Scientists Create Genetically Altered Pigs
Thursday, January 3, 2002
By Rick Weiss,
Washington Post Staff Writer
washtech.com
Scientists for the first time have created genetically engineered pigs whose organs lack a gene that triggers rejection by the human immune system, a key advance toward the goal of developing high-tech hogs bearing organs for transplantation into humans.
The work represents the first time that scientists have successfully substituted one entire gene for another in pigs – in this case, replacing a gene that typically initiates organ rejection with a defective version of the same gene.
The disabled gene is just one of several that scientists believe they would have to alter, or "knock out," if pigs are ever to help meet the growing demand for transplantable organs. Even if scientists overcome the many technical challenges posed by cross-species transplantation – also known as xenotransplantation – many safety and ethics concerns remain unsettled.
But advocates for the fledgling field said they were heartened by the latest feat, which bolsters the feasibility of farm animals serving as organ donors.
"This was a very important experiment to do. It's very important work," said Fritz Bach, a researcher at Harvard Medical School and a pioneer in the field.
The number of patients needing organ transplants has skyrocketed in recent years while the number of organs donated for such purposes has grown only slightly. The great disparity between supply and demand has led doctors and researchers to look beyond the pool of potential human donors to neighboring branches of the mammalian family tree.
Pigs have garnered the most attention as potential sources of organs because they are biologically similar to humans but not as similar as monkeys or apes. Those primates pose problems because they are reservoirs of human diseases and because many people are put off by the idea of conscripting our closest evolutionary cousins as organ donors.
In the new work, scientists at the University of Missouri at Columbia and Immerge BioTherapeutics Inc. of Charlestown, Mass., used cloning technology and a sophisticated gene-swapping technique to grow pigs carrying a dysfunctional version of a gene called GGTA1. The gene helps pigs make an enzyme that attaches sugar molecules to cells in their bodies.
That sugar construct identifies the cells as pig cells. And when pig cells or organs are transplanted into primates, the primate immune system recognizes them as foreign and violently rejects them.
Additional rejection mechanisms come into play even if this most immediate response is blocked. But less is known about those second- and third-tier mechanisms, and scientists hope to be able to study and overcome those hurdles after they take care of GGTA1.
"It's kind of like a brick wall. Until you get past that, you don't know what's on the other side," said Randall S. Prather, the Missouri scientist who led the study. "So we don't know what else has to be altered before these organs can be transplanted successfully."
What's more, there is still work to be done before GGTA1 is completely eliminated as a problem. Two copies reside in virtually every cell in a pig's body – one from each parent. The latest work knocked out only one copy from each cell, so the cells can still make the enzyme that fosters rejection.
The team hopes to have "double-knockout" pigs – in which both GGTA1 genes are replaced – born within the next year or so. The plan is to do so either by further manipulating the genes inside cells or by mating some of the newborn single-knockout animals, which together can give rise to double-knockout offspring.
Organs from animals completely devoid of the culprit gene will be transplanted into non-human primates to see how long the organs live and to identify which mechanisms of the primate immune system interfere with the transplants' long-term survival.
Additional gene knockouts – and perhaps the addition of helpful genes – could take years to complete, and even then it might still be necessary for recipients to stay on drugs that suppress the immune system, as is typical with human organ transplants today. To overcome that last hurdle, the Missouri team and others are experimenting with new methods of inducing immunologic tolerance – a state of suspended discrimination by the immune system that would allow a person to accept an animal organ as though that organ were genetically identical to the recipient.
"The hope is that, over time, you could eliminate immune-suppressing drugs and immune system cells would become tolerant of the transplant," said Immerge President and Chief Executive Julia L. Greenstein.
The method for making knockout pigs is still inefficient. The Missouri group worked with millions of pig cells and made more than 3,100 cloned pig embryos to get just five live-born piglets with the desired genetic knockout. One of those five died inexplicably at 17 days old, the scientists reported yesterday in Science Express, the online version of the journal Science. Others show evidence of other mild abnormalities, as has been the case with many cloned farm animals.
Prather said those abnormalities, including heart and vessel defects, should not affect the animals' usefulness as the founders of a new line of organ-donating pigs, because the offspring of clones typically don't have the same defects carried by their parents. But for opponents of xenotransplantation, the deformities symbolize the field's dubious ethics.
"Pigs are wonderfully inquisitive and naturally intelligent," said Mary Beth Sweetland, president of People for the Ethical Treatment of Animals in Norfolk. "Yet people have no compunction about using these animals as though they were nothing but spare parts. I think it's very irresponsible to go forward with this work when there is so much to be done to improve human organ transplantation."
Others have raised concerns that porcine viruses might hitch a ride on organs and cause infections in recipients. Although recent studies have suggested that risk is small, the Missouri pigs are being housed at an undisclosed location because of fears of protests or vandalism, Prather and Greenstein said.
A second group of scientists reported Wednesday that they, too, had made pigs lacking one copy of the GGTA1 gene. That work, at PPL Therapeutics in Blacksburg, Va., produced five engineered piglets on Dec. 25, about three months after the Missouri piglets were born. The work has yet to pass peer review or be accepted for publication.
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