"If this is correct, it's a great achievement", said virologist Joachim Denner of the Robert Koch Institute in Berlin, an expert in the retroviruses.
Whether or not pig retroviruses would truly pose a risk of causing disease in humans remains controversial.
But scientists argue the pigs involved in their experiments would represent a small percentage of that total, and would be used to save human lives. "If you need to knock [these viruses] out, this is the way to do it, no question", he says.
The world has a big organ shortage problem: nearly 120,000 people are now waiting for a transplant in the U.S., and more than 20 people die each day waiting for a new organ.
Agus tells CBS News in human heart valve replacement, pig valves are used, but they are put in formaldehyde and fixed.
Then CRISPR came along.
Studying the eGenesis-edited pigs will also give researchers the opportunity to see whether editing a significant number of genes with CRISPR causes any long-term problems in mammals.
While not all of the pigs survived, study author George Church argued that they demonstrated what they needed in order to make this method a realistic prospect.
The team first confirmed that PERVs in pig cells can be transmitted to human cells when cultured together. The research identified and deactivated 25 genomic triggers known to activate PERVs, allowing surrogate sows to then be successfully implanted with PERV-free embryos and giving birth to piglets free of the endogenous viruses.
In Australia, 1,713 people received organ donations in 2016. Porcine retroviruses (PERVs) are now one of the big safety barriers preventing us using pigs as organ donors. "We have a pig we are very confident we can make work for kidney transplants", Tector said. "And it will add to the cost of providing pigs for the initial clinical trials".
The first form of xenotransplantation - using animal blood to transfuse humans was first attempted in the 17th century, though as surgeon David Cooper put it in a 2012 review paper, "perhaps not surprisingly, the results were mixed". His group is collaborating with the company United Therapeutics to develop implantable pig hearts.
The PERV family aren't the only pathogen in town, but they are among the most concerning. That could provide an easier source of transplantable organs than cloning. Compared to the PERV feat, Yang says, those compatibility issues are "the second challenge, and probably more challenging".
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