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

Association of Polymorphism within the Putative miRNA Target Site in the 3'UTR Region of the DRD2 Gene with Neuroticism in Patients with Substance Use Disorder

The study aims at looking into associations between the polymorphism rs6276 that occurs in the putative miRNA target site in the 3'UTR region of the DRD2 gene in patients with substance use disorder (SUD) comorbid with a maniacal syndrome (SUD MANIA). In our study, we did not state any essential difference in DRD2 rs6276 genotype frequencies in the studied samples of SUD MANIA, SUD, and control subjects. A significant result was found for the SUD MANIA group vs. SUD vs. controls on the Neuroticism Scale of NEO FFI test, and DRD2 rs6276 (p = 0.0320) accounted for 1.7% of the variance. The G/G homozygous variants were linked with lower results on the neuroticism scale in the SUD MANIA group because G/G alleles may serve a protective role in the expression of neuroticism in patients with SUD MANIA. So far, there have been no data in the literature on the relationship between the miRSNP rs6276 region in the DRD2 gene and neuroticism (personal traits) in patients with a diagnosis of substance use disorder comorbid with the affective, maniacal type disturbances related to SUD. This is the first report on this topic.

Genetic Factors Associated With Tardive Dyskinesia: From Pre-clinical Models to Clinical Studies

Tardive dyskinesia is a severe motor adverse event of antipsychotic medication, characterized by involuntary athetoid movements of the trunk, limbs, and/or orofacial areas. It affects two to ten patients under long-term administration of antipsychotics that do not subside for years even after the drug is stopped. Dopamine, serotonin, cannabinoid receptors, oxidative stress, plasticity factors, signaling cascades, as well as CYP isoenzymes and transporters have been associated with tardive dyskinesia (TD) occurrence in terms of genetic variability and metabolic capacity. Besides the factors related to the drug and the dose and patients’ clinical characteristics, a very crucial variable of TD development is individual susceptibility and genetic predisposition. This review summarizes the studies in experimental animal models and clinical studies focusing on the impact of genetic variations on TD occurrence. We identified eight genes emerging from preclinical findings that also reached statistical significance in at least one clinical study. The results of clinical studies are often conflicting and non-conclusive enough to support implementation in clinical practice.

Chronic methamphetamine interacts with BDNF Val66Met to remodel psychosis pathways in the mesocorticolimbic proteome

Methamphetamine (Meth) abuse has reached epidemic proportions in many countries and can induce psychotic episodes mimicking the clinical profile of schizophrenia. Brain-derived neurotrophic factor (BDNF) is implicated in both Meth effects and schizophrenia. We therefore studied the long-term effects of chronic Meth exposure in transgenic mice engineered to harbor the human BDNF^Val66Met polymorphism expressed via endogenous mouse promoters. These mice were chronically treated with an escalating Meth regime during late adolescence. At least 4 weeks later, all hBDNF^Val66Met Meth-treated mice exhibited sensitization confirming persistent behavioral effects of Meth. We used high-resolution quantitative mass spectrometry-based proteomics to biochemically map the long-term effects of Meth within the brain, resulting in the unbiased detection of 4808 proteins across the mesocorticolimbic circuitry. Meth differentially altered dopamine signaling markers (e.g., Dat, Comt, and Th) between hBDNF^Val/Val and hBDNF^Met/Met mice, implicating involvement of BDNF in Meth-induced reprogramming of the mesolimbic proteome. Targeted analysis of 336 schizophrenia-risk genes, as well as 82 growth factor cascade markers, similarly revealed that hBDNF^Val66Met genotype gated the recruitment of these factors by Meth in a region-specific manner. Cumulatively, these data represent the first comprehensive analysis of the long-term effects of chronic Meth exposure within the mesocorticolimbic circuitry. In addition, these data reveal that long-term Meth-induced brain changes are strongly dependent upon BDNF genetic variation, illustrating how drug-induced psychosis may be modulated at the molecular level by a single genetic locus.

Association of DRD2 gene polymorphisms with schizophrenia in the young Bangladeshi population: A pilot study

Purpose

DRD2 gene is considered one of the most important candidate genes for the schizophrenia (SCZ) development due to its role in dopamine signaling and no genetic association study has been conducted yet on the Bangladeshi SCZ patients. The objective of the present study was to investigate the association of DRD2 genetic polymorphisms (rs4648317, rs4936270, and rs7131056) with SCZ in the Bangladeshi population.

Patients and methods

This case-control study consisted of 101 SCZ patients and 101 controls. Genotyping was performed by the polymerase chain reaction and restriction fragment length polymorphism (PCR–RFLP) method.

Results

The average ages were 22.15 and 22.09 years in patients and controls, respectively (p > 0.05). CT genotype of rs4936270 showed a significantly higher risk for the development of SCZ compared to CC genotype (OR = 2.0, p = 0.023), whereas no association was found for TT genotype. For the dominant model and T allele, rs4936270 showed a higher risk for the development of SCZ (OR = 2.01, p = 0.020; OR = 1.76, p = 0.021, respectively), while the recessive model had no association with SCZ. A statistically significant (OR = 2.70, p = 0.036) higher risk was found for the AA genotype, but no association was found for GA genotype of rs4648317 SNP compared to GG genotype. In case of dominant and recessive models, rs4648317 showed no association with SCZ. ‘A’ allele of rs4648317 SNP was found to be significantly associated with the elevated risk of SCZ (OR = 1.50, p = 0.044). No association with SCZ of rs7131056 SNP was found for AC, CC genotypes, dominant, recessive, and allele models. Furthermore, from the haplotyping analysis, we found that CAA and TAA haplotypes of rs4936270, rs7131056 and rs4648317 SNPs are associated with SCZ (χ2 = 8.26, p = 0.004; χ2 = 5.31, p = 0.021, respectively). After Bonferroni correction, the association of SCZ did not withstand with any genotype, allele and haplotype (p < 0.017) except CAA haplotype.

Conclusion

Our results suggest that DRD2 gene polymorphisms may be associated with the susceptibility of SCZ in the young Bangladeshi population.

Association between DRD2 and ANKK1 polymorphisms with the deficit syndrome in schizophrenia

BACKGROUND

The clinical course of schizophrenia varies among patients and is difficult to predict. Some patient populations present persistent negative symptoms, referred to as the deficit syndrome. Compared to relatives of non-deficit schizophrenia patients, family members of this patient population are at an increased risk of developing schizophrenia. Therefore, the aim of this study was to search for genetic underpinnings of the deficit syndrome in schizophrenia.

METHODS

Three SNPs, i.e., rs1799732 and rs6276 located within DRD2, and rs1800497 within ANKK1, were identified in the DNA samples of 198 schizophrenia probands, including 103 patients with deficit (DS) and 95 patients with non-deficit schizophrenia (NDS). Results: No significant differences concerning any of the analyzed polymorphisms were found between DS and NDS patients. However, significant links were observed between family history of schizophrenia and the deficit syndrome, G/G genotype and rs6276 G allele. In a separate analysis, we identified significant differences in frequencies of rs6276 G allele between DS and NDS patients with family history of schizophrenia. No significant associations were found between DRD2 and ANKK1 SNPs and the age of onset or schizophrenia symptom severity.

CONCLUSIONS

The results of our preliminary study fail to provide evidence of associations between DRD2 and ANKK1 polymorphisms with the deficit syndrome or schizophrenia symptom severity, but suggest potential links between rs6276 in DRD2 and the deficit syndrome in patients with hereditary susceptibility to schizophrenia. However, further studies are necessary to confirm this observation.

Genetic variants in dopamine receptors influence on heterodimerization in the context of antipsychotic drug action

Human dopamine D2 receptor (D₂R) gene has polymorphic variants, three of them alter its amino acid sequence: Val96Ala, Pro310Ser and Ser311Cys. Their functional role never became the object of extensive studies, even though there are some evidence that they correlate with schizophrenia. The present work reviews data indicating that these mutations play a role in dimer formation with dopamine D1 receptor (D₁R), with the strongest effect observed for Ser311Cys variant. Similarly, the affinity for antipsychotic drugs of this genetic variant depends on whether it is expressed together with D₁R or not. Better understanding of altered ability of genetic variants of D₂R to form dimers with D₁R, as well as of altered affinity for antipsychotic drugs, depending on the absence or presence of the second dopamine receptor is of great importance-since these two receptors are not always co-expressed in the same cell. It may well be that targeting new compounds toward the D₁R-D₂R dimers, which the most probably form under conditions of excessive dopamine release, will result in antipsychotic drugs devoid of serious side effects.

Emotional stability is associated with the MAOA promoter uVNTR polymorphism in women

BACKGROUND

Neuroticism is associated with low emotional stability, and it is characterized by a tendency to perceive ordinary situations as threatening and difficult to manage. This personality trait has been associated with psychological distress and predicts some mental disorders. Previous studies have shown that women tend to be more neurotic than men and, in general, females have also a higher incidence of anxious and depressive disorders.

METHODS

We analyzed in a sample of 99 female university students (from 18 to 26 years old) if emotional stability, measured using the Big Five Questionnaire, was linked to polymorphic variants in candidate genes related to dopaminergic and serotonergic systems, and other personality variables.

RESULTS

We found that emotional stability and its subdimensions are genetically associated with MAOA-uVNTR polymorphism. Thus, women carriers of the 3-repeat allele (lower MAO-A expression) showed higher levels of emotional stability. No associations were found with other polymorphisms analyzed, including COMT Val¹⁵⁸ Met, 5-HTTLPR, and DAT 3'UTR VNTR. Furthermore, our results showed a negative correlation between emotional stability and depression, state anxiety, and trait anxiety. In fact, MAOA-uVNTR and trait anxiety also explained emotional stability and its subdimensions. We also found that other genetic characteristic, phenylthiocarbamide tasting, explained impulsivity, specifically tasters controlled impulses better than nontasters.

CONCLUSION

Our results indicate that neuroticism might be regulated by MAOA and could be a common factor between different phenotypes, such as aggressive behaviors or personality disorders, observed in women with higher activity genotype who had been exposed to negative environments during childhood. This study could lead to a better understanding of the basis of emotional stability and could lead to future projects for this purpose.

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.

Evaluation of association of DRD2 TaqIA and -141C InsDel polymorphisms with food intake and anthropometric data in children at the first stages of development

The reward sensation after food intake may be different between individuals and variants in genes related to the dopaminergic system may indicate a different response in people exposed to the same environmental factors. This study investigated the association of TaqIA (rs1800497) and -141C InsDel (rs1799732) variants in DRD2/ANKK1 gene with food intake and adiposity parameters in a cohort of children. The sample consisted of 270 children followed until 7 to 8 years old. DNA was extracted from blood and polymorphisms were detected by PCR-RFLP analysis. Food intake and nutritional status were compared among individuals with different SNP genotypes. Children carrying the A1 allele (TaqIA) had higher energy of lipid dense foods (LDF) when compared with A2/A2 homozygous children at 7 to 8 years old (GLM p=0.004; Mann Whitney p=0.005). No association was detected with -141C Ins/Del polymorphism. To our knowledge, this is the first association study of the DRD2 TaqIA and -141C Ins/Del polymorphism with food intake and anthropometric parameters in children. DRD2 TaqIA polymorphism has been associated with a reduction in D₂ dopamine receptor availability. Therefore, the differences observed in LDF intake in our sample may occur as an effort to compensate the hypodopaminergic functioning.

Neuropsychiatric disease-associated genetic variants of the dopamine transporter display heterogeneous molecular phenotypes

Genetic factors are known to significantly contribute to the etiology of psychiatric diseases such as attention deficit hyperactivity disorder (ADHD) and autism spectrum and bipolar disorders, but the underlying molecular processes remain largely elusive. The dopamine transporter (DAT) has received continuous attention as a potential risk factor for psychiatric disease, as it is critical for dopamine homeostasis and serves as principal target for ADHD medications. Constrain metrics for the DAT-encoding gene, solute carrier family 6 member 3 (SLC6A3), indicate that missense mutations are under strong negative selection, pointing to pathophysiological outcomes when DAT function is compromised. Here, we systematically characterized six rare genetic variants of DAT (I312F, T356M, D421N, A559V, E602G, and R615C) identified in patients with neuropsychiatric disorders. We evaluated dopamine uptake and ligand interactions, along with ion coordination and electrophysiological properties, to elucidate functional phenotypes, and applied Zn²⁺ exposure and a substituted cysteine-accessibility approach to identify shared structural changes. Three variants (I312F, T356M, and D421N) exhibited impaired dopamine uptake associated with changes in ligand binding, ion coordination, and distinct conformational disturbances. Remarkably, we found that all three variants displayed gain-of-function electrophysiological phenotypes. I312F mediated an increased uncoupled anion conductance previously suggested to modulate neuronal excitability. T356M and D421N both mediated a cocaine-sensitive leakage of cations, which for T356M was potentiated by Zn²⁺, concurrent with partial functional rescue. Collectively, our findings support that gain of disruptive functions due to missense mutations in SLC6A3 may be key to understanding how dopaminergic dyshomeostasis arises in heterozygous carriers.

Gene networks associated with non-syndromic intellectual disability

Non-syndromic intellectual disability (NS-ID) is a genetically heterogeneous disorder, with more than 200 candidate genes to date. Despite the increasing number of novel mutations detected, a relatively low number of recurrently mutated genes have been identified, highlighting the complex genetic architecture of the disorder. A systematic search of PubMed and Medline identified 245 genes harbouring non-synonymous variants, insertions or deletions, which were identified as candidate NS-ID genes from case reports or from linkage or pedigree analyses. From this list, 33 genes are common to syndromic intellectual disability (S-ID) and 58 genes are common to certain neurological and neuropsychiatric disorders that often include intellectual disability as a clinical feature. We examined the evolutionary constraint and brain expression of these gene sets, and we performed gene network and protein-protein interaction analyses using GeneGO MetaCore^TM and DAPPLE, respectively. The 245 NS-ID candidate genes were over-represented in axon guidance, synaptogenesis, cell adhesion and neurotransmission pathways, all of which are key neurodevelopmental processes for the establishment of mature neuronal circuitry in the brain. These 245 genes exhibit significantly elevated expression in human brain and are evolutionarily constrained, consistent with expectations for a brain disorder such as NS-ID that is associated with reduced fecundity. In addition, we report enrichment of dopaminergic and glutamatergic pathways for those candidate NS-ID genes that are common to S-ID and/or neurological and neuropsychiatric disorders that exhibit intellectual disability. Collectively, this study provides an overview and analysis of gene networks associated with NS-ID and suggests modulation of neurotransmission, particularly dopaminergic and glutamatergic systems as key contributors to synaptic dysfunction in NS-ID.

Association study of DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A polymorphisms with alcohol dependence

BACKGROUND

The reinforcing effects of alcohol are mediated through complex interactions between multiple neurochemical systems. Genes of dopaminergic (DRD2, DRD3 and DβH), opioid (OPRM1) and glutaminergic (GRIK1) systems mediate the dependent behavior via different mechanisms; however, they all target the serotonergic and dopaminergic pathways in the ventral tegmental area.

METHODS

In the present study, DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A polymorphisms and their interactions were analyzed in 72 alcohol-dependent patients and 74 controls of Greek-Cypriot origin, using the PCR-RFLP method.

RESULTS

No differences were found in the genotype or allele distribution of DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A between alcohol-dependent patients and controls. Additionally, we did not find any gene×gene interactions in association with alcohol dependence in the studied population.

CONCLUSIONS

Alcohol dependence is a complex interaction of genetic and environmental factors. In the present study, we have shown that DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A are not associated with this dependent behavior alone or in interaction.

Pharmacogenetic evaluation to assess breakthrough psychosis with aripiprazole long-acting injection: a case report

BACKGROUND

Given the complex nature of symptom presentation and medication regimens, psychiatric clinics may benefit from additional tools to personalize treatments. Utilizing pharmacogenetic information may be helpful in assessing unique responses to therapy. We report herein a case of wearing-off phenomena during treatment with aripiprazole long-acting injectable (LAI) and a proof of concept strategy of how pharmacogenetic information may be used to assess possible genetic factors and also hypothesize potential mechanisms for further study.

CASE PRESENTATION

A 51-year-old African American male with schizoaffective disorder was referred to a psychiatric clinic for medication management. After unsuccessful trials of multiple antipsychotics, oral aripiprazole was initiated (up to 30 mg/day) and transitioned to aripiprazole LAI with symptom improvement. At a high dose of aripiprazole LAI (400 mg Q3wks), the patient experienced breakthrough symptoms approximately 3 days prior to his next injection. Various considerations were examined to explain his atypical dose requirements, including but not limited to pharmacogenetic influences. Pharmacogenetic testing ruled out genetic influences on drug metabolism but noted a -141C Del variant in the dopamine-D2 receptor (DRD2) gene associated in prior studies of poor-response to antipsychotics. At this time, a new formulation, aripiprazole lauroxil, was explored due to its availability in higher dose options. Transition to the new formulation (882 mg Q4wks) greatly improved and stabilized the patient's symptoms with no breakthrough psychosis. Comparable daily dose equivalents were achieved with two different formulations due to the Q3wks vs Q4wks dosing strategies, although the two agents have some differences in pharmacokinetic profiles.

CONCLUSIONS

We report a case of a patient experiencing wearing-off symptoms with aripiprazole LAI who benefited from switching to aripiprazole lauroxil. Pharmacogenetic testing revealed normal activity for relevant metabolism pathways but a DRD2 -141C variant that may influence brain D2 expression and antipsychotic responsiveness. The clinical utility of DRD2 information and what to do with genotyping results has not been previously addressed, despite availability on clinical test panels. Our case report suggests further investigations of altered dosing strategies and receptor genotype sensitivities to pharmacokinetic factors may be helpful in understanding symptom re-emergence observed in some patients taking LAI antipsychotics.

Significance of Dopaminergic Gene Variants in the Male Biasness of ADHD

OBJECTIVE

ADHD is frequently detected in boys though there is no established cause. One possibility is that genes predisposing to ADHD have sexually dimorphic effects. With an aim to find out the reason for this male biasness, contribution of 14 functional polymorphisms was investigated in ADHD subjects.

METHOD

Genomic DNA of probands, their parents, and ethnically matched controls was subjected to analysis of single-nucleotide polymorphisms and variable number of tandem repeats (VNTRs).

RESULTS

Case-control analysis revealed significant higher occurrence of DAT1 intron 8 VNTR "5R" allele ( p = .028), DBH rs1108580 "A" allele ( p = .027), and MAOA-u VNTR-rs6323 3R-T haplotype ( p = .007) in male probands. Family-based analysis showed significant preferential transmission of Dopamine receptor D4 exon 3 VNTR-rs1800955 7R-T haplotype from parents to male probands ( p = .008). Interaction between DBH gene variants and low enzymatic activity was also noticed, especially in male probands.

CONCLUSION

Data obtained may partly answer the male biasness of ADHD.

No association between dopamine D3 receptor gene Ser9Gly polymorphism (rs6280) and risk of schizophrenia: an updated meta-analysis

OBJECTIVE

Ser9Gly (rs6280) is a functional single-nucleotide polymorphism (SNP) in the dopamine receptor D3 (DRD3) gene that may be associated with schizophrenia. We performed a meta-analysis to determine whether Ser9Gly influences the risk of schizophrenia and examined the relationship between the Ser9Gly SNP and the etiology of schizophrenia.

METHODS

Case-control studies were retrieved from literature databases in accordance with established inclusion criteria. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to evaluate the strength of the association between Ser9Gly and schizophrenia. Subgroup analysis and sensitivity analysis were also performed.

RESULTS

Seventy-three studies comprising 10,634 patients with schizophrenia (cases) and 11,258 controls were included in this meta-analysis. Summary results indicated no association between Ser9Gly and risk of schizophrenia. In the dominant genetic model, the pooled OR using a random effects model was 0.950 (95% CI, 0.847-1.064; P=0.374).

CONCLUSION

Results of this meta-analysis suggest that the Ser9Gly SNP is not associated with schizophrenia. These data provide possible avenues for future case-control studies related to schizophrenia.

Associations between dopamine D2 receptor gene polymorphisms and schizophrenia risk: a PRISMA compliant meta-analysis

OBJECTIVE

To determine the relationships between dopamine D2 receptor gene polymorphisms and the risk of schizophrenia using meta-analysis.

METHOD

The PubMed, Embase, and China National Knowledge Infrastructure databases were searched to identify relevant literature published up to February 2016. The allele contrast model was used. Stata software was used for statistical analysis, with odds ratios (ORs) and 95% confidence intervals (CIs) calculated to evaluate the associations between dopamine D2 receptor gene polymorphisms and the risk of schizophrenia. Meta-regression and publication bias, trim-and-fill, subgroup, sensitivity, cumulative, and fail-safe number analyses were also performed.

RESULTS

This meta-analysis included 81 studies. The rs1801028 and rs1799732 were associated with schizophrenia risk among Asians (P=0.04, OR =1.25, 95% CI =1.01-1.55; P<0.01, OR =0.76, 95% CI =0.63-0.92, respectively), while the rs6277 was associated with schizophrenia risk in Caucasians (P<0.01, OR=0.72, 95% CI =0.66-0.79). The rs1800497 was also associated with schizophrenia risk in population-based controls (P<0.01, OR =0.84, 95% CI =0.72-0.97). The rs6275, rs1079597, and rs1800498 were not associated with schizophrenia risk. In addition, meta-regression indicated that the controls may be sources of heterogeneity for the rs1801028 single-nucleotide polymorphism (SNP), while ethnicity may be sources of heterogeneity for the rs6277 SNP. Publication bias was significant for the rs1801028 SNP, and this result changed after the publication bias was adjusted using the trim-and-fill method.

CONCLUSION

This meta-analysis demonstrated that the rs1801028 may be a risk factor for susceptibility to schizophrenia among Asians, while the rs1799732 may be a protective factor for that population. Large-sample studies are necessary to verify the results of this meta-analysis.

The role of C957T, TaqI and Ser311Cys polymorphisms of the DRD2 gene in schizophrenia: systematic review and meta-analysis

Background

The association between the dopamine D2 receptor (DRD2) gene and schizophrenia has been studied though no conclusive outcomes have been attained. The aim of this study was to perform a systematic review and meta-analysis to explore the relation between three polymorphisms of the DRD2 gene (C957T, TaqI and Ser311Cys) and schizophrenia.

Methods

The search was made in PubMed and EBSCO databases (up to February 2016). The systematic review included 34 case–control association studies (34 for C957T, 16 for TaqI and 36 for Ser311Cys). The association analysis comprised the allelic, additive, dominant, and recessive genetic models. The meta-analysis was performed following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement.

Results

The meta-analysis showed that TaqI (additive model: OR 0.57, 95% CI 0.30–1.14) and C957T (additive model: OR 0.75, 95% OR 0.58–0.97, recessive model: OR 0.79, 95% CI 0.64–0.98) exert a protective effect against developing schizophrenia. However, the sub-analysis for the C957T variant showed that this polymorphism exhibits a risk factor effect on Chinese individuals (allelic model: OR 1.33, 95% CI 1.04–1.70).

Conclusion

Our meta-analysis suggests an association of the DRD2 gene and the risk for schizophrenia, given that TaqI and C957T polymorphisms presented a protective effect against schizophrenia, and in the sub-analyses the C957T variant increased the risk for this disorder in the Chinese population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12993-016-0114-z) contains supplementary material, which is available to authorized users.

-141C insertion/deletion polymorphism of the dopamine D2 receptor gene is associated with schizophrenia in Chinese Han population: Evidence from an ethnic group-specific meta-analysis

Accumulate evidence has implicated dopamine D2 receptor gene polymorphisms in the etiology of schizophrenia. A single nucleotide polymorphism, -141C insertion/deletion (Ins/Del) (rs1799732), in the promoter region of the dopamine D2 receptor gene has been linked to schizophrenia; however, the data are inconclusive. This study investigated whether the -141C polymorphism is associated with the risk of schizophrenia in different ethnic groups by performing a meta-analysis. A total of 24 case-control studies examining the association between -141C Ins/Del polymorphism and schizophrenia were identified according to established inclusion criteria. Significant association was revealed between -141C Ins/Del polymorphism and schizophrenia risk in dominant genetic model (Ins/Ins + Ins/Del versus Del/Del) (odds ratio = 0.33, 95% confidence interval = 0.14-0.81, z = 2.41, P = 0.02) in Chinese Han but not in Caucasian, Japanese or India populations. Our results indicate that -141C Ins/Del polymorphism might be a susceptibility factor for schizophrenia in Chinese Han population.

Child maltreatment, impulsivity, and antisocial behavior in African American children: Moderation effects from a cumulative dopaminergic gene index

A model examining the effects of an increasing number of maltreatment subtypes experienced on antisocial behavior, as mediated by impulsivity and moderated by a polygenic index of dopaminergic genotypes, was investigated. An African American sample of children (N = 1,012, M age = 10.07) with and without maltreatment histories participated. Indicators of aggression, delinquency, and disruptive peer behavior were obtained from peer- and counselor-rated measures to form a latent variable of antisocial behavior; impulsivity was assessed by counselor report. Five genotypes in four dopaminergic genes (dopamine receptors D4, D2, known as DRD4, DRD2; dopamine active transporter 1, known as DAT1; and catechol-O-methyltransferase, known as COMT) conferring heightened environmental sensitivity were combined into one polygenic index. Using structural equation modeling, a first-stage, moderated-mediation model was evaluated. Age and sex were entered as covariates, both as main effects and in interaction with maltreatment and the gene index. The model had excellent fit: χ2 (32, N = 1,012) = 86.51, p < .001; comparative fit index = 0.982, Tucker-Lewis index = 0.977, root mean square error of approximation = 0.041, and standardized root mean square residual = 0.022. The effect of maltreatment subtypes on antisocial behavior was partially mediated by impulsivity (β = 0.173, p < .001), and these relations were moderated by the number of differentiating dopaminergic genotypes. Specifically, a significant Gene × Environment interaction (β = 0.016, p = .013) indicated that the relation between maltreatment and impulsivity was stronger as children evinced more differentiating genotypes, thereby strengthening the mediational effect of impulsivity on antisocial behavior. These findings elucidate the manner by which maltreated children develop early signs of antisocial behavior, and the genetic mechanisms involved in greater vulnerability for maladaptation in impulse control within the context of child maltreatment.

The -141C Ins/Del and Taq1A polymorphism in the dopamine D2 receptor gene may confer susceptibility to schizophrenia in Asian populations

It has been reported that two single nucleotide polymorphisms (SNP) Taq1A and -141C Ins/Del in the DRD2 gene may be associated with susceptibility to schizophrenia. Due to inconclusive and mixed results, a meta-analysis was conducted to further clarify the relationship between the two SNP and schizophrenia susceptibility. A systematic literature search for the association of these two SNP with schizophrenia susceptibility was conducted using PubMed, ScienceDirect, Chinese Biomedical Literature Database, and Chinese National Knowledge Infrastructure. Odds ratios (OR) with 95% confidence intervals (CI) were used to assess the strength of the associations reported. A total of 5558 schizophrenic patients and 6792 healthy controls from 31 articles were included in this study. Evidence regarding the association between -141C Ins/Del polymorphism and schizophrenia was found in the allele frequency comparison (Ins versus Del: OR 1.29, 95% CI 1.06-1.57; p=0.01, Praw=0.1, PFalse Discovery Rate=0.023). In ethnic subgroup analysis, the result revealed that the 141C Ins/Del polymorphism was associated with schizophrenia in all genetic models in Asians, but not in Caucasians. For Taq1A polymorphism, a significant association was found in the allele frequency (A1 versus A2: OR 0.71, 95% CI 0.52-0.98, p=0.03). Stratification by ethnicity indicated an association between the Taq1A polymorphism and schizophrenia in Asians, but not Caucasians. The present study suggests that the -141C Ins/Del polymorphism carries a significantly increased risk of schizophrenia, while the Taq1A polymorphism carries a significantly decreased risk of schizophrenia susceptibility in Asians.

Identification of rare high-risk copy number variants affecting the dopamine transporter gene in mental disorders

BACKGROUND

The dopamine transporter, also known as solute carrier 6A3 (SLC6A3), plays an important role in synaptic transmission by regulating the reuptake of dopamine in the synapses. In line with this, variations in the gene encoding this transporter have been linked to both schizophrenia and affective disorders. Recently, copy number variants (CNVs) in SLC6A3 have been identified in healthy subjects but so far, the implication of CNVs affecting this gene in psychiatric diseases has not been addressed.

AIMS

In the present study, we aimed to investigate whether CNVs affecting SLC6A3 represent rare high-risk variants of psychiatric disorders.

METHODS

We performed a systematic screening for CNVs affecting SLC6A3 in 761 healthy controls, 672 schizophrenia patients, and 194 patients with bipolar disorder in addition to 253 family members from six large pedigrees affected by mental disorders using single nucleotide polymorphism arrays and subsequent verification by real-time polymerase chain reaction.

RESULTS

We identified two duplications and one deletion affecting SLC6A3 in the patients, while no such CNVs were identified in any of the controls. The identified CNVs were of different sizes and two affected several genes in addition to SLC6A3.

CONCLUSION

Our findings suggest that rare high-risk CNVs affecting the gene encoding the dopamine transporter contribute to the pathogenesis of schizophrenia and affective disorders.

DNA microarray unravels rapid changes in transcriptome of MK-801 treated rat brain

AIM: To investigate the impact of MK-801 on gene expression patterns genome wide in rat brain regions.

METHODS: Rats were treated with an intraperitoneal injection of MK-801 [0.08 (low-dose) and 0.16 (high-dose) mg/kg] or NaCl (vehicle control). In a first series of experiment, the frontoparietal electrocorticogram was recorded 15 min before and 60 min after injection. In a second series of experiments, the whole brain of each animal was rapidly removed at 40 min post-injection, and different regions were separated: amygdala, cerebral cortex, hippocampus, hypothalamus, midbrain and ventral striatum on ice followed by DNA microarray (4 × 44 K whole rat genome chip) analysis.

RESULTS: Spectral analysis revealed that a single systemic injection of MK-801 significantly and selectively augmented the power of baseline gamma frequency (30-80 Hz) oscillations in the frontoparietal electroencephalogram. DNA microarray analysis showed the largest number (up- and down- regulations) of gene expressions in the cerebral cortex (378), midbrain (376), hippocampus (375), ventral striatum (353), amygdala (301), and hypothalamus (201) under low-dose (0.08 mg/kg) of MK-801. Under high-dose (0.16 mg/kg), ventral striatum (811) showed the largest number of gene expression changes. Gene expression changes were functionally categorized to reveal expression of genes and function varies with each brain region.

CONCLUSION: Acute MK-801 treatment increases synchrony of baseline gamma oscillations, and causes very early changes in gene expressions in six individual rat brain regions, a first report.

Association between DRD2 (rs1799732 and rs1801028) and ANKK1 (rs1800497) polymorphisms and schizophrenia: a meta-analysis

The role of dopamine D2 receptor (DRD2) polymorphisms in schizophrenia remains controversial. We performed a meta-analysis to determine whether DRD2 polymorphisms influence the risk of schizophrenia and examined the relationship between rs1799732, rs1801028, and rs1800rs028 an23381d rs1800497 genetic variants and the etiology of schizophrenia. Relevant case-control studies were retrieved by database searches and selected according to established inclusion criteria. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to evaluate the strength of the associations. Meta-regression, subgroup analysis, sensitivity analysis, and cumulative meta-analysis were performed. A total of 76 studies with 16096 cases and 18965 controls were included. Specifically, 24 studies with 6075 cases and 6643 controls involved rs1799732, 36 studies with 8043 cases and 10194 controls involved rs1801028 and 16 studies with 1978 cases and 2128 controls involved rs1800497. No significant associations were observed between rs1799732 and rs1800rs732 and rs1800497 and schizophrenia. The rs1801028 locus was associated with schizophrenia, with a pooled OR of 1.221 (95% CI = 1.037-1.438, P = 0.016). This meta-analysis indicates that the rs1801028 locus may be associated with schizophrenia. These data provide possible references for future case-control studies related to schizophrenia.

Hypothesizing dopaminergic genetic antecedents in schizophrenia and substance seeking behavior

The dopamine system has been implicated in both substance use disorder (SUD) and schizophrenia. A recent meta-analysis suggests that A1 allele of the DRD2 gene imposes genetic risk for SUD, especially alcoholism and has been implicated in Reward Deficiency Syndrome (RDS). We hypothesize that dopamine D2 receptor (DRD2) gene Taq1 A2 allele is associated with a subtype of non-SUD schizophrenics and as such may act as a putative protective agent against the development of addiction to alcohol or other drugs of abuse. Schizophrenics with SUD may be carriers of the DRD2 Taq1 A1 allele, and/or other RDS reward polymorphisms and have hypodopaminergic reward function. One plausible mechanism for alcohol seeking in schizophrenics with SUD, based on previous research, may be a deficiency of gamma type endorphins that has been linked to schizophrenic type psychosis. We also propose that alcohol seeking behavior in schizophrenics, may serve as a physiological self-healing process linked to the increased function of the gamma endorphins, thereby reducing abnormal dopaminergic activity at the nucleus accumbens (NAc). These hypotheses warrant further investigation and cautious interpretation. We, therefore, encourage research involving neuroimaging, genome wide association studies (GWAS), and epigenetic investigation into the relationship between neurogenetics and systems biology to unravel the role of dopamine in psychiatric illness and SUD.

Functional −141C Ins/Del polymorphism in the dopamine D2 receptor gene promoter and schizophrenia in a Chinese Han population

Objective

The association between a putative functional promoter polymorphism, −141C insertion/deletion (Ins/Del), in the dopamine receptor D2 gene ( DRD2) and schizophrenia was investigated in a Chinese Han population.

Methods

The polymorphism was studied in unrelated schizophrenia patients and unrelated healthy controls. Linkage relationships were explored in core families of the schizophrenic patients using the transmission disequilibrium test. The Positive and Negative Syndrome Scale was used to evaluate the severity of the disorder.

Results

The Del allele was significantly less frequently found in patients (13/120; 11%) than in controls (18/100; 18%). In the 32 core families studied, 16 parents were Ins/Del heterozygotes. Parents transmitted the Ins and Del alleles to their children in 10 and six cases, respectively. Data from core families did not demonstrate linkage. Age, age at onset of schizophrenia and sex were not significantly different between carriers of the Ins and Del alleles. The group with the Ins allele had a significantly higher positive symptom score (75.3 ± 23.4 versus 53.9 ± 21.9) and excitement score (83.6 ± 16.8 versus 50.3 ± 24.6) than the Del group. Groups did not differ significantly in negative symptom and general psychopathology scores.

Conclusions

The DRD2 −141C Ins/Del polymorphism may affect susceptibility to schizophrenia in a Chinese Han population.

Neurocognitive effects of methylphenidate on ADHD children with different DAT genotypes: a longitudinal open label trial

The variable number of tandem repeat polymorphism in the 3'-untranslated region of the dopamine transporter gene (DAT) may influence the variability of the therapeutic response to methylphenidate (MPH) in Attention Deficit/Hyperactivity Disorder (ADHD). For this reason we evaluated the neuropsychological functioning after a prolonged period of MPH treatment and after a specific time from MPH suspension. Relationship between DAT VNTR genotypes and neurocognitive response to MPH was analyzed in a sample of 108 drug-naive ADHD patients. The performance of children with ADHD on measures of working memory, inhibition and planning was assessed at 4, 8 and 24 weeks and at 8 weeks after MPH withdrawal. Patients with 9/9 genotype evidenced an improvement in response inhibition and working memory only at 4 weeks of treatment, in planning at 24 weeks of therapy and after 8 weeks of MPH suspension. Patients with 9/10 showed an improvement in response inhibition at 4, 8 and 24 weeks of treatment, in planning at 24 weeks and after 8 weeks of MPH suspension. Patients with 10/10 evidenced an improvement in response inhibition and working memory at 4, 8 and 24 weeks of treatment and in planning at 4, 8 and 24 weeks of treatment and after 8 weeks of suspension. These results indicate that the 9/9 ADHD genotype has a different response at 24 weeks treatment with MPH. 10/10 DAT allele seems to be associated with an increased expression level of the dopamine transporter and seems to mediate the MPH treatment response in ADHD patients.

Dopamine transporter DAT and receptor DRD2 variants affect risk of lethal cocaine abuse: a gene-gene-environment interaction

Epistatic gene-gene interactions could contribute to the heritability of complex multigenic disorders, but few examples have been reported. Here, we focus on the role of aberrant dopaminergic signaling, involving the dopamine transporter DAT, a cocaine target, and the dopamine D2 receptor, which physically interacts with DAT. Splicing polymorphism rs2283265 of DRD2, encoding D2 receptors, were shown to confer risk of cocaine overdose/death (odds ratio ∼3) in subjects and controls from the Miami Dade County Brain Bank.(1) Risk of cocaine-related death attributable to the minor allele of rs2283265 was significantly enhanced to OR=7.5 (P=0.0008) in homozygous carriers of the main 6-repeat allele of DAT rs3836790, a regulatory VNTR in intron8 lacking significant effect itself. In contrast, carriers of the minor 5-repeat DAT allele showed no significant risk (OR=1.1, P=0.84). DAT rs3836790 and DRD2 rs2283265 also interacted by modulating DAT protein activity in the ventral putamen of cocaine abusers. In high-linkage disequilibrium with the VNTR, DAT rs6347 in exon9 yielded similar results. Assessing the impact of DAT alone, a rare DAT haplotype formed by the minor alleles of rs3836790 and rs27072, a regulatory DAT variant in the 3'-UTR, occurred in nearly one-third of the cocaine abusers but was absent in African American controls, apparently conferring strong risk. These results demonstrate gene-gene-drug interaction affecting risk of fatal cocaine intoxication.

Dopamine transporter gene variant affecting expression in human brain is associated with bipolar disorder

The gene encoding the dopamine transporter (DAT) has been implicated in CNS disorders, but the responsible polymorphisms remain uncertain. To search for regulatory polymorphisms, we measured allelic DAT mRNA expression in substantia nigra of human autopsy brain tissues, using two marker SNPs (rs6347 in exon 9 and rs27072 in the 3'-UTR). Allelic mRNA expression imbalance (AEI), an indicator of cis-acting regulatory polymorphisms, was observed in all tissues heterozygous for either of the two marker SNPs. SNP scanning of the DAT locus with AEI ratios as the phenotype, followed by in vitro molecular genetics studies, demonstrated that rs27072 C>T affects mRNA expression and translation. Expression of the minor T allele was dynamically regulated in transfected cell cultures, possibly involving microRNA interactions. Both rs6347 and rs3836790 (intron8 5/6 VNTR) also seemed to affect DAT expression, but not the commonly tested 9/10 VNTR in the 3'UTR (rs28363170). All four polymorphisms (rs6347, intron8 5/6 VNTR, rs27072 and 3'UTR 9/10 VNTR) were genotyped in clinical cohorts, representing schizophrenia, bipolar disorder, depression, and controls. Only rs27072 was significantly associated with bipolar disorder (OR = 2.1, p = 0.03). This result was replicated in a second bipolar/control population (OR = 1.65, p = 0.01), supporting a critical role for DAT regulation in bipolar disorder.

Associations between DRDs and schizophrenia in a Korean population: multi-stage association analyses

The dysregulation of the dopaminergic system has been implicated in the pathophysiology of major psychosis, including schizophrenia, with dopamine receptor genes (DRDs) presently targeted as the most promising candidate genes. We investigated DRD1-5 for association with schizophrenia using a multi-stage approach in a Korean sample. One hundred forty-two SNPs in DRD1-5 were selected from the dbSNP, and the associations of each SNP were then screened and typed by MALDI-TOF mass spectrometry using pooled DNA samples from 150 patients with major psychosis and 150 controls. Each of the suggested SNPs was then genotyped and tested for an association within the individual samples comprising each pool. Finally, the positively associated SNPs were genotyped in an extended sample of 270 patients with schizophrenia and 350 controls. Among the 142 SNPs, 88 (62%) SNPs in our Korean population were polymorphic. At the pooling stage, 10 SNPs (DRD1: 2, DRD2: 3, and DRD4: 5) were identified (P<0.05). SNPs rs1799914 of DRD1 (P=0.046) and rs752306 of DRD4 (P=0.017) had significantly different allele frequencies in the individually genotyped samples comprising the pool. In the final stage, with the extended sample, the suggestive association of DRD4 with rs752306 was lost, but the association of DRD1 with rs1799914 gained greater significance (P=0.017). In these large-scale multi-stage analyses, we were able to find a possible association between DRD1 and schizophrenia. These findings suggested the potential contribution of a multi-step strategy for finding genes related to schizophrenia.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.