The new research combines two great achievements in recent years — gene editing and cloning — and is unfolding quickly. But the work is novel and its course unpredictable, Dr. Klassen noted.
It may be years before enough is known about the safety of pig organ transplants to allow them to be used widely.
The idea of using pigs as organ factories has tantalized investigators for decades. Porcine organs can be the right size for human transplantation, and in theory, similar enough to function in patients.
But the prospect also raises thorny questions about animal exploitation and welfare. Already an estimated 100 million pigs are killed in the United States each year for food.
Scientists pursuing this goal argue that the few thousand pigs grown for their organs would represent just a small fraction of that total, and that they would be used to save human lives. The animals would be would be anesthetized and killed humanely.
Major religious groups have already weighed in, generally concluding that pig organs are acceptable for lifesaving transplants, noted Dr. Jay Fishman, co-director of the transplant program at Massachusetts General Hospital. Pig heart valves already are routinely transplanted into patients.
(Some leaders in the Jewish and Muslim communities, though, do not endorse pig kidneys for transplant, reasoning that patients with kidney failure can survive with dialysis.)
Scientists began a pursuing the idea of pig organs for transplant in the 1990s. But in 1998, Dr. Fishman and his colleagues discovered that hidden in pig DNA were genes for viruses that resembled those causing leukemia in monkeys.
When researchers grew pig cells next to human embryonic kidney cells in the laboratory, these viruses — known as retroviruses — spread to the human cells. Once infected, the human cells were able to infect other human cells.
Fears that pig organs would infect humans with bizarre retroviruses brought the research to a halt. But it was never clear how great this threat really was, and as years have gone by, many experts, including Dr. Fishman, have become less concerned.
Some patients with diabetes have received pig pancreas cells, hidden in a sort of sheath so the immune system will not reject them. And burn patients sometimes get grafts made of pig skin. The pig skin is eventually rejected by the body, but it was never meant to be permanent anyway.
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