Dissociation among <i>in vitro</i> telomerase activity, telomere maintenance, and cellular immortalization

TLDR

Immortalization of human cells, a key step in tumorigenesis, requires bypassing senescence and crisis—often driven by telomeric shortening—and is normally achieved by viral oncogenes. Ectopic hTERT expression permits post‑senescent cells to proliferate beyond crisis, yet a C‑terminal mutant that retains enzymatic activity but fails to maintain telomeres prevents immortalization, highlighting the essential role of telomere maintenance.

Abstract

The immortalization of human cells is a critical step during tumorigenesis. In vitro , normal human somatic cells must overcome two proliferative blockades, senescence and crisis, to become immortal. Transformation with viral oncogenes extends the life span of human cells beyond senescence. Such transformed 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 telomerase can allow postsenescent cells to proliferate beyond crisis, the last known proliferative blockade to cellular immortality. Moreover, we demonstrate that alteration of the carboxyl terminus of human telomerase reverse transcriptase does not affect telomerase enzymatic activity but impedes the ability of this enzyme to maintain telomeres. Telomerase-positive cells expressing this mutant enzyme fail to undergo immortalization, further tightening the connection between telomere maintenance and immortalization.