Effect of chronic low dose natural radiation in human peripheral blood mononuclear cells: Evaluation of DNA damage and repair using the alkaline comet assay

Abstract

This study investigates whether peripheral blood mononuclear cells (PBMCs) from inhabitants of Kerala in southwest India, exposed to chronic low dose natural radiation in vivo (>1mSvyear−1), respond with a radioadaptive response to a challenging dose of gamma radiation. Toward this goal, PBMCs isolated from 77 subjects from high-level natural radiation areas (HLNRA) and 37 subjects from a nearby normal level natural radiation area (NLNRA) were challenged with 2Gy and 4Gy gamma radiation. Subjects from HLNRA were classified based on the mean annual effective dose received, into low dose group (LDG) and high dose group (HDG) with mean annual effective doses of 2.69mSv (N=43, range 1.07mSvyear−1 to 5.55mSvyear−1) and 9.62mSv (N=34, range 6.07mSvyear−1 to17.41mSvyear−1), respectively. DNA strand breaks and repair kinetics (at 7min, 15min and 30min after 4Gy) were evaluated using the alkaline single cell gel electrophoresis (comet) assay. Initial levels of DNA strand breaks observed after either a 2Gy or a 4Gy challenging dose were significantly lower in subjects of the HDG from HLNRA compared to subjects of NLNRA (2Gy, P=0.01; 4Gy, P=0.02) and LDG (2Gy P=0.01; 4Gy, P=0.05). Subjects of HDG from HLNRA showed enhanced rejoining of DNA strand breaks (HDG/NLNRA, P=0.06) during the early stage of repair (within 7min). However at later times a similar rate of rejoining of strand breaks was observed across the groups (HDG, LDG and NLNRA). Preliminary results from our study suggest in vivo chronic low-level natural radiation provides an initial exposure that allows an adaptation to a subsequent higher radiation exposure, perhaps through improving DNA repair via an unknown mechanism. Therefore, further investigations would be necessary in this population to understand the biological and health effects of chronic low-level natural radiation exposures.