Gary E. Swan

Variants in Nicotinic Receptors and Risk for Nicotine Dependence

OBJECTIVEnA recent study provisionally identified numerous genetic variants as risk factors for the transition from smoking to the development of nicotine dependence, including an amino acid change in the alpha5 nicotinic cholinergic receptor (CHRNA5). The purpose of this study was to replicate these findings in an independent data set and more thoroughly investigate the role of genetic variation in the cluster of physically linked nicotinic receptors, CHRNA5-CHRNA3-CHRNB4, and the risk of smoking.nnnMETHODnIndividuals from 219 European American families (N=2,284) were genotyped across this gene cluster to test the genetic association with smoking. The frequency of the amino acid variant (rs16969968) was studied in 995 individuals from diverse ethnic populations. In vitro studies were performed to directly test whether the amino acid variant in the CHRNA5 influences receptor function.nnnRESULTSnA genetic variant marking an amino acid change showed association with the smoking phenotype (p=0.007). This variant is within a highly conserved region across nonhuman species, but its frequency varied across human populations (0% in African populations to 37% in European populations). Furthermore, functional studies demonstrated that the risk allele decreased response to a nicotine agonist. A second independent finding was seen at rs578776 (p=0.003), and the functional significance of this association remains unknown.nnnCONCLUSIONSnThis study confirms that at least two independent variants in this nicotinic receptor gene cluster contribute to the development of habitual smoking in some populations, and it underscores the importance of multiple genetic variants contributing to the development of common diseases in various populations.

Genetic influence on smoking--a study of male twins.

BACKGROUNDnThe results of twin and family studies suggest that heredity has a small influence on smoking behavior.nnnMETHODSnWe conducted a genetic analysis of several aspects of smoking behavior among subjects in the National Academy of Sciences-National Research Council Twin Registry. The registry includes male twins who were born in the United States between 1917 and 1927 and who were members of the armed services during World War II. Information on smoking history was available for 4775 pairs of twins, who were first surveyed in 1967 through 1969, when they were 40 to 50 years old, and then re-surveyed in 1983 through 1985, when they were 56 to 66. Eighty percent of the subjects in this cohort had smoked at some time in their lives, 60 percent were smokers in 1967 through 1969, and 39 percent were smoking in 1983 through 1985. Similarities between twins in smoking habits at base line and at the second follow-up 16 years later were examined. The comparison of concordance for smoking between monozygotic and dizygotic twins was used to assess the relative contribution of familial and genetic factors.nnnRESULTSnIn 1967-1969 survey the ratio of observed to expected concordance for smoking was higher among the monozygotic twins than among the dizygotic twins for those who had never smoked (overall rate ratio, 1.38; 95 percent confidence interval, 1.25 to 1.54), for former smokers (overall rate ratio, 1.59; 95 percent confidence interval, 1.35 to 1.85), for current cigarette smokers (overall rate ratio, 1.18; 95 percent confidence interval, 1.11 to 1.26), and for current cigar or pipe smokers (overall rate ratio, 1.60; 95 percent confidence interval, 1.22 to 2.06). The data also suggest genetic influences on quitting smoking. Monozygotic twins were more likely than dizygotic twins to be concordant for quitting smoking (overall rate ratio, 1.24; 95 percent confidence interval, 1.06 to 1.45).nnnCONCLUSIONSnIn this cohort of adult male twins, there were moderate genetic influences on lifetime smoking practices.

Variation in the human immune system is largely driven by non-heritable influences

There is considerable heterogeneity in immunological parameters between individuals, but its sources are largely unknown. To assess the relative contribution of heritable versus non-heritable factors, we have performed a systems-level analysis of 210 healthy twins between 8 and 82 years of age. We measured 204 different parameters, including cell population frequencies, cytokine responses, and serum proteins, and found that 77% of these are dominated (>50% of variance) and 58% almost completely determined (>80% of variance) by non-heritable influences. In addition, some of these parameters become more variable with age, suggesting the cumulative influence of environmental exposure. Similarly, the serological responses to seasonal influenza vaccination are also determined largely by non-heritable factors, likely due to repeated exposure to different strains. Lastly, in MZ twins discordant for cytomegalovirus infection, more than half of all parameters are affected. These results highlight the largely reactive and adaptive nature of the immune system in healthy individuals.

Maternal nutrition at conception modulates DNA methylation of human metastable epialleles

In experimental animals, maternal diet during the periconceptional period influences the establishment of DNA methylation at metastable epialleles in the offspring, with permanent phenotypic consequences. Pronounced naturally occurring seasonal differences in the diet of rural Gambian women allowed us to test this in humans. We show that significant seasonal variations in methyl-donor nutrient intake of mothers around the time of conception influence 13 relevant plasma biomarkers. The level of several of these maternal biomarkers predicts increased/decreased methylation at metastable epialleles in DNA extracted from lymphocytes and hair follicles in infants postnatally. Our results demonstrate that maternal nutritional status during early pregnancy causes persistent and systemic epigenetic changes at human metastable epialleles.

CYP2A6 genotype and the metabolism and disposition kinetics of nicotine.

The liver enzyme cytochrome P450 (CYP) 2A6 is primarily responsible for the metabolism of nicotine. Variants in the CYP2A6 gene have been associated with altered nicotine metabolism and with effects on smoking behavior. Our objective was to determine the relationship between variant CYP2A6 genotypes and the disposition and metabolism of nicotine administered intravenously.

Depressive symptoms and metabolic risk in adult male twins enrolled in the National Heart, Lung, and Blood Institute twin study.

Objective To determine the extent to which depressive symptoms are associated with metabolic risk factors and whether genetic or environmental factors account for this association. Method Twin structural equation modeling was employed to estimate genetic and environmental contributions to the covariation of depressive symptoms, as indexed by the Centers for Epidemiological Studies–Depression Scale, and common variance among blood pressure, body mass index, waist-to-hip ratio, and serum triglycerides and glucose among 87 monozygotic and 86 dizygotic male twin pairs who participated in the NHLBI twin study. Results Depressive symptoms were associated with individual components of the metabolic syndrome and common variance among the risk factors. Twin structural equation modeling indicated that the associations were attributable to environmental (nongenetic) factors. Conclusions These results support the hypothesis that depressive symptoms may increase risk for a pattern of physiological risk consistent with the metabolic syndrome.

Use of the nicotine metabolite ratio as a genetically informed biomarker of response to nicotine patch or varenicline for smoking cessation: a randomised, double-blind placebo-controlled trial.

BACKGROUNDnSubstantial variability exists in therapeutic response and adverse effects with pharmacotherapies for tobacco dependence. Biomarkers to optimise treatment choice for individual smokers might improve treatment outcomes. We tested whether a genetically informed biomarker of nicotine clearance, the nicotine metabolite ratio (NMR; 3-hydroxycotinine:cotinine), predicts response to nicotine patch or varenicline for smoking cessation.nnnMETHODSnWe undertook NMR-stratified multicentre, randomised, placebo-controlled clinical trial from Nov 16, 2010, to Sept 12, 2014, at four sites. Smokers seeking treatment were randomly assigned by baseline NMR status and study site, in blocks of 12 patients (1:1:1 ratio), to 11 weeks of placebo (placebo pill plus placebo patch), nicotine patch (active patch plus placebo pill), or varenicline (active pill plus placebo patch), plus behavioural counselling. Participants and investigators were masked to group allocation and NMR status. An intention-to-treat analysis was done. Participants were followed up for 12 months after the target quit date. The primary endpoint was biochemically verified 7 day point prevalence abstinence at the end of treatment to estimate the pharmacological effect of treatment by NMR. The trial is registered at ClinicalTrials.gov, number NCT01314001.nnnFINDINGSn1246 participants (662 slow metabolisers of nicotine, 584 normal metabolisers of nicotine) were enrolled and randomly assigned to the three interventions (408 placebo, 418 nicotine patch, 420 varenicline). At end of treatment, varenicline was more efficacious than nicotine patch in normal metabolisers (OR 2·17, 95% CI 1·38-3·42; p=0·001), but not in slow metabolisers (OR 1·13, 0·74-1·71; p=0·56). In the longitudinal model including all timepoints, the NMR-by-treatment interaction was significant (ratio of odds ratios [ORR] 1·96, 95% CI 1·11-3·46; p=0·02). An NMR-by-treatment interaction showed that slow (vs normal) metabolisers reported greater overall side-effect severity with varenicline versus placebo (β=-1·06, 95% CI -2·08 to -0·03; p=0·044).nnnINTERPRETATIONnTreating normal metabolisers with varenicline and slow metabolisers with nicotine patch could optimise quit rates while minimising side-effects.nnnFUNDINGnNational Institutes of Health, Canadian Institutes of Health Research, Abramson Cancer Center, Centre for Addiction and Mental Health Foundation, and Pennsylvania Department of Health.

Use of the nicotine metabolite ratio as a genetically informed biomarker of response to nicotine patch or varenicline for smoking cessation

BACKGROUNDnSubstantial variability exists in therapeutic response and adverse effects with pharmacotherapies for tobacco dependence. Biomarkers to optimise treatment choice for individual smokers might improve treatment outcomes. We tested whether a genetically informed biomarker of nicotine clearance, the nicotine metabolite ratio (NMR; 3-hydroxycotinine:cotinine), predicts response to nicotine patch or varenicline for smoking cessation.nnnMETHODSnWe undertook NMR-stratified multicentre, randomised, placebo-controlled clinical trial from Nov 16, 2010, to Sept 12, 2014, at four sites. Smokers seeking treatment were randomly assigned by baseline NMR status and study site, in blocks of 12 patients (1:1:1 ratio), to 11 weeks of placebo (placebo pill plus placebo patch), nicotine patch (active patch plus placebo pill), or varenicline (active pill plus placebo patch), plus behavioural counselling. Participants and investigators were masked to group allocation and NMR status. An intention-to-treat analysis was done. Participants were followed up for 12 months after the target quit date. The primary endpoint was biochemically verified 7 day point prevalence abstinence at the end of treatment to estimate the pharmacological effect of treatment by NMR. The trial is registered at ClinicalTrials.gov, number NCT01314001.nnnFINDINGSn1246 participants (662 slow metabolisers of nicotine, 584 normal metabolisers of nicotine) were enrolled and randomly assigned to the three interventions (408 placebo, 418 nicotine patch, 420 varenicline). At end of treatment, varenicline was more efficacious than nicotine patch in normal metabolisers (OR 2·17, 95% CI 1·38-3·42; p=0·001), but not in slow metabolisers (OR 1·13, 0·74-1·71; p=0·56). In the longitudinal model including all timepoints, the NMR-by-treatment interaction was significant (ratio of odds ratios [ORR] 1·96, 95% CI 1·11-3·46; p=0·02). An NMR-by-treatment interaction showed that slow (vs normal) metabolisers reported greater overall side-effect severity with varenicline versus placebo (β=-1·06, 95% CI -2·08 to -0·03; p=0·044).nnnINTERPRETATIONnTreating normal metabolisers with varenicline and slow metabolisers with nicotine patch could optimise quit rates while minimising side-effects.nnnFUNDINGnNational Institutes of Health, Canadian Institutes of Health Research, Abramson Cancer Center, Centre for Addiction and Mental Health Foundation, and Pennsylvania Department of Health.

Bupropion SR and counseling for smoking cessation in actual practice: Predictors of outcome

To date, only one study has been published on individual characteristics associated with outcome following standard treatment with bupropion SR for smoking cessation. To investigate treatment outcome beyond the 6-week end-of-treatment point, the present study examined characteristics associated with more clinically relevant smoking endpoints following treatment with bupropion SR in a large health care system. A total of 1,524 smokers (649 men and 875 women) of average age 45.1 years were randomized to receive one of four combinations of bupropion SR (150 or 300 mg) and behavioral counseling (tailored mailings or proactive telephone counseling) and assessed for point-prevalent smoking status at 3 and 12 months. Multiple logistic regression analyses of potential risk factors for 12-month point-prevalent smoking and for persistent smoking (point-prevalent smoking at both follow-ups) following treatment were conducted for men and women combined and separately. Risk factors for smoking at both endpoints in the combined sample included treatment with tailored mailings, female gender, younger age, higher levels of tobacco dependence, shorter previous quit attempts, previous use of nicotine replacement therapy, and report of current depressive symptoms or lifetime depression. Risk factors for smoking following treatment identified in women only included treatment with the lower dose of bupropion SR, younger age, and higher perceived stress, whereas those that were unique to men included the presence of lifetime depression. The results are discussed in terms of their implications for the need for more effective treatments in general, and the role of individual differences in the likelihood of returning to smoking following treatment for quitting.

Nicotinic Acetylcholine Receptor Variation and Response to Smoking Cessation Therapies

Objective To evaluate the association of nicotinic acetylcholine receptor (nAChR) single nucleotide polymorphism (SNP) with 7-day point prevalence abstinence (abstinence) in randomized clinical trials of smoking cessation therapies in individuals grouped by pharmacotherapy randomization to inform the development of personalized smoking cessation therapy. Materials and methods We quantified association of four SNPs at three nAChRs with abstinence in eight randomized clinical trials. Participants were 2633 outpatient treatment-seeking, self-identified European ancestry individuals smoking at least 10 cigarettes/day, recruited through advertisement, prescribed pharmacotherapy, and provided with behavioral therapy. Interventions included nicotine replacement therapy (NRT), bupropion, varenicline, placebo (PLA), or combined NRT and bupropion, and five modes of group and individual behavioral therapy. Outcome measures tested in multivariate logistic regression were end of treatment and 6 month (6MO) abstinence, with demographic, behavioral, and genetic covariates. Results ‘Risk’ alleles previously associated with smoking heaviness were significantly (P<0.05) associated with reduced abstinence in the PLA pharmacotherapy group (PG) at 6MO [for rs588765, odds ratio (95% confidence interval) 0.41 (0.17–0.99)], and at end of treatment and at 6MO [for rs1051730, 0.42 (0.19–0.93) and 0.31 (0.12–0.80)], and with increased abstinence in the NRT PG at 6MO [for rs588765, 2.07 (1.11–3.87) and for rs1051730, 2.54 (1.29–4.99)]. We observed significant heterogeneity in rs1051730 effects (F=2.48, P=0.021) between PGs. Conclusion chr15q25.1 nAChR SNP risk alleles for smoking heaviness significantly increase relapse with PLA treatment and significantly increase abstinence with NRT. These SNP–PG associations require replication in independent samples for validation, and testing in larger sample sizes to evaluate whether similar effects occur in other PGs.