Telomeres are caps of repeated DNA sequences at the ends of chromosomes. They shorten with each cell division, a part of the mechanism that ensures somatic cells can only replicate a limited number of times. Telomerase acts to lengthen telomeres, and in humans telomerase is only active in stem cells. Thus our cells exist in a two-tier system, in which only tiny populations of privileged stem cells are permitted unrestricted replication, while the vast majority of somatic cells are limited. Matters are similar across all higher animals, and this state of affairs likely evolved because it keeps cancer to a low enough level, and pushed off far enough into late life, for allow for evolutionary success.

A lot of ink has been spilled on the topic of telomere length because, statistically across large populations, average telomere length and proportion of short telomeres tends to decrease with advancing age. Given that stem cell activity declines with age, this is most likely a reflection of a lower pace of creation of new somatic cells with long telomeres. The human data is complicated by the fact that telomere length is most commonly measured in immune cells from a blood sample, and is thus a very dynamic measure influenced by the day to day reactions of the immune system. In individuals, there isn't much anyone can do with measures of telomere length, given that it is so variable over time and between people of similar health and age: it is a terrible biomarker for any practical purpose.

Further, can we actually use anything that we learn about telomere dynamics in other species? It is well known that mouse telomere dynamics and telomerase expression are quite different from that of humans. This might make us suspect that positive results from telomerase gene therapies in mice, where life span is extended and health improved, without raising the risk of cancer, may not hold up in humans. There is no particular reason why increased cancer risk through putting damaged cells back to work will be balanced in the same way by improved tissue function and improved immune function, from species to species. The research and development community will find out in the years ahead by trying telomerase gene therapies in primates and then humans.

I feel that the open access paper here adds to doubts about the value that the research community can extract from a study of telomeres and telomerase in other mammalian species, though the researchers don't present it in that way. If various short and long lived mammals can have such a range of telomere dynamics, what are we supposed to make of the data resulting from animal studies of any therapeutic approach to targeting telomeres?

Telomeres and Longevity: A Cause or an Effect?