Drug Metabol Drug Interact 2013 ;28(1):23-30
Background: Cytochrome P450 2C19 is responsible for the metabolism of many drugs, including the activation of clopidogrel. The allele CYP2C19*17 is associated with ultra-rapid metabolizer phenotypes by increasing gene transcription. This study tests to what extent CYP2C19*17 enhances CYP2C19 expression in human liver and whether additional regulatory variants contribute to variation in CYP2C19 expression.
Methods: CYP2C19 mRNA was measured with quantitative real-time PCR (qRT-PCR), enzyme activity as metabolic velocity with S-mephenytoin as the substrate and allelic mRNA expression ratio with SNaPshot in human livers. CYP2C19 transcribed exons and a 4kb promoter region were sequenced using IonTorrent PGM or Sanger sequencing and screened for polymorphisms associated with total hepatic CYP2C19 mRNA, enzyme activity and allelic mRNA ratios.
Results: Livers heterozygote and homozygous for CYP2C19*17 had mRNA levels 1.8-fold (p=0.028) and 2.9-fold (p=0.006), respectively, above homozygous reference allele livers. CYP2C19*17 heterozygotes were also associated with increased allelic mRNA expression (allelic ratio ~1.8-fold, SD±0.6, p<0.005), whereas CYP2C19 enzyme activity was elevated 2.3-fold, with borderline significance (p=0.06) in CYP2C19*17 carriers. One liver sample of African ancestry displayed a 2-fold allelic expression ratio, and another sample, a ~12-fold increase in metabolic velocity. Neither case was accounted for by *17, which indicates the presence of additional regulatory variants.
Conclusions: Our findings confirm *17 as a regulatory polymorphism enhancing hepatic CYP2C19 expression 2-fold with potential to compensate for the loss of function allele CYP2C19*2. Additional regulatory factors may also enhance CYP2C19 expression in African American populations.