Characterization of CYP2A13*2, a Variant Cytochrome P450 Allele Previously Found to Be Associated with Decreased Incidences of Lung Adenocarcinoma in Smokers

CYP2A13, a human cytochrome P450 enzyme expressed mainly in the respiratory tract, is believed to play an important role in the initiation of smoking-induced lung cancer. CYP2A13.1 has high efficiency in the metabolic activation of a major tobacco-specific carcinogenic nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). CYP2A13*2, a variant allele, was previously found to be associated with decreased incidence of lung adenocarcinoma in smokers. The aim of the present study was to determine whether the CYP2A13.2 protein has decreased enzyme activity and/or expression levels in the lung, compared with CYP2A13.1. CYP2A13.2 has two sequence variations from CYP2A13.1: R25Q and R257C. We compared the activities of heterologously expressed CYP2A13.1 and CYP2A13.2 toward several known CYP2A13.1 substrates: NNK, N-nitrosomethylphenylamine, N,N-dimethylaniline, 2'-methoxyacetophenone, and hexamethylphosphoramide. Our results indicated that CYP2A13.2 was 20 to 40% less active than CYP2A13.1 with the substrates tested. We also determined the levels of the CYP2A13*2 mRNA, relative to the level of the CYP2A13*1 mRNA, in the lung tissue from *1/*2 heterozygotes. We found that the CYP2A13*2 allele was associated with a level of allelic expression similar to 40% lower than that of the CYP2A13*1 allele. Sequence analysis of the promoter region of the CYP2A13*2 allele identified a 26-nucleotide deletion. Functional analysis of a 2-kilobase pair CYP2A13-luciferase promoter construct indicated that the 26-nucleotide deletion causes decreases in CYP2A13 promoter activity in the A549 human lung cell line. These findings suggest that the reported association of the CYP2A13*2 allele with decreased incidences of lung adenocarcinoma in smokers can be at least partly explained by a decrease in CYP2A13 function.