Telomerase activation during the linear evolution of human fibroblasts to tumorigenicity in nude mice.

The ribonucleoprotein enzyme telomerase is active in most immortal cell lines, most tumors and all tumor-derived cell lines. The enzyme is important because it prevents continual shortening of telomeres and therefore plays a significant role in chromosome maintenance. In man, telomerase is not active in most somal cells with finite lifespans. Using the SV40 T antigen we immortalized and transformed to fully tumorigenic a human fibroblast cell strain. We wished to determine when telomerase was activated during this progression to tumorigenicity. Using the PCR-based TRAP assay we found that eight of eight immortal cell lines that were either not tumorigenic or rarely formed tumors were telomerase positive at the time of inoculation. Additionally, 10 of 11 newly immortal cell lines contained telomerase activity within the first 25-33 population doublings after crisis. None of the precrisis cells from which these immortal cells were derived were positive for telomerase activity. Thus we found that telomerase activation is not the final in vivo step in the transformation of these cells and the window of activation is usually near the escape from crisis or M2. These results strengthen the hypothesis that telomerase activation may allow the rare cell to escape from crisis in those immortal cell populations dependent on telomerase for telomere maintenance.