Scientists have bred the world’s first horses with CRISPR-mediated genomic edits to enhance their muscle power and speed.Credit: Agustin Marcarian/Reuters
These horses might look like ordinary horses, but there is something highly unusual about their genomes. They are the first of their species to have their DNA edited using CRISPR–Cas9, a technique that cuts the genome at a specific location to change gene expression and achieve a desired trait.
The horses are clones of the prize-winning steed Polo Pureza, but they have a tweak to myostatin — a gene involved in regulating muscle development — that is designed to quicken their pace. CRISPR was used in fetal fibroblasts (connective tissue cells) to generate embryos through cloning, and then the embryos were implanted into mares.
The development of these five CRISPR-edited horses ten months ago, by the non-profit research organization Kheiron Biotech in Buenos Aires, Argentina, is proving controversial among horse breeders in Argentina, where polo is extremely popular, Reuters reported on 30 August.
Critics are concerned that the technology threatens people’s livelihoods and that it will compromise the tradition of using selective breeding to generate elite horses. The Argentine Polo Association has now banned the use of gene-edited horses in the sport, following the lead of similar organizations such as the International Federation for Equestrian Sports1, which banned the practice in 2019.
Some scientists, however, welcome the CRISPR horses. “It’s cool to show that CRISPR works and you can create CRISPR-altered horses,” says Molly McCue, a veterinary clinician scientist at the University of Minnesota in Minneapolis. “Horse[riders] often feel very strongly about breeding as an art and not a science, but really it is both together,” she adds.
The CRISPR horses join a menagerie of gene-edited animals that have wide-ranging applications — mostly with the goal of improving agriculture. Such efforts have now transformed from a research promise to a commercial reality, says Tad Sonstegard, chief scientific officer at Acceligen, a company in Eagan, Minnesota that specializes in the precision breeding of livestock. Biological engineering ethicist Jeantine Lunshof, at Harvard Medical School in Boston, Massachusetts, puts the rise of the technology in animals down to improvements in CRISPR techniques and “a broader acceptance of gene editing”.
CRISPR in agriculture
Acceligen is one of a handful of companies that are using CRISPR to alter the genomes of livestock, with the goal of making animal products such as meat and milk in a more efficient manner. Its ‘PRLR-SLICK’ cattle have an edit in the prolactin receptor gene that gives them shorter, slicker hair, providing tolerance to heat stress. Sonstegard says that the short-haired PRLR-SLICK cows should be able to adapt to hotter climates and to tolerate the rising temperatures associated with climate change, increasing the accessibility of meat and dairy products to people around the world. In 2022, the US Food and Drug Administration (FDA) approved the PRLR-SLICK cattle for meat production and human consumption, deeming them a low risk to safety, but it’s unclear when the meat will hit the US market.
With a similar goal, researchers in India announced the first CRISPR-edited sheep earlier this year. As with the Argentine horses, the scientists edited the myostatin gene of the sheep to increase muscle mass — but in this case, the goal was to increase meat production rather than to speed the sheep up.
Scientists have used CRISPR to make pigs more resistant to porcine reproductive and respiratory syndrome, and to make their meat hypoallergenic.Credit: Acceligen
Disease resistance is another reason that scientists are producing CRISPR animals. Genus, a company based in Basingstoke, UK, has used CRISPR to generate pigs that contain a mutation in the CD163 gene. This makes them resistant to a virus that causes porcine reproductive and respiratory syndrome (PRRS), which can be deadly for suckling pigs. In 2020, the FDA approved the gene-edited pigs for sale in the United States, and they are expected to go on the market there in 2026.
Researchers are also using CRISPR to make animal products that are safer for human consumption. GalSafe pigs, made by Revivicor in Blacksburg, Virginia, are designed to be a source of hypoallergenic meat. Researchers use CRISPR to inactivate the GGTA1 gene, which produces alpha-gal, a sugar molecule found in most mammals that can cause allergic reactions in humans who eat red meat. Revivicor is also investigating whether organs from the pigs could be transplanted into people: the company says that the same gene edits might reduce the risk of pig organs being rejected from human bodies. The FDA approved GalSafe pigs for use in biomedical therapeutics and for human consumption in 2020.
