Haplotype structures of EPHX1 and their effects on the metabolism of carbamazepine-10,11-epoxide in Japanese epileptic patients

Objective: Microsomal epoxide hydrolase (mEH) is an enzyme that detoxifies reactive epoxides and catalyzes the biotransformation of carbamazepine-10,11-epoxide (CBZ-epoxide) to carbamazepine-10,11-diol (CBZ-diol). Utilizing single nucleotide polymorphisms (SNPs) of the EPHX1 gene encoding mEH, we identified the haplotypes of EPHX1 blocks and investigated the association between the block haplotypes and CBZ-epoxide metabolism. Methods: SNPs of EPHX1 were analyzed by means of polymerase chain reaction amplification and DNA sequencing using DNA extracted from the blood leukocytes of 96 Japanese epileptic patients, including 58 carbamazepine-administered patients. The plasma concentrations of CBZ and its four metabolites were determined using high-performance liquid chromatography. Results: From sequencing all 9 exons and their surrounding introns, 29 SNPs were found in EPHX1. The SNPs were separated into three blocks on the basis of linkage disequilibrium, and the block haplotype combinations (diplotypes) were assigned. Using plasma CBZ-diol/CBZ-epoxide ratios (diol/epoxide ratios) indicative of the mEH activity, the effects of the diplotypes in each EPHX1 block were analyzed on CBZ-epoxide metabolism. In block 2, the diol/epoxide ratios increased significantly depending on the number of haplotype *2 bearing Y113H (P=0.0241). In block 3, the ratios decreased depending on the number of haplotype *2 bearing H139R (P=0.0351). Also, an increasing effect of a *1 subtype, *1c, was observed on the ratio. Conclusion: These results show that some EPHX1 haplotypes are associated with altered CBZ-epoxide metabolism. This is the first report on the haplotype structures of EPHX1 and their potential in vivo effects.