When experimenting on mice, gene knockdown seems to be a useful technique to deactivate individual genes to study what they are doing. In practice what's the maximum of genes that can be knocked out at the same time?
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2$\begingroup$ I think this question needs clarifications / constraints. Assuming we're talking about mouse knockouts, are you interested in the technical limitations of creating knockouts, or are you asking about the biological limitations of KO mice (i.e. a minimal mouse genome)? Must that mouse live a full mouse life? Should that mouse be able to make more mice? $\endgroup$– acvillJun 7, 2021 at 13:31
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$\begingroup$ My interpretation was that he is asking about transient reduction of expression (e.g. antisense oligos) rather than genomic knockouts. $\endgroup$– ArmandJun 7, 2021 at 21:13
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$\begingroup$ My confusion is the discrepancy between the title (knock down) and the text of the question (knock out), though I think my initial comment stands in either case. $\endgroup$– acvillJun 8, 2021 at 1:20
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1 Answer
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As the OP asked about mice, I am mostly going to talk about organism-level things here.
First of all it is important to distinguish between knock-down and knock-out. Knock-out is where a gene is either removed or made non-functional and as such will never be able to be turned on again. This may in some cases be compensated for by having heterozygous animals, where one allele contains the normal gene and the other allele the non-functional, so that normal expression from the normal allele is enough to produce a normal animal. The heterozygous animals are then mated to produce homozygous (both alleles removed) knock-out progeny.
Knock-down on the other hand is either a transient reduction in expression (mostly done in cells, I don't think there are any methods for this that are viable at the whole mouse sort of level) or a conditional knock-out where expression of the gene is made conditional on a treatment, whereby you remove the treatment and the gene stops expressing. For example you can make a transgenic mouse with a tetracycline dependent promoter for expression (or not) of the transgene (Tet-On/Tet-Off). Knock-down in the strictest sense is only a reduction in the level of expression and may not be complete removal of the expression, which may result in viable cells through residual expression where a knock-out, with complete removal, might not.
However, back to the question at hand:
The answer to this is a rather unsurprising it depends on the genes being knocked down
Knockdown a single gene essential for cellular function such as something in a major metabolic pathway, and cells just won't function, so cells and animals will be non-viable. Knock-out or knock-down of non-essential genes and animals may survive, but also may not. As far as I know at this point in time we have no way of knowing whether knock-down or knock-out of something will result in viable progeny other than experimentation.
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$\begingroup$ Why the downvote? - please let me know how I can improve my answer. $\endgroup$– bob1Jun 8, 2021 at 8:59