Pig-To-Human Transplants Worry Some
By Daniel Q. Haney
AP Medical Editor
Tuesday, February 19, 2002; 8:41 AM
BOSTON -- Transplanting genetically modified hearts and other organs
from pigs to people could be possible in five to seven years, but many
scientific and ethical questions remain, scientists said Sunday.
Researchers are changing pigs' genes to "humanize" their organs, making
them more like people's so they will serve as alternatives to human
cadavers for transplanted organs.
The scientists described the progress toward animal sources - an
approach called xenotransplantation - at a conference in Boston
sponsored by the American Association for the Advancement of Science.
Sunday's meeting followed an important milestone last month. Two
companies said they have produced litters of cloned miniature pigs that
lack one copy of a gene that makes pig parts incompatible with human
immune defenses.
"I strongly believe it will be a major step forward, but it won't be the
only problem to be solved," said Dr. David Cooper of Massachusetts
General Hospital.
Both Cooper and Dr. Fritz H. Bach, another transplant scientist at
Boston's Beth Israel Deaconess Medical Center, predicted that the
transplants could take place in the next five to seven years, if these
problems can be figured out.
In January, Immerge BioTherapeutics and PPL Therapeutics both said they
had created pigs that lack one of the two usual copies of a gene that
makes a sugar called alpha-1-galactose.
This sugar lines pig blood vessels. Because it is nearly identical to a
bacterial sugar, the human immune system attacks it. As a result, pig
organs transplanted into people are destroyed almost instantly.
Immerge's pigs were born in September and October and PPL's in December.
The next step will be to breed these pigs with each other. Cooper said
efforts to do this should begin within two months as the oldest animals
reach sexual maturity.
There is a 1-in-4 chance that each offspring will be born with no copies
of the gene. No one knows for sure whether pigs can survive without
alpha-1-galactose. But if they can, the animals could be raised to
supply hearts and kidneys for human transplants.
Scientists will have to test the process first by transplanting the pig
organs into other animals, such as baboons. Another worry is whether the
organs will carry pig viruses that could be harmful to people,
especially if they spread from the organ recipient to others.
Bach said people should debate whether the risk, however remote, is
worth taking.
"We live in a world of risk, and this is an additional risk," said Bach.
"If the medical establishment imposes this on the public and there is a
potential risk, I feel ethically we have to go to the public."
However, Cooper said the decision should be left to regulatory agencies,
which have the expertise to wade through the highly technical arguments
on both sides.
All pig cells carry a so-called retrovirus that is harmless to them, but
no one knows what it might do to humans. Scientists are working with a
strain of pigs that cannot spread this virus to human cells. However,
Bach said he is concerned about possible pig viruses that have not been
discovered yet.
Cooper responded that scientists can only guard against viruses and
other hazards that have been identified. "You deal with all the known
problems, but if you worry about unknown problems you will never make
any success in any branch of science."
According to the United Network for Organ Sharing, about 77,000
Americans are in line for transplants each year, while 23,000 actually
receive them. Demand is growing five times faster than the supply, and
many who could benefit never even make the waiting list, because they
have other health problems.
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Medical Editor Daniel Q. Haney is a special correspondent for The
Associated Press.