When Dolly the sheep was born 20 years ago, the University of Edinburgh's famous girl was the first mammal created by way of true cloning. Her embryo was created not using the reproductive cells or stem cells of another sheep but from mature cells taken straight from the donor's mammary tissue.
Then, at age 5 — middle age, for a sheep living the good life in a research facility — Dolly developed osteoarthritis. She died at age 6, riddled with joint and lung problems reminiscent of old age. When researchers examined the length of her telomeres — structures at the end of DNA that shorten with each replication, creating cells with shorter telomeres as the body ages — they found that her biological age surpassed her chronological age; her cells looked as if they had been ticking for longer than she was alive. Some worried that this meant clones would age prematurely, carrying the same biological clock as the adult cells they had been created from. From a biomedical standpoint, this would spell disaster, with any tissues or organs created from the cells of healthy adults running on a shortened biological clock.
But according to research published Tuesday in Nature Communications, Dolly's premature aging is not likely to extend to all clones or tissues formed using adult cells. In fact, four sheep clones formed from the same cell line as Dolly seem to be in perfect health and are now pushing age 9.
"When we did the study, these clones were already 2½ years older than Dolly was when she died," lead study author Kevin Sinclair of the University of Nottingham said in an interview. "And they appeared to be perfectly healthy, but we wanted to see if they might be harboring subtle defects."
Sinclair and his colleagues did not, however, measure the telomere lengths of these Dollies (and an additional nine cloned sheep from other cell lines that they studied along with them). They'll investigate molecular aging in about a year, when the sheep — who will then have reached the truly impressive sheep age of 10, a milestone rarely seen in farm animals — are humanely put down.
But the research team undertook the most comprehensive study of physical aging ever in cloned animals (other than mice, according to Sinclair) by looking for signs of noncommunicable diseases, including obesity, hypertension and osteoarthritis. They found some signs of very early osteoarthritis in the sheep, but none was showing symptoms more than three years after their genetic identical had. Now, a year later, one sheep is starting to exhibit signs of stiffness — but given their age, Sinclair said, it would not be unusual for one sheep out of four to have arthritis. The four were completely cleared of all other health issues.
"It was quite obvious that the concerns of Dolly just didn't relate," Sinclair said. "So you can't extend beyond the Dolly experience and say this premature aging applies to all clones."
Some studies have found shorter telomere length in clones than in naturally born animals, but even more have found no difference. Based on the physical evidence, Sinclair and his colleagues expect to find normal telomere lengths when they study Dolly's clones next year, confirming that their cells have aged properly given the lifespans they've had.
So why was Dolly so decrepit? Scientists aren't positive, but it may come down to the very methods with which the cloned embryos are cultured and implanted. The enzyme that causes telomeres to elongate is abundant in a healthy, normal embryo, but a slow or inefficient implantation process might interrupt this, causing embryos to develop with "old" cells instead of ones that totally reset.
"It's a question worth going back and revisiting," Sinclair said.
But the new study suggests that it's possible to completely reprogram an adult cell, Sinclair said, which could mean there's hope for greater batches of healthy stem cells formed without embryos in the future. Pluripotent stem cells are created by reprogramming cells from living humans, forcing them back to a pseudo-embryonic state (just as Dolly's parent cells once were) where they can grow and change into cell types that suit therapeutic purposes. They can be used to provide medical treatments that sometimes seem miraculous — helping stroke patients walk again, for example — but for now, taking stem cells straight from human embryos, cloned or otherwise, is considered more efficient. The obvious ethical debate could be avoided if healthy, truly pliable cells were easier to create from adult source material, and this research suggests that such reprogramming is possible.
"That process is truly the greatest legacy of cloning," Sinclair said. "Cloning was really the inspiration for that."
While other labs around the world are working on cloning (now commonly used to produce other farm animals such as cattle, and even available for grieving dog owners who want genetic copies of their lost pets) to improve efficiency, Sinclair and his team won't be continuing Dolly's legacy for much longer.
"These are the only Dollies left," he said of the four sheep used in the study, "And they're likely to be the last Dollies."
Sometime next year, the famous sheep's copies will meet their end. But the research performed on them — now, and after their death — will continue to push biomedical science into the future for years to come.