Association study of bipolar disorder with candidate genes involved in catecholamine neurotransmission: DRD2, DRD3, DAT1, and TH genes

Zebrafish as a potential non-traditional model organism in translational bipolar disorder research: Genetic and behavioral insights

Bipolar disorder (BD) is a severe and debilitating illness that affects 1-2% of the population worldwide. BD is characterized by recurrent and extreme mood swings, including mania/hypomania and depression. Animal experimental models have been used to elucidate the mechanisms underlying BD and different strategies have been proposed to assess BD-like symptoms. The zebrafish (Danio rerio) has been considered a suitable vertebrate system for modeling BD-like responses, due to the genetic tractability, molecular/physiological conservation, and well-characterized behavioral responses. In this review, we discuss how zebrafish-based models can be successfully used to understand molecular, biochemical, and behavioral alterations paralleling those found in BD. We also outline some advantages and limitations of this aquatic species to examine BD-like phenotypes in translational neurobehavioral research. Overall, we reinforce the use of zebrafish as a promising tool to investigate the neural basis associated with BD-like behaviors, which may foster the discovery of novel pharmacological therapies.

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.

Evaluation of VNTR polymorphisms of dopamine transporter gene and the risk of bipolar disorder in Zahedan, southeast Iran

The exact role of dopamine transporter gene (DAT1) in the pathogenesis of bipolar disorder type 1 (BD) is not understood. In the present study, we aimed to evaluate the possible association between 30, 40 and 63 bp variable number tandem repeat (VNTR) polymorphisms of DAT1 gene and the risk of type 1 (BD) in a sample of Iranian population. This case-control study was performed on 152 BD patients and 153 psychiatrically healthy subjects. Genotyping of the variant was done by polymerase chain reaction method. Totally, the findings did not support an association between DAT1 VNTR polymorphisms and the risk of BD in a sample of southeast Iranian population.

Dopamine transporter gene may be associated with bipolar disorder and its personality traits

Dopamine transporter and its genetic factors have been suggested to play a critical role in the development of bipolar disorder (BPD). However, the importance of the dopamine transporter gene (DAT1) in the pathogenesis of BPD remains unclear. The aims of this study were to assess 18 polymorphisms of the DAT1 gene to determine whether this gene is associated with BPD and whether it influences personality traits of patients with BPD. DAT1 polymorphisms were analyzed in 492 BPD (374 BPDI and 118 BPDII) patients and 436 controls. All participants were screened using the same assessment tool, and all met the criteria for BPD. The Tridimensional Personality Questionnaire was used to assess personality traits in both patients and controls. Several polymorphisms had a weak association with BPD, including rs2550948, rs2652511, and rs2975226 in allele distribution analysis (P < 0.05). Furthermore, the promoter G-A-C-G haplotype (rs6350-rs2975226-rs2652511-rs6413429) was over-represented in the BPD patients compared to the controls (P = 0.007). In personality assessment, the BPDII patients had the highest harm avoidance score, followed by the BPDI patients and controls (P = 3.7 × 10(-32)). In addition, a significant association between rs40184 and harm avoidance was found in the patients with BPD. The DAT1 promoter may be associated with vulnerabilities in BPD. The BPD patients had a higher rate of harm avoidance personality traits than the controls, and DAT1 variants may influence personality traits in patients with BPD.

Interaction of the DRD3 and BDNF gene variants in subtyped bipolar disorder

OBJECTIVES

Bipolar disorder is a severe mental disorder with prominent genetic etiologic factors. Dopaminergic dysfunction has been implicated in the pathogenesis of bipolar disorder, which suggests that the dopamine D3 receptor gene (DRD3) is a strong candidate gene. The brain-derived neurotrophic factor (BDNF) gene has been implicated in the etiology of bipolar disorder. We examined the association between the BDNF Val66Met and DRD3 Ser9Gly polymorphisms with two subtypes of bipolar disorder: bipolar-I and -II. Because BDNF regulates DRD3 expression (1), we also examined possible interactions between these genes.

METHODS

We recruited 964 participants: 268 with bipolar-I, 436 with bipolar-II, and 260 healthy controls. The genotypes of the BDNF Val66Met and DRD3 Ser9Gly polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis.

RESULTS

Logistic regression analysis showed a significant main effect for the Val/Val genotype of the BDNF Val66Met polymorphism (P=0.020), which predicted bipolar-II patients. Significant interaction effects for the BDNF Val66Met Val/Val genotype and both DRD3 Ser9Gly Ser/Ser and Ser/Gly genotypes were found only in bipolar-II patients (P=0.027 and 0.006, respectively).

CONCLUSION

We provide initial evidence that the BDNF Val66Met and DRD3 Ser9Gly genotypes interact only in bipolar-II disorder and that bipolar-I and bipolar-II may be genetically distinct.

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 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.

The COMT and DRD3 genes interacted in bipolar I but not bipolar II disorder

OBJECTIVES. Clarifying the association between bipolar I and bipolar II disorders at the genetic level is essential for improving our understanding of them. In this study, we evaluated the hypothesis that the dopaminergic polymorphisms are risk factors for bipolar disorders. We examined the association between the catechol-O-methyltransferase (COMT) Val158Met and dopamine D3 receptor (DRD3) Ser9Gly polymorphisms and bipolar I and II disorders, as well as possible interactions between these genes. METHODS. Seven hundred and eleven participants were recruited: 205 with bipolar I, 270 with bipolar II, and 236 healthy controls. The genotypes of the COMT Val158Met and DRD3 Ser9Gly polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis. RESULTS. Logistic regression analyses showed a statistically significant main effect for the Met/Met genotype of the COMT Val158Met polymorphism (P=0.032) and a significant interaction effect for the Met/Met genotype of the COMT Val158Met and Ser/Ser genotypes of the DRD3 Ser9Gly polymorphism (P=0.001) predicted bipolar I patients. However, there was no association between the COMT Val158Met or DRD3 Ser9Gly and bipolar II. CONCLUSIONS. We provide initial evidence that the COMT Val158Met and DRD3 Ser9Gly genotypes interact in bipolar I and bipolar II disorders and that bipolar I and bipolar II are genetically distinct.

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.

Neurotransmission and bipolar disorder: a systematic family-based association study

Neurotransmission pathways/systems have been proposed to be involved in the pathophysiology and treatment of bipolar disorder for over 40 years. In order to test the hypothesis that common variants of genes in one or more of five neurotransmission systems confer risk for bipolar disorder, we analyzed 1,005 tag single nucleotide polymorphisms in 90 genes from dopaminergic, serotonergic, noradrenergic, GABAergic, and glutamatergic neurotransmitter systems in 101 trios and 203 quads from Caucasian bipolar families. Our sample has 80% power to detect ORs >or= 1.82 and >or=1.57 for minor allele frequencies of 0.1 and 0.5, respectively. Nominally significant allelic and haplotypic associations were found for genes from each neurotransmission system, with several reaching gene-wide significance (allelic: GRIA1, GRIN2D, and QDPR; haplotypic: GRIN2C, QDPR, and SLC6A3). However, none of these associations survived correction for multiple testing in an individual system, or in all systems considered together. Significant single nucleotide polymorphism associations were not found with sub-phenotypes (alcoholism, psychosis, substance abuse, and suicide attempts) or significant gene-gene interactions. These results suggest that, within the detectable odds ratios of this study, common variants of the selected genes in the five neurotransmission systems do not play major roles in influencing the risk for bipolar disorder or comorbid sub-phenotypes.

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.

Interaction between BDNF Val66Met and dopamine transporter gene variation influences anxiety-related traits

The involvement in neural plasticity and the mediation of effects of repeated stress exposure and long-term antidepressant treatment on hippocampal neurogenesis supports a critical role of brain-derived neurotrophic factor (BDNF) in the pathophysiology of affective and other stress-related disorders. A previously reported valine to methionine substitution at amino-acid position 66 (BDNF Val66Met) seems to account for memory disturbance and hippocampal dysfunction. In the present study, we evaluated the impact of the BDNF Val66Met polymorphism on individual differences in personality traits in a sample of healthy volunteers in relation to other common gene variants thought to be involved in the pathophysiology of affective disorders, such as the serotonin transporter promoter polymorphism (5-HTTLPR) and a variable number of tandem repeat polymorphism of the dopamine transporter gene (DAT VNTR). Personality traits were assessed using the NEO personality inventory (NEO-PI-R) and Tridimensional Personality Questionnaire (TPQ). There was a significant DAT VNTR-dependent association between NEO-PI-R Neuroticism and the BDNF Val66Met polymorphism. Among individuals with at least one copy of the DAT 9-repeat allele, carriers of the BDNF Met allele exhibited significantly lower Neuroticism scores than noncarriers. This interaction was also observed for TPQ Harm Avoidance, a personality dimension related to Neuroticism. Our results support the notion that allelic variation at the BDNF locus--in interaction with other gene variants--influences anxiety- and depression-related personality traits.

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.

Dopamine D2-like receptors and the antidepressant response

Converging lines of evidence suggest a role for the mesolimbic dopamine system in the response to somatic antidepressant therapies. Here, we review evidence suggesting that antidepressant treatments of different types share the effect of increasing the sensitivity of dopamine D2-like receptors in the nucleus accumbens, clinical studies suggesting that activation of these receptors has antidepressant efficacy, as well as relevant imaging and genetic data on the role of this system in the antidepressant response. We then attempt to reconcile this data with evidence of a common target of antidepressant drugs in the cyclic adenosine monophosphate (cAMP) response element binding protein-brain-derived neurotrophic factor (CREB-BDNF) pathway in a model that suggests potential directions for future inquiry.

No allelic association or interaction of three known functional polymorphisms with bipolar disorder in a northern Swedish isolated population

Most genetic association studies in bipolar disorder have focussed on genes involved in major neurotransmitter systems or brain development. Functional polymorphisms in the serotonin transporter (5-HTTLPR), catechol-O-methyltransferase (Val158Met) and dopamine D3 receptor (Ser9Gly) genes have all been associated with bipolar disorder. We aimed at investigating whether these functional variants contribute to the genetic etiology of bipolar disorder in a northern Swedish isolated population. Moreover, we wanted to gain information about the synergistic contribution of these functional variants. Neither of these functional polymorphisms was associated with bipolar disorder in the northern Swedish patient-control sample nor did we find evidence of gene-gene interaction. Together, our data suggest that these functional variants are not involved in the etiology of bipolar disorder in the northern Swedish population nor did gene-gene interaction analysis support a central role of these variants in bipolar disorder.

A 40-basepair VNTR polymorphism in the dopamine transporter (DAT1) gene and the rapid response to antidepressant treatment

Finding predictors of the response to antidepressant therapy is a major goal of molecular psychiatry. The genes encoding the serotonin (SERT) and dopamine (DAT1) transporters are among the possible candidate genes modulating an individual's antidepressant response. In a naturalistic prospective cohort study with a total of 190 fully assessed patients, improvement of depression symptoms during the 3 weeks following initiation of antidepressant therapy was recorded using the 21-item Hamilton Depression Rating Scale (HDRS). The SLC6A3 3' UTR 40-bp variable number of tandem repeats (VNTR) and the SLC6A4 5' 44-bp insertion/deletion polymorphism were analyzed by polymerase chain reaction. There was a significantly smaller number of rapid responders among homozygous carriers of the DAT1 9-repeat allele (9/9) than among heterozygous (9/10) and homozygous (10/10) carriers of the 10-repeat allele (19 versus 37 versus 52%, respectively, P=0.0037). Median decline in HDRS score was 35, 40, and 52% in patients with the 9/9, 9/10, and 10/10 genotypes, respectively (P=0.013). The effect was found in all classes of medications (selective serotonin reuptake inhibitors (SSRIs), tricyclics, mirtazapine, venlafaxine) and statistically significant also within the subgroup of patients having received SSRIs. The serotonin promoter insertion/deletion genotype had no effect in the entire study group, but there was an insignificant trend of better response in the l/l and l/s carriers who received SSRIs or mirtazapine. In conclusion, the dopamine transporter VNTR polymorphism influenced rapid response to antidepressant therapy. Compared with homozygous carriers of the 10-repeat allele, carriers of the 9/10 genotype had an odds ratio (OR) calculated by logistic regression analysis of 1.6 (95% CI 0.8-3.2) and carriers of the 9/9 genotype had an OR of 6.0 (1.5-24.4) for no or poor response. Further studies are required to confirm this clinical association and to elucidate the underlying mechanisms.

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.

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

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

Molecular genetics of affective disorders

Evidence for familial aggregation in Affective Disorders (AD) has been provided in classical studies. Linkage and association genetic studies have been proposed to detect genetic factors implicated in AD. However, findings from molecular genetic studies remain inconclusive. Nevertheless, current research is focusing on the phenotypes, both sub- and endophenotypes. In addition, recent advances in technology, such as microarrays, provide new tools in psychiatric genetics. These different approaches offer a new optimism era in the search of genetic factors in AD.

Evidence that the N-methyl-D-aspartate subunit 1 receptor gene (GRIN1) confers susceptibility to bipolar disorder

There is evidence for the involvement of glutamatergic transmission in the pathogenesis of major psychoses. The two most commonly used mood stabilizers (ie lithium and valproate) have been found to act via the N-methyl-D-aspartate receptor (NMDAR), suggesting a specific role of NMDAR in the pathogenesis of bipolar disorder (BP). The key subunit of the NMDAR, named NMDA-1 receptor, is coded by a gene located on chromosome 9q34.3 (GRIN1). We tested for the presence of linkage disequilibrium between the GRIN1 (1001-G/C, 1970-A/G, and 6608-G/C polymorphisms) and BP. A total of 288 DSM-IV Bipolar I, Bipolar II, or schizoaffective disorder, manic type, probands with their living parents were studied. In all, 73 triads had heterozygous parents for the 1001-G/C polymorphism, 174 for the 1970-A/G, and 48 for the 6608-G/C. These triads were suitable for the final analyses, that is, the transmission disequilibrium test (TDT) and the haplotype-TDT. For the 1001-G/C and the 6608-G/C polymorphisms, we found a preferential transmission of the G allele to the affected individuals (chi(2)=4.765, df=1, P=0.030 and chi(2)= 8.395, df=1, P=0.004, respectively). The 1001G-1970A-6608A and the 1001G-1970A-6608G haplotypes showed the strongest association with BP (global chi(2)=14.12, df=4, P=0.007). If these results are replicated there could be important implications for the involvement of the GRIN1 in the pathogenesis of BP. The role of the gene variants in predicting the response to mood stabilizers in BP should also be investigated.

A novel CpG-associated brain-expressed candidate gene for chromosome 18q-linked bipolar disorder

We previously identified 18q21-q22 as a candidate region for bipolar (BP) disorder and constructed a yeast artificial chromosome (YAC) contig map. Here we identified three potential CpG islands using CCG/CGG YAC fragmentation. Analysis of available genomic sequences using bioinformatic tools identified an exon of 3639 bp downstream of a CpG island of 1.2 kb containing a putative transcription initiation site. The exon contained an open reading frame coding for 1212 amino acids with significant homology to the SART-2 protein; weaker homology was found with a series of sulphotransferases. Alignment of cDNA sequences of corresponding ESTs and RT-PCR sequencing predicted a transcript of 9.5 kb which was confirmed by Northern blot analysis. The transcript was expressed in different brain areas as well as in multiple other peripheral tissues. We performed an extensive mutation analysis in 113 BP patients. A total of nine single nucleotide polymorphisms (SNPs) were identified. Five SNPs predicted an amino acid change, of which two were present in BP patients but not in 163 control individuals.

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.

Post-genomic era and gene discovery for psychiatric diseases: there is a new art of the trade? The example of the HUMTH01 microsatellite in the Tyrosine Hydroxylase gene

The microsatellite HUMTH01, located in the first intron of the Tyrosine Hydroxylase (TH) gene (encoding the rate-limiting enzyme in the synthesis of catecholamines), is characterized by a TCAT repeated motif and has been used in genetic studies of neuropsychiatric and other complex diseases, in which catecholaminergic neurotransmission is implicated. After reporting a positive association between HUMTH01 and bipolar disorder as well as schizophrenia, the authors established that HUMTH01 alleles display the features of regulatory elements. Thereafter, they cloned two proteins (ZNF191 and HBP1), specifically binding to HUMTH01, and demonstrated that allelic variations of HUMTH01 have a quantitative silencing effect on TH gene expression in vitro, and correlate with quantitative and qualitative changes in the binding by ZNF191. The authors aim to characterize the transduction pathway impinging on the HUMTH01 microsatellite and establish its relevance for TH gene regulation in vivo. Since the TCAT repeated sequence is widespread throughout the genome, their approach may lead to the dissection of the mechanisms underlying the quantitative expression of several genes implicated in complex genetic traits, both normal and pathological. Thus, these investigations on the possible contribution and potential role of the HUMTH01 microsatellite in neuro-pathological conditions may represent an example of the different approaches needed to validate genetic targets in the "post-genomic era."

Association study of MAO-A, COMT, 5-HT2A, DRD2, and DRD4 polymorphisms with illness time course in mood disorders

The aim of our study was to investigate a possible influence of monoamine oxydase A (MAO-A), catechol-O-methyltransferase (COMT), serotonin receptor 2A (5-HT2A), dopamine receptor D2 (DRD2), and dopamine receptor D4 (DRD4) gene variants on timing of recurrence in mood disorders. Gene variants were determined using PCR-based techniques in 550 inpatients affected by recurrent mood disorders (major depressives: n = 212; bipolars: n = 338), rapid cycling mood disorder (n = 81), and 663 controls. We investigated possible genetic influences by comparing illness time course of subjects subdivided according to genotype using multivariate analysis of variance (MANOVA). We could not observe a significantly different time course. No demographic and clinical variables such as sex, age or polarity of onset, presence of psychotic features, genetic loading, or education level influenced the observed results. Our results suggest that MAO-A, COMT, 5-HT2A, DRD2, and DRD4 gene variants are not involved in susceptibility toward different time courses in mood disorders.

Dopamine transporter gene (DAT1) VNTR polymorphism in major psychiatric disorders: family-based association study in the Bulgarian population

OBJECTIVE

A 40-bp variable number tandem repeat in the 3'-UTR of dopamine transporter gene (DAT1) has been examined for association with major psychiatric disorders in several case-control studies. No significant results have been found. We used a new collection of parent-offspring trios to test for association with schizophrenia (SZ), bipolar 1 disorder (BPI) and schizoaffective (SA) disorder.

METHOD

We genotyped trios from Bulgarian origin where the proband had SZ (178 trios), BPI (77 trios) and SA (29 trios). Alleles ranging from 5 to 11 repeats were observed. The results were analysed with the extended TDT (ETDT).

RESULTS

No preferential transmission of alleles was observed for any diagnostic group. The presence of allele DAT*10 was associated with the severity and frequency of auditory hallucinations, however, this result is not significant if corrected for multiple testing.

CONCLUSION

Our results are in agreement with previous reports of a lack of association between this polymorphism and major psychiatric disorders.

European combined analysis of the CTG18.1 and the ERDA1 CAG/CTG repeats in bipolar disorder

Several groups have reported association between large CAG/CTG repeats in the genome and BP disorder using the Repeat Expansion Detection (RED) method. Molecular interpretation studies demonstrated that around 90% of the large CAG/CTG repeats detected by RED can by explained by repeat size at either the CTG18.1 or ERDA-1 locus. In this study we report the findings on a large European BP case-control sample analysed for these two frequently expanded repeats. The frequency of expanded alleles (>40 repeats) at the CTG18.1 locus was significantly higher in the subgroup of patients with a more severe phenotype BPI and a positive first degree family history than in a group of matched controls (9% vs 5%). No difference in ERDA-1 expansion frequency was seen between BP cases and matched controls. We conclude that the ERDA-1 locus is not related to the BP phenotype while expanded alleles at the CTG18.1 locus cannot be excluded as a vulnerability factor for BP disorder.

No association between dopamine D(2) and D(4) receptor gene variants and antidepressant activity of two selective serotonin reuptake inhibitors

The possible association of the dopamine receptor D(2) (Ser 311Cys) and D(4) exon 3 (48 base pair repeat) gene variants with the antidepressant activity of selective serotonin reuptake inhibitors (SSRIs) was investigated in a sample of 364 inpatients affected by a major depressive episode treated with fluvoxamine, 300 mg/day (n=266), or paroxetine, 20-40 mg/day (n=98). The severity of depressive symptoms was assessed weekly with the Hamilton Rating Scale for Depression. Dopamine receptor D(2) (DRD2) and dopamine receptor D(4) (DRD4) allelic variants were determined in each subject by polymerase chain reaction. We observed that DRD2 and DRD4 variants were not associated with response to SSRI treatment. Possible stratification factors, such as sex, diagnosis, presence of psychotic features, depressive symptoms at baseline, paroxetine and fluvoxamine plasma levels, and pindolol augmentation did not significantly influence the observed results. The investigated DRD2 and DRD4 gene variants therefore do not seem to play a major role in the antidepressant activity of SSRIs, at least in the present sample.

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.

Molecular genetic and family studies in affective disorders: state of the art

The identification of genes responsible for mood disorders will contribute to significant advances in the awareness of diagnosis (diagnostic process and early recognition), pathophysiology, epidemiology and treatment issues. During the past two decades, the search for genes for mood disorders has mainly contributed to better understand and confirm the genetic complexities inherent to these disorders. The large amount of results available and the difficulty to digest them corroborate this observation. The major contribution of these findings should be integrated in the context of the world-wide efforts to identify the thousands of genes of the human genome. Some of these genes may be identified within the next decade. Several consistent hypotheses are currently being tested and will, hopefully, speed up the process of narrowing the important regions when the complete genome map will be available. The most promising chromosomal regions have been localized on chromosomes 4, 5, 11, 12, 18, 21 and X. A number of candidate genes have also been investigated, some of these are directly linked to neurobiological hypotheses of the aetiology of affective disorders. In parallel, specific hypotheses have been implicated, such as anticipation and dynamic mutations. Further research should concentrate on these hypotheses and confirm positive findings through interdisciplinary and multicenter projects.

Search for the influence of the tyrosine hydroxylase (TCAT)(n) repeat polymorphism on personality traits

A putatively functional tetranucleotide repeat polymorphism in the tyrosine hydroxylase gene (TH) has been investigated with regard to different aspects of psychopathology. We investigated whether reported associations of this TH polymorphism may reflect associations with common personality traits. Personality was assessed by the NEO Personality Inventory-Revised version (NEO PI-R), in 205 healthy Caucasian volunteers. Tendencies for higher scores in the neuroticism (N) facets, Angry hostility (P=0.008) and Vulnerability (P=0.021), were observed among carriers of one of the alleles (T8). Healthy women with the T6/T10 genotype had significantly higher scores (P=0.001) in the Deliberation and Dutifulness facets (P=0.031) (the Conscientiousness dimension, C) and lower scores (P=0.031) in the Feelings facet (the Openness dimension, O). We concluded that: (1) higher mean scores in the Neuroticism facets among T8 allele carriers are consistent with previous data and warrants further research; (2) the T6/T10 genotype may influence personality among women; (3) these data should be cautiously interpreted in the absence of corroborating data.

Systematic screening for DNA sequence variation in the coding region of the human dopamine transporter gene (DAT1)

The dopamine transporter (DAT) plays a central role in dopaminergic neurotransmission in the human brain. Genetic association studies have used a variable number of tandem repeat (VNTR) polymorphism in the 3'-flanking region of the dopamine transporter gene (DAT1) to implicate the DAT in the development of various neuropsychiatric disorders. In this study, we have examined the possibility that a mutation exists in the coding region of the DAT1 gene which through linkage disequilibrium accounts for the observed associations. The complete coding region, as well as exon-intron boundaries, was screened in 91 unrelated individuals including 45 patients with bipolar affective disorder and 46 healthy control individuals by the means of single strand conformation analysis. Our findings suggest that the DAT1 gene is highly conserved since we detected only two rare missense substitutions (Ala559Val, Glu602Gly) and three silent mutations (242C/T, 1342A/G, and 1859C/T) in the whole coding region. Five sequence variants were observed in intronic sequences but none affects known splice sites. The lack of frequent variants of possible functional relevance indicates that genetic variation in the coding region of the DAT1 gene is not responsible for the previously observed associations with neuropsychiatric disorders. The two rare missense substitutions were found in single bipolar patients but not in controls. Investigation of the patients' families revealed independent segregation between the Ala559Val variant and affective disorder. The Glu602Gly variant was inherited by the proband from an affected father. It therefore remains possible that Glu602Gly may be a rare cause of bipolar affective disorder.

Dopamine receptor D2 Ser/Cys 311 variant is associated with delusion and disorganization symptomatology in major psychoses

The D2 receptor (DRD2) is a binding site of many psychoactive drugs and it has been proposed as a genetic risk factor for psychiatric disorders. The aim of this investigation was to study the DRD2 S311C variant in major psychoses. We studied 1182 inpatients with diagnoses of bipolar disorder (n = 480), major depressive disorder (n = 269), schizophrenia (n = 366), delusional disorder (n = 44), psychotic disorder not otherwise specified (n = 23) and 267 healthy controls. Eight hundred and eighty-seven subjects were also scored for their lifetime symptomatology using the the Operational Criteria checklist for psychotic illness (OPCRIT). DRD2 variants were not associated with affected subjects even when possible confounders like gender and onset were considered. When we considered the 887 subjects with the symptomatologic analysis, we observed a significant association of the DRD2 S311C variant with both delusion and disorganization features. The association was present independently from diagnoses. Our results do not show that coding variants of the DRD2 S311C play a major role in conferring susceptibility to major psychoses, but they may be connected with disorganized and delusional symptomatology independently from diagnoses.

Linkage of mood disorders with D2, D3 and TH genes: a multicenter study

BACKGROUND

It has been suggested that the dopaminergic system is involved in the pathophysiology of mood disorders. We conducted a multicenter study of families with mood disorders, to investigate a possible linkage with genes coding for dopamine receptor D2, dopamine receptor D3 and tyrosine hydroxylase (TH).

METHODS

Twenty three mood disorder pedigrees collected within the framework of the European Collaborative Project on Affective Disorders were analyzed with parametric and non-parametric linkage methods. Various potential phenotypes were considered, from a narrow (only bipolar as affected) to a broad (bipolar+major depressive+schizoaffective disorders) definition of affection status.

RESULTS

Parametric analyses excluded linkage for all the candidate genes, even though small positive LOD (Limit of Detection) scores were observed for TH in three families. Non-parametric analyses yielded negative results for all markers.

CONCLUSION

The D2 and D3 dopamine receptors were, therefore, not a major liability factor for mood disorders in our sample, whereas TH may play a role in a subgroup of patients.

Searching high and low: a review of the genetics of bipolar disorder

OBJECTIVES

To review the methodologies and findings in the genetics of bipolar disorder (BPD), and to suggest future directions for research.

METHODS

Reports of family, twin, adoption, linkage, association, cytogenetic, and animal model studies, and segregation analyses in English, were identified from multiple MEDLINE searches. Hand searches were carried out in bibliographies from review articles.

RESULTS

Family, twin, and adoption studies have provided strong evidence for a genetic etiology in BPD. Early reports of linkage of BPD to DNA markers at several chromosomal sites have not proven robust, perhaps because of the complex nature of BPD inheritance. However, linkage findings in the 1990s, on chromosomes 18, 21q, 12q, and 4p, have provided leads that are being pursued through both genetic and physical mapping. No gene has yet been definitively implicated in BPD.

CONCLUSIONS

Strategies for increasing the power to detect BPD genes include: (1) dividing the phenotype into genetically meaningful subtypes to decrease heterogeneity: and (2) ascertaining a very large family sample--a multicenter study now in progress will collect 700 bipolar I sibling pairs. BPD may result from several genes acting in concert so that new multilocus statistical methods could enhance the capacity to detect loci involved. Family-based association studies using a very large number of newly identified single nucleotide polymorphisms (SNPs) may allow for more efficient screening of the genome. As the Human Genome Project approaches its goal of isolating all genes by 2003, the data generated is likely to speed identification of candidate BPD genes.

COMT and DRD3 polymorphisms, environmental exposures, and personality traits related to common mental disorders

In a community sample of 2,327 Caucasians, we tested the hypotheses that polymorphisms in the COMT and DRD3 genes are associated with personality traits conferring vulnerability to anxiety, depression, or alcohol misuse, or with current symptoms of these; and that the association is stronger in persons who also have been exposed to stressor experiences. To conserve resources and to allow replication, the genetic analysis was undertaken in two stages. For the COMT polymorphism, no statistically significant associations were found in the first sample of 862 persons. The remainder of the sample was therefore not analysed for that gene. For the DRD3 polymorphism, those in the first sample with at least one of the Ser(9) alleles had significantly higher scores in neuroticism (p=0.006) and behavioral inhibition (p=0.003). There was a trend, failing to meet the 1% significance criterion, for those with this genotype also to have higher depression and anxiety. The groups did not differ in alcohol use. In persons with the Ser(9) allele who were also exposed to stressors, there was a higher level of depression at the 5% level; and the depression level was higher in homozygotes. But when the remainder of the sample (1,465) was analysed, none of the associations reached statistical significance. We conclude that neither the COMT nor DRD3 polymorphisms are associated with anxiety, depression, or alcohol abuse. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:102-107, 2000

Elevated levels of tyrosine hydroxylase in the locus coeruleus in major depression

BACKGROUND

Levels of tyrosine hydroxylase (TH) are regulated in the noradrenergic locus coeruleus (LC) in response to changes in the activity of LC neurons and in response to changes in brain levels of norepinephrine. To study the potential role of central noradrenergic neurons in the pathobiology of major depression, TH protein was measured in the LC from postmortem brains of 13 subjects with a diagnosis of major depression and 13 age-matched control subjects having no Axis I psychiatric diagnosis. Most of the major depressive subjects died as a result of suicide.

METHODS

Protein from sections cut through multiple rostro-caudal levels of LC was transferred to Immobilon-P membrane, immunoblotted for TH, and quantified autoradiographically.

RESULTS

The distribution of TH-immunoreactivity (TH-ir) along the rostro-caudal axis of the LC was uneven and was paralleled by a similar uneven distribution of neuromelanin-containing cells in both major depressive and psychiatrically normal control subjects. Amounts of TH-ir in the rostral, middle and caudal levels of the LC from major depressive subjects were significantly higher than that of matched control subjects. There were no significant differences in the number of noradrenergic cells at any particular level of the LC comparing major depressive subjects to control subjects.

CONCLUSIONS

Elevated expression of TH in the LC in major depression implies a premortem overactivity of these neurons, or a deficiency of the cognate transmitter, norepinephrine.

Dopamine receptor D2 and D4 genes, GABA(A) alpha-1 subunit genes and response to lithium prophylaxis in mood disorders

Lithium is an effective prophylactic agent in mood disorders, and genetic factors are likely to modulate individual susceptibility to lithium treatment. The aim of this study is to investigate the influence of dopamine receptor D2 (DRD2), D4 exon 3 (DRD4), and gamma-aminobutyric acid type A (GABA(A)) receptor alpha-1 subunit (GABRA1) gene variants on the efficacy of lithium prophylaxis in mood disorders. Patients with mood disorders (N = 125: bipolar subtype, n = 100; major depressive disorder subtype, n = 25) were followed prospectively for an average of 53 months and were typed for DRD2 (Ser311/Cys311: n = 121, VNTR: n = 63), DRD4 (n = 125) and GABRA1 (n = 61) variants using polymerase chain reaction (PCR) techniques. DRD2, DRD4 and GABRA1 variants were not associated with response to lithium. A trend was observed toward a better outcome of DRD4* 2/4 subjects, but it was due to only two subjects. Consideration of possible stratification effects like gender, polarity, family history, age at onset and duration of lithium treatment did not reveal any association either. DRD2, DRD4 and GABRA1 variants therefore do not appear to be associated with the outcome of lithium prophylaxis in mood disorders.

Association analysis between dopamine receptor genes and bipolar affective disorder

We have performed a case-control analysis of dopamine D2-like receptor (DRD2, DRD3 and DRD4) gene polymorphisms in 118 Han Chinese cases with bipolar affective disorder and 196 control subjects, and replication analysis in 157 English cases and 143 control subjects. We found association between a functional DRD2 promoter variant (P = 0.03 by allele) and the DRD2 taq1A polymorphism (P = 0.001 by allele) in Chinese bipolar disorder patients. However, this finding was not replicated in the Caucasian subjects, indicating that the significant association we observed in the Chinese population is a false positive finding. An alternative explanation is that these polymorphisms are risk factors in Chinese but not Caucasian populations, a hypothesis which seems unlikely in view of the similarity of the clinical characteristics of bipolar disorder in the two populations. We also report a novel, rare one-repeat variant of the DRD4 exon 3 VNTR repeat in Chinese populations, which appears to be absent in Caucasians and is not associated with disease.

Molecular genetic findings in mood disorders

Traditional methods used to asses genetic effects, such as twins, adoption and family studies, have demonstrated the role genetic vulnerability factors in the etiology of major psychiatric diseases such as affective disorders and schizophrenia. It remains however impossible, using these methods, to specify the genetic variables involved and the exact mode of transmission of these diseases. New genetic approaches in psychiatry include the use of DNA markers in sophisticated strategies to examine families and populations. Genetic linkage (in families) and allelic association (in unrelated subjects) are the most frequent techniques applied searching for genes in psychiatric diseases. Advances in these methods have permitted their application to complex diseases in which the mode of genetic transmission is unknown. Affective disorders and, in particular, bipolar affective disorder (BPAD) have been examined in many molecular genetic studies which have covered a large part of the genome, specific hypotheses such as mutations have also, been studied. Most recent studies indicate that several chromosomal regions may be involved in the aetiology of affective disorders. Large multi-centre and multi-disciplinary projects are currently underway in Europe and in the US and hopefully will improve our understanding of the genetic factors involved in affective disorders. In parallel to these new developments in molecular genetics, the classical genetic epidemiology, represented by twin, adoption and family studies, have been improved, providing validated models to test the gene-environment interactions.

Analysis and metaanalysis of two polymorphisms within the tyrosine hydroxylase gene in bipolar and unipolar affective disorders

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of dopamine and noradrenaline. While positive associations between TH and bipolar affective disorder have been found in several studies, many studies have failed to reproduce these results. In order to clarify this situation, association studies of bipolar and unipolar affective disorder groups and metaanalyses of published data on the TH tetranucleotide repeat polymorphism were done. The association studies used the TH tetranucleotide repeat polymorphism in intron 1 and a PstI polymorphism at the 3' end of the gene. The study comprised 124 unrelated bipolar patients, 126 unipolar patients, and 242 controls. There was no significant association of either bipolar or unipolar affective disorder with the TH tetranucleotide repeat polymorphism. However, a weak association (chi2 = 3.946, 1 df, P = 0.047; odds ratio, allele 2 vs. allele 1 = 0.71 (95% CI, 0.51-0.996)) was observed in the unipolar sample with the TH-PstI polymorphism. Three metaanalyses of published data on the TH tetranucleotide repeat polymorphism in major affective disorder were performed: bipolar I + II vs. control using 583 cases and 745 controls; unipolar vs. control using 204 cases and 359 controls; and bipolar + unipolar vs. control using 846 cases and 823 controls. In each analysis there was no association of the TH tetranucleotide repeat polymorphism and affective disorder. These results do not support the tyrosine hydroxylase gene having a major role in the etiology of bipolar affective disorder. However, our data suggest that this locus should be examined in larger samples of unipolar affective disorder.