Molecular genetics of addiction and related heritable phenotypes: genome-wide association approaches identify "connectivity constellation" and drug target genes with pleiotropic effects

Generalized genetic liability to substance use disorders

Lifetime and temporal co-occurrence of substance use disorders (SUDs) is common and compared with individual SUDs is characterized by greater severity, additional psychiatric comorbidities, and worse outcomes. Here, we review evidence for the role of generalized genetic liability to various SUDs. Coaggregation of SUDs has familial contributions, with twin studies suggesting a strong contribution of additive genetic influences undergirding use disorders for a variety of substances (including alcohol, nicotine, cannabis, and others). GWAS have documented similarly large genetic correlations between alcohol, cannabis, and opioid use disorders. Extending these findings, recent studies have identified multiple genomic loci that contribute to common risk for these SUDs and problematic tobacco use, implicating dopaminergic regulatory and neuronal development mechanisms in the pathophysiology of generalized SUD genetic liability, with certain signals demonstrating cross-species and translational validity. Overlap with genetic signals for other externalizing behaviors, while substantial, does not explain the entirety of the generalized genetic signal for SUD. Polygenic scores (PGS) derived from the generalized genetic liability to SUDs outperform PGS for individual SUDs in prediction of serious mental health and medical comorbidities. Going forward, it will be important to further elucidate the etiology of generalized SUD genetic liability by incorporating additional SUDs, evaluating clinical presentation across the lifespan, and increasing the granularity of investigation (e.g., specific transdiagnostic criteria) to ultimately improve the nosology, prevention, and treatment of SUDs.

Low Dopamine D2 Receptor Expression Drives Gene Networks Related to GABA, cAMP, Growth and Neuroinflammation in Striatal Indirect Pathway Neurons

Background

A salient effect of addictive drugs is to hijack the dopamine reward system, an evolutionarily conserved driver of goal-directed behavior and learning. Reduced dopamine type 2 receptor availability in the striatum is an important pathophysiological mechanism for addiction that is both consequential and causal for other molecular, cellular, and neuronal network differences etiologic for this disorder. Here, we sought to identify gene expression changes attributable to innate low expression of the Drd2 gene in the striatum and specific to striatal indirect medium spiny neurons (iMSNs).

Methods

Cre-conditional, translating ribosome affinity purification (TRAP) was used to purify and analyze the translatome (ribosome-bound messenger RNA) of iMSNs from mice with low/heterozygous or wild-type Drd2 expression in iMSNs. Complementary electrophysiological recordings and gene expression analysis of postmortem brain tissue from human cocaine users were performed.

Results

Innate low expression of Drd2 in iMSNs led to differential expression of genes involved in GABA (gamma-aminobutyric acid) and cAMP (cyclic adenosine monophosphate) signaling, neural growth, lipid metabolism, neural excitability, and inflammation. Creb1 was identified as a likely upstream regulator, among others. In human brain, expression of FXYD2, a modulatory subunit of the Na/K pump, was negatively correlated with DRD2 messenger RNA expression. In iMSN-TRAP-Drd2HET mice, increased Cartpt and reduced S100a10 (p11) expression recapitulated previous observations in cocaine paradigms. Electrophysiology experiments supported a higher GABA tone in iMSN-Drd2HET mice.

Conclusions

This study provides strong molecular evidence that, in addiction, inhibition by the indirect pathway is constitutively enhanced through neural growth and increased GABA signaling.

Cellular and neuronal mechanisms that underlie addiction - literature review

Introduction: Addictive substances act on a number of neurotransmitter systems, and the end result of this action is the activation of the reward system in the brain. The cellular and neuronal mechanisms that underlie addiction have long been searched for. One of such neurotransmitters is dopamine, a catecholamine synthesized in neurons located mainly in the midbrain.

Material and method: The available literature was reviewed on the Pubmed platform and from other sources. The analysis included original studies, reviews.

The aim of the study was to review the literature on the relationship between the DRD2 gene and the occurrence of substance addiction.

Discussion: This work presents several currently discussed biological mechanisms, especially at the molecular and genetic level, involved in the process of addiction to various psychoactive substances. They discovered the brain structures that are most at risk, as well as other neurotransmitter systems and receptor proteins through which they can exert their pathological effects. It has also been established that exposure to psychoactive substances causes significant changes in expression in over 100 genes (including genes for dopaminergic, serotonergic and signaling pathways). The DRD2 receptor (present, among others, in the nucleus accumbens) plays an important role in the reward system, in the transmission of information. The weakening of this conductivity is a significant risk factor for the onset of clinical features that are associated with reward system deficiency syndrome. The expression of the D2 receptor gene may take up to 2 isoforms: short D2S and long D2L.

Conclusions: Further research at the molecular level may result in the modification of psychotherapy and pharmacotherapy in terms of their personalization.

Application of optogenetics and in vivo imaging approaches for elucidating the neurobiology of addiction

The neurobiology of addiction has been an intense topic of investigation for more than 50 years. Over this time, technological innovation in methods for studying brain function rapidly progressed, leading to increasingly sophisticated experimental approaches. To understand how specific brain regions, cell types, and circuits are affected by drugs of abuse and drive behaviors characteristic of addiction, it is necessary both to observe and manipulate neural activity in addiction-related behavioral paradigms. In pursuit of this goal, there have been several key technological advancements in in vivo imaging and neural circuit modulation in recent years, which have shed light on the cellular and circuit mechanisms of addiction. Here we discuss some of these key technologies, including circuit modulation with optogenetics, in vivo imaging with miniaturized single-photon microscopy (miniscope) and fiber photometry, and how the application of these technologies has garnered novel insights into the neurobiology of addiction.

Cadherin Expression and EMT: A Focus on Gliomas

Cadherins are calcium-binding proteins with a pivotal role in cell adhesion and tissue homeostasis. The cadherin-dependent mechanisms of cell adhesion and migration are exploited by cancer cells, contributing to tumor invasiveness and dissemination. In particular, cadherin switch is a hallmark of epithelial to mesenchymal transition, a complex development process vastly described in the progression of most epithelial cancers. This is characterized by drastic changes in cell polarity, adhesion, and motility, which lead from an E-cadherin positive differentiated epithelial state into a dedifferentiated mesenchymal-like state, prone to metastization and defined by N-cadherin expression. Although vastly explored in epithelial cancers, how these mechanisms contribute to the pathogenesis of other non-epithelial tumor types is poorly understood. Herein, the current knowledge on cadherin expression in normal development in parallel to tumor pathogenesis is reviewed, focusing on epithelial to mesenchymal transition. Emphasis is taken in the unascertained cadherin expression in CNS tumors, particularly in gliomas, where the potential contribution of an epithelial-to-mesenchymal-like process to glioma genesis and how this may be associated with changes in cadherin expression is discussed.

The role of repetitive transcranial magnetic stimulation (rTMS) in the treatment of behavioral addictions: Two case reports and review of the literature

BACKGROUND

Several behaviors, besides consumption of psychoactive substances, produce short-term reward that may lead to persistent aberrant behavior despite adverse consequences. Growing evidence suggests that these behaviors warrant consideration as nonsubstance or "behavioral" addictions, such as pathological gambling, internet gaming disorder and internet addiction.

CASE PRESENTATION

Here, we report two cases of behavioral addictions (BA), compulsive sexual behavior disorder for online porn use and internet gaming disorder. A 57-years-old male referred a loss of control over his online pornography use, started 15 years before, while a 21-years-old male university student reported an excessive online gaming activity undermining his academic productivity and social life. Both patients underwent a high-frequency repetitive transcranial magnetic stimulation (rTMS) protocol over the left dorsolateral prefrontal cortex (l-DLPFC) in a multidisciplinary therapeutic setting. A decrease of addictive symptoms and an improvement of executive control were observed in both cases.

DISCUSSION

Starting from these clinical observations, we provide a systematic review of the literature suggesting that BAs share similar neurobiological mechanisms to those underlying substance use disorders (SUD). Moreover, we discuss whether neurocircuit-based interventions, such as rTMS, might represent a potential effective treatment for BAs.

The Streetlight Effect: Reappraising the Study of Addiction in Light of the Findings of Genome-wide Association Studies

Drug dependence has long been thought to have a genetic component. Research seeking to identify the genetic basis of addiction has gone through important transitions over its history, in part based upon the emergence of new technologies, but also as the result of changing perspectives. Early research approaches were largely dictated by available technology, with technological advancements having highly transformative effects on genetic research, but the limitations of technology also affected modes of thinking about the genetic causes of disease. This review explores these transitions in thinking about the genetic causes of addiction in terms of the "streetlight effect," which is a type of observational bias whereby people search for something only where it is easiest to search. In this way, the genes that were initially studied in the field of addiction genetics were chosen because they were the most "obvious," and formed current understanding of the biological mechanisms underlying the actions of drugs of abuse and drug dependence. The problem with this emphasis is that prior to the genomic era the vast majority of genes and proteins had yet to be identified, much less studied. This review considers how these initial choices, as well as subsequent choices that were also driven by technological limitations, shaped the study of the genetic basis of drug dependence. While genome-wide approaches overcame the initial biases regarding which genes to choose to study inherent in candidate gene studies and other approaches, genome-wide approaches necessitated other assumptions. These included additive genetic causation and limited allelic heterogeneity, which both appear to be incorrect. Thus, the next stage of advancement in this field must overcome these shortcomings through approaches that allow the examination of complex interactive effects, both gene × gene and gene × environment interactions. Techniques for these sorts of studies have recently been developed and represent the next step in our understanding of the genetic basis of drug dependence.

Common genetic substrates of alcohol and substance use disorder severity revealed by pleiotropy detection against GWAS catalog in two populations

Alcohol and other substance use disorders (AUD and SUD) are complex diseases that are postulated to have a polygenic inheritance and are often comorbid with other disorders. The comorbidities may arise partially through genetic pleiotropy. Identification of specific gene variants accounting for large parts of the variance in these disorders has yet to be accomplished. We describe a flexible strategy that takes a variant-trait association database and determines if a subset of disease/straits are potentially pleiotropic with the disorder under study. We demonstrate its usage in a study of use disorders in two independent cohorts: alcohol, stimulants, cannabis (CUD), and multi-substance use disorders (MSUD) in American Indians (AI) and AUD and CUD in Mexican Americans (MA). Using a machine learning method with variants in GWAS catalog, we identified 229 to 246 pleiotropic variants for AI and 153 to 160 for MA for each SUD. Inflammation was the most enriched for MSUD and AUD in AIs. Neurological disorder was the most significantly enriched for CUD in both cohorts, and for AUD and stimulants in AIs. Of the select pleiotropic genes shared among substances-cohorts, multiple biological pathways implicated in SUD and other psychiatric disorders were enriched, including neurotrophic factors, immune responses, extracellular matrix, and circadian regulation. Shared pleiotropic genes were significantly up-regulated in brain regions playing important roles in SUD, down-regulated in esophagus mucosa, and differentially regulated in adrenal gland. This study fills a gap for pleiotropy detection in understudied admixed populations and identifies pleiotropic variants that may be potential targets of interest for SUD.

Attention-deficit/hyperactivity disorder and lifetime cannabis use: genetic overlap and causality

Attention-deficit/hyperactivity disorder (ADHD) is a severely impairing neurodevelopmental disorder with a prevalence of 5% in children and adolescents and of 2.5% in adults. Comorbid conditions in ADHD play a key role in symptom progression, disorder course and outcome. ADHD is associated with a significantly increased risk for substance use, abuse and dependence. ADHD and cannabis use are partly determined by genetic factors; the heritability of ADHD is estimated at 70-80% and of cannabis use initiation at 40-48%. In this study, we used summary statistics from the largest available meta-analyses of genome-wide association studies (GWAS) of ADHD (n = 53,293) and lifetime cannabis use (n = 32,330) to gain insights into the genetic overlap and causal relationship of these two traits. We estimated their genetic correlation to be r2 = 0.29 (P = 1.63 × 10-5) and identified four new genome-wide significant loci in a cross-trait analysis: two in a single variant association analysis (rs145108385, P = 3.30 × 10-8 and rs4259397, P = 4.52 × 10-8) and two in a gene-based association analysis (WDPCP, P = 9.67 × 10-7 and ZNF251, P = 1.62 × 10-6). Using a two-sample Mendelian randomization approach we found support that ADHD is causal for lifetime cannabis use, with an odds ratio of 7.9 for cannabis use in individuals with ADHD in comparison to individuals without ADHD (95% CI (3.72, 15.51), P = 5.88 × 10-5). These results substantiate the temporal relationship between ADHD and future cannabis use and reinforce the need to consider substance misuse in the context of ADHD in clinical interventions.

Selective breeding for high alcohol preference is associated with increased sensitivity to cannabinoid reward within the nucleus accumbens shell

RATIONALE

The rate of cannabinoid intake by those with alcohol use disorder (AUD) exceeds that of the general public. The high prevalence of co-abuse of alcohol and cannabis has been postulated to be predicated upon both a common predisposing genetic factor and the interaction of the drugs within the organism. The current experiments examined the effects of cannabinoids in an animal model of AUD.

OBJECTIVES

The present study assessed the reinforcing properties of a cannabinoid receptor 1 (CB1) agonist self-administered directly into the nucleus accumbens shell (AcbSh) in female Wistar and alcohol-preferring (P) rats.

METHODS

Following guide cannulae surgery aimed at AcbSh, subjects were placed in an operant box equipped with an 'active lever' (fixed ratio 1; FR1) that caused the delivery of the infusate and an 'inactive lever' that did not. Subjects were arbitrarily assigned to one of seven groups that self-administered either artificial cerebrospinal fluid (aCSF), or 3.125, 6.25, 12.5, or 25 pmol/100 nl of O-1057, a water-soluble CB1 agonist, dissolved in aCSF. The first four sessions of acquisition are followed by aCSF only infusates in sessions 5 and 6 during extinction, and finally the acquisition dose of infusate during session 7 as reinstatement.

RESULTS

The CB1 agonist was self-administered directly into the AcbSh. P rats self-administered the CB1 agonist at lower concentrations and at higher rates compared to Wistar rats.

CONCLUSIONS

Overall, the data indicate selective breeding for high alcohol preference has produced rats divergent in response to cannabinoids within the brain reward pathway. The data support the hypothesis that there can be common genetic factors influencing drug addiction.

Impact of Apolipoprotein E gene polymorphism during normal and pathological conditions of the brain across the lifespan

The central nervous system (CNS) is the cellular substrate for the integration of complex, dynamic, constant, and simultaneous interactions among endogenous and exogenous stimuli across the entire human lifespan. Numerous studies on aging-related brain diseases show that some genes identified as risk factors for some of the most common neurodegenerative diseases - such as the allele 4 of APOE gene (APOE4) for Alzheimer's disease (AD) - have a much earlier neuro-anatomical and neuro-physiological impact. The impact of APOE polymorphism appears in fact to start as early as youth and early-adult life. Intriguingly, though, those same genes associated with aging-related brain diseases seem to influence different aspects of the brain functioning much earlier actually, that is, even from the neonatal periods and earlier. The APOE4, an allele classically associated with later-life neurodegenerative disorders as AD, seems in fact to exert a series of very early effects on phenomena of neuroplasticity and synaptogenesis that begin from the earliest periods of life such as the fetal ones.We reviewed some of the findings supporting the hypothesis that APOE polymorphism is an early modifier of various neurobiological aspects across the entire human lifespan - from the in-utero to the centenarian life - during both normal and pathological conditions of the brain.

A multi-ancestry genome-wide study incorporating gene-smoking interactions identifies multiple new loci for pulse pressure and mean arterial pressure

Elevated blood pressure (BP), a leading cause of global morbidity and mortality, is influenced by both genetic and lifestyle factors. Cigarette smoking is one such lifestyle factor. Across five ancestries, we performed a genome-wide gene-smoking interaction study of mean arterial pressure (MAP) and pulse pressure (PP) in 129 913 individuals in stage 1 and follow-up analysis in 480 178 additional individuals in stage 2. We report here 136 loci significantly associated with MAP and/or PP. Of these, 61 were previously published through main-effect analysis of BP traits, 37 were recently reported by us for systolic BP and/or diastolic BP through gene-smoking interaction analysis and 38 were newly identified (P < 5 × 10-8, false discovery rate < 0.05). We also identified nine new signals near known loci. Of the 136 loci, 8 showed significant interaction with smoking status. They include CSMD1 previously reported for insulin resistance and BP in the spontaneously hypertensive rats. Many of the 38 new loci show biologic plausibility for a role in BP regulation. SLC26A7 encodes a chloride/bicarbonate exchanger expressed in the renal outer medullary collecting duct. AVPR1A is widely expressed, including in vascular smooth muscle cells, kidney, myocardium and brain. FHAD1 is a long non-coding RNA overexpressed in heart failure. TMEM51 was associated with contractile function in cardiomyocytes. CASP9 plays a central role in cardiomyocyte apoptosis. Identified only in African ancestry were 30 novel loci. Our findings highlight the value of multi-ancestry investigations, particularly in studies of interaction with lifestyle factors, where genomic and lifestyle differences may contribute to novel findings.

National Institute on Drug Abuse genomics consortium white paper: Coordinating efforts between human and animal addiction studies

The National Institute on Drug Abuse Genetics and Epigenetics Cross-Cutting Research Team convened a diverse group of researchers, clinicians, and healthcare providers on the campus of the University of California, San Diego, in June 2018. The goal was to develop strategies to integrate genetics and phenotypes across species to achieve a better understanding of substance use disorders through associations between genotypes and addictive behaviors. This conference (a) discussed progress in harmonizing large opioid genetics cohorts, (b) discussed phenotypes that are used for genetics studies in humans, (c) examined phenotypes that are used for genetics studies in animal models, (d) identified synergies and gaps in phenotypic analyses of human and animal models and (e) identified strategies to integrate genetics and genomics data with phenotypes across species. The meeting consisted of panels that focused on phenotype harmonization (Dr. Laura Bierut, Dr. Olivier George, Dr. Dan Larach and Dr. Sesh Mudumbai), translating genetic findings between species (Dr. Elissa Chesler, Dr. Gary Peltz and Dr. Abraham Palmer), interpreting and understanding allelic variations (Dr. Vanessa Troiani and Dr. Tamara Richards) and pathway conservation in animal models and human studies (Dr. Robert Hitzemann, Dr. Huda Akil and Dr. Laura Saba). There were also updates that were provided by large consortia (Dr. Susan Tapert, Dr. Danielle Dick, Dr. Howard Edenberg and Dr. Eric Johnson). Collectively, the conference was convened to discuss progress and changes in genome-wide association studies.

Methylomic changes in response to micronutrient supplementation and MTHFR genotype

Exposure times and dosage required for dietary components to modify DNA methylation patterns are largely unknown.

AIM

This exploratory research represents the first genome-wide analysis of DNA methylation changes during a randomized-controlled-trial (RCT) for dietary supplementation with broad spectrum vitamins, minerals and amino acids in humans.

METHODS

Genome-wide changes in methylation from paired, peripheral blood samples were assessed using the Infinium Methylation EPIC 850 K array.

RESULTS

Methylation increased at 84% of the most significant differentially methylated CpGs; however, none showed significance after adjustment for genome-wide testing.

CONCLUSION

Micronutrient supplementation is unlikely to have a substantial biological effect on DNA methylation over 10 weeks; however, the trend toward hypermethylation that we observed is likely to become more marked with longer exposure periods.

Selective breeding for high alcohol consumption and response to nicotine: locomotor activity, dopaminergic in the mesolimbic system, and innate genetic differences in male and female alcohol-preferring, non-preferring, and replicate lines of high-alcohol drinking and low-alcohol drinking rats

RATIONALE

There is evidence for a common genetic link between alcohol and nicotine dependence. Rodents selectively bred for high alcohol consumption/responsivity are also more likely to self-administer nicotine than controls.

OBJECTIVES

The experiments examined the response to systemic nicotine, the effects of nicotine within the drug reward pathway, and innate expression of nicotine-related genes in a brain region regulating drug reward/self-administration in multiple lines of rats selectively bred for high and low alcohol consumption.

METHODS

The experiments examined the effects of systemic administration of nicotine on locomotor activity, the effects of nicotine administered directly into the (posterior ventral tegmental area; pVTA) on dopamine (DA) release in the nucleus accumbens shell (AcbSh), and innate mRNA levels of acetylcholine receptor genes in the pVTA were determined in 6 selectively bred high/low alcohol consuming and Wistar rat lines.

RESULTS

The high alcohol-consuming rat lines had greater nicotine-induced locomotor activity compared to low alcohol-consuming rat lines. Microinjections of nicotine into the pVTA resulted in DA release in the AcbSh with the dose response curves for high alcohol-consuming rats shifted leftward and upward. Genetic analysis of the pVTA indicated P rats expressed higher levels of α2 and β4.

CONCLUSION

Selective breeding for high alcohol preference resulted in a genetically divergent behavioral and neurobiological sensitivity to nicotine. The observed behavioral and neurochemical differences between the rat lines would predict an increased likelihood of nicotine reinforcement. The data support the hypothesis of a common genetic basis for drug addiction and identifies potential receptor targets.

The Role of Cell Adhesion Molecule Genes Regulating Neuroplasticity in Addiction

A variety of genetic approaches, including twin studies, linkage studies, and candidate gene studies, has established a firm genetic basis for addiction. However, there has been difficulty identifying the precise genes that underlie addiction liability using these approaches. This situation became especially clear in genome-wide association studies (GWAS) of addiction. Moreover, the results of GWAS brought into clarity many of the shortcomings of those early genetic approaches. GWAS studies stripped away those preconceived notions, examining genes that would not previously have been considered in the study of addiction, consequently creating a shift in our understanding. Most importantly, those studies implicated a class of genes that had not previously been considered in the study of addiction genetics: cell adhesion molecules (CAMs). Considering the well-documented evidence supporting a role for various CAMs in synaptic plasticity, axonal growth, and regeneration, it is not surprising that allelic variation in CAM genes might also play a role in addiction liability. This review focuses on the role of various cell adhesion molecules in neuroplasticity that might contribute to addictive processes and emphasizes the importance of ongoing research on CAM genes that have been implicated in addiction by GWAS.

ANCO-GeneDB: annotations and comprehensive analysis of candidate genes for alcohol, nicotine, cocaine and opioid dependence

Studies have shown that genetic factors play an important role in the risk to substance addiction and abuse. So far, various genetic and genomic studies have reported the related evidence. These rich, but highly heterogeneous, data provide us an unprecedented opportunity to systematically collect, curate and assess the genetic and genomic signals from published studies and to perform a comprehensive analysis of their features, functional roles and druggability. Such genetic data resources have been made available for other disease or phenotypes but not for major substance dependence yet. Here, we report comprehensive data collection and secondary analyses of four phenotypes of dependence: alcohol dependence, nicotine dependence, cocaine dependence and opioid dependence, collectively named as Alcohol, Nicotine, Cocaine and Opioid (ANCO) dependence. We built the ANCO-GeneDB, an ANCO-dependence-associated gene resource database. ANCO-GeneDB includes resources from genome-wide association studies and candidate gene-based studies, transcriptomic studies, methylation studies, literature mining and drug-target data, as well as the derived data such as spatial-temporal gene expression, promoters, enhancers and expression quantitative trait loci. All associated genes and genetic variants are well annotated by using the collected evidence. Based on the collected data, we performed integrative, secondary analyses to prioritize genes, pathways, eQTLs and tissues that are significantly enriched in ANCO-related phenotypes.

Cadherin-13 Deficiency Increases Dorsal Raphe 5-HT Neuron Density and Prefrontal Cortex Innervation in the Mouse Brain

Background: During early prenatal stages of brain development, serotonin (5-HT)-specific neurons migrate through somal translocation to form the raphe nuclei and subsequently begin to project to their target regions. The rostral cluster of cells, comprising the median and dorsal raphe (DR), innervates anterior regions of the brain, including the prefrontal cortex. Differential analysis of the mouse 5-HT system transcriptome identified enrichment of cell adhesion molecules in 5-HT neurons of the DR. One of these molecules, cadherin-13 (Cdh13) has been shown to play a role in cell migration, axon pathfinding, and synaptogenesis. This study aimed to investigate the contribution of Cdh13 to the development of the murine brain 5-HT system.

Methods: For detection of Cdh13 and components of the 5-HT system at different embryonic developmental stages of the mouse brain, we employed immunofluorescence protocols and imaging techniques, including epifluorescence, confocal and structured illumination microscopy. The consequence of CDH13 loss-of-function mutations on brain 5-HT system development was explored in a mouse model of Cdh13 deficiency.

Results: Our data show that in murine embryonic brain Cdh13 is strongly expressed on 5-HT specific neurons of the DR and in radial glial cells (RGCs), which are critically involved in regulation of neuronal migration. We observed that 5-HT neurons are intertwined with these RGCs, suggesting that these neurons undergo RGC-guided migration. Cdh13 is present at points of intersection between these two cell types. Compared to wildtype controls, Cdh13-deficient mice display increased cell densities in the DR at embryonic stages E13.5, E17.5, and adulthood, and higher serotonergic innervation of the prefrontal cortex at E17.5.

Conclusion: Our findings provide evidence for a role of CDH13 in the development of the serotonergic system in early embryonic stages. Specifically, we indicate that Cdh13 deficiency affects the cell density of the developing DR and the posterior innervation of the prefrontal cortex (PFC), and therefore might be involved in the migration, axonal outgrowth and terminal target finding of DR 5-HT neurons. Dysregulation of CDH13 expression may thus contribute to alterations in this system of neurotransmission, impacting cognitive function, which is frequently impaired in neurodevelopmental disorders including attention-deficit/hyperactivity and autism spectrum disorders.

Exploring the Validity of Proposed Transgenic Animal Models of Attention-Deficit Hyperactivity Disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD) is a common, behavioral, and heterogeneous neurodevelopmental condition characterized by hyperactivity, impulsivity, and inattention. Symptoms of this disorder are managed by treatment with methylphenidate, amphetamine, and/or atomoxetine. The cause of ADHD is unknown, but substantial evidence indicates that this disorder has a significant genetic component. Transgenic animals have become an essential tool in uncovering the genetic factors underlying ADHD. Although they cannot accurately reflect the human condition, they can provide insights into the disorder that cannot be obtained from human studies due to various limitations. An ideal animal model of ADHD must have face (similarity in symptoms), predictive (similarity in response to treatment or medications), and construct (similarity in etiology or underlying pathophysiological mechanism) validity. As the exact etiology of ADHD remains unclear, the construct validity of animal models of ADHD would always be limited. The proposed transgenic animal models of ADHD have substantially increased and diversified over the years. In this paper, we compiled and explored the validity of proposed transgenic animal models of ADHD. Each of the reviewed transgenic animal models has strengths and limitations. Some fulfill most of the validity criteria of an animal model of ADHD and have been extensively used, while there are others that require further validation. Nevertheless, these transgenic animal models of ADHD have provided and will continue to provide valuable insights into the genetic underpinnings of this complex disorder.

Cadherin 13: human cis-regulation and selectively-altered addiction phenotypes and cerebral cortical dopamine in knockout mice

The cadherin 13 (CDH13) gene encodes a cell adhesion molecule likely to influence development and connections of brain circuits that modulate addiction, locomotion and cognition, including those that involve midbrain dopamine neurons. Human CDH13 mRNA expression differs by more than 80% in postmortem cerebral cortical samples from individuals with different CDH13 genotypes, supporting examination of mice with altered Cdh13 expression as models for common human variation at this locus. Constitutive cdh13 knockout mice display evidence for changed cocaine reward: shifted dose response relationship in tests of cocaine-conditioned place preference using doses that do not alter cocaine conditioned taste aversion. Reduced adult Cdh13 expression in conditional knockouts also alters cocaine reward in ways that correlate with individual differences in cortical Cdh13 mRNA levels. In control and comparison behavioral assessments, knockout mice display modestly-quicker acquisition of rotarod and water maze tasks, with a trend toward faster acquisition of 5 choice serial reaction time tasks that otherwise displayed no genotype-related differences. They display significant differences in locomotion in some settings, with larger effects in males. In assessments of brain changes that might contribute to these behavioral differences, there are selective alterations of dopamine levels, dopamine/metabolite ratios, dopaminergic fiber densities and mRNA encoding the activity dependent transcription factor npas4 in cerebral cortex of knockout mice. These novel data and previously reported human associations of CDH13 variants with addiction, individual differences in responses to stimulant administration and attention deficit hyperactivity disorder (ADHD) phenotypes suggest that levels of CDH13 expression, through mechanisms likely to include effects on mesocortical dopamine, influence stimulant reward and may contribute modestly to cognitive and locomotor phenotypes relevant to ADHD.

Quantitative trait locus mapping of acute functional tolerance in the LXS recombinant inbred strains

BACKGROUND

We previously reported that acute functional tolerance (AFT) to the hypnotic effects of alcohol was significantly correlated with drinking in the dark (DID) in the LXS recombinant inbred panel, but only in mice that had been pretreated with alcohol. Here, we have conducted quantitative trait locus (QTL) mapping for AFT. DNA sequencing of the progenitor ILS and ISS strains and microarray analyses were also conducted to identify candidate genes and functional correlates.

METHODS

LXS mice were given either saline or alcohol (5 g/kg) on day 1 and then tested for loss of righting reflex AFT on day 2. QTLs were mapped using standard procedures. Two microarray analyses from brain were conducted: (i) naïve LXS mice and (ii) an alcohol treatment time course in the ILS and ISS. The full genomes of the ILS and ISS were sequenced to a depth of approximately 30×.

RESULTS

A significant QTL for AFT in the alcohol pretreatment group was mapped to distal chromosome 4; numerous suggestive QTLs were also mapped. Preference drinking and DID have previously been mapped to the chromosome 4 locus. The credible interval of the significant chromosome 4 QTL spanned 23 Mb and included 716 annotated genes of which 150 had at least 1 nonsynonymous single nucleotide polymorphism or small indel that differed between the ILS and ISS; expression of 48 of the genes was cis-regulated. Enrichment analysis indicated broad functional categories underlying AFT, including proteolysis, transcription regulation, chromatin modification, protein kinase activity, and apoptosis.

CONCLUSIONS

The chromosome 4 QTL is a key region containing possibly pleiotropic genes for AFT and drinking behavior. Given that the region contains many viable candidates and a large number of the genes in the interval fall into 1 or more of the enriched functional categories, we postulate that many genes of varying effect size contribute to the observed QTL effect.

Reward Sensitivity of ACC as an Intermediate Phenotype between DRD4-521T and Substance Misuse

The development and expression of the midbrain dopamine system is determined in part by genetic factors that vary across individuals such that dopamine-related genes are partly responsible for addiction vulnerability. However, a complete account of how dopamine-related genes predispose individuals to drug addiction remains to be developed. Adopting an intermediate phenotype approach, we investigated whether reward-related electrophysiological activity of ACC—a cortical region said to utilize dopamine reward signals to learn the value of extended, context-specific sequences of goal-directed behaviors—mediates the influence of multiple dopamine-related functional polymorphisms over substance use. We used structural equation modeling to examine whether two related electrophysiological phenomena associated with the control and reinforcement learning functions of ACC—theta power and the reward positivity—mediated the relationship between the degree of substance misuse and genetic polymorphisms that regulate dopamine processing in frontal cortex. Substance use data were collected from 812 undergraduate students. One hundred ninety-six returned on a subsequent day to participate in an electrophysiological experiment and to provide saliva samples for DNA analysis. We found that these electrophysiological signals mediated a relationship between the DRD4-521T dopamine receptor genotype and substance misuse. Our results provide a theoretical framework that bridges the gap between genes and behavior in drug addiction and illustrate how future interventions might be individually tailored for specific genetic and neurocognitive profiles.

Modeling Multiple Responses via Bootstrapping Margins with an Application to Genetic Association Testing

The need for analysis of multiple responses arises from many applications. In behavioral science, for example, comorbidity is a common phenomenon where multiple disorders occur in the same person. The advantage of jointly analyzing multiple correlated responses has been examined and documented. Due to the difficulties of modeling multiple responses, nonparametric tests such as generalized Kendall's Tau have been developed to assess the association between multiple responses and risk factors. These procedures have been applied to genomewide association studies of multiple complex traits. Unfortunately, those nonparametric tests only provide the significance of the association but not the magnitude. We propose a Gaussian copula model with discrete margins for modeling multivariate binary responses. This model separates marginal effects from between-trait correlations. We use a bootstrapping margins approach to constructing Wald's statistic for the association test. Although our derivation is based on the fully parametric Gaussian copula framework for simplicity, the underlying assumptions to apply our method can be weakened. The bootstrapping margins approach only requires the correct specification of the model margins. Our simulation and real data analysis demonstrate that our proposed method not only increases power over some existing association tests, but also provides further insight into genetic association studies of multivariate traits.

Understanding Addiction as a Developmental Disorder: An Argument for a Developmentally Informed Multilevel Approach

Substance abuse and drug addiction are two of the most common psychopathologies among the general population. While a host of risk factors are associated with the onset of drug abuse and drug addiction, there is a growing body of evidence pointing to the powerful influence of early adverse experiences, both child neglect and maltreatment, as well as drug use and abuse in parents and/or primary caretakers. We consider the case for drug addiction as a developmental disorder, outlining the need to consider the role of genetic, epigenetic, and neurobiological factors alongside experiences of adversity at key stages of development. Such a multilevel approach within a developmental framework has the potential to reframe our understanding of how addiction emerges and is maintained, and is essential if we are to identify the mechanisms underlying this disorder to better inform effective treatment and prevention across the generations.

Cadherin-13, a risk gene for ADHD and comorbid disorders, impacts GABAergic function in hippocampus and cognition

Cadherin-13 (CDH13), a unique glycosylphosphatidylinositol-anchored member of the cadherin family of cell adhesion molecules, has been identified as a risk gene for attention-deficit/hyperactivity disorder (ADHD) and various comorbid neurodevelopmental and psychiatric conditions, including depression, substance abuse, autism spectrum disorder and violent behavior, while the mechanism whereby CDH13 dysfunction influences pathogenesis of neuropsychiatric disorders remains elusive. Here we explored the potential role of CDH13 in the inhibitory modulation of brain activity by investigating synaptic function of GABAergic interneurons. Cellular and subcellular distribution of CDH13 was analyzed in the murine hippocampus and a mouse model with a targeted inactivation of Cdh13 was generated to evaluate how CDH13 modulates synaptic activity of hippocampal interneurons and behavioral domains related to psychopathologic (endo)phenotypes. We show that CDH13 expression in the cornu ammonis (CA) region of the hippocampus is confined to distinct classes of interneurons. Specifically, CDH13 is expressed by numerous parvalbumin and somatostatin-expressing interneurons located in the stratum oriens, where it localizes to both the soma and the presynaptic compartment. Cdh13(-/-) mice show an increase in basal inhibitory, but not excitatory, synaptic transmission in CA1 pyramidal neurons. Associated with these alterations in hippocampal function, Cdh13(-/-) mice display deficits in learning and memory. Taken together, our results indicate that CDH13 is a negative regulator of inhibitory synapses in the hippocampus, and provide insights into how CDH13 dysfunction may contribute to the excitatory/inhibitory imbalance observed in neurodevelopmental disorders, such as ADHD and autism.

Mouse Model for Protein Tyrosine Phosphatase D (PTPRD) Associations with Restless Leg Syndrome or Willis-Ekbom Disease and Addiction: Reduced Expression Alters Locomotion, Sleep Behaviors and Cocaine-Conditioned Place Preference

The receptor type protein tyrosine phosphatase D (PTPRD) gene encodes a cell adhesion molecule likely to influence development and connections of addiction-, locomotion- and sleep-related brain circuits in which it is expressed. The PTPRD gene harbors genome-wide association signals in studies of restless leg syndrome (Willis-Ekbom disease [WED]/restless leg syndrome [RLS]; p < 10^-8) and addiction-related phenotypes (clusters of nearby single nucleotide polymorphisms [SNPs] with 10^-2 > p > 10^-8 associations in several reports). We now report work that seeks (a) association between PTPRD genotypes and expression of its mRNA in postmortem human brains and (b) RLS-related, addiction-related and comparison behavioral phenotypes in hetero- and homozygous PTPRD knockout mice. We identify associations between PTPRD SNPs and levels of PTPRD mRNA in human brain samples that support validity of mouse models with altered PTPRD expression. Knockouts display less behaviorally defined sleep at the end of their active periods. Heterozygotes move more despite motor weakness/impersistence. Heterozygotes display shifted dose-response relationships for cocaine reward. They display greater preference for places paired with 5 mg/kg cocaine and less preference for places paired with 10 or 20 mg/kg. The combined data provide support for roles for common, level-of-expression PTPRD variation in locomotor, sleep and drug reward phenotypes relevant to RLS and addiction. Taken together, mouse and human results identify PTPRD as a novel therapeutic target for RLS and addiction phenotypes.

Altered CSMD1 Expression Alters Cocaine-Conditioned Place Preference: Mutual Support for a Complex Locus from Human and Mouse Models

The CUB and sushi multiple domains 1 (CSMD1) gene harbors signals provided by clusters of nearby SNPs with 10-2 > p > 10-8 associations in genome wide association (GWAS) studies of addiction-related phenotypes. A CSMD1 intron 3 SNP displays p < 10-8 association with schizophrenia and more modest associations with individual differences in performance on tests of cognitive abilities. CSDM1 encodes a cell adhesion molecule likely to influence development, connections and plasticity of brain circuits in which it is expressed. We tested association between CSMD1 genotypes and expression of its mRNA in postmortem human brains (n = 181). Expression of CSMD1 mRNA in human postmortem cerebral cortical samples differs 15-25%, in individuals with different alleles of simple sequence length and SNP polymorphisms located in the gene's third/fifth introns, providing nominal though not Bonferroni-corrected significance. These data support mice with altered CSMD1 expression as models for common human CSMD1 allelic variation. We tested baseline and/or cocaine-evoked addiction, emotion, motor and memory-related behaviors in +/- and -/- csmd1 knockout mice on mixed and on C57-backcrossed genetic backgrounds. Initial csmd1 knockout mice on mixed genetic backgrounds displayed a variety of coat colors and sizable individual differences in responses during behavioral testing. Backcrossed mice displayed uniform black coat colors. Cocaine conditioned place preference testing revealed significant influences of genotype (p = 0.02). Homozygote knockouts displayed poorer performance on aspects of the Morris water maze task. They displayed increased locomotion in some, though not all, environments. The combined data thus support roles for common level-of-expression CSMD1 variation in a drug reward phenotype relevant to addiction and in cognitive differences that might be relevant to schizophrenia. Mouse model results can complement data from human association findings of modest magnitude that identify likely polygenic influences.

Human cell adhesion molecules: annotated functional subtypes and overrepresentation of addiction-associated genes

Human cell adhesion molecules (CAMs) are essential for proper development, modulation, and maintenance of interactions between cells and cell-to-cell (and matrix-to-cell) communication about these interactions. Despite the differential functional significance of these roles, there have been surprisingly few systematic studies to enumerate the universe of CAMs and identify specific CAMs in distinct functions. In this paper, we update and review the set of human genes likely to encode CAMs with searches of databases, literature reviews, and annotations. We describe likely CAMs and functional subclasses, including CAMs that have a primary function in information exchange (iCAMs), CAMs involved in focal adhesions, CAM gene products that are preferentially involved with stereotyped and morphologically identifiable connections between cells (e.g., adherens junctions, gap junctions), and smaller numbers of CAM genes in other classes. We discuss a novel proposed mechanism involving selective anchoring of the constituents of iCAM-containing lipid rafts in zones of close neuronal apposition to membranes expressing iCAM binding partners. We also discuss data from genetic and genomic studies of addiction in humans and mouse models to highlight the ways in which CAM variation may contribute to a specific brain-based disorder such as addiction. Specific examples include changes in CAM mRNA splicing mediated by differences in the addiction-associated splicing regulator RBFOX1/A2BP1 and CAM expression in dopamine neurons.

Bipolar disorder and gambling disorder comorbidity: current evidence and implications for pharmacological treatment

BACKGROUND

The co-occurrence of bipolar disorder (BD) and gambling disorder (GD), though of clinical and public health importance, is still scarcely investigated. Comorbid BD-GD subjects experience a more severe course of illness and poorer treatment outcome, due to a range of clinical and psychosocial factors that collectively impede remission and recovery. The aim of our paper is to review the role of pharmacotherapy in the treatment of comorbid BD-GD, in order to support clinical decisions according to the best available evidence.

METHODS

A qualitative systematic review of studies on pharmacological treatment in comorbid BD-GD was performed. A comprehensive literature search of online databases, bibliographies of published articles and gray literature was conducted. Data on efficacy, safety and tolerability were extracted and levels of evidence were assessed. We also provide a brief overview of current epidemiological, neurobiological and clinical findings, with the intention of proposing a dimensional approach to the choice of available drugs.

RESULTS

The only drug with a high level of evidence is lithium. Considering the inclusion of GD in DSM-5 'Substance-related and Addictive Disorders' category, we discuss the use of other drugs with a high level of evidence currently used in BD subjects with co-occurring substance use disorders.

LIMITATIONS

Only few clinical trials are available and the population is limited; therefore no conclusive evidence can be inferred.

CONCLUSIONS

Further randomized controlled trials are required to evaluate the efficacy of pharmacological treatment strategies in large samples of patients with comorbid BD-GD. Also, attempts should be made to identify other shared clinical and psychopathological domains that are amenable to treatment.

Gene-smoking interactions identify several novel blood pressure loci in the Framingham Heart Study

BACKGROUND

Cardiovascular diseases are among the most significant health problems in the United States. Blood pressure (BP) variability has a genetic component, and most of the genetic variance remains to be identified. One promising strategy for gene discovery is genome-wide analysis of interactions between single nucleotide polymorphisms (SNPs) and environmental factors related to cardiovascular diseases.

METHODS

We investigated SNP-smoking interaction effects on BP in genome-wide data in 6,889 participants from the Framingham Heart Study. We performed the standard 1 degree of freedom (df) test of the interaction effect and the joint 2 df test of main and interaction effects. Three smoking measures were used: cigarettes per day (CPD), pack years of smoking, and smoking status.

RESULTS

We identified 7 significant and 21 suggestive BP loci. Identified through the joint 2 df test, significant SBP loci include: rs12149862 (P = 3.65×10(-9)) in CYB5B, rs2268365 (P = 4.85×10(-8)) in LRP2, rs133980 (P = 1.71×10(-8) with CPD and P = 1.07×10(-8) with pack-years) near MN1, and rs12634933 (P = 4.05×10(-8)) in MECOM. Through 1 df interaction analysis, 1 suggestive SBP locus at SNP rs8010717 near NRXN3 was identified using all 3 smoking measures (P = 3.27×10(-7) with CPD, P = 1.03×10(-7) with pack-years, and P = 1.19×10(-7) with smoking status).

CONCLUSIONS

Several of these BP loci are biologically plausible, providing physiological connection to BP regulation. Our study demonstrates that SNP-smoking interactions can enhance gene discovery and provide insight into novel pathways and mechanisms regulating BP.

Chronic methamphetamine abuse and corticostriatal deficits revealed by neuroimaging

Despite aggressive efforts to contain it, methamphetamine use disorder continues to be major public health problem; and with generic behavioral therapies still the mainstay of treatment for methamphetamine abuse, rates of attrition and relapse remain high. This review summarizes the findings of structural, molecular, and functional neuroimaging studies of methamphetamine abusers, focusing on cortical and striatal abnormalities and their potential contributions to cognitive and behavioral phenotypes that can serve to promote compulsive drug use. These studies indicate that individuals with a history of chronic methamphetamine abuse often display several signs of corticostriatal dysfunction, including abnormal gray- and white-matter integrity, monoamine neurotransmitter system deficiencies, neuroinflammation, poor neuronal integrity, and aberrant patterns of brain connectivity and function, both when engaged in cognitive tasks and at rest. More importantly, many of these neural abnormalities were found to be linked with certain addiction-related phenotypes that may influence treatment response (e.g., poor self-control, cognitive inflexibility, maladaptive decision-making), raising the possibility that they may represent novel therapeutic targets.

TARV: tree-based analysis of rare variants identifying risk modifying variants in CTNNA2 and CNTNAP2 for alcohol addiction

Since the development of next generation sequencing (NGS) technology, researchers have been extending their efforts on genome-wide association studies (GWAS) from common variants to rare variants to find the missing inheritance. Although various statistical methods have been proposed to analyze rare variants data, they generally face difficulties for complex disease models involving multiple genes. In this paper, we propose a tree-based analysis of rare variants (TARV) that adopts a nonparametric disease model and is capable of exploring gene-gene interactions. We found that TARV outperforms the sequence kernel association test (SKAT) in most of our simulation scenarios, and by notable margins in some cases. By applying TARV to the study of addiction: genetics and environment (SAGE) data, we successfully detected gene CTNNA2 and its 43 specific variants that increase the risk of alcoholism in women, with an odds ratio (OR) of 1.94. This gene has not been detected in the SAGE data. Post hoc literature search also supports the role of CTNNA2 as a likely risk gene for alcohol addiction. In addition, we also detected a plausible protective gene CNTNAP2, whose 97 rare variants can reduce the risk of alcoholism in women, with an OR of 0.55. These findings suggest that TARV can be effective in dissecting genetic variants for complex diseases using rare variants data.

Association of the HTR2A gene with alcohol and heroin abuse

Positive genetic associations of rs6313 (102T/C at exon 1) and rs6311 (-1438A/G) on the 5-hydroxytryptamine (serotonin) 2A receptor gene (HTR2A or 5-HT2A) were reported for alcohol and drug abuse; however, other association studies failed to produce consistent results supporting the susceptibility of the two single nucleotide polymorphisms (SNPs). To clarify the associations of the HTR2A gene with substance use disorders, we performed a meta-analysis based on the genotypes from the available candidate gene association studies of the two SNPs with alcohol and drug abuse from multiple populations. Evidence of association was found for HTR2A rs6313 in all the combined studies (e.g., allelic P = 0.0048 and OR 0.86, 95 % CI 0.77-0.95) and also in the combined studies of alcohol dependence (abuse) (e.g., allelic P = 0.0001 and OR 0.71, 95 % CI 0.59-0.85). The same association trend was also observed in the Study of Addiction: Genetics and Environment datasets. The meta-analysis supports a contribution of the HTR2A gene to the susceptibility to substance use disorders, particularly alcohol dependence.

Psychometric properties of the Icelandic NEO-FFI in a general population sample compared to a sample recruited for a study on the genetics of addiction

Personality traits are major determinants of social behavior influencing various diseases including addiction. Twin and family studies suggest personality and addiction to be under genetic influence. Identification of DNA susceptibility variants relies on valid and reliable phenotyping approaches. We present results of psychometric testing of the Icelandic NEO-FFI in a population sample (N=657) and a sample recruited for a study on addiction genetics (N=3,804). The Icelandic NEO-FFI demonstrated internal consistency and temporal stability. Factor analyses supported the five-factor structure. Icelandic norms were compared to American norms and language translations selected for geographical and cultural proximity to Iceland. Multiple discriminant function analysis using NEO-FFI trait scores and gender as independent variables predicted membership in recruitment groups for 47.3% of addiction study cases (N=3,804), with accurate predictions made for 69.5% of individuals with treated addiction and 43.3% of their first-degree relatives. Correlations between NEO-FFI scores and the discriminant function suggested a combination of high neuroticism, low conscientiousness and low agreeableness predicted membership in the Treated group.

Allostasis as a conceptual framework linking bipolar disorder and addiction

Bipolar disorders (BDs) and addictions constitute reciprocal risk factors and are best considered under a unitary perspective. The concepts of allostasis and allostatic load (AL) may contribute to the understanding of the complex relationships between BD and addictive behaviors. Allostasis entails the safeguarding of reward function stability by recruitment of changes in the reward and stress system neurocircuitry and it may help to elucidate neurobiological underpinnings of vulnerability to addiction in BD patients. Conceptualizing BD as an illness involving the cumulative build-up of allostatic states, we hypothesize a progressive dysregulation of reward circuits clinically expressed as negative affective states (i.e., anhedonia). Such negative affective states may render BD patients more vulnerable to drug addiction, fostering a very rapid transition from occasional drug use to addiction, through mechanisms of negative reinforcement. The resulting addictive behavior-related ALs, in turn, may contribute to illness progression. This framework could have a heuristic value to enhance research on pathophysiology and treatment of BD and addiction comorbidity.

Curious cases: Altered dose-response relationships in addiction genetics

Dose-response relationships for most addictive substances are "inverted U"-shaped. Addictive substances produce both positive features that include reward, euphoria, anxiolysis, withdrawal-relief, and negative features that include aversion, dysphoria, anxiety and withdrawal symptoms. A simple model differentially associates ascending and descending limbs of dose-response curves with rewarding and aversive influences, respectively. However, Diagnostic and Statistical Manual (DSM) diagnoses of substance dependence fail to incorporate dose-response criteria and don't directly consider balances between euphoric and dysphoric drug effects. Classical genetic studies document substantial heritable influences on DSM substance dependence. Linkage and genome-wide association studies identify modest-sized effects at any locus. Nevertheless, clusters of SNPs within selected genes display 10(-2)>p>10(-8) associations with dependence in many independent samples. For several of these genes, evidence for cis-regulatory, level-of-expression differences supports the validity of mouse models in which levels of expression are also altered. This review documents surprising, recently defined cases in which convergent evidence from humans and mouse models supports central influences of altered dose-response relationships in mediating the impact of relevant genomic variation on addiction phenotypes. For variation at loci for the α5 nicotinic acetylcholine receptor, cadherin 13, receptor type protein tyrosine phosphatase Δ and neuronal cell adhesion molecule genes, changed dose-response relationships conferred by gene knockouts in mice are accompanied by supporting human data. These observations emphasize desirability of carefully elucidating dose-response relationships for both rewarding and aversive features of abused substances wherever possible. They motivate consideration of individual differences in dose-response relationships in addiction nosology and therapeutics.

Multi-cultural association of the serotonin transporter gene (SLC6A4) with substance use disorder

A number of studies have reported associations between the serotonin transporter gene (SLC6A4) and alcohol, heroin, cocaine, or methamphetamine abuse. Other studies have yielded contrary results. There are a number of reasons for non-replication, including inadequate statistical power, population stratification, and poor phenotype definition. This study was to test the association using a meta-analytic approach across a variety of racial and ethnic populations. Using the genotype data of 55 studies (7999 cases, 8264 controls, and 676 families or parent-offspring trios) published in the past 15 years, we have conducted comprehensive meta-analyses to examine the associations of the 5-HTTLPR and STin2 polymorphisms with substance use disorder. The meta-analyses support the associations of 5-HTTLPR with alcohol, heroin, cocaine, and methamphetamine dependence and abuse (eg, the smallest P-values were 0.0058 with odds ratio (OR)=0.54 (0.35, 0.84); 0.0024 with OR=0.77 (0.66, 0.91); 0.018 with OR=1.38 (1.06, 1.81); and 0.028 with OR=0.46 (0.23, 0.92) for alcohol, heroin, cocaine, and methamphetamine dependence/abuse, respectively). When all the phenotypes are combined, the P-value was 0.0006 with OR=0.86 (0.78, 0.94) in the combined European, Asian, and Mexican populations and P-value was 0.0028 with OR=1.41 (1.13, 1.78) in the African populations. Evidence of significant associations was also identified in other subgroup analyses regarding differently combined substance and populations. The effect sizes of 5-HTTLPR were comparable among the European, Asian, and Mexican populations, however, the risk allele was more frequent in Asians than in Europeans and Mexicans. The opposite directions of risk allele in African population might be driven by the opposite directions of risk allele in cocaine dependence. This meta-analysis supports that the association of the SLC6A4 gene with substance use disorder varies depending on substances with different risk allele frequencies in the multi-cultural populations. Further studies using larger sample size are warranted.

Implications of genome wide association studies for addiction: are our a priori assumptions all wrong?

Substantial genetic contributions to addiction vulnerability are supported by data from twin studies, linkage studies, candidate gene association studies and, more recently, Genome Wide Association Studies (GWAS). Parallel to this work, animal studies have attempted to identify the genes that may contribute to responses to addictive drugs and addiction liability, initially focusing upon genes for the targets of the major drugs of abuse. These studies identified genes/proteins that affect responses to drugs of abuse; however, this does not necessarily mean that variation in these genes contributes to the genetic component of addiction liability. One of the major problems with initial linkage and candidate gene studies was an a priori focus on the genes thought to be involved in addiction based upon the known contributions of those proteins to drug actions, making the identification of novel genes unlikely. The GWAS approach is systematic and agnostic to such a priori assumptions. From the numerous GWAS now completed several conclusions may be drawn: (1) addiction is highly polygenic; each allelic variant contributing in a small, additive fashion to addiction vulnerability; (2) unexpected, compared to our a priori assumptions, classes of genes are most important in explaining addiction vulnerability; (3) although substantial genetic heterogeneity exists, there is substantial convergence of GWAS signals on particular genes. This review traces the history of this research; from initial transgenic mouse models based upon candidate gene and linkage studies, through the progression of GWAS for addiction and nicotine cessation, to the current human and transgenic mouse studies post-GWAS.

Genetic matters: thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.

Impact of the ADHD-susceptibility gene CDH13 on development and function of brain networks

Attention-deficit/hyperactivity disorder (ADHD) is a common, early onset and enduring neuropsychiatric disorder characterized by developmentally inappropriate inattention, hyperactivity, increased impulsivity and motivational/emotional dysregulation with similar prevalence rates throughout different cultural settings. Persistence of ADHD into adulthood is associated with considerable risk for co-morbidities such as depression and substance use disorder. Although the substantial heritability of ADHD is well documented the etiology is characterized by a complex coherence of genetic and environmental factors rendering identification of risk genes difficult. Genome-wide linkage as well as single nucleotide polymorphism (SNP) and copy-number variant (CNV) association scans recently allow to reliably define aetiopathogenesis-related genes. A considerable number of novel ADHD risk genes implicate biological processes involved in neurite outgrowth and axon guidance. Here, we focus on the gene encoding Cadherin-13 (CDH13), a cell adhesion molecule which was replicably associated with liability to ADHD and related neuropsychiatric conditions. Based on its unique expression pattern in the brain, we discuss the molecular structure and neuronal mechanisms of Cadherin-13 in relation to other cadherins and the cardiovascular system. An appraisal of various Cadherin-13-modulated signaling pathways impacting proliferation, migration and connectivity of specific neurons is also provided. Finally, we develop an integrative hypothesis of the mechanisms in which Cadherin-13 plays a central role in the regulation of brain network development, plasticity and function. The review concludes with emerging concepts about alterations in Cadherin-13 signaling contributing to the pathophysiology of neurodevelopmental disorders.

The synaptic adhesion molecule SynCAM 1 contributes to cocaine effects on synapse structure and psychostimulant behavior

Drugs of abuse have acute and persistent effects on synapse structure and addiction-related behaviors. Trans-synaptic interactions can control synapse development, and synaptic cell adhesion molecule (SynCAM) proteins (also named nectin-like molecules) are immunoglobulin adhesion proteins that span the synaptic cleft and induce excitatory synapses. Our studies now reveal that the loss of SynCAM 1 in knockout (KO) mice reduces excitatory synapse number in nucleus accumbens (NAc). SynCAM 1 additionally contributes to the structural remodeling of NAc synapses in response to the psychostimulant cocaine. Specifically, we find that cocaine administration increases the density of stubby spines on medium spiny neurons in NAc, and that maintaining this increase requires SynCAM 1. Furthermore, mushroom-type spines on these neurons are structurally more plastic when SynCAM 1 is absent, and challenging drug-withdrawn mice with cocaine shortens these spines in SynCAM 1 KO mice. These effects are correlated with changes on the behavioral level, where SynCAM 1 contributes to the psychostimulant effects of cocaine as measured after acute and repeated administration, and in drug-withdrawn mice. Together, our results provide evidence that the loss of a synapse-organizing adhesion molecule can modulate cocaine effects on spine structures in NAc and increases vulnerability to the behavioral actions of cocaine. SynCAM-dependent pathways may therefore represent novel points of therapeutic intervention after exposure to drugs of abuse.

Associations of the 5-hydroxytryptamine (serotonin) receptor 1B gene (HTR1B) with alcohol, cocaine, and heroin abuse

Abnormal serotonergic pathways are implicated in numerous neuropsychiatric disorders including alcohol and drug dependence (abuse). The human 5-hydroxytryptamine (serotonin) receptor 1B, encoded by the HTR1B (5-HT1B) gene, is a presynaptic serotonin autoreceptor that plays an important role in regulating serotonin synthesis and release. Although there was evidence of associations of the HTR1B gene variants in the etiologies of substance use disorders, negative findings were also reported. To clarify the roles of commonly reported single nucleotide polymorphisms (SNPs) of the HTR1B gene underlying alcohol and drug dependence (abuse), we performed a meta-analysis based on the available genotype data from individual candidate gene-based association studies. Evidence of association was found between the functional SNP -161A>T (rs130058) and alcohol, cocaine, and heroin dependence (e.g., P = 0.03 and odds ratio (OR) = 1.2 (1.02, 1.42) in the combined European, Asian, African, and Hispanic populations). SNP -261T>G (rs11568817) also showed evidence of association but with different directions in Europeans and non-Europeans (e.g., P = 0.0018 with OR = 1.42 (1.14, 1.76) and P = 0.01 with ORs = 0.5 (0.3, 0.85), respectively). This meta-analysis supports the associations of HTR1B -261T>G and -161A>T with alcohol and drug abuse and further investigations are warranted in larger samples.

Cocaine self-administration in mice with forebrain knock-down of trpc5 ion channels

Canonical transient receptor potential (TRPC) channels are a family of non-selective cation channels that play a crucial role in modulating neuronal excitability due to their involvement in intracellular Ca2+ regulation and dendritic growth. TRPC5 channels a) are one of the two most prevalent TRPC channels in the adult rodent brain; b) are densely expressed in deep layer pyramidal neurons of the prefrontal cortex (PFC); and c) modulate neuronal persistent activity necessary for working memory and attention. In order to evaluate the causal role of TRPC5 in motivation/reward-related behaviors, conditional forebrain TRPC5 knock-down (trpc5-KD) mice were generated and trained to nose-poke for intravenous cocaine. Here we present a data set containing the first 6 days of saline or cocaine self-administration in wild type (WT) and trpc5-KD mice. In addition, we also present a data set showing the dose-response to cocaine after both groups had achieved similar levels of cocaine self-administration. Compared to WT mice, trpc5-KD mice exhibited an apparent increase in self-administration on the first day of cocaine testing without prior operant training. There were no apparent differences between WT and trpc5-KD mice for saline responding on the first day of training. Both groups showed similar dose-response sensitivity to cocaine after several days of achieving similar levels of cocaine intake.

Four factors for the initiation of substance use by young adulthood: a 10-year follow-up twin and sibling study of marital conflict, monitoring, siblings, and peers

This study examined genetic and environmental influences on associations among marital conflict about the child, parental monitoring, sibling relationship negativity, and peer delinquency during adolescence and initiation of illegal drug use by young adulthood. The sample comprised data collected longitudinally from same-sex sibling pairs and parents when the siblings were 10-18 years old (M = 14.5 and 12.9 years for Child 1 and Child 2, respectively) and 20-35 years old (M = 26.8 and 25.5 years for Child 1 and Child 2, respectively). Findings indicate four factors that explain the initiation of illegal drug use: two shaped by genetic influences and two shaped by environments shared by siblings. The two genetically shaped factors probably have distinct mechanisms: one a child-initiated coercive process in the family and the other parent and peer processes shaped by the child's disclosure. The environmentally influenced factors seem distinctively shaped by poor parental monitoring of both sibs and the effects of siblings on each other's deviancy.