With the financial aid of a biotechnology executive whose daughter may need a lung transplant, U.S. researchers have been shattering records in xenotransplantation, or between-species organ transplants.
The researchers say they have kept a pig heart alive in a baboon for 945 days and also reported the longest-ever kidney swap between these species, lasting 136 days. The experiments used organs from pigs “humanized” with the addition of as many as five human genes, a strategy designed to stop organ rejection.
The GM pigs are being produced in Blacksburg, Virginia, by Revivicor, a division of the biotechnology company United Therapeutics. That company’s founder and co-CEO, Martine Rothblatt, is a noted futurist who four years ago began spending millions to supply researchers with pig organs and has quickly become the largest commercial backer of xenotransplantation research.
Rothblatt says her goal is to create “an unlimited supply of transplantable organs” and to carry out the first successful pig-to-human lung transplant within a few years. One of her daughters has a usually fatal lung condition called pulmonary arterial hypertension. In addition to GM pigs, her company is carrying out research on tissue-engineered lungs and cryopreservation of organs. “We’re turning xenotransplantation from what looked like a kind of Apollo-level problem into just an engineering task,” she says.
Some researchers agree with Rothblatt that the latest results mean pig-to-human transplants are plausible. “I think it’s possible; it should be considered,” says Leo Bühler, a Swiss transplant surgeon in Geneva. He said he would transplant a genetically engineered pig’s organ into a patient today, were the patient’s situation desperate enough.
And there are desperate cases. In fact, thousands of people die each year while waiting on transplant lists. Donated human organs are scarce, and many that become available don’t end up helping anyone. That is because a heart or kidney lasts only a matter of hours packed in ice, so organs can’t reach any but the closest patients.
“We want to make organs come off the assembly line, a dozen per day,” says Rothblatt. In 2011 her company paid about $8 million to take over Revivicor, and she has outlined plans for a facility able to breed 1,000 pigs a year, complete with a surgical theater and a helipad so organs can be whisked where they are needed.
The problem with xenotransplantation is that animal organs set off a ferocious immune response. Even powerful drugs to block the immune attack can’t entirely stop it. In a famous 1984 case, a California newborn known as “Baby Fae” received a baboon heart. But it lasted only three weeks before failing. The human body reacts even more strongly to pig tissue, since pigs are genetically more distant. All human tests of pig organs have ended quickly, and badly. A Los Angeles woman who got a pig liver in 1992 died within 34 hours. The last time a doctor transplanted a pig heart into a person, in India in 1996, he was arrested for murder.
Researchers continue to work with pigs because they’re in ready supply, and the organs of young pigs are about the right size. In order to beat the rejection problem, researchers began trying to genetically modify the animals. One major step came in 2003 when David Ayares, a cofounder of Revivicor, created pigs whose organs lacked a sugar molecule that normally lines their blood vessels. That molecule was the major culprit behind what’s called hyperacute rejection, which had almost instantaneously destroyed transplanted pig organs.
Removing the sugar molecule helped. But it wasn’t enough. Tests in monkeys showed that other forms of organ rejection still damaged the pig tissue, albeit more slowly. To combat these effects, Ayares’s team has made pigs with more and more human genes. For instance, one gene that’s been added produces the human version of thrombomodulin, a molecule that prevents clotting in blood vessels. Although pigs have their own version of thrombomodulin, it’s the wrong shape and doesn’t work correctly with human blood.
“We are adding the human genes to the pig so you have the organ repressing the immune response, rather than have to give a whopping dose of immune suppressants,” says Ayares. By next year, some of the pigs will have as many as eight added human genes. These genetic changes make their organs more compatible with a human body, but the animals still look and act like normal pigs.
Genetically engineering the pigs isn’t easy. It’s challenging to insert human genes and difficult to get them to function correctly. “You try to put all your genes into one parcel so they go to one place in the genome,” says Bruno Reichart, a professor at the University of Munich, who leads a German consortium developing transgenic pigs. “It’s very cumbersome. Creating a good pig is really like winning the lottery.”
In the United States, leading transplant surgeons have been meeting with Revivicor every few months to plan what genes they’d like to see added next. Since last year, some of the genetic engineering has been carried out in collaboration with Synthetic Genomics, a California company started by DNA sequencing entrepreneur J. Craig Venter. Rothblatt invested $50 million in Venter’s company in 2014, and it has begun designing and building genetic add-ons and inserting them into pig cells. It is left to Revivicor to produce piglets from these engineered cells, using cloning.
Some people involved in the project are more circumspect than Rothblatt about how fast it can succeed. “Every time you relieve one rejection issue, another one comes in behind. You peel back one layer and there is another layer underneath,” says Sean Stevens, who runs the mammalian synthetic biology program for Synthetic Genomics. “No one is so naïve as to think, ‘Oh, we know all the genes—let’s put them in and we are done.’ It’s an iterative process, and no one that I know can say whether we will do two, or five, or 100 iterations.”
Yet surgeons credit the genetically enhanced pigs with some recent successes. Muhammad Mohiuddin, a transplant surgeon and researcher at the National Heart, Lung, and Blood Institute, in Bethesda, Maryland, says a heart from one of Revivicor’s pigs lasted two and a half years inside a baboon. This milestone, reached last month, surpassed a previous record of 179 days, achieved by Massachusetts General Hospital. Also this summer, transplant experts at the University of Pittsburgh said they’d kept a baboon alive with one of Revivicor’s pig kidneys for more than four months. That set a record for the longest “life-sustaining” xenotransplant between a pig and a primate.
The heart transplants were not life-sustaining but “heterotopic”—the pig heart was attached to the baboon’s circulatory system and was able to beat, but it didn’t have to do the work of pumping blood, since the baboon’s own heart remained in place. Mohiuddin says the pig heart gave out only when he decided to stop giving the baboon the novel immune-blocking drugs he had used. “We believe it could have gone on forever,” he says. “I would say 60 percent of the improvement was due to the organ, and 40 percent due to better drugs.”
Reichart calls the survival of these pig hearts “a major breakthrough.” He says, “It gives us all hope that cardiac xenotransplantation works. These hearts remained normal—it’s amazing.” However, he doesn’t think anyone should be forecasting when a transplant into humans could occur. That is because surgeons still need to completely replace a baboon’s heart with one from these pigs and show it keeps the animal alive. “It wouldn’t be serious to give a time line for use in humans,” he says.
Mohiuddin says he’ll soon begin trying to replace baboon hearts entirely. The organs he used before had three genetic alterations, but the next ones will have seven. “If they survive, then we can consider clinical trials,” he says. The first human recipients would be expected to be special cases, like someone who needs an organ as a “bridge” until a human donor becomes available.
Lung transplants will be harder, since lungs are permeated with blood vessels and heavily exposed to the immune system. So far, transplants last only a matter of days, says Rothblatt. She has been financing research at the University of Maryland, where pig lungs are being perfused with human blood in the laboratory as a way of measuring the immune response. “She wants genetically modified lungs for personal reasons, due to personal grief,” says Reichart. “I think that is a great thing, but lungs are very difficult.”
Transplant surgeons say one of the largest obstacles they face is the immense cost of carrying out xenotransplant experiments. A single transplant surgery costs $100,000 and involves eight people. Then there’s the cost of keeping the primates, the red tape of animal regulations, and limited government grants. That’s where Rothblatt’s personal interest and her fortune have made a difference, they say. “She is the one that has rejuvenated the field,” says Mohiuddin. “She has the money and a personal attachment. She wants to get it done fast.”
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