Mary V. Gamble, PhD

Nutritional Influences on Arsenic Toxicity, Bangladesh: The largest known mass exposure to arsenic is presently occurring due to ground water contamination of well water throughout the Ganges-Brahmaputra Delta. There is a significant inter-individual variability in susceptibility to progression from arsenic exposure to clinical manifestations of arsenic toxicity (e.g. skin, lung, liver and bladder cancers). Several observational as well as biochemical studies have led to a prevalent hypothesis that nutritional status may account for a substantial portion of this variability, though no controlled clinical studies have addressed this important hypothesis. Methylation of inorganic arsenic (InAs) has generally been considered to be a detoxification pathway. InAs and DNA are both methylated via one-carbon metabolism, a biochemical pathway which is dependent on folate for de novo generation of one-carbon groups, and also uses vitamins B12 and B6 as cofactors. The primary hypothesis of this proposal is that nutritional regulation of one-carbon metabolism, specifically folate availability, contributes substantially to the large inter-individual variability observed in InAs and DNA methylation, and thus progression from arsenic exposure to toxicity. These hypotheses have been tested in a randomized, controlled folic acid supplementation trial of adults chronically exposed to arsenic-contaminated drinking water in Bangladesh. These studies have also revealed complex interations between arsenic exposure, folate deficiency and DNA methylation that influence risk for development of arsenic-induced premalignant skin lesions.

Folic acid and creatine as therapeutic approaches for lowering blood arsenic, Bangladesh: Arsenic (As) exposure affects more than 140 million people worldwide. Methylation of inorganic-arsenic (InAs) to methylarsonic- (MMA) and dimethylarsinic acids (DMA) relies on folate-dependent one-carbon metabolism and facilitates urinary arsenic (As) elimination. Our studies indicate that folate deficiency and hyperhomocysteinemia (HHcys) are associated with a reduced capacity to methylate As and are risk factors for As-induced skin lesions. Furthermore, folic acid (FA) supplementation significantly lowers blood As concentrations in individuals who are folate deficient. However, several fundamental questions remain to be addressed: a) Does FA supplementation lower blood As concentrations in the general population? b) What is the optimal dose?, c) What is the time-course of the decline in blood As? d) Is there a rebound in blood As after the cessation of FA supplementation due to release of As from tissue stores? e) Can the ability of FA to lower blood As and homocysteine be enhanced by the addition of a novel new alternative approach: reduce methylation demand. The methylation of guanadinoacetate represents the final step in creatine biosynthesis. This reaction is the major consumer of methyl groups and every molecule of creatine synthesized ultimately generates a molecule of homocysteine. Furthermore, creatinine concentrations are a robust predictor of arsenic methylation. We will test the hypothesis that creatine supplementation, which downregulates endogenous creatine biosynthesis, will spare methyl groups, facilitate the methylation of other substrates - including As - and thereby lower blood As. To answer all of these questions, we are conducting a randomized, double-blind, placebo controlled trial of FA (a comparison of two doses and durations), creatine, and creatine plus FA. Positive results of these interventions could have important therapeutic potential for ameliorating the long-term health consequences of As exposure for the many populations at risk.

One-carbon metabolism, oxidative stress and arsenic toxicity in Bangladesh, Bangladesh: Two overarching themes of the biomedical research of our Superfund Basic Research Program addressed in this project relate to a) the metabolism of arsenic (As) and b) As-induced oxidative stress. In this project, we are expanding our studies which have begun to characterize the impact of nutritional regulation of one-carbon metabolism on the inter-individual variability in As methylation. Glutathione (GSH), a key component of the primary antioxidant defense mechanism, and the electron donor for As reduction, is synthesized from homocysteine, and this synthesis is regulated by intermediates of one-carbon metabolism. A great deal of basic research, including salient work from members of our group, points to the growing belief that As depletes glutathione (GSH) and induces oxidative stress. However, this has not been rigorously examined in human populations. We are conducting a nested case-control study to identify modifiable risk factors ? e.g. oxidative stress and/or hyperhomocysteinemia ? related to increased susceptibility to As-induced skin lesions. We are also examining dose-response relationships between As exposure and oxidative stress. These studies have the potential to a) determine whether or not As induces oxidative stress and depletes GSH in humans, b) link As-induced oxidative stress and/or nutritional status to risk for premalignant skin lesions, and c) expand our understanding of the mechanisms underlying these processes. Such findings could have significant implications for the identification of targeted interventions for preventing As-toxicity.