One of the problems holding back the use of other species to grow organs for transplant into humans is the presence of retroviruses in their genomes that could activate in humans and cause a devastating infection. There is even a risk that such an infection could turn out to be transmissable to other humans. This problem has so far ruled out the use of other primate species as a source of organ transplants in spite of their greater genetic similarity to humans than is the case with other types of species. However, a type of miniature pig (mini for a pig still means 250 pounds when fully grown) that has been found to not have viable retroviruses. In particular it doesn't have viable Porcine Endogenous Retrovirus or PERV. For this reason this pig breed has attracted the attention of researchers who want to develop pigs as a source of replacement organs.
One obstacle to the use of pigs is an enzyme that pigs have called a-1,3-galactosyltransferase or GGTA1. GGTA1 puts a sugar of surface of cell membrane proteins that causes human immune systems to recognize those proteins as foreign and to vigorously and rapidly attack them. Some scientists have recently created a pig that lacks this problematic enzyme.
AUCKLAND, NZ, January 13, 2003 -- In a session today at the annual meeting of the International Embryo Transfer Society (IETS), Randall Prather, Ph.D., Distinguished Professor of Reproductive Biotechnology at the University of Missouri-Columbia, announced the successful cloning of the first miniature swine with both copies of a specific gene "knocked out" of its DNA. The ultimate goal of this research, which is being conducted in partnership with Immerge BioTherapeutics, Inc (a BioTransplant Incorporated (Nasdaq:BTRN)/Novartis Pharma AG (NYSE:NYS) joint venture company), is to develop a herd of miniature swine that can be used as a safe source for human transplantation, a process known as xenotransplantation.
"The fact that we have been able to clone this particular strain of miniature swine with both copies of the gene that produces GGTA1 knocked out is a very exciting step for the field of xenotransplantation," said Dr. Prather, a researcher in MU's College of Agriculture, Food and Natural Resources. "Organs from regular swine are too large for human transplant, and this particular strain of miniature swine has been refined for years solely for its potential use in humans."
New options for organ sources are desperately needed to treat the rapidly increasing number of critically ill people on the transplant waiting list (more than 80,000 in the U.S. alone). Researchers have targeted the pig as the best potential candidate for an alternative organ source because of the similarity between human and pig organs and the relative ease of breeding. However, the massive rejection response mounted by the human immune system has been a major hurdle in this research.
A key player in this rejection process is the gene called a-1,3-galactosyltransferase or GGTA1 that produces a sugar molecule. When a foreign organ is introduced, human antibodies attach to the sugar molecule on the surface of pig cells produced from the action of the GGTA1 molecule, thus killing the organ. With both copies of this gene eliminated, the antibodies cannot attach, halting the early rejection process.
Dr. Robert Hawley and scientists at Immerge, in collaboration with Dr. Kenth Gustafsson, first identified the gene that produces GGTA1 and eliminated, or knocked it out, of the DNA of the cells from the miniature swine. This genetic material was then sent to Dr. Prather's lab, where Dr. Liangxue Lai and colleagues implanted it into an egg that had its DNA eliminated. The egg was stimulated to begin dividing and was later implanted into a sow. Prather and Immerge announced in January 2002 in the journal Science that they had successfully cloned the world's first single knock-out miniature swine. The genetic material from these swine was then re-engineered with the aim of knocking out the second copy of this critical gene. These cells were then subjected to another round of nuclear transfer cloning, leading to the birth of the double knock-out piglet on November 18, 2002.
The presence of the sugar on pig organs has provoked such a strong immune reaction in primates that it has not been possible keep pig organs alive in primates for more than a few hours. However, with the removal of this sugar it will likely be possible to test organs for other immune incompatibilities. It may well turn out that there are many other causes of immune incompatibility and it may require a series of cycles of testing, genetic modification of pig genomes, and then recloning to create pigs that are more immunologically compatible. It is difficult to say at this point how many iterations of genetic engineering modiifications, cloning, and testing will be required to make pigs that are immunologically compatible with humans. The process could take several years or even as long as a couple of decades.
Researchers said many issues must be resolved before the promise of transplanting pig organs becomes a reality. They predict it will be at least two or three years before the transplants can be tested in humans, and then only if they can show that the transplanted organs survive in primates for more than six months without requiring such severe suppression of the immune system as to pose a danger to patients.
Another approach would be to use human stem cells to develop into organs in pigs or another species. That way the resulting organ would be more likely to be immunologically compatible with a human recipient. However, then one runs into ethical problems (see the previous post on mini human kidneys grown in mice) because of the methods used to get stem cells that are in the proper genetic regulatory state to be able to become the desired type of organ. Some scientists still think the use of human stem cells will be what wins the race in the long run but others say that the use of pigs will produce useful transplantable organs before a technique utilising human stem cells does.
On the bright side the competition between different technological approaches increases the odds that at least one approach will succeed in producing transplantable organs in 10 or 15 years.
Update: A friend raises an excellent point that I've not seen raised before in discussions about xenotransplantation: xenotransplant organs from a species which has a shorter lifespan than humans (which pretty much describes all species that are candidates for use as xenotransplant organ sources) will probably not last as long as organs grown from human stem cells. Pigs in the wild have a life expectancy of about 25 years and some of their organs will be fairly aged by the time they die. This doesn't seem like a major obstacle to the use of pig organs though. Suppose pig organ transplanted into a human will last 20 years. Someone getting a pig organ transplant in 2010 would have until 2030 to come up with a replacement. By that time it seems very likely that the growth of replacement organs from human stem cells will be possible.
In the longer run the genetic variations that make organs wear out more or less quickly will become identified. DNA sequence comparisons of shorter and longer lived humans will be done once the cost of DNA sequencing drops by orders of magnitude. This will lead to the identification of all genetic variations that affect longevity. This information will be used to do gene therapy treatments to human stem cells to make organs grown from them last for much longer periods of time. Also, entirely new genetic changes will be developed to make organs last far longer than any human's organs can last naturally.
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