ARSENIC, TARGETED METABOLOMICS AND DIABETES-RELATED OUTCOMES: CONNECTING THE DOTS IN THE STRONG HEART STUDY

Diabetes, and related outcomes, is a global epidemic with an enormous cost both to the economy and in terms of lives lost. Exposure to inorganic arsenic, a ubiquitous naturally occurring environmental carcinogen, as well as the efficiency of its metabolism in the body, has been identified as a risk factor for diabetes development. One carbon metabolism (OCM), a biochemical pathway essential to numerous methylation reactions, including arsenic metabolism, appears to play an important role both in arsenic metabolism and diabetes-related outcomes. This dissertation aimed to better understand the relationships between each of these variables and determine whether arsenic metabolism is truly a risk factor for diabetes, or if the association is an epidemiological artifact confounded by OCM status. We used data from both the Strong Heart Study (SHS), a population-based cohort, as well as the Strong Heart Family Study (SHFS), a family-based extension of the SHS. Both populations are comprised of American Indian tribal members from Oklahoma, Arizona and North and South Dakota, exposed to low-moderate arsenic in drinking water and food, with high rates of diabetes and diabetes-related outcomes. First, we conducted a cross-sectional analysis evaluating the association of dietary intake of OCM nutrients (folate and vitamins B2, B6 and B12) with urinary arsenic methylation patterns (iAs%, MMA% and DMA%) in a subset (n=405) of participants from the SHS. Higher vitamin B6 and B2 were associated with higher DMA% and lower MMA% (i.e., a more efficient arsenic metabolism profile). We also observed an antagonistic interaction between folate and vitamin B6 with higher folate being associated with higher DMA% and lower iAs% only in the presence of high vitamin B6. Second, we conducted a prospective analysis in 1,047 SHFS participants free of prevalent metabolic syndrome, evaluating the association of arsenic exposure and arsenic metabolism with incident metabolic syndrome and each of its individual components (elevated waist circumference, elevated triglycerides, reduced HDL, hypertension and elevated fasting plasma glucose (FPG)). Arsenic exposure was associated with increased risk for elevated FPG but not with metabolic syndrome or other individual components. Arsenic metabolism patterns, independent of arsenic exposure, were associated with both incident metabolic syndrome and elevated waist circumference, but not with other components of the syndrome. Third, we conducted a pilot (n=59) cross-sectional targeted metabolomic analysis in the SHFS. Eight metabolites were identified as having significant correlations with both a diabetes-related outcome (HOMA2-IR, FPG, waist circumference) and at least one arsenic metabolism biomarker (iAs%, MMA% or DMA%). Consistent with previous studies, higher MMA% was associated with lower HOMA2-IR and waist circumference, and higher DMA% was associated with higher HOMA2-IR and waist circumference After adjustment for the eight OCM-related metabolites, associations between arsenic metabolism and diabetes-related outcomes were substantially attenuated and no longer significant. Fourth, we conducted a set of analyses using data from the SHFS to better understand the role OCM status plays in arsenic metabolism, diabetes-related outcomes and the relationship between the two. We first conducted cross-sectional analyses evaluating the association between OCM variables (both genetic and nutrient intake) and arsenic metabolism (iAs%, MMA% and DMA%). Next, we evaluated the associations between both OCM nutrients and OCM-related genetic variants with diabetes-related outcomes (diabetes, metabolic syndrome, waist circumference and HOMA2-IR). OCM nutrients were not associated with arsenic metabolism in the SHFS, however, higher vitamin B6 was consistently associated with three of the four diabetes-related outcomes studied (higher HOMA2-IR and increased risk for diabetes and metabolic syndrome). One OCM-related genetic variant (methionine synthase) was associated with both higher MMA% and lower HOMA2-IR per 5 years of follow-up. After adjustment for MMA% the association between the MTR variant and all diabetes-related outcomes were attenuated or reversed direction. In conclusion, arsenic exposure and arsenic metabolism may be risk factors for diabetes-related outcomes, even at low-moderate arsenic exposure. OCM status may also be a risk factor for diabetes-related outcomes as well as for arsenic metabolism, although these associations may differ based on the underlying nutritional state of the population. OCM status, diabetes-related outcomes and arsenic metabolism appear to be linked; more research is needed to understand the direction of the associations, in order leverage these findings into diabetes preventative efforts