Effectiveness of DNA double strand break repair in human lymphocytes exposed to pulsed X-rays depending on reactive oxygen species level
Effects of high doses of ionizing radiation in human cells are well studied while mechanisms of some phenomena in the low dose range, including bystander effect, radiation-induced chromosome instability and hypersensitivity to low doses of ionizing radiation, remain not clear. Moreover, repair of DNA double strand breaks is deficient after exposure of human cells to very low doses of ionizing radiation. Earlier, such reduced DNA double strand break repair was observed in human peripheral blood lymphocytes after exposure to very low dose pulsed X-rays. It was suggested that there was a radiation-induced oxidative threshold for activation of antioxidant system and repair of radiation-induced DNA double strand breaks which is essential for efficient functioning of DNA repair pathways. Thus, in this study we analyzed the effects of exogenously added reactive oxygen species on effectiveness of DNA double strand break repair. With this aim, the level of phosphorylated histone H2AX (fH2AX) foci, which is the most sensitive marker of DNA double strand breaks, was assessed in human lymphocytes incubated with 10 pM H <sub>2</sub>O <sub>2</sub> in the medium with the following exposure to low dose pulsed X-rays. It was observed that 10 pM H <sub>2</sub>O <sub>2</sub> slightly increased the level of yH2AXfoci at 30 min after irradiation in both irradiated and control cells. On the contrary, 10 pMH <sub>2</sub>O <sub>2</sub> did not affect radiosensitivity of human lymphocytes measured as an excess level of fH2AXfoci. Moreover, there was no significant effect of 10 pMH <sub>2</sub>O <sub>2 </sub>on the effectiveness of DNA double strand break repair at 18 h after irradiation. At the same time, the level of radiation-induced yH2AX foci decreased significantly at 18 h for doses higher than 72 mGy in lymphocytes exposed to 10 pMH <sub>2</sub>O <sub>2</sub> while in lymphocytes non-exposed to 10 pM H <sub>2</sub>O <sub>2</sub> such decrease was observed only for doses higher than 160 mGy. Our results are partially inconsistent with the previous study of effectiveness of DNA double strand break repair in human fibroblasts in which the significant effect of reactive oxygen species was observed. High variability of effectiveness of DNA double strand break repair was shown for different donors after irradiation to 12 mGy of pulsed X-ways suggesting that effects of H <sub>2</sub>O <sub>2</sub> on DNA repair probably depend on individual features.