The ATM–Chk2–Cdc25A checkpoint pathway guards against radioresistant DNA synthesis

When exposed to ionizing radiation (IR), eukaryotic cells activate checkpoint pathways to delay the progression of the cell cycle 1 , 2 , 3 . Defects in the IR-induced S-phase checkpoint cause ‘radioresistant DNA synthesis’, a phenomenon that has been identified in cancer-prone patients suffering from ataxia-telangiectasia, a disease caused by mutations in the ATM gene 4 , 5 , 6 . The Cdc25A phosphatase 7 activates the cyclin-dependent kinase 2 (Cdk2) needed for DNA synthesis 8 , 9 , but becomes degraded in response to DNA damage 10 or stalled replication 11 . Here we report a functional link between ATM, the checkpoint signalling kinase Chk2/Cds1 (Chk2) 12 and Cdc25A, and implicate this mechanism in controlling the S-phase checkpoint. We show that IR-induced destruction of Cdc25A requires both ATM and the Chk2-mediated phosphorylation of Cdc25A on serine 123. An IR-induced loss of Cdc25A protein prevents dephosphorylation of Cdk2 and leads to a transient blockade of DNA replication. We also show that tumour-associated Chk2 alleles 13 cannot bind or phosphorylate Cdc25A, and that cells expressing these Chk2 alleles, elevated Cdc25A or a Cdk2 mutant unable to undergo inhibitory phosphorylation (Cdk2AF) fail to inhibit DNA synthesis when irradiated. These results support Chk2 as a candidate tumour suppressor, and identify the ATM–Chk2–Cdc25A–Cdk2 pathway as a genomic integrity checkpoint that prevents radioresistant DNA synthesis.