Decreased coumarin 7-hydroxylase activities and CYP2A6 expression levels in humans caused by genetic polymorphism in CYP2A6promoter region (CYP2A69)

One of seven poor metabolizers of coumarin found in Thai subjects was previously genotyped as heterozygote for the CYP2A64(whole deletion) and CYP2A69. Thus, we aimed to investigate the relationship between the genetic polymorphism in the TATA box of the CYP2A6gene (CYP2A69), expression levels of CYP2A6 mRNA and coumarin 7-hydroxylase activities in human livers. Levels of CYP2A6 mRNA were quantified by real-time quantitative reverse transcriptase-polymerase chain reaction. The mean expression levels of CYP2A6 mRNA in individuals with CYP2A61/4, CYP2A61/9and CYP2A64/9were 58%, 71% and 21% of the individuals genotyped as CYP2A61/1, respectively. The mean in-vitro coumarin 7-hydroxylase activities in subjects carrying CYP2A61/4, CYP2A61/9and CYP2A64/9were 41%, 71% and 12%, respectively, compared to those of the subjects judged as wild-type. Vmaxvalues for coumarin 7-hydroxylation in the liver microsomes from human subjects with genotypes of CYP2A61/1, CYP2A61/4, CYP2A61/9and CYP2A64/9were 0.58, 0.26, 0.44 and 0.13 nmol/min/nmol total P450, respectively. CYP2A6 protein levels in human liver microsomes with the CYP2A64and the CYP2A69alleles were markedly decreased. These results suggest that the genetic polymorphism in the promoter region of the CYP2A6gene (CYP2A69) reduced the expression levels of CYP2A6 mRNA and protein in human livers, resulting in the decrease of coumarin 7-hydroxylase activities. Individuals judged as CYP2A64/9were expected to be poor metabolizers, having extremely low activity of CYP2A6.