Researchers at Francis Crick Institute and the University of Kent have used the CRISPR gene-editing system to produce litters of mice that are 100 percent either male or female. The technique could help prevent the unnecessary culling of animals of unwanted sex in agriculture or scientific experiments.
Only female chickens lay eggs, and only female cows produce milk, meaning the males of both species are usually culled soon after birth. For some experiments, scientists can also require either male or female animals to study sex-specific diseases such as ovarian or prostate cancer. Currently, there aren’t any ways to reliably produce one sex over the other in mammals.
So for this new study, the team investigated how to use CRISPR-Cas9 to control which sex is born from a litter of mice. The aim was to ensure only the desired sex can develop beyond an early embryo stage, and the technique can work to select for either male or female as required.
CRISPR-Cas9 is made up of two main components – the Cas9 enzyme cuts the cell’s DNA, and a guide RNA sequence tells it where to make that edit. To make CRISPR sex selective, the team split those components up among the parent mice. The mother contributes the guide RNA, while Cas9 is placed on one of the father’s chromosomes, depending on which sex is desired for the pups.
Once the two parts combine in the embryo, they edit a gene called Top1, which is essential for DNA replication. With that gene disrupted, the embryo cannot develop beyond 16 to 32 cells. If Cas9 is on the father’s X chromosome, only females will inherit the mutation, resulting in an all-male litter. For an all-female litter, Cas9 is placed on the Y chromosome.
The technique worked extremely well. The team bred 72 litters – 36 of each variation – and every single pup born was the desired sex. The scientists expected that each litter would be smaller, since roughly half of the embryos didn’t develop, but to their surprise the litter sizes were between 61 and 72 percent the size of unedited control litters.
Importantly, the gene edits had no harmful side effects on the health of the mouse pups born. They can go on to have their own litters, which can have natural 50/50 sex ratios or, if selectively bred with a mouse containing the other half of the CRISPR code, have single-sex litters.
The team says that the targeted Top1 gene is conserved across mammals, so the technique could also apply to other cases where it’s useful to control the sex of offspring. Further research will be needed to test how well it works in other animals, but for now it could be applied to laboratory mice.
The research was published in the journal Nature Communications.
Source: Francis Crick Institute
