A genetic association for cigarette smoking behavior

A systems omics-based approach to decode substance use disorders and neuroadaptations

Substance use disorders (SUDs) are a group of neuropsychiatric conditions manifesting due to excessive dependence on potential drugs of abuse such as psychostimulants, opioids including prescription opioids, alcohol, inhalants, etc. Experimental studies have generated enormous data in the area of SUDs, but outcomes from such data have remained largely fragmented. In this review, we attempt to coalesce these data points providing an important first step towards our understanding of the etiology of SUDs. We propose and describe a 'core addictome' pathway that behaves central to all SUDs. Besides, we also have made some notable observations paving way for several hypotheses; MECP2 behaves as a master switch during substance use; five distinct gene clusters were identified based on respective substance addiction; a central cluster of genes serves as a hub of the addiction pathway connecting all other substance addiction clusters. In addition to describing these findings, we have emphasized the importance of some candidate genes that are of substantial interest for further investigation and serve as high-value targets for translational efforts.

Rare and common variant discovery in complex disease: the IBD case study

Complex diseases such as inflammatory bowel disease (IBD), which consists of ulcerative colitis and Crohn's disease, are a significant medical burden-70 000 new cases of IBD are diagnosed in the United States annually. In this review, we examine the history of genetic variant discovery in complex disease with a focus on IBD. We cover methods that have been applied to microsatellite, common variant, targeted resequencing and whole-exome and -genome data, specifically focusing on the progression of technologies towards rare-variant discovery. The inception of these methods combined with better availability of population level variation data has led to rapid discovery of IBD-causative and/or -associated variants at over 200 loci; over time, these methods have grown exponentially in both power and ascertainment to detect rare variation. We highlight rare-variant discoveries critical to the elucidation of the pathogenesis of IBD, including those in NOD2, IL23R, CARD9, RNF186 and ADCY7. We additionally identify the major areas of rare-variant discovery that will evolve in the coming years. A better understanding of the genetic basis of IBD and other complex diseases will lead to improved diagnosis, prognosis, treatment and surveillance.

Prediction and Understanding of Resilience in Albertan Families: Longitudinal Study of Disaster Responses (PURLS) - Protocol

Exposure to a natural disaster in childhood can have serious, long-lasting consequences, impacting physical and mental health, development, and learning. Although many children experience negative effects after a disaster, the majority do not, and what differentiates these groups is not well understood. Some of the factors that influence disaster-related outcomes in the midst of adversity include parents' mental health, the home environment, and socioeconomic status. Furthermore, genetics has also a role to play in how children respond to stressors. We had the opportunity to conduct a natural experiment of disaster recovery following the Alberta 2013 Flood. This paper presents the detailed protocol on prediction of resilience in Albertan families, and validation with cortisol data. In addition, data collection procedures, developing resiliency screening tools, candidate gene identification, genotyping, DNA methylation, and genomic analyses are described to achieve the research objectives. This study produced new knowledge by using pre- and post-disaster information on children's health and development, including children's genetics and responses to stress. This information has been identified as important to governments and other organizations invested in early child development. Our comprehensive research plan generates evidence that can be mobilized population-based approaches to improve child and family resiliency.

An Assessment and Extension of the Mechanism-Based Approach to the Identification of Age-Period-Cohort Models

Many methods have been proposed to solve the age-period-cohort (APC) linear identification problem, but most are not theoretically informed and may lead to biased estimators of APC effects. One exception is the mechanism-based approach recently proposed and based on Pearl’s front-door criterion; this approach ensures consistent APC effect estimators in the presence of a complete set of intermediate variables between one of age, period, cohort, and the outcome of interest, as long as the assumed parametric models for all the relevant causal pathways are correct. Through a simulation study mimicking APC data on cardiovascular mortality, we demonstrate possible pitfalls that users of the mechanism-based approach may encounter under realistic conditions: namely, when (1) the set of available intermediate variables is incomplete, (2) intermediate variables are affected by two or more of the APC variables (while this feature is not acknowledged in the analysis), and (3) unaccounted confounding is present between intermediate variables and the outcome. Furthermore, we show how the mechanism-based approach can be extended beyond the originally proposed linear and probit regression models to incorporate all generalized linear models, as well as nonlinearities in the predictors, using Monte Carlo simulation. Based on the observed biases resulting from departures from underlying assumptions, we formulate guidelines for the application of the mechanism-based approach (extended or not).

Individual differences in the processing of smoking-cessation video messages: An imaging genetics study

Studies testing the benefits of enriching smoking-cessation video ads with attention-grabbing sensory features have yielded variable results. Dopamine transporter gene (DAT1) has been implicated in attention deficits. We hypothesized that DAT1 polymorphism is partially responsible for this variability. Using functional magnetic resonance imaging, we examined brain responses to videos high or low in attention-grabbing features, indexed by "message sensation value" (MSV), in 53 smokers genotyped for DAT1. Compared to other smokers, 10/10 homozygotes showed greater neural response to High- vs. Low-MSV smoking-cessation videos in two a priori regions of interest: the right temporoparietal junction and the right ventrolateral prefrontal cortex. These regions are known to underlie stimulus-driven attentional processing. Exploratory analysis showed that the right temporoparietal response positively predicted follow-up smoking behavior indexed by urine cotinine. Our findings suggest that responses to attention-grabbing features in smoking-cessation messages is affected by the DAT1 genotype.

Polymorphisms in dopaminergic system genes; association with criminal behavior and self-reported aggression in violent prison inmates from Pakistan

Genetic factors contribute to antisocial and criminal behavior. Dopamine transporter DAT-1 (SLC6A3) and DRD2 gene for the dopamine-2 receptor are dopaminergic system genes that regulate dopamine reuptake and signaling, and may be part of the pathogenesis of psychiatric disorders including antisocial behaviors and traits. No previous studies have analyzed DAT-1 and DRD2 polymorphisms in convicted murderers, particularly from Indian subcontinent. In this study we investigated the association of 40 bp VNTR polymorphism of DAT-1 and Taq1 variant of DRD2 gene (rs1800479) with criminal behavior and self-reported aggression in 729 subjects, including 370 men in Pakistani prisons convicted of first degree murder(s) and 359 control men without any history of violence or criminal tendency. The 9R allele of DAT-1 VNTR polymorphism was more prevalent in convicted murderers compared with control samples, for either one or two risk alleles (OR = 1.49 and 3.99 respectively, P = 0.003). This potential association of DAT-1 9R allele polymorphism with murderer phenotype was confirmed assuming different genetic models of inheritance. However, no genetic association was found for DRD2 Taq1 polymorphism. In addition, a combined haplotype (9R-A2) of DAT-1 and DRD2 genes was associated with this murderer phenotype. Further, 9R allele of DAT-1 was also associated with response to verbal abuse and parental marital complications, but not with other measures pertinent to self-reported aggression. These results suggest that 9R allele, which may influence levels of intra-synaptic dopamine in the brain, may contribute to criminal tendency in this sample of violent murderers of Pakistani origin. Future studies are needed to replicate this finding in other populations of murderers and see if this finding extends to other forms of violence and lesser degrees of aggression.

Targeted sequencing identifies genetic polymorphisms of flavin-containing monooxygenase genes contributing to susceptibility of nicotine dependence in European American and African American

BACKGROUND

Smoking is a leading cause of preventable death. Early studies based on samples of twins have linked the lifetime smoking practices to genetic predisposition. The flavin-containing monooxygenase (FMO) protein family consists of a group of enzymes that metabolize drugs and xenobiotics. Both FMO1 and FMO3 were potentially susceptible genes for nicotine metabolism process.

METHODS

In this study, we investigated the potential of FMO genes to confer risk of nicotine dependence via deep targeted sequencing in 2,820 study subjects comprising 1,583 nicotine dependents and 1,237 controls from European American and African American. Specifically, we focused on the two genomic segments including FMO1,FMO3, and pseudo gene FMO6P, and aimed to investigate the potential association between FMO genes and nicotine dependence. Both common and low-frequency/rare variants were analyzed using different algorithms. The potential functional significance of SNPs with association signal was investigated with relevant bioinformatics tools.

RESULTS

We identified different clusters of significant common variants in European (with most significant SNP rs6674596, p = .0004, OR = 0.67, MAF_EA = 0.14, FMO1) and African Americans (with the most significant SNP rs6608453, p = .001, OR = 0.64, MAF_AA = 0.1, FMO6P). No significant signals were identified through haplotype-based analyses. Gene network investigation indicated that both FMO1 and FMO3 have a strong relation with a variety of genes belonging to CYP gene families (with combined score greater than 0.9). Most of the significant variants identified were SNPs located within intron regions or with unknown functional significance, indicating a need for future work to understand the underlying functional significance of these signals.

CONCLUSIONS

Our findings indicated significant association between FMO genes and nicotine dependence. Replications of our findings in other ethnic groups were needed in the future. Most of the significant variants identified were SNPs located within intronic regions or with unknown functional significance, indicating a need for future work to understand the underlying functional significance of these signals.

The SLC6A3 gene possibly affects susceptibility to late-onset alcohol dependence but not specific personality traits in a Han Chinese population

Dopaminergic dysfunction has an important role in the pathoetiology of alcohol dependence (AD). The purpose of this study was to determine whether the solute carrier family 6 member 3 (SLC6A3) gene (also known as the dopamine transporter DAT gene) was associated with AD, and whether variants in the SLC6A3 locus were associated with specific personality traits in patients with AD. Sixteen polymorphisms in SLC6A3 were analyzed using 637 patients with AD and 523 healthy controls. To reduce clinical heterogeneity, patients were classified into two subgroups: early-onset AD (EOAD) and late-onset AD (LOAD). The Tridimensional Personality Questionnaire was used to assess the personality traits novelty seeking (NS) and harm avoidance (HA) in the patients with AD. Using allele frequency and genotype distribution comparisons and logistic regression analysis, we found evidence of association between rs6350 and AD (P < 0.05). Following subgroup analysis, we confirmed evidence of an association in patients with LOAD (P = 0.003), but not in patients with EOAD. Heterozygous carriers of the A allele have a nearly 3 times greater risk to develop LOAD compared to individuals who do not have an A allele. Although we found that patients with AD had higher NS and HA scores compared to controls (P < 0.001), we did not find evidence of association between SLC6A3 polymorphisms and either NS or HA in patients with AD using linear regression analysis. The findings from our study indicate that the SLC6A3 gene may have a role in susceptibility to late-onset AD in the Han Chinese population.

Tobacco Use Cessation and Prevention - A Review

Tobacco use is a major preventable cause of premature death and disease, currently leading to over five million deaths each year worldwide. Smoking or chewing tobacco can seriously affect general, as well as oral health. Oral health professionals play an important role in promoting tobacco free-lifestyles. They should counsel their patients not to smoke; and reinforce the anti-tobacco message and refer the patients to smoking cessation services. Dentists are in a unique position to educate and motivate patients concerning the hazards of tobacco to their oral and systemic health, and to provide intervention programs as a part of routine patient care. Tobacco cessation is necessary to reduce morbidity and mortality related to tobacco use. Strategies for tobacco cessation involves 5 A's and 5 R's approach, quit lines and pharmacotherapy. Additionally, tobacco cessation programs should be conducted at community, state and national levels. Various policies should be employed for better tobacco control. Governments should implement the tobacco control measures to reduce the prevalence of tobacco use and exposure to tobacco smoke. In addition, there should be availability of leaflets, brochures, continuing patient education materials regarding tobacco cessation.

Meta-analysis reveals significant association of 3'-UTR VNTR in SLC6A3 with smoking cessation in Caucasian populations

Many studies have examined the association between SLC6A3 3′-UTR VNTR polymorphism and smoking cessation; however, the results are inconclusive, primarily because of the small to moderate-size samples. The primary goal of this study was to determine whether this polymorphism has any effect on smoking cessation by a meta-analysis of all reported studies. We adopted a 9-repeat dominant model that considers 9-repeat and non 9-repeat as two genotypes and compared their frequencies in former vs. current smokers. Eleven studies with 5,480 participants were included. Considering the presence of study heterogeneity and differences in the availability of information from each study, three separate meta-analyses were performed with the Comprehensive Meta-Analysis statistical software (v. 2.0). The first meta-analysis provided evidence of association between the 9-repeat genotype and smoking cessation under the fixed-effects model (pooled odds ratio [OR] 1.13; 95% confidence interval [CI] 1.01, 1.27; P = 0.037) but not in the random-effects model (pooled OR 1.11; 95% CI 0.96, 1.29; P = 0.159). Given the marginal evidence of heterogeneity among studies (P = 0.10; I2 = 35.9%), which likely was caused by inclusion of an Asian-population treatment study with an opposite effect of the polymorphism on smoking cessation, we excluded these data, revealing a significant association between the 9-repeat genotype and smoking cessation under both the fixed- and random-effects models (pooled OR 1.15; 95% CI 1.02, 1.29; P = 0.02 for both models). By analyzing adjusted and unadjusted results, we performed the third meta-analysis, which showed consistently that the 9-repeat genotype was significantly associated with smoking cessation under both the fixed- and random-effects models (pooled OR 1.17; 95% CI 1.04, 1.31; P = 0.009 for both models). We conclude that the 3′-UTR VNTR polymorphism is significantly associated with smoking cessation, and smokers with one or more 9-repeat alleles have a 17% higher probability of smoking cessation than smokers carrying no such allele.

Genetic influence of dopamine receptor, dopamine transporter, and nicotine metabolism on smoking cessation and nicotine dependence in a Japanese population

Background

This study investigated whether polymorphisms of the ankyrin repeat and kinase domain containing 1 gene (ANKK1), which is adjacent to the dopamine D2 receptor gene (DRD2), and the dopamine transporter (SLC6A3) and cytochrome P450 2A6 (CYP2A6) genes influence smoking cessation and nicotine dependence in a Japanese population. In 96 current and former smokers, genotyping frequencies for the ANKK1/DRD2 TaqIA, SLC6A3 VNTR, and CYP2A6 polymorphisms were subjected to chi-square analysis, and regression analyses were used to determine the association of the genotypes of current smokers with a Heavy Smoking Index, in addition to evaluating the effect of the subjects’ smoking history on the association.

Results

Genotyping results suggested that nicotine dependence among current smokers homozygous for the SLC6A3 10r allele was lower than that of smokers carrying the minor alleles, and that the CYP2A6 polymorphism might mediate this association. Furthermore, the age at which current smokers began smoking might moderate the association between their genetic polymorphisms and nicotine dependence.

Conclusions

This study provides preliminary findings on the influence of genetic variants on the smoking phenotypes in a Japanese population.

What can genes tell us about the relationship between education and health?

We use genome wide data from respondents of the Health and Retirement Study (HRS) to evaluate the possibility that common genetic influences are associated with education and three health outcomes: depression, self-rated health, and body mass index. We use a total of 1.7 million single nucleotide polymorphisms obtained from the Illumina HumanOmni2.5-4v1 chip from 4233 non-Hispanic white respondents to characterize genetic similarities among unrelated persons in the HRS. We then used the Genome Wide Complex Trait Analysis (GCTA) toolkit, to estimate univariate and bivariate heritability. We provide evidence that education (h(2) = 0.33), BMI (h(2) = 0.43), depression (h(2) = 0.19), and self-rated health (h(2) = 0.18) are all moderately heritable phenotypes. We also provide evidence that some of the correlation between depression and education as well as self-rated health and education is due to common genetic factors associated with one or both traits. We find no evidence that the correlation between education and BMI is influenced by common genetic factors.

Smoking-specific parenting and smoking onset in adolescence: the role of genes from the dopaminergic system (DRD2, DRD4, DAT1 genotypes)

Although only few studies have shown direct links between dopaminergic system genes and smoking onset, this does not rule out the effect of a gene-environment interaction on smoking onset. Therefore, the aim of this study was to examine the associations between smoking-specific parenting (i.e., frequency and quality of communication and house rules) and smoking onset while considering the potential moderating role of dopaminergic system genes (i.e., DRD2, DRD4, and DAT1 genotypes). Data from five annual waves of the 'Family and Health' project were used. At time 1, the sample comprised 365 non-smoking adolescents (200 younger adolescents, mean age = 13.31, SD = .48; 165 older adolescents, mean age = 15.19, SD = .57). Advanced longitudinal analyses were used (i.e., logistic regression analyses, (dual) latent growth curves, and cross-lagged path models). The results showed a direct effect of quality of communication on smoking onset. No direct effects were found for frequency of communication and house rules. Furthermore, no direct and moderating effects of the DRD2, DRD4, or DAT1 genotypes were found. In conclusion, the findings indicated that the effects of smoking-specific parenting on smoking are similar for adolescent carriers and non-carriers of the dopaminergic system genes.

Genetics of impulse control disorders in Parkinson's disease

Impulse control disorders (ICD) have been recognised in Parkinson's disease (PD) as adverse effects of dopamine replacement therapy, particularly with dopamine agonists. Although virtually all PD patients are treated with dopaminergic drugs, only a minority will develop hyperdopaminergic states, suggesting predisposing and/or protecting factors. The age at onset, the sex and the dose or type of dopaminergic drugs have been identified as clinical predictive factors. Recent genetic studies have investigated associations between ICD and polymorphisms of genes involved in the dopamine metabolism pathway (COMT, DAT), dopamine receptors (DRD1, DRD2, DRD3, DRD4), serotonin receptors and its transporter (HTR2A, 5HTT), and glutamate receptors (GRIN2B). Although validation in larger and independent cohorts is needed, the results from these studies give us some insights into the pathophysiology of hyperdopaminergic states and may be useful, at term, in personalising antiparkinsonian treatment in clinical practice.

Genetically determined dopamine availability predicts disposition for depression

Although prominent personality theories postulate orthogonality between traits of positive emotionality (PEM) and negative emotionality (NEM), empirical evidence often demonstrates the opposite indicating a negative relationship. Therefore, it is not surprising that dopaminergic (DA) gene loci have been related to traits of positive and of NEM. The present genetic association study investigates the influence of two functional DA gene polymorphisms on Sadness as defined by the Affective Neuroscience Personality Scales (ANPS) in healthy Caucasians (n = 1041). We observed a significant interaction effect between the 10-repeat (10R) allele of the dopamine transporter (DAT1) gene and the methionine (Met) allele of the catechol-O-methyltransferase (COMT) Val158Met polymorphism (F((1,1018)) = 11.11; P < 0.001). Carriers of the 9R/9R and the Val/Val genotype showed dramatically reduced Sadness scores in comparison to the other three genotype configurations. Both the 9R/9R and the Val/Val genotypes characterized by reduced transporter density and high dopamine catabolism, respectively, have been separately related to personality traits of PEM and externalizing behavior in the past. The present findings indicate that gene variations of the DA system previously associated with PEM are at the same time protective against high NEM and can therefore constitute a resilience factor against depression.

The influence of three genes on whether adolescents use contraception, USA 1994-2002

In a further contribution to recent investigations of the relevance of genetic processes for demographic outcomes, we investigate genetic associations with whether adolescents use contraception. Using data from the National Longitudinal Study of Adolescent Health, we find that variants in the dopamine transporter gene DAT1, the dopamine receptor gene DRD2, and the monoamine oxidase gene MAOA are associated with unprotected sexual intercourse. Consistent with previous analyses of these data, the genotypes DRD2*A1/A2, DRD2*A2/A2, DAT1*9R/10R, and MAOA*2R/ are associated with higher odds of unprotected sexual intercourse than other genotypes at these loci. The DRD2 associations apply to both men and women, whereas the other associations apply to women only. These results are robust to controls for population stratification by continental ancestry, do not vary by contraceptive type, and are consistent with previous research showing that these genetic variants are associated with higher rates of impulsivity.

Overnight deprivation from smoking disrupts amygdala responses to fear

Cigarette smoking, a major, yet avoidable, cause of disability and premature death, is the most prevalent form of nicotine addiction. An emerging theme in the neurobiology of nicotine addiction is the integrity of the amygdala. Using functional MRI, amygdala responses during a face perception task were compared between 28 chronic smokers [14 females, 14 males; age, 26.3 (2.8) years; age at onset of smoking, 15.8 (2.6) years; years smoked, 9.1 (2.1); cigarettes per day, 17.1 (3.7); Fagerström test for nicotine dependence score, 4.1 (1.9); exhaled carbon-monoxide level, 17.8 (9.5) ppm] and 28 age- and education-matched nonsmokers [14 females, 14 males; age, 26.9 (2.4) years]. Subjects underwent imaging on two separate occasions 1 week apart: smoking satiety versus overnight smoking deprivation, in a randomized counterbalanced order. Our results show no difference in amygdala responses to faces between nonsmokers and satiated smokers. However, overnight deprivation from smoking was associated with a significantly lowered amygdala response to fear, an effect that was probabilistically mapped to the basolateral amygdala. We suggest that aberrant amygdala reactivity in overnight-deprived smokers may reflect a pre-existing vulnerability to smoking and/or increase the risk of smoking relapse after a cessation attempt.

The dopamine transporter gene, a spectrum of most common risky behaviors, and the legal status of the behaviors

This study tests the specific hypothesis that the 9R/9R genotype in the VNTR of the dopamine transporter gene (DAT1) exerts a general protective effect against a spectrum of risky behaviors in comparison to the 10R/9R and 10R/10R genotypes, drawing on three-time repeated measures of risky behaviors in adolescence and young adulthood on about 822 non-Hispanic white males from the Add Health study. Our data have established two empirical findings. The first is a protective main effect in the DAT1 gene against risky behaviors. The second finding is that the protective effect varies over age, with the effect prominent at ages when a behavior is illegal and the effect largely vanished at ages when the behavior becomes legal or more socially tolerated. Both the protective main effect and the gene-lifecourse interaction effect are replicated across a spectrum of most common risky behaviors: delinquency, variety of sexual partners, binge drinking, drinking quantity, smoking quantity, smoking frequency, marijuana use, cocaine use, other illegal drug use, and seatbelt non-wearing. We also compared individuals with the protective genotype and individuals without it in terms of age, physical maturity, verbal IQ, GPA, received popularity, sent popularity, church attendance, two biological parents, and parental education. These comparisons indicate that the protective effect of DAT1*9R/9R cannot be explained away by these background characteristics. Our work demonstrates how legal/social contexts can enhance or reduce a genetic effect on risky behaviors.

Chromosome 5p Region SNPs Are Associated with Risk of NSCLC among Women

In a population-based case-control study, we explored the associations between 42 polymorphisms in seven genes in this region and non-small cell lung cancer (NSCLC) risk among Caucasian (364 cases; 380 controls) and African American (95 cases; 103 controls) women. Two TERT region SNPs, rs2075786 and rs2853677, conferred an increased risk of developing NSCLC, especially among African American women, and TERT-rs2735940 was associated with a decreased risk of lung cancer among African Americans. Five of the 20 GHR polymorphisms and SEPP1-rs6413428 were associated with a marginally increased risk of NSCLC among Caucasians. Random forest analysis reinforced the importance of GHR among Caucasians and identified AMACR, TERT, and GHR among African Americans, which were also significant using gene-based risk scores. Smoking-SNP interactions were explored, and haplotypes in TERT and GHR associated with NSCLC risk were identified. The roles of TERT, GHR, AMACR and SEPP1 genes in lung carcinogenesis warrant further exploration.

Influences of polymorphic variants of DRD2 and SLC6A3 genes, and their combinations on smoking in Polish population

BACKGROUND

Polymorphisms in dopaminergic genes may influence cigarette smoking by their potential impact on dopamine reward pathway function. A1 allele of DRD2 gene is associated with a reduced dopamine D2 receptor density, and it has been hypothesised that A1 carriers are more vulnerable to smoking. In turn, the 9-repeat allele of dopamine transporter gene (SLC6A3) has been associated with a substantial reduction in dopamine transporter, what might result in the higher level of dopamine in the synaptic cleft, and thereby protective role of this allele from smoking. In the present study we investigated whether polymorphic variants of DRD2 and SLC6A3 genes and their combinations are associated with the smoking habit in the Polish population.

METHODS

Genotyping for TaqIA polymorphism of DRD2 and SLC6A3 VNTR polymorphism was performed in 150 ever-smokers and 158 never-smokers. The association between the smoking status and smoking phenotypes (related to the number of cigarettes smoked daily and age of starting regular smoking), and genotype/genotype combinations was expressed by ORs together with 95% CI. Alpha level of 0.05, with Bonferroni correction whenever appropriate, was used for statistical significance.

RESULTS

At the used alpha levels no association between DRD2 and SLC6A3 genotypes and smoking status was found. However, A1 allele carriers reported longer abstinence periods on quitting attempts than non-carriers (p = 0.049). The ORs for heavier smoking were 0.38 (0.17-0.88), p = 0.023, and 0.39 (0.17-0.88), p = 0.021 in carriers compared to non-carriers of A1 or *9 allele, respectively, and the OR for this smoking phenotype was 8.68 (2.47-30.46), p = 0.0005 for the A1-/9- genotype combination, relatively to the A1+/9+. Carriers of *9 allele of SLC6A3 had over twice a lower risk to start smoking before the age of 20 years compared to non-carriers (sex-adjusted OR = 0.44; 95% CI: 0.22-0.89; p = 0.0017), and subjects with A1-/9- genotype combination had a higher risk for staring regular smoking before the age of 20 years in comparison to subjects with A1+/9+ genotype combination (sex-adjusted OR = 3.79; 95% CI:1.03-13.90; p = 0.003).

CONCLUSION

Polymorphic variants of DRD2 and SLC6A3 genes may influence some aspects of the smoking behavior, including age of starting regular smoking, the level of cigarette consumption, and periods of abstinence. Further large sample studies are needed to verify this hypothesis.

Association of the DRD2 gene Taq1A polymorphism and smoking behavior: a meta-analysis and new data

INTRODUCTION

Many studies have investigated the association of the dopamine type-2 receptor (DRD2) Taq1A polymorphism with tobacco use and cigarette smoking behaviors, but findings remain equivocal. There is a biological basis for considering that this association differs by sex, and differences in subpopulations might explain some of the contradictory evidence.

METHODS

Our a priori hypothesis was that the association of the DRD2 Taq1A polymorphism with smoking behavior would be more prominent in females than males. We therefore investigated the strength of evidence for an association between the DRD2 Taq1A polymorphism and smoking behavior in a large sample of females and used meta-analytic techniques to synthesize existing published data and explore the role of sex in explaining any heterogeneity between studies.

RESULTS

We did not observe any strong evidence of association between the DRD2 Taq1A polymorphism and smoking behavior, including smoking initiation, smoking persistence, and smoking rate, either in our female sample or in our meta-analysis of 29 studies, comprising 28 published studies and the data from the present study. Metaregression suggested an association between the proportion of male participants in a study and the individual study effect size, indicating a larger effect size with a greater proportion of male participants for smoking initiation and smoking persistence. This effect did not appear to be due to the inclusion of the data from the present study.

DISCUSSION

Available evidence does not support an association between the DRD2 Taq1A polymorphism and smoking behavior. Contrary to our a priori hypothesis, we found evidence of a stronger association in males than in females.

Genetic variation in the dopamine pathway and smoking cessation

Twin and family studies have established that genetic factors account for much of the variation in tobacco dependence. Therefore, identification of genetic variants predictive of successful smoking cessation has implications for the future prospect of personalized smoking cessation therapies. Converging data implicate the dopamine pathway as an important neural substrate for tobacco dependence. Several candidate genes within the dopamine pathway (e.g., DRD2 and COMT) have been reported to be associated with the efficacy of bupropion and nicotine replacement therapy, and others (e.g., SLC6A3 and DRD4) have been reported to be associated with smoking cessation independent of pharmacotherapy. However, few of these candidate genes are present within regions of suggestive or significant linkage or overlap with genome-wide linkage or association studies of tobacco dependence or smoking cessation. Future studies should seek to replicate genome-wide association analyses with individual-level genotyping, and use better-defined smoking cessation phenotypes. Once robust evidence for association is established, which may take several more years, further research into the likely cost-effectiveness, feasibility and acceptability of personalized medicine for smoking cessation will be necessary before it can be translated into practice.

Neuroimaging, genetics and the treatment of nicotine addiction

Advances in neuroimaging and genomics provide an unprecedented opportunity to accelerate medication development for nicotine dependence and other addictions. Neuroimaging studies have begun to elucidate the functional neuroanatomy and neurochemistry underlying effects of nicotine and nicotine abstinence. In parallel, genetic studies, including both candidate gene and genome-wide association approaches, are identifying key neurobiological targets and pathways important in addiction to nicotine. To date, only a few neuroimaging studies have explored effects of nicotine or abstinence on brain activity as a function of genotype. Most analyses of genotype are retrospective, resulting in small sample sizes for testing effects of the minor alleles for candidate genes. The purpose of this review is to provide an outline of the work in neuroimaging, genetics, and nicotine dependence, and to explore the potential for increased integration of these approaches to improve nicotine dependence treatment.

Genetics and smoking cessation improving outcomes in smokers at risk

This article reviews evidence supporting the potential utility of a pharmacogenetic approach to the treatment of nicotine dependence. There is substantial evidence that nicotine dependence and smoking persistence are heritable, and are determined by a complex interplay of polygenic and environmental influences. The most robust evidence for specific genetic influences on nicotine dependence is found in studies of genetic variation in nicotine-metabolizing enzymes. Data also support the role of genes in the dopamine and opioid pathways as predictors of dependence and smoking relapse; however, the evidence for genetic associations is not always consistent. Emerging data from pharmacogenetic trials of nicotine-dependence treatment are promising, suggesting that genetic profiles of smokers someday may be used by providers to choose the type, dose, and duration of treatment for individual smokers. However, additional trials including larger and more diverse populations are needed before such data can be translated to practice to reduce smoking prevalence and tobacco-related disease.

Molecular genetics of monoamine transporters: relevance to brain disorders

We have demonstrated in both the human serotonin transporter gene (5HTT) and the dopamine transporter gene (DAT1) that specific polymorphic variants termed Variable Number Tandem Repeats (VNTRs), which correlate with predisposition to a number of neurological and psychiatric disorders, act as transcriptional regulatory domains. We have demonstrated that these domains can act as both tissue-specific and stimulus-inducible regulators of gene expression. As such they can act to be mechanistically associated with the progression or initiation of a behavioural disorder by altering the level of transporter mRNA, which in turn regulates the concentration of transporter in specific cells or in response to a challenge; chemical, environmental or physiological. The synergistic actions of such transcriptional domains will modulate gene expression. Our hypothesis is that these VNTR variants are one mechanism by which nurture can modify concentrations of neurotransmitters in a differential manner.

Pharmacogenetic clinical trial of sustained-release bupropion for smoking cessation

This randomized, double-blinded, placebo-controlled trial examined genetic influences on treatment response to sustained-release bupropion for smoking cessation. Smokers of European ancestry (N = 291), who were randomized to receive bupropion or placebo (12 weeks) plus counseling, were genotyped for the dopamine D2 receptor (DRD2-Taq1A), dopamine transporter (SLC6A3 3' VNTR), and cytochrome P450 2B6 (CYP2B6 1459 CT) polymorphisms. Main outcome measures were cotinine-verified point prevalence of abstinence at end of treatment and at 2-, 6-, and 12-month follow-ups post quit date. Using generalized estimating equations, we found that bupropion, compared with placebo, was associated with significantly greater odds of abstinence at all time points (all p values<.01). We found a significant DRD2 x bupropion interaction (B = 1.49, SE = 0.59, p = .012) [corrected] and a three-way DRD2 x bupropion x craving interaction on 6-month smoking cessation outcomes (B = -0.45, SE = 0.22, p = .038), such that smokers with the A2/A2 genotype demonstrated the greatest craving reduction and the highest abstinence rates with bupropion. Furthermore, there was a significant DRD2 x CYP2B6 interaction (B = 1.43, SE = 0.56, p = .01), such that individuals with the DRD2-Taq1 A2/A2 genotype demonstrated a higher odds of abstinence only if they possessed the CYP2B6 1459 T/T or C/T genotype. Because the sample size of this study was modest for pharmacogenetic investigations, the results should be interpreted with caution. Although these results require replication, the data suggest preliminarily that the DRD2-Taq1A polymorphism may influence treatment response to bupropion for smoking cessation and, further, that exploration of gene x gene and gene x craving interactions in future, larger studies may provide mechanistic insights into the complex pharmacodynamics of bupropion.

Relationship between self-reported task persistence and history of quitting smoking, plans for quitting smoking, and current smoking status in adolescents

The task persistence construct has previously been measured primarily behaviorally (e.g., with a mirror-tracing task, or breath holding), and only in adults. It has been shown to differentiate between adult smokers and non-smokers and to predict smoking cessation in adult smokers trying to quit. This theory-based analysis is the first to examine task persistence in adolescent smokers and to examine a two-item, internally consistent, self-report measure of task persistence. Results indicate that task persistence is greater among adolescent non-smokers as compared to adolescent current smokers, and those planning to quit smoking as compared to those with no plans to quit. Contrary to hypotheses, task persistence was not found to be related to prior successful attempts to quit smoking. Our results suggest that a brief, self-report measure of task persistence may be a methodologically sound, practical clinical tool for this population.

Association between dopamine transporter genotypes and smoking cessation: a meta-analysis

This review assessed the evidence of an association between genotypes of the dopamine transporter (DAT1, SLC6A3) 3' untranslated region (3'UTR) variable number of tandem repeats (VNTR) and smoking cessation. Five studies (seven cohorts) comprising 2155 subjects were included in the meta-analysis. All gave data on the number of smokers who had stopped smoking and the number still smoking for those with one or more variant 9-repeat alleles and other genotypes. Three studies (comprising five cohorts) were cross-sectional population surveys and two were smoking cessation treatment programs with follow-up. Four of the five studies (six of the seven cohorts) showed a trend in favor of cessation when the variant 9-repeat allele was present, although only one study showed a statistically significant effect. The pooled odds ratio in favor of a greater likelihood of cessation for the variant 9-repeat allele was 1.15 [95% confidence interval (CI) = 0.97-1.37]. In a more refined analysis where cohorts within studies were treated as separate samples and adjusted odds ratios were used, the overall pooled odds ratio in favor of cessation with the 9-repeat alleles was 1.20 (95% CI = 1.01-1.43). These results support the hypothesis that the DAT1 3'UTR VNTR polymorphism is associated with smoking cessation. One or more variant 9-repeat alleles relative to the more common 10-repeat allele confers a greater likelihood of cessation, indicative of lower dependence on tobacco. The effect was a 20% increase in the odds of cessation for those with a variant allele.

Genetic polymorphisms in dopamine-related genes and smoking cessation in women: a prospective cohort study

Background

Genes involved in dopaminergic neurotransmission have been suggested as candidates for involvement in smoking behavior. We hypothesized that alleles associated with reduced dopaminergic neurotransmission would be more common in continuing smokers than among women who quit smoking.

Methods

The study included 593 women aged 26–65 years who participated in a twelve month smoking cessation trial conducted in 1993–1994. Participants were contacted three years after the trial to obtain updated smoking history and biological specimens. Seven polymorphisms were assessed in genes involved in dopamine synthesis (tyrosine hydoxylase [TH]), receptor activation (dopamine receptors [DRD2, DRD3, DRD4]), reuptake (dopamine transporter [SLC6A3]), and metabolism (catechol-o-methyltransferase [COMT]). Smoking cessation was assessed as "short-term" quitting (abstinence for the seven days before the conclusion of the trial) and "long-term" quitting (abstinence for the six months before a subsequent interview conducted several years later).

Results

We observed no association of any polymorphism with either short- or long-term quitting. Although some relative risk estimates were consistent with weak associations, either the direction of effect was opposite of that hypothesized, or results of the short- and long-term cessation endpoints differed. However, effect modification on smoking cessation was observed between DRD2 Taq1A and SLC6A3 VNTR polymorphisms, DRD3 Ser/Gly and d,1-fenfluramine, and DRD4 VNTR and d,1-fenfluramine.

Conclusion

Although these results fail to support prior findings of independent associations of these polymorphisms with smoking status, our exploratory findings suggestive of gene-gene and gene-treatment interactions warrants further investigation.

Dopamine receptor genes (DRD2, DRD3 and DRD4) and gene-gene interactions associated with smoking-related behaviors

Cigarette smoking, like many addictive behaviors, has a genetic component, and the dopamine D2-like receptor genes (DRD2, DRD3 and DRD4) are candidates for contributing to these behaviors. Phenotypic information concerning smoking-related behaviors from a nationally representative sample of research volunteers was analyzed for association with polymorphisms in these genes. Genotype status at the DRD2 intron 2 simple tandem repeat was related to cigarettes per day (P = 0.035) and heaviness of smoking index (P = 0.049). The presence of the glycine allele at the S9G polymorphism of the DRD3 gene was associated with frequency/quantity measures of smoking [log-transformed time to first cigarette (P = 0.031) and heaviness of smoking index (P = 0.035)]. There was a trend for DRD4 long alleles of the variable number of tandem repeats polymorphism to be associated with reduced severity of three withdrawal symptoms [desire/craving (P = 0.054); anger/irritability (P = 0.10); and trouble sleeping (P = 0.068)]. Interactions between genotypes at all three genes were associated with nervousness (P = 0.020) and trouble sleeping (P = 0.015). An interaction between DRD2 and DRD3 was found for trouble concentrating (P = 0.020). These relationships present possible dopamine-related responses to nicotine that warrant further study.

Lack of association between polymorphisms of the dopamine receptor D4 and dopamine transporter genes and personality traits in a Korean population

Human personality traits have a considerable genetic component. Cloninger et al. were the first to postulate that certain personality traits, such as novelty seeking, are related to the dopamine neurotransmitter system. In this study, we investigated the associations between dopamine receptor D4 (DRD4) exon III and dopamine transporter (DAT1) polymorphisms and personality traits. The DRD4 and DAT1 gene polymorphisms were genotyped in 214 healthy Korean subjects, whose personality traits were assessed with the Temperament and Character Inventory (TCI). There were no significant differences between scores of TCI temperament dimensions (novelty seeking, harm avoidance, reward dependence, and persistence) and DRD4 gene polymorphism. The DAT1 gene polymorphisms also showed no significant association with any of the temperament subscales of the TCI. These data suggest that DRD4 and DAT1 gene polymorphism may not associated with personality traits in a Korean population.

A genetic association study of dopamine metabolism-related genes and chronic headache with drug abuse

To assess the role of dopamine metabolism-related genes in the genetic liability to chronic headache with drug abuse (DA). We performed a genetic association study using four functional polymorphisms of the dopamine receptor 4 (DRD4), dopamine transporter (DAT), mono-amino-oxidase A (MAOA) and cathecol-O-methyl-transferase (COMT) genes in 103 patients with chronic daily headache associated with DA (CDHDA). Control samples were 117 individuals without headache or DA (controls) and 101 patients with episodic migraine without aura and without DA (MO). No differences were found at the COMT and MAOA genes among the three groups investigated. Allele 4 of DRD4 was significantly overrepresented in patients with MO compared with both controls and CDHDA. Allele 10 of the DAT gene was significantly underrepresented in patients with CDHDA when compared with the MO group. Genetic variability at the DRD4 gene is involved in the predisposition to episodic MO but not to DA, while liability to CDHDA may involve genetic variability at the DAT gene in comparison with episodic MO.

Gene variants of brain dopamine pathways and smoking-induced dopamine release in the ventral caudate/nucleus accumbens

CONTEXT

Preclinical studies demonstrate that nicotine administration leads to dopamine release in the ventral striatum. However, human studies reveal considerable interindividual variability in the extent of smoking-induced dopamine release.

OBJECTIVE

To determine whether common gene variants of the brain dopamine pathway explain this observed phenotypic variability in humans.

DESIGN

Blood samples were drawn to determine gene variants of dopamine system components, and positron emission tomography scanning with the radiotracer raclopride labeled with radioactive carbon (11C) was performed to measure smoking-induced dopamine release.

SETTING

Academic brain imaging center.

PARTICIPANTS

Forty-five tobacco-dependent smokers.

INTERVENTIONS

Subjects either smoked a cigarette (n = 35) or did not smoke (n = 10) during positron emission tomography scanning.

MAIN OUTCOME MEASURES

Gene variants of dopamine system components (the dopamine transporter variable nucleotide tandem repeat, D2 receptor Taq A1/A2, D4 receptor variable nucleotide tandem repeat, and catechol-O-methyltransferase Val158Met polymorphisms) and change in [11C]raclopride binding potential in the ventral caudate/nucleus accumbens on positron emission tomography scans.

RESULTS

For subjects who smoked during scanning, those with at least one 9 allele of the dopamine transporter variable nucleotide tandem repeat, fewer than 7 repeats of the D4 variable nucleotide tandem repeat, and the Val/Val catechol-O-methyltransferase genotype had greater decreases in binding potential (an indirect measure of dopamine release) with smoking than those with the alternate genotypes. An overall decrease in ventral caudate/nucleus accumbens binding potential in those who smoked compared with those who did not smoke was also found but was smaller in magnitude than previously reported.

CONCLUSIONS

Smokers with genes associated with low resting dopamine tone have greater smoking-induced (phasic) dopamine release than those with alternate genotypes. These findings suggest that dopamine system genotype variabilities explain a significant proportion of the interindividual variability in smoking-induced dopamine release and indicate that smoking-induced dopamine release has a genetic predisposition.

An interaction between the serotonin transporter promoter region and dopamine transporter polymorphisms contributes to harm avoidance and reward dependence traits in normal healthy subjects

There is evidence for an association between polymorphisms of serotonin- and dopamine-related genes and temperamental personality traits. Recent findings have shown that interactions between allelic variants of the different genes may contribute to personality traits. We examined the effects of serotonin transporter-linked promoter region (5-HTTLPR) and dopamine transporter (DAT1) gene polymorphisms for associations with the Temperament and Character Inventory (TCI) temperament subscales in 209 Koreans. We found that the variants of 5-HTTLPR interacted with the DAT1 gene polymorphism to influence the HA and RD temperament subscales of TCI. Neither of these two genes affected any subscales of TCI alone.Controlling for the effects of gender and age, we found significant interactions between 5-HTTLPR and DAT1 genes on Harm Avoidance (HA) and Reward Dependence (RD) as measured by the TCI (Hotelling's Trace = 3.0, P = 0.02). In the presence of the DAT1 10/10 genotype, subjects of group L of 5-HTTLPR had a significantly higher HA score and significantly lower RD score than those of group S (F = 5.04, df = 1, p = 0.03 and F = 8.35, df = 1, p = 0.004, respectively). These findings suggest that the variants of 5-HTTLPR interacted with the DAT1 gene polymorphism to influence the HA and RD temperament subscales of TCI.

Symbiotic relationship of pharmacogenetics and drugs of abuse

Pharmacogenetics/pharmacogenomics is the study of how genetic variation affects pharmacology, the use of drugs to treat disease. When drug responses are predicted in advance, it is easier to tailor medications to different diseases and individuals. Pharmacogenetics provides the tools required to identify genetic predictors of probable drug response, drug efficacy, and drug-induced adverse events-identifications that would ideally precede treatment decisions. Drug abuse and addiction genetic data have advanced the field of pharmacogenetics in general. Although major findings have emerged, pharmacotherapy remains hindered by issues such as adverse events, time lag to drug efficacy, and heterogeneity of the disorders being treated. The sequencing of the human genome and high-throughput technologies are enabling pharmacogenetics to have greater influence on treatment approaches. This review highlights key studies and identifies important genes in drug abuse pharmacogenetics that provide a basis for better diagnosis and treatment of drug abuse disorders.

Significant association of interleukin 8 -251T/A polymorphism with smoking behavior in a Japanese population

Accumulating evidence indicates that the genotype may impact on smoking behavior and a deeper understanding of the molecular basis could lead to more effective strategies for preventing initiation of the habit and to help smokers to quit. Since individual variation in airway responsiveness to cigarette smoke might have an important influence, we have focused on associations between smoking behavior and polymorphisms affecting the inflammatory cytokine, IL-8. In the present study, 453 Japanese non-cancer outpatients (191 males and 262 females) who visited Aichi Cancer Center Hospital were genotyped for the IL8 -251T/A polymorphism, and age- and sex-adjusted odds ratios (aORs) for smoking were estimated using a logistic regression model. The aORs for IL8 251-TA and AA combined, genotypes associated with high production of IL-8, were 0.52 (95% CI 0.33-0.82, P=0.004) for ever having smoked and 0.55 (0.33-0.92, P=0.023) for being a current smoker. Our results suggest that the inflammatory-prone genotype of IL8 may act to deter initiation or characteristics of the smoking habit.

Nicotine addiction through a neurogenomic prism: ethics, public health, and smoking

Studies are under way to examine the neurogenetic factors contributing to smoking behaviors. The combined approaches of genomics, molecular biology, neuroscience, and pharmacology are expected to fuel developments in pharmacogenetics, to create new genetic tests, and ultimately to provide the basis for innovative strategies for smoking cessation and prevention. The emergence of a neurogenomic understanding of nicotine addiction is likely to induce fundamental changes in popular, clinical, and public health views of smoking, which could significantly shape existing practices and policies to reduce tobacco use. Still a nascent area of research, nicotine addiction provides an excellent case study through which to anticipate key ethical and policy issues in both behavioral genetics and the neurogenomics of addictive behaviors.

Barriers to translating emerging genetic research on smoking into clinical practice. Perspectives of primary care physicians

OBJECTIVE

Smoking remains the leading cause of preventable death nationally. Emerging research may lead to improved smoking cessation treatment options, including tailoring treatment by genotype. Our objective was to assess primary care physicians' attitudes toward new genetic-based approaches to smoking treatment.

DESIGN AND SETTING

A 2002 national survey of primary care physicians. Respondents were randomly assigned a survey including 1 of 2 scenarios: a scenario in which a new test to tailor smoking treatment was described as a "genetic" test or one in which the new test was described as a "serum protein" test.

PARTICIPANTS

The study sample was randomly drawn from all U.S. primary care physicians in the American Medical Association Masterfile (e.g., those with a primary specialty of internal medicine, family practice, or general practice). Of 2,000 sampled physicians, 1,120 responded, yielding a response rate of 62.3%.

MEASUREMENTS AND MAIN RESULTS

Controlling for physician and practice characteristics, describing a new test as "genetic" resulted in a regression-adjusted mean adoption score of 73.5, compared to a score of 82.5 for a nongenetic test, reflecting an 11% reduction in physicians' likelihood of offering such a test to their patients.

CONCLUSIONS

Merely describing a new test to tailor smoking treatment as "genetic" poses a significant barrier to physician adoption. Considering national estimates of those who smoke on a daily basis, this 11% reduction in adoption scores would translate into 3.9 million smokers who would not be offered a new genetic-based treatment for smoking. While emerging genetic research may lead to improved smoking treatment, the potential of novel interventions will likely go unrealized unless barriers to clinical integration are addressed.

Association of habitual smoking and drinking with single nucleotide polymorphism (SNP) in 40 candidate genes: data from random population-based Japanese samples

Basic information on the association between lifestyle factors and candidate genes is valuable for genetic-environmental study. We screened the association of habitual smoking or drinking with polymorphism in 40 candidate genes for a total of 153 single nucleotide polymorphisms (SNPs) using a sample of 339 middle-aged, randomly selected Japanese men. Smoking and drinking statuses were elicited during questionnaire-based interviews. Genes were selected based on their possible involvement in genetic-environmental, life-style interactions and constitute the genes expressing xenobiotic metabolism enzymes, DNA repair enzymes, and other stress-related proteins. The P values of odds ratios to habitual smoking for CYP17A1, ESR1, EPHX1, GSTT2, ALDH2, NOS2A, OGG1, and SLC6A4 and those of odds ratios to habitual drinking for CYP1B1, ESR1, HSD17B3, GSTM3, COMT, ADH1C, ALDH2, NOS3, and NUDT1 were under 0.05. These variables were included in a stepwise logistic analysis in order to develop a predictive model for smoking or drinking behavior. In the final model, the only significant variables selected for smoking were OGG1, SLC6A4, EPHX1, ESR1, and CYP17A1, and for drinking, ALDH2 and NUDT1. The findings of the present study suggest that polymorphism in associated candidate genes plays a role in the habitual use of tobacco and alcohol among Japanese men.

Genetics of tobacco use

The worldwide prevalence of tobacco use is widespread, resulting in nearly 4.5 million deaths every year. Nicotine is addictive and has psychopharmacological effects that maintain the use of tobacco products. Several studies have documented a strong hereditary component to tobacco use. The present article reviews results from twin and adoption studies and proceeds to present association studies of specific genes that may be involved in tobacco use. Cholinergic receptor nicotinic beta polypeptide 2, serotonin receptor and transporter genes, dopamine receptor and transporter genes, and the cytochrome P450A6 gene are reviewed. Linkage studies help to identify regions of the genome that may be worth further investigation. The paper concludes with a discussion of the limitations of genetic research and the future of genetic epidemiology in this domain.

Association between polymorphism of the dopamine transporter gene and early smoking onset: an interaction risk on nicotine dependence

Previous studies suggested that a polymorphism in the dopamine transporter gene (SLC6A3) is associated with nicotine dependence and age of smoking onset, but the conclusion was controversial. To detect the association of a G-->A polymorphism (NCBI dbSNP cluster ID: rs27072) in 3'-untranslated region of the SLC6A3 with nicotine dependence and early smoking onset, we recruited 253 sibships including 668 nicotine-dependent siblings from a rural district of China. The sibship disequilibrium tests (SDT) showed that the rs27072-A allele is significantly associated with smoking onset < or =18 years (kappa2=9.78, p=0.003 in severely nicotine-dependent smokers, and kappa2=4.24, p=0.058 in total smokers), but not significantly associated with severe nicotine dependence. Conditional logistic regression showed that the risk of early smoking onset by the rs27072-A allele was almost three times greater in severely nicotine-dependent smokers [Odds ratio (OR)=11.3, 95% confidence interval (CI)=1.5-85.6] than that in total smokers. Linear regression showed that rs27072-A allele also increased the risk of nicotine dependence by early smoking onset compared with homozygous rs27072-G genotype. Although these findings are preliminary and need validation, the results suggest that a polymorphism in the SLC6A3 may play important roles in smoking onset, and there may be an interactive effect between the SLC6A3 and early smoking onset on modulating the susceptibility of nicotine dependence.