Medically Reviewed by Gary D. Vogin, MD From the WebMD Archives
Aug. 16, 2000 — Xena the Pig Clone. No, it’s not a rejected summer replacement series — and unlike her warrior-princess namesake, cloned pigs like Xena may one day save the lives of real people. That day may come as soon as four years from now, say researchers who hope that the new technology soon will provide organs and tissues for humans waiting desperately for transplants.
There’s already a problem, however. Every cell in a pig’s body carries the genetic code for making a type of retrovirus called PERV, and a new study shows that this virus can infect mice transplanted with pig cells. The virus doesn’t make the mice sick, however, so efforts to develop genetically engineered pigs move forward.
Xena, born in Japan, has no brothers or sisters, but she is not alone — she was born several months after five other pig clones were born in the U.K. The Japanese and U.K./U.S. research teams used different techniques to overcome the difficult feat of cloning a pig, but both teams’ results are being reported at the same time in two different scientific journals (Science and Nature, in a highly unusual coordinated release).
“In the future, it may be possible for pigs to supply organs for transplantation,” the senior author of the Science article, Akira Onishi, PhD, tells WebMD. Onishi is affiliated with Japan’s National Institute of Animal Industry.
“We expect to be in human clinical trials with genetically modified pigs at the end of four years,” the co-author of the Nature article, Dave Ayares, PhD, tells WebMD. Ayares is vice president of research and development at PPL Therapeutics Inc. in Blacksburg, Va.
Cloning of pigs has proved trickier than cloning of sheep, goats, and cows — but now the Japanese and U.K./U.S. researchers report two different ways to skin the cat. Onishi’s group uses a tiny tube to suck the DNA out of an adult pig cell and place it into an egg cell emptied of its own DNA. The PPL group uses a more complex technique that transfers adult-cell DNA two times, eventually placing it into a fertilized egg cell that has had its original DNA removed. In both techniques, the resulting embryo clones are placed in a surrogate mother pig for gestation.
The Japanese team transferred 110 cloned embryos to four surrogate mothers, one of whom delivered, by vaginal birth, an apparently normal female piglet. Born on July 2, 2000, Xena was named for the scientific term “xenotransplantation,” the transplant of an organ from one type of animal into another. She is genetically identical to the Meishan breed of pig that donated her DNA — and does not at all resemble her surrogate mother, a Landrace breed of pig.
The PPL team transferred 401 of its double nuclear transfer embryos to seven surrogate sows. One carried the pregnancy to term, and in March 2000 gave birth by cesarean section to five live piglets that were about 25% underweight for their kind. Three were clones of one animal, and two were clones of another — but all five were genetically different from their surrogate mother.
“What’s important is that we achieve the end — xenotransplantation — and we are evaluating different means to do that,” Onishi co-author Tony Perry, PhD, tells WebMD. “At this stage, all that can be said is that we have another technique that we hope will allow us to quickly generate engineered animals.”
Onishi says that his group has not yet tried to genetically engineer pig clones. Ayares, however, says that his team already has engineered pig cells lacking the main factor (called alpha-1,3-GT) that makes the human immune system reject pig organs. Eventually, he says, PPL will develop a clone of pigs that lacks all the elements that can complicate pig-to-human transplants.
“This ultimate pig will be ready to go into human clinical trials in about four years,” Ayares says.
Perry, a molecular embryologist at New York’s Rockefeller University, is not sure that this timetable is realistic. “Generating such a pig that is a universal donor is some way off,” he says. “We are doing everything we can to make it happen as soon as possible, but it is many years off. We need support from the medical community to get there.”
One member of the medical community, transplant physician Daniel Salomon, MD, of The Scripps Research Institute in La Jolla, Calif., already has a contribution — in the form of a warning. In the same issue of Nature as the PPL study, Salomon and co-workers report that mice get PERV infection when they are transplanted with the type of pig cells that might one day be transplanted into diabetic patients.
“There is an infection risk with cross-species transplantation,” Salomon tells WebMD. “Transplant of pig tissues is going to result in long-term exposure to the virus. … Now we are telling you that if you do a pig islet [cell] transplant you are going to get this virus in the heart, in the lung, in all kinds of other organs. In all those compartments in which we found pig cells, we found cross-species infection of mouse tissue.” Islet cells are the cells in the pancreas that produce insulin; transplants of these cells have been used to help diabetics.
PERV worries researchers for two reasons. It is closely related to a type of retrovirus that causes leukemia in several types of animal. And HIV, the AIDS virus, is a different type of retrovirus — but, like PERV, it was harmless until it jumped from chimpanzees to humans.
Every cell in a pig’s body carries some 50 copies of PERV. Even so, Salomon says that his findings don’t necessarily mean the end of pig-organ transplantation. Indeed, he notes that the PERV-infected mice remain as healthy as can be.
“None of our animals are sick,” he says. “There are no sick patients. We had to use very sensitive techniques to detect this virus. These animals do get infected, but it would appear that the virus goes quickly dormant. Right now there is nothing about this that is that scary to me.
“Overall, my message is to reassure the public. Our data suggests you will get infection of human tissues — but we have no evidence it will cause any pathology. Of course, we need another year of work to be sure of this.”
