Oncogene-induced telomere dysfunction enforces cellular senescence in human cancer precursor lesionsThis study offers a novel view on the role of telomere attrition in human tumours, providing evidence for tumour suppressor activity resulting from telomere dysfunction-induced DNA damage responses.

Oncogene-induced senescence (OIS) is a critical tumor-suppressing mechanism that restrains cancer progression at premalignant stages, in part by causing telomere dysfunction. Currently it is unknown whether this proliferative arrest presents a stable and therefore irreversible barrier to cancer progression. Here we demonstrate that cells frequently escape OIS induced by oncogenic H-Ras and B-Raf, after a prolonged period in the senescence arrested state. Cells that had escaped senescence displayed high oncogene expression levels, retained functional DNA damage responses, and acquired chromatin changes that promoted c-Myc-dependent expression of the human telomerase reverse transcriptase gene (hTERT). Telomerase was able to resolve existing telomeric DNA damage response foci and suppressed formation of new ones that were generated as a consequence of DNA replication stress and oncogenic signals. Inhibition of MAP kinase signaling, suppressing c-Myc expression, or inhibiting telomerase activity, caused telomere dysfunction and proliferative defects in cells that had escaped senescence, whereas ectopic expression of hTERT facilitated OIS escape. In human early neoplastic skin and breast tissue, hTERT expression was detected in cells that displayed features of senescence, suggesting that reactivation of telomerase expression in senescent cells is an early event during cancer progression in humans. Together, our data demonstrate that cells arrested in OIS retain the potential to escape senescence by mechanisms that involve derepression of hTERT expression.oncogene-induced senescence | telomerase | telomere | senescence escape |

Expressing oncogenes in normal somatic human cells leads to cellular senescence after just a few cell division cycles. In cells that are more resistant to culture stresses, such as human dermal fibroblasts, this oncogene-induced senescence (OIS) is a result of a DNA damage response (DDR) that is activated due to the formation of DNA lesions at both non-telomeric and telomeric DNA sequences. DNA lesions can be visualized as DDR foci by immunofluorescence microscopy using antibodies against a number of DDR factors, including ϒ-H2AX and 53BP1. Over time and as cells remain arrested in OIS, non-telomeric DDR foci progressively become resolved, while telomeric DDR foci, also called dysfunctional telomeres, persist. Here we describe a protocol to detect dysfunctional telomeres in cultured human cells, to monitor a temporal enrichment of dysfunctional telomeres in cells that had undergone OIS, and to detect dysfunctional telomeres in paraffin-embedded and formalin-fixed human tissue.