Gene Editing the Polled Allele

Advances in gene editing allows for introduction of useful alleles.

by Kaci Foraker, editorial intern

BROOKINGS, S.D. (June 19, 2019) — Researchers are using beef cattle genetics to solve an animal welfare issue in dairy cattle.

Alison Van Eenennaam, extension specialist in animal biotechnology and genomics at the University of California–Davis (UCD), presented her latest work in genetic editing at the 2019 Beef Improvement Federation Symposium in Brookings, S.D., June 18-21.

“The overall drive of our selection programs hasn’t changed,” said Van Eenennaam. “Our breeding objectives have remained the same. We are still interested in disease-resistant animals. We are interested in productive animals and trying to go after some welfare traits, but our overall objectives haven’t changed.”

Genetic solution

Van Eenennaam’s group focused recent work on addressing the welfare issue of horned Holsteins. The presence of horns can create a dangerous environment for employees and other cattle, so young cattle must be dehorned.

To create a solution for this problem, a small company, Recombinetics of Minnesota, introduced the polled gene from Angus cattle to Holstein embryos to produce polled Holsteins. The UCD group collaborated on the phenotypic evaluation of the gene-edited cattle.

There are already Holsteins that are polled. However, Van Eenennaam stated that homozygous polled Holsteins net approximately $150 less profit compared to a horned animal. This decrease is partially due to the effects of inbreeding, but producers still don’t want to take a profit decrease, so they stay away from those genetics.

Van Eenennaam’s group tested a data model that projected requiring all Holsteins to be polled. The results of this projection showed the frequency of horned cattle would significantly decrease, but there would be a rapid growth in the occurrence of inbreeding.

“Genetic variation is a breeder’s friend,” said Van Eenennaam. “If you have a really big increase in inbreeding, you are basically taking away your future possibility of genetic improvement, as you are becoming less genetically diverse.”

To solve the negative economic impact that is seen in many polled Holsteins, Van Eenennaam predicted editing the top 1% of sire’s genes would decrease the occurrence of horns while still keeping the same possibility of future genetic gain.

At UCD, a gene-edited homozygous polled bull produced by Recombinetics was bred to horned Herefords and produced offspring without horns. This outcome was expected, as the polled gene is dominant compared to the horned allele.

Approaches

There are two main ways to edit cattle. The first is to edit a somatic cell from an elite animal and then clone that cell. To do this, DNA is first taken from a donor oocyte (egg cell). Then the diploid nucleus from the edited somatic cell is moved into the oocyte and implanted into a surrogate dam, said Van Eenennaam. Nine months later a homozygous non-mosaic edited animal will be produced.

The second approach is to introduce the edit into the developing embryo. In-vitro maturation of the ovocyte uses the DNA from the female, inseminates the egg, produces a zygote and then editing reagents are introduced into the zygote. Seven days later the developing embryo can be transferred into a surrogate dam. The offspring of this approach will most likely be a heterozygous or mosaic animal, and a homozygous animal can be developed through generations of breeding, said Van Eenennaam.

“Genome editing is like sprinkling on top dressing,” said Van Eenennaam “It’s a little bit of a cherry on top of a breeding sundae. Nothing about our traditional breeding programs is going to go away because of editing, but what it does do is offer the opportunity to introduce useful alleles into cattle breeding programs without linkage drag, and enable the precise and rapid transfer of useful genetic variants between different breeds of cattle.”

Van Eenennaam summarized:

• Gene editing offers an approach to precisely introduce useful genetic variation into food animal breeding programs.
• Gene editing is distinct from genetic engineering because it is targeted and does not necessariy involve the introduction of foreign DNA.
• It opens up new opportunities for breeders to address critical problems such as disease resistance, animal welfare and resillience, and product quality traits.

The 2019 BIF Annual Convention was hosted by South Dakota State University and the South Dakota Beef Breeds Council June 18-21 at the University Comfort Suites and Convention Center in Bookings. ANGUS MEDIA® provides comprehensive online coverage of the event at www.BIFconference.com. Visit the Newsroom for summaries, proceedings, PowerPoints, video and/or audio of the sessions and the Awards page for announcements and photos of award winners.

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