Biomarkers for Arsenic Toxicity: Genetics, Epigenetics and Folate

描述（由申请人提供）：目前的估计表明，70多个国家中多达1亿人正在饮用砷浓度高达世界卫生组织（世卫组织）每升10微克指导标准100倍的水。从暴露于砷中毒到疾病临床表现的进展有显著的可变性，据认为遗传和营养因素可能共同解释了这种可变性的很大一部分。因此，本研究旨在开发预测砷中毒发展风险的生物标志物。摄入的无机砷（InAs）通过叶酸依赖的单碳代谢被甲基化成甲基larsonic （MMA）和二甲基larsinic （DMA）酸。将As完全甲基化为DMA的能力降低与尿液中As的消除减少有关。DMAV毒性较小，循环半衰期比InAs短得多，并能迅速从尿液中排出。As甲基化能力存在很大的个体间差异，包括我们自己的流行病学证据表明，“良好的甲基化者”患As诱发的皮肤病变、癌症和心血管疾病的风险较低。然而，越来越多来自体外和实验动物研究的证据表明，三价形式的甲基丙烯酸甲酯是合成甲基丙烯酸甲酯途径中必需的中间体，具有极高的毒性。在过去的几年里，我们在孟加拉国的工作已经取得了开创性的发现，关于单碳代谢的营养调节对As甲基化、血液As水平和砷诱导皮肤病变风险的个体差异的强烈影响，例如，我们的临床试验表明，补充叶酸降低了血液As和血液MMA浓度。我们的第一个目标是对1000例恶性前皮肤病变病例和1000例对照进行巢式病例对照研究，以验证As甲基化能力受As甲基转移酶（AS3MT）基因型的影响，以及AS3MT基因型和As甲基化与皮肤病变风险相关的假设。我们的第二个目标是研究单碳代谢候选基因的多态性是否与高同型半胱氨酸血症（这在该人群中很常见，并且与甲基化a到DMA的能力降低有关）、a的甲基化、白细胞DNA的甲基化以及皮肤病变的风险有关。我们还将研究这些关联是否受到叶酸状态的影响。总的来说，这些目标将使我们能够识别一组生物标志物（如代谢物，白细胞DNA甲基化，叶酸，B12， Hcys和snp），以识别个体的亚群，这些亚群可能会增加砷诱导病理的风险，利用已有的生物标本。这些研究还将解决一个关键问题，即更高的甲基化能力是否有益。这些研究的结果将为评估高危人群的干预措施创造机会。它们还将导致识别生物标记物，以促进利用与易感性有关的机制的靶向治疗，从而产生低风险、低成本的治疗策略，可能降低数十万人的疾病风险。

DESCRIPTION (provided by applicant): Current estimates indicate that as many as 100 million people in over 70 countries are drinking water with arsenic (As) concentrations up to 100 times the World Health Organization (WHO) guideline of 10 ug per liter. There is significant variability in progression from As exposure to clinical manifestations of disease, and it is thought that genetic and nutritional factors may together account for a substantial portion of this variability. Thus, this proposal seeks to develop biomarkers that predict risk for the development of arsenicosis. Ingested inorganic As (InAs) is methylated to methylarsonic (MMA) and dimethylarsinic (DMA) acids via folate- dependent one-carbon metabolism. A reduced capacity to fully methylate As to DMA is associated with reduced urinary elimination of As. DMAV is less toxic and has a much shorter circulating half-live than InAs, and it is rapidly excreted in urine. There is great inter-individual variability in As methylation capacity, and epidemiological evidence, including our own, suggests that people who are "good methylators" are at reduced risk for As-induced skin lesions, cancers and cardiovascular disease. However, a growing body of evidence from in vitro and laboratory animal studies indicates that the trivalent form of MMA, a requisite intermediate in the pathway toward DMA synthesis, is extremely toxic. Our work over the past several years in Bangladesh has made seminal findings regarding the strong impact of nutritional regulation of one-carbon metabolism on the inter-individual variability in As methylation, blood As levels, and risk for arsenic-induced skin lesions, e.g., our clinical trial demonstrated that folic acid supplementation lowered blood As and blood MMA concentrations. Our 1st aim is to conduct a nested case-control study of 1,000 incident premalignant skin lesion cases and 1,000 controls to test the hypotheses that the capacity to methylate As is influenced by As methyltransferase (AS3MT) genotypes and that AS3MT genotypes and As methylation are associated with risk for skin lesions. Our 2nd aim will examine whether polymorphisms in one-carbon metabolism candidate genes are associated with hyperhomocysteinemia (which is very common in this population, and is associated with reduced capacity to methylate As to DMA), with methylation of As, methylation of leukocyte DNA, and with risk for skin lesions. We will also examine whether these associations are influenced by folate status. Collectively, these aims will allow us to identify of a set of biomarkers (As metabolites, leukocyte DNA methylation, folate, B12, Hcys, and SNPs) to identify sub-groups of individuals who may be at increased risk for As-induced pathology utilizing biological specimens that are already in hand. These studies will also resolve the critical issue of whether or not a higher capacity to methylate As is beneficial. Results of these studies will create opportunities for evaluation of interventions among high risk groups. They will also result in the identification of biomarkers to facilitate targeted therapies that exploit the mechanisms involved in susceptibility, resulting in low-risk, low-cost therapeutic strategies that could potentially reduce disease risk for hundreds of thousands of people. PUBLIC HEALTH RELEVANCE: Roughly 140 million people across 70 countries are exposed to excessive arsenic (As) in drinking water. There is significant variability in progression from As exposure to clinical manifestations of disease; genetic and nutritional factors may account for a substantial portion of this variability. Ingested inorganic As is methylated to methylarsonic and dimethylarsinic acids via folate-dependent one-carbon metabolism. A reduced capacity to fully methylate As is associated with reduced urinary elimination of As and is thought by many to be associated with increased risk for the development of an array of adverse As-related health outcomes. We wish to expand our studies which have made seminal findings concerning the strong impact of nutritional regulation of one-carbon metabolism on the inter-individual variability in As methylation, blood As levels, and risk for As-induced skin lesions. Collectively, our aims will allow us to identify of a set of biomarkers (As metabolites, leukocyte DNA methylation, folate, B12, Hcys, and SNPs) to identify sub-groups of individuals who may be at increased risk for As-induced pathology. Studying genetic influences on As metabolism and toxicity will also help us to unravel the basic pathobiology of arsenicosis. The nested case control study of premalignant As-induced skin lesions will help us to elucidate disease mechanisms and create extended opportunities for evaluation of targeted interventions among high risk groups. For example, they will result in the identification of biomarkers to facilitate targeted therapies that exploit the mechanisms involved in susceptibility, resulting in low-risk, low- cost therapeutic strategies that could potentially reduce disease risk for hundreds of thousands of people.