Maintaining experimental mouse colonies is a lot like juggling. There are a lot of balls that you have to keep in the air all the time. You need to evaluate the breeding performance, adjust the colony size to match required production, genotype the mice, and then plan and execute your experiments. It is easy to have concerns about genetic drift far down on your list of priorities. However, for mutant or transgenic mouse strains on inbred backgrounds dealing with potential genetic drift needs to be a priority.
Why do you need to refresh mouse models?
Spontaneous mutations will continually arise in any colony of mice, and through random chance, some of them will spread and become homozygous in all of the mice. If any of those mutations alter the phenotype of your strain (it has happened!), you will have a big problem that will significantly impact your research. Smaller colonies of mice, like a small research colony, will be more affected by genetic drift than large colonies.
There are many, many examples of how genetic changes can lead to phenotypic changes for mutant and transgenic mouse models. The way to mitigate the impact of genetic drift on your mutant and transgenic mouse strain is to refresh the genetic background of your strains every 5-10 generations by backcrossing to the inbred control strain.
8 Steps to genetically refresh your colonies –the right way
Below are detailed steps for refreshing mutant and transgenic mouse strains. Let’s use C57BL/6J as our example, but you can substitute in any appropriate inbred strain.
- Cross females from your mutant or transgenic strain to C57BL/6J males.
- The male progeny from that cross will have a "refreshed" Y chromosome (for lack of a better term).
- Cross those males that carry your mutation or transgene to C57BL/6J females.
- The male progeny from the second backcross will have a "refreshed" X chromosome, Y chromosomes, and mitochondrial genome.
- Cross those males from the second backcross that carry your mutation or transgene to C57BL/6J females.
- Cross the males and females from this backcross together to get homozygotes (if homozygotes are needed).
- Inbreed to maintain/expand refreshed colony.
- Phase refreshed mice into existing colony as older breeders are retired.
If your colony has only been inbred 5 generations since the strain was originally obtained/created, two backcrosses should be sufficient (and you can skip step #5), but if your colony is at 10 or more inbreeding generations, then three backcrosses is the best approach. By regularly refreshing the genetic background of your strains, you will keep them as genetically similar to your control strain as possible, thereby ensuring the reproducibility and validity of your studies.
Genetic drift is inexorable and will impact the phenotype of every live mouse colony if not properly maintained. Just ask some researchers at Harvard! While it cannot be stopped, genetic drift can be limited. The Jackson Laboratory has implemented our unique Genetic Stability Program (GSP) to limit cumulative genetic drift in our most widely used mouse strains, like C57BL/6J, by rebuilding our foundation stocks from cryopreserved, pedigreed embryos every five generations. If the strains you are refreshing happen to be one of our GSP strains, then you will have the added benefit of knowing that your mice will be as genetically similar today as they will be 5 or 10 years from now.