Arsenic exposure and global DNA methylation

Background: It has been hypothesized that inorganic arsenic (iAs), an important environmental carcinogen of global public health significance, induces epigenetic changes including aberrant DNA methylation. Both DNA methylation and arsenic metabolism require S-adenosylmethionine (SAM) as the methyl donor. This competitive demand between arsenic metabolism and DNA methylation for SAM may decrease the percentage of methylated CpG dinucleotides throughout the genome. Arsenic has been shown to produce global hypomethylation in both in vitro and animal models. However, few studies have investigated the effect of iAs exposure on genomic DNA methylation in humans. Methods: We evaluated the relationship between iAs and DNA methylation in maternal and cord blood samples collected as part of a pilot study that recruited 52 pregnant women before their 28th week of gestational age in Serajdikhan, Bangladesh. Of the 52 women recruited into this pilot study, cord blood was successfully collected from 29 of the 44 births attended. DNA was extracted using the Puregene DNA isolation kit (Gentra Systems, Minneapolis, MN). DNA was shipped to University of Milan where genomic DNA was subjected to bisulfite modification and global methylation content was determined by PCR-pyrosequencing analysis for LINE-1 and Alu repeated elements. This technique measures the percentage of methylation in three specific CpG positions in LINE-1 and in three specific CpG positions in Alu repeated elements that are present in several thousand copies in a single genome. Maternal exposure to iAs was measured in drinking water samples collected at the time of enrolment using ICP-MS. Results: Drinking water iAs ranged from <1 to 734 μg/L and the average percent methylation of global LINE-1 and ALU repeats was 78.5% (range: 74.93-88.0%) and 24.57% (range: 20.9-25.9%), respectively. Separate linear regression models evaluated the relationship between the average for LINE-1 and Alu and drinking water arsenic while controlling for exposure to environmental tobacco smoke and chewing betel nuts. We observed that drinking water iAs was associated with decreased methylation at average LINE-1 repeated elements (β = -2.35; SE: 1.07, P-value = 0.04) and for average Alu in cord blood samples (β = -0.67, SE: 0.031, P-value = 0.04). However, no association was observed when DNA methylation was measured in maternal blood. This suggests that the fetus may be the most susceptible to epigenetic effects of iAs exposure. Conclusion: While the biological significance of these findings remains unclear, these results support the concept that arsenic exposure results in global genomic hypomethylation. We are currently analyzing additional samples to confirm the results of this pilot study.