The laboratory mouse (Mus musculus) has emerged as a vital research tool in life science laboratories around the world, contributing to key findings in research for cancer, cardiovascular disease, metabolic disorders, and aging. The mouse’s utility as a model of biological function and disease can be attributed to several factors. First, despite vast phenotypic disparities, the mouse shares a remarkable similarity with humans at a genetic level, possessing roughly 85% homology on average in coding regions. Second, the mouse’s quick estrus cycle, short gestational duration, considerable litter size, and rapid post-partum maturation makes it logistically feasible to generate a large colony of genetically similar (if not identical) experimental specimens within a limited timespan. Finally, the mouse embryo is generally easier to manipulate than that of other species, facilitating the generation of custom transgenic models possessing desired genetic alterations of interest.
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WHAT TYPES OF GENETICALLY ENGINEERED/MUTANT MICE CAN BE CREATED?
Genome manipulation in mice can be performed in a variety of ways. Arguably, the most popular type of transgenic mouse is the knockout model, where genes of interest are inactivated by replacing or disrupting their coding sequences with exogenous DNA. Knockdown models are similar to knockout models, except that the expression of the gene of interest is significantly reduced rather than completely abrogated.
The functional opposite to knockdown and knockout models are gain-of-function models (a.k.a. overexpression or knock-in models), where the introduction of genetic material either results in de novo protein synthesis or increases the amount of protein synthesis to beyond exogenous levels. Gain-of-function models offer the added convenience of being able to insert a tag during the process of manipulating the gene of interest, allowing researchers to both detect the protein of interest and distinguish protein generated as a result of gene alterations from protein produced owing to endogenous responses.
A combination of gain-of-function and loss-of-function models is very useful for delineating the specific role of a given gene and its complementary protein within a homeostatic mechanism, as well as identifying how departures from homeostatic expression levels affect the rest of the mechanism and potentially lead to disease states.
WHAT DO I HAVE TO CONSIDER WHEN DEVELOPING A GENETICALLY ENGINEERED/MUTANT MOUSE MODEL?
Beyond the standard issues that are associated with any process involving transfection and/or transduction, such as gene integration, expression, and stability, researchers also have to take into account the possibility of unintended phenotypic alterations when developing a genetically engineered mouse. Given that gene expression is highly fluid throughout an organism’s lifespan, knocking out a gene can result in unintended consequences, such as embryonic lethality (where the genetically modified embryo/fetus fails to survive gestation), stunted growth post-partum, and sterility.
Workarounds have been devised for such situations, with knockdown models, in particular, proving useful in situations where completely knocking out a gene would result in embryonic lethality. Alternatively, scientists can generate “inducible” models using mechanisms such as the Cre-loxP recombination or the tetracycline-controlled transactivator systems.
These systems can be used for both loss- and gain-of-function. For example, the Cre-loxP system confers loss-of-function by flanking the gene of interest with loxP sites (floxing), typically resulting in sequence deletion upon Cre induction. Alternatively, placing a floxed stop codon ahead of a gene of interest means that Cre induction restores transcription. Finally, by integrating them with specific promoters, induction systems can be used to create cell/tissue-specific genetically engineered mice – something that is very useful for ensuring that a model mimics its associated human condition as closely as possible.
GENETIC MODIFICATION ASIDE, WHAT OTHER PARAMETERS DO I HAVE TO BE AWARE OF?
When using genetically modified mice, it’s important to remember that these are living multicellular organisms, susceptible to not only inherent heterogeneity across individuals and generations, but also capable of undergoing genetic drift, whether naturally or due to improper breeding practices. While genetic drift cannot be completely eliminated, it can be mitigated by detailed record-keeping, careful observation for phenotypic changes, avoiding selection pressure by selecting breeders at random, and “refreshing” your colony by backcrossing breeders with wildtype mice every few generations.
Additionally, the identification and use of proper controls is paramount to delineating a novel phenomenon from variation and noise. It’s clear just from looking at them that there are many different strains of mice, and as such, the wildtype controls for an experiment should be, at minimum, of the same strain as the genetically engineered mice used in that experiment. Ideally, researchers should strive to obtain and use wildtype littermates as controls, with the caveat that a breeding regimen involving heterozygote x heterozygote crosses is not always logistically feasible. If wildtype mice are unavailable, heterozygote littermates may be viable control specimens, provided that no phenotypic deviation from the wildtype is present.
Nowadays, a mouse model exists for just about every known situation, but researchers are continuously pushing the envelope when it comes to asking new questions and requiring novel models. Understanding how mouse models work and the considerations that go into their creation goes a long way in helping each scientist find a model that can help them answer their own research questions.
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