Have A (Pig) Heart

In January the team that helped clone Dolly the sheep revealed another distortion in nature: five genetically modified cloned pigs.

PPL Therapeutics is betting that hearts and other organs from pigs can be transplanted into humans. About the same time, another group at the University of Missouri reported a similar cloning of swine.

About 16 Americans die every day waiting for a transplant donor. Scientists believe pigs are the most likely candidates for xenotransplantation because their organs are biologically similar to those of humans. The biggest obstacle has been rejection by the human immune system. In the new piglet clones, a gene that codes for a sugar on the surface of cells has been turned off.

“Blocking production of this sugar has been the Holy Grail of xenotransplantation, because the molecule causes a hyperacute rejection within the first two or three minutes of putting a pig organ or cell into a human or primate,” said PPL spokesperson David Ayares.

Knocking out one gene, however, doesn’t mean the body will accept a new organ. “We need to have strategies for T-cell-mediated, or chronic, rejection,” Ayares said, which usually occurs within a few days after transplantation. That will involve adding human genes to the DNA of a pig clone so its organs will look more familiar to a human immune system.

And researchers still have to worry about pig viruses that could be transplanted into humans along with the organs. PPL intends to begin trials in primates in the next six to 12 months.

Published in Discover’s Year in Science 2002

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