|Xena, the latest clone.|
Researchers announced last week that a virus found in pigs has the potential to infect humans who receive transplants of pig tissues or organs. Although the virus does not appear to cause disease, the discovery that infection can occur casts a cautionary shadow over the future of cross-species transplantation.
The report, published in the journal Nature, coincided with the publication of results (by two separate teams) confirming the successful cloning of pigs. The cloning teams, who published their results in the journals Nature and Science, are pursuing pig cloning because of the promise of transplanting animal tissues and organs into humans to treat disease.
But the outcome of research to clone pigs as organ donors could be hampered if cross-species transplantation, also called xenotransplantation, itself causes disease.
Virus in every pig
The virus, called porcine endogenous retrovirus (PERV), is found in the DNA of all pigs and doesnít make them ill, and so far doesnít appear to cause harm to other animals either. But the new research shows that cross-species infection can occur. "The implication of our study is that if you transplant pig tissue, there is a possibility that PERV will spread to adjacent tissues and even to tissues far outside the transplant site," says Daniel Salomon, of the Scripps Research Institute in La Jolla, California, who authored the study. "Therefore the risks that have been framed as potential cross-species infection associated with cross-species transplantation are real."
On the other hand, the potential benefits of xenotransplantation are huge. There are more than 70,000 people in the U.S. waiting to receive organ transplants, but 6,000 to 7,000 of them die each year waiting for human donor organs that never arrive. In addition, cells from pigs can be used to treat juvenile onset diabetes, and pig brain cells have the potential replace those damaged by diseases like Parkinsonís.
Salomon and colleagues transplanted insulin-producing cells called islets from pigs into mice and found that PERV infected not only the mices islet cells but also other tissues, including the lungs, spleen, heart and liver. In another experiment, the researchers showed that PERV could also infect human cells.
But is PERV harmful?
Salomonís team is quick to point out that PERV appears to be a benign virus. Some patients have already received islet cells from pigs to treat juvenile onset diabetes, and others suffering from organ failure have undergone experimental procedures in which their blood has been passed through surrogate pig organs. No evidence of PERV infection, or illness, has been found in these patients.
Still, Salomon says that PERV may not be benign forever. "The possibility that pigs can give rise to infections that could spread to the public and infect or even kill people is perfectly possible," he says. He warns that PERV is a type of retrovirus, like HIV, the virus that causes AIDS. Like other pathogens, retroviruses have the ability to mutate rapidly and adapt to a new host, even if that host is a different species of animal. "We donít need to go too far before we realize that even the current HIV epidemic arose in animals," says Salomon. "HIV is another example of cross-species infection that went from animals to humans and then caused the tragic epidemic of AIDS."
Salomon thinks that the discovery of cross-species infection by PERV will provide a scientific model to for examining the risks and benefits of xenotransplantation. "Given the current waiting list, the implications for public health and for individual survival of successful cross-species transplantation is a tremendous benefit to consider," he says. "I believe that it is still reasonable to move forward in very highly supervised, small clinical studies in which everyone who participates, both the patients and the public and of course the medical professionals, are fully aware of the potential risks of PERV infections as well as possibly unknown infections." Elsewhere on the web:
The Centers for Disease Control and Preventions summary of Xenotransplantation
Xenotransplantation as a treatment for AIDS
Opposition to Xenotransplantation