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 γH2AX as a marker, where foci were counted per cell using fluorescence microscopy. RESULTS: Our results revealed that the frequency of γH2AX foci per cell was 0.090 ± 0.051 and 0.096 ± 0.051, respectively in NLNRA and HLNRA individuals, which were not significantly different (t198 = 0.33; P = 0.739). The frequency of γH2AX 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, F4,195 = 2.18, P = 0.072) and suggested non-linearity in dose response. The frequency of γH2AX 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, F5,194 = 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, t198 = 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.

Evaluation of natural chronic low dose radiation exposure on telomere length and transcriptional response of shelterin complex in individuals residing in Kerala coast, India.

The high level natural radiation areas (HLNRA) of Kerala coast provide unique opportunity to study the biological effect of chronic low dose ionizing radiation (LDIR) on human population below 100 mSv. The radiation level in this area varies from < 1.0-45 mGy /year due to patchy distribution of monazite in the sand, which contains 232Th (8-10%), 238U (0.3%), and their decay products. Telomere length attrition has been correlated to DNA damage due to genotoxic agents. The objective of the present study is to evaluate the effect of natural chronic LDIR exposure on telomere length and transcriptional response of telomere specific and DNA damage repair genes in peripheral blood mononuclear cells (PBMCs) of individuals from normal level natural radiation areas (NLNRA) and HLNRA of Kerala coast, southwest India. Blood samples were collected from 71 random male donors (24-80 years) from NLNRA (≤1.50 mGy/year; N = 19) and two HLNRA dose groups [1.51-10 mGy/year (N = 17); > 10 mGy/year, (N = 35)]. Genomic DNA was isolated from PBMCs and relative telomere length (RTL) was determined using real time q-PCR. Radio-adaptive response (RAR) study was carried out in PBMCs of 40 random males from NLNRA (N = 20) and HLNRA (>10 mGy/year; N = 20), where PBMCs were given a challenged dose of 2.0 Gy gamma radiation at 4 h. Transcriptional profile of telomere specific (TRF1, TRF2, POT1, TIN2, TPP1, RAP1), DNA damage response (RAD17, ATM, CHEK1) and base excision repair pathway (BER) (OGG1, XRCC1, NTH1, NEIL1, MUTYH, MBD4) genes were analysed at basal level and after a challenge dose of 2.0 Gy at 4 h. Our results did not show any significant effect of chronic LDR on RTL among the individuals from NLNRA and two HLNRA groups (p = 0.195). However, influence of age on RTL was clearly evident among NLNRA and HLNRA individuals. At basal level, TRF1, TRF2, TIN2, MBD4, NEIL1 and RAD17 showed significant up-regulation, whereas XRCC1 was significantly down regulated in HLNRA individuals. After a challenge dose of 2.0 Gy, significant transcriptional up-regulation was observed at telomere specific (TRF2, POT1) and BER (MBD4, NEIL1) genes in HLNRA individuals as compared to NLNRA suggesting their role in RAR. In conclusion, elevated level of natural chronic LDR exposure did not have any adverse effect on telomere length in Kerala coast. Significant transcriptional response at TRF2, MBD4 and NEIL1 at basal level and with a challenge dose of 2.0 Gy suggested their active involvement in efficient repair and telomere maintenance in individuals from HLNRA of Kerala coast.

Evaluation of the influence of chronic low-dose radiation on DNA repair gene polymorphisms [XRCC1, XRCC3, PRKDC (XRCC7), LIG1, NEIL1] in individuals from normal and high level natural radiation areas of Kerala Coast.

Background: Single Nucleotide Polymorphisms (SNPs) at DNA repair genes are considered as potential biomarkers of radio-sensitivity. The coastal belt of Kerala in south west India has a patchy distribution of monazite in its beach sand that contains Th-232 and its decay products. Thus, radiation levels in this area vary from <1.0mGy to 45.0mGy/year. The areas with external gamma radiation dose >1.5mGy/year are considered as High-Level Natural Radiation Areas (HLNRA) and ≤ 1.5mGy/year are Normal Level Natural Radiation Area (NLNRA).Objective: In the present study, an attempt was made to evaluate the influence of chronic low dose radiation exposure on DNA repair gene polymorphisms in NLNRA and HLNRA population of Kerala coast.Materials and methods: Genomic DNA was isolated from venous blood samples of 246 random, healthy individuals (NLNRA, N = 104; HLNRA, N = 142) and genotyping of five SNPs such as X-ray repair cross complementing 1(XRCC1 Arg399Gln), X-ray repair cross complementing 3 (XRCC3 Thr241Met], Protein kinase, DNA-activated, catalytic subunit (PRKDC) (X-ray repair cross-complementing group 7, XRCC7 G/T), nei like DNA glycosylase 1 (NEIL1 G/T) and DNA ligase 1 (LIG1 A/C) was carried out using PCR based restriction fragment length polymorphism (PCR-RFLP) followed by silver staining.Results: Our results showed no significant difference in genotype frequencies in HLNRA vs NLNRA at three of the five SNPs studied i.e. XRCC1 Arg399Gln (χ2(2) = 5.85, p = .054), XRCC3 Thr241Met (χ2(1) = 0.71, p = .339), PRKDC (XRCC7 G/T) (χ2(2) = 3.72, p = .156), whereas significant difference was observed at NEIL1 G/T (χ2(2) =8.71, p = .013) and LIG1 A/C (χ2(2) = 7.66, p = .022). The odds of heterozygote to homozygote genotypes in HLNRA relative to NLNRA at XRCC1 Arg399Gln (OR = 1.96, 95% CI: 1.13-3.40), XRCC3 Thr241Met (OR = 0.73, 95% CI: 0.41-1.31), PRKDC (XRCC7 G/T), (OR = 0.81; 95% CI: 0.48-1.38), NEIL1 G/T (OR = 0.54; 95% CI: 0.31-0.96) and LIG1 A/C (OR = 1.62; 95% CI: 0.97-2.69) was also not significantly different in HLNRA vs NLNRA, except at XRCC1 and NEIL1.Conclusion: The genotype frequencies at three of these SNPs i.e. XRCC1 Arg399Gln, XRCC3 Thr241Met and PRKDC (XRCC7 G/T) were similar, whereas NEIL1 G/T and LIG1 A/C showed significant difference between HLNRA and NLNRA population. However, further research using more number of SNPs in a larger cohort is required in this study area.

Efficient repair of DNA double strand breaks in individuals from high level natural radiation areas of Kerala coast, south-west India.

High level natural radiation areas (HLNRA) of Kerala coastal strip (55km long and 0.5km wide) in southwest India exhibit wide variations in the level of background dose (< 1.0-45.0mGy/year) due to thorium deposits in the beach sand. The areas with ≤1.5mGy/year are considered as normal level natural radiation area (NLNRA), whereas areas with >1.5mGy/year are HLNRA. Individuals belonging to HLNRA were stratified into two groups, Low dose group (LDG: 1.51-5.0mGy/year) and high dose group (HDG: >5.0mGy/year). The mean annual dose received by the individuals from NLNRA, LDG and HDG was 1.3±0.1, 2.7±0.9 and 9.4±2.3mGy/year, respectively. Induction and repair of DNA double strand breaks (DSBs) in terms of gamma-H2AX positive cells were analysed in peripheral blood mononuclear cells (PBMCs) using flow cytometry. Induction of DSBs was studied at low (0.25Gy) and high challenge doses (1.0 and 2.0Gy) of gamma radiation in 78 individuals {NLNRA, N=23; HLNRA (LDG, N=21 and HDG, N=34)}. Repair kinetics of DSBs were evaluated in PBMCs of 30 individuals belonging to NLNRA (N=8), LDG (N=7) and HDG (N=15) at low (0.25Gy) and high doses (2.0Gy) of gamma radiation. Transcription profile of DNA damage response (DDR) and DSB repair genes involved in non-homologous end joining (NHEJ) and homologous recombination repair (HRR) pathways was analysed after a challenge dose of 2.0Gy in PBMCs of NLNRA (N=10) and HDG, HLNRA (N=10) group. Our results revealed significantly lower induction and efficient repair of DSBs in HLNRA groups as compared to NLNRA. Transcription profile of DCLRE1C, XRCC4, NBS1 and CDK2 showed significant up-regulation (p≤0.05) in HDG at a challenge dose of 2.0Gy indicating active involvement of DDR and DSB repair pathways. In conclusion, lower induction and efficient repair of DNA DSBs in HLNRA groups is suggestive of an in vivo radio-adaptive response due to priming effect of chronic low dose radiation prevailing in this area.

No evidence of telomere length attrition in newborns from high level natural background radiation areas in Kerala coast, south west India.

PURPOSE: The tandemly repeated hexamers (TTAGGG)n present at the telomeric ends protect the human genome from a variety of environmental exposures including ionizing radiation. In order to find out the effect of chronic low dose radiation exposure, we have determined telomere length among newborns from high level natural radiation areas (HLNRA) of the Kerala coast in South west India. METHODOLOGY: Umbilical cord blood samples were collected from 128 newborns from HLNRA and 43 newborns from normal level natural radiation areas (NLNRA) and genomic DNA was isolated using a salt precipitation method. The mean telomere length was determined using SYBR green-based real time quantitative Polymerase chain reaction (qPCR), where telomere gene specific (T) and single copy gene specific (S) primers were used. The average of telomere versus single copy gene (T/S) ratio was calculated which was proportional to telomere length of each individual. RESULTS: The mean relative telomere length was found to be 1.03 ± 0.01 (95% CI, 0.99-1.05) and 1.10 ± 0.03 (95% CI, 1.04-1.17) in HLNRA and NLNRA newborns, respectively (P > 0.05). Based on the level of background radiation dose, samples were divided into four groups, i.e., NLNRA: < 1.50 mGy/year and three HNLRA groups: 1.51-3.00 mGy/year, 3.01-5.00 mGy/year, and > 5.00 mGy/year. The mean relative telomere length in these groups were found to be 1.10 ± 0.03 (95% CI, 1.03-1.17), 0.98 ± 0.01 (95% CI, 0.95-1.01), 1.05 ± 0.02 (95% CI, 1.01-1.10) and 1.07 ± 0.03 (95% CI, 1.04-1.10), respectively. No significant difference was observed between the mean telomere length of male and female newborns. CONCLUSIONS: The elevated level of natural chronic background radiation prevailing in Kerala coast did not show any significant effect on telomere length of newborns. To our knowledge, this is the first report on newborn telomere length from a high level natural radiation area.

Transcription profile of DNA damage response genes at G0 lymphocytes exposed to gamma radiation.

Ionizing radiation induces a plethora of DNA damages in human cells which may alter the level of mRNA expression. We have analyzed mRNA expression profile of DNA damage response genes involved in G(0)/G(1) check point pathway in whole blood to assess their radio-adaptive response, if any, to gamma radiation. Blood samples were collected from twenty-five random, normal, and healthy male donors with written informed consent and irradiated at doses between 0.1 and 2.0 Gy (0.7 Gy/min). DNA strand breaks were studied using comet assay, whereas DNA double-strand breaks were visualized using γH2AX as a biomarker. Dose response if any, at transcriptional level was studied for all these dose groups at 1 and 5-h post-irradiation. Adaptive response at transcriptional level was studied at three different priming doses (0.1, 0.3, and 0.6 Gy) separately followed by a challenging dose of 2.0 Gy after 4 h. For both the experiments, total RNA was isolated from PBMCs obtained from irradiated whole blood and reverse transcribed to cDNA. The level of mRNA expression of ATM, ATR, GADD45A, CDKN1A, P53, CDK2, MDM2, and Cyclin E was studied using real-time quantitative PCR. A significant dose-dependant increase in the percentage of DNA damage in tail was observed using comet assay. Similarly, increased number of foci was observed at γH2AX with increasing dose. At transcriptional level, a significant dose-dependent up-regulation at GADD45A, CDKN1A, and P53 genes up to 1.0 Gy was observed at 5-h post-irradiation (P ≤ 0.05). Radio-adaptive response at mRNA expression level was observed at CDK2, Cyclin E, and P53, whereas ATM, ATR, GADD45A, MDM2, ATM, and ATR have not shown any radio-adaptive changes in the expression profile. DNA damage response genes involved in G(0)/G(1) checkpoint pathway has important implications in terms of radiosensitivity in vivo and changes in the transcriptional profile might throw some new insights to understand the mechanism of adaptive response.

Telomere length in human adults and high level natural background radiation.

BACKGROUND: Telomere length is considered as a biomarker of aging, stress, cancer. It has been associated with many chronic diseases such as hypertension and diabetes. Although, telomere shortening due to ionizing radiation has been reported in vitro, no in vivo data is available on natural background radiation and its effect on telomere length. METHODOLOGY/PRINCIPAL FINDINGS: The present investigation is an attempt to determine the telomere length among human adults residing in high level natural radiation areas (HLNRA) and the adjacent normal level radiation areas (NLNRA) of Kerala coast in Southwest India. Genomic DNA was isolated from the peripheral blood mononuclear cells of 310 individuals (HLNRA: N = 233 and NLNRA: N = 77). Telomere length was determined using real time q-PCR. Both telomere (T) and single copy gene (S) specific primers were used to calculate the relative T/S and expressed as the relative telomere length. The telomere length was determined to be 1.22+/-0.15, 1.12+/-0.15, 1.08+/-0.08, 1.12+/-0.11, respectively, among the four dose groups (5.00 mGy per year), which did not show any dose response. The results suggested that the high level natural chronic radiation did not have significant effect on telomere length among young adult population living in HLNRA, which is indicative of better repair of telomeric ends. No significant difference in telomere length was observed between male and female individuals. In the present investigation, although the determination of telomere length was studied among the adults with an age group between 18 to 40 years (mean maternal age: 26.10+/-4.49), a negative correlation was observed with respect to age. However, inter-individual variation was (0.81-1.68) was clearly observed. CONCLUSIONS/SIGNIFICANCE: In this preliminary investigation, we conclude that elevated level of natural background radiation has no significant effect on telomere length among the adult population residing in HLNRAs of Kerala coast. To our knowledge, this is the first report from HLNRAs of the world where telomere length was determined on human adults. However, more samples from each background dose group and samples from older population need to be studied to derive firm conclusions.