Variability in human sensitivity to 1,3-butadiene: Influence of the allelic variants of the microsomal epoxide hydrolase gene

The carcinogenic effects of 1,3-butadiene (BD), a chemical widely used in the rubber industry, are thought to be due to its epoxide metabolites. In humans, these epoxides are detoxified predominantly by hydrolysis, a reaction mediated by the microsomal epoxide hydrolase (mEH) enzyme. The mEH gene is polymorphic and the most common mEH coding-region variants detected in human populations are the two amino acid polymorphisms Tyr113His and His139Arg. Polymorphic amino acid substitutions at residues 113 and 139 in the human mEH protein can associate in four distinct combinations: Tyr113/His139, Tyr113/Arg139, His113/His139, and His113/Arg139. In vitro studies have shown that each of these genotypes has a unique mEH protein level that can affect net mEH enzymatic activity. In the current study, we examined the relationships among the genotypes involving these two polymorphisms and the mutagenic responses associated with occupational exposure to BD. We studied 49 nonsmoking workers from two styrene-butadiene rubber facilities in southeast Texas using the autoradiographic HPRT mutant lymphocyte assay as a biomarker of genotoxic effect. We genotyped the study participants simultaneously for both polymorphisms, using a multiplex PCR assay developed in our laboratory, and the subjects were assigned to a specific group based on the predicted mEH activity associated with their genotypes (low, intermediate, and high). In the study population, 67% were exposed to low BD levels of < 150 ppb (measured by personal badge dosimeters) and 33% were exposed to > 150 ppb (mean 2,244 ppb). In the BD low-exposure group, the mEH genotypes had no significant effect on the HPRT variant (mutant) frequency (vf). In the high-exposure group (BD > 150 ppb), individuals with genotypes associated with low mEH activity had a significant (P < 0.05) 3-fold increase in HPRTVf (Vf ± SEM = 13.95 ± 2.15 x 10^-6) compared to high-activity individuals (4.41 ± 1.19 x 10^-6), and a 2-fold increase in Vf compared to intermediate-activity individuals (6.44 ± 2.09 x 10^-6). Our results indicate that mEH genotypes may play a significant role in human sensitivity to the genotoxic effects of exposure to BD.