WASHINGTON • In a striking advance that helps open the door to organ transplants from animals, researchers have created gene-edited piglets cleansed of viruses that might cause disease in humans.

The experiments, reported on Thursday in the Science journal, may make it possible one day to transplant livers, hearts and other organs from pigs into humans.

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, non-profit organisation that manages the United States' transplant system.

There were 33,600 organ transplants in the US last year, and 116,800 patients on waiting lists, according to Dr Klassen, who was not involved in the new study.

Dr George Church, a geneticist at Harvard who led the experiments, said the first pig-to-human transplants could occur within two years.

The new research combines two great achievements in recent years - gene editing and cloning. But it may be years before enough is known about the safety of pig organ transplants to allow them to be used widely.

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.

Scientists pursuing this goal argue that the few thousand pigs grown for their organs would represent just a small fraction of the estimated 100 million pigs killed in the US each year for food. The animals would be anaesthetised and killed humanely.

Major religious groups have weighed in, generally concluding that pig organs are acceptable for life-saving transplants, noted Dr Jay Fishman, co-director of the transplant programme at Massachusetts General Hospital.

Pig heart valves are already routinely transplanted into patients. Some Jewish and Muslim leaders, however, do not endorse pig kidneys for transplant, reasoning that patients with kidney failure can survive with dialysis.

In 1998, Dr Fishman and his colleagues discovered that hidden in pig DNA were genes for viruses that resembled those causing leukaemia 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.

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.

"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," Dr Fishman said. "I think the risk to society is very low."

Dr Church and his colleagues thought the retrovirus question could be resolved with CRISPR, the new gene-editing technology. They took cells from pigs and snipped the viral DNA from their genomes. Then the scientists cloned the edited cells.

Each pig cell was brought back to its earliest developmental stage and then slipped into an egg, giving it the genetic material to allow the egg to develop into an embryo. The embryos were implanted in sows and grew into piglets that were genetically identical to the pig that supplied the initial cell.

Dr Church and his colleagues ended up with 15 living piglets, the oldest now four months old. None has the retroviruses.

He has also founded a company, in the hope of selling the genetically altered pig organs.

NYTIMES