Background: Chronic arsenic (As) exposure in a global public health concern. Arsenic exposure through drinking water affects over 140 million people in at least 70 countries, including 40 million people in Bangladesh. In the United States (US), 2.4 million people rely on private wells or public water systems with As levels above the US maximum contaminant level. Ingested inorganic arsenic (InAs) is methylated to monomethyl (MMAs)- and dimethyl (DMAs)-arsenical species using the methyl donor S-adenosylmethionine (SAM). Full methylation of InAs to DMAs decreases As toxicity and facilitates urinary As excretion. Arsenic methylation capacity is influenced by nutrients involved in one-carbon metabolism (OCM), the biochemical pathway that synthesizes SAM. Folate recruits one-carbon units for the remethylation of homocysteine and the synthesis of SAM. The availability of one-carbon units is also impacted by nutrients including the alternative methyl donor betaine, its precursor choline, and possibly the cofactor vitamin B12. In addition, As methylation capacity may also be influenced by creatine; an estimated 50% of SAM is consumed by the final step of endogenous creatine synthesis. The adverse health outcomes associated with chronic As exposure include impaired intellectual function, cardiovascular disease, diabetes, inflammation, and cancers of the bladder, lung, kidney, liver, and skin. In utero As exposure is associated with adverse birth outcomes include decreased birth weight and gestational age. Elevated health risks persist after exposure has been reduced or ended, leading to the hypothesis that epigenetic dysregulation, including changes in DNA methylation, may be a biological mechanism linking As exposure to health outcomes. Objectives: This research has three main objectives: (1) to investigate the influence of OCM nutritional factors on As methylation by evaluating effects of folic acid (FA) and creatine supplementation on As methylation capacity, and effect modification by baseline status of OCM-related nutrients; (2) to examine associations between As exposure and loci-specific DNA methylation in an epigenome-wide association study (EWAS); and (3) to assess mediation of the association between in utero As exposure and birth outcomes (i.e., gestational age and birth weight) by DNA methylation of target genes identified in an EWAS, as well as the candidate gene DNA methyltransferase 3 alpha (DNMT3A), a protein-coding gene involved in de novo DNA methylation. Methods: This research used data from three studies of As-exposed individuals. To address the first objective, we used data from the Folic Acid and Creatine Trial (FACT), a 24-week randomized clinical trial of FA (400 or 800 μg/day) and/or creatine supplementation (3 g/day or 3 g creatine and 400 μg FA/day) among As-exposed adults in Bangladesh recruited independent of folate status (N = 622). We investigated overall FA and creatine treatment effects on mean within-person changes in As metabolite proportions in urine compared to the placebo group (weeks 0 to 12). Rebound of As methylation capacity following the cessation of FA supplementation was assessed from weeks 12 to 24. We also assessed effect modification by baseline choline, betaine, vitamin B12, and plasma folate of treatment effects on changes in homocysteine, guanidinoacetate (GAA) (biomarkers of OCM and endogenous creatine synthesis, respectively), total blood As, and urinary As metabolite proportions and indices. To address the second objective, we used data from the Strong Heart Study (SHS), a population-based prospective cohort of American Indians with low-moderate levels of As exposure. DNA methylation was measured in 2,325 participants using the Illumina MethylationEPIC array, which interrogates > 850,000 loci. We tested for differentially methylated positions (DMPs) and regions (DMRs), and conducted gene ontology (GO) enrichment analysis to understand functions of genes containing differential methylation. To address the third objective, we used data from a prospective birth cohort in Bangladesh. In a discovery phase, an EWAS was conducted to identify CpGs with methylation measured in cord blood that are associated with maternal water As levels and birth outcomes (N = 44). In a validation phase, DNA methylation in cord blood was measured using bisulfite pyrosequencing at three target CpGs annotated to miR124-3, MCC, and GNAL (N = 569). We applied structural equation models (SEMs) to assess mediation of the association between in utero As exposure and gestational age by DNA methylation. In addition, mediation of the association between in utero As exposure and birth outcomes by DNA methylation of the candidate gene DNA methyltransferase alpha (DNMT3A) was assessed. Results: In FACT, the mean within-person decreases %InAs and %MMAs and increase in %DMAs were greater among all groups receiving FA supplementation at weeks 6 and 12 compared to placebo (P < 0.05) (Chapter 3). Stratified by median choline and betaine concentrations at baseline, we observed a trend towards greater FA treatment effects among participants with levels below the median of both nutrients compared to participants above the median (Chapter 4). Among participants who discontinued FA supplementation, at week 24, %InAs and %DMAs were not significantly different than baseline levels, suggesting a rebound in As methylation capacity with cessation of FA supplementation. We observed a significantly greater mean within-person decreases in %MMAs with creatine supplementation compared to placebo at weeks 1, 6, and 12; mean within-person changes in %InAs and %DMAs did not differ significantly between the creatine and placebo groups (Chapter 3). The mean within-person decrease in urinary %MMAs at week 12 with creatine treatment was significantly greater than placebo among participants with baseline choline concentrations below the median, but did not differ from placebo among participants with choline concentrations above the median (Chapter 4). In an EWAS conducted in SHS, we identified 20 DMPs associated with urinary As levels at FDR < 0.05; five DMPs were significant at PBonferroni < 0.05 (Chapter 5). The top significant CpG, cg06690548, was located in solute carrier family 7 member 11 (SLC7A11 ), part of the amino-acid transporter cystine:glutamate antiporter system xc-, which is involved in biosynthesis of the endogenous antioxidant glutathione (GSH). Additional Bonferroni-significant CpGs were located in ANKS3, LINGO3, CSNK1D, and ADAMTSL4. We identified one FDR-significant DMR (chr11:2,322,050-2,323,247) including the open reading frame C11orf21 and tetraspanin 32 (TSPAN32 ). Mediation of the association between in utero As exposure and birth outcomes by cord blood DNA methylation was assessed in a Bangladeshi birth cohort. In the discovery phase (N = 44), the association between maternal water As levels and gestational age was fully mediated by DNA methylation of the top 10 CpGs associated with both variables. In a discovery phase (N = 569), there were significant indirect effects of maternal water As levels on gestational age through DNA methylation of miR124-3 and MCC ; the indirect effect through DNA methylation of GNAL was not significant (Chapter 6). In an adjusted SEM including miR124-3 and MCC, mediation of the association between in utero As exposure and gestational age by DNA methylation of miR124-3 was borderline significant (P = 0.06); DNA methylation of MCC did not act as a mediator. We also assessed mediation by DNA methylation of DNMT3A (Chapter 7). In an adjusted SEM including birth weight and gestational age, there was a significant indirect effect of maternal toenail As levels on gestational age through DNMT3A methylation, the indirect effect on birth weight was borderline significant (P = 0.082). However, the indirect effects of maternal toenail As levels on birth weight through all pathways including gestational age were statistically significant. A doubling in maternal toenail As concentrations had a total effect of a decrease in gestational age of 2.1 days and a decrease in birth weight of 28.9 g. Conclusions: Results from FACT (Chapters 3 and 4) provide evidence of the associations between OCM-related nutrients and As methylation capacity. Specifically, FA and creatine supplementation may increase As methylation capacity by increasing the availability of SAM, and treatment effects may be greater among individuals with low betaine and choline status, respectively. In addition, results reported in Chapters 5-7 support the hypotheses that chronic As exposure is associated with epigenetic dysregulation, and that changes in the epigenome may mediate the association between As exposure and adverse health effects. Findings from the research presented here may help inform public health interventions to reduce the adverse health effects of chronic As exposure. However, further research is needed to fully understand the biological mechanism that influence As methylation and that underlie the associations between chronic As exposure and adverse health outcomes.

Bibliography: leaves 106-114.

Reducing population exposure to inorganic arsenic (iAs), a known carcinogen and highly toxic metalloid of great public health concern, remains an ongoing challenge worldwide and in the United States (US). In the US, the Environmental Protection Agency (EPA) regulates the maximum contaminant level (MCL) for total arsenic in public drinking water supplies through the Safe Drinking Water Act. In 2001, the US EPA implemented the Final Arsenic Rule, which lowered the MCL for arsenic in public drinking water supplies from 50 to 10 µg/L. Reductions in iAs exposure and subsequent related disease associated with this important regulatory change have not been quantified. Currently, no national-level exposure estimates of iAs drinking water exposure are available for US residents reliant on public drinking water. There is a critical need to identify susceptible subgroups of the US population who remain at risk for elevated iAs drinking water exposure. This dissertation aimed to quantify the reduction in drinking water iAs exposure resulting from the US EPA MCL regulatory change, to estimate drinking water iAs exposure for US residents reliant on public drinking water, to identify susceptible subgroups across the US whose water iAs remains high, and to determine if iAs exposure was associated with heart disease mortality in the general US population. Chapter 1 provides background information necessary to contextualize the work contained in this dissertation. In Chapter 2, we conducted a cross-sectional analysis of dietary sources of iAs exposure in the Strong Heart Family Study, a cohort of American Indian adults followed primarily for cardiovascular disease, using a self-reported food frequency questionnaire and urinary iAs measurements. Self-reported intake of rice, organ meat, processed meat, and non-alcoholic drinks was associated with increased urinary iAs concentrations. Diet alone explained only 3% of total variability in urinary iAs concentrations, indicating that the majority of iAs exposure for SHFS participants occurs from drinking water. Second, (in Chapter 3), we explored trends in water iAs exposure in the general US population associated with the EPA’s MCL change using the National Health and Nutrition Examination Survey (NHANES) from 2003-2014, separately for participants reliant on public drinking water vs. private well water (which is not subject to US EPA regulation). We estimated that implementation of the new US EPA MCL was associated with a 17% reduction in drinking water iAs exposure for all participants reliant on public drinking water; the corresponding reduction was 32% for Mexican-American participants. No reduction was observed for participants reliant on private wells. Third (in Chapter 4), we estimated drinking water iAs exposure at the community water system and county-level across the entire US from 2006-2011 using the US EPA’s Six Year Review of Contaminant Occurrence database. We estimated that nationwide public drinking water iAs concentrations decreased by 8.5% and 21.6% at the 80th and 99th percentiles of the water iAs distribution in accordance with the MCL implementation, with significant differences across US subgroups. Greater decreases in iAs concentrations were reported for systems reliant on groundwater, systems serving smaller populations, and systems in the Northeast, Central Midwest, and Southwestern regions of the US. Susceptible subgroups whose public drinking water iAs exposure remains high include populations served by small community water systems reliant on groundwater, communities in the Southwestern US, Semi-Urban, Hispanic communities, and Rural, American Indian communities. Fourth (in Chapter 5), we assessed six-year average arsenic concentrations in community water systems exclusively serving correctional facilities in the US (e.g. prisons, jails, detention centers) compared to other community water systems. Average arsenic concentrations were twice as high in correctional facility community water systems located in the Southwest (6.41 µg/L, 95% CI 3.48, 9.34) compared to all other community water systems in the Southwest (3.11 µg/L, 95% CI 2.97, 3.24). Over a quarter of correctional facility systems in the Southwest reported a six-year average arsenic concentration exceeding the 10 µg/L MCL. Persons incarcerated in the Southwestern US were at disproportionate risk of drinking water arsenic exposure and related disease from 2006-2011. Fifth (in Chapter 6), we multiply imputed urinary arsenic concentrations below the limit of detection (LOD) in NHANES 2003-2016 using a Bayesian Tobit regression model. Epidemiological analyses of urinary arsenic data in NHANES are limited by the relatively high analytical LODs and large proportion of participants with undetectable values. Distributions of urinary arsenic originally reported in NHANES, which replace values below the LOD with the LOD divided by the square root of two, likely overestimate iAs exposure at the lowest exposure levels and may introduce significant bias. Bayesian-multiply imputed datasets may improve the assessment of iAs exposure in cohorts with high analytical LODs for arsenic species. Finally (in Chapter 7), we evaluated the association between urinary iAs concentrations (internal dose) and heart disease mortality as recorded in the National Death Index in NHANES 2003-2014 participants. We found a positive but non-significant prospective association between increasing iAs exposure and heart disease mortality for all participants (hazard ratio 1.15, 95% CI 0.77, 1.70), and a significant positive association for non-Hispanic white participants using flexible spline models. Geometric mean ratios of iAs exposure were higher among cases compared to non-cases, especially for Mexican-American participants (1.30, 95% CI 0.90, 1.88). These findings further support the potential association between low- to moderate- iAs exposure and cardiovascular disease in the US population, and indicate that further high-quality prospective studies of Hispanic and Latino Americans are needed to investigate the potential increased susceptibility of Mexican-Americans to iAs-related cardiovascular disease. Taken together, these studies suggest that while the implementation of the US EPA’s 10 µg/L MCL has reduced drinking water arsenic exposure for many Americans reliant on public drinking water systems, these reductions were not uniform across all US populations. Populations who remain at risk of elevated drinking water arsenic exposure include those reliant on domestic wells, those located in the Southwest, persons incarcerated in the Southwest, tribal communities, and Hispanic communities. Further high-quality epidemiologic research is needed to evaluate the association between low- to moderate iAs exposure and cardiovascular disease in these populations. Stronger federal regulations, targeted compliance enforcement and technical assistance, and other public health interventions are needed to reduce drinking water arsenic exposure in these communities.

Inorganic arsenic is a potent carcinogen and toxicant associated with numerous adverse health outcomes, and is number one on the Agency for Toxic Substances and Disease Registry Substance Priority List. Uranium is also a carcinogen and nephrotoxicant, however health effects at levels experienced by general populations is unclear. Chronic exposure to inorganic arsenic (As) and uranium (U) in the United States (US) occurs from unregulated private wells and federally regulated community water systems (CWSs). Geogenic arsenic contamination typically occurs in groundwater as opposed to surface water supplies. Groundwater is a major source for many CWSs in the US. Although the US Environmental Protection Agency sets the maximum contaminant level (MCL enforceable since 2006: 10 µg/L) for arsenic in CWSs, private wells are not federally regulated. The contribution of drinking water from private wells and regulated CWSs to total inorganic arsenic and uranium exposure is not clear.In the United States (US), type 2 diabetes (T2D) affects approximately 37.3 million people (11.3% of the population), with the highest burden in American Indian communities. Toxic metal exposures have been identified as risk factors of T2D. Most studies rely on biomarkers, which could be affected by early disease processes. Studies directly measuring metals in drinking water in US populations have been limited. In Chapter 2, we evaluated county-level associations between modeled values of the probability of private well arsenic exceeding 10 µg/L and CWS arsenic concentrations for 2,231 counties in the conterminous US, using time invariant private well arsenic estimates and CWS arsenic estimates for two time periods. Nationwide, county-level CWS arsenic concentrations increased by 8.4 µg/L per 100% increase in the probability of private well arsenic exceeding 10 µg/L for 2006 – 2008 (the initial compliance monitoring period after MCL implementation), and by 7.3 µg/L for 2009 – 2011 (the second monitoring period following MCL implementation) (1.1 µg/L mean decline over time). Regional differences in this temporal decline suggest that interventions to implement the MCL were more pronounced in regions served primarily by groundwater. The strong association between private well and CWS arsenic in Rural, American Indian, and Semi Urban, Hispanic counties suggests that future research and regulatory support are needed to reduce water arsenic exposures in these vulnerable subpopulations. This comparison of arsenic exposure values from major private and public drinking water sources nationwide is critical to future assessments of drinking water arsenic exposure and health outcomes. In Chapter 3, we aimed to determine the association between drinking water arsenic estimates and urinary arsenic concentrations in the 2003-2014 National Health and Nutrition Examination Survey (NHANES). We evaluated 11,088 participants from the 2003-2014 NHANES cycles. For each participant, we assigned private well and CWS arsenic levels according to county of residence using estimates previously derived by the U.S. Environmental Protection Agency and U.S. Geological Survey. We used recalibrated urinary dimethylarsinate (rDMA) to reflect the internal dose of estimated water arsenic by applying a previously validated, residual-based method that removes the contribution of dietary arsenic sources. We compared the adjusted geometric mean ratios and corresponding percent change of urinary rDMA across tertiles of private well and CWS arsenic levels, with the lowest tertile as the reference. Comparisons were made overall and stratified by census region and race/ethnicity. Overall, the geometric mean of urinary rDMA was 2.52 (2.30, 2.77) µg/L among private well users and 2.64 (2.57, 2.72) µg/L among CWS users. Urinary rDMA was highest among participants in the West and South, and among Mexican American, Other Hispanic, and Non-Hispanic Other participants. Urinary rDMA levels were 25% (95% confidence interval (CI): 17-34%) and 20% (95% CI: 12-29%) higher comparing the highest to the lowest tertile of CWS and private well arsenic, respectively. The strongest associations between water arsenic and urinary rDMA were observed among participants in the South, West, and among Mexican American and Non-Hispanic White and Black participants. Both private wells and regulated CWSs are associated with inorganic arsenic internal dose as reflected in urine in the general U.S. population. In Chapter 4, our objective was to evaluate regional and sociodemographic inequalities in water arsenic exposure reductions associated with the US Environmental Protection Agency’s Final Arsenic Rule, which lowered the arsenic maximum contaminant level to 10 µg/L in public water systems. We analyzed 8,544 participants from the 2003-14 National Health and Nutrition Examination Survey (NHANES) reliant on community water systems (CWSs). We estimated arsenic exposure from water by recalibrating urinary dimethylarsinate (rDMA) to remove smoking and dietary contributions. We evaluated mean differences and corresponding percent reductions of urinary rDMA comparing subsequent survey cycles to 2003-04 (baseline), stratified by region, race/ethnicity, educational attainment, and tertile of CWS arsenic assigned at the county level. The overall difference (percent reduction) in urine rDMA was 0.32 µg/L (9%) among participants with the highest tertile of CWS arsenic, comparing 2013-14 to 2003-04. Declines in urinary rDMA were largest in regions with the highest water arsenic: the South [0.57 µg/L (16%)] and West [0.46 µg/L, (14%)]. Declines in urinary rDMA levels were significant and largest among Mexican American [0.99 µg/L (26%)] and Non-Hispanic White [0.25 µg/L (10%)] participants. Reductions in rDMA following the Final Arsenic Rule were highest among participants with the highest CWS arsenic concentrations, supporting legislation can benefit those who need it the most, although additional efforts are still needed to address remaining inequalities in CWS arsenic exposure. In Chapter 5, we examined the contribution of water As and U to urinary biomarkers in the Strong Heart Family Study (SHFS), a prospective study of American Indian communities, and the Multi-Ethnic Study of Atherosclerosis (MESA), a prospective study of racially/ethnically diverse urban US communities. We assigned residential zip code-level estimates in CWSs (µg/L) and private wells (90th percentile probability of As >10 µg/L) to up to 1,485 and 6,722 participants with dietary information and urinary biomarkers in the SHFS (2001-2003) and MESA (2000-2002; 2010-2011), respectively. Total inorganic As exposure was estimated as the sum of inorganic and methylated species in urine (urine As). We used linear mixed-effects models to account for participant clustering and removed the effect of dietary sources of As and U via regression adjustment. The median (interquartile range) urine As was 5.32 (3.29, 8.53) and 6.32 (3.34, 12.48) µg/L for SHFS and MESA, respectively, and urine U was 0.037 (0.014, 0.071) and 0.007 (0.003, 0.018) µg/L. In a mixed-effects meta-analysis of pooled effects across the SHFS and MESA, urine As was 11% (95% CI: 3, 20%) higher and urine U was 35% (5, 73%) higher per 2-fold higher CWS As and U, respectively. In the SHFS, CWS and private well As explained >40% of variability in urine As and CWS U explained >20% of urine U. In MESA, CWS As and U explained >50% of urine As and U. Water from public water supplies and private wells represents a major contributor to inorganic As and U exposure in diverse US populations. In Chapter 6, we examined the association of arsenic exposures in community water systems (CWS) and private wells with T2D incidence in the Strong Heart Family Study (SHFS), a prospective cohort of American Indian communities, and the Multi-Ethnic Study of Atherosclerosis (MESA), a prospective study of racially/ethnically diverse urban US communities, to evaluate direct associations between drinking water metal exposures and T2D risk. We evaluated adults in the SHFS free of T2D at baseline (2001-2003) and followed through 2010, with available private well and CWS arsenic (N=1,791) estimates assigned by residential zip code. We also evaluated adults in the MESA free of T2D at baseline (2000-2002) and followed through 2019, with available zip code level CWS arsenic (N=5,577) estimates. We used mixed effects Cox models to account for clustering by family and residential zip code, with adjustment for sex, baseline age, body mass index (BMI), smoking status, and education. T2D incidence in the SHFS was 24.4 cases per 1,000 people (mean follow-up 5.6 years) and T2D incidence in MESA was 11.2 per 1,000 people (mean follow-up 6.0 years). In a meta-analysis of pooled effects across the SHFS and MESA, the corresponding hazard ratio (95% confidence interval) per 2-fold increase in water arsenic was 1.09 (1.01, 1.16). Differences were observed by BMI category and sex; positive associations were observed among participants with BMI <25 kg/m2 and among female participants. In categorical analyses, >10% probability of private well arsenic (<10% reference) in the SHFS and >1 µg/L of CWS arsenic (<1 µg/L reference) in MESA were associated with increased diabetes risk. Low to moderate water arsenic levels in unregulated private wells and federally regulated CWSs were associated with T2D incidence in the SHFS and MESA. In supplementary analyses, we also observed that CWS uranium was associated with T2D risk among SHFS and MESA participants with BMI<25 kg/m2.