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

Catechol-O-Methyltransferase Effects on Smoking: A Review and Proof of Concept of Sex-Sensitive Effects

Purpose of Review

This article reviews recent research on how catechol-O-methyltransferase (COMT) may impact cigarette smoking behavior, and how effects may be sex-sensitive. Preliminary data are presented on sex-sensitive effects of COMT on response to short-term abstinence in individuals who smoke.

Recent Findings

Although research is mixed, functional variants in the COMT gene have been linked with smoking behavior, cessation outcomes and nicotine abstinence-related symptoms. Our proof-of-concept preliminary data from a human laboratory study of individuals who smoke cigarettes found that those with the high COMT enzyme activity genotype (Val/Val) reported more severe smoking urges and withdrawal symptoms following overnight abstinence than Met carriers. These effects were present in women, but not in men and were abstinent-dependent, in that they dissipated following nicotine administration.

Summary

The preliminary data showing sex-sensitive pharmacogenetic effects may shed light on mechanisms contributing to sex differences in barriers to smoking cessation or potential sex-specific treatment options.

Genetic evaluation of the variants using MassARRAY in non-small cell lung cancer among North Indians

Lung cancer is genetically diverse and a major health burden. Non-small cell lung cancer (NSCLC) accounts for 80% of total lung cancer cases and 20% cases are Small cell lung cancer (SCLC). The present case–control association study focused on the cost effective high throughput genotyping using Agena MassARRAY matrix-assisted laser desorption/ionization-time of flight, mass spectrometry (MALDI-TOF) platform to analyze the genetic association of candidate genetic variants. We performed multiplex PCR and genotyped twelve single nucleotide polymorphisms (SNPs) in 723 samples (162 NSCLC cases and 592 healthy controls). These genetic variants were selected from literature for their association with various cancers worldwide and this is the first study from the region to examine these critically important genetic variants. With prospective case–control association study design, twelve variants from ten genes were evaluated. Amongst these six variants, TCF21 (rs12190287), ERCC1 (rs2298881, 11615), ERCC5 (rs751402), ARNTL (rs4757151), BRIP1 (rs4986764) showed significant association with NSCLC risk (p ≤ 0.003) in Jammu and Kashmir population. In-silico findings of these genetic variants showed remarkable functional roles that needs in-vitro validations. It is further anticipated that such case control studies will help us in understanding the missing heritability of non-small cell lung cancer.

The Pan social brain: An evolutionary history of neurochemical receptor genes and their potential impact on sociocognitive differences

Humans have unique cognitive capacities that, compared with apes, are not only simply expressed as a higher level of general intelligence, but also as a quantitative difference in sociocognitive skills. Humans' closest living relatives, bonobos (Pan paniscus), and chimpanzees (Pan troglodytes), show key between-species differences in social cognition despite their close phylogenetic relatedness, with bonobos arguably showing greater similarities to humans. To better understand the evolution of these traits, we investigate the neurochemical mechanisms underlying sociocognitive skills by focusing on variation in genes encoding proteins with well-documented roles in mammalian social cognition: the receptors for vasopressin (AVPR1A), oxytocin (OXTR), serotonin (HTR1A), and dopamine (DRD2). Although these genes have been well studied in humans, little is known about variation in these genes that may underlie differences in social behavior and cognition in apes. We comparatively analyzed sequence data for 33 bonobos and 57 chimpanzees, together with orthologous sequence data for other apes. In all four genes, we describe genetic variants that alter the amino acid sequence of the respective receptors, raising the possibility that ligand binding or signal transduction may be impacted. Overall, bonobos show 57% more fixed substitutions than chimpanzees compared with the ancestral Pan lineage. Chimpanzees, show 31% more polymorphic coding variation, in line with their larger historical effective population size estimates and current wider distribution. An extensive literature review comparing allelic changes in Pan with known human behavioral variants revealed evidence of homologous evolution in bonobos and humans (OXTR rs4686301(T) and rs237897(A)), while humans and chimpanzees shared OXTR rs2228485(A), DRD2 rs6277(A), and DRD2 rs11214613(A) to the exclusion of bonobos. Our results offer the first in-depth comparison of neurochemical receptor gene variation in Pan and put forward new variants for future behavior-genotype association studies in apes, which can increase our understanding of the evolution of social cognition in modern humans.

Polymorphism of the Dopa-Decarboxylase Gene Modifies the Motor Response to Levodopa in Chinese Patients With Parkinson's Disease

Levodopa (L-DOPA) is the most effective drug for Parkinson's disease (PD). However, the response to L-DOPA remains individually variable, which hampers the practical value of L-DOPA in the clinic. Genetic factors play a role in L-DOPA efficacy. This study explored the associations between polymorphisms and motor response to L-DOPA in Chinese patients with PD. A total of 51 Chinese PD patients were enrolled in this study. Patients underwent an acute L-DOPA challenge and were evaluated by the Unified Parkinson Disease Rating Scale (UPDRS) part III at baseline and after L-DOPA administration. Subjects were genotyped for polymorphisms: rs921451 and rs3837091 in the DDC loci, rs3836790 in the SLC6A3 locus, rs4680 in the COMT locus, and rs1799836 in the MAOB locus. We found that patients carrying the DDC CT or TT genotype exhibited a better motor response to L-DOPA than patients with the DDC CC genotype, and there was still a significant difference after adjustment for the L-DOPA dose in the acute challenge. Improvement in the UPDRS III subscores, including bradykinesia and axial symptoms, was significantly lower in patients with the DDC CC genotype than in patients with the CT or TT genotype. There were no significant associations between the motor response to L-DOPA and the rs3837091, rs3836790, rs4680, and rs1799836 variants. The DDC single nucleotide polymorphism rs921451 modulated the motor response to L-DOPA in Chinese PD patients. Our results suggested that DDC may be a modifier gene for the L-DOPA treatment response in PD.

Significant association of mu-opioid receptor 1 haplotype with tobacco smoking in healthy control subjects but not in patients with schizophrenia and alcohol dependence

Tobacco smoking is highly prevalent in patients with schizophrenia and alcohol dependence. The underlying neurobiology of nicotine addiction is complex. Rewarding effects of nicotine from cigarettes are associated, among others, with mu-opioid receptors encoded by the OPRM1 gene. The aim of the study was to evaluate the association between two OPRM1 gene polymorphisms, rs1799971 and rs510769, and tobacco smoking in Caucasian patients with schizophrenia, alcohol dependence, and healthy control subjects. The study included 1058 Caucasians (277 patients with schizophrenia, 359 patients with alcohol dependence, and 422 healthy control subjects), subdivided according to the nicotine dependence into smokers (i.e. current smokers) and non-smokers. A significant association was found between the GC haplotype (OPRM1 rs1799971 and rs510769) and smoking in healthy controls, but not in patients with schizophrenia and alcohol dependence. A nominal association was detected in all cases/controls, but this significance did not survive the correction for the multiple testing. This is the first study to reveal that nicotine dependence is associated with the GC haplotype of the OPRM1 rs1799971 and rs510769 in all subjects or specifically in healthy controls. These results did not confirm the strong connection between OPRM1 polymorphisms and nicotine dependence in schizophrenia or alcohol dependence.

The effect of SLC6A3 variable number of tandem repeats and methylation levels on individual susceptibility to start tobacco smoking and on the ability of smokers to quit smoking

OBJECTIVE

Nicotine acts through the dopamine pathway in the brain affecting reward processing through cigarette consumption. Thus, both genetic and epigenetic factors related to dopamine metabolism may influence individual's smoking behavior.

MATERIALS AND METHODS

We studied variations of two variable numbers of tandem repeats (VNTRs), 40 and 30 bp in length, in SLC6A3 gene together with six DNA methylation sites located in a first intron of the gene in relation to several smoking-related phenotypes in a study population consisting of 1230 Whites of Russian origin.

RESULTS

Both the 5R allele of 30 bp VNTR and the 9R allele of 40 bp VNTR in SLC6A3 were associated with a reduced risk to tobacco smoking [odds ratio (OR) 0.53, 95% confidence interval (CI) 0.37-0.75; OR 0.62, 95% CI 0.43-0.88]. Although the carriers of 9R allele also had high Fagerström test for nicotine dependence scores (OR 1.65, 95% CI 1.04-2.60), they were still more likely to succeed in smoking cessation (OR 0.59, 95% CI 0.40-0.88). Also, current smokers had more than 2.5-fold likelihood to have increased SLC6A3 methylation levels than former smokers (OR 2.72, 95% CI 1.63-4.53).

CONCLUSION

The SLC6A3 5R of 30 bp and 9R of 40 bp VNTR variants may lead to a reduced risk to start smoking through decreased dopamine availability, and can also affect the success in subsequent smoking cessation attempts. Moreover, the elevated mean methylation values in the first intron of SLC6A3 may be related to nicotine dependence via a more active dopamine transporter.

Polymorphism in ASCL1 target gene DDC is associated with clinical outcomes of small cell lung cancer patients

BACKGROUND

Achaete-scute homolog 1 (ASCL1) is a basic helix-loop-helix transcription factor and is essential in the differentiation of neuroendocrine cells and neural tissues. ASCL1 is frequently overexpressed in small cell lung cancer (SCLC) and plays a crucial role in the pathogenesis of SCLC.

METHODS

This study was conducted to identify the association between single nucleotide polymorphisms (SNPs) in ASCL1 target genes and clinical outcomes of patients with SCLC after chemotherapy. A total of 261 patients diagnosed with SCLC were enrolled in this study. The association between 103 SNPs in 58 ASCL1 target genes and the response to chemotherapy and survival of patients with SCLC were analyzed.

RESULTS

Among the 103 SNPs, 10 SNPs were significantly associated with the response to chemotherapy, and 19 SNPs were associated with OS in multivariate analyses. Among these, Dopa Decarboxylase (DDC) rs12666409A>T was significantly associated with both a worse response to chemotherapy and worse OS (adjusted odds ratio [aOR] = 0.40, 95% CI = 0.18-0.90, P = 0.03; adjusted hazard ratio [aHR] = 1.52, 95% CI = 1.10-2.10, P = 0.01, respectively, under a dominant model). In a stage-stratified analysis, the association was significant only in the extensive disease subgroup (aOR = 0.19, 95% CI = 0.06-0.60, P = 0.01; aHR = 1.73, 95% CI = 1.16-2.56, P = 0.01, respectively, under a dominant model), but not in the limited disease subgroup.

CONCLUSION

The results of our study suggest that DDC rs12666409A>T may be useful markers for predicting the clinical outcomes of patients with SCLC undergoing chemotherapy.

ADHD risk genes involved in dopamine signaling and metabolism are associated with reduced estimated life expectancy at young adult follow-up in hyperactive and control children

ADHD is associated with an elevated risk of mortality and reduced estimated life expectancy (ELE) by adulthood. Reduced life expectancy is substantially related to the trait of behavioral disinhibition; a correlate of both ADHD and of several dopamine genes related to dopamine signaling and metabolism. We therefore hypothesized that several ADHD risk genes related to dopamine might also be predictive of reduced ELE. Using a longitudinal study of 131 hyperactive children and 71 control cases followed to young adulthood, we examined whether several polymorphisms involving DRD4, DAT1, and DBH were related to ELE. The homozygous 9/9 allele of DAT1 and the heterozygous allele of DBH TaqI were associated with 5- and 2-year reductions, respectively, in total ELE. They did not operate on ELE through any relationships to ADHD specifically or behavioral disinhibition more generally. Instead, they showed links to alcohol use (DBH), reduced education, smoking, and reduced exercise (DAT1) employed in the computation of ELE. We conclude that polymorphisms of two dopamine genes are linked to reductions in ELE independently of their association with ADHD.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.

A gene-by-sex interaction for nicotine reward: evidence from humanized mice and epidemiology

It has been proposed that vulnerability to nicotine addiction is moderated by variation at the μ-opioid receptor locus (OPRM1), but results from human studies vary and prospective studies based on genotype are lacking. We have developed a humanized mouse model of the most common functional OPRM1 polymorphism rs1799971_A>G (A118G). Here we use this model system together with a cohort of German youth to examine the role of the OPRM1 A118G variation on nicotine reward. Nicotine reinforcement was examined in the humanized mouse model using i.v. self-administration. Male (n=17) and female (n=26) mice homozygous either for the major human A allele (AA) or the minor G allele (GG) underwent eight daily 2 h sessions of nicotine self-administration. Furthermore, male (n=104) and female (n=118) subjects homozygous for the A allele or carrying the G allele from the Mannheim Study of Children at Risk were evaluated for pleasurable and unpleasant experiences during their initial smoking experience. A significant sex-by-genotype effect was observed for nicotine self-administration. Male 118GG mice demonstrated higher nicotine intake than male 118AA mice, suggesting increased nicotine reinforcement. In contrast, there was no genotype effect in female mice. Human male G allele carriers reported increased pleasurable effects from their first smoking experience, as compared to male homozygous A, female G and female homozygous A allele carriers. The 118G allele appears to confer greater sensitivity to nicotine reinforcement in males, but not females.

Nicotine and the adolescent brain

Adolescence encompasses a sensitive developmental period of enhanced clinical vulnerability to nicotine, tobacco, and e-cigarettes. While there are sociocultural influences, data at preclinical and clinical levels indicate that this adolescent sensitivity has strong neurobiological underpinnings. Although definitions of adolescence vary, the hallmark of this period is a profound reorganization of brain regions necessary for mature cognitive and executive function, working memory, reward processing, emotional regulation, and motivated behavior. Regulating critical facets of brain maturation are nicotinic acetylcholine receptors (nAChRs). However, perturbations of cholinergic systems during this time with nicotine, via tobacco or e-cigarettes, have unique consequences on adolescent development. In this review, we highlight recent clinical and preclinical data examining the adolescent brain's distinct neurobiology and unique sensitivity to nicotine. First, we discuss what defines adolescence before reviewing normative structural and neurochemical alterations that persist until early adulthood, with an emphasis on dopaminergic systems. We review how acute exposure to nicotine impacts brain development and how drug responses differ from those seen in adults. Finally, we discuss the persistent alterations in neuronal signaling and cognitive function that result from chronic nicotine exposure, while highlighting a low dose, semi-chronic exposure paradigm that may better model adolescent tobacco use. We argue that nicotine exposure, increasingly occurring as a result of e-cigarette use, may induce epigenetic changes that sensitize the brain to other drugs and prime it for future substance abuse.