Mice, as it happens, have very long telomeres, and the first two generations of mice that lacked the telomerase template gene were unaffected by its lack.
DePinho and his colleagues have now bred third- to sixth-generation mice, with progressively shorter telomeres, and in these mice, especially as they age, short telomeres are a problem.
The third-generation mice developed gray hair, male pattern baldness and erosive skin lesions considerably more often than normal mice of the same age. The sixth generation mice that lacked telomerase displayed these symptoms at an earlier age, going gray at the age of 3 months.
On the other hand, the hearts, livers and kidneys of the mice were normal, and they did not develop thinning of the bones or cataracts.
As to life span, the first to fifth generation mice that lacked telomerase lived as long as normal mice, 50 percent of them living for 24 months. Only in the sixth generation mice, with critically short telomeres, was life span affected; half of these mice were dead at 18 months.
An obvious pattern in these results is that the tissues that demand a steady proliferation of cells -- like skin and hair -- or that need to regenerate under stresses -- like a wound healing -- are the ones that are jeopardized by short telomeres.
A second important finding is that the mice with short telomeres have a higher incidence of cancer, suggesting that short telomeres can destabilize the cell and initiate tumors.
The question now is what relevance do these findings have to aging and cancer in humans. On this point there is a wide range of opinion.
