Atm selectively regulates distinct p53-dependent cell-cycle checkpoint and apoptotic pathways

Atm is part of a pathway that responds to DMA damage from ionizing radiation (IR). This pathway involves p53 as Atm-deficient cell lines and mice are defective in p53 induction after IR1–5. p53 is a multi-functional protein that simultaneously regulates distinct downstream pathways controlling cell-cycle progression and apoptosis6,7. However, the mechanisms by which p53 differentially activates downstream pathways are unknown. To determine the relationship between Atm and p53, we examined cell-cycle and apoptotic responses in Atm-, p53- (ref. 8) and p27-deficient9 mice after IR in the whole animal. As expected, p53 protein levels were not induced by IR in thymus of Atm-deiicient mice. IR-induced cell-cycle checkpoint function was also defective, and induction of p21 was attenuated in thymus from >U/7?-deficient mice. However, IR-induced apoptosis and Bax induction were completely normal; both of which are mediated by p53. IR-induced thymic apoptosis was suppressed in Atm/p53 double-mutant mice but not in Atm/p21 double mutants, demonstrating p53 dependence and Atm independence. Thus, Atm deficiency results in lack of p53 induction by IR, but only selective disruption of p53-dependent functions. Our results support a model in which upstream effectors such as Atm selectively activate p53 to regulate specific downstream pathways, providing a mechanism for controlling distinct cell-cycle and apoptotic responses.