Concerns over these infections being passed from pig to patient have been a major stumbling block in the quest to find an alternative to human organ donation. Researchers in the study used the gene-editing technology to effectively cut out a porcine endogenous retrovirus (PERV) commonly found throughout pig bodies.
"Porcine organs are considered favorable resources for xenotransplantation since they are similar to human organs in size and function, and can be bred in large numbers", they wrote in a study published Thursday in the journal Science.
If pig organs were shown to be safe and effective, "they could be a real game changer", said Dr. David Klassen, chief medical officer at the United Network for Organ Sharing, a private, nonprofit organization that manages the nation's transplant system.
"At least for those who are four months old, we did not observe difference in physiology between the modified piglets and normal ones", Yang said. In their new work the Yang team performed experiments confirming that pig retroviruses can infect human cells-just as another retrovirus, HIV, does with people.
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Next, the team created a map of the PERVs in the genome of pig fibroblast (connective tissue) cells. The same year, they showed that CRISPR could knock out PERV genes at all 62 sites in the pig genome-the most widespread CRISPR editing feat to date. (That's the same technique used to clone Dolly.) This way, they were able to grow "designer piglets" whose PERVs are inactivated and harmless. But they added "ingredients" during the gene modification process - including both growth factors and growth inhibitors - and finally succeeded. But by exposing the cells to a chemical cocktail that encouraged growth and tamped down on a key growth-suppressing gene, the team bumped up the portion of flourishing PERV-free cells in a dish to 100%. Those embryos were implanted into sows and then became piglets. The resulting piglets exhibited no signs of PERVs.
Genetics expert Professor Darren Griffin, from the University of Kent, said: "This represents a significant step forward towards the possibility of making xenotransplantation a reality". Scientists still don't know whether the viral particles they produce can infect humans at all, he notes, much less whether they would cause disease if they did. A chimera is a single organism containing cells (and DNA) from two or more organisms.
Doctors and scientists have been searching for the holy grail of organ replacement-a way to grow usable organs outside the human body-for decades. With only a few more gene editing breakthroughs, we might start seeing pig organs saving thousands of lives in only a few years.
The National Institutes of Health hopes a revised policy will enable research to continue - safely.