A DIRECT link has at last been found between an animal鈥檚 lifespan and the size of DNA sequences called telomeres in their chromosomes. But contrary to expectation, it鈥檚 the rate at which telomeres shorten that matters, rather than their length. The finding is important because telomeres play a key role in cancer and ageing.
Telomeres are repetitive DNA sequences found at the ends of every chromosome. They normally shorten each time a cell divides. When they become too short the cell stops dividing and eventually becomes moribund. So telomere length sets an upper limit on the number of divisions a cell can undergo.
But while this link to cell lifespan and ageing is well established, researchers have had great difficulty linking telomere length directly with an organism鈥檚 lifespan. Now new research suggests they were measuring the wrong thing.
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Mark Haussmann and Carol Vleck at Iowa State University in Ames collected blood samples from birds of known ages and recorded the telomere lengths in DNA from the birds鈥� red blood cells. They looked at five species, each with a different lifespan, and calculated the number of base pairs lost by the telomeres each year.
The researchers found the rate of telomere change per year, rather than overall telomere length, is an excellent predictor of maximum lifespan (see Graph). And in one long-lived species, the Leach鈥檚 storm-petrel, telomere length increased with age, the first time this has been observed, they will report in a future issue of Proceedings of the Royal Society B.
The relationship holds true in mammals, as the researchers found when they analysed data from the scientific literature on eight mammal species. 鈥淭he idea that you come up with the same slope is rather remarkable,鈥� says Bret Weinstein at the University of Michigan, Ann Arbor, who studies the links between telomeres, cancer and ageing.
Weinstein believes the results are evidence of a fundamental trade-off faced by mammals and birds. For if simply adding to telomeres extends lifespan, natural selection should have tended to lengthen them, especially as the enzyme telomerase, which rebuilds telomeres, is found in many different animals.
But other studies have linked telomerase activity to cancer. To continue dividing beyond their normal limits, cancer cells have to switch on the enzyme to prevent telomeres getting any shorter.
Setting a limit to the number of cell divisions, and switching off telomerase, are probably important anti-cancer mechanisms, says Weinstein. Creatures may make a different trade-off depending on their specific environment and ecology.