Biomarkers for Arsenic Toxicity: Genetics, Epigenetics and Folate

描述(由申请人提供)：目前的估计表明，70多个国家有多达1亿人正在饮用砷浓度高达世界卫生组织(WHO)指导方针每升10微克的100倍的水。从接触砷到疾病的临床表现有很大的变异性，人们认为遗传和营养因素可能共同解释了这种变异性的很大一部分原因。因此，这项建议寻求开发预测砷中毒发展风险的生物标记物。摄入的无机砷(InAs)通过叶酸依赖的一碳代谢甲基化为甲基砷酸(MMA)和二甲基砷酸(DMA)。DNA完全甲基化能力的降低与AS尿排泄的减少有关。DMAV毒性较小，循环半衰期比InAs短得多，而且它能迅速从尿液中排泄。AS甲基化能力在个体之间有很大的差异性，流行病学证据，包括我们自己的证据表明，那些“良好的甲基化者”的人患AS引起的皮肤损伤、癌症和心血管疾病的风险较低。然而，越来越多的来自体外和实验室动物研究的证据表明，MMA的三价形式是合成DMA过程中必不可少的中间体，具有极强的毒性。我们过去几年在孟加拉国的工作取得了开创性的发现，即一碳代谢的营养调节对砷甲基化、血液砷水平和砷致皮肤损害风险的个体间变异性具有强烈影响，例如，我们的临床试验表明，补充叶酸可以降低血液中砷和MMA的浓度。我们的第一个目标是对1,000例癌前皮损病例和1,000名对照进行嵌套式病例对照研究，以检验AS甲基化能力受AS甲基转移酶(AS3MT)基因影响的假设，以及AS3MT基因和AS甲基化与皮肤损害风险相关的假设。我们的第二个目标是检查单碳代谢候选基因的多态是否与高同型半胱氨酸血症(这在该人群中非常常见，并与DMA甲基化能力降低有关)、AS的甲基化、白细胞DNA的甲基化以及皮肤损害的风险有关。我们还将研究这些联系是否受叶酸状况的影响。总而言之，这些目标将使我们能够识别一套生物标记物(如代谢物、白细胞DNA甲基化、叶酸、B12、Hcys和SNPs)，以利用已有的生物标本识别可能处于AS诱导病理风险增加的个体亚群。这些研究还将解决关键问题，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中的干预措施创造机会。它们还将导致识别生物标记物，以促进利用易感性相关机制的靶向治疗，从而产生低风险、低成本的治疗策略，有可能降低数十万人的疾病风险。 与公共卫生相关：70个国家的大约1.4亿人暴露在饮用水中过量的砷(As)中。从AS暴露到疾病的临床表现的进展有很大的变异性；遗传和营养因素可能是这种变异性的主要原因。摄入的无机砷通过叶酸依赖的一碳代谢甲基化为甲基砷酸和二甲基砷酸。AS完全甲基化能力的降低与AS尿排泄减少有关，许多人认为这与发生一系列与AS相关的不良健康后果的风险增加有关。我们希望扩大我们的研究，这些研究已经取得了开创性的发现，即一碳代谢的营养调节对AS甲基化、血液AS水平和AS诱导的皮肤损害风险的个体间变异性的强烈影响。总而言之，我们的目标将使我们能够识别一组生物标记物(作为代谢物、白细胞DNA甲基化、叶酸、B12、Hcys和SNPs)，以识别可能处于AS诱导病理风险增加的个体的亚群。研究遗传对砷代谢和毒性的影响也将有助于我们揭示砷中毒的基本病理生物学。癌前AS诱发皮肤损害的嵌套病例对照研究将有助于我们阐明疾病机制，并为在高危人群中评估有针对性的干预措施创造更多机会。例如，它们将导致识别生物标记物，以促进利用易感性相关机制的靶向治疗，从而产生低风险、低成本的治疗策略，有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.