Genetics of dopamine receptors and drug addiction

Dopamine plays a key role in reward behavior, yet the association of drug dependence as a chronic, relapsing disorder with the genes encoding the various dopaminergic receptor subtypes remains difficult to delineate. In the context of subsequent genome-wide association (GWAS) research and post-GWAS investigations, we summarize the novel data that link genes encoding molecules involved in the dopaminergic system (dopamine receptors, transporter and enzymes in charge of its metabolism) to drug addiction. Recent reports indicate that the heritability of drug addiction should be high enough to allow a significant role for a specific set of genes, and the available genetic studies, which might not be already conclusive because of the heterogeneity of designs, methods and recruited samples, should support the idea of a significant role of at least one gene related to dopaminergic system. Evolutionary changes in primates and non-primate animals of genes coding for molecules involved in dopaminergic system highlight why addictive disorders are mainly limited to humans. Restricting the analyses to more specific intermediate phenotypes (or endophenotypes) such as attention allocation, stress reactivity, novelty seeking, behavioral disinhibition and impulsivity, instead of the broad addictive disorder concept can be instrumental to identify novel genes associated with these traits in the context of genome-wide studies.     

Linking genes to brain,behavior and neurological diseases: what can we learn from zebrafish?

How our brain is wired and subsequently generates functional output, ranging from sensing and locomotion to emotion, decision-making and learning and memory, remains poorly understood. Dys-regulation of these processes can lead to neurodegenerative, as well as neuro-psychiatric, disorders. Molecular genetic together with behavioral analyses in model organisms identify genes involved in the formation of neuronal circuits, the execution of behavior and mechanisms involved in neuro-pathogenesis. In this review I will discuss the current progress and future potential for study in a newly established vertebrate model organism for genetics, the zebrafish Danio rerio . Where available, schemes and results of genetic screens will be reviewed concerning the sensory, motor and neuromodulatory monoamine systems. Genetic analyses in zebrafish have the potential to provide important insights into the relationship between genes, neuronal circuits and behavior in normal as well as diseased states.     

Translational genetic approaches to substance use disorders: bridging the gap between mice and humans

While substance abuse disorders only occur in humans, mice and other model organisms can make valuable contributions to genetic studies of these disorders. In this review, we consider a few specific examples of how model organisms have been used in conjunction with studies in humans to study the role of genetic factors in substance use disorders. In some examples genes that were first discovered in mice were subsequently studied in humans. In other examples genes or specific polymorphisms in genes were first studied in humans and then modeled in mice. Using anatomically and temporally specific genetic, pharmacological and other environmental manipulations in conjunction with histological analyses, mechanistic insights that would be difficult to obtain in humans have been obtained in mice. We hope these examples illustrate how novel biological insights about the effect of genes on substance use disorders can be obtained when mouse and human genetic studies are successfully integrated.     

Review. Genetics of addictions: strategies for addressing heterogeneity and polygenicity of substance use disorders

Addictions are common psychiatric disorders that exert high cost to the individual and to society. Addictions are a result of the interplay of multiple genetic and environmental factors. They are characterized by phenotypic and genetic heterogeneity as well as polygenicity, implying a contribution of different neurobiological mechanisms to the clinical diagnosis. Therefore, treatments for most substance use disorders are often only partially effective, with a substantial proportion of patients failing to respond. To address heterogeneity and polygenicity, strategies have been developed to identify more homogeneous subgroups of patients and to characterize genes contributing to their phenotype. These include genetic linkage and association studies as well as functional genetic analysis using endophenotypes and animal behavioural experimentation. Applying these strategies in a translational context aims at improving therapeutic response by the identification of subgroups of addiction patients for individualized, targeted treatment strategies. This article aims to discuss strategies addressing heterogeneity and polygenicity of substance use disorders by presenting results of recent research on genetic and environmental components of addiction. It will also introduce the European IMAGEN study that aims to integrate methodical approaches discussed in order to identify the genetic and neurobiological basis of behavioural traits relevant to the development of addictions.     

Zebrafish as a Model for Human Ciliopathies

Cilia, microtubule-based structures found on the surface of almost all vertebrate cells, play an array of diverse biological functions. Abnormal ciliary axonemal structure and function can result in a class of genetic disorders that are collectively termed ciliopathies. Model organisms,including Chlamydomonas reinhardtii and Caenorhabditis elegans have been widely used to study the complex genetic basis of ciliopathies.Here, we review the advantages of the zebrafish as a vertebrate model for human ciliopathies. We summarize the features of zebrafish cilia, and the major findings and contributions of the zebrafish model in recent studies of human ciliopathies. We also discuss the new genome editing approaches being efficiently used in zebrafish, and the exciting prospects of these approaches in modeling human ciliopathies.     

The role of α-synuclein in the pathophysiology of alcoholism

Alcoholism has complex etiology and there is evidence for both genetic and environmental factors in its pathophysiology. Chronic, long-term alcohol abuse and alcohol dependence are associated with neuronal loss with the prefrontal cortex being particularly susceptible to neurotoxic damage. This brain region is involved in the development and persistence of alcohol addiction and neurotoxic damage is likely to exacerbate the reinforcing effects of alcohol and may hinder treatment. Understanding the mechanism of alcohol’s neurotoxic effects on the brain and the genetic risk factors associated with alcohol abuse are the focus of current research. Because of its well-established role in neurodegenerative and neuropsychological disorders, and its emerging role in the pathophysiology of addiction, here we review the genetic and epigenetic factors involved in regulating α-synuclein expression and its potential role in the pathophysiology of chronic alcohol abuse. Elucidation of the mechanisms of α-synuclein regulation may prove beneficial in understanding the role of this key synaptic protein in disease and its potential for therapeutic modulation in the treatment of substance use disorders as well as other neurodegenerative diseases.     

Zebrafish models flex their muscles to shed light on muscular dystrophies

Muscular dystrophies are a group of genetic disorders that specifically affect skeletal muscle and are characterized by progressive muscle degeneration and weakening. To develop therapies and treatments for these diseases, a better understanding of the molecular basis of muscular dystrophies is required. Thus, identification of causative genes mutated in specific disorders and the study of relevant animal models are imperative. Zebrafish genetic models of human muscle disorders often closely resemble disease pathogenesis, and the optical clarity of zebrafish embryos and larvae enables visualization of dynamic molecular processes in vivo. As an adjunct tool, morpholino studies provide insight into the molecular function of genes and allow rapid assessment of candidate genes for human muscular dystrophies. This unique set of attributes makes the zebrafish model system particularly valuable for the study of muscle diseases. This review discusses how recent research using zebrafish has shed light on the pathological basis of muscular dystrophies, with particular focus on the muscle cell membrane and the linkage between the myofibre cytoskeleton and the extracellular matrix.     

G-protein-coupled receptors and asthma endophenotypes: the cysteinyl leukotriene system in perspective

Genetic variation in specific G-protein coupled receptors (GPCRs) is associated with a spectrum of respiratory disease predispositions and drug response phenotypes. Although certain GPCR gene variants can be disease-causing through the expression of inactive, overactive, or constitutively active receptor proteins, many more GPCR gene variants confer risk for potentially deleterious endophenotypes. Endophenotypes are traits, such as bronchiole hyperactivity, atopy, and aspirin intolerant asthma, which have a strong genetic component and are risk factors for a variety of more complex outcomes that may include disease states. GPCR genes implicated in asthma endophenotypes include variants of the cysteinyl leukotriene receptors (CYSLTR1 and CYSLTR2), and prostaglandin D2 receptors (PTGDR and CRTH2), thromboxane A2 receptor (TBXA2R), beta2-adrenergic receptor (ADRB2), chemokine receptor 5 (CCR5), and the G protein-coupled receptor associated with asthma (GPRA). This review of the contribution of variability in these genes places the contribution of the cysteinyl leukotriene system to respiratory endophenotypes in perspective. The genetic variant(s) of receptors that are associated with endophenotypes are discussed in the context of the extent to which they contribute to a disease phenotype or altered drug efficacy.     

Endophenotypes as a measure of suicidality

Suicide is thought to result from the harmful interaction of multiple factors that have social, environmental, neurobiological, and genetic backgrounds. Recent studies have suggested that genetic predisposition to suicidal behavior may be independent of the risk of suicide associated to mental disorders, such as affective disorders, schizophrenia, or alcohol dependence. Given the suicidal behavior heterogeneity and its hereditary complexity, the need to find demonstrable intermediate phenotypes that may make it possible to establish links between genes and suicide behaviors (endophenotypes) seems to be necessary. The main objective of this review was to consider the candidate endophenotypes of suicidal behaviors. Due to the recent advances in neuroimaging, we also characterize brain regions implicated in vulnerability to suicide behavior that are influenced by gene polymorphisms associated with suicidal behavior.     

In search of predictive endophenotypes in addiction: insights from preclinical research

Drug addiction is widely recognized to afflict some but not all individuals by virtue of underlying risk markers and traits involving multifaceted interactions between polygenic and external factors. Remarkably, only a small proportion of individuals exposed to licit and illicit drugs develop compulsive drug‐seeking behavior, maintained in the face of adverse consequences and associated detrimental patterns of drug intake involving extended and repeated bouts of binge intoxication, withdrawal and relapse. As a consequence, research has increasingly endeavored to identify distinctive neurobehavioral mechanisms and endophenotypes that predispose individuals to compulsive drug use. However, research in active drug users is hampered by the difficulty in categorizing putatively causal behavioral traits prior to the initiation of drug use. By contrast, research in experimental animals is often hindered by the validity of approaches used to investigate the neural and psychological mechanisms of compulsive drug‐seeking habits in humans. Herein, we survey and discuss the principal findings emanating from preclinical animal research on addiction and highlight how specific behavioral endophenotypes of presumed genetic origin (e.g. trait anxiety, novelty preference and impulsivity) differentially contribute to compulsive forms of drug seeking and taking and, in particular, how these differentiate between different classes of stimulant and non‐stimulant drugs of abuse.     

1001 model organisms to study cilia and flagella

Most mammalian cell types have the potential to assemble at least one cilium. Immotile cilia participate in numerous sensing processes, while motile cilia are involved in cell motility and movement of extracellular fluid. The functional importance of cilia and flagella is highlighted by the growing list of diseases due to cilia defects. These ciliopathies are marked by an amazing diversity of clinical manifestations and an often complex genetic aetiology. To understand these pathologies, a precise comprehension of the biology of cilia and flagella is required. These organelles are remarkably well conserved throughout eukaryotic evolution. In this review, we describe the strengths of various model organisms to decipher diverse aspects of cilia and flagella biology: molecular composition, mode of assembly, sensing and motility mechanisms and functions. Pioneering studies carried out in the green alga Chlamydomonas established the link between cilia and several genetic diseases. Moreover, multicellular organisms such as mouse, zebrafish, Xenopus, Caenorhabditis elegans or Drosophila, and protists such as Paramecium, Tetrahymena and Trypanosoma or Leishmania each bring specific advantages to the study of cilium biology. For example, the function of genes involved in primary ciliary dyskinesia (due to defects in ciliary motility) can be efficiently assessed in trypanosomes.     

Dissecting cause and effect in the pathogenesis of psychiatric disorders: genes, environment and behaviour

It has long been established that the development of psychiatric illness results from a complex interplay between genetic and environmental factors. Postmortem and genetic linkage studies have identified a number of promising candidate genes which have been reinforced by replication and functional studies. However, the fact that concordance rates for monozygotic twins rarely approach 100% highlights the involvement of environmental factors. Whilst epidemiological studies of psychiatric cohorts have demonstrated potential risk factors, such studies are clearly limited and in many cases the potential mechanism linking a given risk factor with pathogenesis remains unclear. A very powerful method of elucidating the mechanisms underlying gene-environment interactions is the use of appropriate animal models of psychiatric pathology. Whilst animals cannot be used to map the entire complexity of diseases such as schizophrenia, dissecting the symptom profile into more simply encapsulated traits or endophenotypes has proved to be a successful approach. Such endophenotypes provide a measurable link between aetiological factors and phenotypic outcome. Given the potential for the careful control and modification of an experimental animal's environment, the combination of studies of candidate genes with investigations of environmental factors is an effective heuristic tool, allowing examination of behavioural endophenotypes in conjunction with cellular and molecular outcomes. This review will consider the extant genetic, molecular, pharmacological and lesion-based models of psychiatric disorders, and the relevant methods of environmental manipulation appearing in the literature. We will discuss studies where such models have been combined, and the potential for future experimentation in this area.     

“Casting” light on the role of glycosylation during embryonic development: Insights from zebrafish

Zebrafish (Danio rerio) remains a versatile model organism for the investigation of early development and organogenesis, and has emerged as a valuable platform for drug discovery and toxicity evaluation [1–6]. Harnessing the genetic power and experimental accessibility of this system, three decades of research have identified key genes and pathways that control the development of multiple organ systems and tissues, including the heart, kidney, and craniofacial cartilage, as well as the hematopoietic, vascular, and central and peripheral nervous systems [7–31]. In addition to their application in large mutagenic screens, zebrafish has been used to model a variety of diseases such as diabetes, polycystic kidney disease, muscular dystrophy and cancer [32–36]. As this work continues to intersect with cellular pathways and processes such as lipid metabolism, glycosylation and vesicle trafficking, investigators are often faced with the challenge of determining the degree to which these pathways are functionally conserved in zebrafish. While they share a high degree of genetic homology with mouse and human, the manner in which cellular pathways are regulated in zebrafish during early development, and the differences in the organ physiology, warrant consideration before functional studies can be effectively interpreted and compared with other vertebrate systems. This point is particularly relevant for glycosylation since an understanding of the glycan diversity and the mechanisms that control glycan biosynthesis during zebrafish embryogenesis (as in many organisms) is still developing. Nonetheless, a growing number of studies in zebrafish have begun to cast light on the functional roles of specific classes of glycans during organ and tissue development. While many of the initial efforts involved characterizing identified mutants in a number of glycosylation pathways, the use of reverse genetic approaches to directly model glycosylation-related disorders is now increasingly popular. In this review, the glycomics of zebrafish and the developmental expression of their glycans will be briefly summarized along with recent chemical biology approaches to visualize certain classes of glycans within developing embryos. Work regarding the role of protein-bound glycans and glycosaminoglycans (GAG) in zebrafish development and organogenesis will also be highlighted. Lastly, future opportunities and challenges in the expanding field of zebrafish glycobiology are discussed.     

The genetics of the opioid system and specific drug addictions

Addiction to drugs is a chronic, relapsing brain disease that has major medical, social, and economic complications. It has been established that genetic factors contribute to the vulnerability to develop drug addiction and to the effectiveness of its treatment. Identification of these factors may increase our understanding of the disorders, help in the development of new treatments and advance personalized medicine. In this review, we will describe the genetics of the major genes of the opioid system (opioid receptors and their endogenous ligands) in connection to addiction to opioids, cocaine, alcohol and methamphetamines. Particular emphasis is given to association and functional studies of specific variants. We will provide information on the sample populations and the size of each study, as well as a list of the variants implicated in association with addiction-related phenotypes, and with the effectiveness of pharmacotherapy for addiction.     

Heroin addiction in African Americans: a hypothesis-driven association study

Heroin addiction is a chronic complex disease with a substantial genetic contribution. This study was designed to identify gene variants associated with heroin addiction in African Americans. The emphasis was on genes involved in reward modulation, behavioral control, cognitive function, signal transduction and stress response. We have performed a case–control association analysis by screening with 1350 variants of 130 genes. The sample consisted of 202 former severe heroin addicts in methadone treatment and 167 healthy controls with no history of drug abuse. Single nucleotide polymorphism (SNP), haplotype and multi-SNP genotype pattern analyses were performed. Seventeen SNPs showed point-wise significant association with heroin addiction (nominal P < 0.01). These SNPs are from genes encoding several receptors: adrenergic ( ADRA1A ), arginine vasopressin ( AVPR1A ), cholinergic ( CHRM2 ), dopamine (DRD1 ), GABA-A ( GABRB3 ), glutamate ( GRIN2A ) and serotonin ( HTR3A ) as well as alcohol dehydrogenase ( ADH7 ), glutamic acid decarboxylase ( GAD1 and GAD2 ), the nucleoside transporter ( SLC29A1 ) and diazepam-binding inhibitor ( DBI ). The most significant result of the analyses was obtained for the GRIN2A haplotype G-A-T (rs4587976-rs1071502-rs1366076) with protective effect ( P _uncorrected = 9.6E- 05, P _corrected = 0.058). This study corroborates several reported associations with alcohol and drug addiction as well as other related disorders and extends the list of variants that may affect the development of heroin addiction. Further studies will be necessary to replicate these associations and to elucidate the roles of these variants in drug addiction vulnerability.     

Phenotype Ontologies and Cross-Species Analysis for Translational Research

The use of model organisms as tools for the investigation of human genetic variation has significantly and rapidly advanced our understanding of the aetiologies underlying hereditary traits. However, while equivalences in the DNA sequence of two species may be readily inferred through evolutionary models, the identification of equivalence in the phenotypic consequences resulting from comparable genetic variation is far from straightforward, limiting the value of the modelling paradigm. In this review, we provide an overview of the emerging statistical and computational approaches to objectively identify phenotypic equivalence between human and model organisms with examples from the vertebrate models, mouse and zebrafish. Firstly, we discuss enrichment approaches, which deem the most frequent phenotype among the orthologues of a set of genes associated with a common human phenotype as the orthologous phenotype, or phenolog, in the model species. Secondly, we introduce and discuss computational reasoning approaches to identify phenotypic equivalences made possible through the development of intra- and interspecies ontologies. Finally, we consider the particular challenges involved in modelling neuropsychiatric disorders, which illustrate many of the remaining difficulties in developing comprehensive and unequivocal interspecies phenotype mappings.     

Use of herpes virus amplicon vectors to study brain disorders

There is an enormous initiative to establish the genetic basis for disorders of brain function. Unfortunately, genetic intervention is not accomplished easily in the nervous system. One strategy is to engineer and deliver to neurons specialized viral vectors that carry a gene (or genes) of interest, thereby exploiting the natural ability of viruses to insert genetic material into cells. When delivered to brain cells, these vectors cause infected cells to increase the expression of the genes of interest. The ability to deliver genes into neurons in vitro and in vivo with herpes simplex virus (HSV) amplicon vectors has made it possible to carry out exactly these sorts of experiments. This technology has the potential to offer new insights into the etiology of a wide variety of neuropsychiatric disorders. We describe the use of HSV amplicon vectors to study Alzheimer disease, drug addiction, and depression, and discuss the considerations that enter into the use of these vectors both in vitro and in vivo. The HSV amplicon virus is a user-friendly vector for the delivery of genes into neurons that has come of age for the study of brain function.     

The zebrafish as a model system for assessing the reinforcing properties of drugs of abuse

Recent reports make use of the zebrafish to study complex behavior such as addiction, anxiety, or learning and memory. We have established reliable tests and appropriate controls to measure these behavioral parameters in the zebrafish adult. Our assays are robust enough to permit the detection of dominant mutations affecting drug-induced reward, and therefore can be used in forward genetic screens. We provide the reader with the technical details of these tests, as well as their appropriate and crucial, although often overlooked, control assays. In particular, our results make it possible to use the zebrafish as a promising model to identify new genetic components of the reward pathway, or other measurable behaviors.     

Zebrafish as a model organism for nutrition and growth: towards comparative studies of nutritional genomics applied to aquacultured fishes

Zebrafish (Danio rerio) is a common research model in fish studies of toxicology, developmental biology, neurobiology and molecular genetics; it has been proposed as a possible model organism for nutrition and growth studies in fish. The advantages of working with zebrafish in these areas are their small size, short generation time (12–14 weeks) and their capacity to produce numerous eggs (100–200 eggs/clutch). Since a wide variety of molecular tools and information are available for genomic analysis, zebrafish has also been proposed as a model for nutritional genomic studies in fish. The detailed study of every species employed as a model organism is important because these species are used to generalize how several biological processes occur in related organisms, and contribute considerably toward improving our understanding of the mechanisms involved in nutrition and growth. The objective of this review is to show the relevant aspects of the nutrition and growth in zebrafish that support its utility as a model organism for nutritional genomics studies. We made a particular emphasis that gene expression and genetic variants in response to zebrafish nutrition will shed light on similar processes in aquacultured fish.