Biomarkers for Arsenic Toxicity: Genetics, Epigenetics and Folate

描述（由申请人提供）：目前的估计表明，在70多个国家中，有多达1亿人饮用的水中砷（As）浓度高达世界卫生组织（WHO）指南（10 μ g/L）的100倍。从As暴露到疾病临床表现的进展存在显著的变异性，并且认为遗传和营养因素可能共同导致这种变异性的很大一部分。因此，该提案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）通过叶酸依赖的一碳代谢甲基化为甲基胂酸（MMA）和二甲基胂酸（DMA）。完全甲基化为DMA的能力降低与As的尿消除减少有关。DMAV的毒性较低，循环半衰期比InAs短得多，并迅速经尿液排泄。有很大的个体间变异的甲基化能力，流行病学证据，包括我们自己的，表明人谁是“好的甲基化”是在降低风险的作为诱导的皮肤病变，癌症和心血管疾病。然而，来自体外和实验室动物研究的越来越多的证据表明，MMA的三价形式是DMA合成途径中的必要中间体，具有极高的毒性。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、血液砷水平和砷诱导的皮肤病变风险的个体间变异性产生了强烈影响，例如，我们的临床试验证明，叶酸补充剂降低了血液中As和血液中MMA的浓度。我们的第一个目标是进行巢式病例对照研究，1,000例发病癌前皮肤病变病例和1,000例对照，以检验假设，即甲基化的能力是由作为甲基转移酶（AS 3 MT）基因型的影响，以及AS 3 MT基因型和作为甲基化与皮肤病变的风险。我们的第二个目标是研究一碳代谢候选基因的多态性是否与高同型半胱氨酸血症（这在该人群中非常常见，并且与As甲基化为DMA的能力降低有关），As甲基化，白细胞DNA甲基化以及皮肤病变风险相关。我们还将研究这些协会是否受叶酸状态的影响。总的来说，这些目标将使我们能够确定一组生物标志物（As代谢物，白细胞DNA甲基化，叶酸，B12，HCys和SNP），以确定亚组的个人谁可能是在增加风险的As诱导的病理利用生物标本已经在手。这些研究还将解决更高的甲基化As能力是否有益的关键问题。这些研究的结果将为评估高危群体的干预措施创造机会。它们还将导致生物标志物的鉴定，以促进利用易感性机制的靶向治疗，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.