Assessing the Involvement of Telomerase in Stem Cell Biology

Highly proliferative cells including stem cells and cancer cells express high levels of telomerase, an enzyme activity that adds a six-base DNA repeat sequence (TTAGGG) to chromosome ends and thereby prevents their shortening during successive rounds of mitosis (1 ,2 ). Telomerase activity decreases in association with cell differentiation and is generally absent from most somatic cells in the adult; shortening of telomeres in such somatic cells may trigger cell cycle arrest in the G1 phase (cellular senescence). In this way, telomere shortening effectively limits the proliferative potential of cells, functions as a tumor suppressor mechanism and may contribute to the aging process (3 –6 ). Telomerase consists of an RNA template (TR) and a protein called TERT that possesses reverse transcriptase activity. Several telomere-associated proteins have been identified including TRF1 (telomere repeat-binding factor 1) that may inhibit telomerase activity and promote telomere shortening, and TRF2 which may promote maintenance of telomeres (7 ,8 ). Data obtained during the past several years have provided evidence that telomerase can play important roles in the regulation of cell proliferation, differentiation, and survival. Examples include overexpression of hTERT can immortalize cultured fibroblasts and epithelial cells (4 ); telomerase is downregulated during muscle cell differentiation (9 ); and TERT promotes cell survival (prevents apoptosis) of developing mouse and rat brain neurons (10 –12 ).