Oncogene-induced senescence is a DNA damage response triggered by DNA hyper-replication

Abstract

Cancer is commonly thought of as uncontrolled cellular proliferation, but in the early stages of many cancers, oncogene expression is associated with cellular senescence. A possible explanation for this has now been found. Two groups report a link between oncogene-induced senescence and the DNA damage response. Activated oncogenes can cause aberrant DNA replication and thereby DNA damage that can lead to cell senescence. Cellular senescence was found previously to be a barrier to tumorigenesis in vivo, so oncogene-induced senescence may be an innate defence against cancer. But its effectiveness is often disabled by further mutations. Understanding the relationship between cell senescence and tumour formation may aid in the development of diagnostic and prognostic tools based on senescence markers. Early tumorigenesis is associated with the engagement of the DNA-damage checkpoint response (DDR)1,2. Cell proliferation and transformation induced by oncogene activation are restrained by cellular senescence3,4,5,6. 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.