Abstract

A genetic polymorphism in the metabolism of the anticonvulsant drugS-mephenytoin has been attributed to defectiveCYP2C19 alleles. This genetic polymorphism displays large interracial differences with the poor metabolizer (PM) phenotype representing 2–5% of Caucasian and 13–23% of Oriental populations. In the present study, we identified two new mutations inCYP2C19 in a single Swiss Caucasian PM outlier (JOB 1) whose apparent genotype (CYP2C19*1/CYP2C19*2) did not agree with his PM phenotype. These mutations consisted of a single base pair mutation (G₃₉₅A) in exon 3 resulting in an Arg₁₃₂→Gln coding change and a (G₂₇₆C) mutation in exon 2 resulting in a coding change Glu₉₂→Asp. However, the G₂₇₆C mutation and the G₃₉₅A mutation resided on separate alleles. Genotyping tests of a family study of JOB1 showed that the exon 2 change occurred on the CYP2C19*2 allele, which also contained the known splice mutation in exon 5 (this variant is termedCYP2C19*2B to distinguish it from the original splice variant now termed CYP2C19*2A). The exon 3 mutation resided on a separate allele (termed CYP2C19*6). In all other respects this allele was identical to one of two wild-type alleles, CYP2C19*1B. The incidence ofCYP2C19*6 in a European Caucasian population phenotyped for mephenytoin metabolism was 0/344 (99% confidence limits of 0 to 0.9%). Seven of 46 Caucasian CYP2C19*2 alleles wereCYP2C19*2B(15%) and 85% wereCYP2C19*2A. The Arg₁₃₂Gln mutation was produced by site-directed mutatgenesis and the recombinant protein expressed in a bacterial cDNA expression system. Recombinant CYP2C19 6 had negligible catalytic activity toward S-mephenytoin compared with CYP2C19 1B, which is consistent with the conclusion thatCYP2C19*6 represents a PM allele. Thus, the newCYP2C19*6 allele contributes to the PM phenotype in Caucasians.