Interindividual Variability in Human Microsomal Epoxide Hydrolase

描述（由申请人提供）：外源代谢主要导致解毒。然而，在某些情况下，会产生具有生物活性和剧毒的中间体。由化学代谢产生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到亲电反应代谢物才能发挥其致突变和致瘤活性。微粒体环氧化物水解酶EPHX1生物激活多环芳烃，促进海湾地区和峡湾地区形成高活性和致癌的环氧二醇中间体。此前，我们对人类EPXH1的基因结构进行了表征，发现了两个结构多态性，一个是编码Y/H替换的第3外显子多态性，编码第113位氨基酸的Y/H替换，另一个是编码第139位氨基酸的第4外显子H/R替换。许多分子流行病学研究结果表明，EPHX1等位基因变异是多种疾病的危险因素，尤其是肺癌。最近，我们实验室的一份范式转换报告表明，人类EPHX1基因的表达是由替代启动子的使用驱动的，在大多数人类组织中，一个远上游的启动子，称为E1-b，优先驱动EPHX1的表达。新的研究结果表明，E1-b启动子本身具有遗传多态性，并可通过膳食化学预防剂（如萝卜硫素）进行转录调节。在这个研究项目中，我们设计了一些实验来测试几个关键和综合的假设，包括：1)EPHX1最常见的结构遗传变异（Y113H; H139R）通过它们在多环芳烃环氧化物之间的底物特异性催化活性，特别是对峡湾地区表现出极高致瘤潜力的环氧化物，赋予它们与多环芳烃诱导的癌症风险的关联；2) E1-b启动子区域的相关转录活性受到作为外源诱导毒性主要靶点的组织中遗传多态性和转座遗传元件的相互调节；3)人体组织中EPHX1的表达受暴露于异硫氰酸酯衍生物的差异调控，异硫氰酸酯衍生物是目前研究中的化学预防剂。这些研究的结果将描述人类EPXH1的酶学、转录调控和遗传学的关键特征，并阐明这些过程作为人类疾病潜在风险调节剂的功能作用。公共卫生相关性：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中起关键作用。这种酶的遗传和调控可能在一定程度上决定了化学物质暴露（包括癌症）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这些差异的关键特征，并研究它们作为人类疾病风险调节剂的作用。

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.