In another refutation of the notion that cloned animals age prematurely, researchers report that mice cloned from clones--for up to six generations--show no indication of early aging. The findings, in the 21 September issue of Nature, are consistent with results reported for cloned cows earlier this year (Science, 28 April, p. 586).
After Scottish researchers cloned the sheep Dolly 3 years ago, scientists discovered that the caps on the ends of her chromosomes--her telomeres--were shorter than usual. Telomere length is one gauge of a cell's age, and some researchers speculated that the process of injecting DNA from an adult cell into an egg and prodding it to develop would create organisms with tissues and organs that wouldn't last long enough for use in humans.
To test how "old" mice appear after generations of cloning, a group of researchers led by Teruhiko Wakayama of the Rockefeller University in New York City cloned successive generations of mice and measured their telomere lengths. By taking DNA from an already-cloned adult animal to make another clone, the researchers thought the technique would cause any age-related changes to accumulate. However, instead of finding signs of aging in the telomeres of each successive generation, Wakayama and colleagues found just the opposite. The telomeres of the cloned mice appeared to get slightly longer with each generation, and co-author Tony Perry of the Rockefeller University doesn't understand why. "I think it's important to make this report," he says, "but I think it's premature to speculate on an apparent phenomenon."
Other cloning researchers praised the group's results. "We've shown [premature aging doesn't occur] in bovines; they've shown it in mice. It's terrific," says Jose Cibelli of Advanced Cell Technology in Worcester, Massachusetts. But some expressed caution about reading too much into the lengthened telomeres in later generations. Because the researchers disregarded the age of the mice when comparing the lengths of their telomeres, they couldn't accurately compare different generations, says Xiangzhong Yang of the University of Connecticut, Storrs.
Cloning site from the Pharmaceutical Research and Manufacturers of America