Telomeres and telomerase in human leukemias.

There is increasing evidence supporting the hypothesis that telomere shortening both in vitro and in vivo, is the clock that counts cell divisions and determines the onset of cellular senescence. Cells that overcome the normal senescence mechanisms do so by stabilizing telomere length, probably due to the activity of telomerase, a ribonucleoprotein enzyme that synthesizes telomeric repeats. Most human primary tumors contain telomerase, while the cells of most normal tissues lack this activity. A hypothesis gaining prominence is that the activation of telomerase is necessary for the sustained growth of most solid tumors. Since normal hematopoietic stem cells and some of their progeny already express telomerase activity, it is important to consider whether or not telomere shortening and telomerase activity play any role in cancer progression in various forms of leukemia. This review includes a discussion of the utility of telomere length and/or telomerase activity measurements in the diagnosis and prognosis of leukemia as well as the potential value of antitelomerase therapy for the leukemias.