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August 15, 2017

New research could lead to pig-to-human organ transplants, experts say

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Scientists have edited piglets' genes to eradicate viruses that could cause disease in humans, according to research published Thursday in the journal Science, and some experts say the advancement eventually could make it possible to transplant organs from pigs into humans.

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Background: Fear of retroviruses halts research on pig-to-human transplants

According to New York Times, researchers in the past discovered that pig DNA contained genes for viruses, called retroviruses, that resembled those that can cause leukemia in monkeys. Further, when scientists grew pig cells next to human embryonic kidney cells, the retroviruses spread to the human cells. Those infected human cells then were able to infect other human cells, the Times reports.

The discovery prompted fear that pig organs could infect humans with the retroviruses, and research on pig-to-human organ transplants halted. However, recent advances in gene-editing technology prompted some scientists to revisit the topic to see if they could edit away the potentially harmful pig genes.

New research edits retroviruses out of pig DNA

In the new research, scientists used the gene-editing technology CRISPR to remove the retroviruses from pigs' genomes. The scientists then cloned the edited cells and reverted the cells back to their earliest form of development before placing them into an egg. According to the Times, the cells gave the eggs the genetic material needed to develop into an embryo. Scientists then implanted the embryos into 17 sows.

According to the Times, most of the embryos and fetuses the scientists created died before being born, and some of the piglets died soon after being born. Of the 37 piglets that were born, 15 survived for up to four months. The piglets were genetically identical to the pigs from which the scientists took the initial cells—but they no longer carried the retroviruses.


George Church, a geneticist at Harvard University who led the study, said the first pig-to-human transplant could take place within two years. According to the Times, Church has founded a company, called eGenesis, that could create pigs with organs that are compatible with human patients and sell the genetically altered organs.

David Klassen, CMO at the United Network for Organ Sharing who was not involved in the study, said it would "be a real game changer" if pig organs were shown to be safe and effective when transplanted into humans. He explained, "There's a big gap between organ supply and organ demand," noting that 116,800 patients were on transplant waiting lists last year, while just 33,600 organ transplants occurred.

However, some health care providers have said there is not enough evidence that the retroviruses are actually harmful. Jay Fishman, co-director of Massachusetts General Hospital's transplant program, said, "We don't know that if we transplant pig organs with the viruses that they will transmit infections, and we don’t know that the infections are dangerous," adding, "I think the risk to society is very low."

For instance, according to the Times, pig heart valves routinely are transplanted into humans, some diabetes patients have received pancreas cells from pigs, and some burn patients have been treated with grafts made from pig skin. According to the Times, there is no evidence such patients were infected with the retroviruses found in pig DNA.

Joseph Tector, a transplant surgeon and professor of surgery at the University of Alabama at Birmingham School of Medicine, said given the potentially low risk, it remains unclear if FDA would require the viruses to be removed before a transplant. However, he said, "If you need to knock [these viruses] out, this is the way to do it, no question" (Kolata, New York Times, 8/10; Weintraub, Scientific American, 8/10).

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