Dissociation among in vitro telomerase activity, telomere maintenance, andcellular immortalization

The immortalization of human cells is a critical step during tumorigenesis. In vitro, normal human somatic cells must overcome two proliferative block ades, senescence and crisis, to become immortal. Transformation with viral oncogenes extends the life span of human cells beyond senescence. Such tran sformed cells eventually succumb to crisis, a period of widespread cellular death that has been proposed to be the result of telomeric shortening. We now show that ectopic expression of the telomerase catalytic subunit (human telomerase reverse transcriptase or hTERT) and subsequent activation of te lomerase can allow postsenescent cells to proliferate beyond crisis, the la st known proliferative blockade to cellular immortality. Moreover, we demon strate that alteration of the carboxyl terminus of human telomerase reverse transcriptase does not affect telomerase enzymatic activity but impedes th e ability of this enzyme to maintain telomeres. Telomerase-positive cells e xpressing this mutant enzyme fail to undergo immortalization, further tight ening the connection between telomere maintenance and immortalization.