By Lisa Seachrist
Washington Editor
The scientific team that collaborated in bringing the world Dolly in 1997 has now cloned five pigs that could speed the progress toward using pig organs for human transplants.
PPL Therapeutics Inc. produced the pigs from the DNA of an adult pig using a similar technique to the one that created the cloned sheep named Dolly. The ability to clone pigs opens up new avenues of genetic manipulation. It could eventually result in modified pigs whose organs and cells can successfully be transplanted into humans.
¿We are very excited,¿ said David Ayares, vice president of research at PPL in Blacksburg, Va. ¿This gives us the ability to wholly transform the field of xenotransplantation. Plus, it¿s an important milestone for us as we are having discussions with development partners.¿
Should all go well with the research and financing of the project, Ayares said PPL anticipates entering human clinical trials to test modified pig organs in four years. Ayares noted there were 2,000 heart transplants last year in the U.S. He estimated there would have been 45,000 such transplants if organs were available.
The ability to clone pigs will allow the researchers to create gene ¿knockouts.¿ One of the biggest hurdles in using pigs as the source of organs for human transplant is the activity of the enzyme alpha 1-3 gal transferase, an enzyme that modifies newly created pig proteins by adding sugar molecules to the protein. Those sugars alert the human immune system that a newly transplanted organ is from a pig, not a human.
¿Knocking out alpha 1-3 gal transferase would be the big step that needs to be taken for pig organs to be available for humans,¿ said Jeffrey Platt, professor of surgery, immunology and pediatrics and director of transplant biology at the Mayo Clinic in Rochester, Minn. ¿Currently, we are pretty good at inserting new genes into these animals, but taking something away is much more difficult. The ability to clone the pig now makes that step possible.¿
Creating the cloned pigs proved much more complicated than creating Dolly. PPL researchers used somatic cell nuclear transfer to produce the pigs, but needed to make several adjustments to the procedure. Ayares said pig embryos proved very difficult to manipulate and keep viable because they are so fragile. In addition, in order for pigs to carry a pregnancy, there must be several viable fetuses since a single fetus is insufficient for the mother to maintain a pregnancy.
The pigs were born on March 5 and PPL confirmed the animals were clones with the assistance of DNA analyses by Celera-AgGEN, of Rockville, Md.
While the five piglets ¿ Christa, Alexis, Carrel, Millie and Dotcom ¿ are all clones of a normal pig, Ayares noted PPL has already created cells with the alpha 1-3 gal transferase knockout. From those cells, the company plans to create cloned pigs.
Once PPL perfects cloned knockout pigs, the company will need to address other issues that lead to organ rejection. Ayares said the company plans to up-regulate complement inhibitors in the pig organs. Complement is a series of proteins that initiate the immune attack on the transplanted organs. The company will also need to up-regulate the production of anticoagulant molecules and down-regulate the production of cell surface receptors known as V-CAM, which draw inflammatory white cells to the graft.
The company received $2 million over three years in funding from the National Institute of Standards and Technology in Gaithersburg, Md., to create the production of a knockout pig.
PPL Inc. is a subsidiary of PPL Therapeutics plc based in Edinburgh, Scotland. The company¿s shares (PTH) gained 88 pence Tuesday, or 54 percent, to close at 252 pence in trading on the London Stock Exchange.
PPL collaborated with the Roslin Institute in Edinburgh, Scotland, to produce Dolly. Roslin since has been acquired by Geron Corp., of Menlo Park, Calif.