Polymorphisms in genes involved in neurotransmission in relation to smoking

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.

Nicotinic acetylcholine gene cluster CHRNA5-A3-B4 variants influence smoking status in a Bangladeshi population

BACKGROUND

Past studies have established the association of CHRNA5-A3-B4 gene cluster variants with various smoking behaviors in different ethnicities, yet no such study has been reported in Bengali ethnicity to date.

METHODS

A case-control study with 129 smokers and 111 non-smokers was conducted and genotyped using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method aimed to manifest the association of three SNPs in this gene cluster with smoking status (SS) in a Bangladeshi population.

RESULTS

The non-synonymous CHRNA5 rs1s6969968 and 3'-UTR variant CHRNA3 rs578776 polymorphisms were found to have a strong association with SS. Carriers of polymorphic 'A' allele of rs16969968 showed 1.51-fold more risk of being smokers (adjusted OR = 1.51, 95% CI 0.88-2.57, p = 0.128); whereas, rs578776 polymorphic 'A' allele carriers showed 0.595-fold less risk of being smokers (adjusted OR = 1.51, 95% CI 0.88-2.57, p = 0.006). Comparing smokers and non-smokers, A/A mutant homozygous genotypes of rs578776 and rs16969968 variants pose 0.369-fold (95% CI 0.177-0.77, p = 0.008) and 3.3-fold (95% CI 0.66-16.46, p = 0.14) more risk for positive SS, respectively. No genotypic association for SS was found with intronic variant CHRNB4 rs11072768 (T/G; adjusted OR = 0.827, 95% CI 0.457-1.499, p = 0.532 and G/G; adjusted OR = 0.992, 95% CI 0.455-2.167, p = 0.985). Combination of rs16969968-positive/rs578776-negative polymorphic variants possesses the risk of positive SS in young adults. Furthermore, two new haplotypes (AAT and AAG) were identified in Bangladeshi population and GAG (OR = 0.45, 95% CI 0.25-0.8, p = 0.006) haplotype was found to be a protective factor for SS.

CONCLUSION

Nicotinic acetylcholine gene cluster CHRNA5-A3-B4 variants rs16969968 and rs578776 are associated with SS in a Bangladeshi population. Large-scale studies are warranted to establish this genotype-phenotype correlation.

Genetic polymorphisms in CYP2A6 are associated with a risk of cigarette smoking and predispose to smoking at younger ages

Nicotine is the main component of cigarettes that causes addiction, which is considered a complex disease, and genetic factors have been proposed to be involved in the development of addiction. The CYP2A6 gene encodes the main enzyme responsible for nicotine metabolism. Depending on the study population, different genetic variants of CYP2A6 associated with cigarette smoking have been described. Therefore, we evaluated the possible association between SNPs in CYP2A6 with cigarette smoking and nicotine addiction-related variables in Mexican mestizo smokers. We performed a genetic association study comparing light smokers (LS, n=349), heavy smokers (HS, n=351) and never-smokers (NS, n=394). SNPs rs1137115, rs4105144, rs1801272 and rs28399433 were genotyped in the CYP2A6 gene. We found that the A allele of rs1137115 (OR=1.41) in exon 1 of CYP2A6 and the T allele of rs4105144 (OR=1.32) in the 5' UTR of the gene are associated with the risk of cigarette smoking (p<0.05); rs1137115 affects the level of alternative splicing, resulting in a CYP2A6 isoform with low enzymatic activity, whereas rs4105144 is likely to be in a binding site for the transcription factor for glucocorticoids receptor (GR) and regulates the expression of CYP2A6. In addition, having a greater number of risk alleles (rs1137115 (A), rs4105144 (T) and rs28399433 (G)) is associated with a younger age at onset. The present study shows that in Mexican mestizos, the analyzed SNPs confer greater risk in terms of consumption and age of onset.

Distribution of polymorphic variants of CYP2A6 and their involvement in nicotine addiction

Tobacco consumption has become a major public health issue, which has motivated studies to identify and understand the biological processes involved in the smoking behavior for prevention and smoking cessation treatments. CYP2A6 has been identified as the main gene that codifies the enzyme that metabolizes nicotine. Many alleles have been identified after the discovery of CYP2A6, suggesting a wide interethnic variability and a diverse smoking behavior of the allele carrying individuals. The main purpose of this review is to update and highlight the effects of the CYP2A6 gene variability related to tobacco consumption reported from diverse human populations. The review further aims to consider CYP2A6 in future studies as a possible genetic marker for the prevention and treatment of nicotine addiction. Therefore, we analyzed several population studies and their importance at addressing and characterizing a population using specific parameters. Our efforts may contribute to a personalized system for detecting, preventing and treating populations at a higher risk of smoking to avoid diseases related to tobacco consumption.

Nine generations of selection for high and low nicotine intake in outbred Sprague-Dawley rats

Previous animal studies have revealed significant involvement of genetics in nicotine intake; however, the extent of the genetic contribution to this behavior has not been well addressed. We report the first study of nine generations of selection for high and low voluntary nicotine intake in outbred Sprague-Dawley rats. Bidirectional mass selection resulted in progressively greater nicotine consumption in the high nicotine-preferring line but no decrease in nicotine intake in the low nicotine-preferring line across generations. Our estimated realized heritability for high voluntary nicotine intake is 0.26 vs close to zero for low voluntary nicotine intake. In contrast, we found no differences between the lines across generations for saccharine intake. These selected lines may provide useful animal models for identifying susceptibility and resistance genes and variants for controlling voluntary nicotine intake in rodents, although we recognize that more generations of selection of these two lines and independent replication of our selection for high and low nicotine-preferring lines are needed.

Exploring alternate processes contributing to the association between maternal smoking and the smoking behavior among young adult offspring

INTRODUCTION

Maternal smoking during pregnancy (MSP) is a known risk factor for regular smoking in young adulthood and may pose a risk independently of mother's lifetime smoking. The processes through which MSP exerts this influence are unknown but may occur through greater smoking quantity and frequency following initiation early in adolescence or increased sensitivity to nicotine dependence (ND) at low levels of smoking.

METHODS

This study used path analysis to investigate adolescent smoking quantity, smoking frequency, and ND as potential simultaneous mediating pathways through which MSP and mother's lifetime smoking (whether she has ever smoked) increase the risk of smoking in young adulthood among experimenters (at baseline, <100 cigarettes/lifetime) and current smokers (>100 cigarettes/lifetime).

RESULTS

For experimenters, MSP was directly associated with more frequent young adult smoking and was not mediated by adolescent smoking behavior or ND. Independently of MSP, the effect of mother's lifetime smoking was fully mediated through frequent smoking and was heightened ND during adolescence. Controlling for MSP eliminated a previously observed direct association between mother's lifetime smoking and future smoking among experimenters. For current smokers, only prior smoking behavior was associated with future smoking frequency.

CONCLUSIONS

These results seem to rule out sensitivity to ND and increased smoking behavior as contributing pathways of MSP. Further, the impact of MSP on young adult smoking extends beyond that of having an ever-smoking mother. Future work should test other possible mediators; for example, MSP-related epigenetic changes or gene variants influencing the brain's nicotine response.

Risk factors for adolescent smoking: parental smoking and the mediating role of nicotine dependence

BACKGROUND

Parental smoking and early-emerging nicotine dependence symptoms are well-documented risk factors for adolescent smoking. However, very little is known about the mediating pathways through which these risk factors may act, or whether parental smoking may cause or signal early-emerging nicotine dependence symptoms.

METHODS

Data were drawn from the longitudinal Social and Emotional Contexts of Adolescent Smoking Patterns Study. Adolescents who had smoked under 100 cigarettes in their lifetime (n=594; low-exposure group) and adolescents who had smoked over 100 cigarettes, but fewer than 5 cigarettes per day (n=152) were included in the analyses. Path analysis was performed on longitudinal data to investigate the association between parental smoking and smoking frequency at the 48 months follow-up, both directly and through mediating variables of smoking frequency, smoking quantity, and nicotine dependence.

RESULTS

Father's smoking was associated with higher adolescent nicotine dependence scores at the baseline assessment wave. Structural equation modeling revealed that mother's smoking at baseline was associated with adolescent's smoking frequency at the 48-month follow-up, and its effect was partially mediated by both smoking frequency and nicotine dependence among low-exposure adolescent smokers.

CONCLUSIONS

Parental smoking is a risk factor for future smoking in low-exposure adolescent smokers, above and beyond the risks posed by smoking behavior and nicotine dependence. Moreover, parental smoking is associated with early-onset nicotine dependence in low-exposure adolescent smokers. As an easily measureable risk factor, parent smoking status can be used to identify and intervene with novice adolescent smokers who are at high risk for chronic smoking behavior.

Association of the 5-HTT gene-linked promoter region (5-HTTLPR) polymorphism with psychiatric disorders: review of psychopathology and pharmacotherapy

Serotonin (5-HT) regulates important biological and psychological processes including mood, and may be associated with the development of several psychiatric disorders. An association between psychopathology and genes that regulate 5-HT neurotransmission is a robust area of research. Identification of the genes responsible for the predisposition, development, and pharmacological response of various psychiatric disorders is crucial to the advancement of our understanding of their underlying neurobiology. This review highlights research investigating 5-HT transporter (5-HTTLPR) polymorphism, because studies investigating the impact of the 5-HTTLPR polymorphism have demonstrated significant associations with many psychiatric disorders. Decreased transcriptional activity of the S allele ("risk allele") may be associated with a heightened amygdala response leading to anxiety-related personality traits, major depressive disorder, suicide attempts, and bipolar disorder. By contrast, increased transcriptional activity of the L allele is considered protective for depression but is also associated with completed suicide, nicotine dependence, and attention deficit hyperactivity disorder. For some disorders, such as post-traumatic stress disorder and major depressive disorder, the research suggests that treatment response may vary by allele (such as an enhanced response to serotonin specific reuptake inhibitors in patients with major depressive disorder and post-traumatic stress disorder with L alleles), and for alcohol dependence, the association and treatment for S or L alleles may vary with alcoholic subtype. While some studies suggest that 5-HTTLPR polymorphism can moderate the response to pharmacotherapy, the association between 5-HTTLPR alleles and therapeutic outcomes is inconsistent. The discovery of triallelic 5-HTTLPR alleles (L(A)/L(G)/S) may help to explain some of the conflicting results of many past association studies, while concurrently providing more meaningful data in the future. Studies assessing 5-HTTLPR as the solitary genetic factor contributing to the etiology of psychiatric disorders continue to face the challenges of statistically small effect sizes and limited replication.

Pharmacogenetics of smoking cessation therapy

Nicotine dependence is a major health problem, with a large amount of smoking-related premature deaths and disabilities. The dependence mechanism of nicotine is especially complex and is under strong genetic influence. Smoking cessation is associated with substantial health benefits. Evidence from animal and human studies suggests that genetic polymorphisms influencing pharmacokinetics and pharmacodynamics of nicotine may have great potential for aiding smoking treatment. There are more than 30 association studies and one genome-wide association study (GWAS) between genetic polymorphisms and smoking cessation following nicotine replacement therapy (NRT) and/or bupropion therapy. However, only a few candidate genes or regions were analyzed more than twice and even these genes require additional investigations in different therapeutic schemes. There are a growing number of new pharmacologic options that have not been pharmacogenetically assessed according to published literature. In addition, molecular genetics studies are needed to assess the functional mechanisms of some putative association results. Taken together, the preliminary findings are promising but raise the need for new studies with adequate sample sizes and adjustment for several potential confounding factors frequently neglected, such as comorbidity and sociodemographic factors. The current state of the art in the field encourages an optimist view that personalized treatment approaches may become possible. However, the current scientific evidence still does not support the use of pharmacogenetic tests in routine smoking cessation therapy.

Constitutional mechanisms of vulnerability and resilience to nicotine dependence

The core nature of nicotine dependence is evident in wide variations in how individuals become and remain smokers. Individuals with pre-existing behavioral traits are more likely to develop nicotine dependence and experience difficulty when attempting to quit. Many molecular factors likely contribute to individual variations in the development of nicotine dependence and behavioral traits in complex manners. However, the identification of such molecules has been hampered by the phenotypic complexity of nicotine dependence and the complex ways molecules affect elements of nicotine dependence. We hypothesize that nicotine dependence is, in part, a result of interactions between nicotine and pre-existing behavioral traits. This perspective suggests that the identification of the molecular bases of such pre-existing behavioral traits will contribute to the development of effective methods for reducing smoking dependence and for helping smokers to quit.

The impact of genetic variation in DRD2 and SLC6A3 on smoking cessation in a cohort of participants 1 year after enrollment in a lung cancer screening study

Smoking cessation strategies continue to have disappointing results. By determining the interindividual genetic differences that influence smoking behaviors, we may be able to develop tailored strategies that increase the likelihood of successful cessation. This study attempts to determine genetic influences on the relationship between the dopamine pathway and smoking cessation by examining associations with a variable number tandem repeat variation in SLC6A3 and the DRD2 variants TaqIA (A2 vs. A1), TaqIB (B2 vs. B1), C957T (C vs. T), and -141C Ins/Del (C vs. Del). Baseline smokers in the Pittsburgh Lung Screening Study who provided information on smoking status 1 year later were evaluated. We frequency-matched those who were not abstinent at 1 year to those who were abstinent at 1 year by gender, decade of age, and time of enrollment (3-month intervals) in a 3:1 ratio (N = 881). Logistic regression was used to identify the effect of genotype on abstinence at 1 year. In a model containing the matching variables and other genotypes, DRD2 TaqIA was significantly associated with being abstinent at 1 year (P = 0.01). Compared to participants who were homozygous for the TaqIA major allele (A2A2), participants who carried at least one minor allele (A1) were less likely to quit (Odds Ratio: 0.47, 95% CI: 0.24-0.94). The other dopamine receptor genotypes and the SLC6A3 genotype were not associated with smoking status at 1 year. The association between DRD2 TaqIA and smoking cessation supports the hypothesis that genetic variation in the dopamine pathway influences smoking cessation.

Smoking as a product of gene-environment interaction

A strong hereditary influence on smoking has been demonstrated. As one of the candidate genes in relation to smoking, the serotonin transporter gene (5-HTTLPR) has been suggested, however with conflicting results. In recent studies, it has been shown that genotypic and environmental (G*E) factors interact in the shaping of a variety of phenotypic expressions. The objective of the present study was to investigate the interaction between a variation in the 5-HTTLPR and family environment in relation to smoking habits, nicotine dependence, and nicotine and cotinine levels in hair samples. A random Swedish adolescent population sample (n = 785), from which 200 individuals were stratified regarding behaviour, was genotyped for 5-HTTLPR and assessed with semi-structured interviews, a questionnaire, and hair analyses of nicotine and cotinine. The 5-HTTLPR gene interacted with a poor family environment to predict smoking habits, as well as nicotine and cotinine levels. The risk of being a smoker was increased 13 times for an individual with a combination of the 5-HTTLPR LS genotype and a poor family environment in comparison with the Homozygous Long-Long (LL) genotype and a good family environment.

Addiction science and its genetics

AIM

To assess the progress and impact of genetic studies in the addictions arena and to present this information in a form accessible to the general readership of Addiction.

METHODS

Review of the evidence that genes are involved in addiction, approaches to their identification, current findings and the potential implications.

RESULTS

Family, twin and adoption studies provide strong evidence that addiction runs in families and that this is determined in part by genetic factors. Two main molecular genetic approaches, namely linkage and association, have been adopted to identify the specific genes involved. Both methods are fraught with problems. Linkage is limited by issues of sensitivity, and association by false positives. Perhaps the strongest finding in psychiatric genetics to date is the impressive effect that a single genetic variant, in the aldehyde dehydrogenase 2 gene, has on drinking behaviour and reducing the risk of developing alcohol dependence. Other findings are currently less robust; however, the implications of elucidating the genetic underpinning of addiction will be profound.

CONCLUSIONS

Addiction genetics is a developing science that has yet to prove its worth in the clinical setting.

Reduced CYP2D6 activity is a negative risk factor for methamphetamine dependence

Because methamphetamine (METH) is metabolized by CYP2D6 at the first step of hydroxylation and demethylation, it is possible that functional variants of CYP2D6 alter susceptibility to methamphetamine-induced dependence. We genotyped CYP2D6*1, *4, *5, *10, and *14 for 202 patients with METH dependence and 337 controls in a Japanese population and found a significant association of the CYP2D6 gene with METH dependence (p=0.0299). The patients had fewer *10 and *14 alleles, which are hypofunction alleles, than the controls. CYP2D6 genotypes were divided into three phenotypes: extensive metabolizers, intermediate metabolizers, and poor metabolizers. There was no poor metabolizer among our Japanese subjects, and intermediate metabolizers of CYP2D6 were significantly fewer in methamphetamine-dependent subjects than in controls (p=0.0212), with an odds ratio of 0.62 (95% confidence interval: 0.51-0.76). The present study demonstrated that reduced CYP2D6 activity was a negative risk factor for methamphetamine dependence.

Pleiotropic impact of constitutive fosB inactivation on nicotine-induced behavioral alterations and stress-related traits in mice

Multiple genes are thought to influence both susceptibility to nicotine dependence and its comorbid behavioral traits in humans. However, which specific genes contribute to this pleiotropic effect is poorly understood. Previous rodent studies have shown that many addictive substances and stressful stimuli increase the expression of the transcription factor FosB in limbic and associated regions and that the protein products of fosB contribute to certain behavioral effects of cocaine and morphine. However, the role of this gene in nicotine-regulated behaviors and dependence-related behavioral traits is unknown. We tested the hypothesis that a constitutive level of FosB affects nicotine-regulated behaviors and comorbid behavioral traits using constitutive fosB knockout (KO) mice. Following repeated or prolonged nicotine administration, but not a single acute administration, KO mice were impaired in conditioned place preference, oral nicotine intake and motor suppression. In wild-type mice, repeated nicotine injections, but not a single acute injection, increased the expression of FosB and its truncated variant DeltaFosB in the targets but not at the origins of the mesolimbic and nigrostriatal dopamine pathways; no detectable level of FosB/DeltaFosB was found in KO mice. In tasks designed to assess behavioral traits, KO mice exhibited more pronounced behavioral abnormalities when stress levels were high than when they were minimized. Our results suggest that the constitutive absence of fosB has a pleiotropic influence on the behavioral effects of repeated or prolonged nicotine administration and on stress-related behavioral traits in mice.

The influence of genetics on nicotine dependence and the role of pharmacogenetics in treating the smoking habit

Despite the considerable efforts made in the fight against smoking in the last decades, there are still substantial numbers of people who, in full knowledge of the health hazards, begin smoking or continue smoking. Recent studies have focused on the genetic bases of the nicotine addiction. Various genetic polymorphisms have been associated with smoking. However, environmental factors have also been shown to play a role. In this review, we present some of the principal data collected in genetic studies of smoking behavior. The results obtained through this line of research will eventually aid clinicians in individualizing the type, dosage and duration of treatment for patients with nicotine dependence in accordance with the genotype of each smoker, thereby maximizing the efficacy of the proposed treatment regimen.

Why do young women smoke? IV. Role of genetic variation in the dopamine transporter and lifetime traumatic experience

Cigarette smoking is a complex behavior to which environmental, psychological, and genetic factors contribute. Applying a multifactorial model, we examined the role of genetic variation in the dopamine transporter (DAT1) in smoking initiation (SI) and nicotine dependence. The participants were female college students who had never smoked (n = 148) or had smoked daily for at least a year (n = 242). All participants provided extensive background information and completed a series of psychological instruments. Five SNPs were genotyped in the 3' and 5' regions of DAT1. Data were analyzed by logistic regression. The best fitting model for SI (P = 1.9 x 10(-17), Nagelkerke R2 = 0.33) revealed novelty seeking (OR = 1.14, P = 0.000004) and lifetime traumatic experience (OR = 2.3, P = 0.001) as risk factors and a DAT1_E15 + 274-DAT1_VNTR G-9 haplotype as protective (OR = 0.57, P = 0.03). In the model for nicotine dependence (P = 1.4 x 10(-8), Nagelkerke R2 = 0.27) novelty seeking was a risk factor (OR = 1.07, P = 0.03); the DAT1_E15+274-DAT1_VNTR G-9 haplotype (OR = 0.37, P = 0.001) and the interaction between trauma and a DAT1_E15 + 274-DAT1_VNTR C-9 haplotype (OR = 0.15, P = 0.01) were protective. Lifetime experience of trauma was associated with high nicotine dependence among non-carriers of the C-9 haplotype but not among carriers of this haplotype. These findings indicate that in the context of a multifactorial model, haplotypes in the 3' region of DAT1 influence the propensity of young women to initiate smoking as well as the severity of nicotine dependence once the habit is established. A haplotype in the 3' untranslated region of DAT1 modifies the effect of lifetime traumatic experience on the severity of nicotine dependence.

Overview of the pharmacogenomics of cigarette smoking

Cigarette smoking increases the risk of numerous health problems, including cancer, cardiovascular and pulmonary disorders, making smoking the leading cause of preventable death in the world. Nicotine is primarily responsible for the highly addictive properties of cigarettes. Although the majority of smokers express a desire to quit, few are successful in doing so. Twin and family studies have indicated substantial genetic contributions to smoking behaviors. One major research focus has been to elucidate the specific genes involved; this has been accomplished primarily through genome-wide linkage analyses and candidate gene association studies. Much attention has focused on genes involved in the neurotransmitter pathways for the brain reward system and genes altering nicotine metabolism. This paper reviews the current state of knowledge for genetic factors implicated in smoking behaviors, and examines how genetic variations may affect therapeutic outcomes for drugs used to assist smoking cessation.

The genetics of nicotine dependence: Relationship to pancreatic cancer

Smoking of tobacco products continues to be a major cause of worldwide health problems. Epidemiological studies have shown that tobacco smoking is the greatest risk factor for the development of pancreatic cancer. Smokers who are able to quit smoking can reduce their risk of pancreatic cancer by nearly 50% within two years, however, their risk of developing pancreatic cancer remains higher than that of non-smokers for 10 years. Nicotine is the major psychoactive substance in tobacco, and is responsible for tobacco dependence and addiction. Recent evidence suggests that individuals have genetically based differences in their ability to metabolize nicotine, as well as genetic differences in the psychological reward pathways that may influence individual response to smoking initiation, dependence, addiction and cessation. Numerous associations have been reported between smoking behavior and genetic polymorphisms in genes that are responsible for nicotine metabolism. In addition, polymorphisms in genes that encode neurotransmitters and transporters that function in psychological reward pathways have been implicated in differences in smoking behavior. However, there is a large degree of between-study variability that demonstrates the need for larger, well-controlled case-control studies to identify target genes and deduce mechanisms that account for the genetic basis of inter-individual differences in smoking behavior. Understanding the genetic factors that increase susceptibility to tobacco addiction may result in more effective tobacco cessation programs which will, in turn, reduce the incidence of tobacco related disease, including pancreatic cancer.

Monoamine oxidase A knockout mice exhibit impaired nicotine preference but normal responses to novel stimuli

Nicotine is thought to act on brain monoamine systems that normally mediate diverse motivational behaviors. How monoamine-related genes contribute to behavioral traits (e.g. responses to novel stimuli) comorbid with the susceptibility to nicotine addiction is still poorly understood. We examined the impact of constitutive monoamine oxidase A (MAOA) deficiency in mice on nicotine reward and responses to novel stimuli. Age-matched, male Maoa-knockout (KO) mice and wild-type (WT) littermates were tested for nicotine-induced conditioned place preference (CPP); voluntary oral nicotine preference/intake; spontaneous locomotor activity in a novel, inescapable open field; and novelty place preference. Nicotine preference in WT mice was reduced in Maoa-KO mice in the CPP and oral preference/intake tests. Control experiments showed that these phenotypes were not due to abnormalities in nicotine metabolism, fluid intake or response to taste. In contrast, Maoa-KO mice were normal in their behavioral response to a novel, inescapable open field and in their preference for a novel place. The observed phenotypes suggest that a constitutive deficiency of MAOA reduces the rewarding effects of nicotine without altering behavioral responses to novel stimuli in mice. Constitutive MAOA activity levels are likely to contribute to the vulnerability or resiliency to nicotine addiction by altering the rewarding effects of nicotine.

Why do young women smoke? II. Role of traumatic life experience, psychological characteristics and serotonergic genes

Cigarette smoking is a complex behavioral phenotype to which environmental, psychological and genetic factors contribute. The purpose of this study was to investigate these multifactorial effects with a specific focus on young women and on genes that encode serotonin (5-HT) receptors and the 5-HT transporter. A case-control sample of female Israeli college students provided comprehensive background data and details of cigarette smoking and completed a battery of psychological instruments. They were divided into smoking initiators (SI, n=242) or non-initiators (NI, n=148); SI were further subdivided into high (HND, n=127) and low nicotine-dependent smokers (LND, n=115) on the basis of their scores on the Fagerstrom Tolerance Questionnaire (FTQ). Single-nucleotide polymorphisms (SNPs) in five serotonin receptor genes (HTR1A, HTR1B, HTR2A, HTR2C and HTR6) and the 5-HT transporter-linked polymorphic region (5-HTTLPR) were genotyped. In a logistic regression model for SI (chi2=117.90, P=1.6 x 10(-19), Nagelkerke R2=0.42), novelty seeking (odds ratio (OR)=1.134, P=0.00009) was a significant risk factor. A five SNP CACCC haplotype in HTR6 was a strong protective factor against SI (OR=0.26; P=0.007). The interaction of HTR6-C276T genotype and lifetime traumatic experience contributed strongly to the risk of SI (OR=13.88, P=0.0001). Specifically, subjects homozygous for the HTR6-C276T C allele showed significantly increased risk of SI if they had experienced trauma. Although significant (chi2=42.85, P=1.00 x 10(-7)), the best-fitting model for ND was less predictive than the model for SI (Nagelkerke R2=0.24). HTR1B-G861C GG genotype (OR=2.29, P=0.01) was a significant risk factor for HND. Further studies should consider the interactive contribution of life events and relevant gene variants to cigarette smoking and other complex behavioral traits.

The serotonin transporter gene polymorphism is not associated with smoking behavior

Nicotine increases serotonin release in the brain and symptoms of nicotine withdrawal may be modulated by diminished serotonergic neurotransmission. The promoter region of the serotonin transporter gene, solute carrier family neurotransmitter transporter member 4 (SLC6A4), contains a functional tandem repeat polymorphism. The long (L) variant is more actively transcribed than the short (S) variant and is associated with a higher serotonin uptake. To investigate the potential role of this polymorphism for smoking behavior, SLC6A4 genotypes were determined in two different studies, the SMOKING GENES case-control study (470 current smokers and 419 subjects who had never smoked) and the cross-sectional Ludwigshafen risk and cardiovascular health (LURIC) study (777 current smokers and 1178 subjects who had never smoked). In the SMOKING GENES case-control study, SLC6A4 genotype frequencies were not statistically different between smokers (LL: 30.9%; LS: 46.8%; SS: 16.4%) and non-smokers (LL: 36.3%; LS: 41.8%; SS: 14.3%; P=0.13). Similar results were obtained in the cross-sectional LURIC study (smokers: LL, 36.5%, LS, 45.6%, SS, 17.9%; non-smokers: LL, 33.6%, LS, 48.9%, SS, 17.6%; P=0.33). SLC6A4 genotypes were furthermore not associated with Fagerstrom Tolerance Questionnaire score, packyears, number of cigarettes smoked per day or previous attempts to quit smoking. We conclude that the SLC6A4 promoter polymorphism is not a major determinant of smoking behavior in Caucasian.

Canine tyrosine hydroxylase (TH) gene and dopamine beta -hydroxylase (DBH) gene: their sequences, genetic polymorphisms, and diversities among five different dog breeds

Dopamine and noradrenaline are catecholamine neurotransmitters that are produced by biosynthetic enzymes such as tyrosine hydroxylase (TH) and dopamine beta -hydroxylase (DBH). As a first step to elucidate the genetic background of canine behavioral traits, we selected these genes as targets and sequenced these canine genes, and found that both were highly homologous with those of human beings. Then brain cDNAs derived from ten unrelated Beagles were used to search for polymorphisms in these genes. Four single nucleotide polymorphisms (SNPs) (C97T, G168A, G180A and C264T), one of which (C97T) will cause amino acid substitution in the TH gene, and two SNPs (C789A and A1819G), both of which will cause amino acid substitutions in the DBH gene were identified. The allelic frequencies among five dog breeds (47 Golden Retrievers, 41 Labrador Retrievers, 40 Malteses, 26 Miniature Schnauzers, and 39 Shibas) were examined and found to have significant variation between them with regards to all these SNPs, except for C97T in the TH gene and A1819G in the DBH gene. The polymorphisms of C97T and A1819G were found only in the Shiba. The present results suggest that the polymorphisms of the genes encoding catecholamine biosynthetic enzymes may become important markers for examining the genetic background of behavioral characteristics in dogs.

DARPP-32 phosphorylation opposes the behavioral effects of nicotine

BACKGROUND

The addictive properties of nicotine are mediated via dopaminergic pathways and their post-synaptic neurons in the striatum. Because post-synaptic neurons within the striatum contain high levels of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), we hypothesized that DARPP-32 may functionally contribute to the behavioral effects of nicotine.

METHODS

We examined the behavioral effects of nicotine and the phosphorylation state of DARPP-32 in wild-type (WT) and DARPP-32 knockout (KO) mice. In one experiment, we assessed voluntary nicotine intake (0-50 microg/ml) of WT and KO mice in a two-bottle choice paradigm. In a separate experiment, the motor-depressant effects of acute and repeated nicotine injections (0-.8 mg/kg, subcutaneously [SC]) were assessed. The phosphorylation of DARPP-32 at threonine34 and threonine75 were examined using Western blotting.

RESULTS

A heightened responsiveness to nicotine was seen in KO mice when compared with WT mice in oral intake and motor depression. The enhanced responsiveness in KO mice was not due to alterations in taste sensations, fluid intake, or blood nicotine or cotinine levels. Systemic injections of nicotine resulted in increased striatal DARPP-32 phosphorylation at threonine34 and threonine75.

CONCLUSIONS

DARPP-32 opposes the behavioral effects of nicotine possibly via concurrent phosphorylation at the two threonine sites.

Genetic Influences on Smoking

Smoking is a complex behavior influenced by environ-mental and genetic factors. A number of twin studies demonstrate genetic influences on multiple aspects of smoking including initiation, dependence, continuation, amount consumed, and cessation. It is likely that this genetic risk involves a complex interaction between multiple genes in different biologic pathways. These may include genes involved in the general aspect of dependence (eg, genes within the dopaminergic system) and those more specific to nicotine dependence (eg, genes encoding nicotinic acetylcholine receptors and nicotine-metabolizing enzymes). Genome-wide linkage analyses and gene association studies provide approaches for identifying these genes. This review provides a brief highlight of data from some of the candidate gene association studies and discusses some of the discrepancies found in the literature. In addition, the authors have reviewed some recent studies examining how genetic variations in drug-metabolizing enzymes and drug targets can influence the response to treatment with drugs such as bupropion and nicotine.These latter studies provide the potential for maximizing treatment efficacy by individualizing the type, dosage, and duration of the treatment based on an individual's genotype/phenotype. Significant advances have been made in understanding the genetic contribution to smoking behaviors. These and future studies will provide potential directions for novel prevention and treatment strategies.

Novel Strategies for the Early Detection and Prevention of Lung Cancer

Lung cancer is the leading cause of cancer death in the United States. Despite evidence of molecular abnormalities in biological specimens, progress in this disease is hampered by the lack of diagnostic markers useful for clinical practice. The majority of patients with lung cancer are still diagnosed at an advanced stage, when prognosis is poor. This article reviews new strategies being studied for the early detection of lung cancer. These strategies involve new methods of imaging (including low-dose computed tomography [CT] scanning), DNA analysis, and proteomic-based techniques. These strategies have not only improved our understanding of lung cancer but show promise in offering better survival to patients with this deadly disease. Of paramount importance in the search for methods of early detection is the need for the identification of the ideal population to screen, a multidisciplinary approach, and validation of promising techniques.

Association of the serotonin transporter promoter polymorphism with smoking behavior among adolescents

Serotonin transporter promoter polymorphism (5-HTTLPR) genotype was previously found associated with smoking behavior, difficulty in quitting smoking, and nicotine addiction; with non-replicated findings and contrasting results. Aim of the present study was to evaluate the possible association between 5-HTTLPR genotype and smoking behavior among adolescents, in relationship with psychological characteristics. Two hundred and ten Caucasian high school students (aged 14-19 years); 103 non-smokers, who have never smoked nicotine; and 107 tobacco smokers have been genotyped. Aggressiveness levels and temperamental traits were measured in both smokers and non-smokers, respectively, utilizing Buss-Durkee Hostility Inventory (BDHI) and Cloninger Three-Dimensional Personality Questionnaire (TPQ). Data about school performance have been also collected. The short-short (SS) genotype frequency was significantly higher among smokers compared with non-smokers (P = 0.023). The odds ratio for the SS genotype versus the long-long (LL) genotype frequency was 1.17 [95% CL (0.30-2.05)], when smokers were compared with non-smokers. The SS genotype frequency was significantly higher among heavy smokers with early onset, compared with moderate smokers with late onset (P = 0.042). BDHI irritability scores, NS scores at TPQ, and school failure frequency were significantly higher in smokers than in non-smokers. Multivariate model-fitting analysis evidenced a significantly greater relationship of genotype with irritability levels (BDHI scores) (0.34, P < 0.001) and temperament traits (NS scores) (0.36, P < 0.001), than with school performance (rate of school under-achievements) (0.18, P < 0.05) and nicotine smoking (number of cigarettes) (0.24, P < 0.01). Accordingly, factor-analysis showed that gene polymorphism contributes more directly to BDHI scores and NS scores (0.73; 0.71) than to smoking behavior and school under-achievement (0.54; 0.51). Our data suggest that a decreased expression of the gene encoding the 5-HTT transporter, due to "S" promoter polymorphism, may be associated with smoking behavior among adolescents and increased risk to develop nicotine dependence, possibly in relationship to personality traits, temperamental characteristics, and school under-achievements.

ADVANCES IN CANCER EPIDEMIOLOGY: Understanding Causal Mechanisms and the Evidence for Implementing Interventions

In a worldwide population of 6 billion, in the year 2000, approximately 10 million cancers were diagnosed, and there were an estimated 6.2 million cancer deaths. Whereas the universality of cancer incidence and mortality is established, the burden of cancer by type or organ site is distributed unequally between developing and industrialized nations. Populations in developing countries are disproportionately affected by cancers in which infectious agents are causal. Our review of advances in cancer epidemiology underscores the complexity of pathogenic mechanisms mediated by chronic inflammation, obesity, and gene-environment interactions as in tobacco and alcohol carcinogenesis. Ultimately, the implementation of effective cancer control interventions that will serve to alleviate the cancer burden must integrate basic and applied research in the behavioral, social, biomedical, and population sciences.

Genetic susceptibility to substance dependence

Despite what is often believed, the majority of those who experiment with substances with a dependence potential do not develop dependence. However, there is a subpopulation of users that easily becomes dependent on substances, and these individuals exhibit pre-existing comorbid traits, including novelty seeking and antisocial behavior. There appears to be a genetic basis for the susceptibility to dependence and these comorbid traits. Animal studies have identified specific genes that can alter susceptibility to dependence and response to novelty. The mechanisms underlying the genetic susceptibility to dependence and response to novelty are complex, but genetic susceptibility plays a significant role in the transition from substance use to dependence and from chronic use to addiction. We discuss two models to explain how genetic variations alter dependence susceptibility. Identification of the specific genes involved in these processes would help to identify individuals that are vulnerable to dependence/addiction and to devise novel treatment strategies.

Implications of CYP2A6 genetic variation for smoking behaviors and nicotine dependence

Nicotine is the primary addictive compound in tobacco smoke. In this review we summarize nicotine dependence and the genetics of smoking in brief before focusing on cytochrome P450 (CYP) 2A6. In humans nicotine is mainly inactivated to cotinine and CYP2A6 mediates approximately 90% of this conversion. Some, but not all, studies suggest that genetic variation in CYP2A6 may play a role in smoking. We review some of the recent findings on the influence of CYP2A6 genetic polymorphisms on nicotine kinetics, smoking behaviors, and how the gene appears to exert differential effects during various stages of smoking (eg, initiation, conversion to dependence, amount smoked during dependence, and quitting). These new findings will be put in the context of the discrepancies found in the literature. Implications of these recent findings on current and novel treatment approaches for smoking cessation and tobacco-related lung cancer will also be discussed.

Depression vulnerability, cigarette smoking, and the serotonin transporter gene

People with current or past depression are more likely to have been smokers at some point in their lives. Smokers with depression histories are also less likely to quit. Attempts to understand this relationship are important insofar as they can help treatment efficacy for this group of smokers. Prior research indicates that different genetic variations affect the relationship between smoking and neuroticism. This study examined whether people with a short serotonin transporter genotype would likewise show a stronger relationship between depression vulnerability and smoking behavior than those with the long genotype. Although depression vulnerability was associated with smoking behaviors, genotype did not significantly affect this relationship. Discussion centered on possible reasons for varying results across conceptually similar studies.

Bases genéticas del hábito tabáquico

Dependence on tobacco is one of the most important health problems in our society due to the direct relationship with lung cancer. Early studies in twins revealed that genetic factors modify the susceptibility to develop tobacco dependence. Different studies are underway to try to find an association between polymorphisms of genes involved either in nicotine metabolization or in neural transmission and the initiation and maintenance of the dependence on tobacco. Here we review the studies performed so far and discuss new perspectives for future studies.

CYP2A6 genetic polymorphisms and correlation with smoking status in Brazilians

We investigated polymorphisms of cytochrome P450 2A6 (CYP2A6) and its association with smoking habits in 412 healthy Brazilians, self-recognized as white (n=147), black (n=123) and intermediate (n=142), and classified as smokers (n=205, including 61 ex-smokers) and nonsmokers (n=207). The frequencies of the variant alleles CYP2A6(*)1B, CYP2A6(*)2, CYP2A6(*)4 and CYP2A6(*)9 in the overall study population were 29.9, 1.7, 0.5 and 5.7%, respectively. Significant differences in the CYP2A6 allelic distribution were observed across the three population subgroups. There was a statistically significant trend for decreasing frequency of CYP2A6(*)1B from white to intermediate and to black persons. An association between CYP2A6 genotype and smoking dependence was detected, which could not be explained by the expected phenotypic activity of CYP2A6. In white and intermediate persons, the odds ratio (OR) of being smokers vs nonsmokers was 0.07 (95% CI 0.02-0.20; P<0.001) and 0.27 (95% CI 0.12-0.61; P<0.001), respectively, for genotypes including allele CYP2A6(*)1B, as compared to wild-type homozygous. In contrast, the corresponding OR in black Brazilians was 1.34 (95% CI 0.57-3.17; P=0.46). These data suggest that the CYP2A6(*)1B is associated with smoking dependence in white and intermediate, but not black Brazilians.

MAO-B knockout mice exhibit deficient habituation of locomotor activity but normal nicotine intake

Low levels of monoamine oxidase-B (MAO-B) activity, such as those observed in smokers, are also associated with behavioral traits such as a heightened responsiveness to novelty. However, the exact mechanism by which low MAO-B activity influences smoking and heightened responsiveness to novelty is still poorly understood. We used MAO-B knockout (KO) mice to test the hypothesis that MAO-B concomitantly affects locomotor responses in a novel inescapable open field and nicotine intake. Male wild-type (WT) and MAO-B KO mice were placed in an inescapable open field and their horizontal locomotor activity was measured for 30 min per day for 5 days. MAO-B KO mice exhibited impaired within-session habituation of locomotor activity, as compared to WT mice. Separate groups of male WT and MAO-B KO mice were individually housed in their home cages with two water bottles. One of the bottles contained tap water and the other contained nicotine (0, 3.125, 6.25, 12.5, 25, 50 or 100 micro g/ml). The total amount of water and nicotine solution consumed was measured every three days for 16 days. MAO-B KO mice and WT mice consumed equal amounts of nicotine and exhibited comparable concentration-dependent nicotine preference and aversion over a period of 16 days. The data suggest that the absence of MAO-B impairs the ability of mice to habituate in the inescapable environment, but does not alter their nicotine intake.

Polymorphism of 5HT2A serotonin receptor gene is implicated in smoking addiction

Smoking behavior is influenced by genetic factors. Polymorphisms affecting the dopaminergic system have been linked to smoking habits. The aim of this study was to investigate if the T102C polymorphism of the 5-HT(2A) receptor gene is related to tobacco use, since this receptor modulates the mesolimbic dopamine system and the C allele is associated with reduced receptor gene expression. A sample of 625 subjects were genotyped and classified according to their smoking behavior (never, former, or current smokers). We found differences in the distribution of the genotypes when the current smokers were compared with the never + former smokers, suggesting that T102C polymorphism is associated with maintenance, but not with initiation of the smoking habit. The CC genotype was more frequent in the current smokers than in the never + former smokers (chi(2) = 6.825, P = 0.03). The odds ratio of being a current smoker with a CC genotype was 1.63, 95% CI 1.06-2.51.

Results of a genomewide linkage scan: support for chromosomes 9 and 11 loci increasing risk for cigarette smoking

Cigarette smoking is highly destructive to individuals and society, and is moderately heritable. We completed a genomewide linkage scan to map loci increasing risk for cigarette smoking in a set of families originally identified because they segregate panic disorder (PD). One hundred forty two genotyped individuals in a total of 12 families were studied (214 subjects analyzed, including non-genotyped individuals). Of these individuals, 69 were "affected" with habitual cigarette smoking (i.e., they smoked more than one pack per day for at least a year, or at least 1/2 pack per day for at least 10 years), 49 were "unaffected" (i.e., they smoked less than 1/2 pack per day for less than 1 year), and 24 were scored as "unknown." Nine families from the panic series were excluded from these analyses because they lacked multiple affected individuals with habitual cigarette smoking. In an initial genomewide scan, we genotyped a total of 416 markers (398 autosomal, 18 X-chromosome) with an average spacing of less than 10 cM, spanning the genome. Linkage analysis (pairwise, or single-point, and multi-point) was performed using ALLEGRO. An additional 14 markers were genotyped in a high-density panel to follow-up on an identified region of interest on chromosome 11p. The three highest multi-point Zlr scores (3.43, 3.04, and 3.01; P = 0.0003, P = 0.0012, and P = 0.0013, respectively), which each reflect "suggestive" evidence for linkage, were observed in multi-point linkage analyses using Allegro on chromosomes 11p and 9, near markers D11S4046, D9S283, and D9S1677, respectively. D11S4046 is in a region where linkage to alcohol dependence and linkage disequilibrium to substance dependence have previously been identified. The chromosome 9 region we identified as possibly linked to cigarette smoking in anxiety families, was previously identified as significantly linked to PD in Icelandic pedigrees. We also identified evidence supporting linkage (Zlr score > 2.3, P < 0.01) to regions of chromosomes 14, 16, and X. There was a significant phenotypic association between PD and cigarette smoking (P < 0.001).

CONCLUSIONS

We identified evidence for two loci increasing risk for cigarette smoking that map to chromosomes 9 and 11. There is now evidence supporting linkage or association of chromosome 11 markers with alcohol dependence, illegal drug abuse and dependence, and cigarette smoking. Interestingly, one of our most promising linkage regions, includes a region previously identified as linked to PD.

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.

Nicotine dependence in a prospective population-based study of adolescents

Dopamine is a key neurotransmitter of the mesolimbic reward pathway in the human brain, and tyrosine hydroxylase (TH) is the rate-limiting enzyme in dopamine biosynthesis. Consequently, the gene encoding TH is a strong candidate for involvement in the genetic component of addiction. The importance of this gene in nicotine dependence is supported by many studies showing a link between nicotine administration and TH expression. A functional tetranucleotide repeat polymorphism within intron 1 of the TH gene (HUMTH01-VNTR) has been shown to modify tobacco use in two independent Caucasian samples from the USA and Australia. Using information drawn from an eight-wave Australian population-based longitudinal study of adolescent health, we tested the effect of the HUMTH01-VNTR on nicotine dependence. Comparisons were made between dependent smokers and non-dependent smokers. These data provide further support for a protective association between the K4 allele and dependent smoking (odds ratio 0.54, 95% confidence interval 0.28-1.0). No associations were observed at any of three other common TH polymorphisms (rs6356, rs6357 and HUMTH01-PstI). Including these data, three independent studies, two of which use identical phenotypes, have now identified a protective relationship between the K4 allele of the functional HUMTH01-VNTR polymorphism and high-level smoking.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Lung cancer. 9: Molecular biology of lung cancer: clinical implications

It has been hypothesised that clinically evident lung cancers have accumulated many different genetic or epigenetic abnormalities in oncogenes and/or tumour suppressor genes. This notion has important clinical ramifications. Recent developments in our knowledge of the molecular biology of lung cancer are reviewed, with particular reference to genetic abnormalities in tumour suppressor gene inactivation and overactivity of growth promoting oncogenes. These changes lead to the "hallmarks of lung cancer". These hallmarks are the new rational targets for early detection, prevention, and treatment of lung cancer.

Emerging ethical issues in pharmacogenomics: from research to clinical practice

Pharmacogenomics holds much promise for the application of genetic information to the improvement of clinical care. Ethical issues for pharmacogenomics arise at the intersection of the spheres of drug development and genetic testing. Clinical drug trial designs which use subject selection based on genotype must consider the features of scientific validity, social value and risk-benefit ratio, and later, the impact of this strategy on post-market studies and clinical use of drugs. Although the testing context for pharmacogenomic tests is different from other genetic tests, decisions to use any new clinical tests in medical practice will require evaluation of not only the benefit linked to improved drug use but also the risks arising in part from the scale of testing, predictive value and collateral potential of the genetic test. Integration of pharmacogenomic information into clinical practice will require clinical trials to assess their clinical usefulness, including the impact of tests on therapeutic outcomes. Trials will also be needed to demonstrate the effectiveness of education, informed consent and counseling.

The genetic determinants of smoking

Dependence on tobacco, like many other drug dependencies, is a complex behavior with both genetic and environmental factors contributing to the variance. The heritability estimates for smoking in twin studies have ranged from 46 to 84%, indicating a substantial genetic component to smoking. Candidate gene studies have detected functional polymorphisms in genes coding for the cytochrome P450 enzymes, and variations in these genes that lead to more rapid nicotine metabolism have been implicated in smoking. Similarly, smoking has been associated with polymorphisms in dopaminergic genes that may influence the dopamine receptor number and/or function. Animal experiments have localized specific subunits of the nicotinic receptors that may mediate the reinforcing properties of nicotine and have investigated their role in nicotine dependence. However, environmental factors have also been found to contribute to the risk of initiation and persistence of smoking. We review the scientific evidence that supports a role for genetic influences on smoking, discuss the specific genetic and neurobiological mechanisms that may mediate susceptibility to nicotine dependence, identify possible gene/environmental interactions that may be important in understanding smoking behavior, and suggest directions for future research. Insights into the genetic contributions to smoking can potentially lead to more effective strategies to reduce smoking.

Interindividual differences in nicotine metabolism and genetic polymorphisms of human CYP2A6

Nicotine is widely consumed throughout the world, and exerts a number of physiological effects. After nicotine is absorbed through the lungs by cigarette smoking, it undergoes extensive metabolism in humans. Nicotine is mainly metabolized to cotinine by cytochrome P450 (CYP) 2A6. CYP2A6 can metabolize some pharmaceutical agents such as halothane, valproic acid, and fadrozole, and activate tobacco-specific nitrosamines. There are large interindividual differences in nicotine metabolism, and it has been found that the interindividual differences are attributed to the genetic polymorphisms of CYP2A6 gene. This review describes the techniques for determination of in vivo nicotine metabolism, characteristics of each human CYP2A6 alleles, and ethnic differences. The relationship between CYP2A6 genetic polymorphism and potency of nicotine metabolism, smoking behavior, and cancer risk are extensively reviewed. Finally, the usefulness of nicotine metabolism for phenotyping of CYP2A6 in individuals and implication of the significance of CYP2A6 genetic polymorphism in a clinical perspective are discussed.

Applying genetic approaches to the treatment of nicotine dependence

To advance the science of tobacco control, an enhanced understanding of the bio-behavioral basis of nicotine addiction is needed. In this study, we provide an overview of data from investigations of genetic factors in smoking behavior, discuss potential bio-behavioral mechanisms and effect modifiers, and suggest avenues for pharmacogenetics research in the area of smoking cessation treatment. The evidence to date is very consistent with respect to the significance of genetic contributions to smoking behavior. However, attempts to elucidate the role of specific genetic variants have met with mixed success. Explanations for the lack of consistency in the results of genetic association studies include biases in ascertainment, ethnic admixture, lack of attention to co-variates or modifiers of genetic risk, and the need for more refined phenotypes. As the field of genetics and smoking research progresses, increasing attention is being devoted to gene-environment interactions, with particular attention to the identification of genetic variants that may modify the effects of pharmacological treatment for smoking. With advances in molecular biology and genomics technology, individualized tailoring of smoking cessation therapy to genotype is within our grasp. Such research has the potential to improve treatment outcome, thereby reducing morbidity and mortality from smoking-related disease.

Ibogaine signals addiction genes and methamphetamine alteration of long-term potentiation

The mapping of the human genetic code will enable us to identify potential gene products involved in human addictions and diseases that have hereditary components. Thus, large-scale, parallel gene-expression studies, made possible by advances in microarray technologies, have shown insights into the connection between specific genes, or sets of genes, and human diseases. The compulsive use of addictive substances despite adverse consequences continues to affect society, and the science underlying these addictions in general is intensively studied. Pharmacological treatment of drug and alcohol addiction has largely been disappointing, and new therapeutic targets and hypotheses are needed. As the usefulness of the pharmacotherapy of addiction has been limited, an emerging potential, yet controversial, therapeutic agent is the natural alkaloid ibogaine. We have continued to investigate programs of gene expression and the putative signaling molecules used by psychostimulants such as amphetamine in in vivo and in vitro models. Our work and that of others reveal that complex but defined signal transduction pathways are associated with psychostimulant administration and that there is broad-spectrum regulation of these signals by ibogaine. We report that the actions of methamphetamine were similar to those of cocaine, including the propensity to alter long-term potentiation (LTP) in the hippocampus of the rat brain. This action suggests that there may be a "threshold" beyond which the excessive brain stimulation that probably occurs with compulsive psychostimulant use results in the occlusion of LTP. The influence of ibogaine on immediate early genes (IEGs) and other candidate genes possibly regulated by psychostimulants and other abused substances requires further evaluation in compulsive use, reward, relapse, tolerance, craving and withdrawal reactions. It is therefore tempting to suggest that ibogaine signals addiction gene products.

The role of pharmacogenetically-variable cytochrome P450 enzymes in drug abuse and dependence

The risk of drug dependence is determined by the interaction of drug, individual and environment. 'Pharmacogenetics' is the study of the influence of heredity on the response to drugs and their fate in the body; these studies aim to improve the understanding of inter-individual variability in drug response. The authors have applied this research approach to the study of drug metabolism and dependence. Specifically the interaction of genetically variable hepatic cytochrome P450 (CYP) enzymes and their effect on self-administration of drugs has been examined. Many drugs of abuse are substrates (e.g., amphetamines, codeine, nicotine) or inhibitors (e.g., (-)-cocaine) of polymorphic CYPs. Drug metabolism by genetically polymorphic enzymes can have significant clinical implications relating to drug toxicity, therapeutic failure, drug-drug interactions, disease susceptibility and abuse liability. There is good evidence that drug metabolism by genetically variable CYPs can influence the risk of drug dependence, the amount of drug consumed by dependent individuals and some of the toxicities associated with drug-taking behavior. It is anticipated that pharmacogenetics will be used to identify individuals at a greater risk for specific drug dependencies, provide information that can lead to novel treatment and prevention approaches as well as provide guidance for individualization of treatment choice.