The dopamine transporter gene (SLC6A3) variable number of tandem repeats domain enhances transcription in dopamine neurons

Identification of additional variants within the human dopamine transporter gene provides further evidence for an association with bipolar disorder in two independent samples

The dopamine transporter (DAT) is the site of action of stimulants, and variations in the human DAT gene (DAT1) have been associated with susceptibility to several psychiatric disorders including attention deficit hyperactivity disorder (ADHD) and bipolar disorder. We have previously reported the association of bipolar disorder to novel SNPs in the 3' end of DAT1. We now report the identification of 20 additional SNPs in DAT1 for a total of 63 variants. We also report evidence for association to bipolar disorder in a second independent sample of families. Eight newly identified SNPs and 14 previously identified SNPs were analyzed in two independent samples of 50 and 70 families each using the transmission disequilibrium test. Two of the eight new SNPs, one in intron 8 and one in intron 13, were found to be moderately associated with bipolar disorder, each in one of the two independent samples. Analysis of haplotypes comprised of all 22 SNPs in sliding windows of five adjacent SNPs revealed an association to the region near introns 7 and 8 in both samples (empirical P-values 0.002 and 0.001, respectively, for the same window). The haplotype block structure observed in the gene in our previous study was confirmed in this sample with greater resolution allowing for discrimination of a third haplotype block in the middle of the gene. Together, these data are consistent with the presence of multiple variants in DAT1 that convey susceptibility to bipolar disorder.

A dopamine transporter mutation associated with bipolar affective disorder causes inhibition of transporter cell surface expression

The dopamine transporter (DAT) plays a crucial role in dopaminergic neurotransmission as it clears the extracellular space of dopamine (DA) and thus controls the concentration of active neurotransmitter. Genetic association studies have reported a variable number of tandem repeat polymorphisms in the 3'-noncoding region of the DAT gene implicating this protein in the development of various psychiatric disorders. In a sample of bipolar patients, two rare missense substitutions (A559V and E602G) have been identified, one of which (E602G) was inherited by the patient from her affected father. None of these mutations had been identified in any control subjects of this survey. Using a heterologous cellular expression system, we have analysed possible consequences of these mutations on functional properties of the encoded DAT protein. DA transport measurements and antagonist binding revealed that the A559V mutant protein is fully functional, whereas the E602G mutant is not. Further analyses by confocal microscopy showed that the E602G protein is transcribed and translated but not delivered to the cell surface. Taken together, our results suggest that this missense mutation has functional consequences thus supporting the need to screen larger samples of patients and their relatives for this rare but bipolar disorder-associated mutation in the DAT gene.

The variable number of tandem repeats element in DAT1 regulates in vitro dopamine transporter density

BACKGROUND

A 40-bp variable number of tandem repeats (VNTR) polymorphism exists in the 15th exon of DAT1, the gene encoding the human dopamine transporter (DAT). Though the VNTR resides in a region encoding the 3' untranslated region (UTR) and does not alter the protein's amino acid sequence, the prevalent 10-repeat variant has shown both linkage and association to Attention Deficit Hyperactivity Disorder (ADHD). In this study, we examined the effects of the DAT1 VNTR on measures of in vitro DAT expression and pharmacology. A series of four DAT1 constructs, each containing the DAT1 coding region, but varying with respect to the downstream presence or content of the 3'UTR, were engineered and stably transfected into an HEK-293 variant using Flp-In integration, an enzyme-mediated, site-specific recombination technology.

RESULTS

[3H] Win 35,428 saturation binding assays and DAT immunoblots revealed statistically significant differences in DAT expression attributable to DAT1 genotype. Cells harboring the 10-repeat DAT1 variant were characterized by a Bmax approximately 50% greater than cells with the 9-repeat VNTR; those containing only the DAT1 coding region or the coding region flanked by a truncated 3' UTR resulted in greater DAT density than either of the naturalistic 9- and 10-repeat variants. Competition binding assays showed no statistically significant DAT1 genotype effects on the DAT affinity for methylphenidate, a finding consistent with the positional location of the VNTR.

CONCLUSION

This study identified the DAT1 VNTR as a functional polymorphism and provides an interpretive framework for its association with behavioral phenotypes.

ASSOCIATION OF THE DOPAMINE TRANSPORTER GENE WITH ALCOHOLISM

AIMS

It was investigated whether the allele A9 of the dopamine transporter gene (DAT1; SLC6A3) is associated with alcoholism, delirium tremens (DT), alcohol withdrawal seizures (AWS), or the daily alcohol intake.

METHODS

A group of 102 healthy subjects and 216 alcoholics, including 97 patients with a history of mild withdrawal symptoms, 65 with a history of AWS and 83 with a history of DT were genotyped and personal data were achieved for statistical evaluation in a case-control design.

RESULTS

The frequency of individuals carrying the allele A9 [f(A9+)] was significantly higher (P = 0.01) in the group of alcoholics [f(A9+) = 0.48] compared with healthy controls [f(A9+) = 0.32]. There was no significant association of the allele A9 with severe withdrawal symptoms or the daily amount of alcohol consumed.

CONCLUSIONS

Our results reveal that the allele A9 is strongly associated with alcoholism but not with withdrawal symptoms or daily alcohol intake.

The dopamine transporter and attention-deficit/hyperactivity disorder

The high incidence of attention-deficit/hyperactivity disorder (ADHD) and escalating use of ADHD medications present a compelling case for clarifying the pathophysiology of, and developing laboratory or radiologic tests for, ADHD. Currently, the majority of specific genes implicated in ADHD encode components of catecholamine signaling systems. Of these, the dopamine transporter (DAT) is a principal target of the most widely used antihyperactivity medications (amphetamine and methylphenidate); the DAT gene is associated with ADHD, and some studies have detected abnormal levels of the DAT in brain striatum of ADHD subjects. Medications for ADHD interfere with dopamine transport by brain-region- and drug-specific mechanisms, indirectly activating dopamine- and possibly norepinephrine-receptor subtypes that are implicated in enhancing attention and experiential salience. The most commonly used DAT-selective ADHD medications raise extracellular dopamine levels in DAT-rich brain regions. In brain regions expressing both the DAT and the norepinephrine transporter (NET), the relative contributions of dopamine and norepinephrine to ADHD pathophysiology and therapeutic response are obfuscated by the capacity of the NET to clear dopamine as well as norepinephrine. Thus, ADHD medications targeting DAT or NET might disperse dopamine widely and consign dopamine storage and release to regulation by noradrenergic, as well as dopaminergic neurons.

Effects of dopamine D2 receptor (DRD2) and transporter (SLC6A3) polymorphisms on smoking cue-induced cigarette craving among African-American smokers

Cue-induced craving for addictive substances has long been known to contribute to the problem of persistent addiction in humans. Research in animals over the past decade has solidly established the central role of dopamine in cue-induced craving for addictive substances, including nicotine. Analogous studies in humans, however, are lacking, especially among African-American smokers, who have lower quit rates than Caucasian smokers. Based on the animal literature, the study's objective was to test the hypothesis that smokers carrying specific variants in dopamine-related genes previously associated with risk for addictive behaviors would exhibit heightened levels of cigarette craving following laboratory exposure to cues. To this end, cigarette craving was induced in healthy African-American smokers (n=88) through laboratory exposure to smoking cues. Smokers carrying either the DRD2 (D2 dopamine receptor gene) TaqI A1 RFLP or the SLC6A3 (dopamine transporter gene) 9-repeat VNTR polymorphisms had stronger cue-induced cravings than noncarriers (Ps <0.05 and 0.01, respectively). Consistent with the separate biological pathways involved (receptor, transporter), carriers of both polymorphisms had markedly higher craving responses compared to those with neither (P<0.0006), reflecting additive effects. Findings provide support for the role of dopamine in cue-induced craving in humans, and suggest a possible genetic risk factor for persistent smoking behavior in African-American smokers.

Restoration of the cystic fibrosis transmembrane conductance regulator function by splicing modulation

A significant fraction of disease-causing mutations affects pre-mRNA splicing. These mutations can generate both aberrant and correct transcripts, the level of which varies among different patients. An inverse correlation was found between this level and disease severity, suggesting a role for splicing regulation as a genetic modifier. Overexpression of splicing factors increased the level of correctly spliced RNA, transcribed from minigenes carrying disease-causing splicing mutations. However, whether this increase could restore the protein function was unknown. Here, we demonstrate that overexpression of Htra2-beta1 and SC35 increases the level of normal cystic fibrosis transmembrane conductance regulator (CFTR) transcripts in cystic-fibrosis-derived epithelial cells carrying the 3849+10 kb C --> T splicing mutation. This led to activation of the CFTR channel and restoration of its function. Restoration was also obtained by sodium butyrate, a histone deacetylase inhibitor, known to upregulate the expression of splicing factors. These results highlight the therapeutic potential of splicing modulation for genetic diseases caused by splicing mutations.

Transient expression analysis of allelic variants of a VNTR in the dopamine transporter gene (DAT1)

Background

The 10-repeat allele of a variable number tandem repeat (VNTR) polymorphism in the 3'-untranslated region of the dopamine transporter gene (DAT1) has been associated with a range of psychiatric phenotypes, most notably attention-deficit hyperactivity disorder. The mechanism for this association is not yet understood, although several lines of evidence implicate variation in gene expression. In this study we have characterised the genomic structure of the 9- and 10-repeat VNTR alleles, and directly examined the role of the polymorphism in mediating gene expression by measuring comparative in vitro cellular expression using a reporter-gene assay system.

Results

Differences in the sequence of the 9- and 10- repeat alleles were confirmed but no polymorphic differences were observed between individuals. There was no difference in expression of reporter gene constructs containing the two alleles.

Conclusions

Our data suggests that this VNTR polymorphism may not have a direct effect on DAT1 expression and that the associations observed with psychiatric phenotypes may be mediated via linkage disequilibrium with other functional polymorphisms.

DAT1, DRD4, and DRD5 polymorphisms are not associated with ADHD in Dutch families

Recent meta-analyses have indicated that the dopamine transporter gene (DAT1) and the dopamine receptor genes D4 (DRD4) and D5 (DRD5) are associated with attention-deficit hyperactivity disorder (ADHD), although single studies frequently failed to show significant association. In a family-based sample of 236 Dutch children with ADHD, we have investigated the previously described variable number of tandem repeat (VNTR) polymorphisms and two additional microsatellites at the DAT1 and DRD4 loci. DRD5 was investigated using the microsatellite that was previously found to be associated. Transmission disequilibrium tests (TDTs) did not show preferential transmission of alleles or two-marker haplotypes to affected offspring. These data suggest that DAT1, DRD4, and DRD5 do not contribute substantially to ADHD in the Dutch population.

Functional polymorphisms in dopamine and serotonin pathway genes

There is mounting evidence on the functional significance of single nucleotide and simple repeat sequence polymorphisms in both the coding and regulatory regions of genes in the monoamine neurotransmitter pathways. Many of these gene variants have been associated with human behavioral disorders and traits, and thus have important clinical relevance. This review summarizes the literature on the published functional studies from a molecular, cellular, and neurobiological perspective, and notes their possible behavioral consequences. Functional studies have adopted a variety of strategies. Pharmacological studies have focused on the effects of gene variation at the protein level in terms of binding to ligands or drugs. Other key investigations have determined effects on gene expression at the level of transcription in mammalian cell cultures, lymphoblasts, and/or human postmortem brain tissue. This has enabled the comparison of in vitro and in vivo data, and furthermore provides an improved perceptive of their respective advantages. Additionally, molecular biological approaches have identified transcription factors (DNA-binding proteins) that interact with the motifs within the polymorphisms themselves. Various neuroimaging studies have further determined the relationship of genotype with protein availability in the brain, and thus have contributed to our understanding of the in vivo functional significance of gene variants. Finally, there is growing evidence from both human and animal studies on the interaction of functional polymorphisms with the environment in determining a behavioral outcome. Taken together, these findings have contributed to a greater understanding of the plausible molecular mechanisms that underpin the functional significance of polymorphisms in monoamine neurotransmitter pathway genes, and how they may influence behavioral phenotypes.

Attention-Deficit Hyperactivity Disorder in the post-genomic era

BACKGROUND

ADHD is a common and complex genetic disorder. Genetic risk factors are expected to be multiple, have small effect sizes when considered individually and to interact with each other and with environmental factors.

OBJECTIVE

To describe the difficulties involved in the genetic investigation of such a complex disorder and give a prospective for the future.

METHODS

Review based on empirical literature and project description.

RESULTS

Considerable progress has been achieved through the association analysis of candidate gene loci. Linkage scans using affected sibling pairs have identified a number of potential loci that may lead to the identification of novel genes of moderate effect size. Quantitative trait locus (QTL) approaches provide powerful complementary strategies that have the potential to link the categorical disorder to continuously distributed traits associated more closely with underlying genetic liability in the general population. Success in identifying some associated genes has been complemented by functional studies that seek to understand the mode of action of such genes.

CONCLUSION

Progress in understanding the mechanisms involved has not been straightforward and many inconsistencies have arisen. In order to take advantage of the potential for progress that stems from the genetic findings it will be important to draw upon a variety of approaches and experimental paradigms. A functional genomic approach to ADHD means that investigation of gene function is carried out at various levels of analysis, not only at the level of molecular and cellular function but also at the level of psychological processes, neuronal networks, environmental interactions and behavioural outcomes.

Stress-induced cigarette craving: effects of the DRD2 TaqI RFLP and SLC6A3 VNTR polymorphisms

Animal models have long implicated dopamine in stress-induced craving for a variety of addictive substances. However, translational studies of dopamine, stress and craving in humans are lacking. Based on the animal literature, this study's objective was to test the hypothesis that cigarette smokers carrying specific variants in dopamine-related genes would have heightened levels of cigarette craving following exposure to a laboratory stressor. Cigarette craving induced by controlled exposure to a laboratory stressor was assessed in healthy adult smokers (n=108) recruited by advertisement. Significantly stronger stress-induced cigarette craving was found for individuals carrying either the DRD2 (D2 dopamine receptor gene) A1, or the SLC6A3 (dopamine transporter gene) nine-repeat allelic variants. Stress-induced craving was markedly higher for those carrying both alleles, compared to those with neither, consistent with the separate biological pathways involved (receptor, transporter). Findings provide strong support for the possibility that dopamine involvement in stress-induced craving well established in animal models also applies to humans, and suggest a potential genetic risk factor for persistent smoking behavior.

Dopamine transporter gene polymorphism, spect imaging, and levodopa response in patients with Parkinson disease

OBJECTIVES

To assess the potential association between dopamine transporter (DAT) genotype, single photon emission CT (SPECT) measures using [123I]-N-omega-fluoropropyl-2beta-carbomethoxy-3beta-(4-iodophenyl)nortropane ([123I]-FP-CIT) of striatal dopaminergic function, and oral levodopa response pattern in a cohort of patients with Parkinson disease.

METHODS

Thirty-six patients at different disease stages enrolled in the study. Each patient was examined by [123I]-FP-CIT SPECT and a standardized oral levodopa test on 2 separate days in a randomized order within 3 weeks. The main outcome variables were the specific-to-nonspecific tracer uptake ratio in the contralateral putamen for SPECT analysis; latency, duration, and magnitude of the motor effect; and presence of dyskinesias for the levodopa test. The variable number of tandem repeat (VNTR) polymorphisms of the gene coding for DAT were detected for each patient by standard methods.

RESULTS

Contralateral putamen [123I]-FP-CIT uptake ratios were similar in the patients carrying the 9-copy allele (n=20) of the DAT VNTR compared with 10-repeat homozygotes (n=16). No significant difference was found in levodopa main outcome variables and dyskinesia incidence between the two groups of patients stratified by DAT VNTR polymorphism.

CONCLUSIONS

The study did not identify clinically relevant in vivo DAT neurochemical function phenotypes or levodopa response patterns associated with the DAT polymorphism.

Brain-specific RGS9-2 is localized to the nucleus via its unique proline-rich domain

Brain-specific regulator of G protein signaling 9 (RGS9-2) is a member of a family of proteins that can function as GTPase-activating proteins for heterotrimeric G proteins. In the present study, we examined the intracellular distribution of RGS9-2 in native brain tissue and transfected cells. Immunocytochemical and immunoblot experiments revealed an unexpectedly high proportion of RGS9-2 within the nuclei of forebrain neurons. A similar intracellular distribution was seen in transfected COS-7 cells. The RGS9 binding partner G(beta5) further enhanced the nuclear localization of RGS9-2, but did not affect the strongly cytoplasmic localization of RGS9-1, the retinal form of RGS9. Deletion construct analysis revealed that the unique polyproline-rich C-terminus of brain-specific RGS9-2 contains sequences necessary and sufficient to target RGS9 to the nucleus of COS-7 cells, as well as cultured striatal neurons. Furthermore, RGS9-2 transfection increased the transcriptional activity of a neuronal gene construct normally expressed in RGS9-positive neurons, suggesting that nuclear RGS9 directly or indirectly regulates transcription in vivo. The nuclear localization of RGS9-2 suggests a heretofore-unanticipated role for this brain-specific protein in transducing signals to the nuclei of forebrain neurons.

Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and parkinsonism

We review the basic science of the dopamine transporter (DAT), a key neurotransmitter for locomotor control and reward systems, including those lost or deranged in Parkinson's disease (PD). Physiology, pharmaceutical features, expression, cDNA, protein structure/function relationships, and phosphorylation and regulation are discussed. The localization of DAT provides the best marker for the integrity of just the pre-synaptic dopaminergic systems that are most affected in PD. Its function is key for the actions of several toxins that provide some of the best current models for idiopathic parkinsonism, and its variation can clearly alter movement. The wealth of information about this interesting molecule that has been developed over the last 12 years has led to increased interest in DAT among workers interested in both normal and abnormal movement.

Polymorphisms of the Dopamine Transporter Gene

Attention deficit-hyperactivity disorder (ADHD) is a very common and heterogeneous childhood-onset psychiatric disorder, affecting between 3% and 5% of school age children worldwide. Although the neurobiology of ADHD is not completely understood, imbalances in both dopaminergic and noradrenergic systems have been implicated in the origin and persistence of core symptoms, which include inattention, hyperactivity, and impulsivity. The role of a genetic component in its etiology is strongly supported by genetic studies, and several investigations have suggested that the dopamine transporter gene (DAT1; SLC6A3 locus) may be a small-effect susceptibility gene for ADHD. Stimulant medication has a well-documented efficacy in reducing ADHD symptoms. Methylphenidate, the most prescribed stimulant, seems to act mainly by inhibiting the dopamine transporter protein and dopamine reuptake. In fact, its effect is probably related to an increase in extracellular levels of dopamine, especially in brain regions enriched in this protein (i.e. striatum). It is also important to note that dopamine transporter densities seem to be particularly elevated in the brain of ADHD patients, decreasing after treatment with methylphenidate. Altogether, these observations suggest that the dopamine transporter does play a major role in ADHD. Among the several polymorphisms already described in the SLC6A3 locus, a 40 bp variable number of tandem repeats (VNTR) polymorphism has been extensively investigated in association studies with ADHD. Although there are some negative results, the findings from these reports indicate the allele with ten copies of the 40 bp sequence (10-repeat allele) as the risk allele for ADHD. Some investigations have suggested that this polymorphism can be implicated in dopamine transporter gene expression in vitro and dopamine transporter density in vivo, even though it is located in a non-coding region of the SLC6A3 locus. Despite all these data, few studies have addressed the relationship between genetic markers (specifically the VNTR) at the SLC6A3 locus and response to methylphenidate in ADHD patients. A significant effect of the 40 bp VNTR on response to methylphenidate has been detected in most of these reports. However, the findings are inconsistent regarding both the allele (or genotype) involved and the direction of this influence (better or worse response). Thus, further investigations are required to determine if genetic variation due to the VNTR in the dopamine transporter gene is able to predict different levels of clinical response and palatability to methylphenidate in patients with ADHD, and how this information would be useful in clinical practice.

Promoter and intronic variants affect the transcriptional regulation of the human dopamine transporter gene

We have attempted to identify regions involved in the transcriptional regulation of the DAT1 (HUGO approved symbol SLC6A3) gene that may harbor functional variants predisposing to several neuropsychiatric disorders by examining haplotypes of various 5' and intronic regions for their effect on expression in a dopaminergic cell line. A 1.5-fold difference in regulatory activity was observed between haplotypes of the proximal promoter/intron 1 region, representing the two previously identified 5' clades. Although we found no effect on transcription with inclusion of the 9- and 10-repeat alleles of the 3' VNTR, introns 9, 12, and 14 appear to contain enhancer elements capable of increasing expression approximately 2-fold with respect to the promoter constructs. Differences in expression were also observed between two alleles of intron 14. These results thus suggest that it may be the particular combination of polymorphisms in a haplotype across the gene that ultimately affects DAT1 gene expression.

Genetics of posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is a prevalent anxiety disorder marked by behavioral, physiologic, and hormonal alterations. PTSD is disabling and commonly follows a chronic course. The etiology of PTSD is unknown, although exposure to a traumatic event constitutes a necessary, but not sufficient, factor. A twin study of Vietnam veterans has shown significant genetic contribution to PTSD. The fact that PTSD's underlying genotypic vulnerability is only expressed following trauma exposure limits the usefulness of family-based linkage approaches. In contrast to the other major psychiatric disorders, large studies for the search of underlying genes have not been described in PTSD to date. Complementary approaches for locating involved genes include association-based studies employing case-control or parental genotypes for transmission dysequilibrium analysis and quantitative trait loci studies in animal models. Identification of susceptibility genes will increase our understanding of traumatic stress disorders and help to elucidate their molecular basis. The current review provides an up-to-date outline of progress in the field of PTSD.

Pharmacogenetics of monoamine transporters

The monoamine transporters are the sites of action of the most commonly used psychoactive compounds in therapeutic use today as well as the psychostimulant drugs of abuse. These transporters have been the focus of a large number of genetic association studies of complex behavioral phenomena. More recently, pharmacogenetic studies have suggested an association between a functional regulatory polymorphism in the serotonin transporter gene and antidepressant response. This review will discuss the clinical pharmacology of the monoamine transporters, their molecular genetic variability and the results of several association studies of the transporters and psychoactive drug response.

Association of the 480 bp DAT1 allele with methylphenidate response in a sample of Irish children with ADHD

Several studies have implicated the dopamine transporter gene (DAT1) as conferring susceptibility to attention deficit hyperactivity disorder (ADHD), in particular, a VNTR situated at the 3' end of the gene. In addition, the 10-repeat VNTR allele associated with ADHD has been reported to be associated with an over-active transporter protein (DAT). Thus children possessing this variant might be particularly responsive to methylphenidate, a drug known to act by blocking DAT. We have examined this hypothesis and now report an association between the 10-repeat VNTR DAT1 polymorphism and retrospectively rated methylphenidate response in a sample of 119 Irish children with ADHD (chi(2) = 7.918, df = 1, P = 0.005). Our findings suggest a role for the 10-repeat DAT1 risk allele in medication response and may help to predict positive clinical outcome in ADHD.

Role of dopamine transporter genotype and maternal prenatal smoking in childhood hyperactive-impulsive, inattentive, and oppositional behaviors

OBJECTIVE

To examine the joint effects of a dopamine transporter (DAT) polymorphism and maternal prenatal smoking on childhood hyperactivity-impulsivity and inattentiveness.

STUDY DESIGN

A cohort of 161 children was followed prospectively from age 6 months to 60 months. Primary outcomes were the DSM-IV hyperactive-impulsive and inattentive scales of the Conners' Parent Rating Scale Revised-Long Version (CPRS R:L). A secondary outcome was the oppositional scale. Predictors included DAT genotype and maternal report of prenatal smoking. Children homozygous for the 480-bp DAT allele (DAT +/+) were compared with all other children (DAT +/- or -/-).

RESULTS

In multivariate analyses, children with both prenatal smoke exposure and the DAT +/+ genotype had significantly elevated hyperactive-impulsive scores (beta, 7.5; SE, 2.9; P<.01) compared with children with no smoke exposure and DAT +/- or -/-. Inattentive scores were not significantly elevated in this group, but oppositional scores were a full standard deviation higher. Neither prenatal smoke exposure alone nor DAT +/+ genotype alone was significantly associated with increased scores.

CONCLUSIONS

Child hyperactivity-impulsivity and oppositional behaviors were associated with a DAT polymorphism but only when the child also had exposure to maternal prenatal smoking. This study emphasizes the importance of incorporating environmental cofactors in genetic studies of attention deficit hyperactivity disorder.

Elucidating the role of genetic factors in smoking behavior and nicotine dependence

Despite over a decade of intensive tobacco control efforts, a substantial proportion of the world's population continues to smoke. With advances in molecular biology and genomics technology, attention is focusing on the genetic basis of addiction to nicotine. The hope is that by better understanding the biology of nicotine addiction, novel prevention and cessation strategies can be developed and targeted to individuals most likely to benefit. This article provides an overview of currently available data from genetics research on smoking behavior and discusses study design issues to be addressed to make progress in this line of research.

Animal models of attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) involves clinically heterogeneous dysfunctions of sustained attention, with behavioral overactivity and impulsivity, of juvenile onset. Experimental models, in addition to mimicking syndromal features, should resemble the clinical condition in pathophysiology, and predict potential new treatments. One of the most extensively evaluated animal models of ADHD is the spontaneously hypertensive rat. Other models include additional genetic variants (dopamine transporter gene knock-out mouse, coloboma mouse, Naples hyperexcitable rat, acallosal mouse, hyposexual rat, and population-extreme rodents), neonatal lesioning of dopamine neurons with 6-hydroxydopamine, and exposure to other neurotoxins or hippocampal irradiation. None is fully comparable to clinical ADHD. The pathophysiology involved varies, including both deficient and excessive dopaminergic functioning, and probable involvement of other monoamine neurotransmitters. Improved models as well as further testing of their ability to predict treatment responses are required.

Monoamine transporter gene structure and polymorphisms in relation to psychiatric and other complex disorders

The norepinephrine, dopamine and serotonin transporters (NET, DAT and SERT, respectively), limit cellular signaling by recapturing released neurotransmitter, and serve as targets for antidepressants and drugs of abuse, emphasizing the integral role these molecules play in neurotransmission and pathology. This has compelled researchers to search for polymorphisms in monoamine (MA) transporter genes. Studies support linkage and association of MA transporter genetic variation in psychiatric and other complex disorders. Understanding the contribution of MA transporter polymorphisms to human behavior, disease susceptibility and response to pharmacotherapies will involve further progress in linkage and association that will be aided by both definition of highly selective phenotypes and utilization of a large number of polymorphic markers. The relationship of polymorphisms to alterations in transport capacity, likely a complex interaction, involving genetic background, disease state, and medication, will elucidate the means by which MA transporter genetic variability contributes to our individuality.

The serotonin transporter intronic VNTR enhancer correlated with a predisposition to affective disorders has distinct regulatory elements within the domain based on the primary DNA sequence of the repeat unit

We have demonstrated that a variable number tandem repeat domain (VNTR) within intron 2 of the serotonin transporter gene is a transcriptional regulatory domain which is potentially correlated with a predisposition to affective disorders and other behavioural conditions. This correlation based on copy number of the VNTR alone (nine, 10 or 12 copies of 16/17 base-pair element) has been controversial and not reproduced in all studies. We demonstrate that individual repeat elements within the VNTR domain differ in their enhancer activity in an embryonic stem cell model. This has implications for both the mechanism by which these VNTRs are correlated with the progression of the disease and suggests that clinical analysis should now be extended to correlate sequence variation within the VNTR with the disorder. The latter may resolve some of the conflicting data published to date.

The A9 allele of the dopamine transporter gene is associated with delirium tremens and alcohol-withdrawal seizure

BACKGROUND

The dopamine transporter (DAT) plays a key role in homeostatic regulation of dopaminergic neurotransmission and could thus be involved in the variability of two severe alcohol-withdrawal symptoms, alcohol-withdrawal seizure (AWS) and delirium tremens (DT). Interestingly, an association was found between the DAT gene (9-copy repeat) and the risk for these symptoms in two previous case-control studies.

METHODS

We reanalyzed the role of the DAT gene in the lifetime risk for AWS and DT in 120 alcohol-dependent patients, taking into account potentially confounding factors.

RESULTS

Alcohol-dependent patients with the A(9) allele had experienced AWS or DT at least once (odds ratio [OR] = 2.52, p =.03). This association persisted when excluding patients with antisocial personality comorbidity (OR = 3.48, p =.02) or limiting the analysis to older patients (OR = 8.3, p =.0008).

CONCLUSIONS

This study provides convergent data in favor of a significant role of the DAT gene in the risk for some severe withdrawal symptoms. If further replicated in larger samples, the DAT genetic polymorphism could be one of the factors to be analyzed to further assess the risk of some severe alcohol-withdrawal symptoms.

Expression of the dopamine transporter gene is regulated by the 3' UTR VNTR: Evidence from brain and lymphocytes using quantitative RT-PCR

Genetic association studies provide considerable evidence that the 10-repeat allele of a variable number tandem repeat (VNTR) in the 3'-untranslated region (3'-UTR) of the dopamine transporter gene (DAT1) is associated with a range of psychiatric phenotypes, most notably, attention deficit hyperactivity disorder. The mechanism for this association is not yet understood, although several lines of evidence implicate variation in gene expression. In this study, we measured DAT1 messenger RNA levels in cerebellum, temporal lobe, and lymphocytes using quantitative real-time reverse-transcription polymerase chain reaction. Relative to a set of four control housekeeping genes (beta-actin, GAPD, ribosomal 18S, and beta2-microglobulin) we observed that increased levels of DAT1 expression were associated with the number of 10-repeat alleles. These data provide direct evidence that the VNTR, or another polymorphism in linkage disequilibrium with the VNTR, is involved in regulating expression of this gene.

Variation in the vasopressin V1a receptor promoter and expression: implications for inter- and intraspecific variation in social behaviour

Instability in highly repetitive microsatellite DNA located in the regulatory regions of genes may be a major factor producing diversity in both region-specific gene expression and the resulting phenotypes. Polymorphisms in promoter regions affecting expression of genes involved in regulating behaviour may play a role in generating individual variation in behaviour, including psychopathologies in humans, and probably are also important for the evolution of behaviour. Here we discuss the prairie vole vasopressin V1a receptor gene as a model that may be useful for understanding the evolution of promoter sequences and the relationship between gene sequence, expression and behavioural phenotype.