Activation of ATM and Chk2 kinases in relation to the amount of DNA strand breaks

The diverse checkpoint responses to DNA damage may reflect differential sensitivities by molecular components of the damage-signalling network to the type and amount of lesions. Here, we determined the kinetics of activation of the checkpoint kinases ATM and Chk2 ( the latter substrate of ATM) in relation to the initial yield of genomic DNA single-strand (SSBs) and double-strand breaks (DSBs). We show that doses of gamma-radiation (IR) as low as 0.25 Gy, which generate vast numbers of SSBs but only a few DSBs per cell (19 DSBs per cell (e.g. 1 Gy), which cause Chk2 autophosphorylation on Thr387, Chk2-dependent accumulation of p21(waf1) and checkpoint arrest in the S phase. Our results indicate that, in contrast to ATM, Chk2 activity is triggered by a greater number of DSBs, implying that, below a certain threshold level of lesions (