Abstract
Risk estimation for low dose of radiation is based on extrapolations from existing high dose data. This idea assumes that cellular responses, including DNA repair, play an equally efficient role at low and high radiation doses. However, the validity of this assumption in the recent year is challenged by some radiobiological phenomena, such as the bystander effect and radio-adaptive response. These phenomena have a significant impact on assessment of risk of exposure to low doses of ionizing radiation. In this study, to investigate the biological effect of low dose radiation, we analyzed the amount of initial phosphorylated ATM forms discrete foci in non-dividing primary normal human fibroblasts (MRC-5 cells) that irradiated of X-rays with doses ranging for 1.2 mGy to 1000 mGy and showed that phosphorylated ATM forms discrete foci are capable of detecting the radiation-induced DNA damage after radiation doses as low as 1 mGy. Also the induction of foci showed sharp linear dose-response relationship with doses ranging for 1.2 mGy to 5 mGy. This result indicated that very low dose of x-ray might occur bystander effect. In addition we investigated the time course analysis of radiation-induced DNA damage based on phosphorylated ATM forms discrete foci in MRC-5 cells. Here we present evidence that foci of phosphorylated ATM focus induced in culture of non-dividing MRC-5 cells by very low dose of X-rays remain unrepaired for a long time. This result suggested that the threshold in the repair of DNA damage exists.
