Pig and human organs are similar in size and structure. But pig organs carry sugar molecules that act as red flags, triggering destructive attacks by the human immune system. If cloned pigs could be genetically designed not to carry these flags, then the humanized animals could “donate” hearts, livers and kidneys to people. In studies released last week, teams based in Scotland and Japan used different methods to make genetic modification possible.
Writing in the journal Nature, researchers at the Scottish biotech firm PPL Therapeutics (cocreators of Dolly the sheep) said they had cloned five piglets using a two-step process. As in past experiments, they fused an adult cell, complete with its DNA, with a waiting egg cell whose own DNA had been removed. Then they transferred that genetic material into another empty egg–this one fertilized and ready to develop. In the other study, reported in the journal Science, Japanese and American researchers used a technique called microinjection. Like the PPL team, they transferred cellular DNA into a hollowed-out egg, but instead of fusing two cells, they used a tiny device to remove the genetic material from one and inject it into the other.
It’s too early to say which method will prove most useful in the long run, and pig-organ transplants won’t happen overnight. Scientists still have to learn how to breed pigs whose organs can survive in the human body. And doctors must contend with another threat. All pigs carry strands of genetic material with viruslike qualities. In a third study reported last week in Nature, scientists confirmed that these “porcine endogenous retroviruses” (PERVs) can infect human cells in a test tube. Researchers at the Scripps Research Institute in La Jolla, Calif., also transferred pig cells into mice whose own immune systems had been knocked out. After two months the PERVs had spread to mouse organs. The mice didn’t get sick, but researchers are wary, because humans can fall victim to animal viruses that mutate when placed in new hosts. Daniel Salomon of Scripps says his data doesn’t necessarily spell doom for pig-organ transplants. But his results injected a note of caution into a week otherwise filled with oinking bundles of joy.
