Association analysis between dopamine receptor genes and bipolar affective disorder

Association of 11 variants of the dopaminergic and cognitive pathways genes with major depression, schizophrenia and bipolar disorder in the Pakistani population

Background: The dopaminergic pathways control neural signals that modulate mood and behaviour along and have a vital role in the aetiology of major depression (MDD), schizophrenia (SHZ) and bipolar disorder (BD). Genome-wide association studies (GWAS) have reported several dopaminergic and cognitive pathway genes association with these disorders however, no such comprehensive data was available regarding the Pakistani population.Objective: The present study was conducted to analyse the 11 genetic variants of dopaminergic and cognitive system genes in MDD, SHZ, and BD in the Pakistani population.Methods: A total of 1237 subjects [MDD n = 479; BD n = 222; SHZ n = 146; and controls n = 390], were screened for 11 genetic variants through polymerase chain reaction (PCR) techniques. Univariant followed by multivariant logistic regression analysis was applied to determine the genetic association.Results: Significant risk associations were observed for rs4532 and rs1799732 with MDD; and rs1006737 and rs2238056 with BD. However, after applying multiple test corrections rs4532 and rs1799732 association did not remain significant for MDD. Moreover, a protective association was found for three variants; DRD4-120bp, rs10033951 and rs2388334 in the current cohort.Conclusions: The present study revealed the risk association of single nucleotide polymorphisms (SNPs) rs1006737 and rs2238056 with BD and the protective effect of the DRD4-120bp variant in MDD and BD, of rs2388334 in BD and of rs10033951 in MDD, BD, and SHZ in the current Pakistani cohort. Thus, the study is valuable in understanding the genetic basis of MDD, BD and SHZ in the Pakistani population, which may pave the way for future functional studies.

Age at onset in bipolar disorder: Investigation of the role of TaqIA polymorphism of DRD2 gene in a Sardinian sample

Bipolar disorder (BD) is a highly heterogeneous and heritable psychiatric illness. Age at onset has been shown to be a powerful tool for dissecting both the phenotypic and genetic complexity of BD. In this article, we present findings from an association study between the DRD2 TaqIA polymorphism and age at onset, showing that both alleles and genotypes at this locus associate with early onset BD.

Role of DRD2 and ALDH2 genes in bipolar II disorder with and without comorbid anxiety disorder

The presence of comorbid anxiety disorders (AD) and bipolar II disorders (BP-II) compounds disability complicates treatment, worsens prognosis, and has been understudied. The genes involved in metabolizing dopamine and encoding dopamine receptors, such as aldehyde dehydrogenase 2 (ALDH2) and dopamine D2 receptor (DRD2) genes, may be important to the pathogenesis of BP-II comorbid with AD. We aimed to clarify ALDH2 and DRD2 genes for predisposition to BP-II comorbid with and without AD. The sample consisted of 335 subjects BP-II without AD, 127 subjects BP-II with AD and 348 healthy subjects as normal control. The genotypes of the ALDH2 and DRD2 Taq-IA polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis. Logistic regression analysis showed a statistically significant association between DRD2 Taq-I A1/A2 genotype and BP-II with AD (OR = 2.231, P = 0.021). Moreover, a significant interaction of the DRD2 Taq-I A1/A1 and the ALDH2*1*1 genotypes in BP-II without AD was revealed (OR = 5.623, P = 0.001) compared with normal control. Our findings support the hypothesis that a unique genetic distinction between BP-II with and without AD, and suggest a novel association between DRD2 Taq-I A1/A2 genotype and BP-II with AD. Our study also provides further evidence that the ALDH2 and DRD2 genes interact in BP-II, particularly BP-II without AD.

Genetic Variation of the Mu Opioid Receptor (OPRM1) and Dopamine D2 Receptor (DRD2) is Related to Smoking Differences in Patients with Schizophrenia but not Bipolar Disorder

It is not known why mentally ill persons smoke excessively. Inasmuch as endogenous opioid and dopaminergic systems are involved in smoking reinforcement, it is important to study mu opioid receptor (OPRM1) A118G (rs1799971), dopamine D2 receptor (DRD2) Taq1A (rs1800497) genotypes, and sex differences among patients with schizophrenia or bipolar disorder. Smokers and nonsmokers with schizophrenia (n=177) and bipolar disorder (n=113) were recruited and genotyped. They were classified into three groups: current smoker, former smoker, and never smoker by tobacco smoking status self-report. The number of cigarettes smoked per day was used as the major tobacco smoking parameter. In patients with schizophrenia, tobacco smoking prevalence was greater in males than in females as expected, but women had greater daily cigarette consumption (p<0.01). Subjects with schizophrenia who had the OPRM1 *G genotype smoked more cigarettes per day than the AA allele carriers with schizophrenia (p<0.05). DRD2 Taq1A genotype differences had no effect on the number of cigarettes smoked per day. However, female smokers with schizophrenia who were GG homozygous of the DRD2 receptor smoked more than the *A male smokers with schizophrenia (p<0.05). In bipolar patients, there were no OPRM1 and DRD2 Taq1A genotype differences in smoking status. There also were no sex differences for smoking behavior among the bipolar patients. The results of this study indicate that single nucleotide polymorphism (SNP) of the less functional mu opioid receptor increases tobacco smoking in patients with schizophrenia. Alteration of DRD2 receptor function also increased smoking behavior in females with schizophrenia.

Association between the dopamine D4 receptor gene exon III variable number of tandem repeats and political attitudes in female Han Chinese

Twin and family studies suggest that political attitudes are partially determined by an individual's genotype. The dopamine D4 receptor gene (DRD4) exon III repeat region that has been extensively studied in connection with human behaviour, is a plausible candidate to contribute to individual differences in political attitudes. A first United States study provisionally identified this gene with political attitude along a liberal-conservative axis albeit contingent upon number of friends. In a large sample of 1771 Han Chinese university students in Singapore, we observed a significant main effect of association between the DRD4 exon III variable number of tandem repeats and political attitude. Subjects with two copies of the 4-repeat allele (4R/4R) were significantly more conservative. Our results provided evidence for a role of the DRD4 gene variants in contributing to individual differences in political attitude particularly in females and more generally suggested that associations between individual genes, and neurochemical pathways, contributing to traits relevant to the social sciences can be provisionally identified.

Interaction of DRD2TaqI, COMT, and ALDH2 genes associated with bipolar II disorder comorbid with anxiety disorders in Han Chinese in Taiwan

It is hypothesized that dopaminergic genes-dopamine type-2 receptor (DRD2), aldehyde dehydrogenase 2 (ALDH2), and catechol-O-methyltransferase (COMT)-are associated with bipolar disorder (BP) and anxiety disorder (AD). Bipolar II (BP-II) is reported to be highly comorbid with AD. We examined whether interactions among these three genes are susceptibility factors in BP-II with AD (BP-II(+AD)) and without AD (BP-II(-AD)). In this study, we hypothesize that the interaction of the dopaminergic genes between BP-II(+AD) and BP-II(-AD) is significant different. We recruited 1260 participants: 495 with BP-II(-AD), 170 with BP-II(+AD), and 595 healthy controls without BP-II or AD. Genotyping was done using polymerase chain reactions plus restriction fragment length polymorphism analysis. Genotypic frequencies of the DRD2TaqIA, COMT, and ALDH2 polymorphisms between the two BP-II groups were nonsignificant. In logistic regression, the ALDH2 and DRD2TaqIA genes showed a main effect that was protective against BP-II(-AD) (odds ratio [OR] = 0.497, p = 0.010, and OR = 0.415, p = 0.017, respectively). The interaction of DRD2TaqIA A1/A1 and ALDH2*1/*1 had a significant risk effect on the BP-II(-AD) group (OR = 7.177, p < 0.001). However, the interaction of DRD2TaqIA A1/A1, ALDH2*1/*1, and COMTMet/Met&Val/Met become a weak protective factor against BP-II(-AD) (OR = 0.205, p = 0.047). All of the significant results described above are found only in BP-II(-AD). This study supports the hypothesis the interaction of the dopaminergic genes between BP-II(+AD) and BP-II(-AD) is significant different,, and provides additional evidence that the DRD2TaqIA A1/A1, ALDH2*1/*1 and COMT genes interact in BP-II(-AD) but not in BP-II(+AD).

Association study of dopamine receptor genes polymorphism with cognitive functions in bipolar I disorder patients

OBJECTIVE

To determine the correlation among the polymorphisms of dopamine receptor genes, cognitive function of Bipolar disorder (BD) patients, and BD.

METHODS

Twenty-three Single Nucleotide Polymorphisms (SNPs) of dopamine receptor genes were genotyped using Illumina GoldenGate genotyping assay in 375 patients with bipolar I disorder (BD-I) (patients group) and 475 healthy controls (control group). Cognitive function tests were performed in 158 patients who were clinically stable and 307 healthy controls who were matched with the patients in age, sex, and education.

RESULTS

The allele frequencies of rs3758653 in the promoter region of the DRD4 gene were significantly different between patients group and control group (χ(2)=9.386, Corrected P=0.046). This significant difference was also observed between BD-I patients with psychotic symptoms and healthy controls (χ(2)=9.27, Corrected P=0.049). Patients with BD-I performed significantly worse than healthy controls in all cognitive domains (p<0.01) except TMTA errors and illegal time. Significant interactions between polymorphisms of rs5326 in DRD1 gene and phenotype (affected or unaffected with BD-I) were found in non-perseverative errors (β=3.20 and Corrected P=0.0034) on the Wisconsin Card Sorting Test (WCST). The allele of this SNP denoted the positive effect on the WCST non-perseverative errors in BD-I patients group (β=2.80 and Corrected P=0.017). The genotypic association analyses also supported the findings (F=4.24 and P=0.007), but this effect was not found in controls.

LIMITATIONS

The sample size was relatively small and the SNP coverage was limited, making it very important to be cautious when drawing a conclusion.

CONCLUSIONS

DRD4 gene may play an important role in psychotic symptomatology rather than in unique diagnosis, BD, for example. A genetic association exists between DRD1 gene and impaired cognition in BD.

Updated findings of the association and functional studies of DRD2/ANKK1 variants with addictions

Both nicotine and alcohol addictions are severe public health hazards worldwide. Various twin and family studies have demonstrated that genetic factors contribute to vulnerability to these addictions; however, the susceptibility genes and the variants underlying them remain largely unknown. Of susceptibility genes investigated for addictions, DRD2 has received much attention. Considering new evidence supporting the association of DRD2 and its adjacent gene ankyrin repeat and kinase domain containing 1 (ANKK1) with various addictions, in this paper, we provide an updated view of the involvement of variants in DRD2 and ANKK1 in the etiology of nicotine dependence (ND) and alcohol dependence (AD) based on linkage, association, and molecular studies. This evidence shows that both genes are significantly associated with addictions; however the association with ANKK1 appears to be stronger. Thus, both more replication studies in independent samples and functional studies of some of these variants are warranted.

The DRD2 rs1800497 polymorphism increase the risk of mood disorder: evidence from an update meta-analysis

BACKGROUND

Growing studies have revealed the association between rs1800497 polymorphism in the dopamine receptor D2 (DRD2) and susceptibility to mood disorder (MD). However, the results remained inconsistent.

METHODS

To assess the effect of DRD2 rs1800497 polymorphism on MD. We performed a meta-analysis based on eight case-control studies, including a total of 2097 MD cases and 1681 controls. Summary odds ratios (OR) and corresponding 95% confidence intervals (CIs) for DRD2 rs1800497 polymorphism and MD risk were estimated.

RESULTS

Our meta-analysis indicated that DRD2 rs1800497 was associated with an increased MD risk, especially in Asians. Moreover, in the subgroup analysis by the type of MD, DRD2 rs1800497 polymorphism was observed to increase risk in BP.

LIMITATIONS

The results should be treated with caution for lacking of data to perform gene-gene and gene-environment interaction.

CONCLUSIONS

Our results indicated that polymorphism in DRD2 rs1800497 may play a role in development of MD.

Gender-specific association of the SLC6A4 and DRD2 gene variants in bipolar disorder

Findings on the association between the risk for developing bipolar disorder and the functions of the serotonin transporter-linked polymorphic region gene (5-HTTLPR) and dopamine D2 receptor gene (DRD2) variants are contradictory. One explanation for this is that a gender difference may exist for genetic contributions. We compared the gender-related main effects and the gene-to-gene interaction between serotonin transporter gene (SLC6A4) and DRD2 in adult male and female patients with bipolar I (BP-I) and bipolar II (BP-II) disorder. Patients with BP-I (n = 400) and BP-II (n = 493), and healthy controls (n = 442) were recruited from Taiwan's Han Chinese population. The genotypes of the 5-HTTLPR and DRD2 Taq-IA polymorphisms were determined using polymerase chain reaction-restriction fragment length polymorphism analysis. Logistic regression analysis showed a significant gender-specific association of the DRD2 A1/A1 and the 5-HTTLPR S/S, S/LG , and LG/LG (S+) (p = 0.01) genotypes in men with BP-I (p = 0.002 and 0.01, respectively) and BP-II (p = 0.001 and 0.007, respectively), but not in women. A significant interaction for the DRD2 A1/A1 and 5-HTTLPR S+ polymorphisms was also found only in men with BP-I and BP-II (p = 0.003 and 0.001, respectively). We provide preliminary evidence for a gender-specific effect of the SLC6A4 and DRD2 gene variants for the risk of BP-I and of BP-II. We also found gender-specific interaction between 5-HTTLPR and DRD2 Taq-IA polymorphisms in patients with bipolar disorder.

Association study of DRD2 and MAOA genes with subtyped alcoholism comorbid with bipolar disorder in Han Chinese

BACKGROUND

Several studies have hypothesized that genes involved in the dopamine system, including dopamine type-2 receptor (DRD2)-related TaqIA polymorphism and monoamine oxidase-A upstream variable number tandem repeat (uVNTR), may be associated with alcoholism. But their results were contradictory because of alcoholism's heterogeneity. Therefore, we examined whether the DRD2TaqIA and MAOA-uVNTR gene polymorphisms are susceptibility factors for alcoholism comorbid with bipolar disorder (ALC+BP) in Han Chinese in Taiwan.

METHODS

We recruited 101 Han Chinese men with comorbid alcoholism and bipolar disorder, and 328 healthy male controls from the community. Genotyping was done using PCR-RFLP.

RESULTS

There were no significant differences in the genotypic frequencies of the DRD2TaqIA or the MAOA-uVNTR polymorphisms between the 2 groups. The MAOA-uVNTR 3-repeat had a significant protective effect on the ALC+BP (odds ratio=0.432, p=0.035) but not on the healthy controls. However, the interaction between the MAOA-uVNTR 3-repeat and DRD2 A1/A2 was a risk factor in the ALC+BP (odds ratio=3.451, p=0.018).

CONCLUSIONS

We indicated the impact of the association between MAOA-uVNTR 3-repeat and DRD2 A1/A2 with ALC+BP.

Meta-analysis of genetic association studies on bipolar disorder

Numerous candidate gene association studies of bipolar disorder (BP) have been carried out, but the results have been inconsistent. Individual studies are typically underpowered to detect associations with genes of small effect sizes. We conducted a meta-analysis of published candidate gene studies to evaluate the cumulative evidence. We systematically searched for all published candidate gene association studies of BP. We then carried out a random-effects meta-analysis on all polymorphisms that were reported on by three or more case-control studies. The results from meta-analyses of these genes were compared with the findings from a recent mega-analysis of eleven genome-wide association studies (GWAS) in BP performed by the Psychiatric GWAS Consortium (PGC). A total of 487 articles were included in our review. Among these, 33 polymorphisms in 18 genes were reported on by three or more case-control studies and included in the random-effects meta-analysis. Polymorphisms in BDNF, DRD4, DAOA, and TPH1, were found to be nominally significant with a P-value < 0.05. However, none of the findings were significant after correction for multiple testing. Moreover, none of these polymorphisms were nominally significant in the PGC-BP GWAS. A number of plausible candidate genes have been previously associated with BP. However, the lack of robust findings in our review of these candidate genes highlights the need for more atheoretical approaches to study the genetics of BP afforded by GWAS. The results of this meta-analysis and from other on-going genomic experiments in BP are available online at Metamoodics (http://metamoodics.igm.jhmi.edu).

The interaction between BDNF and DRD2 in bipolar II disorder but not in bipolar I disorder

Bipolar I (BP-I) and bipolar II (BP-II) disorders are the two most common subtypes of bipolar disorder. However, most studies have not differentiated bipolar disorder into BP-I and BP-II groups, for which the underlying etiology differentiating these two subtypes remains unclear. The genetic association between both subtypes is essential for improving our understanding. The dopamine D2 receptor/ankyrin repeat and kinase domain containing 1 (DRD2/ANKK1), one of the dopaminergic pathways, as well as the brain-derived neurotrophic factor (BDNF) gene, were reported as candidate genes in the etiology of bipolar disorder. Therefore, we examined the contribution of the BDNF and DRD2/ANKK1 genes and their interaction to the differentiation of BP-I and BP-II. Seven hundred ninety-two participants were recruited: 208 with BP-I, 329 with BP-II, and 255 healthy controls. The genotypes of the BDNF and DRD2/ANKK1 Taq1A polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis. A significant main effect for the Val/Val genotype of the BDNF Val66Met polymorphism predicted BP-II patients. The significant interaction effect for the Val/Val genotype of the BDNF Val66Met polymorphism and A1/A2 genotype of DRD2/ANKK1 Taq1A polymorphism was found only in BP-II patients. We provide initial evidence that the BDNF Val66Me and DRD2/ANKK1 Taq1A polymorphisms interact only in BP-II disorder and that BP-I and BP-II are genetically distinct.

The influence of five monoamine genes on trajectories of depressive symptoms across adolescence and young adulthood

The influence of five monoamine candidate genes on depressive symptom trajectories in adolescence and young adulthood were examined in the Add Health genetic sample. Results indicated that, for all respondents, carriers of the dopamine receptor D4 5-repeat allele were characterized by distinct depressive symptom trajectories across adolescence and early adulthood. Similarly, for males, individuals with the monoamine oxidase A 3.5-repeat allele exhibited unique depressive symptom trajectories. Specifically, the trajectories of those with the dopamine receptor D4 5-repeat allele were characterized by rising levels in the transition to adulthood, while their peers were experiencing a normative drop in depressive symptom frequency. Conversely, males with the monoamine oxidase A 3.5-repeat allele were shown to experience increased distress in late adolescence. An empirical method for examining a wide array of allelic combinations was employed, and false discovery rate methods were used to control the risk of false positives due to multiple testing. Special attention was given to thoroughly interrogate the robustness of the putative genetic effects. These results demonstrate the value of combining dynamic developmental perspectives with statistical genetic methods to optimize the search for genetic influences on psychopathology across the life course.

The DRD2/ANKK1 gene is associated with response to add-on dextromethorphan treatment in bipolar disorder

Dextromethorphan (DM) is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist that may be neuroprotective for monoamine neurons. We hypothesized that adding DM to valproate (VPA) treatment would attenuate bipolar disorder (BP) symptoms. We evaluated in BP patients the association between the DRD2/ANKK1 TaqIA polymorphism with treatment response to VPA+add-on DM and to VPA+placebo. This double-blind, stratified, randomized study ran from January 2007 through December 2010. BP patients undergoing regular VPA treatments were randomly assigned to groups given either add-on DM (60 mg/day) (n=167) or placebo (n=83) for 12 weeks. The Young Mania Rating Scale (YMRS) and Hamilton Depression Rating Scale (HDRS) were used to evaluate clinical response. The genotypes of the DRD2/ANKK1 TaqIA polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis. To adjust within-subject dependence over repeated assessments, multiple linear regression with generalized estimating equation methods was used to analyze the effects of the DRD2/ANKK1 TaqIA polymorphism on clinical performance. Both groups showed significantly decreased YMRS and HDRS scores after 12 weeks of treatment; the differences between groups were non-significant. Decreases in YMRS scores were greater in patients with the A1A1 (P=0.004) genotypes than with the A2A2 genotype. We conclude that the DRD2/ANKK1 TaqIA polymorphism influenced responses to DM by decreasing manic symptoms in BP patients.

Association of DRD2 gene polymorphisms with mood disorders: a meta-analysis

BACKGROUND

In the past few decades, a number of studies have investigated the association of dopamine D2 receptor (DRD2) gene polymorphisms with mood disorders, but the findings are not always consistent. The aim of our study was to assess the association between DRD2 gene polymorphisms and mood disorders by using a meta-analysis.

METHODS

Data were collected from the following electronic databases: PubMed, Elsevier Science Direct, Cochrane Library, Chinese Biomedical Literature Database, China National Knowledge Infrastructure, and Wanfang, with the last report up to June 2010. Meta-analysis was performed in a fixed/random effect model by using the software Review Manager 4.2.

RESULTS

We identified 19 separate studies using search, but only 14 separate studies (2157 cases and 3272 controls) were included in the current study. Meta-analysis was performed for three DRD2 gene polymorphisms (-141Cins/del, Ser311/Cys311, and TaqI A1). We performed meta-analysis in overall, Caucasian, and Asian populations. We also performed disease-specific meta-analysis in unipolar disorder and bipolar disorder. We found no association between DRD2 gene -141Cins/del polymorphism and mood disorders in overall and Caucasian populations (P>0.05). We also found no association between DRD2 gene Ser311/Cys311 polymorphism and mood disorders in overall, Caucasian, and Asian populations (P>0.05). An association of DRD2 gene TaqI A1 polymorphism with mood disorders was found in overall population, and the individuals with A1A1 genotype were more susceptible to mood disorders in comparison to those with A2A1 and A2A2 genotypes (OR=1.84, 95% CI=1.07-3.17, P=0.03).

LIMITATION

Meta-analysis is retrospective research that is subject to the methodological deficiencies of the included studies.

CONCLUSION

This meta-analysis suggests that mood disorders may be associated with DRD2 gene TaqI A1 polymorphism, but not -141Cins/del and Ser311/Cys311.

Striatal structure and function in mood disorders: a comprehensive review

OBJECTIVES

  A large and diverse literature has implicated abnormalities of striatal structure and function in both unipolar and bipolar disorder. Recent functional imaging studies have greatly expanded this body of research. The aim of this review is to provide a comprehensive and critical appraisal of the relevant literature.

METHODS

  A total of 331 relevant articles were reviewed to develop an integrated overview of striatal function in mood disorders.

RESULTS

  There is compelling evidence from multiple studies that functional abnormalities of the striatum and greater corticostriatal circuitry exist in at least some forms of affective illness. The literature does not yet provide data to determine whether these aberrations represent primary pathology or they contribute directly to symptom expression. Finally, there is considerable evidence that bipolar disorder may be associated with striatal hyperactivity and some suggestion that unipolar illness may be associated with hypoactivation.

CONCLUSIONS

  Additional research investigating striatal function in affective disorders will be critical to the development of comprehensive models of the neurobiology of these conditions.

The role of dopamine in bipolar disorder

OBJECTIVE

Despite effective pharmacological treatments for bipolar disorder, we still lack a comprehensive pathophysiological model of the illness. Recent neurobiological research has implicated a number of key brain regions and neuronal components in the behavioural and cognitive manifestations of bipolar disorder. Dopamine has previously been investigated in some depth in bipolar disorder, but of late has not been a primary focus of attention. This article examines the role of dopamine in bipolar disorder, incorporating recent advances into established models where possible.

METHODS

A critical evaluation of the literature was undertaken, including a review of behavioural, neurochemical, receptor, and imaging studies, as well as genetic studies focusing on dopamine receptors and related metabolic pathways. In addition, pharmacologic manipulation of the central dopaminergic pathways and comparisons with other disease states such as schizophrenia were considered, principally as a means of exploring the hypothesised models.

RESULTS

Multiple lines of evidence, including data from pharmacological interventions and structural and functional magnetic resonance imaging studies, suggest that the dopaminergic system may play a central role in bipolar disorder.

CONCLUSION

Future research into the pathophysiological mechanisms of bipolar disorder and the development of new treatments for bipolar disorder should focus on the dopaminergic system.

Convergent functional genomics of genome-wide association data for bipolar disorder: comprehensive identification of candidate genes, pathways and mechanisms

Given the mounting convergent evidence implicating many more genes in complex disorders such as bipolar disorder than the small number identified unambiguously by the first-generation Genome-Wide Association studies (GWAS) to date, there is a strong need for improvements in methodology. One strategy is to include in the next generation GWAS larger numbers of subjects, and/or to pool independent studies into meta-analyses. We propose and provide proof of principle for the use of a complementary approach, convergent functional genomics (CFG), as a way of mining the existing GWAS datasets for signals that are there already, but did not reach significance using a genetics-only approach. With the CFG approach, the integration of genetics with genomics, of human and animal model data, and of multiple independent lines of evidence converging on the same genes offers a way of extracting signal from noise and prioritizing candidates. In essence our analysis is the most comprehensive integration of genetics and functional genomics to date in the field of bipolar disorder, yielding a series of novel (such as Klf12, Aldh1a1, A2bp1, Ak3l1, Rorb, Rora) and previously known (such as Bdnf, Arntl, Gsk3b, Disc1, Nrg1, Htr2a) candidate genes, blood biomarkers, as well as a comprehensive identification of pathways and mechanisms. These become prime targets for hypothesis driven follow-up studies, new drug development and personalized medicine approaches.

The genetics of bipolar disorder: genome 'hot regions,' genes, new potential candidates and future directions

Bipolar disorder (BP) is a complex disorder caused by a number of liability genes interacting with the environment. In recent years, a large number of linkage and association studies have been conducted producing an extremely large number of findings often not replicated or partially replicated. Further, results from linkage and association studies are not always easily comparable. Unfortunately, at present a comprehensive coverage of available evidence is still lacking. In the present paper, we summarized results obtained from both linkage and association studies in BP. Further, we indicated new potential interesting genes, located in genome 'hot regions' for BP and being expressed in the brain. We reviewed published studies on the subject till December 2007. We precisely localized regions where positive linkage has been found, by the NCBI Map viewer (http://www.ncbi.nlm.nih.gov/mapview/); further, we identified genes located in interesting areas and expressed in the brain, by the Entrez gene, Unigene databases (http://www.ncbi.nlm.nih.gov/entrez/) and Human Protein Reference Database (http://www.hprd.org); these genes could be of interest in future investigations. The review of association studies gave interesting results, as a number of genes seem to be definitively involved in BP, such as SLC6A4, TPH2, DRD4, SLC6A3, DAOA, DTNBP1, NRG1, DISC1 and BDNF. A number of promising genes, which received independent confirmations, and genes that have to be further investigated in BP, have been also systematically listed. In conclusion, the combination of linkage and association approaches provided a number of liability genes. Nevertheless, other approaches are required to disentangle conflicting findings, such as gene interaction analyses, interaction with psychosocial and environmental factors and, finally, endophenotype investigations.

Bidirectional genetic and environmental influences on mother and child behavior: the family system as the unit of analyses

Family systems theory proposes that an individual's functioning depends on interactive processes within the self and within the context of dyadic family subsystems. Previous research on these processes has focused largely on behavioral, cognitive, and psychophysiological properties of the individual and the dyad. The goals of this study were to explore genetic and environmental interactions within the family system by examining how the dopamine receptor D2 gene (DRD2) A1+ polymorphism in mothers and children relates to maternal sensitivity, how maternal and child characteristics might mediate those effects, and whether maternal sensitivity moderates the association between DRD2 A1+ and child affective problems. Evidence is found for an evocative effect of child polymorphism on parenting behavior, and for a moderating effect of child polymorphism on the association between maternal sensitivity and later child affective problems. Findings are discussed from a family systems perspective, highlighting the role of the family as a context for gene expression in both mothers and children.

Genetic variants in the DRD2 gene moderate the relationship between stressful life events and depressive symptoms in adults: cardiovascular risk in young Finns study

OBJECTIVE

To examine the potential moderating role of DRD2 polymorphism (rs1800497) in the association between stressful life events and depressive symptoms among young adults. Although stressful life events, such as divorce, unemployment, and serious illness in the family, are generally associated with negative health outcomes, including depressive symptoms, there are large individual differences in coping with such events. A number of studies suggest that variants in dopamine receptor genes, such as DRD2, are associated with depression but it is unclear if such variants also modify the association between life events and depression.

METHODS

We analyzed the prospective data on life events and depressive symptoms in 1992 and 2001 related to 1611 young adults (672 men and 939 women, aged 15-30 years at baseline) who participated in the ongoing population-based cardiovascular risk in young Finns study.

RESULTS

Occurrence of stressful life events was associated with increased risk of subsequent depressive symptoms in men and women. However, this association was seen only among those who carried A2/A2 (n = 872) genotype. No such association was detected in participants carrying A1/A1 or A1/A2 (n = 486) genotype.

CONCLUSION

DRD2 polymorphism moderates the effect of stressful life events on depressive symptoms and those who carry A2/A2 DRD2 genotypes may be more vulnerable than others.

G protein-coupled receptors in major psychiatric disorders

Although the molecular mechanisms underlying psychiatric illnesses such as depression, bipolar disorder and schizophrenia remain incompletely understood, there is increasing clinical, pharmacologic, and genetic evidence that G protein-coupled receptors (GPCRs) play critical roles in these disorders and their treatments. This perspectives paper reviews and synthesizes the available data. Dysfunction of multiple neurotransmitter and neuropeptide GPCRs in frontal cortex and limbic-related regions, such as the hippocampus, hypothalamus and brainstem, likely underlies the complex clinical picture that includes cognitive, perceptual, affective and motoric symptoms. The future development of novel agents targeting GPCR signaling cascades remains an exciting prospect for patients refractory to existing therapeutics.

Genetic variation and shared biological susceptibility underlying comorbidity in neuropsychiatry

Genetic factors underlying alcoholism, substance abuse, antisocial and violent behaviour, psychosis, schizophrenia and psychopathy are emerging to implicate dopaminergic and cannabinoid, but also monoaminergic and glutamatergic systems through the maze of promoter genes and polymorphisms. Candidate gene association studies suggest the involvement of a range of genes in different disorders of CNS structure and function. Indices of comorbidity both complicate the array of gene-involvement and provide a substrate of hazardous interactivity. The putative role of the serotonin transporter gene in affective-dissociative spectrum disorders presents both plausible genetic variation and complication of comorbidity The position of genetic variation is further complicated through ethnic, contextual and social factors that provide geometric progressions in the comordity already underlying diagnostic obstacles. The concept of shared biological susceptibility to two or more disorder conditions of comorbidity seems a recurring observation, e.g., bipolar disorder with alcoholism or schizophrenia with alcohol/substance abuse or diabetes with schizopsychotic disorder. Several lines of evidence seem to suggest that the factors influencing variation in one set of symptoms and those affecting one or more disorders are observed to a marked extent which ought to facilitate the search for susceptibility genes in comorbid brain disorders. Identification of regional genetic factors is awaited for a more compelling outline that ought eventually to lead to greater efficacy of symptom-disorder arrangements and an augmentation of current pharmacological treatment therapies.

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.

The dopamine D4 receptor gene 48-base-pair-repeat polymorphism and mood disorders: a meta-analysis

BACKGROUND

We conducted a meta-analysis to re-evaluate the role of the dopamine D4 receptor gene 48-base-pair- repeat (DRD4) polymorphism in mood disorders.

METHODS

DRD4 allele frequencies were compared between 917 patients with unipolar (UP) or bipolar affective disorder (BP) and 1164 control subjects from 12 samples, using the Cochrane Review Manager.

RESULTS

An association was found between all mood disorder groups and DRD4.2. After correcting for multiple testing, the association between DRD4.2 and BP dropped to insignificance; however, the evidence of an association between the DRD4.2 allele and UP (p < .001) and the combined group (p < .001) remained. There was no evidence for heterogeneity or publication bias.

CONCLUSIONS

These findings suggest that the DRD4.2 allele is a risk allele for depression symptomatology. Meta-analysis may be a valuable objective tool for a quantitative summary of evidence for association studies in psychiatric genetics.

An overview of the genetics of psychotic mood disorders

This article presents a conceptual review of the genetic underpinnings of psychotic mood disorders. Both unipolar and bipolar forms of mood disorder sometimes feature psychotic symptoms. Some evidence from epidemiological research suggests that psychotic forms of mood disorder specifically might be heritable. Linkage studies of mood disorders in general have also provided some support for that notion, as have associated studies involving serotonin and dopamine genes and psychotic mood disorder. Some research suggests there might be a genetic connection between schizophrenia and bipolar disorder, undermining the Kraepelinian dichotomous classification of the psychoses. Future research should continue to examine psychotic forms of mood disorder using both epidemiological and molecular approaches.

Epigenetics and bipolar disorder: New opportunities and challenges

Despite significant effort, understanding of the molecular causes and mechanisms of bipolar disorder (BD) remains a major challenge. Numerous molecular genetic linkage and association studies have been conducted over the last two decades; however, the data are quite inconsistent or even controversial. This article develops an argument that molecular studies of BD would benefit significantly from adding an epigenetic (epiG) perspective. EpiG factors refer to modifications of DNA and chromatin that "orchestrate" the activity of the genome, including regulation of gene expression. EpiG mechanisms are consistent with various non-Mendelian features of BD such as the relatively high degree of discordance in monozygotic (MZ) twins, the critical age group for susceptibility to the disease, clinical differences in males and females, and fluctuation of the disease course, including interchanges of manic and depressive phases, among others. Apart from the phenomenological consistency, molecular epiG peculiarities may shed new light on the understanding of controversial molecular genetic findings. The relevance of epigenetics for the molecular studies of BD is demonstrated using the examples of genetic studies of BD on chromosome 11p and the X chromosome. A spectrum of epiG mechanisms such as genomic imprinting, tissue-specific effects, paramutagenesis, and epiG polymorphism, as well as epiG regulation of X chromosome inactivation, is introduced. All this serves the goal of demonstrating that epiG factors cannot be ignored anymore in complex phenotypes such as BD, and systematic large-scale epiG studies of BD have to be initiated.

Genome scan of a second wave of NIMH genetics initiative bipolar pedigrees: chromosomes 2, 11, 13, 14, and X

As part of the on-going NIMH Genetics Initiative on Bipolar Disorder, we have ascertained 153 multiplex bipolar pedigrees and genotyped them in two waves. We report here the genome scan results for chromosomes 2, 11, 13, 14, and X in the second wave of 56 families. A total of 354 individuals were genotyped and included in the current analyses, including 5 with schizoaffective/bipolar (SA/BP), 139 with bipolar I disorder (BPI), 41 with bipolar II disorder (BPII), and 43 with recurrent unipolar depression (RUP). Linkage analyses were carried out with multi-point parametric and non-parametric affected relative pair methods using three different definitions of the affected phenotype: (model 1) SA/BP and BPI; (model 2) SA/BP, BPI, and BPII; and (model 3) SA/BP, BPI, BPII, and RUP. The best findings were on 11p15.5 (NPL = 2.96, P = 0.002) and Xp11.3 (NPL = 2.19, P = 0.01). These findings did not reach conventional criteria for significance, but they were located near regions that have been identified in previous genetic studies of bipolar disorder. The relatively modest but consistent findings across studies may suggest that these loci harbor susceptibility genes of modest effect in a subset of families. Large samples such as that being collected by the NIMH Initiative will be necessary to examine the heterogeneity and identify these susceptibility genes.

Dopamine D4 receptor and tyrosine hydroxylase genes in bipolar disorder: evidence for a role of DRD4

The involvement of the mesocorticolimbic dopamine system in behaviors that are compromised in patients with mood disorder has led to the investigation of dopamine system genes as candidates for bipolar disorder. In particular, the functional VNTRs in the exon III of the dopamine D4 (DRD4) and in intron I of the tyrosine hydroxylase (TH) genes have been investigated in numerous association studies that have produced contrasting results. Likewise, linkage studies in multiplex bipolar families have shown both positive and negative results for markers in close proximity to DRD4 and TH on 11p15.5. We performed a linkage disequilibrium analysis of the DRD4 and TH VNTRs in a sample of 145 nuclear families comprised of DSM-IV bipolar probands and their biological parents. An excess of transmissions and non transmissions was observed for the DRD4 4- and 2-repeat alleles respectively. The biased transmission showed a parent of origin effect (POE) since it was derived almost exclusively from the maternal meiosis (4-repeat allele maternally transmitted 40 times vs 20 times non-transmitted; chi(2) = 6.667; df = 1; P = 0.009; while paternally transmitted 26 times vs 21 times non-transmitted; chi(2) = 0.531; df = 1; P = 0.46). The analysis of TH did not reveal biased transmission of intron I VNTR alleles. Although replication of our study is necessary, the fact that DRD4 exhibit POE and is located on 11p15.5, in close proximity to a cluster of imprinted genes, suggests that genomic imprinting may be operating in bipolar disorder.

D2 dopamine receptor gene in psychiatric and neurologic disorders and its phenotypes

The D2 dopamine receptor (DRD2) has been one of the most extensively investigated gene in neuropsychiatric disorders. After the first association of the TaqI A DRD2 minor (A1) allele with severe alcoholism in 1990, a large number of international studies have followed. A meta-analysis of these studies of Caucasians showed a significantly higher DRD2 A1 allelic frequency and prevalence in alcoholics when compared to controls. Variants of the DRD2 gene have also been associated with other addictive disorders including cocaine, nicotine and opioid dependence and obesity. It is hypothesized that the DRD2 is a reinforcement or reward gene. The DRD2 gene has also been implicated in schizophrenia, posttraumatic stress disorder, movement disorders and migraine. Phenotypic differences have been associated with DRD2 variants. These include reduced D2 dopamine receptor numbers and diminished glucose metabolism in brains of subjects who carry the DRD2 A1 allele. In addition, pleiotropic effects of DRD2 variants have been observed in neurophysiologic, neuropsychologic, stress response, personality and treatment outcome characteristics. The involvement of the DRD2 gene in certain neuropsychiatric disorders opens up the potential of a targeted pharmacogenomic approach to the treatment of these disorders.

Family-based association study of 5-HTTLPR, TPH, MAO-A, and DRD4 polymorphisms in mood disorders

Variants of the functional polymorphism in the serotonin transporter (upstream regulatory region: 5-HTTLPR), the tryptophan hydroxylase (TPH), the monoamine oxidase A (MAO-A), and the dopamine receptor D4 (DRD4) genes have all been associated with mood disorders. The aim of this study was to test those hypotheses by using a family-based association approach. Both diagnoses and psychopathology were used for phenotype definitions. A total of 134 nuclear families with mood disorders, with probands affected by bipolar (n = 103) or major depressive (n = 58) disorders, were included in the study. All subjects were typed for the above-mentioned gene variants using polymerase chain reaction (PCR) technique. No significant transmission disequilibrium was found in the overall sample for any polymorphism. A separate analysis of bipolar subjects only, or the use of continuous psychopathologic traits as affectation status did not influence the observed results. Our study did not support the involvement of 5-HTTLPR, TPH, MAO-A, or DRD4 polymorphisms in mood disorders.

DRD4 exon 3 variants associated with delusional symptomatology in major psychoses: a study on 2,011 affected subjects

We previously reported an association of DRD4 exon3 long allele variants with delusional symptomatology independently from diagnoses. The aim of this investigation was to study DRD4 in major psychoses and to test the association in a larger sample. We studied 2,011 inpatients affected by bipolar disorder (n = 811), major depressive disorder (n = 635), schizophrenia (n = 419), delusional disorder (n = 104), psychotic disorder not otherwise specified (n = 42), and 601 healthy controls. A subsample of 1,264 patients were evaluated using the OPCRIT checklist and differences of symptomatology factor scores among genetic variants were assessed using one-way analysis of variance (ANOVA). DRD4 allele and genotype frequencies in bipolars, schizophrenics, delusionals, and psychotic NOS were not significantly different from controls; major depressives showed a trend toward an excess of DRD4*Short and DRD4*Short/Short variants versus controls. The ANOVA on factor scores in the whole subsample of 1,264 subjects showed a significant difference on delusion factor in allele analysis (P = 0.007), and in genotype one (P = 0.018), with DRD4*Long containing variants associated with severe symptomatology. The analysis in the replication subjects only (n = 803) showed a trend in the same direction, though not reaching the significance level. This analysis in an enlarged sample suggests that DRD4*Long alleles exert a small but significant influence on the delusional symptomatology in subjects affected by major psychoses.

Allelic variation of a BalI polymorphism in the DRD3 gene does not influence susceptibility to bipolar disorder: results of analysis and meta-analysis

Bipolar disorder is a major psychiatric illness that has evidence for a significant genetic contribution toward its development. In recent years, the BalI RFLP (restriction fragment length polymorphism) in the dopamine D3 receptor gene has been examined as a possible susceptibility factor for both schizophrenia and bipolar disorder. While analysis in schizophrenia has produced examples of increased homozygosity in patients, less encouraging results have been found for bipolar disorder. Recently, however, a family-based association study has found a significant excess of allele 1 and allele 1-containing genotypes in transmitted alleles to bipolar probands over nontransmitted controls. In a large bipolar case control sample (n = 454), we have been unable to replicate the family-based association study (chi-square = 0.137, P = 0.71, 1 df) or detect an effect similar to the positive homozygosity findings in schizophrenia (chi-square = 0.463, P = 0.50, 1 df). A meta-analysis of previous association studies also revealed no difference in allele distributions between bipolar patients and controls for this polymorphism in ethnically homogeneous samples (odds ratio, OR, = 1.04; P = 0.60; 95% confidence interval, CI, = 0.89-1.20). In view of this evidence, we conclude that variation at the BalI RFLP is not an important factor influencing the susceptibility to bipolar disorder. It remains possible, however, that other sequence variations within the DRD3 gene could play a role.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

Genetics of bipolar affective disorder

Bipolar affective disorder is a highly heritable condition, as demonstrated in twin, family, and adoption studies. Morbid risk in first degree relatives is four to six times higher than the population prevalence of about 1%. However, the mode of inheritance is complex, and linkage findings have been difficult to replicate. Despite these limitations, consistent linkage findings have emerged on several chromosomes, notably 18p, 18q, 21q, 12q, 4p, and Xq. Two additional areas, 10p and 13q, have shown linkage in regions that appear to overlap with significant linkage findings in schizophrenia. Separate linkage studies in schizophrenia also have targeted the replicated bipolar linkages on 18p and 22q. New methods are being developed for fine mapping and candidate identification. Recent candidate gene studies include some positive results for the serotonin transporter gene on 17q and the catechol-o-methyltransferase gene on 22q. No other candidate gene studies are yet showing replicated results. A convincing demonstration for a susceptibility gene will probably require a mixture of case- control studies, family-based association methods, and pathophysiologic studies.