Genetic polymorphism of human CYP2E1: characterization of two variant alleles

Ethanol-inducible CYP2E1 is an enzyme of major toxicological interest because it metabolizes several precarcinogens, drugs, and solvents to reactive metabolites. CYP2E1 has also been implicated in alcohol liver disease because of its contribution to oxidative stress. Previously, polymorphic alleles with mutations in introns and in the 5'-flanking regulatory region have been described, and their presence has been related to the incidence of alcohol liver disease and lung cancer. In the present investigation, we investigated whether any functional mutations are linked to the above-mentioned rare alleles and also screened for mutations in the open reading frame using single-stranded conformation polymorphism and genomic DNA from almost 200 individuals belonging to either a Chinese, an Italian, or a Swedish population. Two new CYP2E1 gene variants were found with functional mutations: one (CYP2E1*2) in which a G1168A point mutation in exon 2 caused an R76H amino acid substitution, and the other (CYP2E1*3) in which a G10059A base substitution in exon 8 yielded a V3891 amino acid exchange. The corresponding CYP2E1 cDNAs were constructed, subcloned into the pCMV4 expression vector, and expressed in COS-1 cells. The cellular levels of CYP2E1 mRNA, CYP2E1 protein, and rate of chlorzoxazone hydroxylation were monitored. The CYP2E1*3 cDNA variant was indistinguishable from the wild-type cDNA on all variables investigated, whereas CYP2E1*2 cDNA, although yielding similar amounts of mRNA, only caused 37% of the protein expression and 36% of the catalytic activity compared with the wild-type cDNA. Complete screening by single-stranded conformation polymorphism of the three populations studied revealed that these variant alleles were rare. We conclude that the human CYP2E1 gene is functionally surprisingly well conserved compared with other cytochrome P450 enzymes active in drug metabolism, which suggests an important endogenous function in humans.