Family-based association study of 76 candidate genes in bipolar disorder: BDNF is a potential risk locus. Brain-derived neutrophic factor

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.

Neurotrophin receptor activation and expression in human postmortem brain: effect of suicide

BACKGROUND

The physiological functions of neurotrophins occur through binding to two receptors: pan75 neurotrophin receptor (p75(NTR)) and a family of tropomyosin receptor kinases (Trks A, B, and C). We recently reported that expression of neurotrophins and TrkB were reduced in brains of suicide subjects. This study examines whether expression and activation of Trk receptors and expression of p75(NTR) are altered in brain of these subjects.

METHODS

Expression levels of TrkA, B, C, and of p75(NTR) were measured by quantitative reverse transcription polymerase chain reaction and Western blot in prefrontal cortex (PFC) and hippocampus of suicide and normal control subjects. The activation of Trks was determined by immunoprecipitation followed by Western blotting using phosphotyrosine antibody.

RESULTS

In hippocampus, lower mRNA levels of TrkA and TrkC were observed in suicide subjects. In the PFC, the mRNA level of TrkA was decreased, without any change in TrkC. However, the mRNA level of p75(NTR) was increased in both PFC and hippocampus. Immunolabeling studies showed similar results as observed for the mRNAs. In addition, phosphorylation of all Trks was decreased in hippocampus, but in PFC, decreased phosphorylation was noted only for TrkA and B. Increased expression ratios of p75(NTR) to Trks were also observed in PFC and hippocampus of suicide subjects.

CONCLUSIONS

Our results suggest not only reduced functioning of Trks in brains of suicide subjects but also that increased ratios of p75(NTR) to Trks indicate possible activation of pathways that are apoptotic in nature. These findings may be crucial in the pathophysiology of suicide.

Family-based association study of the BDNF, COMT and serotonin transporter genes and DSM-IV bipolar-I disorder in children

BACKGROUND

Over the past decade pediatric bipolar disorder has gained recognition as a potentially more severe and heritable form of the disorder. In this report we test for association with genes coding brain-derived neurotrophic factor (BDNF), the serotonin transporter (SLC6A4), and catechol-O-methyltransferase (COMT).

METHODS

Bipolar-I affected offspring triads (N = 173) were drawn from 522 individuals with 2 parents in 332 nuclear families recruited for genetic studies of pediatric psychopathology at the Clinical and Research Program in Pediatric Psychopharmacology and Adult ADHD at Massachusetts General Hospital.

RESULTS

We failed to identify an association with the val66 allele in BDNF (OR = 1.23, p = 0.36), the COMT-l allele (OR = 1.27, p = 0.1), or the HTTLPR short allele (OR = 0.87, p = 0.38).

CONCLUSION

Our study suggests that the markers examined thus far in COMT and SLC6A4 are not associated with pediatric bipolar disorder and that if the val66met marker in BDNF is associated with pediatric bipolar disorder the magnitude of the association is much smaller than first reported.

Association between Val66Met brain-derived neurotrophic factor (BDNF) gene polymorphism and post-treatment relapse in alcohol dependence

BACKGROUND

The purpose of this study was to examine relationships between genetic markers of central serotonin (5-HT) and dopamine function, and risk for post-treatment relapse, in a sample of alcohol-dependent patients.

METHODS

The study included 154 patients from addiction treatment programs in Poland, who met DSM-IV criteria for alcohol dependence. After assessing demographics, severity of alcohol use, suicidality, impulsivity, depression, hopelessness, and severity of alcohol use at baseline, patients were followed for approximately 1 year to evaluate treatment outcomes. Genetic polymorphisms in several genes (TPH2, SLC6A4, HTR1A, HTR2A, COMT, and BDNF) were tested as predictors of relapse (defined as any drinking during follow-up) while controlling for baseline measures.

RESULTS

Of 154 eligible patients, 123 (80%) completed follow-up and 48% (n = 59) of these individuals relapsed. Patients with the Val allele in the Val66Met BDNF polymorphism and the Met allele in the Val158Met COMT polymorphism were more likely to relapse. Only the BDNF Val/Val genotype predicted post-treatment relapse [odds ratio (OR) = 2.62; p = 0.019], and time to relapse (OR = 2.57; p = 0.002), after adjusting for baseline measures and other significant genetic markers. When the analysis was restricted to patients with a family history of alcohol dependence (n = 73), the associations between the BDNF Val/Val genotype and relapse (OR = 5.76, p = 0.0045) and time to relapse (hazard ratio = 4.93, p = 0.001) were even stronger.

CONCLUSIONS

The Val66Met BDNF gene polymorphism was associated with a higher risk and earlier occurrence of relapse among patients treated for alcohol dependence. The study suggests a relationship between genetic markers and treatment outcomes in alcohol dependence. Because a large number of statistical tests were conducted for this study and the literature on genetics and relapse is so novel, the results should be considered as hypothesis generating and need to be replicated in independent studies.

Genetics of bipolar disorder: successful start to a long journey

Family and twin studies attest to the importance of genetic factors influencing susceptibility to bipolar disorder and to its genetic and phenotypic complexity. Although linkage and candidate gene association studies have repeatedly implicated some chromosome regions and certain genes, they have not produced the level of unambiguous support required to confirm the involvement of any specific gene or sequence variant in the pathogenesis of bipolar disorder. However, strong associations have recently been reported in meta-analyses of genome-wide association studies and the systematic study of structural variation is ongoing. These findings indicate that the study of large, phenotypically well-characterized samples will make an important contribution to delineating the etiology and pathogenesis of bipolar disorder and thereby pave the way for major improvements in clinical management.

Sexually dimorphic effects of four genes (COMT, SLC6A2, MAOA, SLC6A4) in genetic associations of ADHD: a preliminary study

A growing body of literature finds gender differences in ADHD. However, little is known about the causes of these differences. One possibility is that ADHD risk genes have sexually dimorphic effects. We have investigated four ADHD candidate genes (COMT, SLC6A2, MAOA, SLC6A4) for which there is evidence of sexually dimorphic effects. Past neurobiological and genetic studies suggest that COMT, and SLC6A4 variants may have a greater influence on males and that SLC6A2, and MAOA variants may have a greater influence on females. Our results indicate that genetic associations are stronger when stratified by sex and in the same direction as the previous neurobiological studies indicate: associations were stronger in males for COMT, SLC6A4 and stronger in females for SLC6A2, MAOA. Moreover, we found a statistically significant gender effect in the case of COMT (P = 0.007) when we pooled our work with a prior study. In conclusion, we have found some evidence suggesting that the genetic association for these genes with ADHD may be influenced by the sex of the affected individual. Although our results are not fully validated yet, they should motivate further investigation of gender effects in ADHD genetic association studies.

Evidence of association between brain-derived neurotrophic factor gene and bipolar disorder

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) plays an important role in the survival, differentiation, and outgrowth of select peripheral and central neurons throughout adulthood. Growing evidence suggests that BDNF is involved in the pathophysiology of mood disorders.

METHODS

Ten single nucleotide polymorphisms (SNPs) across the BDNF gene were genotyped in a sample of 1749 Caucasian Americans from 250 multiplex bipolar families. Family-based association analysis was used with three hierarchical bipolar disorder models to test for an association between SNPs in BDNF and the risk of bipolar disorder. In addition, an exploratory analysis was performed to test for an association of the SNPs in BDNF and the phenotypes of rapid cycling and episode frequency.

RESULTS

Evidence of association (P<0.05) was found with several of the SNPs using multiple models of bipolar disorder; one of these SNPs also showed evidence of association (P<0.05) with rapid cycling.

CONCLUSION

These results provide further evidence that variation in BDNF affects the risk for bipolar disorder.

Impact of the brain-derived neurotrophic factor Val66Met polymorphism on levels of hippocampal N-acetyl-aspartate assessed by magnetic resonance spectroscopic imaging at 3 Tesla

BACKGROUND

This study was conducted to corroborate prior evidence of an effect of the brain-derived neurotrophic factor (BDNF) valine (val) to methionine (met) amino acid substitution at codon 66 (val66met) polymorphism on measures of N-acetyl-aspartate (NAA) containing compounds in healthy subjects.

METHODS

The NAA to creatine (Cre) ratio (NAA/Cre), NAA to choline (Cho) ratio (NAA/Cho), and Cho to Cre ratio (Cho/Cre) were measured in the left and right hippocampi, left and right dorsolateral prefrontal cortices, occipital lobe, anterior cingulate, and white matter of the centrum semiovale of 69 carefully screened healthy volunteers utilizing proton magnetic resonance spectroscopic imaging (MRSI) at 3 Tesla (T).

RESULTS

Val/met subjects exhibited significantly reduced levels of left hippocampal NAA/Cre and NAA/Cho compared with val/val subjects. This effect was independent of age, IQ, number of voxels, hippocampal volume, or gray matter content in the voxels of interest. Analysis of other brain regions showed no effect of BDNF genotype on NAA measures.

CONCLUSIONS

We confirmed the association between the met-BDNF variant and reduced levels of hippocampal NAA found with a similar technique at 1.5T. The consonance of our results with prior findings adds to the evidence that the BDNF val/met genotype affects hippocampal biology with implications for a variety of neuropsychiatric disorders.

From synapse to nucleus: calcium-dependent gene transcription in the control of synapse development and function

One of the unique characteristics of higher organisms is their ability to learn and adapt to changes in their environment. This plasticity is largely a result of the brain's ability to convert transient stimuli into long-lasting alterations in neuronal structure and function. This process is complex and involves changes in receptor trafficking, local mRNA translation, protein turnover, and new gene synthesis. Here, we review how neuronal activity triggers calcium-dependent gene expression to regulate synapse development, maturation, and refinement. Interestingly, many components of the activity-dependent gene expression program are mutated in human cognitive disorders, which suggest that this program is essential for proper brain development and function.

Pharmacogenetic analysis of genes implicated in rodent models of antidepressant response: association of TREK1 and treatment resistance in the STAR(*)D study

Recent rodent models of antidepressant response implicate a novel set of genes in mechanisms of antidepressant action. The authors examined variants in four such genes (KCNK2 (TREK1), SLC18A2 (VMAT2), S100A10, and HDAC5) for association with remission in a large effectiveness trial of antidepressant treatments. Subjects were drawn from the Sequenced Treatment Alternatives to Relieve Depression (STAR(*)D) study, a multicenter, prospective, effectiveness trial in major depressive disorder (MDD). Outpatients with nonpsychotic MDD were initially treated with citalopram for up to 14 weeks; those who did not remit with citalopram were sequentially randomized to a series of next-step treatments, each for up to 12 weeks. Single-nucleotide polymorphisms in four genes were examined for association with remission, defined as a clinician-rated Quick Inventory of Depressive Symptomatology (QIDS-C(16)) score < or =5. Of 1554 participants for whom DNA was available, 565 (36%) reached remission with citalopram treatment. No association with any of the four genes was identified. However, among the 751 who entered next-step treatment, variants in KCNK2 were associated with treatment response (Bonferroni-corrected, gene-based empirical p<0.001). In follow-up analyses, KCNK2 was also associated with effects of similar magnitude for third-step treatment among those with unsatisfactory benefit to both citalopram and one next-step pharmacotherapy (n=225). These findings indicate that genetic variation in KCNK2 may identify individuals at risk for treatment resistance. More broadly, they indicate the utility of animal models in identifying genes for pharmacogenetic studies of antidepressant response.

Neuropeptides in depression: role of VGF

The monoamine hypothesis of depression is increasingly called into question by newer theories that revolve around changes in neuronal plasticity, primarily in the hippocampus, at both the structural and the functional levels. Chronic stress negatively regulates hippocampal function while antidepressants ameliorate the effects of stress on neuronal morphology and activity. Both stress and antidepressants have been shown to affect levels of brain-derived neurotrophic factor (BDNF) whose transcription is dependent on cAMP response element binding protein (CREB). BDNF itself has antidepressant-like actions and can induce transcription of a number of molecules. One class of genes regulated by both BDNF and serotonin (5-HT) are neuropeptides including VGF (non-acryonimic) which has a novel role in depression. Neuropeptides are important modulators of neuronal function but their role in affective disorders is just emerging. Recent studies demonstrate that VGF, which is also a CREB-dependent gene, is upregulated by antidepressant drugs and voluntary exercise and is reduced in animal models of depression. VGF enhances hippocampal synaptic plasticity as well as neurogenesis in the dentate gyrus but the mechanisms of antidepressant-like actions of VGF in behavioral paradigms are not known. We summarize experimental data describing the roles of BDNF, VGF and other neuropeptides in depression and how they may be acting through the generation of new neurons and altered synaptic activity. Understanding the molecular and cellular changes that underlie the actions of neuropeptides and how these adaptations result in antidepressant-like effects will aid in developing drugs that target novel pathways for major depressive disorders.

Family based association study of pediatric bipolar disorder and the dopamine transporter gene (SLC6A3)

The dopamine transporter gene (SLC6A3) is a compelling candidate for pediatric bipolar disorder because (a) it has been associated with ADHD, (b) bipolar comorbidity with ADHD has been hypothesized to be an etiologically distinct familial subtype (c) blockade of the dopamine transporter with psychostimulants can induce mania in susceptible individuals and (d) previous studies have implicated the gene in bipolar disorder in adults. We conducted a family-based association study of SLC6A3 in 170 affected offspring trios defined by a child (12.9 +/- 5.3 years of age)with DSM-IV Bipolar-I disorder. Twenty-eight tag SNPs were chosen from the CEU (European) population of the International HapMap project (www.hapmap.org). Results indicated nominally positive association for 4 SNPs (rs40184, rs11133767, rs3776512, and rs464049), but only rs40184 survived correction for multiple statistical comparisons (P = 0.038). This is the first examination of the association with SLC6A3 and bipolar disorder in children and, like previous findings in adults with bipolar disorder, we found evidence of association with SNPs in the 3' region of the gene. These data provide suggestive evidence supporting a role for SLC6A3 in the etiology of pediatric bipolar disorder.

Personality endophenotypes for bipolar affective disorder: a family-based genetic association analysis

Genetic analyses of complex conditions such as bipolar disorder (BD) may be facilitated by the use of intermediate phenotypes. Various personality traits are overrepresented in people with BD and their unaffected relatives, and may constitute genetically transmitted risk factors or endophenotypes of the illness. In this study, we administered a battery of seven different personality questionnaires comprising 19 subscales to 31 Caucasian BD families (n = 241). Ten of these personality traits showed significant evidence of heritability and were therefore selected as candidate endophenotypes. In addition, a principal components analysis produced two heritable components (negative affect and appetitive drive), which accounted for a considerable proportion of the variance (29% + 12%) and were also used in the analysis. A family-based quantitative association study was carried out using the orthogonal model from the quantitative transmission disequilibrium tests (QTDT) program. Monte Carlo permutations (M = 500), which allow for non-normal data and provide a global P value, corrected for multiple testing, were used to calculate empirical P values for the within-family component of association. The 3' untranslated region repeat polymorphism of the dopamine transporter gene (SLC6A3) was associated with self-directedness (P < 0.0001) and negative affect (P = 0.010). The short allele of the serotonin transporter gene (SLC6A4) promoter polymorphism showed a trend toward association with higher harm avoidance (P = 0.016) and negative affect (P = 0.028). The catechol-o-methyltransferase val158met polymorphism was weakly associated with the personality traits, 'Spirituality' (P = 0.040) and irritable temperament (P = 0.022). Furthermore, the met allele of the brain-derived neurotrophic factor val66met polymorphism was associated with higher hyperthymic temperament scores. We raise the possibility that the 10R allele of the SLC6A3 repeat polymorphism and the short allele of the SLC6A4 promoter variant constitute risk factors for irritable-aggressive and anxious-dysthymic subtypes of BD, respectively.

Attenuated BDNF-induced upregulation of GABAergic markers in neurons lacking Xbp1

XBP1 is a transcription factor induced by unconventional splicing associated with endoplasmic reticulum stress and plays a role in development. Brain-derived neurotrophic factor (BDNF) causes splicing of Xbp1 mRNA in neurites, and Xbp1 is required for BDNF-induced neurite extension and branching. To search for the molecular mechanisms of how Xbp1 plays a role in neural development, comprehensive gene expression analysis was performed in primary telencephalic neurons obtained from Xbp1 knockout mice at embryonic day 12.5. By searching for the genes induced by BDNF in wild type neurons but not in Xbp1 knockout mice, we found that upregulation of three GABAergic markers, somatostatin (Sst), neuropeptide Y (Npy), and calbindin (Calb1), were compromised in Xbp1 knockout neurons. Attenuated upregulation of Npy and Calb1 in Xbp1 knockout neurons was confirmed by quantitative RT-PCR. This finding may be relevant to impaired BDNF-induced neurite extension in Xbp1 knockout neurons.

Disturbances in selective information processing associated with the BDNF Val66Met polymorphism: evidence from cognition, the P300 and fronto-hippocampal systems

In this study, we examined whether the Met allele of the BDNF Val66Met polymorphism is associated with selective disruptions to task-relevant information processing. In 475 non-clinical participants for whom BDNF genotype status was determined we used the 'IntegNeuro' computerized battery of neuropsychological tests to assess cognitive performance, an auditory oddball task to elicit the P300 event-related potential (ERP) and, in smaller subsets of these subjects, high resolution structural MRI imaging to quantify fronto-hippocampal grey matter (n=161), and functional magnetic resonance imaging to assess fronto-hippocampal BOLD activation (n=37). Met/Met (MM) homozygotes had higher verbal recall errors, in the absence of differences in attention, executive function, verbal ability or sensori-motor function. Further, MM homozygotes demonstrated a slowed P300 ERP during the oddball task, with corresponding alterations in hippocampal and lateral prefrontal activation, and a localized reduction in hippocampal grey matter. These results are consistent with a subtle impact of the Met allele on fronto-hippocampal systems involved in selective information processing of stimulus context and memory updating within the normal population. The findings also indicate that heritable endophenotypes such as the P300 have value in elucidating genotype-phenotype relationships.

Serum brain-derived neurotrophic factor, depression, and antidepressant medications: meta-analyses and implications

BACKGROUND

Converging lines of evidence implicate the neurotrophin brain-derived neurotrophic factor (BDNF) in the pathophysiology of major depression. Recent studies have begun to explore the relationship between serum BDNF and depression.

METHODS

We conducted meta-analyses of 11 studies examining differences in serum BDNF content between depressed and nondepressed subjects (N = 748), and eight studies comparing pre- and post-antidepressant treatment serum BDNF content (N = 220).

RESULTS

The meta-analysis revealed strong evidence that BDNF levels were lower in depressed subjects than healthy control subjects (p < 6.8 x 10(-8)). Similarly, the second meta-analysis found significantly higher BDNF levels after antidepressant treatment (p = .003). There was no evidence of publication bias in the first (p = .376) or second (p = .571) meta-analysis and no evidence that either meta-analysis was unduly influenced by any one study.

CONCLUSIONS

These findings provide strong evidence to suggest that serum BDNF levels are abnormally low in patients suffering from major depressive disorder and that the BDNF levels are elevated following a course of antidepressant treatment. Although the relationship of our findings to pathophysiology of depression and the mechanism of drug action remains to be determined, the measure may have potential use as a biomarker for psychiatric disorders or as a predictor of antidepressant efficacy.

Genomic imprinting in the development and evolution of psychotic spectrum conditions

I review and evaluate genetic and genomic evidence salient to the hypothesis that the development and evolution of psychotic spectrum conditions have been mediated in part by alterations of imprinted genes expressed in the brain. Evidence from the genetics and genomics of schizophrenia, bipolar disorder, major depression, Prader-Willi syndrome, Klinefelter syndrome, and other neurogenetic conditions support the hypothesis that the etiologies of psychotic spectrum conditions commonly involve genetic and epigenetic imbalances in the effects of imprinted genes, with a bias towards increased relative effects from imprinted genes with maternal expression or other genes favouring maternal interests. By contrast, autistic spectrum conditions, including Kanner autism, Asperger syndrome, Rett syndrome, Turner syndrome, Angelman syndrome, and Beckwith-Wiedemann syndrome, commonly engender increased relative effects from paternally expressed imprinted genes, or reduced effects from genes favouring maternal interests. Imprinted-gene effects on the etiologies of autistic and psychotic spectrum conditions parallel the diametric effects of imprinted genes in placental and foetal development, in that psychotic spectrum conditions tend to be associated with undergrowth and relatively-slow brain development, whereas some autistic spectrum conditions involve brain and body overgrowth, especially in foetal development and early childhood. An important role for imprinted genes in the etiologies of psychotic and autistic spectrum conditions is consistent with neurodevelopmental models of these disorders, and with predictions from the conflict theory of genomic imprinting.

A simulated genetic structure for bipolar illness

Bipolar illness is conceptualized as a polygenic condition. Based on candidate gene findings from the literature to date, up to 22% of the genetic risk in affected persons may be explained by six gene variants with an average allele frequency of 0.59 in cases and 0.54 in controls. The mean allele specific relative risk (ASRR) for these variants is 1.42 (range 1.1-1.8). Initial results from genome-wide association studies tend to confirm this estimate of effect size. Using the characteristics of these variants as a guide, a 30 allele model for bipolar illness is presented in which the modal affected person would carry 22 susceptibility variants, and the median unaffected person would carry 15. In a comparable model with 100 alleles, the modal affected person would carry 62 susceptibility variants compared with a median of 50 in unaffecteds. To the extent that common gene variants are associated with bipolar disorder they may be expected to also be widely distributed in the general population. As the neurobiology of replicated candidate genes is considered, models of potentially relevant biological pathways may be constructed.

Early life stress as a risk factor for mental health: role of neurotrophins from rodents to non-human primates

Early adverse events can enhance stress responsiveness and lead to greater susceptibility for psychopathology at adulthood. The epigenetic factors involved in transducing specific features of the rearing environment into stable changes in brain and behavioural plasticity have only begun to be elucidated. Neurotrophic factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), are affected by stress and play a major role in brain development and in the trophism of specific neuronal networks involved in cognitive function and in mood disorders. In addition to the central nervous system, these effectors are produced by peripheral tissues, thus being in a position to integrate the response to external challenges. In this paper we will review data, obtained from animal models, indicating that early maternal deprivation stress can affect neurotrophin levels. Maladaptive or repeated activation of NGF and BDNF, early during postnatal life, may influence stress sensitivity at adulthood and increase vulnerability for stress-related psychopathology.

Brain-derived neurotrophic factor gene expression in pediatric bipolar disorder: effects of treatment and clinical response

OBJECTIVE

Pediatric bipolar disorder (PBD) is a major public health concern; however, little is known about the cellular and genetic factors that are involved in the pathophysiology of this illness. The observed structural abnormality in the brains of patients with mood disorders has been related to abnormal brain-derived neurotrophic factor (BDNF) function, suggesting an important role for BDNF in these disorders.

METHOD

We determined the gene expression of BDNF in lymphocytes obtained from 26 PBD subjects during a drug-free baseline period and during the eighth week of treatment (n = 19) and from 21 medication-free normal control subjects. We also determined the protein levels of BDNF in platelets of patients with PBD and normal control subjects. Subjects were diagnosed according to DSM-IV diagnostic criteria using the Washington University at St. Louis Schedule for Affective Disorders and Schizophrenia.

RESULTS

The mRNA levels of BDNF in lymphocytes of PBD subjects were significantly decreased compared with those of normal control subjects and were significantly higher in 19 subjects after 8 weeks of treatment than the pretreatment drug-free baseline levels and similar to those of normal controls. Similarly, protein levels of BDNF were decreased in platelets of patients with PBD.

CONCLUSIONS

These studies suggest that BDNF levels may be a potential biomarker for PBD. BDNF levels may also serve as a potential treatment predictor and prognostic indicator in PBD.

Association of a polymorphism near CREB1 with differential aversion processing in the insula of healthy participants

CONTEXT

Previous functional neuroimaging studies have identified a network of brain regions that process aversive stimuli, including anger. A polymorphism near the cyclic adenosine monophosphate response element binding protein gene (CREB1) has recently been associated with greater self-reported effort at anger control as well as risk for antidepressant treatment-emergent suicidality in men with major depressive disorder, but its functional effects have not been studied.

OBJECTIVE

To determine whether this genetic variant is associated with altered brain processing of and behavioral avoidance responses to angry facial expressions.

DESIGN AND PARTICIPANTS

A total of 28 white participants (mean age, 29.2 years; 13 women) were screened using the Structured Clinical Interview for DSM-IV to exclude any lifetime Axis I psychiatric disorder and were genotyped for rs4675690, a single-nucleotide polymorphism near CREB1.

MAIN OUTCOME MEASURES

Blood oxygenation level-dependent signal by functional magnetic resonance imaging in the amygdala, insula, anterior cingulate, and orbitofrontal cortex during passive viewing of photographs of faces with emotional expressions. To measure approach and avoidance responses to anger, an off-line key-press task that traded effort for viewing time assessed valuation of angry faces compared with other expressions.

RESULTS

The CREB1-linked single-nucleotide polymorphism was associated with significant differential activation in an extended neural network responding to angry and other facial expressions. The CREB1-associated insular activation was coincident with activation associated with behavioral avoidance of angry faces.

CONCLUSIONS

A polymorphism near CREB1 is associated with responsiveness to angry faces in a brain network implicated in processing aversion. Coincident activation in the left insula is further associated with behavioral avoidance of these stimuli.

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.

Bipolar disorder as maladaptive arousal: a behavioral model and evidence

Bipolar disorder can be understood as a disorder of behavioral regulation. Manic and depressed individuals are impaired in the titration of appetitive arousal, possibly at the level of neuronal plasticity. An experiment in which fixed 5% CO2 stimulates respiration and blocks satiety tests the regulation of appetitive arousal. In preliminary analysis of data from 35 individuals (24 with bipolar disorder) individuals with bipolar disorder were more likely to fail to find a stable state of respiratory adjustment to CO2. If confirmed, the unstable respiratory response to CO2 may prove useful as a bipolar-disorder endophenotype.

Association between brain-derived neurotrophic factor gene and a severe form of bipolar disorder, but no interaction with the serotonin transporter gene

BACKGROUND

Recent data suggest that brain-derived neurotrophic factor (BDNF) and the serotonergic system are involved and interact in major depressive disorder and suicidal behavior (SB). Several family and population-based studies have reported associations between the BDNF gene and serotonin-related genes, specifically the serotonin transporter (5HTT) gene, with bipolar disorder (BD) and SB. However, despite the fact that gene-by-gene interaction between BDNF and 5HTT has been demonstrated in monoamine deficiencies in animals, this kind of interaction has never been tested in humans. Our hypothesis is that some BDNF and 5HTT polymorphisms might confer increased risk for BD and SB and that both genes may interact with each other.

METHODS

To test this hypothesis, we genotyped the most common BDNF polymorphisms, G196A (Val66Met), A-633T and BDNF-LCPR, as well as 5HTT (5HTT-LPR), in 447 BD patients and 370 controls.

RESULTS

We replicated the association previously reported between BDNF G196A (Val66Met) polymorphism and BD. We also observed a correlation between the number of G196 alleles and short alleles of 5HTT-LPR and the severity of SB in BD. However, we found no significant interaction between these two markers.

CONCLUSIONS

These results suggest that BDNF G196A as well as 5HTT-LPR polymorphisms confer risk for SB in BD, but we did not observe any evidence for an interaction between them.

Evidence of biologic epistasis between BDNF and SLC6A4 and implications for depression

Complex genetic disorders such as depression likely exhibit epistasis, but neural mechanisms of such gene-gene interactions are incompletely understood. 5-HTTLPR and BDNF VAL66MET, functional polymorphisms of the serotonin (5-HT) transporter (SLC6A4) and brain-derived neurotrophic factor (BDNF) gene, impact on two distinct, but interacting signaling systems, which have been related to depression and to the modulation of neurogenesis and plasticity of circuitries of emotion processing. Recent clinical studies suggest that the BDNF MET allele, which shows abnormal intracellular trafficking and regulated secretion, has a protective effect regarding the development of depression and in mice of social defeat stress. Here we show, using anatomical neuroimaging techniques in a sample of healthy subjects (n=111), that the BDNF MET allele, which is predicted to have reduced responsivity to 5-HT signaling, protects against 5-HTTLPR S allele-induced effects on a brain circuitry encompassing the amygdala and the subgenual portion of the anterior cingulate (rAC). Our analyses revealed no effect of the 5-HTTLPR S allele on rAC volume in the presence of BDNF MET alleles, whereas a significant volume reduction (P<0.001) was seen on BDNF VAL/VAL background. Interacting genotype effects were also found in structural connectivity between amygdala and rAC (P=0.002). These data provide in vivo evidence of biologic epistasis between SLC6A4 and BDNF in the human brain by identifying a neural mechanism linking serotonergic and neurotrophic signaling on the neural systems level, and have implications for personalized treatment planning in depression.

[The genetics of depressive disorders]

Among the most common severe psychiatric disorders worldwide, depressive disorders are a leading cause of morbidity, the onset usually occurring during childhood or adolescence. Symptomatology, prevalence, outcome and treatment differentiate depressive disorder nosologically as being either unipolar depression or bipolar disorder, which is characterized by one or more episodes of mania with or without episodes of depression. Genetic factors decisively influence the susceptibility to depressive disorders. Family studies and twin studies have been essential in defining the magnitude of familial risk and liability to heritability, particularly in the case of bipolar disorder. In recent years, linkage and association studies have made great strides towards identifying candidate genes. Particularly the s-allele of the serotonin transporter has been repeatedly confirmed to be a risk factor. Meta-analyses suggest, however, that the genetic contributions of the ascertained loci are relatively small. Along with genetic factors, environmental factors are heavily involved. Gene-environment action plays a pivotal role, particularly in unipolar depression. The genetic disposition seems to be modulated by a protective or pathogenic environment. Early-onset disorders must be further investigated in future as studies to date are somewhat limited.

Association study of theta EEG asymmetry and brain-derived neurotrophic factor gene variants in childhood-onset mood disorder

BACKGROUND

Childhood-onset mood disorders (COMD) include various serious, disabling psychiatric conditions that are heterogeneous in presentation and etiology. Because intermediate phenotypes may help to identify genetic contributors to COMD, we tested for an association between variants in the brain-derived neurotrophic factor (BDNF) gene and theta EEG asymmetry, both of which have been independently implicated in affective disorders.

METHODS

Theta EEG asymmetry measures were calculated for a total of 191 individuals with COMD and 93 controls, who were also genotyped at seven BDNF single-nucleotide polymorphism (SNPs), two intergenic flanking SNPs, and one SNP in the lin-7 homolog C (Caenorhabditis elegans) (LIN7C) gene.

RESULTS

Adjusting for sex and ethnicity in linear models of asymmetry scores at ten brain regions, significant genotype and genotype-by-ethnicity interactions were observed for marker Val66Met in two parietal (P3/4 and P7/8) regions in the depressed group only.

CONCLUSIONS

Our results suggest that the functional Val66Met polymorphism affects theta EEG asymmetry in parietal brain regions specifically in individuals with COMD.

Whole-genome association study of bipolar disorder

We performed a genome-wide association scan in 1461 patients with bipolar (BP) 1 disorder, 2008 controls drawn from the Systematic Treatment Enhancement Program for Bipolar Disorder and the University College London sample collections with successful genotyping for 372,193 single nucleotide polymorphisms (SNPs). Our strongest single SNP results are found in myosin5B (MYO5B; P=1.66 x 10(-7)) and tetraspanin-8 (TSPAN8; P=6.11 x 10(-7)). Haplotype analysis further supported single SNP results highlighting MYO5B, TSPAN8 and the epidermal growth factor receptor (MYO5B; P=2.04 x 10(-8), TSPAN8; P=7.57 x 10(-7) and EGFR; P=8.36 x 10(-8)). For replication, we genotyped 304 SNPs in family-based NIMH samples (n=409 trios) and University of Edinburgh case-control samples (n=365 cases, 351 controls) that did not provide independent replication after correction for multiple testing. A comparison of our strongest associations with the genome-wide scan of 1868 patients with BP disorder and 2938 controls who completed the scan as part of the Wellcome Trust Case-Control Consortium indicates concordant signals for SNPs within the voltage-dependent calcium channel, L-type, alpha 1C subunit (CACNA1C) gene. Given the heritability of BP disorder, the lack of agreement between studies emphasizes that susceptibility alleles are likely to be modest in effect size and require even larger samples for detection.

Further evidence of association between two NET single-nucleotide polymorphisms with ADHD

The norepinephrine transporter (NET) gene is an attractive candidate gene for attention-deficit hyperactivity disorder (ADHD). Noradrenergic systems are critical to higher brain functions such as attention and executive function, which are defective in ADHD. The clinical efficacy of medications that target NET also supports its role in the etiology of ADHD. Here, we have applied a dense mapping strategy to capture all genetic variations within the NET gene in a large number of ADHD families (474 trios). As a result, we found association of the same alleles from two single-nucleotide polymorphisms (rs3785143 and rs11568324) previously identified in another large-scale ADHD genetic study (International Multisite ADHD Geneproject). Furthermore, the effect sizes were consistent across both studies. This is the first time that identical alleles of NET from different studies were implicated, and thus our report provides further evidence that the NET gene is involved in the etiology of ADHD.

Dose-dependent effect of the Val66Met polymorphism of the brain-derived neurotrophic factor gene on memory-related hippocampal activity

Brain-derived neurotrophic factor (BDNF) plays a critical role in activity-dependent neuroplasticity underlying learning and memory in the hippocampus. Recent human studies have indicated that a common single nucleotide polymorphism of the BDNF gene, the Val66Met polymorphism, has impact on episodic memory, hippocampal morphology and memory-related hippocampal activity measured by functional magnetic resonance imaging (fMRI). However, two issues remain to be clarified: (1) whether the genotype effect of this polymorphism on memory-related brain activity is allele dose dependent and (2) whether the effect of this polymorphism in Asian population is the same as effects observed in Caucasian sample. To clarify these issues, we studied the relationship of the Val66Met polymorphism genotype and hippocampal activity during episodic memory task using fMRI in healthy 58 biologically unrelated Japanese. Although there was no genotype effect on episodic memory function obtained by behavioral assessments, fMRI measurements revealed a significantly negative correlation between the dose of Met-BDNF allele and encoding related brain activity in the bilateral hippocampi and right parahippocampal gyrus. There was no genotype effect on retrieval related brain activity. These data indicated a genetic mechanism for normal variation in human memory and suggest effects of BDNF signaling on hippocampal function in humans.

Ethanol-BDNF interactions: still more questions than answers

Brain-derived neurotrophic factor (BDNF) has emerged as a regulator of development, plasticity and, recently, addiction. Decreased neurotrophic activity may be involved in ethanol-induced neurodegeneration in the adult brain and in the etiology of alcohol-related neurodevelopmental disorders. This can occur through decreased expression of BDNF or through inability of the receptor to transduce signals in the presence of ethanol. In contrast, recent studies implicate region-specific up-regulation of BDNF and associated signaling pathways in anxiety, addiction and homeostasis after ethanol exposure. Anxiety and depression are precipitating factors for substance abuse and these disorders also involve region-specific changes in BDNF in both pathogenesis and response to pharmacotherapy. Polymorphisms in the genes coding for BDNF and its receptor TrkB are linked to affective, substance abuse and appetitive disorders and therefore may play a role in the development of alcoholism. This review summarizes historical and pre-clinical data on BDNF and TrkB as it relates to ethanol toxicity and addiction. Many unresolved questions about region-specific changes in BDNF expression and the precise role of BDNF in neuropsychiatric disorders and addiction remain to be elucidated. Resolution of these questions will require significant integration of the literature on addiction and comorbid psychiatric disorders that contribute to the development of alcoholism.

Decreased levels of serum brain-derived neurotrophic factor in both depressed and euthymic patients with unipolar depression and in euthymic patients with bipolar I and II disorders

OBJECTIVES

Brain-derived neurotrophic factor (BDNF) has been proposed as a candidate molecule in the pathophysiology of major depressive disorder (MDD) and bipolar disorders (BD). Reduced levels of peripheral BDNF have been found in drug-free MDD patients, in drug-treated depressed or manic patients with BD type I (BD-I), but not in drug-treated euthymic BD-I individuals. No study has been done in patients with BD type II (BD-II). Moreover, the influence of Axis I psychiatric comorbidity on circulating BDNF in affective patients has never been evaluated. Therefore, in the present study, we aimed: (i) to confirm previous findings on peripheral BDNF in MDD and BD-I patients; (ii) to assess whether changes in circulating BDNF occur also in patients with BD-II; and (iii) to exclude the possibility that comorbid psychiatric disorders exerted an effect on BDNF levels in affective patients.

METHODS

We measured serum BDNF concentrations by an enzyme-linked immunosorbent assay method in 85 subjects, including 24 euthymic patients with unipolar depression (UD), 17 euthymic patients with BD-I, 11 euthymic patients with BD-II, 11 UD patients with a current major depressive episode and 22 drug-free healthy controls. At the time of the study, 15 patients were drug-treated; the remaining ones were drug-free for at least four weeks.

RESULTS

Compared to healthy controls, serum BDNF concentrations were significantly reduced in all the patient groups (F(4,80) = 3.840, p = 0.006) with no significant difference among them. Drug treatments and comorbid psychiatric disorders had no effect on lowered circulating BDNF levels in affective patients.

CONCLUSIONS

Present results confirm previous independent findings of reduced circulating BDNF in patients with MDD and report, for the first time, decreased serum BDNF levels in euthymic patients with UD, BD-I and BD-II, independently from drug treatment status and concomitant Axis I psychiatric disorders.

Adenylyl cyclase-cyclicAMP signaling in mood disorders: role of the crucial phosphorylating enzyme protein kinase A

Mood disorders are among the most prevalent and recurrent forms of psychiatric illnesses. In the last decade, there has been increased understanding of the biological basis of mood disorders. In fact, novel mechanistic concepts of the neurobiology of unipolar and bipolar disorders are evolving based on recent pre-clinical and clinical studies, most of which now focus on the role of signal transduction mechanisms in these psychiatric illnesses. Particular investigative emphasis has been given to the role of phosphorylating enzymes, which are crucial in regulating gene expression and neuronal and synaptic plasticity. Among the most important phosphorylating enzyme is protein kinase A (PKA), a component of adenylyl cyclase-cyclic adenosine monophosphate (AC-cAMP) signaling system. In this review, we critically and comprehensively discuss the role of various components of AC-cAMP signaling in mood disorders, with a special focus on PKA, because of the interesting observation that have been made about its involvement in unipolar and bipolar disorders. We also discuss the functional significance of the findings regarding PKA by discussing the role of important PKA substrates, namely, Rap-1, cyclicAMP-response element binding protein, and brain-derived neurotrophic factor. These studies suggest the interesting possibility that PKA and related signaling molecules may serve as important neurobiological factors in mood disorders and may be relevant in target-specific therapeutic interventions for these disorders.

Association of the brain-derived neurotrophic factor gene and bipolar disorder with early age of onset in mainland China

Several evidences have suggested that the brain-derived neurotrophic factor (BDNF) gene may be involved in the pathogenesis of bipolar disorder (BPD), but not all studies get the same result. The paper investigated two genetic polymorphisms of BDNF, C-270T and Val66Met, in a case-control design for their association with BPD. Sixty-seven patients of early age of onset and 130 patients of late age of onset were selected for study and 208 healthy individuals were used as controls. No significantly statistical differences of these two polymorphisms were found in genotypes or allele frequencies between either overall patients or late age of onset patients and normal control subjects. However, the frequency of the Val allele of the Val66Met polymorphism was found to have significantly increased in the subgroup patients with early age of onset as compared with the controls (genotype: chi(2)=6.602, d.f.=2, P=0.037; allele: chi(2)=6.223, d.f.=1, P=0.015). The study demonstrates that the BDNF C-270T and Val66Met polymorphisms are unlikely to contribute to the genetic predisposition to BPD as a whole. But Val66Met may be associated with susceptibility to the early age of onset subset of the disorder, further studies designed to explore the relationship in a larger population may be warranted.

New insights into BDNF function in depression and anxiety

The 'neurotrophin hypothesis of depression' is based largely on correlations between stress or antidepressant treatment and down- or upregulation, respectively, of brain-derived neurotrophic factor (BDNF). Genetic disruption of the signaling pathways involving BDNF and its receptor, the tyrosine kinase TrkB, does not seem to cause depressive behaviors, but does hamper the effect of antidepressant drugs. Thus, BDNF may be a target of antidepressants, but not the sole mediator of depression or anxiety. Advances in BDNF cell biology, including its transcription through multiple promoters, trafficking and secretion, may provide new insights into its role in mood disorders. Moreover, as the precursor proBDNF and the mature protein mBDNF can elicit opposite effects on cellular functions, the impact of proBDNF and its cleavage on mood should be considered. Opposing influences of mBDNF and proBDNF on long-term potentiation and long-term depression might contribute to the dichotomy of BDNF actions on behaviors mediated by the brain stress and reward systems.

Interaction between BDNF and serotonin: role in mood disorders

Brain-derived neurotrophic factor (BDNF) and serotonin (5-hydroxytryptamine, 5-HT) are two seemingly distinct signaling systems that play regulatory roles in many neuronal functions including survival, neurogenesis, and synaptic plasticity. A common feature of the two systems is their ability to regulate the development and plasticity of neural circuits involved in mood disorders such as depression and anxiety. BDNF promotes the survival and differentiation of 5-HT neurons. Conversely, administration of antidepressant selective serotonin reuptake inhibitors (SSRIs) enhances BDNF gene expression. There is also evidence for synergism between the two systems in affective behaviors and genetic epitasis between BDNF and the serotonin transporter genes.

Association study of the brain-derived neurotropic factor (BDNF) gene in attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a prevalent neurodevelopmental childhood psychiatric disorder. Brain-derived neurotropic factor (BDNF) has been suggested to play a role in the pathogenesis of ADHD and two family-based association studies demonstrated an association of BDNF polymorphisms with ADHD. The aim of the current study was to investigate the BDNF gene for association with ADHD in a large sample of families from Toronto. The transmission of three polymorphisms of the BDNF gene (rs6265, rs11030104, and rs2049046) was examined in 266 nuclear families ascertained through a proband with ADHD (315 affected children) using the transmission/disequilibrium test (TDT). In addition, we conducted quantitative analysis to assess the relationship between these marker alleles and the symptom dimensions of ADHD (inattention and hyperactivity/impulsivity) and cognitive measures of working memory. None of the individual marker alleles showed significant evidence of association with ADHD, dimensional symptom scores, or working memory ability in our sample of ADHD families. There was no significant evidence for biased transmission of individual haplotypes with frequency >10% (global chi2 for these three haplotypes: chi2 = 6.349, df = 3, P = 0.096). One uncommon haplotype (A-G-G; frequency 2.2%) showed a significant association with ADHD in the categorical (chi2 = 5.293, df = 1, P = 0.021) and quantitative analyses (parents' rated inattention: Z = -2.504, P = 0.012; and hyperactivity/impulsivity: Z = -2.651, P = 0.008). These results should be interpreted cautiously, however, because of the low haplotype frequency. In light of the evidence for an involvement of BDNF in ADHD, further analysis of the BDNF gene in ADHD is warranted.

The Val66Met polymorphism of the brain-derived neurotrophic factor gene is associated with psychotic feature and suicidal behavior in Japanese major depressive patients

Recent researches have suggested that brain-derived neurotrophic factor (BDNF) may be implicated in the pathophysiology of mood disorder. This study examined the association between the BDNF Val66Met polymorphism and major depressive disorder (MDD) in a Japanese population. We genotyped the BDNF Val66Met polymorphism in 154 major depressive patients and 154 age- and sex-matched control subjects. The genotypic distributions and allele frequencies were similar among the patients and control subjects. When the relationships of the polymorphism with several clinical variables (i.e., age, sex, age of onset, number of episode, presence of psychotic features, suicidal behavior, and family history) were examined, the dose of Met allele had significant effects on psychotic feature and suicidal behavior and family history. These results suggest that the BDNF Val66Met polymorphism is not related to the development of MDD but related to clinical features of MDD in a Japanese population.

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.

Sudden glory revisited: cognitive contents of hypomania

INTRODUCTION

Cognitive aspects of hypomania have been historically neglected. Although they do not have an etiological role, they may be essential to understand factors underlying the hypomanic ascent in bipolar disorders and constitute key modulators of the course of illness.

METHODS

We have performed a critical review of the existing literature on the role of cognition during hypomania, considering outputs coming from very different fields of knowledge.

RESULTS

There is a nuclear cognitive change occurring in most hypomanic phases that we have defined as 'anastrophic' thinking. This key cognitive procedure has several implications--going from social sciences and philosophy to basic sciences.

CONCLUSIONS

Hypomania has received certain attention from cognitive theorists. Unfortunately, this attention has not been translated into a cognitive model that is as robust as the one seen in depression. The inclusion of certain psychological aspects in models of hypomania should give rise, as occurred with depression, to an increased emphasis on psychoeducation and cognitive modification of behavioral patterns in the management of this disorder with combined psychological and pharmacological tools.