Abstract
CYP2C9 metabolizes approximately 20% of clinically administered drugs. Several single-nucleotide polymorphisms (SNPs) of CYP2C9 (e.g., *2, *3, *8, and rs12777823) are used as biomarkers to predict CYP2C9 activity. However, a large proportion of variability in CYP2C9 expression remains unexplained. BACKGROUND/OBJECTIVES: We previously identified a variable number tandem repeat (pVNTR) polymorphism in the CYP2C9 promoter. The short repeat (pVNTR-S) showed reduced transcriptional activity in reporter gene assays and was associated with decreased CYP2C9 mRNA expression. However, because pVNTR-S is in high linkage disequilibrium (LD) with CYP2C9*3 in the European population, whether pVNTR-S directly impacts CYP2C9 expression remains unclear. The objective of this study was to clarify the association between the pVNTR-S and CYP2C9 mRNA expression in human liver samples and to assess its impact on CYP2C9 expression independently of known CYP2C9 biomarkers. METHODS: Gene expression was measured by real-time qPCR. SNPs and pVNTRs were genotyped using SNapShot assays and fragment analysis, respectively. Associations between CYP2C9 and the pVNTR-S or SNPs were analyzed using multiple linear regression. RESULTS: Our results showed that pVNTR-S was associated with lower CYP2C9 expression (34% reduction, p-value = 0.032) in human liver samples (n = 247), while the known CYP2C9 biomarkers (CYP2C9*2, *3, *8, or rs12777823) were not. These results suggest that pVNTR-S reduces CYP2C9 expression independently of known biomarkers. Therefore, pVNTR-S may explain additional variability in CYP2C9 expression when present alone or in conjunction with other CYP2C9 alleles.