Telomerase is the ribonucleoprotein enzyme complex that maintains telomere length and confers immortality to 85-90% of human cancers. Telomerase recruitment to telomeres is a complex cell-cycle regulated process important for telomere length control. Understanding this process is important for the development of inhibitors of telomerase action at telomeres as potential anti-cancer therapies.
The yeast homologue of the DNA damage protein ATM is involved in telomerase recruitment1, but there is evidence in mouse cells that ATM is not involved in this process2; the potential conservation of this pathway is therefore uncertain. In human cell lines, the core telomere binding protein TRF1 is phosphorylated by ATM, leading to telomere elongation3. We therefore directly tested the hypothesis that ATM mediates human telomerase recruitment to the telomere via phosphorylation of TRF1. We employed an hTR/telomere fluorescence in situ hybridisation (FISH) assay that allows the direct visualisation of endogenous telomerase to confirm that telomerase is recruited to the telomere at mid-S phase in human cells. ATM silencing resulted in decreased telomerase recruitment across the cell cycle, while TRF1 knockdown resulted in retention of telomerase at the telomere in G2 and M phases. Co-knockdown of TRF1 partially rescued the phenotype of diminished telomerase recruitment resulting from ATM knockdown alone, demonstrating that these two proteins act in the same pathway. The kinase activity of ATM is vital in determining telomerase localisation to the telomere, and mutagenesis studies established that this is mediated by phosphorylation of TRF at amino acid S367. Agents such as aphidicolin that interfere with DNA replication cause an increase in telomerase recruitment. This study demonstrates that replicating telomeric DNA is recognised as a form of DNA damage in immortal human cells, and this sends a signal that allows telomerase action at telomeres.