Meta-analysis shows schizophrenia is not associated with the 40-base-pair repeat polymorphism of the dopamine transporter gene

Statistical Validation of Risk Alleles in Genetic Addiction Risk Severity (GARS) Test: Early Identification of Risk for Alcohol Use Disorder (AUD) in 74,566 Case–Control Subjects

Since 1990, when our laboratory published the association of the DRD2 Taq A1 allele and severe alcoholism in JAMA, there has been an explosion of genetic candidate association studies, including GWAS. To develop an accurate test to help identify those at risk for at least Alcohol Use Disorder (AUD), Blum’s group developed the Genetic Addiction Risk Severity (GARS) test, consisting of ten genes and eleven associated risk alleles. In order to statistically validate the selection of these risk alleles measured by GARS, we applied strict analysis to studies that investigated the association of each polymorphism with AUD or AUD-related conditions published from 1990 until 2021. This analysis calculated the Hardy–Weinberg Equilibrium of each polymorphism in cases and controls. If available, the Pearson’s χ2 test or Fisher’s exact test was applied to comparisons of the gender, genotype, and allele distribution. The statistical analyses found the OR, 95% CI for OR, and a post-risk for 8% estimation of the population’s alcoholism prevalence revealed a significant detection. The OR results showed significance for DRD2, DRD3, DRD4, DAT1, COMT, OPRM1, and 5HTT at 5%. While most of the research related to GARS is derived from our laboratory, we are encouraging more independent research to confirm our findings.

Revisiting tandem repeats in psychiatric disorders from perspectives of genetics, physiology, and brain evolution

Genome-wide association studies (GWASs) have revealed substantial genetic components comprised of single nucleotide polymorphisms (SNPs) in the heritable risk of psychiatric disorders. However, genetic risk factors not covered by GWAS also play pivotal roles in these illnesses. Tandem repeats, which are likely functional but frequently overlooked by GWAS, may account for an important proportion in the "missing heritability" of psychiatric disorders. Despite difficulties in characterizing and quantifying tandem repeats in the genome, studies have been carried out in an attempt to describe impact of tandem repeats on gene regulation and human phenotypes. In this review, we have introduced recent research progress regarding the genomic distribution and regulatory mechanisms of tandem repeats. We have also summarized the current knowledge of the genetic architecture and biological underpinnings of psychiatric disorders brought by studies of tandem repeats. These findings suggest that tandem repeats, in candidate psychiatric risk genes or in different levels of linkage disequilibrium (LD) with psychiatric GWAS SNPs and haplotypes, may modulate biological phenotypes related to psychiatric disorders (e.g., cognitive function and brain physiology) through regulating alternative splicing, promoter activity, enhancer activity and so on. In addition, many tandem repeats undergo tight natural selection in the human lineage, and likely exert crucial roles in human brain evolution. Taken together, the putative roles of tandem repeats in the pathogenesis of psychiatric disorders is strongly implicated, and using examples from previous literatures, we wish to call for further attention to tandem repeats in the post-GWAS era of psychiatric disorders.

A Meta-analysis of the Association Between SLC6A3 Gene Polymorphisms and Schizophrenia

The dopamine transporter is coded by the SLC6A3 gene and plays an important role in regulation of the neurotransmitter dopamine. To detect the association between the SLC6A3 gene and the risk of schizophrenia, 31 case-control articles were included in this meta-analysis. There were 23 studies with 40 bp VNTR (3246 cases and 3639 controls), 4 studies with rs40184 (2020 cases and 1674 controls), rs6347 (1317 cases and 1917 controls), rs403636 (2045 cases and 1704 controls), and rs2975226 (849 cases and 904 controls); and 3 studies with rs12516948 (1920 cases and 1569 controls), rs27072 (984 cases and 1015 controls), rs6869645 (1142 cases and 1082 controls), rs37022 (1168 cases and 1091 controls), rs464049 (1169cases and 1096 controls), rs2652511 (707 cases and 714 controls), and rs3756450 (1176 cases and 1096 controls). Pooled, subgroup, and sensitivity analyses were performed, and the results were visualized by forest and funnel plots. In the dominant genetic model, the genotype AA+AT of rs2975226 in the Indian population (P_z = 0, odds ratio [OR] = 3.245, 95% confidence interval [CI] = 1.806-5.831), TT of rs464049 (P_z = 0.002, OR = 1.389, 95% CI = 1.129-1.708), and TT of rs3756450 (P_z = 0.014, OR = 1.251, 95% CI = 1.047-1.495) might be risk factors for schizophrenia. Additionally, no other single nucleotide polymorphisms were observed. These results indicate that more functional studies are warranted.

Presence of recombination hotspots throughout SLC6A3

The human dopamine transporter gene SLC6A3 is involved in substance use disorders (SUDs) among many other common neuropsychiatric illnesses but allelic association results including those with its classic genetic markers 3'VNTR or Int8VNTR remain mixed and unexplainable. To better understand the genetics for reproducible association signals, we report the presence of recombination hotspots based on sequencing of the entire 5' promoter regions in two small SUDs cohorts, 30 African Americans (AAs) and 30 European Americans (EAs). Recombination rate was the highest near the transcription start site (TSS) in both cohorts. In addition, each cohort carried 57 different promoter haplotypes out of 60 and no haplotypes were shared between the two ethnicities. A quarter of the haplotypes evolved in an ethnicity-specific manner. Finally, analysis of five hundred subjects of European ancestry, from the 1000 Genome Project, confirmed the promoter recombination hotspots and also revealed several additional ones in non-coding regions only. These findings provide an explanation for the mixed results as well as guidance for selection of effective markers to be used in next generation association validation (NGAV), facilitating the delineation of pathogenic variation in this critical neuropsychiatric gene.

Association study and a systematic meta-analysis of the VNTR polymorphism in the 3'-UTR of dopamine transporter gene and attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) has been postulated to associate with dopaminergic dysfunction, including the dopamine transporter (DAT1). Several meta-analyses showed small but significant association between the 10-repeat allele in the DAT1 gene in 3'-untranslated region variant number tandem repeat polymorphism and child and adolescent ADHD, whereas in adult ADHD the 9-repeat allele was suggested to confer as risk allele. Interestingly, recent evidence indicated that the long-allele variants (10 repeats and longer) might confer to lower expression of the transporter in comparison to the short-allele. Therefore, we assessed here the association in samples consisting of families with child and adolescent ADHD as well as a case-control sample, using either the 10- versus 9-repeat or the long- versus short-allele approach. Following, we conducted a systematic review and meta-analysis, including family and case-control studies, using the two aforementioned approaches as well as stratifying to age and ethnicity. The first approach (10-repeat) resulted in nominal significant association in child and adolescent ADHD (OR 1.1050 p = 0.0128), that became significant stratifying to European population (OR 1.1301 p = 0.0085). The second approach (long-allele) resulted in significant association with the whole ADHD population (OR 1.1046 p = 0.0048), followed by significant association for child and adolescent ADHD (OR 1.1602 p = 0.0006) and in Caucasian and in European child and adolescent ADHD (OR 1.1310 p = 0.0114; OR 1.1661 p = 0.0061; respectively). We were not able to confirm the association reported in adults using both approaches. In conclusion, we found further indication for a possible DAT1 gene involvement; however, further studies should be conducted with stringent phenotyping to reduce heterogeneity, a limitation observed in most included studies.

Depressive symptoms in schizophrenia and dopamine and serotonin gene polymorphisms

Although depressive symptoms seem to be frequent in schizophrenia they have received significantly less attention than other symptom domains. As impaired serotonergic and dopaminergic neurotransmission is implicated in the pathogenesis of depression and schizophrenia this study sought to investigate the putative association between several functional gene polymorphisms (SERT 5-HTTLPR, MAO-A VNTR, COMT Val158Met and DAT VNTR) and schizophrenia. Other objectives of this study were to closely examine schizophrenia symptom domains by performing factor analysis of the two most used instruments in this setting (Positive and negative syndrome scale - PANSS and Calgary depression rating scale - CDSS) and to examine the influence of investigated gene polymorphisms on the schizophrenia symptom domains, focusing on depressive scores. A total of 591 participants were included in the study (300 schizophrenic patients and 291 healthy volunteers). 192 (64%) of schizophrenic patients had significant depressive symptoms. Genotype distribution revealed no significant differences regarding all investigated polymorphisms except the separate gender analysis for MAO-A gene polymorphism which revealed significantly more allele 3 carriers in schizophrenic males. Factor analysis of the PANSS scale revealed the existence of five separate factors (symptom domains), while the CDSS scale revealed two distinct factors. Several investigated gene polymorphisms (mostly SERT and MAO-A, but also COMT) significantly influenced two factors from the PANSS (aggressive/impulsive and negative symptoms) and one from the CDSS scale (suicidality), respectively. Depressive symptoms in schizophrenic patients may be influenced by functional gene polymorphisms, especially those implicated in serotonergic neurotransmission.

Evaluating the dopamine hypothesis of schizophrenia in a large-scale genome-wide association study

BACKGROUND

The dopamine hypothesis, which posits that dysregulation of the dopaminergic system is etiologic for schizophrenia, is among the most enduring biological theories in psychiatry. Although variation within genes related to dopaminergic functioning has been associated with schizophrenia, an aggregate test of variation, using the largest publicly available schizophrenia dataset, has not previously been conducted.

METHODS

We first identified a core set of 11 genes involved in the synthesis, metabolism, and neurotransmission of dopamine. We then extracted summary statistics of markers falling within, or flanking, these genes from the Psychiatric Genomics Consortium's most recent schizophrenia mega-analysis results. We conducted aggregate tests for enrichment of dopamine-related pathways for association with schizophrenia.

RESULTS

We did not detect significant enrichment of signals across the core set of dopamine-related genes. However, we did observe modest to strong enrichment of genetic signals within the DRD2 locus.

CONCLUSIONS

Within the limits of available power, common sequence variation within core genes of the dopaminergic system is not related to risk of schizophrenia. This does not preclude a role of dopamine, or dopamine-related genes, in the clinical presentation of schizophrenia or in treatment response. However, it does suggest that the genetic risk for schizophrenia is not substantially affected by common variation in those genes which, collectively, critically impact dopaminergic functioning.

Meta-Analysis Reveals Significant Association of the 3'-UTR VNTR in SLC6A3 with Alcohol Dependence

BACKGROUND

Although many studies have analyzed the association of 3'-untranslated region variable-number tandem repeat (VNTR) polymorphism in SLC6A3 with alcohol dependence (AD), the results remain controversial. This study aimed to determine whether this variant indeed has any genetic effect on AD by integrating 17 reported studies with 5,929 participants included.

METHODS

The A9-dominant genetic model that considers A9-repeat and non-A9 repeat as 2 genotypes and compared their frequencies in alcoholics with that in controls was adopted. Considering the potential influence of ethnicity, differences in diagnostic criteria of AD, and alcoholic subgroups, stratified meta-analyses were conducted. There existed no evidence for the presence of heterogeneity among the studied samples, indicating the results under the fixed-effects model are acceptable.

RESULTS

We found a significant association of VNTR A9 genotypes with AD in all ethnic populations (pooled odds ratio [OR] 1.12; 95% confidence interval [CI] 1.00, 1.25; p = 0.045) and the Caucasian population (pooled OR 1.15; 95% CI 1.01, 1.31; p = 0.036). We also found VNTR A9 genotypes to be significantly associated with alcoholism as defined by the DSM-IV criteria (pooled OR 1.18; 95% CI 1.03, 1.36; p = 0.02). Further, we found a significant association between VNTR A9 genotypes and alcoholism associated with alcohol withdrawal seizure or delirium tremens (pooled OR 1.55; 95% CI 1.24, 1.92; p = 1.0 × 10(-4) ). In all these meta-analyses, no evidence of publication bias was detected.

CONCLUSIONS

We concluded that the VNTR polymorphism has an important role in the etiology of AD, and individuals with at least 1 A9 allele are more likely to be dependent on alcohol than persons carrying the non-A9 allele.

Functional effects of dopamine transporter gene genotypes on in vivo dopamine transporter functioning: a meta-analysis

Much psychiatric genetic research has focused on a 40-base pair variable number of tandem repeats (VNTR) polymorphism located in the 3'-untranslated region (3'UTR) of the dopamine active transporter (DAT) gene (SLC6A3). This variant produces two common alleles with 9- and 10-repeats (9R and 10R). Studies associating this variant with in vivo DAT activity in humans have had mixed results. We searched for studies using positron emission tomography (PET) or single-photon emission computed tomography (SPECT) to evaluate this association. Random effects meta-analyses assessed the association of the 3'UTR variant with DAT activity. We also evaluated heterogeneity among studies and evidence for publication bias. We found twelve studies comprising 511 subjects, 125 from PET studies and 386 from SPECT studies. The PET studies provided highly significant evidence that the 9R allele was associated with increased DAT activity in human adults. The SPECT studies were highly heterogeneous. As a group, they suggested no association between the 3'UTR polymorphism and DAT activity. When the analysis was limited to the most commonly used ligand, [123I]β-CIT, stratification by affection status dramatically reduced heterogeneity and revealed a significant association of the 9R allele with increased DAT activity for healthy subjects. In humans, the 9R allele of the 3'UTR polymorphism of SLC6A3 regulates dopamine activity in the striatal brain regions independent of the presence of neuropsychiatric illness. Differences in study methodology account for the heterogeneous results across individual studies.

The association study of polymorphisms in DAT, DRD2, and COMT genes and acute extrapyramidal adverse effects in male schizophrenic patients treated with haloperidol

Extrapyramidal symptoms (EPSs) are common adverse effects of antipsychotics. The development of acute EPSs could depend on the activity of dopaminergic system and its gene variants. The aim of this study was to determine the association between dopaminergic type 2 receptor (DRD2) dopamine transporter (SLC6A3) and catechol-O-methyltransferase (COMT) gene polymorphisms and acute EPSs in 240 male schizophrenic patients treated with haloperidol (15-mg/d) over a period of 2 weeks. Acute EPSs were assessed with Simpson-Angus Scale. Three dopaminergic gene polymorphisms, the DRD2 Taq1A, the SLC6A3 VNTR, and the COMT Val158Met, were determined. Extrapyramidal symptoms occurred in 116 (48.3%) of patients. Statistically significant associations were found for SLC6A3 VNTR and COMT Val158Met polymorphisms and EPS susceptibility. Patients with SLC6A3 9/10 genotype had almost twice the odds to develop EPSs compared with those with all other SLC6A3 genotypes (odds ratio, 1.9; 95% confidence interval, 1.13-3.30), and patients with COMT Val/Met genotype had 1.7 times greater odds to develop EPSs than those with all other COMT genotypes (odds ratio, 1.7; 95% confidence interval, 1.01-2.88). There was no statistically significant association between genotype and allele frequencies of DRD2, SLC6A3, or COMT polymorphisms and the development of particular EPSs.In conclusion, the results of the present study showed for the first time the association between acute haloperidol-induced EPSs and SLC6A3 VNTR and COMT Val158Met polymorphisms. Although the precise biological mechanisms underlying these findings are not yet understood, the results suggest that the dopaminergic gene variations could predict the vulnerability to the development of the acute EPSs in haloperidol-treated schizophrenic patients.

Interaction between effects of genes coding for dopamine and glutamate transmission on striatal and parahippocampal function

The genes for the dopamine transporter (DAT) and the D-Amino acid oxidase activator (DAOA or G72) have been independently implicated in the risk for schizophrenia and in bipolar disorder and/or their related intermediate phenotypes. DAT and G72 respectively modulate central dopamine and glutamate transmission, the two systems most robustly implicated in these disorders. Contemporary studies have demonstrated that elevated dopamine function is associated with glutamatergic dysfunction in psychotic disorders. Using functional magnetic resonance imaging we examined whether there was an interaction between the effects of genes that influence dopamine and glutamate transmission (DAT and G72) on regional brain activation during verbal fluency, which is known to be abnormal in psychosis, in 80 healthy volunteers. Significant interactions between the effects of G72 and DAT polymorphisms on activation were evident in the striatum, parahippocampal gyrus, and supramarginal/angular gyri bilaterally, the right insula, in the right pre-/postcentral and the left posterior cingulate/retrosplenial gyri (P < 0.05, FDR-corrected across the whole brain). This provides evidence that interactions between the dopamine and the glutamate system, thought to be altered in psychosis, have an impact in executive processing which can be modulated by common genetic variation.

Association of intron 1 variants of the dopamine transporter gene with schizophrenia

The dopamine transporter (DAT1) gene has been implicated in the pathogenesis of many neuropsychiatric disorders, including schizophrenia. The present study aimed to investigate association of the DAT1 gene polymorphisms with schizophrenia in a Han Chinese population. Two single nucleotide polymorphisms (SNPs) in the DAT1 gene (rs2975223 and rs2455391) were tested in 368 patients with schizophrenia and 420 healthy controls, of whom 293 patients underwent an assessment of psychotic symptoms through the positive and negative syndrome scale (PANSS). The chi-square test (χ(2)) showed disease association for rs2455391 (corrected p=0.023 for allelic association and p=0.034 for genotypic association, respectively). The rs2975223(G)-rs2455391(C) haplotype was associated with increased risk of the illness (p=0.0012, OR=2.09, 95% CI=1.28-3.42). Quantitative trait analysis showed that rs2455391 was associated with positive symptoms, general symptoms and global symptoms but not with negative symptoms. The present results suggest that the DAT1 gene may be mainly involved in the development of the positive symptoms in the Chinese population.

Genetic influences of dopamine transport gene on alcohol dependence: a pooled analysis of 13 studies with 2483 cases and 1753 controls

Previous genetic association studies have reported a possible role of the dopamine transporter (DAT, gene symbol: SLC6A3) gene in the etiology of alcohol dependence, but the results were conflicting with each other. We conducted a pooled analysis of published population-based case-control genetic studies investigating associations between polymorphisms in SLC6A3 and alcohol dependence. We also explored whether geographic area, ethnicity, gender, and diagnostic criteria moderated any association by using stratified analysis. Through combining 13 studies with 2483 cases and 1753 controls, the 40-base pair variable number tandem repeat (VNTR) in the 3' un-translated region, the well studied polymorphism in SLC6A3, did not show any association with alcohol dependence in general or in stratified analyses according to geographic area, ethnicity, gender, and diagnostic criteria. Due to limited studies focused on polymorphisms in other regions of the SLC6A3 gene, we cannot rule out the role of the SLC6A3 gene in the involvement of the genetic risk of alcohol dependence. Further clarification of the genetic role of SLC6A3 in the susceptibility to alcohol dependence should be centered on other potential functional regions of the SLC6A3 gene.

No influence of SLC6A3 40 base VNTR polymorphism on the response to risperidone

Abstract Objectives. The SLC6A3 40 base variable number of tandem repeats (VNTR) polymorphism has been associated with several clinical phenotypes associated with dysregulation of dopamine transmission. However, there is only little evidence about a possible influence of such genetic variant on the response to antipsychotics. The aim of the present study is to investigate whether SLC6A3 40 base VNTR polymorphism could modulate response to risperidone in a sample of Korean schizophrenia subjects. Methods. One hundred and forty-two schizophrenia inpatients were treated with a flexible dose of risperidone. Efficacy was assessed at baseline and at discharge using the scores of the Clinical Global Impression-severity (CGI-S), Brief Psychiatric Rating Scale (BPRS) and Positive and Negative Symptom Score (PANSS). Multivariate analysis of covariance was used to test possible influences of SLC6A3 VNTR variants on clinical scores. Results. None of the genotypes and of the alleles under investigation was associated with clinical scores at discharge or with changes of clinical scores over time. In addition, we also failed to find any association between genotypes and allele frequency distribution in accordance with treatment response defined as a 20% (or 30%) or more reduction in the total PANSS scores from the baseline to the end of treatment. Conclusion. Our findings do not suggest a possible association between SLC6A3 40 base VNTR polymorphism and response to risperidone. However, because of several limitations including the investigation of a single drug, the flexible design of the present study and the absence of a complete coverage of features which could influence the response, further investigations could be required.

Pharacogenetic effects of dopamine transporter gene polymorphisms on response to chlorpromazine and clozapine and on extrapyramidal syndrome in schizophrenia

A number of studies have investigated the effectiveness of the dopamine transporter (SLC6A3) Gene as an antipsychotic target. However, the focus has mainly been on a 40-bp variable number of a tandem repeat (VNTR) in the 3'-region and results have been inconsistent. To fully evaluate SLC6A3 as a therapeutic antipshycotic target we investigated association of the gene with responses to chlorpromazine and clozapine and with chlorpromazine-induced extrapyramidal syndrome (EPS) in the Chinese schizophrenia population. Six polymorphisms across the whole region of this gene were analyzed, namely rs2652511 (T-844C) and rs2975226 (T-71A) in the 5'-regulatory region, rs2963238 (A1491C) in intron 1, a 30-bp VNTR in intron 8, rs27072 and the 40-bp VNTR in the 3'-region. We found that the polymorphic marker, rs2975226, showed significant association of allele and genotype frequencies with response to clozapine (allele-wise: adjusted p=0.00404; genotype-wise: adjusted p=0.024), and that patients with the T allele had a better response to the drug. The haplotype block constructed from the first three markers near the 5'-region showed significant association with response to clozapine (for haplotype T-T-A: p=0.0085; for haplotype C-A-C: p=0.0092). We did not identify any significant association of the six genetic variants or haplotypes with EPS after Bonferoni correction. Our findings suggest that the 5'-regulatory region of SLC6A3 plays an important role in response to clozapine and that its role in EPS needs to be replicated in a large-scale well designed study.

Genetic polymorphisms in the dopamine-2 receptor (DRD2), dopamine-3 receptor (DRD3), and dopamine transporter (SLC6A3) genes in schizophrenia: Data from an association study

OBJECTIVE

To investigate the association between dopaminergic polymorphisms [DRD2 -141C Ins/Del, DRD3 Ser9Gly, and SLC6A3 VNTR] and schizophrenia.

METHODS

Two hundred and eighty-eight outpatients with schizophrenia (DSM-IV criteria) [mean age (SD)=36.4 (12.4), 60.1% males] and 421 unrelated healthy controls [mean age (SD)=40.6 (11.3), 51.3% males] from a homogeneous Spanish Caucasian population were genotyped using standard methods.

RESULTS

There was a significant difference in genotype distribution for the DRD2 -141C Ins/Del polymorphism [(chi(2) (2)=12.35, corrected p=0.012]. The -141C Del allele was more common in patients than in controls [0.19 vs. 0.13; chi(2) (1)=9.14, corrected p=0.018, OR (95% CI)=1.57 (1.17-2.10)]. Genotype and allele distributions for DRD3 Ser9Gly and SLC6A3 VNTR polymorphisms were similar in both groups. However, there was tentative evidence of an interaction effect between DRD3 Ser9Gly and SLC6A3 VNTR [Wald=9.56 (4), p=0.049]. Compared to the SLC6A3 10/10 genotype category, the risk of schizophrenia was halved among those with 9/10 [OR=0.51 (95% CI=0.30-0.89), p=0.017]. This protective effect was only present in combination with DRD3 Ser/Ser genotype because of the significant interaction between 9/10 and both Ser/Gly [OR=2.45 (95% CI=1.16-5.17), p=0.019] and Gly/Gly [OR=3.80 (95% CI=1.24-11.63), p=0.019].

CONCLUSIONS

This study provides evidence that a genetic variant in the DRD2 gene and possible interaction between DRD3 and SLC6A3 genes are associated with schizophrenia. These findings warrant examination in replication studies.

Association of promoter variants of human dopamine transporter gene with schizophrenia in Han Chinese

OBJECTIVE

Although dopamine was implicated in the etiology of schizophrenia, the human dopamine transporter gene (DAT1; SLC6A3) has not consistently been associated with schizophrenia. The purpose of this study was to examine whether six polymorphisms within the DAT1 gene are associated with schizophrenia.

METHODS

Six polymorphisms of the DAT1 gene (3 SNPs [rs6413429, rs2652511, and rs2975226] in the promoter region, one SNP [rs6347] in exon 9, and one SNP [rs27072]/one variable number tandem repeat [VNTR] in exon 15) were analyzed in 352 Chinese patients with schizophrenia and in 311 healthy controls. Pretreatment psychopathology was assessed using the Positive and Negative Syndrome Scale in a subset of 160 hospitalized schizophrenia patients who were drug-free or drug-naïve.

RESULTS

A statistically significant difference in two polymorphisms (rs2652511 and rs2975226) and a promoter region haplotype (rs2652511, rs2975226, and rs6413429) was found between patients and healthy controls. No association with schizophrenia was found for other polymorphisms and another haplotype (3' region). Symptoms severity (PANSS global, positive, negative and general symptoms scores) was similar regardless of DAT1 polymorphism.

CONCLUSION

The promoter region of the DAT1 gene may play a role in increasing susceptibility to schizophrenia, but does not affect the severity of psychotic symptoms in Han Chinese.

Molecular genetic case-control women investigation from the first Brazilian high-risk study on functional psychosis

OBJECTIVE

Data from epidemiological studies have demonstrated that genetics is an important risk factor for psychosis. The present study is part of a larger project, pioneer in Brazil, which has been conducted by other researchers who intend to follow a high-risk population (children) for the development of schizophrenia and bipolar disorder. In this first phase of the project, the objective was to investigate the distribution of four candidate genetic polymorphisms for functional psychosis (Ser9Gly DRD3, 5HTTLPR, the VNTR 3'-UTR SLC6A3 and Val66Met BDNF) in a case-control sample.

METHOD

A total of 105 women (58 with schizophrenia and 47 with bipolar disorder) and 62 gender-matched controls were investigated.

RESULTS

Allele and genotype distributions of all identified functional polymorphisms did not differ statistically between cases and controls.

CONCLUSIONS

These results suggest that the investigated polymorphisms were not related to susceptibility to functional psychoses in our Brazilian sample. These findings need to be validated in larger and independent studies.

Dopamine transporter polymorphism modulates oculomotor function and DAT1 mRNA expression in schizophrenia

Smooth pursuit eye movement (SPEM) deficit is an established schizophrenia endophenotype with a similar neurocognitive construct to working memory. Frontal eye field (FEF) neurons controlling SPEM maintain firing when visual sensory information is removed, and their firing rates directly correlate with SPEM velocity. We previously demonstrated a paradoxical association between a functional polymorphism of dopamine signaling (COMT gene) and SPEM. Recent evidence implicates the dopamine transporter gene (DAT1) in modulating cortical dopamine and associated neurocognitive functions. We hypothesized that DAT1 10/10 genotype, which reduces dopamine transporter expression and increases extracellular dopamine, would affect SPEM. We examined the effects of DAT1 genotype on: Clinical diagnosis in the study sample (n = 418; 190 with schizophrenia), SPEM measures in a subgroup with completed oculomotor measures (n = 200; 87 schizophrenia), and DAT1 gene expression in FEF tissue obtained from postmortem brain samples (n = 32; 16 schizophrenia). DAT1 genotype was not associated with schizophrenia. DAT1 10/10 genotype was associated with better SPEM in healthy controls, intermediate SPEM in unaffected first-degree relatives of schizophrenia subjects, and worse SPEM in schizophrenia subjects. In the gene expression study, DAT1 10/10 genotype was associated with significantly reduced DAT1 mRNA transcript in FEF tissue from healthy control donors (P < 0.05), but higher expression in schizophrenia donors. Findings suggest regulatory effects of another gene(s) or etiological factor in schizophrenia, which modulate DAT1 gene function.

Dopamine transporter polymorphisms and risperidone response in Chinese schizophrenia patients: an association study

Risperidone is a widely used atypical antipsychotic medication and there is currently considerable interest in individual differences in patient response to it. In this study, we investigated the pharmacogenetic correlates of SLC6A3 and response to risperidone treatment in 130 Chinese schizophrenia patients. We selected six polymorphisms, including two SNPs in the 5'-regulatory regions, two SNPs in intron 1, one SNP and a variable number tandem repeat in the 3'-flanking region of SLC6A3 for this study and analyzed the differences in the reduction of Positive and Negative Syndrome Scale (PANSS) scores among the subgroups with different genotypes and diplotypes after 8 weeks of risperidone treatment. The confounding effects of nongenetic factors were estimated and the baseline symptom score was included as a covariate for adjustment. We found no significant differences in response to treatment in terms of PANSS or subscores improvements among the subgroups according to different genotypes and diplotypes. In addition, we have found no significant differences between different diplotypic groups in the plasma levels of risperidone and 9-hydroxyrisperidone. Further studies on larger groups and on the effects of longer-term risperidone treatment are needed to confirm these results.

A network of dopaminergic gene variations implicated as risk factors for schizophrenia

We evaluated the hypothesis that dopaminergic polymorphisms are risk factors for schizophrenia (SZ). In stage I, we screened 18 dopamine-related genes in two independent US Caucasian samples: 150 trios and 328 cases/501 controls. The most promising associations were detected with SLC6A3 (alias DAT), DRD3, COMT and SLC18A2 (alias VMAT2). In stage II, we comprehensively evaluated these four genes by genotyping 68 SNPs in all 478 cases and 501 controls from stage I. Fifteen (23.1%) significant associations were found (p < or = 0.05). We sought epistasis between pairs of SNPs providing evidence of a main effect and observed 17 significant interactions (169 tests); 41.2% of significant interactions involved rs3756450 (5' near promoter) or rs464049 (intron 4) at SLC6A3. In stage III, we confirmed our findings by genotyping 65 SNPs among 659 Bulgarian trios. Both SLC6A3 variants implicated in the US interactions were overtransmitted in this cohort (rs3756450, p = 0.035; rs464049, p = 0.011). Joint analyses from stages II and III identified associations at all four genes (p(joint) < 0.05). We tested 29 putative interactions from stage II and detected replication between seven locus pairs (p < or = 0.05). Simulations suggested our stage II and stage III interaction results were unlikely to have occurred by chance (p = 0.008 and 0.001, respectively). In stage IV we evaluated rs464049 and rs3756450 for functional effects and found significant allele-specific differences at rs3756450 using electrophoretic mobility shift assays and dual-luciferase promoter assays. Our data suggest that a network of dopaminergic polymorphisms increase risk for SZ.

Dopamine genes and schizophrenia: case closed or evidence pending?

The dopamine hypothesis of schizophrenia (SZ) has motivated a large number of genetic association studies but few if any dopaminergic (DA) polymorphisms are accepted as credible risk factors at present. To evaluate whether dopamine-related genes have been investigated adequately, we surveyed public genetic databases and published SZ association studies with regard to 14 conventional DA genes and 7 selected dopamine-interacting proteins. We estimate that 325 polymorphisms would be required to evaluate the impact of common variation on SZ risk among Caucasian samples. To date, 98 polymorphisms have been analyzed in published association studies. We estimate that only 19 of these variations have been evaluated in samples with at least 50% power to detect an association of the effect size commonly found in genetically complex disorders. While it is possible that DA genes do not harbor genetic risk factors for SZ, our review suggests that satisfactory conclusions for most genes cannot be drawn at present. Whole-genome association studies have begun to fill this void, but additional analyses are likely to be needed. Recommendations for future association studies include analysis of adequately powered samples, judiciously selected polymorphisms, multiple ethnic groups, and concurrent evaluation of function at associated single-nucleotide polymorphisms.

Family-based study of markers at the 5'-flanking region of the human dopamine transporter gene reveals potential association with schizophrenic psychoses

The dopamine hypothesis of schizophrenia proposes an inherited or acquired presynaptic hyperactivity of dopaminergic neurons. The human dopamine transporter gene (hSLC6A3; hDAT) represents one major mechanism for the termination of dopaminergic neurotransmission. This study examines the degree of genetic association of the 5'-untranslated region (5'-UTR) of the hSLC6A3 to schizophrenia in a family-based association design. Five single nucleotide polymorphisms (SNPs) derived by a previous systematic mutation scan approximately 1.2 kb of the 5'-UTR of the hSLC6A3 locus were genotyped for transmission disequilibrium between 82 index cases (56 males) with schizophrenia and their biological parents. We observed no preferential transmission of alleles from heterozygous parents to affected offspring. Five estimated haplotypes accounted for a frequency of 90% in the index cases, and were identical in cases and non-transmitted parental control haplotypes. Distinct five-locus-genotypes accumulated in schizophrenia compared to parental controls at P-value 0.0038 with odds-ratio of 2.02 (95% CI 0.99-4.14). In conclusion, our present findings support the genetic involvement of distinct hSLC6A3 genotypes in schizophrenia. We propose replication in extended samples and examination of the functional relevance of the associated genotypes on human dopamine transporter expression.

Association study of 5'-UTR polymorphisms of the human dopamine transporter gene with manic depression

OBJECTIVES

To determine the degree of association of five single nucleotide polymorphisms at the 5'-untranslated region (5'-UTR) of the human dopamine transporter gene (hSLC6A3; hDAT1) in bipolar affective disorder.

METHODS

In a case-control design study, the polymorphisms were genotyped for allelic and genotypic distribution between 105 index cases (50 males) with bipolar affective disorder according to DSM IV and 199 unaffected control subjects (120 males).

RESULTS

At the 5'-UTR locus of hSLC6A3, no significant allelic or genotypic differences were observed between index cases and controls. However, distinct 5-locus genotypes accumulated in subjects with bipolar affective disorder compared to control subjects (p = 0.029, odds ratio 1.84, 95% confidence interval 1.12-3.02).

CONCLUSIONS

In conclusion, our data do not provide evidence for a major role of the 5'-UTR of the dopamine transporter gene in bipolar affective disorder. A minor contribution of distinct genotypes may be possible and warrants replication in extended samples.

Evaluation of a susceptibility gene for schizophrenia: genotype based meta-analysis of RGS4 polymorphisms from thirteen independent samples

BACKGROUND

Associations between schizophrenia (SCZ) and polymorphisms at the regulator of G-protein signaling 4 (RGS4) gene have been reported (single nucleotide polymorphisms [SNPs] 1, 4, 7, and 18). Yet, similar to other SCZ candidate genes, studies have been inconsistent with respect to the associated alleles.

METHODS

In an effort to resolve the role for RGS4 in SCZ susceptibility, we undertook a genotype-based meta-analysis using both published and unpublished family-based and case-control samples (total n = 13,807).

RESULTS

The family-based dataset consisted of 10 samples (2160 families). Significant associations with individual SNPs/haplotypes were not observed. In contrast, global analysis revealed significant transmission distortion (p = .0009). Specifically, analyses suggested overtransmission of two common haplotypes that account for the vast majority of all haplotypes. Separate analyses of 3486 cases and 3755 control samples (eight samples) detected a significant association with SNP 4 (p = .01). Individual haplotype analyses were not significant, but evaluation of test statistics from individual samples suggested significant associations.

CONCLUSIONS

Our collaborative meta-analysis represents one of the largest SCZ association studies to date. No individual risk factor arose from our analyses, but interpretation of these results is not straightforward. Our analyses suggest risk due to at least two common haplotypes in the presence of heterogeneity. Similar analysis for other putative susceptibility genes is warranted.

Major affective disorders and schizophrenia: a common molecular signature?

Psychiatric disorders, including affective disorders (AD) and schizophrenia (SZ) are among the most common disabling brain diseases in Western populations and result in high costs in terms of morbidity as well as mortality. Although their etiology and pathophysiology is largely unknown, family-, twin-, and adoption studies argue for a strong genetic determination of these disorders. These studies indicate that there is between 40 and 85% heritability for these disorders but point also to the importance of environmental factors. Therefore, any research strategy aiming at the identification of genes involved in the development of AD and SZ should account for the complex nature (multifactorial) of these disorders. During the last decade, molecular genetic studies have contributed a great deal to the identification of genetic factors involved in complex disorders. Here we provide a comprehensive review of the most promising genes for AD and SZ, and the methods and approaches that were used for their identification. Also, we discuss the current knowledge and hypotheses that have been formulated regarding the effect of variations on protein functioning as well as recent observations that point to common molecular mechanisms.