Abstract
Recent studies indicate that IR-induced nuclear foci of phosphorylated ATM (S1981), 53BP1, and NBS1 are involved in the induction of DNA-damage checkpoint. Although most of the initial foci disappear in a few hours after irradiation, there are foci remaining long after irradiation. To examine the relationship between residual foci and G1 arrest, we analysed dimensional dynamics of the remaining foci of phosphorylated ATM, 53BP1, and NBS1 proteins. Twenty-four hours after 1 Gy of X-rays, phosphorylated ATM foci were remained in 63% of irradiated normal human diploid cells, and 32% of the foci-positive cells have enlarged foci, whose sizes were more than 1 µm in diameter. When synchronized cells at G0 were irradiated with 1 Gy of X-rays, and were released, these enlarged foci were exclusively detected in cells that did not proceed to the S phase. Interestingly, enlarged foci of phosphorylated ATM showed 100% colocalization with large foci of 53BP1 and NBS1. Furthermore, cells with 53BP1/NBS1 large foci were not increased in 1 Gy-irradiated A-T cells (4.5% in control, 3.6% at 24h after IR), whereas they were significantly increased in 1 Gy-irradiated normal human cells (4.5% in control, 26% at 24h after IR). These results indicate that growing of 53BP1 and NBS1 foci size, which depends on enlarged phosphorylated ATM foci remained several hours after irradiation, might play a critical role in G1 checkpoint induction.
