Hepatic CYP2A6 levels and nicotine metabolism: impact of genetic, physiological, environmental, and epigenetic factors.

PURPOSE: We investigated the role of genetic, physiological, environmental, and epigenetic factors in regulating CYP2A6 expression and nicotine metabolism. METHODS: Human livers (n = 67) were genotyped for CYP2A6 alleles and assessed for nicotine metabolism and CYP2A6 expression (mRNA and protein). In addition, a subset of livers (n = 18), human cryopreserved hepatocytes (n = 2), and HepG2 cells were used for DNA methylation analyses. RESULTS: Liver samples with variant CYP2A6 alleles had significantly lower CYP2A6 protein expression, nicotine C-oxidation activity, and affinity for nicotine. Female livers had significantly higher CYP2A6 protein and mRNA expression compared to male livers. Livers exposed to dexamethasone and phenobarbital had higher CYP2A6 expression and activity, however the difference was not statistically significant. Age and DNA methylation status of the CpG island and a regulatory site were not associated with altered CYP2A6. CONCLUSIONS: We identified genotype, gender, and exposure to inducers as sources of variation in CYP2A6 expression and activity, but much variation remains to be accounted for.

Novel and established CYP2A6 alleles impair in vivo nicotine metabolism in a population of Black African descent.

Cytochrome P450 2A6 (CYP2A6) is a human enzyme best known for metabolizing tobacco-related compounds, such as nicotine, cotinine (COT), and nitrosamine procarcinogens. CYP2A6 genetic variants have been associated with smoking status, cigarette consumption, and tobacco-related cancers. Our objective was to functionally characterize four nonsynonymous CYP2A6 sequence variants with respect to their haplotype, allele frequency, and association with in vivo CYP2A6 activity. In vivo, nicotine was administered orally to 281 volunteers of Black African descent. Blood samples were collected for kinetic phenotyping and CYP2A6 genotyping. In vitro, nicotine C-oxidation catalytic efficiencies of heterologously expressed variant enzymes were assessed. The four uncharacterized sequence variants were found in seven novel alleles CYP2A6(*)24A&B ; (*)25, (*)26, (*)27, and *28A&B, most were associated with impaired in vivo CYP2A6 activity. Nicotine metabolism groupings, based on the in vivo data of variant alleles, were created. Mean trans-3'-hydroxycotinine/cotinine (3HC/COT) differed (P<0.001) between normal (100%), intermediate (64%), and slow (40%) groups. Systemic exposure to nicotine following oral administration also differed (P<0.001) between normal (100%), intermediate (139%), and slow (162%) metabolism groups. In addition, alleles of individuals with unusual phenotype-genotype relationships were sequenced, resulting in the discovery of five novel uncharacterized alleles and at least one novel duplication allele. A total of 7% of this population of Black African descent had at least one of the eight novel characterized alleles and 29% had at least one previously established allele. These findings are important for increasing the accuracy of association studies between CYP2A6 genotype and behavioral, disease, or pharmacological phenotypes.

Ethnic variation in CYP2A6 and association of genetically slow nicotine metabolism and smoking in adult Caucasians.

Genetically variable CYP2A6 is the primary enzyme that inactivates nicotine to cotinine. Our objective was to investigate allele frequencies among five ethnic groups and to investigate the relationship between genetically slow nicotine metabolic inactivation and smoking status, cigarette consumption, age of first smoking and duration of smoking. Chinese, Japanese, Canadian Native Indian, African-North American and Caucasian DNA samples were assessed for CYP2A6 allelic frequencies (CYP2A6*1B-*12,*1x2). Adult Caucasian non-smokers (n = 224) (1-99 cigarettes/lifetime) and smokers (n = 375) (> or = 100 cigarettes/lifetime) were assessed for demographics, tobacco/drug use history and DSM-IV dependence and genotyped for CYP2A6 alleles associated with decreased nicotine metabolism (CYP2A6*2, CYP2A6*4, CYP2A6*9, CYP2A6*12). CYP2A6 allele frequencies varied substantially among the ethnic groups. The proportion of Caucasian slow nicotine inactivators was significantly lower in current, DSM-IV dependent smokers compared to non-smokers [7.0% and 12.5%, respectively, P = 0.03, odds ratio (OR) = 0.52; 95% confidence interval (CI) 0.29-0.95]; non-dependent smokers showed similar results. Daily cigarette consumption (cigarettes/day) was significantly (P = 0.003) lower for slow (21.3; 95% CI 17.4-25.2) compared to normal inactivators (28.2; 95% CI 26.4-29.9); this was observed only in DSM-IV dependent smokers. Slow inactivators had a significantly (P = 0.03) lower age of first smoking compared to normal inactivators (13.0 years of age; 95% CI 12.1-14.0 versus 14.2; 95% CI 13.8-14.6), and a trend towards smoking for a shorter duration. This study demonstrates that slow nicotine inactivators are less likely to be adult smokers (dependent or non-dependent). Slow inactivators also smoked fewer cigarettes per day and had an earlier age of first smoking (only dependent smokers).