Abstract
Ionizing radiation stimulates auto-phosphorylation of ATM protein at serine 1981, and phosphorylated ATM forms discrete foci. We found, in normal human diploid cells, that the initial phosphorylated ATM foci grew rapidly concurrently with DNA repair, and the foci were colocalized with the foci of phosphorylated histone H2AX, 53BP1 and NBS1. While most of the initial foci disappeared gradually after X-irradiation, there were persistent foci, whose size reached approximately 1 micron. To know biological significance of the grown foci we treated exposed cells with wortmannin, and found that number of the initial and residual phosphorylated ATM foci decreased similarly. Moreover, the effect was suppressed by calyculin A, which is a well known inhibitor of the protein phosphatase 1 and 2A. This suggests that altered chromatin structure persists if DNA broken ends are rejoined. Therefore, we next examined phosphorylated ATM foci on metaphase chromosomes isolated from X-irradiated CHO cells. We found that more than 60% of the foci were on chromosomes without detectable breaks. These results indicate that growing and persistent phosphorylated ATM foci were colocalized with damaged chromatin domains when the broken ends are rejoined, and they could mediate amplification of DNA damage checkpoint signals.
