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

Abstract

When exposed to ionizing radiation (IR), eukaryotic cells activate checkpoint pathways to delay the progression of the cell cycle1,2,3. 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 gene4,5,6. The Cdc25A phosphatase7 activates the cyclin-dependent kinase 2 (Cdk2) needed for DNA synthesis8,9, but becomes degraded in response to DNA damage10 or stalled replication11. 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 alleles13 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.

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Figure 1: Effects and regulation of IR-induced destruction of Cdc25A.
Figure 2: Failure of Chk2 mutants to bind and induce degradation of Cdc25A abrogates the S-phase checkpoint.
Figure 3: IR-induced destruction of Cdc25A requires functional ATM and Chk2.
Figure 4: Chk2 phosphorylates Cdc25A on Ser 123 and triggers its IR-induced destruction.
Figure 5: Model of the IR-induced S-phase checkpoint pathway in normal (left) versus checkpoint-deficient (right) cells.

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Acknowledgements

We thank S. Elledge, G. Evan, S. I. Reed and Y. Shiloh for providing reagents; K. Hansen for advice; and the Danish Cancer Society, the Human Frontier Science Programme, Alfred Benzon's Fund, the European Commission, the Danish Medical Research Council and the Danish Cancer Research Fund for financial support.

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Correspondence to Jiri Bartek.

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Falck, J., Mailand, N., Syljuåsen, R. et al. The ATM–Chk2–Cdc25A checkpoint pathway guards against radioresistant DNA synthesis. Nature 410, 842–847 (2001). https://doi.org/10.1038/35071124

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