Genetic variation in dopamine pathways differentially associated with smoking progression in adolescence

The association of genetic polymorphisms within the dopaminergic system with nicotine dependence: A narrative review

Nicotine, the main compound in cigarettes, leads to smoking addiction. Nicotine acts on the limbic dopamine reward loop in the midbrain by binding to nicotinic acetylcholine receptors, promoting the release of dopamine, and resulting in a rewarding effect or satisfaction. This satisfaction is essential for continued and compulsive tobacco use, and therefore dopamine plays a crucial role in nicotine dependence. Numerous studies have identified genetic polymorphisms of dopaminergic pathways which may influence susceptibility to nicotine addiction. Dopamine levels are greatly influenced by synthesis, storage, release, degradation, and reuptake-related genes, including genes encoding tyrosine hydroxylase, dopamine decarboxylase, dopamine transporter, dopamine receptor, dopamine 3-hydroxylase, catechol-O-methyltransferase, and monoamine oxidase. In this paper, we review research progress on the effects of polymorphisms in the above genes on downstream smoking behavior and nicotine dependence, to offer a theoretical basis for the elucidation of the genetic mechanism underlying nicotine dependence and future personalized treatment for smoking cessation.

Associations of timing, level, and pattern of secondhand smoke exposure with early alcohol initiation: A cohort study

BACKGROUND

Few studies explored the longitudinal link between early-life secondhand smoke (SHS) exposure and later alcohol initiation despite its risk for child behavioral difficulties. We examined the associations of the timing, level, and pattern of SHS exposure from pregnancy to childhood with early alcohol initiation and evaluated the sex differences in these associations.

METHODS

Data were from 16,440 participants of the Taiwan Birth Cohort Study conducted when the children were aged 6 months, 18 months, 3 years, 5.5 years, 8 years, and 12 years. Group-based trajectory modeling was applied to identified patterns of SHS exposure. A series of multiple logistic regression were conducted to examine study hypotheses.

RESULTS

Exposure to prenatal SHS was associated with an increased risk of early alcohol initiation (adjusted odds ratio [aOR] = 1.17, 95% confidence interval [CI] = 1.06, 1.30). Compared with the adolescents with a persistent-low-exposure trajectory, those who exhibited prenatal-high-decreasing (aOR = 1.18, 95% CI = 1.04, 1.35) or persistent-high-exposure (aOR = 1.27, 95% CI = 1.12, 1.45) patterns exhibited increased risks of early alcohol initiation. Those with higher cumulative levels of SHS exposure also exhibited an increased risk of early alcohol initiation (aOR = 1.03, 95% CI = 1.01, 1.04). Sex differences were also observed.

CONCLUSIONS

Varying timing, levels, and longitudinal patterns of SHS exposure during early life had differential effects on early alcohol initiation, with the effects differing by sex. Targeting SHS exposure while considering the nature of exposure and sex differences could help prevent and curb alcohol use in adolescents.

Genetic influences impacting nicotine use and abuse during adolescence: Insights from human and rodent studies

Nicotine use continues to be a major public health concern, with an alarming recent rise in electronic cigarette consumption. Heritability estimates of nicotine use and abuse range from 40% to 80%, providing strong evidence that genetic factors impact nicotine addiction-relevant phenotypes. Although nicotine use during adolescence is a key factor in the development of addiction, it remains unclear how genetic factors impact adolescent nicotine use and abuse. This review will discuss studies investigating genetic factors impacting nicotine use during adolescence. Evidence from both rodent and human studies will be summarized and integrated when possible. Human adolescent studies have largely included candidate gene studies for genes identified in adult populations, such as genes involved in nicotine metabolism, nicotinic acetylcholine receptor signaling, dopaminergic signaling, and other neurotransmitter signaling systems. Alternatively, rodent studies have largely taken a discovery-based approach identifying strain differences in adolescent nicotine addiction-relevant behaviors. Here, we aim to answer the following three questions by integrating human and rodent findings: (1) Are there genetic variants that uniquely impact nicotine use during adolescence? (2) Are there genetic variants that impact both adolescent and adult nicotine use? and (3) Do genetic factors in adolescence significantly impact long-term consequences of adolescent nicotine use? Determining answers for these three questions will be critical for the development of preventative measures and treatments for adolescent nicotine use and addiction.

Chronic pain and delinquency partially explain the effect of the DRD4 gene polymorphism on adult substance use

Background: The dopamine receptor D4 [DRD4] has been reported to be associated with substance use. Yet, the roles that health conditions and behaviors may play in such association are understudied.Objective: This longitudinal study investigated the potential mediation effects of chronic pain and delinquency in adolescence on the association between the DRD4 2-repeat allele and substance use in adulthood. Sex, witnessing violence, and experiencing violence were also examined as potential moderators for the mediation pathways.Methods: We used the restricted and candidate gene data from the National Longitudinal Study of Adolescent to Adult Health (Waves I-IV) to conduct secondary analysis (N = 8,671; 47% male). A two-step approach was adopted to examine the mediation effects regarding four substance use outcomes in adulthood: number of lifetime alcohol use disorder symptoms, lifetime regular smoker status, past-month smoking, and lifetime "pain killer" misuse. The moderation effects were investigated using stratification and permutation.Results: The DRD4 2-repeat allele was associated with all adulthood substance use outcomes through adolescent chronic pain and delinquency (AORs/IRR range 1.08-3.78; all ps<0.01). The association between delinquency and smoking was higher among females. The association between delinquency and substance use was lower among the participants who witnessed violence in adolescence.Conclusions: This study identified modifiable mediators underlying the association between the DRD4 2-repeat allele and substance use behaviors, concluding that chronic pain and delinquency partially explain the effect of the DRD4 gene polymorphism on adult substance use.

Genetic and Epigenetic Analysis Revealing Variants in the NCAM1-TTC12-ANKK1-DRD2 Cluster Associated Significantly With Nicotine Dependence in Chinese Han Smokers

BACKGROUNDS

Although studies have demonstrated that the NCAM1-TTC12-ANKK1-DRD2 gene cluster plays essential roles in addictions in subjects of European and African origin, study of Chinese Han subjects is limited. Further, the underlying biological mechanisms of detected associations are largely unknown.

METHODS

Sixty-four single-nucleotide polymorphisms (SNPs) in this cluster were analyzed for association with Fagerstrőm Test for Nicotine Dependence score (FTND) and cigarettes per day (CPD) in male Chinese Han smokers (N = 2616). Next-generation bisulfite sequencing was used to discover smoking-associated differentially methylated regions (DMRs). Both cis-eQTL and cis-mQTL analyses were applied to assess the cis-regulatory effects of these risk SNPs.

RESULTS

Association analysis revealed that rs4648317 was significantly associated with FTND and CPD (p = .00018; p = .00072). Moreover, 14 additional SNPs were marginally significantly associated with FTND or CPD (p = .05-.01). Haplotype-based association analysis showed that one haplotype in DRD2, C-T-A-G, formed by rs4245148, rs4581480, rs4648317, and rs11214613, was significantly associated with CPD (p = .0005) and marginally associated with FTND (p = .003). Further, we identified four significant smoking-associated DMRs, three of which are located in the DRD2/ANKK1 region (p = .0012-.00005). Finally, we found five significant CpG-SNP pairs (p = 7.9 × 10-9-6.6 × 10-6) formed by risk SNPs rs4648317, rs11604671, and rs2734849 and three methylation loci.

CONCLUSIONS

We found two missense variants (rs11604671; rs2734849) and an intronic variant (rs4648317) with significant effects on ND and further explored their mechanisms of action through expression and methylation analysis. We found the majority of smoking-related DMRs are located in the ANKK1/DRD2 region, indicating a likely causative relation between non-synonymous SNPs and DMRs.

IMPLICATIONS

This study shows that there exist significant association of variants and haplotypes in ANKK1/DRD2 region with ND in Chinese male smokers. Further, this study also shows that DNA methylation plays an important role in mediating such associations.

Genetic association of human Corticotropin-Releasing Hormone Receptor 1 (CRHR1) with Internet gaming addiction in Korean male adolescents

BACKGROUND

The number of people with Internet gaming addiction (IGA) is increasing around the world. IGA is known to be associated with personal characteristics, psychosocial factors, and physiological factors. However, few studies have examined the genetic factors related to IGA. This study aimed to investigate the association between IGA and stress-related genetic variants.

METHODS

This cross-sectional study was conducted with 230 male high school students in a South Korean city. We selected five stress-related candidate genes: DAT1, DRD4, NET8, CHRNA4, and CRHR1. The DAT1 and DRD4 genes were genotyped by polymerase chain reaction, and the NET8, CHRNA4, and CRHR1 genes were genotyped by pyrosequencing analysis. We performed a Chi-square test to examine the relationship of these five candidate genes to IGA.

RESULTS

Having the AA genotype and the A allele of the CRHR1 gene (rs28364027) was associated with higher odds of belonging to the IGA participant group (p = .016 and p = .021, respectively) than to the non-IGA group. By contrast, the DAT1, DRD4, NET8, and CHRNA4 gene polymorphisms showed no significant difference between the IGA group and control group.

CONCLUSIONS

These results indicate that polymorphism of the CRHR1 gene may play an important role in IGA susceptibility in the Korean adolescent male population. These findings provide a justification and foundation for further investigation of genetic factors related to IGA.

Affiliation with substance-using peers: Examining gene-environment correlations among parent monitoring, polygenic risk, and children's impulsivity

Parental monitoring can buffer the effect of deviant peers on adolescents' substance use by reducing affiliation with substance-using peers. However, children's genetic predispositions may evoke poorer monitoring, contributing to negative child outcomes. We examined evocative genotype-environment correlations underlying children's genetic predisposition for behavioral undercontrol and parental monitoring in early adolescence via children's impulsivity in middle childhood, and the influence of parental monitoring on affiliation with substance-using peers a year and a half later (n = 359). Genetic predisposition for behavioral undercontrol was captured using a polygenic risk score, and a portion of passive rGE was controlled by including parents' polygenic risk scores. Children's polygenic risk predicted poorer parental monitoring via greater children's impulsivity, indicating evocative rGE, controlling for a portion of passive rGE. Poorer parental monitoring predicted greater children's affiliation with substance-using peers a year and a half later. Results are discussed with respect to gene-environment correlations within developmental cascades.

The association of dopamine pathway gene score, nicotine dependence and smoking cessation in a rural male population of Shandong, China

BACKGROUND AND OBJECTIVES

Smoking and smoking cessation are both psychological and physiological traits. We aimed to investigate the interaction between dopamine pathway gene scores and nicotine dependence on smoking cessation in a rural Chinese male population.

METHODS

Participants were recruited from 17 villages in Shandong, China. DNA was extracted from blood sample of 819 participants. 25 single nucleotide polymorphisms (SNPs) in 8 dopamine (DA) pathway genes were genotyped. Weighted gene score of each gene is used to analyze the whole gene effect. Logistic regression was used to calculate odds ratios (OR) and multivariate-adjusted OR of each gene score for smoking cessation. Multiplicative model interaction was assessed through a cross-product interaction term of gene score by nicotine dependence in a multivariate logistic regression model.

RESULTS

After adjusting for age, occupation, education, marital status, self-rating anxiety score, and disease status, we observed significant negative associations of catechol-O-methyltransferase (COMT), dopamine receptor D2 (DRD2) gene score and smoking cessation, as well as significant positive associations between ankyrin repeat and kinase domain containing 1 (ANKK1), dopamine transporter (SLC6A3), dopamine receptor D4 (DRD4) gene score and smoking cessation. A significant multiplicative model interaction between nicotine dependence and the SLC6A3 gene score on smoking cessation was also observed (p = .03).

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

There is a significant multiplicative model interaction of SLC6A3 gene score and nicotine dependence on smoking cessation. This finding could help to identify smokers who may be at high risk of relapse, and thus to develop more professional and personalized smoking cessation treatment. (Am J Addict 2016;25:493-498).

Predicting substance use in emerging adulthood: A genetically informed study of developmental transactions between impulsivity and family conflict

Deviance proneness models propose a multilevel interplay in which transactions among genetic, individual, and family risk factors place children at increased risk for substance use. We examined bidirectional transactions between impulsivity and family conflict from middle childhood to adolescence and their contributions to substance use in adolescence and emerging adulthood (n = 380). Moreover, we examined children's, mothers', and fathers' polygenic risk scores for behavioral undercontrol, and mothers' and fathers' interparental conflict and substance disorder diagnoses as predictors of these transactions. The results support a developmental cascade model in which children's polygenic risk scores predicted greater impulsivity in middle childhood. Impulsivity in middle childhood predicted greater family conflict in late childhood, which in turn predicted greater impulsivity in late adolescence. Adolescent impulsivity subsequently predicted greater substance use in emerging adulthood. Results are discussed with respect to evocative genotype-environment correlations within developmental cascades and applications to prevention efforts.

Nicotinic cholinergic and dopaminergic receptor mRNA expression in male and female rats with high or low preference for nicotine

BACKGROUND

Nicotine exerts its central actions through nicotinic acetylcholine receptors (nAChRs), which in turn regulate major neurotransmitter systems including dopamine. Nicotinic and dopaminergic systems play significant roles in physiological functions, neuropsychiatric disorders, and addiction.

OBJECTIVES

To evaluate possible differences in the expression of nAChR subunit and dopamine receptor (DR) mRNAs following voluntary nicotine intake.

METHODS

Male and female rats (n = 67) were exposed to long-term free-choice oral nicotine (24 hours/day, 6 weeks); rats with maximum and minimum nicotine preference/intake were selected. The mRNA levels of genes encoding α4,β2,α5, and α7 nAChR subunits and DR Drd1and Drd2 subtypes were evaluated in the striatum (STR), prefrontal cortex (PFC), and hippocampus using quantitative real-time polymerase chain reaction in selected rats (n = 30) and their control groups (n = 15).

RESULTS

In addition to baseline differences, expression changes were observed in the mRNA levels of evaluated genes in rats exposed to voluntary oral nicotine in a brain region-, sex-, and preference-related manner. Nicotine intake is correlated negatively with Chrnb2, Chrna7 and positively with Drd1 expression. In the cholinergic system, regional differences in Chnrb2 and Chrna5, sex differences in Chrna4 and Chrna5, and nicotine preference effects in the expression of all subunits except α4 were observed. Chrna5 was lower in maximum than in minimum preferring, and in male than female rats, supporting the inhibitory role of the α5 subunit in nicotine dependence. Nicotine increased Drd2 mRNA expression only in minimum preferring female rats in STR and PFC.

CONCLUSION

Modulation of nAChR and DR gene expression by nicotine may have clinical implications and aid drug development. Pharmaceuticals targeting the nicotinic cholinergic and dopaminergic systems might be expected to have differential efficacy that varies with the patient's sex or smoking status.

Genome-wide association study of lifetime cannabis use based on a large meta-analytic sample of 32 330 subjects from the International Cannabis Consortium

Cannabis is the most widely produced and consumed illicit psychoactive substance worldwide. Occasional cannabis use can progress to frequent use, abuse and dependence with all known adverse physical, psychological and social consequences. Individual differences in cannabis initiation are heritable (40-48%). The International Cannabis Consortium was established with the aim to identify genetic risk variants of cannabis use. We conducted a meta-analysis of genome-wide association data of 13 cohorts (N=32 330) and four replication samples (N=5627). In addition, we performed a gene-based test of association, estimated single-nucleotide polymorphism (SNP)-based heritability and explored the genetic correlation between lifetime cannabis use and cigarette use using LD score regression. No individual SNPs reached genome-wide significance. Nonetheless, gene-based tests identified four genes significantly associated with lifetime cannabis use: NCAM1, CADM2, SCOC and KCNT2. Previous studies reported associations of NCAM1 with cigarette smoking and other substance use, and those of CADM2 with body mass index, processing speed and autism disorders, which are phenotypes previously reported to be associated with cannabis use. Furthermore, we showed that, combined across the genome, all common SNPs explained 13-20% (P<0.001) of the liability of lifetime cannabis use. Finally, there was a strong genetic correlation (rg=0.83; P=1.85 × 10(-8)) between lifetime cannabis use and lifetime cigarette smoking implying that the SNP effect sizes of the two traits are highly correlated. This is the largest meta-analysis of cannabis GWA studies to date, revealing important new insights into the genetic pathways of lifetime cannabis use. Future functional studies should explore the impact of the identified genes on the biological mechanisms of cannabis use.

Parental smoke exposure and the development of nicotine craving in adolescent novice smokers: the roles of DRD2, DRD4, and OPRM1 genotypes

BACKGROUND

Among adolescent novice smokers, craving is often the first, and is the most reported, symptom of nicotine dependence. Until now, little has been known about the development of craving symptoms in novice smokers. The aim of this study was to identify specific genetic (i.e., DRD2 Taq1A, DRD4 48 bp VNTR, and OPRM1 A118G polymorphisms) and environmental mechanisms that underlie the emergence of both cue-induced and cognitive craving among adolescent novice smokers.

METHOD

A five-wave longitudinal, genetically-informed survey study was conducted with intervals of four months. The sample included 376 early adolescent smokers (12-13 years of age at baseline). Self-report questionnaires were completed regarding smoking behavior, observed parental smoking behavior, and both cue-induced and cognitive craving.

RESULTS

Data were analyzed with a latent growth curve approach. For both cue-induced and cognitive craving, significant interaction effects were found for DRD2 Taq1A with parental smoke exposure. A1-allele carriers did not seem to be influenced by the environment with regard to craving development. Adolescents who are homozygous for the A2-allele and who are more exposed to parental smoking experience the highest levels of both types of craving over time. No significant interaction effects were found between parental smoke exposure and DRD4 48 bp VNTR or OPRM1 A118G.

CONCLUSIONS

Previous studies identified DRD2 Taq1A A1-allele carriers as vulnerable to developing nicotine dependence. However, this study showed that parental smoking increased the chances of developing dependence more rapidly for early adolescents who are considered to be less sensitive to the rewarding effects of nicotine according to their DRD2 Taq1A genotype. It is thus especially important that these young people not be exposed to smoking in their social environment.

Contribution of BDNF and DRD2 genetic polymorphisms to continued opioid use in patients receiving methadone treatment for opioid use disorder: an observational study

BACKGROUND

The heritability of opioid use disorder has been widely investigated; however, the influence of specific genes on methadone treatment outcomes is not well understood. The association between response to methadone treatment and genes that are involved in substance use behaviors and reward mechanisms is poorly understood, despite evidence suggesting their contribution to opioid use disorder. The aim of this study was to investigate the effect of brain-derived neurotrophic factor (BDNF) and dopamine receptor D2 (DRD2) polymorphisms on continued opioid use among patients on methadone treatment for opioid use disorder.

METHODS

BDNF 196G>A (rs6265) and DRD2-241A>G (rs1799978) genetic variants were examined in patients with opioid use disorder who were recruited from methadone treatment clinics across Southern Ontario, Canada. We collected demographic information, substance use history, blood for genetic analysis, and urine to measure opioid use. We used regression analysis to examine the association between continued opioid use and genetic variants, adjusting for age, sex, ethnicity, methadone dose, duration in treatment, and number of urine screens.

RESULTS

Among 240 patients treated with methadone for opioid use disorder, 36.3 percent (n = 87) and 11.3 percent (n = 27) had at least one risk allele for rs6265 and rs1799978, respectively. These genetic variants were not significantly associated with continued opioid use while on methadone maintenance treatment [rs6265: odds ratio (OR) = 1.37, 95 % confidence interval (CI) = 0.792, 2.371, p = 0.264; rs1799978: OR 1.27, 95 % CI 0.511, 3.182, p = 0.603].

CONCLUSIONS

Despite an association of BDNF rs6265 and DRD2 rs1799978 with addictive behaviors, these variants were not associated with continued illicit opioid use in patients treated with methadone. Problematic use of opioids throughout treatment with methadone may be attributed to nongenetic factors or a polygenic effect requiring further exploration. Additional research should focus on investigating these findings in larger samples and different populations.

An initial investigation of associations between dopamine-linked genetic variation and smoking motives in African Americans

Nicotine dependence (ND) is a heterogeneous phenotype with complex genetic influences that may vary across ethnicities. The use of intermediate phenotypes may clarify genetic influences and reveal specific etiological pathways. Prior work in European Americans has found that the four Primary Dependence Motives (PDM) subscales (Automaticity, Craving, Loss of Control, and Tolerance) of the Wisconsin Inventory of Smoking Motives represent core features of nicotine dependence and are promising intermediate phenotypes for understanding genetic pathways to ND. However, no studies have examined PDM as an intermediate phenotype in African American smokers, an ethnic population that displays unique patterns of smoking and genetic variation. In the current study, 268 African American daily smokers completed a phenotypic assessment and provided a sample of DNA. Associations among haplotypes in the NCAM1-TTC12-ANKK1-DRD2 gene cluster, a dopamine-related gene region associated with ND, PDM intermediate phenotypes, and ND were examined. Dopamine-related genetic variation in the DBH and COMT genes was also considered on an exploratory basis. Mediational analysis was used to test the indirect pathway from genetic variation to smoking motives to nicotine dependence. NCAM1-TTC12-ANKK1-DRD2 region variation was significantly associated with the Automaticity subscale and, further, Automaticity significantly mediated associations among NCAM1-TTC12-ANKK1-DRD2 cluster variants and ND. DBH was also significantly associated with Automaticity, Craving, and Tolerance; Automaticity and Tolerance also served as mediators of the DBH-ND relationship. These results suggest that PDM, Automaticity in particular, may be a viable intermediate phenotype for understanding dopamine-related genetic influences on ND in African American smokers. Findings support a model in which putatively dopaminergic variants exert influence on ND through an effect on patterns of automatic routinized smoking.

Genetic and Clinical Factors Associated with Chronic Postsurgical Pain after Hernia Repair, Hysterectomy, and Thoracotomy

Background:

Chronic postsurgical pain (CPSP) has been linked to many surgical settings. The authors aimed to analyze functional genetic polymorphisms and clinical factors that might identify CPSP risk after inguinal hernia repair, hysterectomy, and thoracotomy.

Methods:

This prospective multicenter cohort study enrolled 2,929 patients scheduled for inguinal hernia repair, hysterectomy (vaginal or abdominal), or thoracotomy. The main outcome was the incidence of CPSP confirmed by physical examination 4 months after surgery. The secondary outcome was CPSP incidences at 12 and 24 months. The authors also tested the associations between CPSP and 90 genetic markers plus a series of clinical factors and built a CPSP risk model.

Results:

Within a median of 4.4 months, CPSP had developed in 527 patients (18.0%), in 13.6% after hernia repair, 11.8% after vaginal hysterectomy, 25.1% after abdominal hysterectomy, and 37.6% after thoracotomy. CPSP persisted after a median of 14.6 months and 26.3 months in 6.2% and 4.1%, respectively, after hernia repair, 4.1% and 2.2% after vaginal hysterectomy, 9.9% and 6.7% after abdominal hysterectomy, and 19.1% and 13.2% after thoracotomy. No significant genetic differences between cases and controls were identified. The risk model included six clinical predictors: (1) surgical procedure, (2) age, (3) physical health (Short Form Health Survey-12), (4) mental health (Short Form Health Survey-12), (5) preoperative pain in the surgical field, and (6) preoperative pain in another area. Discrimination was moderate (c-statistic, 0.731; 95% CI, 0.705 to 0.755).

Conclusions:

Until unequivocal genetic predictors of CPSP are understood, the authors encourage systematic use of clinical factors for predicting and managing CPSP risk.

Gender differences in the effect of tobacco use on brain phosphocreatine levels in methamphetamine-dependent subjects

BACKGROUND

A high prevalence of tobacco smoking has been observed in methamphetamine users, but there have been no in vivo brain neurochemistry studies addressing gender effects of tobacco smoking in methamphetamine users. Methamphetamine addiction is associated with increased risk of depression and anxiety in females. There is increasing evidence that selective analogues of nicotine, a principal active component of tobacco smoking, may ease depression and improve cognitive performance in animals and humans.

OBJECTIVES

To investigate the effects of tobacco smoking and gender on brain phosphocreatine (PCr) levels, a marker of brain energy metabolism reported to be reduced in methamphetamine-dependent subjects.

METHODS

Thirty female and 27 male methamphetamine-dependent subjects were evaluated with phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS) to measure PCr levels within the pregenual anterior cingulate, which has been implicated in methamphetamine neurotoxicity.

RESULTS

Analysis of covariance revealed that there were statistically significant slope (PCr versus lifetime amount of tobacco smoking) differences between female and male methamphetamine-dependent subjects (p = 0.03). In females, there was also a statistically significant interaction between lifetime amounts of tobacco smoking and methamphetamine in regard to PCr levels (p = 0.01), which suggests that tobacco smoking may have a more significant positive impact on brain PCr levels in heavy, as opposed to light to moderate, methamphetamine-dependent females.

CONCLUSION

These results indicate that tobacco smoking has gender-specific effects in terms of increased anterior cingulate high energy PCr levels in methamphetamine-dependent subjects. Cigarette smoking in methamphetamine-dependent women, particularly those with heavy methamphetamine use, may have a potentially protective effect upon neuronal metabolism.

NCAM1-TTC12-ANKK1-DRD2 variants and smoking motives as intermediate phenotypes for nicotine dependence

RATIONALE

Nicotine dependence (ND) is a heterogeneous phenotype with complex genetic influences. The use of intermediate ND phenotypes may clarify genetic influences and reveal specific etiological pathways. Prior work has found that the four Primary Dependence Motives (PDM) subscales (Automaticity, Craving, Loss of Control, and Tolerance) of the Wisconsin Inventory of Smoking Motives (WISDM) represent heavy, pervasive smoking, which is a core feature of nicotine dependence, making these motives strong candidates as intermediate phenotypes.

OBJECTIVE

This study examines the WISDM PDM as a novel intermediate phenotype of nicotine dependence.

METHODS

The study used data from 734 European Americans who smoked at least 5 cigs/day [M = 16.2 (SD = 9.5) cigs/day], completed a phenotypic assessment, and provided a sample of DNA. Based on prior evidence of the role of genetic variation in the NCAM1-TTC12-ANKK1-DRD2 region on chromosome 11q23 in smoking behavior, associations among 12 region loci with nicotine dependence and PDM phenotypes were examined using haplotype and individual loci approaches. In addition, mediational analysis tested the indirect pathway from genetic variation to smoking motives to nicotine dependence.

RESULTS

NCAM1-TTC12-ANKK1-DRD2 region loci and haplotypes were significantly associated with the motive of Automaticity and, further, Automaticity significantly mediated associations among NCAM1-TTC12-ANKK1-DRD2 cluster variants and nicotine dependence.

CONCLUSIONS

These results suggest that motives related to automaticity are a viable intermediate phenotype for understanding genetic contributions to nicotine dependence. Further, NCAM1-TTC12-ANKK1-DRD2 variants may increase the likelihood that a person will become dependent via a highly automatic smoking ritual that can be elicited with little awareness.

Pharmacogenetics of nicotine and associated smoking behaviors

This chapter summarizes genetic factors that contribute to variation in nicotine pharmacokinetics and nicotine's pharmacological action in the central nervous system (CNS), and how this in turn influences smoking behaviors. Nicotine, the major psychoactive compound in cigarette smoke, is metabolized by a number of enzymes, including CYP2A6, CYP2B6, FMOs, and UGTs, among others. Variation in the genes encoding these enzymes, in particular CYP2A6, can alter the rate of nicotine metabolism and smoking behaviors. Faster nicotine metabolism is associated with higher cigarette consumption and nicotine dependence, as well as lower quit rates. Variation in nicotine's CNS targets and downstream signaling pathways can also contribute to interindividual differences in smoking patterns. Binding of nicotine to neuronal nicotinic acetylcholine receptors (nAChRs) mediates the release of several neurotransmitters including dopamine and serotonin. Genetic variation in nAChRs, and in transporter and enzyme systems that leads to altered CNS levels of dopamine and serotonin, is associated with a number of smoking behaviors. To date, the precise mechanism underpinning many of these findings remains unknown. Considering the complex etiology of nicotine addiction, a more comprehensive approach that assesses the contribution of multiple gene variants, and their interaction with environmental factors, will likely improve personalized therapeutic approaches and increase smoking cessation rates.

Genetic variants and early cigarette smoking and nicotine dependence phenotypes in adolescents

Background

While the heritability of cigarette smoking and nicotine dependence (ND) is well-documented, the contribution of specific genetic variants to specific phenotypes has not been closely examined. The objectives of this study were to test the associations between 321 tagging single-nucleotide polymorphisms (SNPs) that capture common genetic variation in 24 genes, and early smoking and ND phenotypes in novice adolescent smokers, and to assess if genetic predictors differ across these phenotypes.

Methods

In a prospective study of 1294 adolescents aged 12–13 years recruited from ten Montreal-area secondary schools, 544 participants who had smoked at least once during the 7–8 year follow-up provided DNA. 321 single-nucleotide polymorphisms (SNPs) in 24 candidate genes were tested for an association with number of cigarettes smoked in the past 3 months, and with five ND phenotypes (a modified version of the Fagerstrom Tolerance Questionnaire, the ICD-10 and three clusters of ND symptoms representing withdrawal symptoms, use of nicotine for self-medication, and a general ND/craving symptom indicator).

Results

The pattern of SNP-gene associations differed across phenotypes. Sixteen SNPs in seven genes (ANKK1, CHRNA7, DDC, DRD2, COMT, OPRM1, SLC6A3 (also known as DAT1)) were associated with at least one phenotype with a p-value <0.01 using linear mixed models. After permutation and FDR adjustment, none of the associations remained statistically significant, although the p-values for the association between rs557748 in OPRM1 and the ND/craving and self-medication phenotypes were both 0.076.

Conclusions

Because the genetic predictors differ, specific cigarette smoking and ND phenotypes should be distinguished in genetic studies in adolescents. Fifteen of the 16 top-ranked SNPs identified in this study were from loci involved in dopaminergic pathways (ANKK1/DRD2, DDC, COMT, OPRM1, and SLC6A3).

Impact

Dopaminergic pathways may be salient during early smoking and the development of ND.

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.

COMT met allele differentially predicts risk versus severity of aberrant eating in a large community sample

Prefrontal dopamine (DA) transmission participates in the reinforcement of reward-driven behaviors like eating. Because catechol-O-methyltransferase (COMT) degrades DA and is expressed in the prefrontal cortex, variation in the COMT gene may modulate eating behavior. Previous studies have shown that the met allele of the COMT val158met single nucleotide polymorphism (SNP) is associated with Bulimia Nervosa (BN). The specific aim of this study was to test whether the met allele increased risk for, and severity of, eating disorder symptomatology in community volunteers. Caucasian adults (N=1003; 51.2% female) from the University of Pittsburgh Adult Health and Behavior Project (AHAB) were genotyped and completed the Eating Disorders Inventory (EDI). Logistic and Poisson regression analyses assessed genotype-dependent presence and severity of eating disorder symptomatology. The met allele was significantly associated with the presence of symptoms on the Bulimia subscale and the severity of Body Dissatisfaction scores. All EDI subscales significantly predicted BMI. To our knowledge, these are the first data indicating that the COMT met allele increases risk for some symptoms of disordered eating, while increasing severity of others, in a community sample. These novel findings may have important implications for understanding the etiology of heterogeneous disordered eating phenotypes.

Effects of nicotine deprivation and replacement on BOLD-fMRI response to smoking cues as a function of DRD4 VNTR genotype

INTRODUCTION

Reactivity to smoking cues is an important factor in the motivation to smoke and has been associated with the dopamine receptor 4 variable number tandem repeat (DRD4 exon III VNTR) polymorphism. However, little is known about the associated neural mechanisms.

METHODS

Non-treatment-seeking Caucasian smokers completed overnight abstinence and viewed smoking and neutral cues during 2 separate functional magnetic resonance imaging scans while wearing either a nicotine or placebo patch (order randomized) and were genotyped for the DRD4 VNTR. We conducted mixed-effects repeated-measures analyses of variance (within-subject factor: nicotine or placebo patch; between-subject factor: DRD4 long [L: ≥ 1 copy of ≥ 7 repeats] or short [S: 2 copies ≤ 6 repeats] genotype) of 6 a priori regions of interest.

RESULTS

Relative to neutral cues, smoking cues elicited greater activity in bilateral ventral striatum and left amygdala during nicotine replacement and deactivation in these regions during nicotine deprivation. A patch × DRD4 interaction was observed in the left amygdala, an area associated with appetitive reinforcement and relapse risk, such that S allele carriers demonstrated greater activation on active patch than on placebo patch.

CONCLUSIONS

Brain systems associated with reward salience may become primed and overreactive at nicotine replacement doses intended for the first step of smoking cessation and may become inhibited during nicotine withdrawal in DRD4 S but not in DRD4 L carriers. These findings are consistent with the role of these regions in drug reinforcement and suggest a differential influence of nicotine replacement on amygdala activation in the association of incentive salience with smoking stimuli across DRD4 genotypes.

Initial responses to the first dose of nicotine in novel smokers: the role of exposure to environmental smoking and genetic predisposition

BACKGROUND

Sensitivity to initial smoking constitutes an early predictor of the risk of dependence. We investigated the role of exposure to smoking (by parents, siblings, and peers) and reward-related candidate gene polymorphisms (OPRM1 A118G, DRD2 TaqlA and DRD4 bp VNTR) in adolescents' responses to initial smoking.

METHODS

We used cross-sectional survey data and saliva samples from 171 Dutch students who had never inhaled on a cigarette (mean age: 13.9 years). The outcome measure was adolescents' self-reported responses to initial smoking.

RESULTS

Exposure to peer smoking was associated with increased liking (OR = 1.74, CI = 1.13-2.70) and more pleasant sensations (β = .21, p = .01). Exposure to maternal smoking was associated with less unpleasant sensations (β = -.20, p = .01). Adolescents carrying the G-variant of the OPRM1 A118G polymorphism were more likely to report liking (OR = 2.50, CI = 1.09-5.73) and adolescents homozygous for the C-variant of the DRD2 TaqlA polymorphism reported less unpleasant sensations (β = .18, p = .04).

CONCLUSION

Although preliminary, these findings suggest that exposure to environmental smoking and polymorphisms in the OPRM1 and DRD2 gene may affect initial sensitivity to nicotine, an early phenotype of the risk of dependence. In the future, collaborative efforts to combine data from multiple studies in meta-analyses are needed to improve accuracy of estimated effects in genetic studies.

Environmental smoking and smoking onset in adolescence: the role of dopamine-related genes. Findings from two longitudinal studies

Although environmental smoking (i.e., paternal and maternal smoking, sibling smoking, and peer smoking) is one of the most important factors for explaining adolescent smoking behavior, not all adolescents are similarly affected. The extent to which individuals are vulnerable to smoking in their environment might depend on genetic factors. The aim of this study was to examine the interplay between environmental smoking and genes encoding components of the dopaminergic system (i.e., dopamine receptor D2, D4, and dopamine transporter DAT1) in adolescent smoking onset. Data from two longitudinal studies were used. Study 1 consisted of 991 non-smoking early adolescents (mean age = 12.52, SD = .57) whereas study 2 consisted of 365 non-smoking middle to late adolescents (mean age = 14.16, SD = 1.07) who were followed for 16 and 48 months, respectively. Logistic regression analyses were conducted using Mplus. In study 1, we found positive associations between parents' and friends' smoking at the first measurement and smoking status 16 months later. In study 2 we found a positive association between friends' smoking and smoking onset 48 months later. Neither study demonstrated any interaction effects of the DRD2, DRD4, or DAT1 genotypes. In conclusion, the effects of environmental smoking on smoking onset are similar for adolescent carriers and non-carriers of these specific genes related to the dopaminergic system.

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.

Lack of association of a functional catechol-O-methyltransferase gene polymorphism with risk of tobacco smoking: results from a multicenter case-control study

BACKGROUND

The catechol-O-methyltransferase (COMT) modulates dopaminergic neurotransmission in the prefrontal cortex as well as in the mesolimbic reward system. Since the reward system mediates addictive behavior, the COMT gene is a strong candidate gene regarding the pathophysiology of tobacco dependence and smoking behavior. Because of rather conflicting results in previous studies, the purpose of the present study was to test for association between a functional genetic variant in the COMT gene (single nucleotide polymorphism [SNP] rs4680) and tobacco smoking behavior.

METHODS

In a population-based case-control multicenter study designed for tobacco addiction research, a total of 551 current smokers of European ancestry and 548 age-matched healthy volunteers (never-smokers) were genotyped for SNP rs4680 and extensively characterized concerning their smoking behavior.

RESULTS

We found no association between smoking status and SNP rs4680 genotype nor did we find a significant association to the degree of tobacco dependence.

CONCLUSIONS

Although prefrontal cortical and ventral striatal activity are highly relevant for addictive behavior, and under partial control of COMT rs4680 genotype, no association between COMT and smoking behavior was observed. Other genetic variants may account for the high heritability of behavioral smoking phenotypes.

Genetic Analysis of Polymorphisms in Dopamine Receptor and Transporter Genes for Association with Smoking among Cancer Patients

BACKGROUND

Smoking among Russian cancer patients may be related to variations in the DRD2/ANKK1 (Taq1), DRD4 (exon III VNTR), and SLC6A3 genes.

METHODS

Seven hundred fifty patients provided smoking history and DNA.

RESULTS

Current smokers were more likely to be DRD2 A2 allele carriers versus nonsmokers (former/never smokers; 69 vs. 56%; OR = 1.69; 95% CI 1.13-2.53, p = 0.01) and former smokers (69 vs. 59%; OR = 1.54; 95% CI 0.97-2.46, p = 0.07). Ever smokers (current/former smokers) were more likely to be DRD2 A2 allele carriers versus never smokers (65 vs. 55%; OR = 1.50; 95% CI 1.00-2.27, p = 0.05). The risk of current smoking among DRD2 A2 allele carriers was present if the DRD4 short allele was also present (OR = 1.76; 95% CI 1.12-2.78, p = 0.02), and the risk of ever smoking among DRD2 A2 allele carriers was present if the DRD4 short allele was also present (OR = 1.62; 95% CI 1.02-2.55, p = 0.04). DRD2 A2 allele carriers had a shorter period of previous abstinence versus DRD2 A1 carriers (p = 0.02). Effects were not statistically significant when controlling for multiple comparisons.

CONCLUSIONS

The DRD2 A2 allele may increase the risk of smoking among cancer patients, convergent with studies using non-Western samples. However, additional replication is needed.

A prospective study of the effects of the DRD2/ANKK1 TaqIA polymorphism and impulsivity on smoking initiation

This study tested whether DRD2/ANKK1 TaqIA genotype predicted smoking initiation and subsequent use, and effects were mediated by sensation seeking and negative urgency. Between 2009 and 2012, college never smokers (n = 387) completed six assessments over 15 months; those who reported smoking were classified as initiators. Logistic regression indicated that the A1 allele was associated with initiation (p = .003). This effect was partially mediated by sensation seeking and negative urgency. Effects were stronger in Asian Americans. Findings have implications for improving prevention by including elements focused on urges to seek positive or negative reinforcement. Limitations and future directions are discussed.

Sensorimotor gating and D2 receptor signalling: evidence from a molecular genetic approach

Converging evidence from pharmacological investigations, genetic association studies and schizophrenia research indicates an important influence of the dopamine system on sensorimotor gating as measured by prepulse inhibition (PPI) of the acoustic startle response. In particular, D2 receptor agonists have been shown to disrupt PPI in humans and rodents. In the present study, we investigated the associations of two functional DRD2 related single nucleotide polymorphisms (rs4648317 and rs1800497, the latter also known as DRD2/ANKK1 Taq1A) with PPI in two independent healthy human samples (overall n=197; Munich n=101; London n=96). Taq1A is a prominent marker of striatal D2 receptor signalling and was therefore hypothesized to impact on PPI. In line with our hypothesis, we report here reduced PPI levels in individuals with higher striatal D2 receptor signalling as indicated by the Taq1A genotype. Meta-analysis across both samples confirmed this finding. In contrast, an association between rs4648317 and PPI found in the Munich sample could not be confirmed in the London sample. Overall, the present study helps to bridge the gap between pharmacological manipulations of PPI and molecular genetics of the dopaminergic system.

Assessment between Dopamine Receptor D2 (DRD2) Polymorphisms and Schizophrenia in Korean Population

OBJECTIVE

The aim of this study was to investigate whether single nucleotide polymorphisms (SNPs) of dopamine receptor D2 (DRD2) are associated with schizophrenia in Korean population.

METHODS

Four SNPs (rs4648317, rs7131056, rs4936270, and rs1076562) of DRD2 were selected and genotyped by direct sequencing in 197 schizophrenia patients and 370 control subjects. SNPAnalyzer, SNPStats, and Haploview version 4.2 programs were performed to analyze the genetic data. Multiple logistic regression models (codominant1, codominant2, dominant, recessive, overdominant, and log-additive) were used to evaluate the odds ratios (ORs), 95% confidence intervals (CIs), and p values. For multiple testing, p values (p(c)) were re-evaluated by Bonferroni's correction.

RESULTS

The genotype frequency of DRD2 rs4936270 SNP was associated with the development of schizophrenia (p=0.0007, OR=1.71, 95% CI=1.16-2.52 in the codominant1 model; p=0.011, OR=1.63, 95% CI=1.12-2.37 in the dominant model; p=0.035, OR=1.41, 95% CI=1.03-1.95 in the log-additive model). The allele frequency of rs4936270 was also associated with the development of schizophrenia (p=0.024, OR=1.45, 95% CI=1.05-1.98). After Bonferroni's correction, the genotype distribution of rs4936270 was still related to the development of schizophrenia (p(c)=0.0028 in the codominant1 model; p(c)=0.044 in the dominant model). A linkage disequilibrium block consisted of rs4648317, rs7131056, and rs4936270. The CAT haplotype frequency was different between schizophrenia and controls (p=0.039).

CONCLUSION

These results suggest that DRD2 SNPs may be associated with the development of schizophrenia in Korean population.

Lack of association between dopamine receptor D4 variable numbers of tandem repeats gene polymorphism and smoking

Nicotine addiction, related to cigarette smoking, develops as a product of the complex interactions between social, environmental and genetic factors. Genes encoding the components of the dopaminergic system are thought to be associated with smoking. Literature data showed not only an association, but also a lack of association between variable number of tandem repeats (VNTR) polymorphism located in the third exon of dopamine D4 receptor (DRD4) gene and smoking. Repetitive sequence of DRD4 VNTR is 48 bp long and maximum 11 tandem copies were reported in humans. Presence of alleles with 6 and more repeats (i.e. long alleles) was associated with greater tendency to novelty seeking and addictive behaviors than the presence of 5 and less alleles (short alleles). The aim of this study was to determine the association between VNTR in DRD4 gene and present smoking status in ethnically homogenous Caucasian population from the Eastern European (Croatian) origin. Genotyping was done in 565 healthy subjects, 511 men and 54 women, respectively, who were subdivided into 176 smokers and 389 nonsmokers. Logistic regression analyses, adjusted for age and sex, revealed the lack of significant (p>0.05) effect of the 4/4, 4/7 and 7/7 genotypes, or carriers of the long and short allele, or all genotypes of the DRD4 VNTR on smoking status. The results of this study failed to confirm the hypothesis that long allele of the DRD4 VNTR is associated with smoking status in Caucasian subjects.

Bidirectional influences between parents and children in smoking behavior: a longitudinal full-family model

BACKGROUND

The present study investigated longitudinal associations and bidirectional influences between family members in smoking behavior using a longitudinal full-family design. Family systems provide a powerful social context in which modeling and imitation take place. In current literature, however, bidirectional associations between parents and children in smoking behavior are seldom considered.

METHODS

Participants were 426 families, including mothers, fathers, and 2 adolescent children. Associations were assessed over 5 waves with yearly intervals using a cross-lagged model in structural equation modeling.

RESULTS

Findings demonstrate that families resemble an interactive system affording smoking contagion across family members. Results suggest that associations between parents and children are bidirectional, that is, parental smoking behavior influences adolescent smoking behavior and adolescent smoking behavior influences parental smoking behavior. There is insufficient evidence to conclude that longitudinal associations between family members are generally bidirectional, as only unidirectional longitudinal associations were found between between siblings and partners.

CONCLUSIONS

The present study extends previous research on the intergenerational transmission of smoking behavior by demonstrating bidirectional influences between parents and children in smoking behavior. Moreover, the present study suggests that family members may be susceptible to adjust their smoking behavior across time in response to smoking behavior within the family.

Genetic factors modulating the response to stimulant drugs in humans

Individuals vary in their responses to stimulant drugs, and several lines of evidence suggest that the basis for this variation is at least partially genetic in origin. Association studies have examined the effects of polymorphisms in specific genes on acute and chronic responses to stimulant drugs. Several of these genetic polymorphisms are also associated with other psychiatric dimensions and disorders.This chapter examines the evidence for genetic associations between the genes that have been most carefully examined for their influence on the response to stimulant drugs.

Impact of genetic polymorphisms in ABCB1, CYP2B6, OPRM1, ANKK1 and DRD2 genes on methadone therapy in Han Chinese patients

AIM

The present study explored the integrative effect of genes encoding methadone pharmacokinetic and pharmacodynamic pathways on methadone maintenance doses in Han Chinese Patients.

MATERIALS & METHODS

Genomic DNA was extracted from 321 opioid-dependent patients and 202 healthy controls, and realtime-PCR and PCR-RFLP were conducted to determine the genotypes.

RESULTS

Pair-wise comparisons revealed that carriers of the variants ABCB1 3435C>T or CYP2B6 516G>T alleles were more likely to require a higher methadone dose than noncarriers (both p < 0.0001). On the other hand, carriers of the variant DRD2 -214A>G or 939C>T allele had a twofold chance of requiring a lower methadone dose than noncarriers (p = 0.001). Proportional odds regression with adjustment of cofactors demonstrated that ABCB1, CYP2B6, OPRM1, ANKK1 and DRD2 genetic variants were jointly correlated with optimal methadone dose (adjusted r(2) = 53%).

CONCLUSIONS

These findings provide new insight to the fact that the interindividual variability of methadone dosage requirement is polygenetic and cannot be explained by a single-gene effect.

Cigarette experimentation in Mexican origin youth: psychosocial and genetic determinants

BACKGROUND

Established psychosocial risk factors increase the risk for experimentation among Mexican origin youth. Now, we comprehensively investigate the added contribution of select polymorphisms in candidate genetic pathways associated with sensation seeking, risk taking, and smoking phenotypes to predict experimentation.

METHODS

Participants (N = 1,118 Mexican origin youth) recruited from a large population-based cohort study in Houston, TX, provided prospective data on cigarette experimentation over 3 years. Psychosocial data were elicited twice-baseline and final follow-up. Participants were genotyped for 672 functional and tagging variants in the dopamine, serotonin, and opioid pathways.

RESULTS

After adjusting for gender and age, with a Bayesian False Discovery Probability set at 0.8 and prior probability of 0.05, six gene variants were significantly associated with risk of experimentation. After controlling for established risk factors, multivariable analyses revealed that participants with six or more risk alleles were 2.25 [95% confidence interval (CI), 1.62-3.13] times more likely to have experimented since baseline than participants with five or fewer. Among committed never-smokers (N = 872), three genes (OPRM1, SNAP25, HTR1B) were associated with experimentation as were all psychosocial factors. Among susceptible youth (N = 246), older age at baseline, living with a smoker, and three different genes (HTR2A, DRD2, SLC6A3) predicted experimentation.

CONCLUSIONS

Our findings, which have implications for development of culturally specific interventions, need to be validated in other ethnic groups.

IMPACT

These results suggest that variations in select genes interact with a cognitive predisposition toward smoking. In susceptible adolescents, the impact of the genetic variants appears to be larger than committed never-smokers.

Variant within the promoter region of the CHRNA3 gene associated with FTN dependence is not related to self-reported willingness to quit smoking

INTRODUCTION

Common variation in the CHRNA5-CHRNA3-CHRNB4 gene region is robustly associated with smoking quantity. Conversely, the association between one of the most significant single nucleotide polymorphisms (SNPs; rs1051730 within the CHRNA3 gene) with perceived difficulty or willingness to quit smoking among current smokers is unknown.

METHODS

Cross-sectional study including current smokers, 502 women, and 552 men. Heaviness of smoking index (HSI), difficulty, attempting, and intention to quit smoking were assessed by questionnaire.

RESULTS

The rs1051730 SNP was associated with increased HSI (age, gender, and education-adjusted mean ± SE: 2.6 ± 0.1, 2.2 ± 0.1, and 2.0 ± 0.1 for AA, AG, and GG genotypes, respectively, p < .01). Multivariate logistic regression adjusting for gender, age, education, leisure-time physical activity, and personal history of cardiovascular or lung disease showed rs1051730 to be associated with higher smoking dependence (odds ratio [OR] and 95% CI for each additional A-allele: 1.38 [1.11-1.72] for smoking more than 20 cigarette equivalents/day; 1.31 [1.00-1.71] for an HSI ≥5 and 1.32 [1.05-1.65] for smoking 5 min after waking up) and borderline associated with difficulty to quit (OR = 1.29 [0.98-1.70]), but this relationship was no longer significant after adjusting for nicotine dependence. Also, no relationship was found with willingness (OR = 1.03 [0.85-1.26]), attempt (OR = 1.00 [0.83-1.20]), or preparation (OR = 0.95 [0.38-2.38]) to quit. Similar findings were obtained for other SNPs, but their effect on nicotine dependence was no longer significant after adjusting for rs1051730.

CONCLUSIONS

These data confirm the effect of rs1051730 on nicotine dependence but failed to find any relationship with difficulty, willingness, and motivation to quit.

Psychopathological aspects of dopaminergic gene polymorphisms in adolescence and young adulthood

Dopamine hypotheses of several psychiatric disorders are based upon the clinical benefits of drugs affecting dopamine transporter or receptors, and have prompted intensive candidate gene research within the dopaminergic system during the last two decades. The aim of this review is to survey the most important findings concerning dopaminergic gene polymorphisms in attention deficit hyperactivity disorder (ADHD), Tourette syndrome (TS), obsessive compulsive disorder, and substance abuse. Also, genetic findings of related phenotypes, such as inattention, impulsivity, aggressive behavior, and novelty seeking personality trait are presented, because recent studies have applied quantitative trait measures using questionnaires, symptom scales, or other objective endophenotypes. Unfortunately, genetic variants with minor effects are problematic to detect in these complex inheritance disorders, often leading to contradictory results. The most consistent association findings relate to ADHD and the dopamine transporter and the dopamine D4 receptor genes. Meta-analyses also support the association between substance abuse and the D2 receptor gene. The dopamine catabolizing enzyme genes, such as monoamine oxidase (MAO) A and catechol-O-methyltransferase (COMT) genes, have been linked to aggressive behaviors.

Effect of model choice in genetic association studies: DRD4 exon III VNTR and cigarette use in young adults

A major concern with the vast literature associating the highly polymorphic 48 bp VNTR in exon III of the human dopamine receptor D4 gene (DRD4) with various behavioral phenotypes is the lack of concordance between studies. Part of the problem arises from the absence of a universally accepted scheme for pooling the large number of low frequency genotypes into appropriate categories. Here, we investigated the effect of different pooling strategies and genetic models on the reported association between DRD4-exIII-VNTR polymorphism and cigarette smoking. Genotyping was performed on a large randomly selected community-based sample of 2,274 individuals aged 20-24 years. Participants were grouped into sub-samples based on their genotypes to test specific genetic models. Multiple regression analyses were used to assess the relationship between DRD4-exIII-VNTR genotype and cigarette smoking measures while controlling for confounders. While smoking status and age at start of smoking were not associated with the genotype, a significantly (P = 0.006) higher rate of cigarette consumption was observed among carriers of the 7-repeat (7r) allele. Thus, 7r carriers were not more likely to be smokers but if they did smoke they consumed significantly more cigarettes per day than 4r carriers. Unlike previous studies this association was observed only when comparing carriers of the 7r with the 4r but not the other repeat alleles. Our study demonstrates the need for caution when grouping functionally different DRD4-exIII-VNTR alleles in association studies. It particularly highlights the requirement for better functional characterization of the DRD4-exIII-VNTR alleles for interpreting results from association studies.

TTC12-ANKK1-DRD2 and CHRNA5-CHRNA3-CHRNB4 influence different pathways leading to smoking behavior from adolescence to mid-adulthood

BACKGROUND

CHRNA5-CHRNA3-CHRNB4 and TTC12-ANKK1-DRD2 gene-clusters influence smoking behavior. Our aim was to test developmental changes in their effects as well as the interplays between them and with nongenetic factors.

METHODS

Participants included 4762 subjects from a general population-based, prospective Northern Finland 1966 Birth Cohort (NFBC 1966). Smoking behavior was collected at age 14 and 31 years. Information on maternal smoking, socioeconomic status, and novelty seeking were also collected. Structural equation modeling was used to construct an integrative etiologic model including genetic and nongenetic factors.

RESULTS

Several single nucleotide polymorphisms in both gene-clusters were significantly associated with smoking. The most significant were in CHRNA3 (rs1051730, p = 1.1 × 10(-5)) and in TTC12 (rs10502172, p = 9.1 × 10(-6)). CHRNA3-rs1051730[A] was more common among heavy/regular smokers than nonsmokers with similar effect-sizes at age 14 years (odds ratio [95% CI]: 1.27 [1.06-1.52]) and 31 years (1.28 [1.13-1.44]). TTC12-rs10502172[G] was more common among smokers than nonsmokers with stronger association at 14 years (1.33 [1.11-1.60]) than 31 years (1.14 [1.02-1.28]). In adolescence, carriers of three-four risk alleles at either CHRNA3-rs1051730 or TTC12-rs10502172 had almost threefold odds of smoking regularly than subjects with no risk alleles. TTC12-rs10502172 effect on smoking in adulthood was mediated by its effect on smoking in adolescence and via novelty seeking. Effect of CHRNA3-rs1051730 on smoking in adulthood was direct.

CONCLUSIONS

TTC12-ANKK1-DRD2s seemed to influence smoking behavior mainly in adolescence, and its effect is partially mediated by personality characteristics promoting drug-seeking behavior. In contrast, CHRNA5-CHRNA3-CHRNB4 is involved in the transition toward heavy smoking in mid-adulthood and in smoking persistence. Factors related to familial and social disadvantages were strong independent predictors of smoking.

Addiction as a systems failure: focus on adolescence and smoking

OBJECTIVE

Scientific advances in the field of addiction have forever debunked the notion that addiction reflects a character flaw under voluntary control, demonstrating instead that it is a bona fide disease of the brain. The aim of this review is to go beyond this consensus understanding and explore the most current evidence regarding the vast number of genetic, developmental, and environmental factors whose complex interactions modulate addiction risk and trajectory.

METHOD

Focusing on childhood and adolescent smoking as a paradigm, we review the important risk factors for the development of addictions, starting at the level of genetics and closing with a focus on sociocultural and policy factors.

RESULTS

A critical review of the pertinent literature provides a detailed view of the cumulative power of risk and protection factors across different phenomenological levels to modulate the risk of undesirable outcomes, particularly for young people. The result represents a compelling argument for the need to engage in comprehensive, multilevel approaches to promoting health.

CONCLUSIONS

Today, the field of medicine understands more about disease than about health; however it need not be that way. The view of drug addiction as a systems failure should help refocus our general approach to developing dynamic models and early comprehensive interventions that optimize the ways in which we prevent and treat a complex, developmental disorder such as drug addiction.

Neural markers of genetic vulnerability to drug addiction

This chapter will summarize genetics findings derived from various strategies and highlight important neural markers (or correlates) in some specific and extensively studied genes. Most studies highlighted here focus on alcohol and nicotine dependence (AD and ND, respectively). AD and ND are among the most prevalent addictive disorders worldwide, are among the best studied, and are also associated globally with the largest socioeconomic impact.We describe different mechanisms through which genes can have an impact on the addictive behaviors, distinguishing between the genes that inscribe the proteins affecting the metabolism of the addictive substance (e.g., ADH/ALDH for alcohol or CYP2A6 for nicotine) and genes that code for the brain transmitter systems, such as genes involved in cerebral neurotransmission thought to be involved in addiction (e.g., brain reward system, mood regulation, opioid system). Strategies include linkage analyses, association studies, whole genome association studies as well as intermediate/endophenotype studies. Moreover, some important findings derived from animal studies and from neuroimaging studies are highlighted. In conclusion, we provide the reader with an overview of most important studies related to AD and ND and give an outlook how these findings may become useful and beneficial in the future.

A genetic instrumental variables analysis of the effects of prenatal smoking on birth weight: evidence from two samples

There is a large literature showing the detrimental effects of prenatal smoking on birth and childhood health outcomes. It is somewhat unclear though, whether these effects are causal or reflect other characteristics and choices by mothers who choose to smoke that may also affect child health outcomes or biased reporting of smoking. In this paper we use genetic markers that predict smoking behaviors as instruments to address the endogeneity of smoking choices in the production of birth and childhood health outcomes. Our results indicate that prenatal smoking produces more dramatic declines in birth weight than estimates that ignore the endogeneity of prenatal smoking, which is consistent with previous studies with non-genetic instruments. We use data from two distinct samples from Norway and the United States with different measured instruments and find nearly identical results. The study provides a novel application that can be extended to study several behavioral impacts on health and social and economic outcomes.

Dopamine D4 receptor gene variation moderates the efficacy of bupropion for smoking cessation

Smokers (≥10 cig/day; N =331) of European ancestry taking part in a double-blind placebo-controlled randomized trial of 12 weeks of treatment with bupropion plus counseling for smoking cessation were genotyped for a VNTR polymorphism in Exon-III of the Dopamine D4 receptor (DRD4) gene. Generalized estimating equations predicting point-prevalence abstinence at end of treatment and 2, 6, and 12-months post-end of treatment indicated that bupropion (vs. placebo) predicted increased odds of abstinence. The main effect of Genotype was not significant. A Genotype × Treatment interaction (p=.005) showed that bupropion predicted increased odds of abstinence in long-allele carriers (OR=1.31, p<.0001), whereas bupropion was not associated with abstinence among short-allele homozygotes (OR=1.06, p=.23). The Genotype × Treatment interaction remained when controlling for demographic and clinical covariates (p=.01) and in analyses predicting continuous abstinence (ps≤.054). Bupropion may be more efficacious for smokers who carry the long-allele, which is relevant to personalized pharmacogenetic treatment approaches.

Association study of a functional catechol-O-methyltransferase polymorphism and smoking in healthy Caucasian subjects

Tobacco smoking is a global health problem. The association of a functional common polymorphism in the catechol-o-methyltransferase gene (COMT Val158Met) with smoking behavior has been extensively studied, but with divergent findings. In the present study the frequency of COMT genotypes and alleles was evaluated in 578 male and a smaller group of 79 female unrelated, medication-free Caucasian healthy subjects of Croatian origin. Smokers were classified as subjects smoking <or=10 cigarettes per day, while subjects who never smoked in their life were regarded as nonsmokers. A chi(2)-test with standardized residuals and Bonferroni correction revealed significant (P=0.017) differences in Met/Met, Met/Val or Val/Val genotype frequency between male smokers and nonsmokers. This significant association between COMT Val158Met polymorphism and smoking was not detected in female subjects, due to the small number of women, which represents a limitation of the study. Our results confirmed the significant association between COMT variants and smoking, which was due to the higher frequency of Val/Val homozygotes in male smokers compared to male nonsmokers. These results suggest that carriers of the high activity COMT variant are more prone to develop a higher level of nicotine dependence, or that they release more dopamine than carriers of Met/Met or Met/Val genotypes.