Duplications and defects in the CYP2A6 gene: Identification, genotyping, and in vivo effects on smoking

In humans, 80% of nicotine is metabolized to the inactive metabolite cotini ne by the enzyme CYP2A6, which can also activate tobacco smoke procarcinoge ns (e.g., 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone). Previously, we d emonstrated that individuals who are nicotine-dependent and have defective CYP2A6 alleles (* 2, *3) smoked fewer cigarettes; however, we recognize tha t the genotyping method used for the CYP2A6*3 allele gave a high false-posi tive rate. In the current study we used improved genotyping methods to exam ine the effects of the defective CYP2A6*2 and CYP2A6*4 alleles on smoking b ehavior. We found that those with the defective alleles (N = 14) smoked few er cigarettes per day than those homozygous (N = 277) for wild-type alleles (19 versus 28 cigarettes per day, P < .001). In addition, we identified a duplicated form of the CYP2A6 gene, corresponding to the gene deletion CYP2 A6*4 allele, developed a genotyping assay, assessed the gene copy number, a nd examined its prevalence in Caucasian smokers (N = 296). We observed an a scending rank order for plasma cotinine and breath carbon monoxide levels ( an index of smoke inhalation) in individuals with null (CYP2A6*2 and CYP2A6 *4) alleles (N = 14), those homozygous for wild-type (CYP2A6*1/*1) alleles (N = 277), and those with our newly identified CYP2A6 gene duplication (N = 5). The phenotype, as determined by plasma nicotine/cotinine ratios, had a descending rank order for these three genotype groups that did not reach s ignificance. Although further characterization is required for the duplicat ion gene variant, these results extend our previous findings and suggest a substantial influence of CYP2A6 genotype and phenotype on smoking behavior.