ATM promotes apoptosis and suppresses tumorigenesis in response to Myc
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Precancerous lesions from a variety of human tissues display markers of DNA damage suggesting that genetic instability occurs early during the process of carcinogenesis. Consistent with this, several oncogenes can activate ATM and other components of the DNA damage response pathway when expressed in cultured cells. Here we demonstrate that preneoplastic epithelial tissues from four different transgenic mouse models expressing the oncogenes c-myc, SV40 T antigen, human papilloma virus (HPV) E7, or E2F3a display [gamma]-H2AX foci and other markers of DNA damage. Moreover, transgenic expression of these oncogenes leads to increased levels of damaged DNA as measured by the comet assay. In at least the Myc transgenic model, the formation of [gamma]-H2AX foci is dependent on functional ATM. Inactivation of Atm also impairs p53 activation and reduces the level of apoptosis observed in transgenic tissue overexpressing Myc. This correlates with accelerated tumor development in Myc transgenic mice lacking ATM. To understand the mechanism by which oncogenes induce DNA damage, we employed an adenoviral overexpression system. Under conditions in which Myc or E2F3a induced replication is inhibited, we see a reduction in the DNA damage induced by these oncogenes both by comet assay and levels of [gamma]-H2AX. Moreover, Myc and E2F3a induced increased levels of the Cdt1 protein, a replication origin- licensing factor implicated in aberrant DNA replication. Taken together, these findings suggest that deregulated oncogenes induce unscheduled DNA replication leading to DNA damage and activation of the ATM DNA damage response pathway, which is important for the activation of p53, induction of apoptosis and the suppression of tumorigenesis.