HERDS of cloned pigs may one day be supplying organs suitable for
transplantation into people. That鈥檚 the view of the British company that last
week unveiled its litter of five cloned piglets.
鈥淎n end to the chronic organ shortage is now in sight,鈥� says Ron James,
managing director of PPL Therapeutics, the company that helped produce Dolly the
sheep. But the finding is bound to add heat to the controversy surrounding such
animal-to-human 鈥渪enotransplants鈥�, which critics fear might introduce animal
viruses into humans
(快猫短视频, 4 September 1999, p 18).
Pigs are an attractive source for transplants because their organs are of a
similar size to those of humans, but normally pig organs are rejected by the
human immune system within an hour. The pig cells carry sugars on their surface
that act as red rags to human immune cells.
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Genetic engineers can create pig cells without these sugars by disrupting the
genes that encode the enzymes which process them. However, they haven鈥檛 been
able to turn those cells into whole animals. Nuclear transfer, the technique
used to produce Dolly, was a promising approach, but making the technology work
in pigs turned out to be problematic.
Sows require several viable embryos in a litter to carry a pregnancy to term.
And the electric pulse or chemicals that PPL used on other species to activate
the egg with its implanted nucleus to begin forming an embryo did not work in
pigs.
Dave Ayares of PPL says that they had to modify every step of their normal
nuclear transfer process鈥攚hich cells to use as donors, how to grow them
and how to unite egg and nucleus. Even so, the cloning efficiency was so low
that they had to put 鈥渂ucketloads of embryos鈥� into the surrogate sow to produce
five female clones.
鈥淚t鈥檚 a real step forward, very impressive,鈥� says Randall Prather, who
studies pig reproduction at the University of Missouri at Columbia. The next
step for PPL is to produce 鈥渒nockout鈥� pigs with sugar-free cells, and then add
three other genes to help combat a slower immune reaction that can take weeks to
develop.