to human transplants
The possibility of pig to human organ transplants took a leap forward on Christmas Day when a litter of genetically modified pigs was born in Virginia. Simon Jeffery explains
Thursday January 3, 2002
What is special about the pigs?
The five cloned piglets - Noel, Angel, Star, Joy and Mary - have been genetically modified so humans will not reject their internal organs. This opens up the possibility of pig to human transplants, which may save the lives of many seriously ill people.
Can you give pig organs to humans?
The animals are sometimes called "horizontal humans". Although they are more distantly related to us than, for example, the great apes - pigs are about the right size, and so are their organs. A 75kg pig has the same-sized heart as a 75kg human, with the same pumping capacity. In theory it should be possible to farm pigs for their organs, much as we now farm them for bacon. But there are problems.
Our immune system is designed to attack any foreign material that enters the body. Many human to human transplants are only possible with powerful drugs that suppress the immune system and prevent it from treating the new organ or tissue as a huge infection and rejecting it. Doctors try to match donors to recipients to keep rejection to a minimum, but the problems are greater with pigs. If an unmodified pig heart were given to a human, the reaction would be so violent that the heart would turn black in 15 minutes and be virtually destroyed in 30.
How do the genetically modified piglets get round this?
They are an important step towards "knock-out" pigs, meaning animals where the specific gene that causes the human immune system to reject pig organs is made inactive - or knocked out. The all-female litter of five lacks one of its two alpha gal genes (which put a sugar on pig cells to which the human immune system reacts aggressively). When an all-male litter is born next month (also lacking one of its two alpha gal genes) it will be possible to mate the two animals. A new breed of pig will be created where every fourth offspring will have no alpha gal gene at all, making it a true "knock out" and an ideal source of spare parts for humans.
Is that all?
No. David Ayares, vice president of research for PPL Therapeutics (the company that helped clone Dolly the Sheep), says it is only the first step and speaks of a need for further modifications before the company can create the "ultimate pig". It will take until at least 2005 to figure out how to deal with adverse immune reactions and conduct trials with primates before human clinical trials can begin. There will also be a need to ensure that pig diseases do not cross to humans, and to establish whether a heart that will serve a pig for its 30-year life span will last longer in humans.
Are pig to human transplants necessary?
There are currently more than 5,500 people in Britain waiting for a transplant of a kidney, pancreas, heart, lungs, liver, or a combination of these organs. On average, adults have to wait more than 500 days for a new kidney. There is a clear demand. Efforts are being made to increase the donation of human organs - the supply is still not high enough, though some argue that pig to human transplants would be unnecessary if the taking of healthy organs from the dead was mandatory.
Would people accept pig organs?
There may be a degree
of revulsion at killing an animal to save a human, but some could feel
happier carrying the organ of a dead pig than a dead human. Pigs are already
bred and killed for food, but some vegetarians and vegans might feel uneasy
about making such use of an animal. People who follow Jewish and Muslim
eating codes forbidding the consumption of pork may also have objections.