Cloning Sheep

Major development in altering sheep genes

June 28, 2000 - AP

Scientists have developed a way to alter and insert new genes in sheep with unprecedented precision, a step toward creating healthier livestock and using animals as organ banks.

This is the first time so-called gene targeting has been done in a mammal other than a mouse.

"For some of us, this was sort of the Holy Grail, the ability to achieve this kind of modification," said Alan Colman, research director at PPL Therapeutics in Edinburgh, Scotland - the same laboratory that helped produce Dolly the cloned sheep in 1996.

Colman and colleagues present their research in Thursday's issue of the journal Nature.

They altered the DNA in sheep cells, then used the cells to create sheep by cloning. Three female sheep produced last year with genetic changes are still alive.

Two of the sheep carry a human gene that makes them produce milk with a certain human protein. The third animal's DNA contains a deliberate disruption in its natural DNA code.

Livestock clones with genetic changes have been created before by simply inserting human genes into a fertilized egg. But that approach gave scientists no control over where in the animal's body the inserted gene would turn on or how active it would be. Moreover, the technique wouldn't permit making specific changes in the animals' own genes.

The new approach largely solves those problems.

It allows scientists to insert new genes or introduce changes into a specific part of sheep DNA and then create clones carrying the altered genes.

This "is a significant advance," said Randall Prather, a professor of animal science at the University of Missouri at Columbia, Mo. "It's something that people have talked about and said, `Yeah, we can do it.' But this is the first time it's been shown that it can be done."

Within a few years, the work will probably lead to sheep stripped of undesirable genes, such as one that causes scrapie, a disorder linked to mad cow disease, predicted Mario Capecchi, a professor of human genetics at the University of Utah's School of Medicine.

Favorable traits could also be added to animals to make their meat or milk tastier, he said.

PPL Therapeutics hopes to use the new gene targeting process to create genetically altered pig clones to provide organs for transplant into humans.

Genetic modification would be used on the pigs to remove a chemical red flag that alerts the human immune system to foreign tissue. That would reduce transplant patients' risk of suffering organ rejection.

Still, the new process isn't without its problems.

Among the 80 genetically altered embryos that were implanted in ewes, only 14 survived to birth. And only three of the 14 lived past six months.

Colman said the high death rate appears to be due to factors in the cloning process, not the genetic manipulation.

He said gene targeting could also let scientists mimic human diseases in a wider variety of animals to test new therapies. Mice are now widely used for such research, but human diseases often progress differently in mice than in people because of physiological differences.

An approach used since the mid-1980s to genetically alter mice hasn't worked in livestock, so PPL's scientists turned to a different technique.

They created altered copies of a stretch of sheep DNA, then inserted them into sheep cells called fibroblasts. In some of those cells, the altered DNA pieces took the place of their natural counterparts.

From those cells, scientists removed the nucleus -- the part that contains the DNA. They inserted each nucleus into a fertilized sheep egg stripped of its own nucleus. These eggs were then grown into embryos and implanted in ewes.