A new genetic defect in human CYP2C19: mutation of the initiation codon is responsible for poor metabolism of S-mephenytoin.

The 4'-hydroxylation of the S-enantiomer of the anticonvulsant drug mephenytoin exhibits a genetic polymorphism in humans. This polymorphism shows marked interracial heterogeneity, with the poor metabolizer (PM) phenotype representing 2 to 5% of Caucasian and 13 to 23% of Asian populations. Two defective CYP2C19 alleles, CYP2C192 and CYP2C193, have been described which account for approximately 87% of Caucasian and > 99% of Oriental PM alleles. The present study identifies a new allele (CYP2C194) in Caucasian PMs which contains an A-->G mutation in the initiation codon. A new polymerase chain reaction-restriction fragment length polymorphism genotyping test was developed, and the incidence of this allele was examined in a European Caucasian population which had been phenotyped for mephenytoin metabolism. One of nine putative PMs was heterozygous for CYP2C192/CYP2C194, which suggests that CYP2C194 represents a defective allele. Six of the seven remaining putative PMs available for genotyping were explained by CYP2C192. The frequency of the CYP2C194 allele in Caucasians was 0.6%. An additional Caucasian PM from a separate study was also heterozygous for CYP2C192 and CYP2C194. To verify that CYP2C194 represented a defective CYP2C19 allele, the initiation codon of the normal CYP2C191 cDNA was mutated to a GTG, and both cDNAs were expressed in yeast. Recombinant CYP2C19 protein was detected by Western blot analysis of colonies transformed with CYP2C191 cDNA, but not in those transformed with CYP2C194 cDNA. The two cDNAs were also used in an in vitro coupled transcription/translation assay. CYP2C19 protein was translated only from the CYP2C191 allele. These data indicate that CYP2C194 represents a new PM allele.