Non-linear dose response of DNA double strand breaks in response to chronic low dose radiation in individuals from high level natural radiation areas of Kerala coast

Background The human population living in high level natural radiation areas (HLNRAs) of Kerala coast provide unique opportunities to study the biological effects of low dose and low dose rate ionizing radiation below 100 mGy. The level of radiation in this area varies from < 1.0 to 45 mGy/year. The areas with <= 1.50 mGy/year are considered as normal level natural radiation areas (NLNRA) and > 1.50 mGy/year, as high level natural radiation areas (HLNRA). The present study evaluated dose response relationship between DNA double strand breaks (DSBs) and background radiation dose in individuals residing in Kerala coast. Venous blood samples were collected from 200 individuals belonging to NLNRA (n = 50) and four dose groups of HLNRA; 1.51-5.0 mGy/year (n = 50), 5.01-10.0 mGy/year (n = 30), 10.01-15.0 mGy/year (n = 33), > 15.0 mGy/year (n = 37) with written informed consent. The mean dose of NLNRA and four HLNRA dose groups studied are 1.21 +/- 0.21 (range: 0.57-1.49), 3.02 +/- 0.95 (range: 1.57-4.93), 7.43 +/- 1.48 (range: 5.01-9.75), 12.22 +/- 1.47 (range: 10.21-14.99), 21.64 +/- 6.28 (range: 15.26-39.88) mGy/year, respectively. DNA DSBs were quantified using gamma H2AX as a marker, where foci were counted per cell using fluorescence microscopy. Results Our results revealed that the frequency of gamma H2AX foci per cell was 0.090 +/- 0.051 and 0.096 +/- 0.051, respectively in NLNRA and HLNRA individuals, which were not significantly different (t(198) = 0.33; P = 0.739). The frequency of gamma H2AX foci was observed to be 0.090 +/- 0.051, 0.096 +/- 0.051, 0.076 +/- 0.036, 0.087 +/- 0.042, 0.108 +/- 0.046 per cell, respectively in different dose groups of <= 1.50, 1.51-5.0, 5.01-10.0, 10.01-15.0, > 15.0mGy/year (ANOVA, F-4,F-195 = 2.18, P = 0.072) and suggested non-linearity in dose response. The frequency of gamma H2AX foci was observed to be 0.098 +/- 0.042, 0.078 +/- 0.037, 0.084 +/- 0.042, 0.099 +/- 0.058, 0.097 +/- 0.06 and 0.114 +/- 0.033 per cell in the age groups of <= 29, 30-34, 35-39, 40-44, 45-49 and >= 50 years, respectively (ANOVA, F-5,F-194 = 2.17, P = 0.059), which suggested marginal influence of age on the baseline of DSBs. Personal habits such as smoking (No v/s Yes: 0.092 +/- 0.047 v/s 0.093 +/- 0.048, t(198) = 0.13; P = 0.895) and drinking alcohol (No v/s Yes: 0.096 +/- 0.052 v/s 0.091 +/- 0.045, t198 = 0.62; P = 0.538) did not show any influence on DSBs in the population. Conclusion The present study did not show any increase in DSBs in different dose groups of HLNRA compared to NLNRA, however, it suggested a non-linear dose response between DNA DSBs and chronic low dose radiation.