Effects upon in-vivo nicotine metabolism reveal functional variation in FMO3 associated with cigarette consumption

Abstract

Background: Flavin-containing monooxygenases (FMO) catalyze the metabolism of nucleophilic heteroatom-containing drugs and xenobiotics, including nicotine. Rare mutations in FMO3 are responsible for defective N-oxidation of dietary trimethylamine leading to trimethylaminuria, and common genetic variation in FMO3 has been linked to interindividual variability in metabolic function that may be substrate specific.

Methods: A genetic model of CYP2A6 function is used as a covariate to reveal functional polymorphism in FMO3 that indirectly influences the ratio of deuterated nicotine metabolized to cotinine following oral administration. The association is tested between FMO3 haplotype and cigarette consumption in a set of nicotine-dependent smokers.

Results: FMO3 haplotype, based on all common coding variants in Europeans, significantly predicts nicotine metabolism and accounts for ∼2% of variance in the apparent percent of nicotine metabolized to cotinine. The metabolic ratio is not associated with FMO2 haplotype or an FMO1 expression quantitative trait locus. Cross-validation demonstrates calculated FMO3 haplotype parameters to be robust and significantly improve the predictive nicotine metabolism model over CYP2A6 genotype alone. Functional classes of FMO3 haplotypes, as determined by their influence on nicotine metabolism to cotinine, are also significantly associated with cigarettes per day in nicotine-dependent European Americans (n=1025, P=0.04), and significantly interact (P=0.016) with CYP2A6 genotype to predict cigarettes per day.

Conclusion: These findings suggest that common polymorphisms in FMO3 influence nicotine clearance and that these genetic variants in turn influence cigarette consumption.

Figure 1

Cigarettes per day among COGEND European American nicotine dependent smokers, divided by FMO3 diplotype, ± 95% confidence intervals. 1/3 homozygotes carry only haplotypes 1 and 3 (Table 3), heterozygotes carry one haplotype 1 or 3 and one of the other haplotypes (2,4–10), 2/4/5 homozygotes carry only all other FMO3 haplotypes (2,4&5, as well as the rare haplotypes 6–10). 1/3 homozygotes vs. 2/4/5 homozygotes, p=0.017; 2/4/5 homozygotes vs. heterozygotes, p=0.038

Figure 2

Cigarettes per day among COGEND European American nicotine dependent smokers, divided by predicted CYP2A6 function based on CYP2A6 genotype, and FMO3 diplotype, ± 95% confidence intervals. 1/3 homozygotes carry only FMO3 haplotypes 1 and 3 (Table 3), heterozygotes carry one FMO3 haplotype 1 or 3 and one of the other haplotypes (2,4–10), 2/4/5 homozygotess carry only all other FMO3 haplotypes (2,4&5, as well as the rare haplotypes 6–10). ‘CYP2A6 slow’ are subjects with a predicted metabolism metric <0.85 [28], corresponding to all carriers of CYP2A6*2,*4,*9,*12, or *38 alleles, and *1A homozygotes; ‘CYP2A6 fast’ are all other subjects with a predicted metabolism metric >0.85. CYP2A6 fast/FMO3 2/4/5 homozygotess vs. CYP2A6 fast/FMO3 1/3 homozygotes p=0.006; CYP2A6 fast/FMO3 2/4/5 homozygotes vs. CYP2A6 fast/FMO3 heterozygotes p=0.027.