Determinants of the telomerase protein component required for telomere maintenance, immortalization, protein stability and cancer

Telomerase maintains cell viability and chromosomal stability through the addition of telomere repeats to chromosome ends. Reactivation of telomerase through upregulation of TERT, the telomerase protein subunit, is an important step during cancer development, yet TERT protein function and regulation remain incompletely understood. Despite close sequence similarity to human TERT (hTERT), we find that mouse TERT (mTERT) does not immortalize primary human fibroblasts. Here we exploit these differences in activity to understand TERT protein function by creating chimeric mouse-human TERT proteins. Through analysis of these chimeric TERT proteins, we find that sequences in the human carboxy-terminal domain are critical for telomere maintenance in human fibroblasts. Substitution of the human carboxy-terminal sequences into the mouse TERT protein is sufficient to confer immortalization and maintenance of telomere length and function. We also identify a subset of these human sequences that are required for tumorigenesis but not immortalization, identifying the first TERT mutant that uncouples telomerase's role in immortalization and transformation. Strikingly, we also find that the c-terminal region regulates protein accumulation, with hTERT accumulating to markedly higher levels than mTERT. This disparity in steady-state protein levels correlates with the determined inherent stability of the mouse and human proteins. Thus, the TERT carboxy-terminus contains sequences that allow tumorigenesis, regulate TERT protein levels and determinants required for telomere maintenance.