Host: The Japan Radiation Research Society
Co-host: Asian Association for Radiation Research
Ionizing radiation causes DNA double strand breaks (DSBs), which are the cause of detrimental effects of radiation. DNA DSBs are recognized by ataxia-telangiectasia mutated (ATM) protein, and ATM activated through auto-phosphorylation at serine 1981 (S1981) transduces DNA damage signals via phosphorylation of the down-stream effectors. Recently, the effects of low dose ionizing radiation have been received much scientific attention. To understand low dose effects it is indispensable to analyze radiation response in individual cells. In the present study, we examined the induction of DNA damage checkpoint signal at a single cell level using anti-S1981 phosphorylated ATM antibody. Normal human diploid cells are irradiated with various doses of X-rays, and phosphorylated ATM foci were detected by immunofluorescent assay. We found liner dose-dependent induction of phosphorylated ATM foci with doses between 10 mGy and 1 Gy. Interestingly, the mean size of the initial foci (approx. 0.1 micron) changed to 1.0 micron 30 minutes after X-irradiation. Similar foci growth was observed in the foci of phosphorylated histone H2AX at serine 139, 53BP1, and NBS1. Furthermore, we confirmed that the mean size of the phosphorylated ATM foci induced by 50 mGy of X-rays was comparable to that induced by 1 Gy, indicating that phosphorylation-dependent amplification of the signal is involved in DNA damage checkpoint. These results indicate that even a single DSB could be detectable efficiently though signal amplification.
