Transient telomerase inhibition alters cell cycle kinetics

Telomerase is the ribonucleoprotein reverse transcriptase that catalyzes the synthesis of telomeres at the ends of linear human chromosomes and contributes to proper chromosomal capping function. Formation of the telomere-loop (T-loop), an obligate step before cell division can proceed, requires the generation of a 3′-overhang on the G-rich strand of telomeric DNA via telomerase or C-strand specific nucleases. Here, we discover telomerase activity is critical for efficient cell cycle progression using transient chemical inhibition by the telomerase inhibitor imetelstat. Telomerase inhibition caused changes in cell cycle kinetics and increased the proportion of cells in G2 phase, suggesting delayed clearance through this checkpoint. Investigating the possible contribution of unstructured telomere ends to these cell cycle distribution changes, we observed that imetelstat treatment induced γH2AX DNA damage foci in a subset of telomerase-positive cells but not telomerase-negative primary human fibroblasts. Chromatin-immunoprecipitation with γH2AX antibodies demonstrated imetelstat treatment-dependent enrichment of this DNA damage marker at telomeres. Notably, the effects of telomerase inhibition on cell cycle profile alterations were abrogated by pharmacological inhibition of the DNA-damage-repair transducer ATM. Additionally, imetelstat potentiation of etoposide, a DNA-damaging drug that acts preferentially during S/G2 phases of the cell cycle, also depended on functional ATM signaling. Our results suggest that telomerase inhibition delays the kinetics of T-loop formation in telomerase-positive cancer cells, resulting in the engagement of an ATM-dependent DNA-damage signal that prevents cell cycle progression. This demonstrates for the first time that telomerase activity directly modulates the progression of the cell cycle through facilitation of T-loop formation.