Increased Gamma-H2a.X Intensity In Response To Chronic Medium-Dose-Rate Gamma-Ray Irradiation

Background: The molecular mechanisms of DNA repair following chronic medium-dose-rate (MDR) gamma-ray-induced damage remain largely unknown.Methodology/Principal Findings: We used a cell function imager to quantitatively measure the fluorescence intensity of gamma-H2A.X foci in MDR (0.015 Gy/h and 0.06 Gy/h) or high-dose-rate (HDR) (54 Gy/h) gamma-ray irradiated embryonic fibroblasts derived from DNA-dependent protein kinase mutated mice (scid/scid mouse embryonic fibroblasts (scid/scid MEFs)). The obtained results are as follows: (1) Automatic measurement of the intensity of radiation-induced gamma-H2A.X foci by the cell function imager provides more accurate results compared to manual counting of gamma-H2A.X foci. (2) In high-dose-rate (HDR) irradiation, gamma-H2A.X foci with high fluorescence intensity were observed at 1 h after irradiation in both scid/scid and wild-type MEFs. These foci were gradually reduced through de-phosphorylation at 24 h or 72 h after irradiation. Furthermore, the fluorescence intensity at 24 h increased to a significantly greater extent in scid/scid MEFs than in wild-type MEFs in the G(1) phase, although no significant difference was observed in G(2)/M-phase MEFs, suggesting that DNA-PKcs might be associated with non-homologous-end-joining-dependent DNA repair in the G(1) phase following HDR c-ray irradiation. (3) The intensity of gamma-H2A.X foci for continuous MDR (0.06 Gy/h and 0.015 Gy/h) irradiation increased significantly and in a dose-dependent fashion. Furthermore, unlike HDR-irradiated scid/scid MEFs, the intensity of gamma-H2A.X foci in MDR-irradiated scid/scid MEFs showed no significant increase in the G(1) phase at 24 h, indicating that DNA repair systems using proteins other than DNA-PKcs might induce cell functioning that are subjected to MDR gamma-ray irradiation.Conclusions: Our results indicate that the mechanism of phosphorylation or de-phosphorylation of gamma-H2A.X foci induced by chronic MDR gamma-ray irradiation might be different from those induced by HDR gamma-ray irradiation.