The revelation that Dolly the cloned sheep has developed arthritis represents bad news for those promoting reproductive cloning. If Dolly's apparent premature ageing is linked to her status as a clone, it would prove that there are hidden dangers in this technology and be a fundamental setback for advocates of xenotransplantation, writes Dick Ahlstrom, Science Editor
Dolly, now 5½ years old, has developed arthritis in her left hind-quarters. Although sheep of her age can develop the disease, it is not the norm.
"We are very disappointed and we will have to keep a careful eye on her," Dolly's creator, Prof Ian Wilmut, of the Edinburgh-based Roslin Institute, said yesterday.
The possibility that Dolly might age more quickly than her non-clone peers first arose in May 1999. As the most famous sheep ever born, she was the subject of intensive study by scientists at the Roslin Institute. They discovered that Dolly's DNA looked more like that of an older animal, possibly as a result of her method of conception. Her fame lies in the fact that she was produced using genetic material taken from a six-year-old ewe. She started life with genes which were already aged and yesterday's announcement may be an indication that she is older than her years suggest.
The 1999 discovery focused on a component of DNA known as a telomere. Found at the ends of chromosomes, telomeres can be likened to the small plastic tabs at the ends of shoelaces. Telomeres help to keep the DNA ends from unravelling. They are at maximum length at birth, but shorten with age. Dolly's telomeres at birth were like those of a six-year-old animal. The scientists were unsure what to make of this, however.
"It doesn't necessarily follow that she will age prematurely, because this link between the telomeres and ageing is not proven, it's just an interesting theory," said Dr Alan Coleman, research director at PPL Therapeutics, the associated company commercialising cloning technology.
If it is proved that Dolly has aged prematurely, this will not end the researchers' interest in cloning. Dolly was unique as the first mammal cloned from old cells, but most of the world's existing animal clones were grown from embryonic or infant cells. Short telomeres are less of an issue when young cells are used.
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