By Matthew Willett
For now the old wives are safe in their saying: You still can't make a silk purse out of a sow's ear. Scottish scientists, however, are working on it, if you count a transplantable organ from a porcine donor as a proverbial silk purse.
The five pigs whose birth PPL Therapeutics plc announced Wednesday won't need to guard their ears, but the next generation might.
Privately held PPL's announcement of the successful cloning of five transgenic pigs that carry a marker gene, pigs just one generation removed from pigs that bear transplantable organs, marked another step in livestock cloning.
PPL said the production of genetically altered piglets means proof of principle for the Scottish scientists who created Dolly, the world's first cloned ewe, further promising that a genetically altered "knockout pig," one that lacks an organ rejection-triggering gene, can be produced.
Molecular embryologist Alan Colman, at PPL in Roslin, Scotland, is senior vice president of research at the multinational company's facilities in Virginia and New Zealand. He said xenotransplant-feasible pigs are near.
"The work shows that genetic manipulation of cells is still compatible with the birth of healthy animals," Colman told BioWorld Today. "I liken this to the penultimate station in a long journey in which the final destination is the birth of a pig with a gene knocked out. We announced some time ago that we'd knocked out the gene in cultured cells."
PPL scientists first cloned pigs last August using a "double nuclear transfer" cloning method that puts a somatic cell nucleus into an unfertilized egg, simulates fertilization activation and replaces the nucleus, which considers itself an embryonic germ line nucleus, into a normally fertilized embryo from which all genetic information has been removed.
Producing genetically altered pigs with no rejection-producing genes is the next step. The gene in question, the a-1,3-galactosyl transferase gene, produces the organ surface compound that sounds the alarm in humans that an organ is foreign, triggering near-instant rejection.
"This puts a galactose sugar on the surface of pig organs," Colman said. "Humans have the gene, but it doesn't work in humans; there's a natural gene inactivation already. We do have lots of antibodies in the blood that recognize this sugar, so if you expose a pig organ or any organ to human blood there will be a rapid binding of that antibody that triggers hyperactue rejection. It takes only about 20 minutes."
The trick, Colman said, was genetically altering the porcine genome in a way that would still produce healthy organisms. As a proof-of-principle trial the scientists inserted a gene for the production of jellyfish green fluorescent protein.
"The next stage is that if we want to tinker with the cells, the question is would the genetic engineering we're doing ruin the chances of the cells providing an animal by the end of the day," he said.
Should the scientists produce genetically altered knockout pigs, pigs without genes to express the a-1,3-galactosyl protein, he said, the resulting pigs, living organ factories, could meet the clinical needs of potential transplant recipients.
"The aim is to provide organs for transplantation to humans and deal with unmet clinical needs," Colman said. "The reasons we chose pigs are, firstly, that they're physiologically closely matched to humans, secondly that pigs have large litters, and thirdl, that it's easy to keep pigs in a clean, pathogen-free facility, and that's much more difficult with wool or hair-bearing animals."
Colman said work to clone the knockout pigs is under way, but that he couldn't estimate a time frame for completion of the project.
In unrelated news, PPL withdrew its planned #45 million (US$64.6 million) stock offer.
PPL has funding to last it less than one year. CEO Ron James said a better plan appears to be to raising money in two rounds instead of one.