Regulation and polymorphism of CYP2A6, CYP2B6 and CYP2E1 : functional and clinical aspects

Abstract

Cytochrome P450s constitute a superfamily of heme-binding monooxygenases that play important roles in the metabolism of endogenous substrates and a wide variety of exogenous substances, such as toxins and drugs, and participate as well in the biosynthesis of steroids, fatty acids, vitamins and bile acids. The P450s genes are subjects for large inter-individual variability in expression, which subsequently cause inter-individual differences in drug metabolism. A significant amount of the inter-individual variation can be attributed to genetic polymorphisms of P450 genes. The gene regulation of P450 expression occurs at multiple levels, e.g. transcription, RNA processing, translation and post-translation.

Novel functional polymorphisms altering P450 expression were here identified in CYP2B6 and CYP2A6 genes. A novel CYP2B6*16 allele was found to carry two SNPs, 983T>C and 785A>G, and was present in 5 subjects having high plasma concentrations of efavirenz, all of African origin. This allele was found to significantly impair the expression level of CYP2B6, especially at the protein level, indicating impaired post-translational processing. Examination of the flanking regions of CYP2A6 gene revealed two functionally polymorphic elements. A strong enhancer was identified between -1005 and -1019, containing a SNP, -1013A>G. A known polymorphism in the 3´-UTR, CYP2A6*1B characterized by the gene conversion with CYP2A7 3´-UTR, was shown to be related to increased mRNA and protein levels and catalytic activity of coumarin. In vitro transfection experiments indicated that the stabilization of the CYP2A6*1B mRNA resulted in higher protein expression and also a higher rate of nicotine metabolism in vivo as shown by others.

IL4 transcriptionally activates the expression of CYP2E1 gene, which is an important mechanism in P450 regulation by inflammation and infection. The regulation was shown to be mediated by two independent signaling pathways, the JAK-STAT6 and IRS1/2 pathways. These two pathways activate specific cis- and trans-acting elements, indicating the significance of the regulation. IL4-activated signaling and the possible feedback mechanisms revealed the complexity of P450 gene regulation.

In summary, our studies contribute to the understanding of the regulatory mechanisms underlying the variable expression of P450 genes. Genetic polymorphisms contribute to a large extent of the variability, via multiple mechanisms. We have also shown that the regulation of CYP2E1 is mediated by a complex intracellular network.