Candidate gene studies of bipolar disorder

Genetics of bipolar disorder: insights into its complex architecture and biology from common and rare variants

Bipolar disorder (BD) is a common mental disorder characterized by recurrent mood episodes, which causes major socioeconomic burdens globally. Though its disease pathogenesis is largely unknown, the high heritability of BD indicates strong contributions from genetic factors. In this review, we summarize the recent achievements in the genetics of BD, particularly those from genome-wide association study (GWAS) of common variants and next-generation sequencing analysis of rare variants. These include the identification of dozens of robust disease-associated loci, deepening of our understanding of the biology of BD, objective description of correlations with other psychiatric disorders and behavioral traits, formulation of methods for predicting disease risk and drug response, and the discovery of a single gene associated with bipolar disorder and schizophrenia spectrum with a large effect size. On the other hand, the findings to date have not yet made a clear contribution to the improvement of clinical psychiatry of BD. We overview the remaining challenges as well as possible paths to resolve them, referring to studies of other major neuropsychiatric disorders.

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.

Oxidative Stress Implications in the Affective Disorders: Main Biomarkers, Animal Models Relevance, Genetic Perspectives, and Antioxidant Approaches

The correlation between the affective disorders and the almost ubiquitous pathological oxidative stress can be described in a multifactorial way, as an important mechanism of central nervous system impairment. Whether the obvious changes which occur in oxidative balance of the affective disorders are a part of the constitutive mechanism or a collateral effect yet remains as an interesting question. However it is now clear that oxidative stress is a component of these disorders, being characterized by different aspects in a disease-dependent manner. Still, there are a lot of controversies regarding the relevance of the oxidative stress status in most of the affective disorders and despite the fact that most of the studies are showing that the affective disorders development can be correlated to increased oxidative levels, there are various studies stating that oxidative stress is not linked with the mood changing tendencies. Thus, in this minireview we decided to describe the way in which oxidative stress is involved in the affective disorders development, by focusing on the main oxidative stress markers that could be used mechanistically and therapeutically in these deficiencies, the genetic perspectives, some antioxidant approaches, and the relevance of some animal models studies in this context.

Acute inflammatory biomarker profiles predict depression risk following moderate to severe traumatic brain injury

OBJECTIVE

To examine whether acute inflammation profiles predict posttraumatic depression (PTD) risk 6 and 12 months after traumatic brain injury.

SETTING

University-affiliated level 1 trauma center and community.

PARTICIPANTS

Adults with moderate to severe traumatic brain injury (acute serum levels: n = 50; acute cerebrospinal fluid (CSF) levels: n = 41).

DESIGN

Prospective cohort study.

MAIN MEASURES

Patient Health Questionnaire; inflammatory biomarkers (interleukin [IL]-1β, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12, tumor necrosis factor α, soluble vascular adhesion molecule [sVCAM-1], soluble intracellular adhesion molecule [sICAM-1], soluble Fas [sFAS]).

RESULTS

Higher levels of acute CSF cytokine surface markers (sVCAM-1, sICAM-1, and sFAS) in an inflammatory biomarker risk (IBR) score were associated with a 3.920-fold increase in the odds of developing PTD at 6 months (95% confidence interval: 1.163-8.672). Having sICAM-1, sVCAM-1, or sFAS above the 75th percentile had a positive predictive value of 85.7% for PTD risk at 6 months. An IBR score including inflammatory biomarkers IL-7 and IL-8 showed a trending association with 12-month PTD risk (odds ratio = 3.166, 95% confidence interval: 0.936-10.708).

CONCLUSION

Acute CSF IBR scores show promise for identifying individuals at risk for PTD. Further research should assess acute CSF inflammatory biomarkers' relationships to chronic inflammation as a mechanism of PTD and should explore anti-inflammatory treatments for PTD, as well as prevention and screening protocols, and link inflammatory biomarkers to symptom tracking.

Genetic and neurocognitive foundations of emotion abnormalities in bipolar disorder

INTRODUCTION

Bipolar Disorder (BD) is a serious mood disorder, the aetiology of which is still unclear. The disorder is characterised by extreme mood variability in which patients fluctuate between markedly euphoric, irritable, and elevated states to periods of severe depression. The current research literature shows that BD patients demonstrate compromised neurocognitive ability in addition to these mood symptoms. Viable candidate genes implicated in neurocognitive and socioemotional processes may explain the development of these core emotion abnormalities. Additionally, links between faulty neurocognition and impaired socioemotional ability complement genetic explanations of BD pathogenesis. This review examines associations between cognition indexing prefrontal neural regions and socioemotional impairments including emotion processing and regulation. A review of the effect of COMT and TPH2 on these functions is also explored.

METHODS

Major computer databases including PsycINFO, Google Scholar, and Medline were consulted in order to conduct a comprehensive review of the genetic and cognitive literature in BD.

RESULTS

This review determines that COMT and TPH2 genetic variants contribute susceptibility to abnormal prefrontal neurocognitive function which oversees the processing and regulation of emotion. This provides for greater understanding of some of the emotional and cognitive symptoms in BD.

CONCLUSIONS

Current findings in this direction show promise, although the literature is still in its infancy and further empirical research is required to investigate these links explicitly.

Investigation of the ZNF804A gene polymorphism with genetic risk for bipolar disorder in attention deficit hyperactivity disorder

BACKGROUND

Genome-wide association studies (GWAS) have been conducted on many psychiatric disorders. Evidence from large GWAS indicates that the single nucleotide polymorphism (SNP) rs1344706 in the zinc-finger protein 804A gene (ZNF804A) is associated with psychotic disorders including bipolar disorder and schizophrenia. One study also found significant association between rs1344706 and the executive control network of attention. In this study we examine the role of the rs1344706 polymorphism that previously showed association with BD and is known to alter expression of the gene in two clinical family-based ADHD samples from the UK and Taiwan.

FINDINGS

To investigate the association between rs1344706 and ADHD, two family samples of ADHD probands from the United Kingdom (n = 180) and Taiwan (n = 212) were genotyped using TaqMan SNP genotyping assays and analysed using within-family transmission disequilibrium test. No significant associations were found between rs1344706 polymorphism and ADHD in either of the samples from Taiwan (P = 0.91) and UK (P = 0.41). Even combining the two datasets together the A allele of rs1344706 SNP was still not significantly over-transmitted to affected probands (P = 0.50). Furthermore, there was no evidence of association with the specific symptoms subgroups of inattention or hyperactivity-impulsivity.

CONCLUSIONS

In this study we used family-based ADHD data in the UK and Taiwanese population to test for an association between rs1344706 SNP in the ZNF804A gene and ADHD. Results showed no significant association of rs1344706 with ADHD in UK and Taiwanese samples.

Pathophysiology of depression and mechanisms of treatment

Major depression is a serious disorder of enormous sociological and clinical relevance. The discovery of antidepressant drugs in the 1950s led to the first biochemical hypothesis of depression, which suggested that an impairment in central monoaminergic function was the major lesion underlying the disorder. Basic research in all fields of neuroscience (including genetics) and the discovery of new antidepressant drugs have revolutionized our understanding of the mechanisms underlying depression and drug action. There is no doubt that the monoaminergic system is one of the cornerstones of these mechanisms, but multiple interactions with other brain systems and the regulation of central nervous system function must also be taken into account In spite of all the progress achieved so far, we must be aware that many open questions remain to be resolved in the future.

The severity of mental disorders is linked to interaction among candidate genes

There is a considerable overlap in the manifestation of symptoms in three mental disorders namely unipolar disorder, bipolar disorder and schizophrenia. A gene coexpression network was developed based on a mutual information approach including four candidate genes (NRG1, DISC1, BDNF and COMT) along with other coexpressing genes in unipolar disorder, bipolar disorder and schizophrenia. There is a significant difference in the degree distribution of nodes between normal and bipolar disorder network and bipolar disorder network and schizophrenia network. Moreover, there is a differential direct connectivity among candidate genes in various mental disorders and between normal and mental disorders. All candidate genes are directly connected to each other in schizophrenia except one pair (NRG1-BDNF) indicating a strong role of inter-gene interactions in the manifestation of severe symptoms in this disease. DISC1 and NRG1 are key hub genes in the unipolar disorder network and the bipolar disorder network but have lost the role of hub genes in schizophrenia network, despite their significant association with schizophrenia. This study indicates that the three psychiatric diseases may not have discrete classes but three phenotypic manifestations of the same continuous disease based on severity.

Pathways-based analyses of whole-genome association study data in bipolar disorder reveal genes mediating ion channel activity and synaptic neurotransmission

Despite known heritability, the complex genetic architecture of bipolar disorder (likely including trait, locus and allelic heterogeneity, as well as genetic interactions) has confounded genetic discovery for many years. Even modern day whole genome association studies (WGAS) using over half a million common SNPs have implicated only a handful of genes at the genomewide level. Temporally coincident with this series of WGAS, a host of pathways-based analyses (PBAs) have emerged as novel computational approaches in the examination of large-scale datasets, but thus far rarely have been applied to WGAS data in psychiatric disorders. Here, we report a series of PBAs conducted using exploratory visual analysis, an analytic and visualization software tool for examining genomic data, to examine results from the National Institutes of Mental Health and Wellcome-Trust Case Control Consortium WGAS in bipolar disorder. Consistent with a host of prior linkage findings, some candidate gene association studies, and recent WGAS, our strongest findings suggest involvement of ion channel structural and regulatory genes, including voltage-gated ion channels and the broader ion channel group that comprises both voltage- and ligand-gated channels. Moreover, we found only modest overlap in the particular genes driving the significance of these gene sets across the analyses. This observation strongly suggests that variation in ion channel genes, as a class of genes, may contribute to the susceptibility of bipolar disorder and that heterogeneity may figure prominently in the genetic architecture of this susceptibility.

Update on psychiatric genetics

Genetic factors play a fundamental role in the genesis of many mental disorders. The identification of the underlying genetic variation will therefore transform parts of psychiatry toward a neuroscience-based discipline. With the sequence of the human genome now available, the majority of common variations identified, and new high-throughput technologies arriving in academic research laboratories, the identification of genes is expected to explain a large proportion of the risk of developing mental disorders. So far, a number of risk genes have been identified, but no major gene has emerged. The majority of these genes participate in the regulation of biogenic amines that play critical roles in affect modulation and reward systems. The identification of genetic variations associated with mental disorders should provide an approach to evaluate risk for mental disorders, adjust pharmacotherapy on the individual level, and even allow for preventive interactions. New targets for the development of treatment are anticipated to derive from results of genetic studies. In this review, we summarize the current state of psychiatric genetics, underscore current discussions, and predict where the field is expected to move in the near future.

Genetics of stress response and stress-related disorders

The major findings regarding the genetics of stress response and stress-related disorders are: (i) variations in genes involved in the sympathetic system or in the hypothalamic-pituitary-adrenocortical axis are associated with altered stress responses; (ii) genes related to the renin-angiotensin-aldosterone system or inflammation/immune response show associations with cardiovascular disorders; (iii) genes involved in monoaminergic neurotransmitter systems are associated with bipolar disorder and unipolar depression. The vast majority of these association studies followed a conventional hypothesis-driven approach, restricting the gene selection to established candidates. This very conservative approach retarded our understanding of the complex interplay between genetic factors, stress response, and stress-related disorders. Chip-based whole-genome technologies will open up access to new unbiased and statistically efficient approaches that will help to identify new candidate genes, which should be thoroughly validated in clinical and preclinical confirmatory studies. This, together with the use of new text- and information-mining tools, will bring us closer to integrating all the findings into sophisticated models delineating the pathways from genes to stress response and stress-related disorders.

Atypical antipsychotics in bipolar depression: neurobiological basis and clinical implications

The use of atypical antipsychotics is rapidly expanding in the management of bipolar disorder. This therapeutic class appears to have benefits across the spectrum of moods found in bipolar disorder, and a re-examination of the monoamine hypothesis is required. This paper reviews the evidence for the role of monoamines, and particularly dopamine, in bipolar depression and its implications in the treatment of patients, focusing upon the response to atypical antipsychotics. Relevant papers were identified undertaking a literature search using PubMed: preclinical and clinical studies that incriminate the dopaminergic system in bipolar depression, and recent controlled trials supporting the use of atypical antipsychotics, are reviewed. There is substantial evidence indicating that not only serotonin, but also dopamine may still play an important role as a mediator of antidepressant response in bipolar depression. A regionally selective balance between the dopamine and serotonin systems may account for the mood-stabilizing properties of these drugs, and in fact, a low D(2) occupancy might be more relevant to bipolar depression than greater 5-HT(2) action. This mechanism might correlate with the ability to induce neurogenetic effects. Hence, the greater the atypical profile, the more suitable for bipolar depression. Further studies are needed to confirm this hypothesis.

Cytokine, sickness behavior, and depression

Sufficient evidence is now available to accept the concept that the brain recognizes cytokines as molecular signals of sickness. Clarifying the way the brain processes information generated by the innate immune system is accompanied by a progressive elucidation of the cellular and molecular components of the intricate system that mediates cytokine-induced sickness behavior. We are still far, however, from understanding the whole. Among the hundreds of genes that proinflammatory cytokines can induce in their cellular targets, only a handful has been examined functionally. In addition, a dynamic view of the cellular interactions that occur at the brain sites of cytokine production and action is missing, together with a clarification of the mechanisms that favor the transition toward pathology.

Co-occurrence of bipolar and attention-deficit hyperactivity disorders in children

OBJECTIVES

Pediatric bipolar disorder (BPD) and attention-deficit hyperactivity disorder (ADHD) co-occur more frequently than expected by chance. In this review, we examine 4 potential explanations for the high rate of this common co-occurrence: (i) BPD symptom expression leads to overdiagnosis of ADHD in BPD youth; (ii) ADHD is a prodromal or early manifestation of pediatric-onset BPD; (iii) ADHD and associated factors (e.g., psychostimulants) lead to the onset of pediatric BPD; and (iv) ADHD and BPD share an underlying biological etiology (i.e., a common familial or genetic risk or underlying neurophysiology).

METHODS

Peer-reviewed publications of studies of children and adolescents with comorbid BPD and ADHD were reviewed.

RESULTS

There is a bidirectional overlap between BPD and ADHD in youth, with high rates of ADHD present in children with BPD (up to 85%), and elevated rates of BPD in children with ADHD (up to 22%). Phenomenologic, genetic, family, neuroimaging, and treatment studies revealed that BPD and ADHD have both common and distinct characteristics. While there are data to support all 4 explanations postulated in this paper, the literature most strongly suggests that ADHD symptoms represent a prodromal or early manifestation of pediatric-onset BPD in certain at-risk individuals. Bipolar disorder with comorbid ADHD may thus represent a developmentally specific phenotype of early-onset BPD.

CONCLUSIONS

The etiology of comorbid BPD and ADHD is likely multifactorial. Additional longitudinal and biological studies are warranted to clarify the relationships between BPD and ADHD since they may have important diagnostic and treatment implications.

Genetics of affective (mood) disorders

The enormous public health importance of mood disorders, when considered alongside their substantial heritabilities, has stimulated much work, predominantly in bipolar disorder but increasingly in unipolar depression, aimed at identifying susceptibility genes using both positional and functional molecular genetic approaches. Several regions of interest have emerged in linkage studies and, recently, evidence implicating specific genes has been reported; the best supported include BDNF and DAOA but further replications are required and phenotypic relationships and biological mechanisms need investigation. The complexity of psychiatric phenotypes is demonstrated by (a) the evidence accumulating for an overlap in genetic susceptibility across the traditional classification systems that divide disorders into schizophrenia and mood disorders, and (b) evidence suggestive of gene-environment interactions.

The genetics of schizophrenia and bipolar disorder: dissecting psychosis

Much work has been done to identify susceptibility genes in schizophrenia and bipolar disorder. Several well established linkages have emerged in schizophrenia. Strongly supported regions are 6p24-22, 1q21-22, and 13q32-34, while other promising regions include 8p21-22, 6q16-25, 22q11-12, 5q21-q33, 10p15-p11, and 1q42. Genomic regions of interest in bipolar disorder include 6q16-q22, 12q23-q24, and regions of 9p22-p21, 10q21-q22, 14q24-q32, 13q32-q34, 22q11-q22, and chromosome 18. Recently, specific genes or loci have been implicated in both disorders and, crucially, replicated. Current evidence supports NRG1, DTNBP1, DISC1, DAOA(G72), DAO, and RGS4 as schizophrenia susceptibility loci. For bipolar disorder the strongest evidence supports DAOA(G72) and BDNF. Increasing evidence suggests an overlap in genetic susceptibility across the traditional classification systems that dichotomised psychotic disorders into schizophrenia or bipolar disorder, most notably with association findings at DAOA(G72), DISC1, and NRG1. Future identification of psychosis susceptibility genes will have a major impact on our understanding of disease pathophysiology and will lead to changes in classification and the clinical practice of psychiatry.

Bipolar affective disorder: advances in genetics and mood-stabilising medication

The lifetime prevalence of bipolar affective disorder is between 1 % and 2%. This educational review paper focuses on two areas of interest and relevance to trainees preparing for the membership examination of the Royal College of Psychiatrists: (a) advances in the genetics of bipolar affective disorder; and (b) mood-stabilising medication in bipolar affective disorder.

Translating knowledge of genetics and pharmacology into improving everyday practice

Bipolar disorder is associated with significant morbidity and mortality; however, many aspects of this disorder remain poorly understood. It is likely that rapid advances in molecular genetics and neuroimaging will play a major role in advancing our understanding of bipolar disorder in future. Molecular genetics studies have already identified some candidate genes; for example, the BDNF, G72 and XBP1 genes, and chromosomal 'hot spots', which may confer a predisposition to development of bipolar disorder. Such advances may facilitate earlier, easier and more accurate diagnosis and provide novel targets for the treatment of this condition. Brain imaging studies using positron emission tomography and single photon emission computed tomography have shown that reduction in brain 5-hydroxytryptamine type 2 (5-HT(2)) receptors may be associated with prevention of or relief from depressive symptoms. Similarly, other imaging studies suggest that increased dopamine levels in the synapse mediate the symptoms of bipolar mania and that reduction in dopamine transmission through reduction in dopamine synthesis or blockade of dopamine D(2) receptors may be associated with antimanic effects. The ability of atypical antipsychotics to block both 5-HT(2) and D(2) receptors and downregulate 5-HT(2) receptors may explain how these drugs treat both the depressive and manic symptoms of bipolar disorder. Although molecular genetics and imaging techniques are not yet used as clinical tools for bipolar disorder, they provide valuable data to improve the understanding of the pathophysiology of bipolar disorder and should lead to new treatments and potentially episode prevention.

Genetic linkage and association studies in bipolar affective disorder: a time for optimism

Genetic research on complex diseases is beginning to bear fruit, with the successful identification of candidate susceptibility genes in diabetes, asthma, and other illnesses. Similar success is on the horizon for bipolar affective disorder (BPAD), but significant challenges remain. In this review, we outline the basic concepts of linkage and association mapping for complex phenotypes. We point out important caveats inherent in both approaches, and review guidelines on the interpretation of linkage statistics and significance thresholds. We then apply these concepts to an evaluation of the present status of genetic linkage and association studies in BPAD. The challenges posed by locus heterogeneity, phenotype definition, and sample size requirements are given a detailed treatment. Despite these challenges, we argue that the way ahead remains firmly rooted in linkage studies, complemented by association studies in linked regions. This is the only truly genome-wide approach currently available; it has succeeded in other complex phenotypes, and it is the surest strategy for mapping susceptibility genes in BPAD. Once these genes are identified, genetic mapping methods will yield to the other methods of 21st-century molecular biology as we begin to elucidate the pathophysiology of BPAD.

Brain-derived neurotrophic factor as a potential risk locus for bipolar disorder: evidence, limitations, and implications

Brain-derived neurotrophic factor (BDNF) plays an important role in promoting and modifying growth, development, and survival of neuronal populations, and, in the mature nervous system, is involved in activity-dependent neuronal plasticity. Based on several lines of evidence, BDNF has been hypothesized to play an important role in the pathogenesis of mood disorder and the therapeutic action of at least some effective treatments. The gene encoding BDNF lies on the short arm of chromosome 11 in a region where some linkage studies of bipolar disorder have reported evidence for a susceptibility gene. BDNF can, thus, be considered as an attractive candidate gene for involvement in the pathogenesis of bipolar disorder, and two recent family-based association studies have provided evidence that one or more sequence variants within or near the BDNF gene show an association with disease susceptibility. These findings are of great interest and may open up a new chapter in the understanding of the causation and treatment of bipolar disorder. However, it is still early in the genetic investigation of BDNF in bipolar disorder, and it is important that these findings are replicated in large independent samples and that functional studies can confirm and characterize the pathogenic relevance of this genetic variation.

No association between two polymorphisms at the 5HT2A gene and bipolar affective puerperal psychosis

OBJECTIVE

To examine whether variation at two common polymorphisms, T102C and -1438AG, of the serotonin 2A gene (5HT2A) are involved in the puerperal triggering mechanism of bipolar affective puerperal psychosis.

METHOD

A total of 242 parous women diagnosed with bipolar disorder were genotyped for the two polymorphisms. Of these, 165 women had experienced a manic or psychotic episode, according to DSM-IV criteria, within 6 weeks of childbirth (the puerperal psychosis group). The comparison group comprised of 77 parous women who had not experienced psychiatric disturbance following childbirth.

RESULTS

No significant differences between genotype or allelic frequencies were found between the two groups for either polymorphism.

CONCLUSION

The results indicate that variation at two common polymorphisms of the 5HT2A gene does not appear to play a major role in the development of bipolar affective puerperal psychosis.

Triplet repeats and bipolar disorder

Anticipation, the phenomenon of a disease becoming more severe or having earlier onset as it is transmitted down the generations, was originally described in families with psychiatric illness but was thought due to ascertainment bias and became forgotten. Interest was rekindled when a number of neurodegenerative disorders that show this phenomenon, were found to be due to a novel form of mutation--unstable triplet repeats showing intergenerational expansion. Some recent studies of anticipation are consistent with its occurrence in bipolar disorder but are still associated with methodological problems making interpretation difficult. A number of case-control studies employing the repeat expansion detection (RED) technique have found longer repeats in bipolar probands but other studies have found no such association. Despite a large number of studies examining the role of various repeat containing candidate genes, a pathogenic triplet repeat has yet to be found for bipolar disorder. It is likely that the controversy surrounding anticipation and the existence of triplet repeats will only finally be resolved with the demonstration of such a mutation in the aetiology of bipolar disorder.

Molecular genetic approaches to puerperal psychosis

Puerperal psychosis, an episode of mania or psychosis precipitated by childbirth follows approximately one in 1000 deliveries. The evidence of clinical, outcome and genetic studies supports the hypothesis that the majority of puerperal psychotic episodes are manifestations of an affective disorder diathesis with a puerperal trigger. Furthermore the available evidence supports the hypothesis that genes are involved in susceptibility to both diathesis and trigger. For complex genetic disorders such as affective illness there are marked benefits in focussing on a homogeneous subtype which allows a subset of hypotheses to be tested. Molecular genetic studies of puerperal psychosis provide an excellent example of this strategy, allowing a hierarchy of hypotheses concerning the involvement of neurosteroid pathways in pathophysiology to be tested. Puerperal psychosis results in considerable suffering to a woman and her family. Elucidating the pathophysiological basis of this disorder will lead to better prevention and treatment and, it is anticipated, inform research on affective disorders more generally.