Primates are desired over pigs for transplants because they are more similar in many respects, however this raises the ethical question of the value of life. Primates have been proved to be 99.4% similar to humans genetically; because of this the rapidly growing xenotransplant research industry is very interested in primate transplant possibilities. Primates have been subject to experiments involving the transfer of pig hearts and other organs; these unfortunately resulted in disastrous consequences with 473 higher primates dead and a public scandal that threatened to bring down the xenotransplantation industry (S2, S3). The rate of deaths among primates was 100% and no subjects survived trails. This is because the immune system recognizes and attacks any foreign bodies including entire organs and they are completely destroyed, this is a large blockade for xenotransplants because the patient cannot easily accept organs. A large cocktail of immune system repressant drugs are required to disable the immune system enough to allow the organ to function unfortunately this leaves the body very susceptible to disease.
The science behind this resides in a complex molecule known as GAL - something which is also contained on the surface of a great many bacteria and viruses. Other animals have this, but primates like humans don't, which means that our immune systems are essentially wired to destroy GAL-containing organs in a process known as hyperacute rejection.
Because of this, it has been a major goal of scientists to find a way to genetically engineer pigs so that the GAL molecule is eliminated or replaced. This, then, would end the problem of hyperacute rejection and move us back to the problem of chronic rejection, something which already exists with human transplants and which we can handle to some degree.
That goal has very nearly been achieved. Two different companies have announced that they have produced litters of cloned miniature pigs lacking one copy of a gene that produces the GAL molecules. According to them, pigs with transplantable organs could exist within seven years. (S4)
This is an interesting thought. Pigs now exist that have been unnaturally engineered to serve as donors for humans. The ethics behind this issue are very complex. However the scientific dangers are also very large as introducing a foreign body into a human could bring about transmission and develop of new diseases. One example of a disease that jumps from animals to humans is the well-known influenza virus. Most new strains of the flu originate in the Far East where large numbers of people live in close, unsanitary conditions with pigs and chickens. In 1998, authorities in Hong Kong had thousands of chickens killed to prevent the spread of a fatal flu virus designated H5N1, which started out killing poultry but then began killing people. Fortunately, it only infected those who came into direct contact with diseased animals; but if it had been transmittable through coughs and sneezes, it might have become a pandemic killing tens of millions around the world.
Pigs raised for their organs may be protected from flu viruses, but that won't protect humans. All pig cells carry a known retrovirus known as Porcine Endogenous Retrovirus, or PERV. It is harmless to them, but no one knows what it might do to humans (although it has been shown that it can infect human cells). Retroviruses like this can incorporate themselves into a host's DNA, meaning that once a host has it, it is impossible to eliminate it entirely, and that is the case with this one. This is not something that can be eliminated in the same way GAL-producing genes have. [S5]
So what will happen if PERV is introduced into human bodies? Could it mutate into something that would directly infect humans - including humans who have not received transplants? Yes, that is possible. Could it mutate into something which is not neutral, as with pigs, but which is instead harmful? Yes, that is possible.
We just don't know what will happen. We can't. So what do we do? We could save hundreds of thousands of lives if we are able to use pig organs in xenotransplants that is pretty certain. We could kill millions the same way however. How do we weigh the risks? How do we balance the interests of those who would be saved against those who might die?
Xenotransplants for humans will rely on human genes being added to the animal in order to make the animal cells "feel" human enough to be accepted. Scientists involved always stress the tiny amount of human genetic material used but the process poses profound philosophical and religious questions. Is it right to blur the distinctives between animals and humans? Is there a limit morally that ought to be placed on the percentage of an animal that can be humanised? If so, what is it? Given monkeys and humans only differ by 3% in their genes, presumably a transfer of just 0.5% could be highly significant, perhaps enough to give a monkey speech? We do not know the answers to these questions. We now have the technology to produce a 50:50 mix of any mammals we like. Geep have already been born (combined sheep and goat). Humonkeys would be relatively simple to make. How many human genes does an animal have to have to gain human rights? And what about animal rights? One may argue that these questions are far removed from humanising a pig with a minute amount of human genetic material but the underlying issues will never go away. [S6]
Also if we are prepared to use advanced primate organs for xenotransplants why not severely disabled humans? In many respects of intelligence they are similar, what makes a brain damaged human more valuable than an ape? These questions are very hard to answer and are best sometimes not answered. Perhaps life should not be so easily given and taken?
In conclusion then the truth is that at present xenotransplants are unsafe, unreliable and pose unresolved ethical questions. These questions need urgent debate because the speed of progress is likely to change views on safety and efficacy within five years.
[S1] Xenotransplantation Action Plan FDA Approach to the Regulation of Xenotransplantation (U.S. food and drug administration website)
[S2] news website
[S3] September 22nd 2000 Daily Mail
[S4] “The genetic revolution” by Dr Patrick Dixon
[S5] information document
[S6] by Alison Plinick
