Multicistronic Vector IRES Non-ATG Marker Selection
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Solution Overview
Problem
The unpredictable nature of protein expression in host cells due to gene silencing issues makes it challenging to select for high-level expression of recombinant proteins, requiring extensive testing of many host cell clones.
Innovation Solution
A DNA molecule with a multicistronic transcription unit is used, featuring a polypeptide of interest and a selectable marker polypeptide with a non-ATG start codon, an internal ribosome entry site (IRES), and chromatin control elements, allowing for selective expression of high levels of the protein of interest by impairing translation of the selectable marker.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional selection methods are used to identify host cells expressing high levels of recombinant protein, then the probability of finding high-expression clones increases, but the time and resources required for testing many clones increases significantly
Solution Approach 1:
The patent applies preliminary action by designing the expression vector with an impaired selectable marker translation initiation sequence before host cell transformation. This pre-configured impairment ensures that only clones with high-level transgene expression will produce sufficient selectable marker protein for survival, thereby filtering out low-expression clones before they consume valuable culture resources and time.
Solution Approach 2:
The patent changes the translation efficiency parameter of the selectable marker by using non-optimal start codons (GTG, TTG, CTG, ATT, or ACG) or impaired Kozak sequences. This parameter modification creates a threshold effect where only cells with high transgene expression levels can overcome the impaired translation and survive selection, thereby accelerating the identification of high-expression clones.
2Manufacturing precision
If the selectable marker translation is impaired to increase selection stringency, then the probability of selecting high-expression clones increases, but the risk of false negatives (missing viable clones) increases
Solution Approach 1:
The patent applies partial impairment rather than complete abolition of selectable marker translation. By using non-optimal start codons or impaired Kozak sequences instead of deleting the marker entirely, the system maintains a gradient of selection pressure that allows viable high-expression clones to survive while still filtering out low-expression clones, thereby avoiding false negatives.
Solution Approach 2:
The impaired translation initiation sequence creates a feedback mechanism where the level of selectable marker expression directly reflects the level of transgene expression. Cells that successfully express high levels of the transgene will also produce sufficient selectable marker protein to overcome the impairment and survive, providing real-time feedback on expression levels during the selection process.
3Reliability
If multiple clones are tested to overcome gene silencing unpredictability, then the likelihood of finding stable high-expression clones increases, but the cost and complexity of the selection process increases
Solution Approach 1:
The patent applies preliminary action by pre-configuring the expression vector with sequence elements (non-optimal start codons, impaired Kozak sequences, IRES elements) that are designed to minimize gene silencing effects and ensure stable expression. This pre-optimization reduces the need to test numerous clones, thereby simplifying the overall selection process while maintaining reliability.
Solution Approach 2:
The patent combines multiple functional elements (coding sequence for protein of interest, impaired selectable marker, IRES element, polyadenylation signal) into a single bicistronic expression unit. This merging ensures coordinated expression of both the transgene and selectable marker from a single transcriptional event, reducing variability and simplifying the selection process while improving expression stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly increases the chances of obtaining host cells expressing high levels of the protein of interest, providing a more efficient and stringent selection system with stable expression and reduced gene silencing.
Implementation Method 1
an internal ribosome entry site (IRES) is present downstream from the coding sequence for the polypeptide of interest and upstream from the coding sequence for the selectable marker polypeptide
Implementation Method 2
the nucleic acid sequence coding for the selectable marker polypeptide in the coding strand comprises a translation start sequence chosen from the group consisting of: a) a GTG start codon; b) a TTG start codon; c) a CTG start codon; d) a ATT start codon; and e) a ACG start codon
Data Source
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AI summary
The invention provides a DNA molecule comprising a multicistronic transcription unit coding for i) a polypeptide of interest, and for ii) a selectable marker polypeptide functional in a eukaryotic host cell, wherein the polypeptide of interest has a translation initiation sequence separate from that of the selectable marker polypeptide, and wherein the coding sequence for the polypeptide of interest is upstream from the coding sequence for the selectable marker polypeptide in said multicistronic transcription unit, and wherein an internal ribosome entry site (IRES) is present downstream from the coding sequence for the polypeptide of interest and upstream from the coding sequence for the selectable marker polypeptide, and wherein the nucleic acid sequence coding for the selectable marker polypeptide in the coding strand comprises a GTG or a TTG startcodon. The invention also provides methods for obtaining host cells expressing a polypeptide of interest, said host cells comprising the DNA molecules of the invention. The invention further provides the production of polypeptides of interest, comprising culturing host cells comprising the DNA molecules according to the invention.