Addicted to Nicotine
A National Research Forum

Section II: Nicotine-Individual Risk Factors for Initiation
Richard R. Clayton, Ph.D., Chair

A COMMON GENETIC DEFECT IN NICOTINE METABOLISM DECREASES SMOKING

Rachel F. Tyndale, Ph.D. (Contributor: Edward M. Sellers, Ph.D., University of Toronto)
Department of Pharmacology
University of Toronto

Introduction

This presentation will outline what we know about one gene, CYP2A6, its role in the risk for becoming a nicotine-dependent smoker, and how its function alters the number of cigarettes consumed by smokers.

What We Know

Approximately one-third of the global population older than age 15 smokes. Smoking is associated with a higher incidence of many diseases, including cancers and respiratory and cardiovascular diseases. Approximately 50 percent of initiation into smoking dependence is genetically influenced, while persistence of smoking and the amount smoked have approximately a 70-percent genetic contribution.

Nicotine is the primary compound in tobacco that establishes and maintains tobacco dependence. In humans, 60 to 80 percent of nicotine is metabolized and inactivated to cotinine, principally by the genetically polymorphic CYP2A6 enzyme. Three CYP2A6 alleles have been identified: wild-type (CYP2A6*1) and two defective alleles (CYP2A6*2 and CYP2A6*3). Each individual has two copies of this gene, one each from the maternal and paternal sides. An individual can have two active forms of the gene and have normal nicotine removal (metabolism), one active and one defective copy and have reduced nicotine removal, or two defective copies, which we predict will drastically reduce their nicotine removal.

• Protection Against Becoming a Smoker. We hypothesized that individuals with impaired nicotine metabolism (carriers of a defective CYP2A6 allele[s]) would be protected from becoming tobacco dependent. When learning to smoke, individuals often find the nicotine unpleasant (e.g., causing dizziness or nausea), and where nicotine metabolism is decreased in individuals, the aversive effects might last longer. Therefore, these individuals may be less likely to become dependent smokers. We found that 20 percent of the nonsmoking population were carriers of defective CYP2A6 alleles. In contrast, in dependent smokers (DSM-IV and Fagerstrom criteria) with or without alcohol dependence, only 10 percent of the individuals had CYP2A6 defective alleles (20.1 percent [N = 213] versus 11.7 percent [N = 317], p < 0.01, chi-square; OR = 1.9, 95-percent confidence interval 1.2-3.2). These data demonstrate that impaired nicotine metabolism protects against becoming a dependent smoker. In fact, even a single CYP2A6 defective allele (i.e., heterozygosity) is sufficient to significantly reduce by twofold the risk of tobacco dependence. This protects approximately seven million North Americans from becoming smokers. Individuals with two defective alleles (i.e., homozygosity for defective alleles) may be protected to an even greater extent. The individuals with two defective alleles represent about 1.5 percent of the nonsmoking population; assessing the degree of protection from smoking in these double-defect individuals will require evaluation of over a thousand smokers.

• Decreased Cigarette Smoking. Dependent smokers adjust their smoking to maintain constant blood and brain nicotine concentrations, suggesting that dependent smokers with impaired nicotine metabolism need to smoke fewer cigarettes. In other words, because nicotine is removed more slowly in these people, the restocking of the nicotine can be done at longer intervals, resulting in their smoking fewer cigarettes. Within the tobacco-dependent group, those who had one defective and one active CYP2A6 gene copy smoked significantly fewer cigarettes per day and per week than smokers without impaired nicotine metabolism ([carriers of two CYP2A6 active alleles], 129 versus 159 cigarettes/week, t-test p < 0.02). Again, these data confirm that CYP2A6-mediated nicotine metabolism is a significant determinant of smoking behavior; heterozygosity in a single gene, the CYP2A6 gene, significantly decreases both initiation of dependence and drug-taking behavior. Interestingly, the decrease in cigarette smoking was observed to a far greater extent in male smokers than in female smokers. This might be caused by kinetic differences between genders. A more likely explanation is that females regulate their smoking less specifically for nicotine than do males. For example, preloading females with nicotine has less effect on reducing their smoking and craving than it does in males. Nicotine replacement therapies also fail more often in females than in males.

What We Need To Know More About

While these data clearly demonstrate a role for genetically variable nicotine metabolism (as mediated by the CYP2A6 gene) in smoking dependence and tobacco carcinogen exposure, many issues still remain to be elucidated.

• How Does Defective Nicotine Metabolism Protect Against Nicotine Dependence? While we know that nicotine metabolism is reduced in individuals with a defective CYP2A6 allele(s) and that these people are protected against tobacco dependence, we have not demonstrated how this protection is imparted. We need to gain a better understanding of the role that gender plays in smoking behavior and nicotine regulation. The data clearly show that females are equally protected by this gene defect from becoming dependent smokers, but that the effect is smaller than in males with respect to cigarette consumption. In addition, this gene has only recently been characterized; therefore, many new defective alleles will be identified over the next few years. In keeping with other genes in the CYP family, the frequency of defective alleles varies profoundly across ethnic groups. This may be part of the reason for ethnic differences in smoking and tobacco-related diseases.

• Cancer Rates. Tobacco smoke contains nitrosamines that can be activated to carcinogens by CYP2A6; therefore, individuals who carry CYP2A6 defective alleles may also be less efficient at bioactivating tobacco smoke procarcinogens to carcinogens. Thus, individuals carrying CYP2A6 defective alleles may have a decreased risk of developing tobacco-related cancers and other medical complications for three reasons: (1) They have a decreased risk of becoming a smoker; (2) if they do become tobacco dependent, they smoke less than those without impaired nicotine metabolism; and (3) they may activate fewer tobacco-related procarincogens. These three factors suggest a significant reduction in tobacco-related cancers for carriers of a CYP2A6 defective allele(s) that must be confirmed experimentally.

• Existing and Novel Therapies. The CYP2A6 genotype (whether one is a carrier of defective copies or not) may significantly affect nicotine levels from nicotine sources other than cigarettes, such as existing nicotine replacement therapies (NRTs) (e.g., patch, gum, nasal spray). This may be of growing importance because NRTs are used increasingly for long-term tobacco dependence maintenance and for treatment of other syndromes (e.g., Alzheimer's disease, Tourette's syndrome, ulcerative colitis). The protective effect of impaired nicotine metabolism (carriers of CYP2A6 null alleles) on the risk for becoming tobacco dependent and in lowering the number of cigarettes smoked, as well as in reduced procarcinogen activation, suggests that inhibiting this enzyme may provide novel therapeutic approaches to prevention and treatment of tobacco smoking. The manipulation of CYP2A6 activity must be explored.

Recommended Reading

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Fernandez-Salguero, P.; Hoffman, S.M.; Cholerton, S.; Mohrenweiser, H.; Raunio, H.; Rautio, A.; Pelkonen, O.; Huang, J.D.; Evans, W.E.; Idle, J.R.; et al. A genetic polymorphism in coumarin 7-hydroxylation: Sequence of the human CYP2A genes and identification of variant CYP2A6 alleles. Am J Hum Genet 57(3):651-660, 1995.

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Messina, E.S.; Tyndale, R.F.; and Sellers, E.M. A major role for CYP2A6 in nicotine C-oxidation by human liver microsomes. J Pharmacol Exp Ther 282(3):1608-1614, 1997.

Perkins, K.; Grobe, J.; and Fonte, C. Influence of acute smoking exposure on the subsequent reinforcing value of smoking. Exp Clin Psychopharmacol 5(3):277-285, 1997.

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