If you or a loved one has a heart that's failing or kidneys that are giving out, you already know the grim statistics on transplants. A new organ can turn a death sentence into a full and healthy life--but the supply of replacement body parts lags far behind the demand. According to the nonprofit United Network for Organ Sharing, which maintains the nation's transplant waiting lists, nearly 80,000 patients in the U.S. alone are standing by for new organs--and more than 5,000 people die each year before their turn comes.
Those dismal numbers have prompted scientists to consider organs from other mammals--especially pigs, which are easily bred and whose physiology is similar to ours. But pig biology is different enough from human biology that rejection, a surmountable problem in human-to-human transplants, is disastrous in so-called xenotransplants. (Humans can receive pigs' heart valves and other tissues because they are treated to suppress the immune problem. That doesn't work for whole organs.)
But a solution may be on the horizon. Last week two research teams said they had removed the pig gene responsible for the most severe form of rejection. Not only that. Both teams--one from PPL Therapeutics, which in 1996 helped make Dolly the sheep, the first mammal cloned from an adult animal, and the other from the University of Missouri-Columbia and Immerge BioTherapeutics, of Charlestown, Mass.--then cloned their little pigs, producing five and four piglets, respectively.
By itself, this breakthrough won't lead directly to pig transplants. For one thing, pigs carry two copies of the gene, called GGTA1; the scientists knocked out only one. Researchers expect to create "double-knockout" pigs within a year, but until they do, rejection will remain an insurmountable problem. And even if they do eliminate the most problematic form of inter-species rejection, others exist, and they will have to be dealt with.
Another danger arises because animals carry viruses that are harmless to their hosts but can turn deadly in another species. If such a virus hitchhiked aboard transplanted tissue, it could not only infect its new host but also spread to other humans--much as HIV did when it jumped from monkeys to man.
Finally, cloning often produces animals that are deformed or die young; they may age prematurely as well. Just last week researchers at Scotland's Roslin Institute, PPL's partner in the Dolly experiment, reported that their famous ewe has come down with arthritis at age 5 1/2--a condition that may be related to her cloning.
Still, interspecies transplantation is so promising that researchers are determined to tackle each hurdle as it comes. It could be a decade or more before clinical trials become routine and even longer for transplants. But they seem to be on their way.