GENETICS ▶▶▶
Gene editing offers avenues to end castration
New gene editing developments could be moving the global pork industry closer to the end of pig castration. Especially in the European Union, this leads to an interesting discussion as gene editing is still not permitted. Will the possibility of “solving” the boar taint issue change that?
BY TREENA HEIN, CORRESPONDENT A
SRY-KO pigs have a female phenotype but their genotype is male.
cross the globe, many groups are working on af- fordable and reliable ways for the pork industry to avoid the practice of physical castration. Castra- tion is a painful procedure done on tens of mil-
lions of young male pigs around the world each year. It pre- vents the meat of these pigs from having boar taint, an offensive odour or taste caused by the release of certain natu- ral hormones after a male pig reaches puberty. Gene editing, using tools such as CRISPR/Cas, offers several avenues to prevent the need for castration, and there are sev- eral exciting developments to report on. Dr Björn Petersen and his colleagues at the Friedrich-Loeffler-Institut (Federal Research Institute for Animal Health) in Germany has edited out a gene called SRY in pigs, a gene found on the Y (male) chromosome of all mammals and the main “genetic switch” for male gender development. This research builds on previ- ous studies showing that “knocking out” the SRY gene in male mice and rabbits resulted in suppressed testis develop- ment and female sex organs. These mammals thus have what
is called a female phenotype but their genotype is male. If no testes develop in pigs, no boar taint results.
Edited pigs These edited pigs, after developing from embryos in recipient gilts, had a vulva, uterus, ovaries and other female reproduc- tive parts. They were checked for signs of oestrus at six months and showed none. In post-mortem examinations at nine months, Petersen reports that “we found no functional follicles on the ovaries and the uteri had not matured”. To achieve commercial production of these pigs, there are two possible approaches. “The first is to do the edit not in embryos but in normal adult boars so that their sperm carries the edit,” Petersen explains. “We are trying this by integrating the CRISPR/Cas vector onto the Y-chromosome of boars, under control of a spermatogenesis-specific promoter, and then testing to see if the sperm produced have the edit. When this sperm is used to impregnate a normal female, she will give birth to female pigs and to ‘male’ pigs with a female phenotype.”
Destroying the Y chromosome The second approach is to completely destroy the Y chromo- some in the sperm. “Basically, you create so many breaks that it falls apart,” says Petersen. “This has already been done in mice. However, in ‘cutting’ apart the Y chromosome, you need to do the cutting at genes that don’t exist on the X (female) chromosome. The X and Y chromosomes share many genes and we need the X chromosome to stay intact.” Again, the CRISPR/Cas vector would be integrated under the control of a spermatogenesis-specific promoter, leading to non-function- al Y-chromosomal sperm and intact X-chromosomal sperm. This approach could be described, says Petersen, as “some sort of in vivo sperm sorting.” Petersen favours the latter approach as it results in offspring with no gene edits, offspring that are all phenotypically and genotypically female, with one X chromosome from their mothers and one from their fathers, as is the case with all nor- mal female pigs (and indeed, all normal female mammals). There should be no difference, he adds, in meat quality and reproductive performance.
Regulatory and consumer acceptance Returning to the gene-editing approach, Petersen makes several comments on regulatory and consumer acceptance.
30 ▶PIG PROGRESS | Volume 36, No. 5, 2020
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