Early tumorigenesis is associated with the engagement of the DNA-damage checkpoint response (DDR). Cell proliferation and transformation induced by oncogene activation are restrained by cellular senescence. It is unclear whether DDR activation and oncogene-induced senescence (OIS) are causally linked. Here we show that senescence, triggered by the expression of an activated oncogene (H-RasV12) in normal human cells, is a consequence of the activation of a robust DDR. Experimental inactivation of DDR abrogates OIS and promotes cell transformation. DDR and OIS are established after a hyper-replicative phase occurring immediately after oncogene expression. Senescent cells arrest with partly replicated DNA and with DNA replication origins having fired multiple times. In vivo DNA labelling and molecular DNA combing reveal that oncogene activation leads to augmented numbers of active replicons and to alterations in DNA replication fork progression. We also show that oncogene expression does not trigger a DDR in the absence of DNA replication. Last, we show that oncogene activation is associated with DDR activation in a mouse model in vivo. We propose that OIS results from the enforcement of a DDR triggered by oncogene-induced DNA hyper-replication.

Cells enter senescence, a state of stable proliferative arrest, in response to a variety of cellular stresses, including telomere erosion, DNA damage, and oncogenic signaling, which acts as a barrier against malignant transformation in vivo. To identify genes controlling senescence, we conducted an unbiased screen for small hairpin RNAs that extend the life span of primary human fibroblasts. Here, we report that knocking down the chemokine receptor CXCR2 (IL8RB) alleviates both replicative and oncogene-induced senescence (OIS) and diminishes the DNA-damage response. Conversely, ectopic expression of CXCR2 results in premature senescence via a p53-dependent mechanism. Cells undergoing OIS secrete multiple CXCR2-binding chemokines in a program that is regulated by the NF-kappaB and C/EBPbeta transcription factors and coordinately induce CXCR2 expression. CXCR2 upregulation is also observed in preneoplastic lesions in vivo. These results suggest that senescent cells activate a self-amplifying secretory network in which CXCR2-binding chemokines reinforce growth arrest.

The DNA damage response (DDR) arrests cell-cycle progression until damage is removed. DNA damage-induced cellular senescence is associated with persistent DDR. The molecular bases that distinguish transient from persistent DDR are unknown. Here we show that a large fraction of exogenously-induced persistent DDR markers are associated with telomeric DNA in cultured cells and mammalian tissues. In yeast, a chromosomal DNA double-strand break (DSB) next to telomeric sequences resists repair and impairs DNA ligase 4 recruitment. In mammalian cells, ectopic localization of telomeric factor TRF2 next to a DSB induces persistent DNA damage and DDR. Linear telomeric DNA, but not circular or scrambled DNA, induces a prolonged checkpoint in normal cells. In terminally-differentiated tissues of old primates, DDR markers accumulate at telomeres which are not critically short. We propose that linear genomes are not uniformly reparable and telomeric DNA tracts, if damaged, are irreparable and trigger persistent DDR and cellular senescence.

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.