Telomeres and telomerase in stem cells during aging and disease.

Cell cycle checkpoints induced by telomere dysfunction represent one of the major in vivo tumor suppressor mechanisms preventing cancer but at the same time provoking age dependent decline in self-renewal and regeneration of tissues and organs. On the other hand, telomere shortening contributes to the initiation of cancer by inducing chromosomal instability. Telomere function and telomerase activity are mainly associated with actively proliferating cells. Since stem cells are continuously proliferating throughout lifetime, it is of great interest to explore the role of telomeres and telomerase in stem cells. Although most stem cell compartments express telomerase, the level of telomerase activity is not sufficient to maintain telomere length of stem cells during aging. Stem cells appear to have tighter DNAdamage checkpoint control in comparison to somatic cells, which may reflect the need to protect this long lasting cell compartment against malignant transformation. These enhanced checkpoint responses may have a detrimental impact on stem cell function, by causing increased sensitivity towards senescence or apoptosis induced by telomere shortening. This review summarizes our knowledge on telomere dynamics and its functional impact on stem cells during aging and transformation.