A genetically edited bull successfully passed down to his offspring a gene that scientists had introduced into his genome as an embryo so that he would not grow horns as an adult, scientists reported Monday (October 7) in Nature Biotechnology. The six calves—which, at two years of age, are perfectly healthy, the researchers report—are expected to lack the dangerous horns that are often removed from dairy cows in a process that is labor-intensive, costly, and dangerous itself.
Four of the calves also inherited a bit of the plasmid that had ferried the hornless gene, called POLLED, into the bull’s genome. The unintended presence of foreign DNA has derailed their route to market in Brazil and the US.
“We’ve demonstrated that healthy hornless calves with only the intended edit can be produced, and we provided data to help inform the process for evaluating genome-edited animals,” coauthor Alison Van Eenennaam, a researcher at the University of California, Davis, says in a press release sent to The Scientist. “Our data indicates the need to screen for plasmid integration when they’re used in the editing process.”
Spotigy and Buri, two bulls that had the POLLLED gene inserted into their genomes from a hornless breed of cattle, were born in 2015. Buri then moved to Davis to be monitored and bred, ultimately siring six calves in September 2017.
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Van Eenennaam’s initial analysis found that the calves were heterozygous for the dominant POLLED allele—just as scientists had hoped they would be. In a number of examinations since they were born, the animals have continued to show good health and no abnormalities, except the lack of horns.
But as she and her colleagues were preparing their manuscript, the US Food and Drug Administration’s (FDA) review of the calves’ genomes spotted the plasmid sequence, inserted right next to the POLLED gene, Wired reports. The agency posted its findings to bioRxiv in July of this year.
The presence of the plasmid felled a partnership between Brazil and Recombinetics, the biotech that developed the hornless cows, and the company is no longer seeking US approval of its modified dairy cattle. To Van Eenennaam’s disappointment, the FDA has decided Buri’s calves cannot be slaughtered for market—even those lacking the plasmid in their genomes—and must be incinerated instead, according to Wired.
Van Eenennaam’s paper was revised to include the discovery of the plasmid sequence in some of the calves—a finding she tells Wired indicates proper oversight of genetically edited animals.
Kerry Grens is a senior editor and the news director of The Scientist. Email her at email@example.com.