A far upstream and genetically polymorphic alternative promoter drives expression of the human microsomal epoxide hydrolase (EPHX1) gene

Abstract

Microsomal epoxide hydrolase (EPHX1) plays an important role in the bioactivation of polycyclic aromatic hydrocarbons (PAH). The genetics and regulation of EPHX1 in humans is likely an important determinant of interindividual differences in toxicity outcomes related to chemical exposure. Previously, we identified two EPHX1 genetic polymorphisms that alter the amino acid structure of the enzyme. Subsequently, a number of epidemiology investigations have associated EPHX1 coding region polymorphisms as a risk factor for lung cancer. The initially defined EPHX1 exon 1, E1, is derived from the use of a promoter directly proximal to exon 2 of the EPHX1 coding region. Remarkably, the E1 promoter directs expression only in the liver. We have now identified an alternative EPHX1 promoter, E1‐b, localized ~18.5 kb upstream of exon 2. Recently, real‐time RT‐ PCR assays were specifically designed for E1 and E1‐b, and the expression of each transcript was quantified relative to a standard curve. The results demonstrate that the E1‐b promoter is the primary driver of EPHX1 gene expression in liver as well as extrahepatic tissues, with the expression of E1‐b in lung comparable to levels in liver. Additionally, E1‐b promoter region polymorphisms at −85 and −137, as well as the genetically variable presence of Alu repetitive elements appear to function as determinants of interindividual differences in EPHX1 transcript expression.