Researchers at the Francis Crick Institute and the University of Kent have used gene-editing technology to create female-only and male-only mice litters with 100% efficiency.
Despite ongoing ethical discussions regarding their use, it is clear that animals have been critical models for understanding human health and disease. Some applications of these models require either females (XX) or males (XY), such as studying X-chromosome inactivation or sex-limited cancers. As a result, the unrequired sex is needlessly generated and subsequently culled. This issue is particularly prominent in agriculture, for example, the dairy industry requires females. Consequently, each year around 95,000 male calves are culled in the UK, 200,000 in Germany and 500,000 in Australia.
The ‘Replacement, Reduction and Refinement’ (3Rs) global guidelines in laboratory research promote the efficient use of animals. Nonetheless, there are still pitfalls within this system. The ethical and financial burden of unnecessarily producing and culling animals of the undesired sex is significant. As such, there is an urgent need for the development of new methods to generate single sex litters in research and agriculture.
Using CRISPR to generate single sex litters
In a recent paper, published in Nature Communications, researchers detailed a novel technology that could be used to improve animal welfare in scientific research, and potentially agriculture as well. More specifically, the new method uses a two-part genetic system to inactivate embryos shortly after fertilisation. The team placed one element of the CRISPR-Cas9 system on the father’s X or Y chromosome. This means that only female or male embryos will inherit it, respectively. The other element of the system is contributed by the mother and is inherited by all embryos. The researchers targeted the Top1 gene, which is critical for DNA replication and repair. Disruption of this gene in mice causes embryonic lethality at the 4-16 cell stage.
Using this technology, the researchers were able to control the sex of a litter with 100% efficiency. In order to produce a male-only litter, the team edited the father’s X chromosome, meaning only females inherited the deleterious Top1 mutation. For a female-only litter, they edited the Y chromosome. The Top1 gene is well conserved across mammals; therefore, this technology may also be applicable to other animals.
Offspring that survive only contain half of the CRISPR-Cas9 system, which prevents the sex-selection being passed down to further generations. The authors also noted that there were no harmful effects of the gene edit in the surviving offspring.
Peter Ellis, author and senior lecturer in molecular genetics and reproduction at University of Kent, said:
“The implications of this work are potentially far-reaching when it comes to improving animal welfare but should be considered at ethical and regulatory levels.
In particular, before any potential use in agriculture, there would need to be extensive public conversation and debate, as well as changes to legislation. On the scientific side, there is also much work to be done over a number of years. Further research is needed, first to develop the particular gene editing toolkits for different species, and then to check they are safe and effective.”
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