Interindividual Variability in Human Microsomal Epoxide Hydrolase

描述(申请人提供)：异种生物代谢的主要结果是解毒。然而，在某些情况下，会产生生物活性和剧毒的中间体。化学新陈代谢导致的细胞水平的环氧化物部分似乎是毒性损伤的关键启动者，包括基因突变。例如，多环芳烃(PAHs)是有机物不完全燃烧的产物，是广泛存在的环境污染物，被认为是前致癌物，因为它们需要代谢激活亲电活性的反应性代谢物来发挥其诱变和致癌活性。微粒体环氧化物水解酶EPHX1激活多环芳烃，有助于形成高活性和致癌的湾区和峡湾区二醇环氧化物中间体。在此之前，我们研究了人EPXH1的基因结构，发现了两个结构多态，一个是编码第113位氨基酸的Y/H替换的外显子3多态，另一个是第139位编码外显子4的H/R替换。许多分子流行病学研究的结果表明，EPHX1等位基因变异是各种疾病的危险因素，尤其是肺癌。最近，我们实验室的一份范式转换报告表明，人EPHX1基因的表达是由替代启动子的使用驱动的，其中有一个远上游的启动子，称为E1-b，在大多数人类组织中优先驱动EPHX1的表达。新的发现表明，e1-b启动子本身在基因上是多态的，并受到饮食化学预防药物(如萝卜硫素)的转录调控。在这个研究计划中，我们设计了实验来检验几个关键的和综合的假设，包括：1)EPHX1(Y113H；H139R)最常见的结构遗传变异通过它们在多环芳烃环氧化物中的底物特异性催化活性，特别是对具有极高致癌潜力的峡区环氧化物的底物特异性催化活性，而与多环芳烃致癌风险相关；2)e1-b启动子区域的相关转录活性是由组织中存在的遗传多态和可转座的遗传元件相互调节的，这些基因元件是异种生物诱导毒性的主要靶标；以及，3)EPHX1在人体组织中的表达受暴露于异硫氰酸酯衍生物的差异调节，异硫氰酸酯衍生物是目前研究中作为化学防腐剂的饮食物质。这些研究的结果将描述包括人类EPXH1的酶学、转录调控和遗传学在内的关键特征，并阐明这些过程作为人类疾病潜在风险修饰物的功能作用。与公共健康相关：微粒体环氧化物水解酶是一种存在于大多数组织中的酶，在代谢许多环境化学物质方面发挥关键作用。这种酶的遗传和调控可能在一定程度上决定了化学暴露(包括癌症)所导致的毒性的个体间差异的性质和程度。这项研究计划将描述解释这些差异的关键特征，并调查它们作为人类疾病风险修饰物的作用。

DESCRIPTION (provided by applicant): Xenobiotic metabolism results principally in detoxication. However, in certain instances, bioactivated and highly toxic intermediates are generated. Cellular levels of epoxide moieties resulting from chemical metabolism appear to be critical initiators of toxic damage, including genetic mutation. As a case in point, polyaromatic hydrocarbons (PAHs) are products of incomplete combustion of organic matter, are widespread environmental contaminants and are considered procarcinogens because they require metabolic activation to electrophilic reactive metabolites to exert their mutagenic and tumorigenic activities. Microsomal epoxide hydrolase, EPHX1, bioactivates PAHs, contributing to the formation of highly reactive and carcinogenic bay region and fjord region diol-epoxide intermediates. Previously, we characterized the gene structure for human EPXH1 and identified two structural polymorphisms, an exon 3 polymorphism encoding Y/H substitutions at amino acid position 113, and an exon 4 H/R substitution at position 139. Results of many molecular epidemiological studies have since associated EPHX1 allelic variation as a risk factor for various diseases, most notably lung cancer. Recently, a paradigm-shifting report from our laboratory demonstrated that the expression of the human EPHX1 gene is driven by the use of alternative promoters, with a far upstream promoter, termed E1-b, preferentially driving expression EPHX1 in most human tissues. New findings demonstrate that the E1-b promoter is itself genetically polymorphic and is transcriptionally modulated by dietary chemopreventive agents, such as sulforaphane. In this research program, we have designed experiments to test several key and integrated hypotheses, including: 1) that the most common structural genetic variants of EPHX1 (Y113H; H139R) confer their association with PAH-induced cancer risk by nature of their substrate-specific catalytic activity among PAH epoxides, specifically towards fjord-region epoxides that exhibit extraordinarily high tumorigenic potential; 2) that the associated transcriptional activity of the E1-b promoter region is interindividually regulated by the presence of genetic polymorphism and transposable genetic elements in tissues that are principle targets of xenobiotic-induced toxicity; and, 3) that EPHX1 expression in human tissues is differentially regulated by exposures to isothiocyanate derivatives, dietary substances under current study as chemoprevention agents. The results of these investigations will delineate critical features comprising the enzymology, transcriptional regulation and genetics of human EPXH1 and elucidate the functional roles of these processes as potential risk modifiers of human disease. PUBLIC HEALTH RELEVANCE: Microsomal epoxide hydrolase is an enzyme that is present in most tissues and plays a key role in metabolizing numerous environmental chemicals. The genetics and regulation of this enzyme likely determines, in part, the nature and extent of interindividual differences in toxicity that result from chemical exposures, including cancers. This research program will characterize the critical features that account for these differences and investigate their roles as risk modifiers of human diseases.