive promoter, it is preferable that the expression vector does not contain another promoter that controls the expression of the H1foo gene.

[0034] Examples of promoters include the SRα promoter, SV40 early promoter, retroviral LTR, CMV (cytomegalovirus) promoter, RSV (Rous sarcoma virus) promoter, HSV-TK (herpes simplex virus thymidine kinase) promoter, EF1α promoter, metallothionein promoter, and heat shock promoter. The enhancer of the IE gene of human CMV may also be used in combination with a promoter. For example, the CAG promoter (containing the cytomegalovirus enhancer, chicken β-actin promoter, and β-globin gene poly(A) signal site) can be used. Furthermore, the stimulus-responsive promoters listed above in the section "Regulatory Sequence" may also be used.

[0035] In addition to a promoter, an expression vector may optionally contain an enhancer, a poly(A) addition signal, a marker gene, a replication origin, a gene encoding a protein that binds to the replication origin and controls replication, and the like. A marker gene is a gene that enables cell sorting or selection by introducing the marker gene into cells. Specific examples of marker genes include drug resistance genes, fluorescent protein genes, luciferase genes, and chromogenic enzyme genes. These may be used alone or in combination. Specific examples of drug resistance genes include neomycin resistance genes, tetracycline resistance genes, kanamycin resistance genes, zeocin resistance genes, hygromycin resistance genes, and puromycin resistance genes. Specific examples of fluorescent protein genes include green fluorescent protein (GFP) genes, yellow fluorescent protein (YFP) genes, and red fluorescent protein (RFP) genes. Specific examples of luciferase genes include luciferase genes. Specific examples of chromogenic enzyme genes include β-galactosidase genes, β-glucuronidase genes, and alkaline phosphatase genes. When a stimulus-responsive promoter is used as the promoter, the expression vector may contain an enhancer gene or repressor gene that binds to the promoter in response to the stimulus. For example, in the case of a tetracycline-responsive promoter, the expression vector may contain a gene such as reverse tetracycline-controlled transactivator (rtTA) or tetracycline-controlled transactivator (rTA). These enhancer genes or repressor genes may be contained in an expression vector separate from the expression vector containing the H1foo gene.

[0036] The type of expression vector is not particularly limited, and any known expression vector can be used, including, for example, episomal vectors, artificial chromosome vectors, plasmid vectors, and viral vectors.

[0037] Episomal vectors are vectors capable of autonomous replication outside of a chromosome. Specific methods for using episomal vectors are disclosed in Yu et al., Science, 324, 797-801 (2009). For example, an episomal vector can be used in which loxP sequences are placed in the same orientation on the 5' and 3' ends of vector elements necessary for episomal vector replication. Because episomal vectors can autonomously replicate outside of a chromosome, they can provide stable expression in host cells even without being integrated into the genome. However, once iPS cells are established, it is desirable to promptly remove the vector. By flanking the vector elements necessary for episomal vector replication between two loxP sequences and then excising the vector elements with Cre recombinase, the autonomous replication ability of the episomal vector can be lost, allowing the vector to be rapidly eliminated from iPS cells.

[0038] Examples of episomal vectors include vectors that contain, as vector elements, sequences necessary for autonomous replication derived from EBV, SV40, etc. Specific examples of vector elements necessary for autonomous replication include a replication origin and a gene encoding a protein that binds to the replication origin and controls replication, such as the replication origin oriP and EBNA-1 gene for EBV, and the replication origin ori and SV40LT gene for SV40.

[0039] Examples of artificial chromosome vectors include YAC (Yeast artificial chromosome) vectors, BAC (Bacterial artificial chromosome) vectors, and PAC (P1-derived artificial chromosome) vectors.

[0040] The plasmid vector is not particularly limited as long as it can be expressed in the somatic cells to be introduced. When the somatic cells to be introduced are mammalian, a plasmid vector commonly used for expression in animal cells can be used. Examples of plasmid vectors for expression in animal cells include pA1-11, pXT1, pRc / CMV, pRc / RSV, and pcDNAI / Neo.

[0041] Examples of viral vectors include retroviral (including lentiviral) vectors, adenoviral vectors, adeno-associated viral vectors, Sendai viral vectors, herpes viral vectors, vaccinia viral vectors, pox viral vectors, polio viral vectors, Silvis viral vectors, rhabdoviral vectors, paramyxoviral vectors, and orthomyxoviral vectors. As used herein, a viral vector refers to a vector that has genomic nucleic acid derived from the virus and can express a gene by incorporating the gene into the nucleic acid.

[0042] A preferred viral vector is a Sendai virus vector. Sendai virus is a virus of the order Mononegavirales and belongs to the family Paramyxoviridae (including genera Paramyxovirus, Morbillivirus, Rubulavirus, and Pnemovirus). Its genome contains a single minus-strand RNA (the antisense strand to the sense strand encoding the viral protein). Minus-strand RNA is also called negative-strand RNA. Sendai virus vectors are non-chromosomally integrated viral vectors, and the vector is expressed in the cytoplasm. Therefore, there is no risk of the introduced gene being integrated into the host chromosome. This makes them highly safe, and the vector can be removed from the introduced cells after the intended purpose is achieved.

[0043] Sendai virus vectors include infectious virus particles as well as complexes consisting of a virus core, a complex of a virus genome and virus proteins, or non-infectious virus particles, which are capable of expressing the gene carried by the vector when introduced into cells. For example, ribonucleoproteins (the core portion of the virus) consisting of the Sendai virus genome and the Sendai virus proteins (NP, P, and L proteins) that bind to it can express introduced genes within cells when introduced into the cells (WO 00 / 70055). Introduction into cells can be carried out using an appropriate transfection reagent or the like. Therefore, such ribonucleoproteins (RNPs) are also encompassed within the Sendai virus vector.

[0044] The Sendai virus genome contains, from the 3' to the 5' end, the NP (nucleocapsid) gene, P (phospho) gene, M (matrix) gene, F (fusion) gene, HN (hemagglutinin / neuraminidase) gene, and L (large) gene. Of these, the Sendai virus can function as a vector sufficiently with the NP, P, and L genes, replicating its genome in cells and expressing the genes it carries. Because the Sendai virus has a negative-strand RNA genome, the 3' end of the genome is upstream and the 5' end is downstream, which is the opposite of the usual genome.

[0045] The accession numbers of the nucleotide sequences of the above genes of Sendai virus in the database (GenBank) are, for example, M29343, M30202, M30203, M30204, M51331, M55565, M69046, X17218 for the NP gene; M30202, M30203, M30204, M55565, M69046, X00583, X17007, X17008 for the P gene; and D11446, K02742, M30202, M30203, M30204, M69046, U31956, X0 for the M gene. 0584, X53056; for the F gene, D00152, D11446, D17334, D17335, M30202, M30203, M30204, M69046, X00152, X02131; for the HN gene, D26475, M12397, M30202, M30203, M30204, M69046, X00586, X02808, X56131; for the L gene, D00053, M30202, M30203, M30204, M69040, X00587, X58886 can be identified by reference. However, multiple strains of Sendai virus are known, and depending on the strain, genes with sequences other than those listed above may exist. Sendai virus vectors carrying viral genes derived from any of these genes are also useful as Sendai virus vectors. For example, a Sendai virus vector may contain a nucleotide sequence that has 90% or more, preferably 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more, sequence identity to the coding sequence of any of the viral genes listed above. Furthermore, a Sendai virus vector may contain a nucleotide sequence encoding an amino acid sequence that has 90% or more, preferably 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more, identity to the amino acid sequence encoded by the coding sequence of any of the viral genes listed above. Furthermore, the Sendai virus vector may contain, for example, a base sequence encoding a polypeptide that retains the function of each gene product, which is an amino acid sequence in which 10 or less, preferably 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less, 2 or less, or 1 amino acid has been substituted, inserted, deleted, and / or added in the amino acid sequence encoded by the coding sequence of any of the above-mentioned viral genes.

[0046] The sequences referred to in this specification, such as nucleotide sequences and amino acid sequences, have database accession numbers that refer to the sequences as of the filing date of this application. The sequences at each time point can be identified by referring to the database revision history.

[0047] The Sendai virus vector used in this embodiment may be a derivative. Derivatives of Sendai virus vectors include viruses whose viral genes have been modified and viruses that have been chemically modified so as not to impair the gene transfer ability of the Sendai virus.

[0048] Sendai viruses may be derived from natural strains, wild-type strains, mutant strains, laboratory-passaged strains, or artificially constructed strains. For example, the Z strain can be used (Medical Journal of Osaka University, Vol. 6, No. 1, March 1955, pp. 1-15). In other words, as long as the virus can achieve the intended function, it may be a viral vector with a structure similar to that of a virus isolated from nature, or a virus artificially modified by genetic recombination. For example, a virus with a mutation or deletion in any of the genes contained in a wild-type virus may be used. Incomplete viruses such as DI particles (J. Virol. 68: 8413-8417, 1994) can also be used. For example, viruses with a mutation or deletion in at least one gene encoding the viral envelope protein or coat protein can be used preferably. Such viral vectors are capable of replicating their genome in infected cells but are unable to form infectious viral particles. Such propagation-deficient viral vectors are highly safe because they eliminate the risk of spreading infection to the surrounding area. For example, viral vectors can be used that lack at least one gene encoding an envelope protein such as F and / or HN, or a spike protein, or a combination thereof (WO 00 / 70055, WO 00 / 70070, Li, H.-O. et al., J. Virol. 74(14) 6564-6569 (2000)). If the genomic RNA encodes proteins necessary for genome replication (e.g., NP, P, and L proteins), the genome can be amplified in infected cells. Defective viruses can be produced, for example, by exogenously supplying the defective gene product or a protein capable of complementing it to virus-producing cells (WO 00 / 70055, WO 00 / 70070, Li, H.-O. et al., J. Virol. 74(14) 6564-6569 (2000)). When a viral vector is recovered as RNP (for example, RNP consisting of N, L, and P proteins and genomic RNA), the vector can be produced without complementing the envelope protein.

[0049] Suitable Sendai virus vectors include, for example, an F gene-deleted Sendai virus vector (e.g., Z strain) having G69E, T116A, and A183S mutations in the M protein, A262T, G264, and K461G mutations in the HN protein, L511F mutation in the P protein, and N1197S and K1795E mutations in the L protein, or vectors into which TS7, TS12, TS13, TS14, or TS15 mutations have been introduced. Specific examples include, but are not limited to, SeV18+ / TSΔF (WO 2010 / 008054, WO 2003 / 025570), SeV(PM) / TSΔF, and vectors into which TS7, TS12, TS13, TS14, or TS15 mutations have been introduced. "TSΔF" refers to a strain that has G69E, T116A, and A183S mutations in the M protein, A262T, G264R, and K461G mutations in the HN protein, L511F mutation in the P protein, and N1197S and K1795E mutations in the L protein, and lacks the F gene.

[0050] Suitable Sendai virus vectors include, for example, vectors in which a degron sequence has been added to the P protein to facilitate its removal from infected cells (WO 2016 / 125364). Examples of degrons include, but are not limited to, the mTOR degron (U.S. Patent Application Publication No. 2009 / 0215169), dihydrofolate reductase (DHFR) degron (U.S. Patent Application Publication No. 2012 / 0178168), PEST (WO 99 / 54348), TetR degron (WO 2007 / 032555), and auxin-inducible degron (AID) (WO 2010 / 125620). For example, a vector in which a DD-tag, a type of mTOR degron, has been added to the C-terminus of the P protein is suitable.

[0051] Reconstitution of a recombinant Sendai virus vector carrying the polynucleotide to be introduced (H1foo gene, or H1foo gene and regulatory sequence) can be carried out using known methods. Specifically, the virus can be produced by the following steps: (a) transcribing cDNA encoding Sendai virus genomic RNA (minus strand) or its complementary strand (plus strand) in cells expressing viral proteins (N, P, and L) required for virus particle formation; and (b) recovering the culture supernatant containing the produced virus. The viral proteins required for particle formation may be expressed from the transcribed viral genomic RNA or may be supplied in trans from a source other than the genomic RNA. For example, they can be supplied by introducing an expression plasmid encoding the N, P, and L proteins into cells. If the genomic RNA lacks a viral gene required for particle formation, the viral gene can be separately expressed in virus-producing cells to complement particle formation. To express viral proteins or RNA genomes in cells, a vector containing DNA encoding the protein or genomic RNA ligated downstream of an appropriate promoter functional in the host cell is introduced into the host cell. The transcribed genomic RNA replicates in the presence of viral proteins, resulting in the formation of infectious virus particles. When producing a defective virus lacking a gene for an envelope protein or the like, the defective protein or another viral protein capable of complementing its function can be expressed in virus-producing cells.

[0052] Furthermore, Sendai virus can be produced using the following known methods (WO 97 / 16539; WO 97 / 16538; WO 00 / 70055; WO 00 / 70070; WO 01 / 18223; WO 03 / 025570; WO 2005 / 071092; WO 2006 / 137517; WO 2007 / 083644; WO 2008 / 007581; Hasan, M. K. et al., J. Gen. Virol. 78: 2813-2820, 1997; Kato, A. et al., 1997, EMBO J. 16: 578-587; and Yu, D. et al., 1997, Genes Cells 2: 457-466;Durbin, AP et al., 1997, Virology 235: 323-332;Whelan, SP et al., 1995, Proc. Natl. Acad. Sci. USA 92: 8388-8392;Schnell. MJ et al., 1994, EMBO J. 13: 4195-4203;Radecke, F. et al., 1995, EMBO J. 14: 5773-5784;Lawson, ND et al., Proc. Natl. Acad. Sci. USA 92: 4477-4481;Garcin, D. et al., 1995, EMBO J. 14: 6087-6094;Kato, A.et al.,1996, Genes Cells 1: 569-579;Baron, MD and Barrett, T., 1997, J. Virol. 71: 1265-1271;Bridgen, A. and Elliott, RM, 1996, Proc. Natl. Acad. Sci. USA 93: 15400-15404;Tokusumi, T. et al. Virus Res. 2002: 86; 33-38, Li, H.-O. et al., J. Virol. 2000: 74; 6564-6569).

[0053] When a Sendai virus vector is used, the H1foo gene, etc. may be located at any position in the Sendai virus genomic RNA (minus strand) as long as it does not disrupt each gene of the Sendai virus. For example, the H1foo gene, etc. may be located 3' closer to the NP gene, between the NP gene and the P gene, between the P gene and the M gene, between the M gene and the F gene, between the F gene and the HN gene (between the M gene and the HN gene if the F gene is not present), between the HN gene and the L gene, or 5' closer to the L gene. In order to increase the expression level of the H1foo gene, etc., it is preferable to position the H1foo gene, etc. closer to the 3' side. For example, when the H1foo gene, etc. is positioned closest to the 3' side of the Sendai virus genomic RNA (minus strand) (e.g., 3' closer to the NP gene), the expression level of the H1foo gene, etc. is maximized. When a Sendai virus vector is used, it is preferable to position the H1foo gene, etc. closer to the 3' side than all of the genes of the Sendai virus. That is, it is preferable to position the H1foo gene, etc. closest to the 3' side. This facilitates maintaining an appropriate balance between the expression level of H1foo protein and the degradation of H1foo protein promoted by the destabilization domain. This allows for the efficient production of higher quality iPS cells. Furthermore, by infecting target cells with Sendai virus, higher quality iPS cells can be produced efficiently.

[0054] When a viral vector is used as an expression vector, viral particles obtained using packaging cells may be used. Packaging cells are cells into which a gene encoding a viral structural protein has been introduced. When a recombinant viral vector incorporating a gene of interest is introduced into such cells, recombinant viral particles incorporating the gene of interest are produced. Packaging cells are not particularly limited and can be selected appropriately depending on the purpose. Examples include packaging cells based on human kidney-derived HEK293 cells or mouse fibroblast-derived NIH3T3 cells; PLAT-E cells engineered to express an envelope glycoprotein derived from an ecotropic virus; PLAT-A cells engineered to express an envelope glycoprotein derived from an amphotropic virus; and PLAT-GP cells engineered to express an envelope glycoprotein derived from a vesicular stomatitis virus. When the target of viral vector introduction is human somatic cells, PLAT-A cells, PLAT-GP cells, and the like are preferred as packaging cells in terms of host tropism. The method for introducing the viral vector into packaging cells is not particularly limited and can be selected appropriately depending on the purpose. For example, lipofection, electroporation, calcium phosphate method, etc. may be mentioned.

[0055] The expression vector may contain a nuclear reprogramming substance, which will be described later. When the expression vector contains a nuclear reprogramming substance, the nuclear reprogramming substance may be one type or two or more types. When the expression vector has a nuclear reprogramming substance together with the H1foo gene or the like, a base sequence encoding a self-cleaving peptide such as a 2A peptide, or an IRES (Internal Ribozyme Entry Site) sequence, or the like, may be interposed between the H1foo gene or the like and the nuclear reprogramming substance. By interposing these sequences, multiple proteins can be expressed independently from one promoter.

[0056] In a preferred embodiment, the quality improvement agent of this embodiment is a Sendai virus vector carrying, in an expressible form, a polynucleotide encoding a fusion protein of an H1foo protein and a destabilization domain. In a further preferred embodiment, the quality improvement agent of this embodiment is a Sendai virus vector carrying, in an expressible form, a polynucleotide encoding a fusion protein of an H1foo protein and a destabilization domain derived from FKBP12. Specific examples of the fusion protein containing H1FOO include proteins comprising the amino acid sequence set forth in SEQ ID NO: 16 or SEQ ID NO: 18. Specific examples of polynucleotides encoding these proteins include polynucleotides comprising the nucleotide sequence set forth in SEQ ID NO: 15 or SEQ ID NO: 17.

[0057] Second Embodiment In one embodiment, the present invention provides an agent for improving the quality of iPS cells, comprising a fusion protein of an H1foo protein and a destabilization domain, wherein the destabilization domain induces degradation of the fusion protein by the proteasome.

[0058] (H1foo protein) H1foo protein is a protein produced by transcription and translation from the H1foo gene described above in "<First Embodiment>" and has H1foo activity. The biological species from which H1foo protein is derived is not particularly limited and can be selected appropriately depending on the purpose, and examples include any mammalian species such as human, mouse, rat, cow, sheep, horse, and monkey. The amino acid sequence of human H1foo protein is exemplified in SEQ ID NO: 2. The amino acid sequence of mouse H1foo protein is exemplified in SEQ ID NO: 4.

[0059] The H1foo protein is not limited to wild-type H1foo protein, but may contain mutations (deletions, substitutions, insertions, and additions, or a combination thereof). The number of mutations is not particularly limited as long as the protein has H1foo activity.

[0060] Examples of H1foo proteins include the following (a) to (c): (a) Wild-type H1foo protein (e.g., a polypeptide consisting of the amino acid sequence represented by SEQ ID NO: 2 or SEQ ID NO: 4) (b) a protein having an amino acid sequence in which one or more amino acids have been mutated in the amino acid sequence of the wild-type H1foo protein (e.g., the amino acid sequence represented by SEQ ID NO: 2 or 4), and having H1foo activity; (c) A protein having an amino acid sequence that has 70% or more sequence identity with the amino acid sequence of the wild-type H1foo protein (for example, the amino acid sequence represented by SEQ ID NO: 2 or 4) and has H1foo activity.

[0061] In the above (b), the "mutation" may be any of deletion, substitution, addition, and insertion, or a combination thereof. In (b) above, the term "multiple" is not particularly limited as long as the resulting protein has H1foo activity, but examples include 2 to 30, with 2 to 20 being preferred, 2 to 10 being more preferred, 2 to 5 being even more preferred, and 2 or 3 being particularly preferred. In (c) above, the sequence identity is not particularly limited as long as it is 70% or more, but 80% or more is preferred, 85% or more is more preferred, and 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more is even more preferred.

[0062] (destabilizing domain) The destabilization domain is the same as that explained in the above "<First embodiment>", and examples thereof include those similar to those exemplified in the above "<First embodiment>".

[0063] (fusion protein) A fusion protein of an H1foo protein and a destabilization domain has a destabilization domain linked to at least one of the N-terminus and C-terminus of the H1foo protein. The destabilization domain may be linked to either the N-terminus or the C-terminus of the H1foo protein, or may be linked to both the N-terminus and the C-terminus.

[0064] A fusion protein of the H1foo protein and a destabilization domain can be produced using an expression vector containing a polynucleotide in which a destabilization domain gene is linked to at least one of the 5' end or 3' end of the H1foo gene. The fusion protein can be obtained by introducing the expression vector into appropriate cells, culturing the cells, and isolating and purifying the fusion protein from the culture supernatant or cultured bacteria or cells.

[0065] The fusion protein of the H1foo protein and the destabilization domain may further contain a protein transduction domain (PTD). PTDs using the cell passage domains of proteins such as Drosophila-derived AntP, HIV-derived TAT, and HSV-derived VP22 have been developed. The inclusion of a PTD allows the fusion protein to be introduced into cells without the use of a protein transduction reagent.

[0066] When introduced into somatic cells, a fusion protein of H1foo protein and a destabilization domain is rapidly degraded by the proteasome due to the presence of the destabilization domain. The amount and timing of the presence of a fusion protein containing a destabilization domain within the cell can be controlled by adding a stabilizing compound. Therefore, the amount and timing of the presence of H1foo protein within the cell can be controlled. After introducing the iPS cell quality improving agent of this embodiment into somatic cells together with a nuclear reprogramming substance described below, high-quality iPS cells can be produced by controlling the presence of the fusion protein of H1foo protein and a destabilization domain within the cell for, for example, a desired period of time immediately after introduction.

[0067] In the iPS cell quality improver and iPS cell production method of the present invention, instead of the polynucleotide containing the H1foo gene and a regulatory sequence, a substance can be introduced into somatic cells together with a nuclear reprogramming substance and simultaneously suppress the innate immune response in the cells upon initiation of cell reprogramming. Suppression of the innate immune response specifically refers to suppression of expression of innate immune response markers such as IFIT1 and IFNA. More specifically, it refers to increasing FKBP1A expression in the cells by at least 2-fold, preferably 2-20-fold, and more preferably 5-15-fold, compared to when only the nuclear reprogramming substance is introduced. Here, "the time when cell reprogramming begins" refers to 2-15 days, preferably 3-6 days, after introduction of the nuclear reprogramming substance. Performing this reprogramming step not only increases the efficiency of iPS cell establishment, but also ensures that the resulting iPS cell population contains many iPS cells with high quality, such as high pluripotency. Therefore, this method is effective as a method for producing allogeneic iPS cells, but is particularly effective when producing autologous iPS cells without establishing a cell line and handling them in bulk. The substance having the function of suppressing an innate immune response is not particularly limited as long as it can suppress the expression of an innate immune response marker. Examples of the substance having the function of suppressing an innate immune response include, but are not limited to, siRNA or antisense RNA against an innate immune response marker gene, a transcription inhibitor of an innate immune response marker gene, FKBP1A, the FKBP1A gene, and a transcription promoter of FKBP1A.

[0068] [Method of producing iPS cells] First Embodiment In one embodiment, the present invention provides a method for producing iPS cells, comprising the step of introducing a nuclear reprogramming substance and the agent for improving iPS cell quality of the above embodiment into somatic cells.

[0069] ≪Introduction process≫ The present embodiment provides a method for producing iPS cells, which includes the step of introducing a nuclear reprogramming substance and the iPS cell quality improving agent of the above embodiment into somatic cells.

[0070] (iPS cell quality improver) The agent for improving iPS cell quality may be either the first or second embodiment described above in "[Agent for improving iPS cell quality]." In addition, both the agents for improving iPS cell quality of the first and second embodiments may be used in combination.

[0071] (nuclear reprogramming substance) As used herein, "nuclear reprogramming substance" refers to a substance (or substances) that can be introduced into somatic cells to induce the somatic cells into iPS cells. The nuclear reprogramming substance is not particularly limited, as long as it is a substance (or substances) that can induce iPS cells from somatic cells. The nuclear reprogramming substance may be any substance, such as a gene (including a form incorporated into an expression vector) or its gene product, or a low-molecular-weight compound. "Gene product" refers to mRNA transcribed from a gene and a protein translated from the mRNA. The gene product used as a nuclear reprogramming substance may be mRNA, a protein, or both. A gene that is a nuclear reprogramming substance refers to a polynucleotide that encodes a protein that is a nuclear reprogramming substance.

[0072] When the nuclear reprogramming substance is a gene or its gene product, it may be at least one selected from the group consisting of Oct gene family genes, Sox gene family genes, Klf gene family genes, Myc gene family genes, Lin gene family genes, and Nanog genes, and their gene products (WO 2007 / 69666; Japanese Patent No. 5696282; Science, 2007, 318:1917-1920). Among these, it is preferably at least one selected from the group consisting of Oct gene family genes, Sox gene family genes, Klf gene family genes, and Myc gene family genes, and their gene products.

[0073] Specific examples of genes in these families and their combinations are listed below. Note that although only the names of the genes are listed below, this also includes cases where the gene products are used.

[0074] (a) A nuclear reprogramming substance consisting of a gene of the Oct gene family; (b) A combination of two nuclear reprogramming factors consisting of genes from the Oct gene family and genes from the Sox gene family; (c) A combination of two nuclear reprogramming factors consisting of genes from the Oct gene family and genes from the Klf gene family; (d) A combination of two nuclear reprogramming factors consisting of a gene of the Oct gene family and a Nanog gene; (e) A combination of three nuclear reprogramming factors consisting of genes from the Oct gene family, genes from the Sox gene family, and genes from the Klf gene family; (f) A combination of three nuclear reprogramming substances consisting of genes from the Oct gene family, Klf gene family, and Myc gene family. (g) a combination of four nuclear reprogramming substances consisting of Oct gene family genes, Sox gene family genes, Klf gene family genes, and Myc gene family genes; and (h) A combination of four nuclear reprogramming factors consisting of genes from the Oct gene family, genes from the Sox gene family, genes from the Lin gene family, and the Nanog gene.

[0075] More specifically, examples of combinations include, but are not limited to, the following: In the following combinations, the Sox2 gene can be replaced with the Sox1 gene, the Sox3 gene, the Sox15 gene, the Sox17 gene, or the Sox18 gene; the Klf4 gene can be replaced with the Klf1 gene, the Klf2 gene, or the Klf5 gene; and the c-Myc gene can be replaced with the T58A (active mutant) gene, the N-Myc gene, or the L-Myc gene. (1) Oct3 / 4 gene, Klf4 gene, c-Myc gene (2) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene (3) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, Fbx15 gene, Nanog gene, Eras gene, ECAT15-2 gene, TclI gene, β-catenin (active mutant S33Y) (4) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, hTERT gene, SV40 Large T antigen (SV40LT) gene (5) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, hTERT gene, HPV16 E6 gene (6) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, hTERT gene, HPV16 E7 gene (7) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, hTERT gene, HPV6 E6 gene, HPV16 E7 gene (8) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, hTERT gene, Bmil gene (For the combinations (1) to (8) above, see International Publication No. 2007 / 069666 (however, for the substitution of the Sox2 gene with the Sox18 gene and the substitution of the Klf4 gene with the Klf1 gene or the Klf5 gene in the combination (2) above, see Nature Biotechnology, 26, 101-106 (2008)). For the combination of "Oct3 / 4 gene, Sox2 gene, Klf4 gene, and c-Myc gene," see also Cell, 126, 663-676 (2006) and Cell, 131, 861-872 (2007), etc. For the combination of "Oct3 / 4 gene, Sox2 gene, Klf2 (or Klf5) gene, and c-Myc gene," see Nat. Cell Biol., 11, 197-203 (2009) See also Nature, 451, 141-146 (2008) for the combination of "Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, hTERT gene, and SV40LT gene." (9) Oct3 / 4 gene, Sox2 gene, Klf4 gene (see Nature Biotechnology, 26, 101-106 (2008)) (10) Oct3 / 4 gene, Sox2 gene, Nanog gene, Lin28 gene (see Science, 318, 1917-1920 (2007)) (11) Oct3 / 4 gene, Sox2 gene, Nanog gene, Lin28 gene, hTERT gene, SV40LT gene (see Stem Cells, 26, 1998-2005 (2008)) (12) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, Nanog gene, Lin28 gene (see Cell Research (2008) 600-603) (13) Oct3 / 4 gene, Sox2 gene, Klf4 gene, c-Myc gene, SV40LT gene (see also Stem Cells, 26, 1998-2005 (2008)) (14) Oct3 / 4 gene, Klf4 gene (see Nature 454:646-650 (2008), Cell Stem Cell, 2:525-528 (2008)) (15) Oct3 / 4 gene, c-Myc gene (see Nature 454:646-650 (2008)) (16) Oct3 / 4 gene, Sox2 gene (Nature, 451, 141-146 (2008), International Publication No. 2008 / 118820) (17) Oct3 / 4 gene, Sox2 gene, Nanog gene (see International Publication No. 2008 / 118820) (18) Oct3 / 4 gene, Sox2 gene, Lin28 gene (see International Publication No. 2008 / 118820) (19) Oct3 / 4 gene, Sox2 gene, c-Myc gene, Esrrb gene (Esrrb gene can be replaced with Esrrg gene. See Nat. Cell Biol., 11, 197-203 (2009)) (20) Oct3 / 4 gene, Sox2 gene, Esrrb gene (see Nat. Cell Biol., 11, 197-203 (2009)) (21) Oct3 / 4 gene, Klf4 gene, L-Myc gene (22) Oct3 / 4 gene, Nanog gene (23) Oct3 / 4 gene (24) Oct3 / 4 gene, Klf4 gene, c-Myc gene, Sox2 gene, Nanog gene, Lin28 gene, SV40LT gene (see Science, 324: 797-801 (2009))

[0076] In the combinations (1) to (24) above, the Oct3 / 4 gene may be replaced with another member gene of the Oct gene family (e.g., Oct1A, Oct6, etc.). The Sox2 gene (or the Sox1 gene, Sox3 gene, Sox15 gene, Sox17 gene, or Sox18 gene) may be replaced with another member gene of the Sox gene family (e.g., Sox7 gene, etc.). The Lin28 gene may be replaced with another member gene of the Lin gene family (e.g., Lin28b gene, etc.).

[0077] Combinations that do not fall under the above combinations (1) to (24) but contain all of the components of any of them and further contain any other substance may also be included in the category of "nuclear reprogramming substances" in the present invention. Under conditions in which somatic cells to be subjected to nuclear reprogramming endogenously express some of the components of any of the above combinations (1) to (24) at levels sufficient for nuclear reprogramming, combinations containing only the remaining components excluding those components may also be included in the category of "nuclear reprogramming substances" in the present invention.

[0078] In addition to the above nuclear reprogramming substances, one or more nuclear reprogramming substances selected from the group consisting of the Fbx15 gene, ERas gene, ECAT15-2 gene, Tcl1 gene, and β-catenin gene may be combined, and / or one or more nuclear reprogramming substances selected from the group consisting of the ECAT1 gene, Esg1 gene, Dnmt3L gene, ECAT8 gene, Gdf3 gene, Mybl2 gene, ECAT15-1 gene, Fth117 gene, Sall4 gene, Rex1 gene, UTF1 gene, Stella gene, Stat3 gene, and Grb2 gene may be combined. These combinations are specifically described in WO 2007 / 69666.

[0079] Preferred nuclear reprogramming substances include at least one selected from the group consisting of the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, the c-Myc gene (or the L-Myc gene), the Lin28 gene, the Nanog gene, and gene products of these genes. Preferably, a combination of two or more selected from the group consisting of the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, the c-Myc gene (or the L-Myc gene), the Lin28 gene, the Nanog gene, and gene products thereof, more preferably a combination of three or more selected from the group consisting of the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, the c-Myc gene (or the L-Myc gene), the Lin28 gene, the Nanog gene, and gene products thereof. Among these, combinations of nuclear reprogramming substances that are preferably introduced include (1) the Oct3 / 4 gene or its gene product, the Sox2 gene or its gene product, and the Klf4 gene or its gene product, (2) the Oct3 / 4 gene or its gene product, the Sox2 gene or its gene product, the Klf4 gene or its gene product, and the c-Myc gene or its gene product, and (3) the Oct3 / 4 gene or its gene product, the Sox2 gene or its gene product, the Klf4 gene or its gene product, and the L-Myc gene or its gene product. Among these, combinations of the Oct3 / 4 gene or its gene product, the Sox2 gene or its gene product, and the Klf4 gene or its gene product, and combinations of the Oct3 / 4 gene or its gene product, the Sox2 gene or its gene product, the Klf4 gene or its gene product, and the L-Myc gene or its gene product are preferred. Among these, a combination of the Oct3 / 4 gene, the Sox2 gene, and the Klf4 gene, and a combination of the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, and the L-Myc gene are more preferable.

[0080] When the nuclear reprogramming substance is a gene or its gene product, the biological species from which the gene is derived is not particularly limited and can be appropriately selected depending on the purpose, and examples include any mammal such as human, mouse, rat, cow, sheep, horse, or monkey.

[0081] The cDNA sequence information of each of the above nuclear reprogramming substances can be obtained from publicly known databases. For example, the GenBank accession numbers described in International Publication No. 2007 / 069666 may be referenced. The Nanog gene is described in the publication under the name "ECAT4."

[0082] Among the above nuclear reprogramming substances, information on the mouse and human cDNA sequences of four particularly preferred genes (Oct3 / 4 gene, Sox2 gene, Klf4 gene, and L-Myc gene) is provided below. Gene Name Mouse Human Oct3 / 4 NM_013633 NM_002701 Sox2 NM_011443 NM_003106 Klf4 NM_010637 NM_004235 L-Myc NM_008506 NM_001033081

[0083] The cDNA sequence of the human Oct3 / 4 gene registered under the above GenBank accession numbers is shown in SEQ ID NO: 5, and the amino acid sequence of the human Oct3 / 4 protein is shown in SEQ ID NO: 6. The cDNA sequence of the human Sox2 gene is shown in SEQ ID NO: 7, and the amino acid sequence of the human Sox2 protein is shown in SEQ ID NO: 8. The cDNA sequence of the human Klf4 gene is shown in SEQ ID NO: 9, and the amino acid sequence of the human Klf4 protein is shown in SEQ ID NO: 10. The cDNA sequence of the human L-Myc gene is shown in SEQ ID NO: 11, and the amino acid sequence of the human L-Myc protein is shown in SEQ ID NO: 12.

[0084] Furthermore, among the above-mentioned nuclear reprogramming substances, mouse and human cDNA sequence information for genes for which GenBank accession numbers are not listed in WO 2007 / 069666 is listed below. Gene Name Mouse Human Lin28 NM_145833 NM_024674 Lin28b NM_001031772 NM_001004317 Esrrb NM_011934 NM_004452 Esrrg NM_011935 NM_001438

[0085] The cDNA of each of the above nuclear reprogramming substances can be easily isolated from the cells of the organism from which the sequence is derived using known methods such as PCR, based on the above cDNA sequence information or sequence information registered in a known database.

[0086] When the nuclear reprogramming substance is any of the above genes or their mRNAs, their nucleotide sequences are not limited to those of wild-type genes and may contain mutations as long as they have the nuclear reprogramming effect. The nucleotide sequences of the above genes may consist of only the protein-coding sequence, or may contain other portions (e.g., introns, 5'UTR, 3'UTR, etc.). When the nuclear reprogramming substance is a protein encoded by each of the above genes, its amino acid sequence is not limited to the amino acid sequence of the wild-type protein, and may contain mutations as long as it has the nuclear reprogramming effect. As used herein, the term "nuclear reprogramming effect" refers to the effect of inducing somatic cells to become iPS cells by introducing the somatic cells into the cells.

[0087] When the nuclear reprogramming substance is a gene or mRNA, examples of the nuclear reprogramming substance include the following (a) to (g). (a) a wild-type gene or wild-type mRNA exemplified as the nuclear reprogramming substance; (b) a polynucleotide consisting of a nucleotide sequence encoding a wild-type protein exemplified as the nuclear reprogramming substance; (c) a polynucleotide encoding a protein having a nuclear reprogramming activity, the protein having an amino acid sequence in which one or more amino acids are mutated in the wild-type protein exemplified as the nuclear reprogramming substance. (d) a polynucleotide encoding a protein having a nuclear reprogramming activity and consisting of an amino acid sequence having 70% or more sequence identity with the amino acid sequence of the wild-type protein exemplified as the nuclear reprogramming substance. (e) A polynucleotide encoding a protein having a nuclear reprogramming activity, which consists of a nucleotide sequence in which one or more nucleotides are mutated in the nucleotide sequence of the wild-type gene exemplified as the nuclear reprogramming substance. (f) A polynucleotide that consists of a nucleotide sequence that has 70% or more sequence identity with the nucleotide sequence of the wild-type gene exemplified as the nuclear reprogramming substance and encodes a protein that has nuclear reprogramming activity. (g) a polynucleotide that hybridizes under stringent conditions with the nucleotide sequence of the wild-type gene exemplified as the nuclear reprogramming substance and encodes a protein that has nuclear reprogramming activity.

[0088] In the above (c) and (e), the "mutation" may be any of deletion, substitution, addition, and insertion, or a combination thereof. The above (c) "plurality" is not particularly limited as long as the resulting protein has nuclear reprogramming activity, but examples include 2 to 30, preferably 2 to 20, more preferably 2 to 10, even more preferably 2 to 5, and particularly preferably 2 or 3. In the above (e), the term "multiple" is not particularly limited as long as the resulting polynucleotide encodes a protein having nuclear reprogramming activity, but examples include 2 to 60, with 2 to 50 being preferred, 2 to 40 or 2 to 30 being more preferred, 2 to 20 or 2 to 10 being even more preferred, and 2 to 5, or 2 or 3 being particularly preferred. In the above (d) and (f), the sequence identity is not particularly limited as long as it is 70% or more, but 80% or more is preferable, 85% or more is more preferable, and 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more is even more preferable. In (g) above, "stringent conditions" include the same conditions as those exemplified in the section "Agent for improving the quality of iPS cells," <First embodiment>, (H1foo gene)" above.

[0089] In the above (b) to (e), the degenerate codons used are preferably those with high codon usage frequency in the somatic cells in which the iPS cell quality improving agent of this embodiment is used. For example, when used in human somatic cells, it is preferable to use codons with high usage frequency in human cells. That is, it is preferable to optimize the codons to human codons. Furthermore, when used in mouse somatic cells, it is preferable to use codons with high usage frequency in mouse cells. That is, it is preferable to optimize the codons to mouse codons.

[0090] When the nuclear reprogramming substance is a gene, the gene is preferably incorporated into an expression vector. Examples of expression vectors include those listed above in the section "Agent for improving iPS cell quality (Polynucleotide)." Among these, Sendai virus vectors are preferred as expression vectors. When an expression vector is used, only one type of nuclear reprogramming substance may be incorporated into a single expression vector, or two or more types of nuclear reprogramming substances may be incorporated into a single expression vector. When an expression vector contains two or more types of nuclear reprogramming substances, a base sequence encoding a self-cleaving peptide such as 2A peptide, an IRES sequence, or the like may be interposed between the two or more genes that are nuclear reprogramming substances.

[0091] When the nuclear reprogramming substance is a protein, examples of the nuclear reprogramming substance include the following (A) to (C). (A) Wild-type proteins exemplified as nuclear reprogramming substances (B) A protein having an amino acid sequence in which one or more amino acids are mutated in the amino acid sequence of the wild-type protein exemplified as the nuclear reprogramming substance, and having nuclear reprogramming activity. (C) A protein having an amino acid sequence with 70% or more sequence identity to the wild-type protein exemplified as the nuclear reprogramming substance and having nuclear reprogramming activity.

[0092] In the above (B), the "mutation" may be any of deletion, substitution, addition, and insertion, or a combination thereof. In the above (B), the term "multiple" is not particularly limited as long as the resulting protein has nuclear reprogramming activity, but examples include 2 to 30, with 2 to 20 being preferred, 2 to 10 being more preferred, 2 to 5 being even more preferred, and 2 or 3 being particularly preferred. In (C) above, the sequence identity is not particularly limited as long as it is 70% or more, but 80% or more is preferred, 85% or more is more preferred, and 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more is even more preferred.

[0093] When the nuclear reprogramming substance is a protein, a protein transduction domain (PTD) may be linked to it for introduction into somatic cells.

[0094] When the nuclear reprogramming substance is a gene or its gene product, it may be an embodiment in which only the gene is used, or only the gene product, or both the gene and its gene product are used. When the nuclear reprogramming substance is a gene or its gene product and two or more types of "genes or their gene products" are used in combination, it may be an embodiment in which a gene product is used for one gene and a gene is used for another gene. The nuclear reprogramming substance is preferably one of the above-mentioned genes or a combination of genes, and more preferably in the form of an expression vector.

[0095] (somatic cells) The somatic cells are not particularly limited and can be appropriately selected depending on the purpose. Examples of somatic cells include fetal somatic cells and mature somatic cells. Specific examples of mature somatic cells include tissue stem cells (somatic stem cells) such as mesenchymal stem cells, hematopoietic stem cells, adipose tissue-derived stromal cells, adipose tissue-derived stromal stem cells, neural stem cells, and spermatogonial stem cells; tissue progenitor cells; and already differentiated cells such as fibroblasts, epithelial cells, lymphocytes, and muscle cells.

[0096] The biological species from which somatic cells are derived is not particularly limited and can be appropriately selected depending on the purpose. Examples of biological species from which somatic cells are derived include any mammals, such as humans, mice, rats, cattle, sheep, horses, and monkeys. The biological species from which somatic cells are derived and the biological species from which nuclear reprogramming substances are derived may not be the same, but are preferably the same. The biological species from which the H1foo gene or H1foo protein contained in the iPS cell quality improvement agent of the above embodiment is derived and the biological species from which somatic cells are derived may not be the same, but are preferably the same.

[0097] The individual from which the somatic cells are derived is not particularly limited and can be appropriately selected depending on the purpose. However, when iPS cells produced by the method of this embodiment are used for regenerative medicine, from the viewpoint of rejection, the individual to be treated with the regenerative medicine or another individual having the same or substantially the same MHC type as that of the individual to be treated with the regenerative medicine is preferred. Here, "substantially the same MHC type" means that when cells obtained by inducing differentiation from iPS cells derived from the somatic cells are transplanted into an individual, the MHC type matches to an extent that the transplanted cells can survive with the use of immunosuppressants, etc.

[0098] The somatic cells may be recombinant cells into which allogeneic genes have been introduced to facilitate the selection of iPS cells. Specific examples of recombinant cells include recombinant cells into which at least one of a reporter gene and a drug resistance gene has been integrated into the locus of a gene that is specifically and highly expressed in pluripotent cells. Examples of genes that are specifically and highly expressed in pluripotent cells include the Fbx15 gene, the Nanog gene, and the Oct3 / 4 gene. Examples of reporter genes include the green fluorescent protein (GFP) gene, the luciferase gene, and the β-galactosidase gene. Examples of the drug resistance gene include the blastocidin gene, the hygromycin gene, the puromycin resistance gene, and the neomycin resistance gene.

[0099] The culture conditions for somatic cells are not particularly limited and can be selected appropriately depending on the purpose. For example, the culture temperature may be about 37°C, the CO2 concentration about 2% to 5%, and the O2 concentration about 5% to 20%. The medium used for culturing somatic cells is not particularly limited and can be selected appropriately depending on the purpose. For example, minimum essential medium (MEM), Dulbecco's modified medium (DMEM), RPMI1640 medium, 199 medium, F12 medium, and the like containing 5% to 20% by mass of serum are included. In this specification, when a concentration is simply expressed as "% by volume," it means "% by volume."

[0100] (Introduction into somatic cells) The method for introducing the nuclear reprogramming substance and the iPS cell quality improvement agent of the above embodiment (hereinafter simply referred to as "quality improvement agent") into somatic cells is not particularly limited and can be selected appropriately depending on the purpose. Considering the ease of introduction into somatic cells, it is preferable to introduce the nuclear reprogramming substance into somatic cells in the form of a gene or mRNA of the gene rather than in the form of a protein, and it is even more preferable to introduce it into somatic cells in the form of a gene. In particular, it is preferable to introduce the nuclear reprogramming substance into somatic cells in the form of an expression vector. Similarly, it is preferable that the polynucleotide contained in the quality improvement agent is also in the form of an expression vector.

[0101] The method for introducing an expression vector into somatic cells is not particularly limited and can be appropriately selected depending on the purpose. Examples of methods for introducing an expression vector into somatic cells include lipofection, microinjection, DEAE-dextran, gene gun, electroporation, and calcium phosphate. When the expression vector is a viral vector, examples of methods for infecting somatic cells with the viral vector include the polybrene method.

[0102] When a Sendai virus vector is used as an expression vector, the Sendai virus vector carrying a nuclear reprogramming substance or a quality improving agent, or a nuclear reprogramming substance and a quality improving agent, is introduced into somatic cells by adding the vector (Sendai virus particles) to a medium containing somatic cells and infecting the somatic cells. The dose of the Sendai virus vector can be adjusted appropriately, but for example, the infection can be carried out at a multiplicity of infection (MOI) of 0.1 or more, preferably 0.3 or more, 0.5 or more, 1 or more, 2 or more, or 3 or more, and 100 or less, preferably 90 or less, 80 or less, 70 or less, 60 or less, 50 or less, 40 or less, 30 or less, 20 or less, 10 or less, or 5 or less. Infection is preferably carried out at an MOI of 0.3 to 100, more preferably 0.5 to 50, 1 to 40, 1 to 30, 2 to 30, or 3 to 30. When the Sendai virus vector is in the form of RNP, it can be introduced into cells by techniques such as electroporation, lipofection, and microinjection.

[0103] When the nuclear reprogramming substance is mRNA, the method for introducing mRNA (messenger RNA) into somatic cells is not particularly limited, and any known method can be appropriately selected and used. For example, mRNA can be introduced into somatic cells using a commercially available RNA transfection reagent such as Lipofectamine (registered trademark) MessengerMAX (manufactured by Life Technologies).

[0104] When the nuclear reprogramming substance is a protein, the method for introducing the protein into somatic cells is not particularly limited, and any known method can be appropriately selected and used. Examples of such methods include a method using a protein introduction reagent, a method using a protein introduction domain (PTD) fusion protein, and microinjection. When the quality improvement agent contains a fusion protein of an H1foo protein and a destabilization domain, it can also be introduced into somatic cells using any known method.

[0105] Commercially available protein delivery reagents include the cationic lipid-based BioPOTER® Protein Delivery Reagent (Gene Therapy Systems) and Pro-Ject™ Protein Transfection Reagent (PIERCE); the lipid-based Profect-1 (Targeting Systems); the membrane-permeable peptide-based Penetratin Peptide (Q Biogene) and Chariot Kit (Active Motif); and GenomONE (Ishihara Sangyo Kaisha), which utilizes the HVJ envelope (inactivated Sendai virus). Protein delivery can be performed according to the protocols provided with these reagents, but the general procedure is as follows: The protein is diluted in an appropriate solvent (e.g., a buffer solution such as PBS or HEPES), the delivery reagent is added, and the mixture is incubated at room temperature for approximately 5 to 15 minutes to form a complex. This complex is then added to cells in serum-free medium and incubated at 37°C for one to several hours. The medium is then removed and replaced with serum-containing medium.

[0106] Examples of PTDs include those listed in the section "Agent for improving iPS cell quality, <Second embodiment>, (Fusion protein)." When a PTD is used, the procedure can be the same as described above, except that no protein introduction reagent is added.

[0107] Microinjection is a method in which a protein solution is placed in a glass needle with a tip diameter of approximately 1 μm and then punctured into cells, ensuring reliable introduction of proteins into cells. Previously, methods have been developed to establish iPS cells in mice and humans by injecting nuclear reprogramming substances in the form of proteins together with CPPs (cell penetrating peptides) such as polyarginine or TAT, and these methods can also be used (Cell Stem Cell, 4:381-384 (2009)).

[0108] In the method for producing iPS cells of this embodiment, the nuclear reprogramming substance and the quality improving agent may be introduced into somatic cells once or twice or more times. The timing of introduction is not particularly limited and can be selected appropriately depending on the purpose. All of the nuclear reprogramming substances and the quality improving agents may be introduced at the same time, or some or all of them may be introduced at different times. It is preferable that all of the nuclear reprogramming substances and the quality improving agents are introduced into somatic cells at the same time. The number of introductions is preferably one.

[0109] When the nuclear reprogramming substance is a gene or its gene product, the amount of nuclear reprogramming substance introduced into somatic cells is not particularly limited as long as it can reprogram the nucleus of such somatic cells, and all genes or their gene products used may be introduced in equal amounts or in different amounts. An example of using genes as the genes or their gene products is a method of introducing the Oct3 / 4 gene in a larger amount (for example, about three times the amount) than the Sox2 gene, the Klf4 gene, or the c-Myc gene or the L-Myc gene (PNAS 106(31):12759-12764 (2009), J.Biol.Chem.287(43):36273-36282 (2012)).

[0110] When the nuclear reprogramming substance used in the iPS cell production method of this embodiment is a low-molecular-weight compound, the low-molecular-weight compound can be contacted with somatic cells by dissolving the low-molecular-weight compound at an appropriate concentration in an aqueous or non-aqueous solvent, adding the low-molecular-weight compound solution to a medium suitable for culturing somatic cells (e.g., minimum essential medium (MEM) containing approximately 5-20% fetal bovine serum, Dulbecco's modified Eagle's medium (DMEM), RPMI 1640 medium, 199 medium, F12 medium, etc.) at a concentration sufficient to induce nuclear reprogramming in somatic cells without causing cytotoxicity, and culturing the cells for a certain period of time. The concentration of the low-molecular-weight compound serving as a nuclear reprogramming substance varies depending on the type of low-molecular-weight compound used, but can be appropriately selected from the range of approximately 0.1 nM to approximately 100 nM. There are no particular limitations on the contact period as long as it is long enough to achieve nuclear reprogramming in the cells, but it is usually sufficient to allow the low-molecular-weight compound to coexist in the medium until positive colonies appear.

[0111] <Other processes> In addition to the introduction step, the method for producing iPS cells of this embodiment may further include other steps as necessary. There are no particular limitations on the other steps as long as they do not impair the effects of the present invention, and they can be selected appropriately depending on the purpose. For example, there may be mentioned a step of culturing somatic cells into which a nuclear reprogramming substance and a quality improving agent have been introduced (hereinafter also simply referred to as "introduced cells") (hereinafter also simply referred to as "introduced cell culture step").

[0112] (Transduced cell culture process) The transfected cell culture step is a step of culturing the transfected cells. The culture conditions for the transfected cells are not particularly limited, and commonly used stem cell culture conditions can be used. For example, the culture conditions can be those suitable for culturing ES cells. Examples of such conditions include a culture temperature of approximately 37°C, a CO2 concentration of approximately 2% to 5%, and an O2 concentration of approximately 5% to 20%. The medium used for culturing the transfected cells is not particularly limited, and can be appropriately selected depending on the purpose. Mouse cells are cultured in a standard medium supplemented with leukemia inhibitory factor (LIF) as a differentiation inhibitor. Human cells are cultured with LIF, basic fibroblast growth factor (bFGF), and / or stem cell factor (SCF) depending on the culture conditions. Typically, cells are cultured in the presence of mouse embryonic fibroblasts (MEFs), which have been treated with radiation or antibiotics to stop cell division, as feeder cells. STO cells and the like are commonly used as MEFs, but SNL cells (McMahon, AP & Bradley, A. Cell 62, 1073-1085 (1990)) and the like are often used to induce iPS cells. Co-culture with feeder cells may be initiated before, at the time of, or after (e.g., 1 to 10 days after) the introduction of the nuclear reprogramming substance and the quality improving agent.

[0113] The period of the above-mentioned introduced cell culture step is not particularly limited and can be appropriately selected depending on the purpose.

[0114] In the method for producing iPS cells of this embodiment, after the nuclear reprogramming substance and the quality improvement agent are introduced into somatic cells, the regulatory sequence contained in the quality improvement agent can control at least one of the timing and amount of H1foo protein present in the cells, thereby enabling appropriate control of the timing and amount of H1foo protein present, resulting in the production of high-quality iPS cells.

[0115] Second Embodiment In one embodiment, the present invention provides a method for producing iPS cells, comprising the step of introducing a nuclear reprogramming substance and an H1foo gene into somatic cells, wherein at least one of the timing and amount of the H1foo protein expressed from the H1foo gene introduced into the somatic cells is controlled within the somatic cells.

[0116] ≪Introduction process≫ The production method of this embodiment includes the step of introducing a nuclear reprogramming substance and an H1foo gene into somatic cells.

[0117] (nuclear reprogramming substance) The nuclear reprogramming material is the same as that explained above in "<First embodiment>." Preferred nuclear reprogramming materials are also the same as those listed above in "<First embodiment>."

[0118] (H1foo gene) The H1foo gene is the same as that described above in "[Agent for improving quality of iPS cells], <First embodiment> (H1foo gene)." Preferred H1foo genes include those similar to those listed above in "[Agent for improving quality of iPS cells], <First embodiment> (H1foo gene)."

[0119] In the method for producing iPS cells of this embodiment, the H1foo protein expressed from the H1foo gene introduced into somatic cells is regulated in at least one of its timing and amount within the somatic cells. The method for regulating the timing and amount of the H1foo protein within the somatic cells is not particularly limited, and known methods can be used.

[0120] A preferred method for controlling the timing and amount of H1foo protein is to use the control sequence listed in "Agent for Improving iPS Cell Quality, <First Embodiment> (Control Sequence)." For example, a method is exemplified in which a destabilization domain gene is linked to the H1foo gene, and when introduced into somatic cells, the H1foo protein is expressed as a fusion protein of the H1foo protein and the destabilization domain. In this case, since the fusion protein has a destabilization domain, it is rapidly degraded by the proteasome after its production within the cell. The time required for the fusion protein to be produced by transcription and translation from the fusion gene of the H1foo gene and the destabilization domain gene and then degraded by the proteasome after production is, for example, within 6 hours, preferably within 4 hours, more preferably within 3 hours, and particularly preferably within 2 hours. The timing and amount of the fusion protein present may be controlled by adding a stabilizing substance. For example, the fusion protein can be made to exist in the cells at a desired time by adding a stabilizing substance to the medium of the transfected cells, or by recovering the cells from the medium and transferring them to a medium that does not contain a stabilizing substance. Furthermore, the amount of the fusion protein present in the cells can be controlled by adjusting the amount of the stabilizing substance added.

[0121] Alternatively, a stimulus-responsive promoter may be linked upstream of the H1foo gene so that when introduced into somatic cells, the H1foo gene is expressed under the control of the stimulus-responsive promoter. In this case, the timing and amount of H1foo protein present can be controlled by providing the introduced cells with a "stimulus" to which the stimulus-responsive promoter responds. If the stimulus-responsive promoter is a tetracycline-responsive promoter, the timing and amount of expression of the H1foo gene can be controlled using tetracycline or its derivatives (e.g., doxycycline), thereby controlling the timing and amount of H1foo protein present in the introduced cells.

[0122] (Introduction into somatic cells) Methods for introducing a nuclear reprogramming substance and an H1foo gene into somatic cells include the same methods as those described above in "<First Embodiment>".

[0123] <Other processes> The method for producing iPS cells of this embodiment may further include other steps as necessary in addition to the above-described introduction step. The other steps are not particularly limited as long as they do not impair the effects of the present invention, and can be selected appropriately depending on the purpose. Examples of other steps include an introduction cell culture step, as in the above "<First Embodiment>". The introduction cell culture step can be carried out in the same manner as in the above "<First Embodiment>".

[0124] The method for producing iPS cells of this embodiment may include a step of controlling at least one of the amount and timing of H1foo protein in the introduced cells (hereinafter referred to as the "H1foo protein control step"). The amount and timing of H1foo protein can be controlled using a stabilizing substance, for example, when the H1foo protein is expressed as a fusion protein with a destabilization domain. Furthermore, for example, when the H1foo gene is expressed under the control of a stimulus-responsive promoter, the amount and timing of H1foo protein can be controlled by providing the introduced cells with a stimulus to which the responsive promoter responds.

[0125] When producing iPS cells, the amount and timing of H1foo protein in the introduced cells are controlled so that it is 50% or less, preferably 30% or less, and more preferably 20% or less of the maximum expression level by the time somatic cell reprogramming is completed. More specifically, the H1foo protein introduced into the cells is controlled so that 24 hours after its expression in the cells reaches its maximum, it is 50% or less, preferably 30% or less, and more preferably 20% or less. Alternatively, the duration of expression of the H1foo gene after introduction of a nuclear reprogramming substance into somatic cells may be within 24 hours after the introduction. Alternatively, the amount of H1foo protein in the cells into which the nuclear reprogramming substance has been introduced may be less than about 50% of the maximum amount of H1foo protein after the introduction, approximately 5 days after the introduction.

[0126] The method for producing iPS cells of this embodiment makes it possible to appropriately control at least one of the timing and amount of H1foo protein present in cells, thereby producing high-quality iPS cells.

[0127] [iPS cells] In one embodiment, the present invention provides iPS cells produced by the method for producing iPS cells according to the above embodiment.

[0128] iPS cells are pluripotent stem cells induced from somatic cells and possess pluripotency and self-renewal capabilities. Pluripotency refers to the ability to differentiate into all three germ layers. Self-renewal capabilities refer to the ability to proliferate while maintaining an undifferentiated state.

[0129] Whether or not the cells produced by the method for producing iPS cells according to the above embodiment are iPS cells can be confirmed by known methods, such as by the expression of undifferentiation markers, the ability to form embryoid bodies, and / or the ability to form chimeras.

[0130] The cells produced by the iPS cell production method of the above embodiment may be iPS cells comprising a nuclear reprogramming substance and the iPS cell quality improving agent of the above embodiment. The cells produced by the iPS cell production method of the above embodiment may be cells comprising a nuclear reprogramming substance, an H1foo gene, and a regulatory sequence. The cells produced by the iPS cell production method of the above embodiment may be cells comprising a nuclear reprogramming substance and a fusion gene of the H1foo gene and a regulatory sequence (e.g., a destabilization domain gene). The cells produced by the iPS cell production method of the above embodiment may be cells comprising a nuclear reprogramming substance and an H1foo gene operably linked to a stimulus-responsive promoter.

[0131] iPS cells produced by the production method of the above embodiment are of higher quality than iPS cells produced by conventional methods because the timing and amount of H1foo protein present in the introduced cells are appropriately controlled. In particular, the iPS cells of this embodiment form an improved number of colonies expressing Tra-1-60. They also have improved ability to generate naive iPS cells. Naive iPS cells can be confirmed by the dome-shaped colonies characteristic of naive iPS cells and the high expression of genes specific to naive iPS cells. They also have improved ability to differentiate into target cells (e.g., cardiomyocytes, primitive endoderm cells). Furthermore, the cell population of iPS cells produced by the production method of the above embodiment can be a homogeneous cell population with no variation in gene expression.

[0132] [Composition for iPS cell production] In one embodiment, the present invention provides a composition for producing iPS cells, which comprises a nuclear reprogramming substance and the iPS cell quality improving agent of the above embodiment.

[0133] The nuclear reprogramming substances are the same as those described above in "Method for producing iPS cells: <First embodiment>." Preferred nuclear reprogramming substances are also the same as those listed above in "<First embodiment>." The agent for improving the quality of iPS cells is the same as that described above in "[Agent for improving the quality of iPS cells]." Preferred agents for improving the quality of iPS cells are also the same as those listed above in "[Agent for improving the quality of iPS cells]."

[0134] In the composition for iPS cell production of this embodiment, the amounts of the nuclear reprogramming substances and quality improving agents are not particularly limited and may all be equal or different. For example, when the Oct3 / 4 gene, the Sox2 gene, the Klf4 gene, the c-Myc gene, or the L-Myc gene are used as nuclear reprogramming substances, the amount of the Oct3 / 4 gene may be greater than that of the Sox2 gene, the Klf4 gene, the c-Myc gene, or the L-Myc gene, for example, about three times as much.

[0135] The composition for iPS cell production of this embodiment may contain other components in addition to the nuclear reprogramming substance and the quality improving agent, including, but not limited to, a buffer, a gene transfer reagent, a protein transfer reagent, etc.

[0136] [Other embodiments] In another embodiment, the present invention provides a method for improving the quality of iPS cells, comprising the step of introducing the agent for improving iPS cell quality according to the above embodiment into somatic cells. In another embodiment, the present invention provides use of a fusion gene of the H1foo gene and a destabilization domain gene in the production of an agent for improving the quality of iPS cells. In another embodiment, the present invention provides use of a fusion protein of an H1foo protein and a destabilization domain in the manufacture of an agent for improving the quality of iPS cells. In another embodiment, the present invention provides use of an H1foo gene linked downstream of a stimulus-responsive promoter in the production of an agent for improving the quality of iPS cells. In another embodiment, the present invention provides use of a nuclear reprogramming substance and a fusion gene of the H1foo gene and a destabilization domain in the production of a composition for producing iPS cells. In another embodiment, the present invention provides use of a nuclear reprogramming substance and a fusion protein of an H1foo protein and a destabilization domain in the production of a composition for producing iPS cells. In another embodiment, the present invention provides use of a nuclear reprogramming substance and an H1foo gene linked downstream of a stimulus-responsive promoter in the production of a composition for producing iPS cells. In another embodiment, the present invention provides an iPS cell production kit comprising a nuclear reprogramming substance and an iPS cell quality improvement agent according to the above embodiment. The nuclear reprogramming substance and the quality improvement agent may be contained in separate containers, or may be contained in a single container, or any number of containers may be contained in the same container. The kit may also contain a gene transfer reagent, a protein transfer reagent, packaging cells, a buffer, a diluent, etc. [Example]

[0137] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. All experiments described below were performed in accordance with the guidelines for animal and DNA experiments of Keio University and Kyoto University, were approved by the ethics committees of Keio University and Kyoto University, and conformed to the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health.

[0138] [Example 1] Construction of a Sendai virus vector containing the H1FOO gene Sendai virus vector (SeV18; ID Pharma) and ProteoTuner TM Using the Shield System (Clontech), three Sendai virus vectors containing H1FOO cDNA (Teranishi, T., et al. Rapid replacement of somatic linker histones with the oocyte-specific linker histone H1foo in nuclear transfer. Developmental Biology 266, 76-86 (2004)) were constructed (Figure 1). (a) SeV18+H1FOO / TS15ΔF was analyzed using ProteoTuner TM This vector does not contain the destabilized domain (DD) of the Shield System. (b) SeV18+H1FOO-DD / TS15ΔF is a vector containing a sequence in which the DD coding sequence was added to the 3' end of the H1FOO cDNA. Vector (b) expresses a fusion protein in which DD was added to the C-terminus of H1FOO, promoting the degradation of H1FOO. (c) SeV18+DD-H1FOO / TS15ΔF is a vector containing a sequence in which the DD coding sequence was added to the 5' end of the H1FOO cDNA. Vector (c) expresses a fusion protein in which DD was added to the N-terminus of H1FOO, promoting the degradation of H1FOO.

[0139] The sequences in the above vectors (a) to (c) are shown in the following SEQ ID NOs. DD coding sequence: SEQ ID NO: 13 H1FOO-DD coding sequence: SEQ ID NO: 15 DD-H1FOO coding sequence: SEQ ID NO: 17 Table 1 shows the primer sequences used to prepare the above vectors (a) to (c).

[0140] [Table 1]

[0141] [Example 2] Western blotting of H1FOO protein Each of the Sendai virus vectors (a) to (c) containing the H1FOO gene was introduced into human skin fibroblasts (TIG120 strain, provided by the Tokyo Metropolitan Institute of Gerontology; Kondo et al., Exp Cell Res. 1995 Oct;220(2):501-4) at a multiplicity of infection (MOI) of 3. After culturing for 5 days, each cell was harvested and subjected to Western blotting using an anti-H1FOO antibody (HPA037992; Sigma-Aldrich). The results are shown in Figure 2. In Figure 2, (1) to (4) indicate the results for human dermal fibroblasts transfected with (1) a control containing no H1FOO, (2) H1FOO (vector (a)), (3) H1FOO-DD (vector (b)), and (4) DD-H1FOO (vector (c)). Five days later, the expression level of H1FOO protein was still significantly high in (2) H1FOO. On the other hand, the expression levels of H1FOO-DD fusion protein and DD-H1FOO fusion protein were reduced in (3) H1FOO-DD and (4) DD-H1FOO. (1) For the control not containing H1FOO, a Sendai virus vector containing the Azami-Green gene instead of the H1FOO gene in the vector (a) above was used.

[0142] [Example 3] Preparation of primed human iPS cells and culture conditions Human iPS cells were generated according to the protocol described in the literature (Ban, H., Nishishita, N., Fusaki, N., Tabata, T., Saeki, K., Shikamura, M., Takada, N., Inoue, M., Hasegawa, M., Kawamata, S., Nishikawa, S., Efficient generation of transgene-free human induced pluripotent stem cells (iPSCs) by temperature-sensitive Sendai virus vectors, Proc Natl Acad Sci USA 108, 14234-9, 2011; Seki, T., Yuasa, S., Fukuda, K., Generation of induced pluripotent stem cells from a small amount of human peripheral blood using a combination of activated T cells and Sendai virus, Nature Protocols 7, 718-728, 2012). Primed iPS cells were generated from human skin fibroblasts (TIG120 strain) and peripheral blood mononuclear cells (CTL-UP1, Lot: HHU20140120, Cellular Technology Ltd.) using either one of the Sendai virus vectors (a) to (c) prepared in Example 1, together with a Sendai virus vector containing the Oct3 / 4, Sox2, Klf4, and L-Myc genes (infected at a multiplicity of infection of 0.1 to 0.3). As a control, iPS cells (Control-iPS) were generated using a Sendai virus vector containing the Azami-Green gene instead of the H1FOO gene in the vector (a). This experiment was conducted in accordance with the guidelines for genetic recombination experiments of Keio University and Kyoto University.

[0143] The human iPS cells prepared as described above were cultured and maintained in StemFit AK02N (Ajinomoto Co.) medium (hereafter referred to as "primed human iPS cell culture medium") on culture dishes coated with iMatrix-511 (Nippon Pharmaceuticals). The culture medium for primed human iPS cells was changed every two days, and the cells were passaged every 5 to 7 days using StemPro Accutase (Gibco).

[0144] [Example 4] Preparation of naive human iPS cells and culture conditions Human iPS cells were generated according to the protocol described in the literature (Ban, H. et al., Proc Natl Acad Sci USA 108, 14234-9, 2011; Seki, T. et al., Nature Protocols 7, 718-728, 2012; Liu, X. et al., Nature Methods 14, 1055-1062, 2017). Naive iPS cells were generated from human skin fibroblasts and human peripheral blood mononuclear cells using a Sendai virus vector containing the Oct3 / 4, Sox2, Klf4, and L-Myc genes, as well as any of the Sendai virus vectors (a) to (c) prepared in Example 1 (multiplicity of infection: 0.1 to 0.3). The identity of the iPS cells was confirmed by the colony's dome-shaped naive-type shape and high expression of genes specific to the naive-type (KLF17, TFCP2L1, PRDM14, and DPPA3 genes, confirmed by quantitative PCR and immunostaining). As a control, iPS cells (Control-iPS) were generated using a Sendai virus vector containing the Azami-Green gene instead of the H1FOO gene in the vector (a) above. This experiment was performed in accordance with the guidelines for genetic recombination experiments of Keio University and Kyoto University.

[0145] The human iPS cells generated as described above were cultured and maintained on culture dishes seeded with irradiated mouse fetal fibroblasts in a culture medium (hereafter referred to as "naive human iPS cell culture medium") containing NDiff227 (Takara Bio) supplemented with CHIR99021 (Sigma-Aldrich), PD0325901 (Sigma-Aldrich), Go6983 (Sigma-Aldrich), and human recombinant leukemia inhibitory factor (HLEF) (Fujifilm Wako Pure Chemical Industries, Ltd.). The culture medium for naive human iPS cells was changed every two days, and the cells were passaged every three to four days using StemPro Accutase (Gibco). Culture and passage were performed in a hypoxic environment with an oxygen concentration of 5% in an oxygen-controlled incubator.

[0146] [Example 5] Comparison of iPS cell establishment efficiency by counting the number of ALP-positive iPS cell colonies Primed human dermal fibroblasts (iPSCs) were generated from human dermal fibroblasts using the method described in Example 3. (1) Control-iPSCs, (2) H1FOO-iPSCs, (3) H1FOO-DD-iPSCs, and (4) DD-H1FOO-iPSCs were then cultured in primed human iPS cell culture medium for 15 days, and the number of ALP-positive colonies was counted. ALP is a known stem cell marker.

[0147] The results are shown in Figure 3. (3) H1FOO-DD-iPS and (4) DD-H1FOO-iPS showed a significant increase in the number of ALP-positive cell colonies compared to (1) Control-iPS.

[0148] To verify the effect of the original cell type, primed (1) Control-iPS and (3) H1FOO-DD-iPS cells were generated from human peripheral blood mononuclear cells (PBMCs) and cultured in primed human iPS cell culture medium for 15 days, and the number of ALP-positive colonies was counted. Furthermore, to compare the efficiency of naive iPS cell establishment, naive (1) Control-iPS and (3) H1FOO-DD-iPS cells were generated from human skin fibroblasts and human peripheral blood mononuclear cells, respectively, according to the method described in Example 4 above. They were cultured in naive human iPS cell culture medium for 15 days, and the number of ALP-positive colonies was counted.

[0149] The results are shown in Figures 4A to 4C. Figure 4A (primed human iPS cells established from human peripheral blood mononuclear cells), Figure 4B (naive human iPS cells established from human skin fibroblasts), and Figure 4C (naive human iPS cells established from human peripheral blood mononuclear cells) all showed that (3) H1FOO-DD-iPS cells significantly increased the number of ALP-positive cell colonies compared to (1) Control-iPS cells, similar to the results in Figure 3. These results demonstrate that H1FOO-DD improves the efficiency of human iPS cell establishment, regardless of the original cell type or the type of iPS cells established.

[0150] [Example 6] Comparison of variation in gene expression levels Primed human dermal fibroblasts (1) Control-iPSCs and (3) H1FOO-DD-iPSCs were generated from human dermal fibroblasts according to the method described in Example 3 above. Primed human iPSCs were cultured in a primed human iPS cell culture medium for approximately 210 days (28 passages), and eight clones from each of (1) and (3) were selected and subjected to RNA-seq. Based on the results of RNA-seq data analysis, the number of genes with a mean absolute error (MAE) of gene expression levels greater than 2.0 (large variability) was determined for the eight clones from (1). Using a similar method, the number of genes with an MAE greater than 2.0 was also determined for the eight clones from (3). The results are shown in Figure 5A.

[0151] Naive (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human dermal fibroblasts according to the method described in Example 4 above. Naive iPS cells were cultured in naive human iPS cell culture medium for approximately 190 days (45 passages), and eight clones for each of (1) and (3) were selected and subjected to RNA-seq. Using the same method as for the primed human iPS cells described above, the number of genes with an MAE greater than 2.0 was determined for each of the eight clones for (1) and (3). The results are shown in Figure 5B.

[0152] In both primed and naive human iPS cells, (3) H1FOO-DD-iPS cells suppressed the number of genes with large inter-clonal variability in expression levels by approximately half compared to (1) Control-iPS cells. These results indicate that global gene expression is more uniform among clones in H1FOO-DD-iPS cells.

[0153] [Example 7] Comparison of DNA methylation variation DNA methylation was analyzed using a DNA methylation array (Infinium Human Methylation 450K BeadChip Kit, Illumina) for the eight clones of primed human iPS cells ((1) Control-iPS) and (3) H1FOO-DD-iPS) selected in Example 6 above. Based on the results of the DNA methylation array analysis, the number of methylated probes with a mean absolute error (MAE) greater than 2.0 (large variability) was determined for the eight clones ((1)). Using a similar method, the number of genes with an MAE greater than 2.0 was also determined for the eight clones ((3)). The results are shown in Figure 6A.

[0154] DNA methylation was analyzed using a DNA methylation array for the eight clones of naive human iPS cells ((1) Control-iPS) and eight clones of (3) H1FOO-DD-iPS selected in Example 6 above. The number of methylated probes with MAE greater than 2.0 was determined for each of the eight clones ((1) and (3)) using the same method as for the primed human iPS cells. The results are shown in Figure 6B.

[0155] In both primed and naive human iPS cells, (3) H1FOO-DD-iPS cells showed approximately half the number of methylated probes, which had a large inter-clonal variability, compared with (1) Control-iPS cells. These results indicate that DNA methylation is more uniform among clones in H1FOO-DD-iPS cells.

[0156] [Example 8] Comparison of cardiomyocyte differentiation potential Primed (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human peripheral blood mononuclear cells according to the method described in Example 3 above. Three clones from each of (1) and (3) were selected and cultured in a cardiac differentiation-inducing medium to induce differentiation into cardiomyocytes (Tohyama, S., et al. (2013). Cell Stem Cell 12(1): 127-137.). Cells were harvested on day 10 after the start of culture in the cardiac differentiation-inducing medium. TNNT2 expression in the harvested cells was detected using a flow cytometer, and the percentage of TNN2-positive cells was calculated. TNNT2 is a representative cardiomyocyte marker. Each clone was subjected to six experiments to induce differentiation into cardiomyocytes.

[0157] Figure 7A shows an example of measurement of the percentage (%) of TNNT2-positive cells using a flow cytometer. The left figure shows an example of measurement for (1) Control-iPS, and the right figure shows an example of measurement for (3) H1FOO-DD-iPS. In (1) Control-iPS, only approximately 6% of all cells were TNNT2-positive, indicating poor differentiation into cardiomyocytes. In (3) H1FOO-DD-iPS, approximately 95% of all cells were strongly TNNT2-positive, indicating good cardiomyocyte differentiation potential.

[0158] Figure 7B shows the percentage (%) of TNNT2-positive cells in each clone. (3) H1FOO-DD-iPS cells had a higher percentage of TNNT2-positive cells than (1) Control-iPS cells. Furthermore, (3) H1FOO-DD-iPS cells showed less variability between cardiomyocyte differentiation experiments than (1) Control-iPS cells, confirming that cardiomyocyte differentiation can be induced more stably. These results indicate that H1FOO-DD-iPS cells have superior cardiomyocyte differentiation potential and can differentiate into cardiomyocytes with greater uniformity.

[0159] [Example 9] Comparison of endoderm differentiation potential Primed (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human dermal fibroblasts according to the method described in Example 3 above. Eight clones each for (1) and (3) were selected and induced to differentiate into three germ layer components using the STEMdiff Trilineage Differentiation Kit (Cat. ST-05230, VERITAS). Cells were harvested on day 5 after the start of differentiation induction. The harvested cells were subjected to semi-comprehensive qPCR analysis to evaluate the expression of endoderm-related markers using the TaqMan hPSC Scorecard Kit (Cat. A15876, ThermoFisher Scientific).

[0160] The results are shown in Figure 8. (3) H1FOO-DD-iPS cells had a higher algorithmic score than (1) Control-iPS cells. Furthermore, (3) H1FOO-DD-iPS cells showed less variation between clones than (1) Control-iPS cells, confirming that they could induce endoderm differentiation more stably. These results indicate that H1FOO-DD-iPS cells have excellent endoderm differentiation potential and can differentiate into endoderm with a higher degree of uniformity.

[0161] [Example 10] Comparison of hepatocyte differentiation potential (1) Primed (1) Control-iPSCs and (3) H1FOO-DD-iPSCs were generated from human peripheral blood mononuclear cells (PBMCs) according to the method described in Example 3 above. Ten clones each from (1) and (3) were selected and induced to differentiate into hepatocytes (a type of terminally differentiated endodermal cell) (Kajiwara, M., et al. (2012). Proc Natl Acad Sci USA 109(31): 12538-12543.; Takebe, T., et al. (2017). Cell Rep 21(10): 2661-2670.). Cells were harvested 21 days after the start of differentiation induction. Expression of ASGR1, a representative mature hepatocyte marker, was quantitatively assessed in the harvested cells by qRT-PCR.

[0162] The results are shown in Figure 9. Compared with (1) Control-iPS, (3) H1FOO-DD-iPS showed higher overall AGSR1 expression levels, including higher mean and average values. Furthermore, (3) H1FOO-DD-iPS showed smaller variability (STDVP, CV) among clones compared with (1) Control-iPS, confirming that hepatocyte differentiation can be induced more stably. These results indicate that H1FOO-DD-iPS have superior hepatocyte differentiation potential and can differentiate into hepatocytes with greater uniformity.

[0163] [Example 11] Comparison of hepatocyte differentiation potential (2) Primed (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human peripheral blood mononuclear cells according to the method described in Example 3 above. Ten clones each of (1) and (3) were selected and induced to differentiate into hepatocytes as in Example 11. Culture supernatants were collected 21 days after the start of differentiation induction. The amount of secreted albumin in the collected culture supernatants was quantitatively assessed by ELISA. Because mature hepatocytes produce and secrete albumin, albumin, like ASGR1, is used as a marker for mature hepatocytes.

[0164] The results are shown in Figure 10. Compared with (1) Control-iPS, (3) H1FOO-DD-iPS showed higher albumin secretion levels overall, with a higher median. Furthermore, compared with (1) Control-iPS, (3) H1FOO-DD-iPS showed smaller variability (STDVP, CV) among clones, confirming that hepatocyte differentiation can be induced more stably. These results indicate that H1FOO-DD-iPS have superior hepatocyte differentiation potential and can differentiate into hepatocytes with greater uniformity.

[0165] [Example 12] Comparison of primitive endoderm differentiation potential Naive (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human dermal fibroblasts and human peripheral blood mononuclear cells (PBMCs) according to the method described in Example 4 above. Six clones (four clones derived from human dermal fibroblasts and two clones derived from human peripheral blood mononuclear cells) were selected for each of (1) and (3) and cultured in a primitive endoderm differentiation-inducing medium to induce differentiation into primitive endoderm (WO 2019 / 093340). Cells were harvested on day 3 after the start of culture in the primitive endoderm differentiation-inducing medium. PDGFRA and ANPEP expression in the harvested cells was detected using a flow cytometer, and the percentages of PDGFRA-positive and ANPEP-positive cells were calculated. PDGFRA and ANPEP are representative primitive endoderm markers. Each clone was subjected to a primitive endoderm differentiation induction test three times. Primitive endoderm differentiation medium was prepared by adding 25 ng / mL FGF4 (Peprotech), 1 μg / mL Heparin (Wako), 10 ng / mL BMP4 (R&D), 10 ng / mL PDGFRA (Peprotech), 1 μM XAV939 (Wako), 3 μM A83-01 (Wako), and 0.1 μM Retinoic Acid (Sigma) to N2B27 medium (NDiff227). Two days after the start of differentiation induction, 10 ng / mL IL-6 (R&D) was added.

[0166] Figure 11A shows an example of flow cytometric measurement of the percentage (%) of PDGFRA- and ANPEP-positive cells. The left panel shows an example of measurement of (1) Control-iPS cells. The percentage of cells positive for both PDGFRA and ANPEP was approximately 19% of the total cells. The figure on the right shows an example of the measurement of (3) H1FOO-DD-iPS. The percentage of cells positive for both PDGFRA and ANPEP was approximately 37% of the total cells.

[0167] Figure 11B shows the percentage (%) of cells positive for both PDGFRA and ANPEP in each clone. (3) H1FOO-DD-iPS cells had a higher percentage of positive cells than (1) Control-iPS cells. Furthermore, (3) H1FOO-DD-iPS cells showed less variability among clones than (1) Control-iPS cells, confirming that they could induce primitive endoderm differentiation more stably. These results indicate that H1FOO-DD-iPS cells have excellent primitive endoderm differentiation potential and can differentiate into primitive endoderm with higher uniformity.

[0168] [Example 13] Verification of metabolic function Naive (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human dermal fibroblasts and human peripheral blood mononuclear cells according to the method described in Example 4. Six clones were selected for each of (1) and (3), and spare respiratory capacity (evaluation of electron transport chain function) was measured using an Agilent Seahorse XF96 Extracellular Flux Analyzer (Prime Tech Co., Ltd.).

[0169] The results are shown in Figure 12A. (3) H1FOO-DD-iPS cells had a higher spare respiratory capacity than (1) Control-iPS cells.

[0170] Naive (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human dermal fibroblasts or human peripheral blood mononuclear cells according to the method described in Example 4. Six clones were selected for each of (1) and (3), and the oxygen consumption rate (OCR, a functional evaluation of the electron transport chain) and extracellular acidification rate (ECAR, a functional evaluation of the glycolysis pathway) were measured using an Agilent Seahorse XF96 Extracellular Flux Analyzer (Prime Tech Co., Ltd.).

[0171] The results are shown in Figure 12B. The X-axis indicates ECAR, which reflects anaerobic metabolic capacity. The Y-axis indicates OCR, which reflects aerobic metabolic capacity. Each plot shows the average of six measurements for one clone. In (3) H1FOO-DD-iPS cells, the plots were generally located in the upper right corner compared to (1) Control-iPS cells, and both ECAR and OCR were higher. These results indicate that H1FOO-DD-iPS cells have a more naive metabolic capacity.

[0172] [Example 14] Verification of X chromosome activation In female-derived somatic cells and primed iPS / ES cells, one X chromosome is generally inactivated. On the other hand, in naive iPS cells, especially in preimplantation epiblasts, both X chromosomes are known to be activated. To verify this, we performed mRNA-FISH on four naive iPS clones, (1) Control-iPS and (3) H1FOO-DD-iPS, and compared the expression of three markers.

[0173] Naive (1) Control-iPS cells and (3) H1FOO-DD-iPS cells were generated from human dermal fibroblasts and human peripheral blood mononuclear cells (PBMCs) according to the method described in Example 4. Four clones were selected from each of (1) and (3), and the expression of UTX, HUWE1, and XIST was confirmed by mRNA-FISH (Sahakyan, A., et al. (2017). Cell Stem Cell 20(1): 87-101). Currently available naive human iPS cells generate a large number of polyploid cells, so for accurate evaluation, it is necessary to select cells with a normal chromosome number. UTX is expressed regardless of whether X chromosome activation is present or not. Therefore, cells expressing two copies of UTX were selected as normal cells. Next, the expression of HUWE1 (expressed when X chromosome is active) and XIST (originally involved in X chromosome inactivation, but known to be expressed on both X chromosomes in the preimplantation epiblast) was examined using mRNA-FISH.

[0174] Figure 13A shows examples of fluorescent microscopy images of mRNA-FISH. The left image shows an example of HUWEI+ / +XIST+ / +, and the right image shows an example of HUWEI+ / -XIST- / -.

[0175] Figure 13B shows the HUWE1 and XIST expression patterns for 100 cells from each clone. The numbers below the bars indicate the clone numbers for (1) Control-iPS and (3) H1FOO-DD-iPS. Clones 1 and 2 were derived from skin fibroblasts, and clones 5 and 6 were derived from human peripheral blood mononuclear cells. The phenotype closest to that of the preimplantation epiblast was HUWE1+ / + and XIST+ / +, and the corresponding phenotype was HUWE1+ / + and XIST+ / -. (1) In the Control-iPS cells, some clones (clones 1 and 2) contained a majority of cells with a phenotype close to that of the primed type. On the other hand, (3) in all clones of H1FOO-DD-iPS cells, cells with a naive phenotype dominated. These results indicate that H1FOO-DD-iPS cells have a phenotype closer to that of the naive type.

[0176] [Example 15] Comparison of FKBP1A expression levels H1FOO-DD was introduced into human fibroblasts (TIG120) using the Sendai virus vector (c) prepared in Example 1 (H1FOO OE HDF). Oct3 / 4, Sox2, Klf4, and L-Myc genes were introduced into human fibroblasts (TIG120) using the Sendai virus vector, and iPS cells were generated (OSKL). H1FOO-DD-iPS cells were generated by introducing the Oct3 / 4, Sox2, Klf4, and L-Myc genes into human fibroblasts (TIG120) together with the Sendai virus vector (c) prepared in Example 1 above, and introducing these genes into human fibroblasts (TIG120).

[0177] The H1FOO OE HDFs, OSKL, and OSKLH prepared above were cultured, and the cells were harvested two days after transfection of each gene. The expression levels of FKBP1A were then measured by qRT-PCR. As controls, the expression levels of FKBP1A were also measured in human fibroblasts (HDFs) and H9 ES cells (H9 ESCs). The expression levels of FKBP1A were normalized by the expression levels of GAPDH.

[0178] The results are shown in Figure 14. In the figure, HDF refers to human dermal fibroblasts, H9 ESC refers to H9 ES cells, H1FOO OE HDF refers to H1FOO OE HDF cells cultured for 2 days after production, OSKL day2 refers to OSKL cells cultured for 2 days after production, and OSKLH day2 refers to OSKLH cells cultured for 2 days after production. The expression level of FKBP1A is shown as a relative expression level, with the expression level of OSKLH day2 set to 1.0. As shown in Figure 14, FKBP1A expression was significantly higher only in OSKLH day 2 cells. In OSKLH day 2 cells, FKBP1A expression was approximately 10-fold higher compared to other cells. This result indicates that the introduction of H1FOO-DD together with nuclear reprogramming substances increases FKBP1A expression in cells at the early stage of nuclear reprogramming induction. Furthermore, although the results are omitted here, FKBP1A was confirmed to suppress the expression of innate immune response markers, IFIT1 and IFNA. Therefore, it is speculated that the introduction of H1FOO-DD together with nuclear reprogramming substances suppresses the expression of innate immune response markers. These results suggest that suppressing the expression of innate immune response markers may contribute to improving the quality of iPS cells. [Industrial Applicability]

[0179] According to the present invention, there are provided an agent for improving iPS cell quality, which enables the production of high-quality iPS cells, a method for producing iPS cells, iPS cells produced by such a production method, and a composition for producing iPS cells.

[0180] While preferred embodiments of the present invention have been described and illustrated, it should be understood that these are exemplary of the present invention and should not be considered limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the present invention is not to be deemed limited by the foregoing description, but is limited only by the scope of the appended claims.

Claims

1. The H1foo gene, a control sequence that can control at least one of the amount and timing of the H1foo protein expressed from the H1foo gene in cultured cells when the H1foo gene is introduced into the cultured cells; a polynucleotide having the formula: the control sequence comprises a nucleotide sequence encoding a destabilization domain, the destabilization domain being a domain that promotes proteasomal degradation of a fusion protein comprising the destabilization domain; the control sequence is linked to the H1foo gene so as to express a fusion protein of the destabilization domain and the H1foo protein; The destabilization domain has at least one property selected from the group consisting of: (a) when a fusion gene in which a gene for the destabilization domain is linked to the H1foo gene so as to be able to express the fusion protein of the destabilization domain and the H1foo protein is introduced into the cultured cells, the amount of the H1foo protein present in the cells 5 days after the introduction is less than 50% of the maximum amount of the H1foo protein present after the introduction; and (b) when a fusion gene in which a gene for the destabilization domain is linked to the H1foo gene so as to express the fusion protein of the destabilization domain and the H1foo protein, and a nuclear reprogramming substance, are introduced into the cultured cells, the ability to generate primed iPS cells that express alkaline phosphatase is improved compared to the cultured cells introduced with the H1foo gene and the nuclear reprogramming substance. An agent for improving the quality of iPS cells.

2. The agent for improving iPS cell quality according to claim 1 , wherein the destabilizing domain is selected from the group consisting of a destabilizing domain derived from FKBP12, a destabilizing domain derived from ecDHFR, and a Tegron sequence.

3. The agent for improving iPS cell quality according to claim 1 , wherein the destabilization domain derived from FKBP12 has the amino acid sequence set forth in SEQ ID NO:

14.

4. The iPS cell quality improvement agent described in claim 1, wherein the polynucleotide is inserted into an expression vector in a state in which a gene encoding a fusion protein of the destabilization domain and H1foo protein can be expressed in the cultured cells into which it is introduced.

5. The agent for improving iPS cell quality according to claim 4 , wherein the expression vector is a Sendai virus vector.

6. The iPS cell quality improving agent according to any one of claims 1 to 5, wherein the control sequence comprises a promoter sequence that controls transcription of the H1foo gene in response to a chemical stimulus.

7. Nuclear reprogramming substances, The agent for improving iPS cell quality according to any one of claims 1 to 6, A method for producing iPS cells, comprising the step of introducing the above into cultured somatic cells.

8. Before the introducing step, (a) selecting a destabilization domain that, when a fusion gene in which a gene for the destabilization domain is linked to the H1foo gene so as to be able to express the fusion protein of the destabilization domain and the H1foo protein is introduced into the cultured cells, reduces the amount of the H1foo protein present in the cells 5 days after the introduction to less than 50% of the maximum amount of the H1foo protein present after the introduction; or (b) selecting a destabilization domain that, when a fusion gene in which a gene for the destabilization domain is linked to the H1foo gene so as to express the fusion protein of the destabilization domain and the H1foo protein and a nuclear reprogramming substance are introduced into the cultured cells, improves the ability to generate primed iPS cells that express alkaline phosphatase, compared to the cultured cells introduced with the H1foo gene and a nuclear reprogramming substance; The method for producing iPS cells according to claim 7, further comprising:

9. The method for producing iPS cells according to claim 7 or 8, wherein the iPS cells are primed or naive iPS cells.

10. The nuclear reprogramming substance comprises at least one selected from the group consisting of a gene of the Oct gene family, a gene of the Sox gene family, a gene of the Klf gene family, a gene of the Myc gene family, a gene of the Lin gene family, and a Nanog gene, and gene products thereof. The method for producing iPS cells according to any one of claims 7 to 9.

11. The nuclear reprogramming substance is an Oct3 / 4 gene, a Sox2 gene, a Klf4 gene, L-Myc or c-Myc, or a gene product thereof. The method for producing iPS cells according to any one of claims 7 to 10.

12. the nuclear reprogramming substance is at least one gene selected from the group consisting of an Oct gene family gene, a Sox gene family gene, a Klf gene family gene, a Myc gene family gene, a Lin gene family gene, and a Nanog gene; the at least one gene is inserted into an expression vector in a state in which the at least one gene can be expressed in a cell into which the at least one gene is introduced; The method for producing iPS cells according to any one of claims 7 to 10.

13. The method for producing iPS cells according to claim 12 , wherein the expression vector is a Sendai virus vector.

14. Nuclear reprogramming substances, The agent for improving iPS cell quality according to any one of claims 1 to 6, A composition for producing iPS cells comprising:

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