Characterization of the microsomal epoxide hydrolase gene in patients with anticonvulsant adverse drug reactions

Abstract

Therapy with the aromatic anticonvulsants phenytoin, phenobarbital and carbamazepine has been associated with the occurrence of rare idiosyncratic hypersensitivity reactions. These drugs are thought to be activated to potentially reactive arene oxide (epoxide) metabolites by cytochrome P450-dependent monooxygenation, while liver microsomal epoxide hydrolase (mEH) plays a detoxifying role by converting such reactive intermediates to non-toxic dihydrodiols. Evidence from in vitro lymphocyte toxicity tests and enzyme inhibitor studies has suggested that an inherited defect in mEH function may be responsible for the enhanced drug toxicity observed in affected individuals. To test this hypothesis we designed methods to directly compare mEH gene structure in patients presenting with anticonvulsant adverse reactions and in control subjects in which no in vivo or in vitro toxicity to anticonvulsants could be demonstrated. Southern analysis of peripheral lymphocyte DNA using a full-length mEH cDNA as hybridization probe revealed no gross differences in mEH gene structure in selected patients when compared with DNA samples from unaffected control subjects. The human mEH gene was then cloned and characterized from a control individual. Nine exons were identified within a 22 kb region and sequences of selected regions, including all exons, were determined. Single strand conformation polymorphism (SSCP) analysis was performed on all exonic regions in genomic DNA from each of 26 subjects, including six unrelated patients with previous toxicity to anticonvulsant therapy and seven siblings (three of whom had displayed toxicity). Several distinct SSCP patterns could be observed among the subjects tested, each corresponding to a specific point mutation within one of the amplified fragments of the mEH gene. However, none of the SSCP patterns reflecting point mutations was correlated with the occurrence of anticonvulsant toxicity. From these observations we conclude that a genetic defect altering the structure and function of the mEH protein is unlikely to be responsible for predisposing patients to anticonvulsant adverse reactions.