Preliminary genome-wide association study of bipolar disorder in the Japanese population

Genome wide association studies (GWAS) and copy number variation (CNV) studies of the major psychoses: what have we learnt?

Schizophrenia (SZ) and bipolar disorder (BPD) have high heritabilities and are clinically and genetically complex. Genome wide association studies (GWAS) and studies of copy number variations (CNV) in SZ and BPD have allowed probing of their underlying genetic risks. In this systematic review, we assess extant genetic signals from published GWAS and CNV studies of SZ and BPD up till March 2011. Risk genes associated with SZ at genome wide significance level (p value<7.2 × 10(-8)) include zinc finger binding protein 804A (ZNF804A), major histocompatibility (MHC) region on chromosome 6, neurogranin (NRGN) and transcription factor 4 (TCF4). Risk genes associated with BPD include ankyrin 3, node of Ranvier (ANK3), calcium channel, voltage dependent, L type, alpha 1C subunit (CACNA1C), diacylglycerol kinase eta (DGKH), gene locus on chromosome 16p12, and polybromo-1 (PBRM1) and very recently neurocan gene (NCAN). Possible common genes underlying psychosis include ZNF804A, CACNA1C, NRGN and PBRM1. The CNV studies suggest that whilst CNVs are found in both SZ and BPD, the large deletions and duplications are more likely found in SZ rather than BPD. The validation of any genetic signal is likely confounded by genetic and phenotypic heterogeneities which are influenced by epistatic, epigenetic and gene-environment interactions. There is a pressing need to better integrate the multiple research platforms including systems biology computational models, genomics, cross disorder phenotyping studies, transcriptomics, proteomics, metabolomics, neuroimaging and clinical correlations in order to get us closer to a more enlightened understanding of the genetic and biological basis underlying these potentially crippling conditions.

Association study of Nogo-related genes with schizophrenia in a Japanese case-control sample

Many studies have suggested that myelin dysfunction may be causally involved in the pathogenesis of schizophrenia. Nogo (RTN4), myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMG) all bind to the common receptor, Nogo-66 receptor 1 (RTN4R). We examined 68 single nucleotide polymorphisms (SNPs) (51 with genotyping and 17 with imputation analysis) from these four genes for genetic association with schizophrenia, using a 2,120 case-control sample from the Japanese population. Allelic tests showed nominally significant association of two RTN4 SNPs (P = 0.047 and 0.037 for rs11894868 and rs2968804, respectively) and two MAG SNPs (P = 0.034 and 0.029 for rs7249617 and rs16970218, respectively) with schizophrenia. The MAG SNP rs7249617 also showed nominal significance in a genotypic test (P = 0.017). In haplotype analysis, the MAG haplotype block including rs7249617 and rs16970218 showed nominal significance (P = 0.008). These associations did not remain significant after correction for multiple testing, possibly due to their small genetic effect. In the imputation analysis of RTN4, the untyped SNP rs2972090 showed nominally significant association (P = 0.032) and several imputed SNPs showed marginal associations. Moreover, in silico analysis (PolyPhen) of a missense variant (rs11677099: Asp357Val), which is in strong linkage disequilibrium with rs11894868, predicted a deleterious effect on Nogo protein function. Despite a failure to detect robust associations in this Japanese cohort, our nominally positive signals, taken together with previously reported biological and genetic findings, add further support to the "disturbed myelin system theory of schizophrenia" across different populations.

Comparative linkage meta-analysis reveals regionally-distinct, disparate genetic architectures: application to bipolar disorder and schizophrenia

New high-throughput, population-based methods and next-generation sequencing capabilities hold great promise in the quest for common and rare variant discovery and in the search for ”missing heritability.” However, the optimal analytic strategies for approaching such data are still actively debated, representing the latest rate-limiting step in genetic progress. Since it is likely a majority of common variants of modest effect have been identified through the application of tagSNP-based microarray platforms (i.e., GWAS), alternative approaches robust to detection of low-frequency (1–5% MAF) and rare (<1%) variants are of great importance. Of direct relevance, we have available an accumulated wealth of linkage data collected through traditional genetic methods over several decades, the full value of which has not been exhausted. To that end, we compare results from two different linkage meta-analysis methods—GSMA and MSP—applied to the same set of 13 bipolar disorder and 16 schizophrenia GWLS datasets. Interestingly, we find that the two methods implicate distinct, largely non-overlapping, genomic regions. Furthermore, based on the statistical methods themselves and our contextualization of these results within the larger genetic literatures, our findings suggest, for each disorder, distinct genetic architectures may reside within disparate genomic regions. Thus, comparative linkage meta-analysis (CLMA) may be used to optimize low-frequency and rare variant discovery in the modern genomic era.

Convergent functional genomic studies of ω-3 fatty acids in stress reactivity, bipolar disorder and alcoholism

Omega-3 fatty acids have been proposed as an adjuvant treatment option in psychiatric disorders. Given their other health benefits and their relative lack of toxicity, teratogenicity and side effects, they may be particularly useful in children and in females of child-bearing age, especially during pregnancy and postpartum. A comprehensive mechanistic understanding of their effects is needed. Here we report translational studies demonstrating the phenotypic normalization and gene expression effects of dietary omega-3 fatty acids, specifically docosahexaenoic acid (DHA), in a stress-reactive knockout mouse model of bipolar disorder and co-morbid alcoholism, using a bioinformatic convergent functional genomics approach integrating animal model and human data to prioritize disease-relevant genes. Additionally, to validate at a behavioral level the novel observed effects on decreasing alcohol consumption, we also tested the effects of DHA in an independent animal model, alcohol-preferring (P) rats, a well-established animal model of alcoholism. Our studies uncover sex differences, brain region-specific effects and blood biomarkers that may underpin the effects of DHA. Of note, DHA modulates some of the same genes targeted by current psychotropic medications, as well as increases myelin-related gene expression. Myelin-related gene expression decrease is a common, if nonspecific, denominator of neuropsychiatric disorders. In conclusion, our work supports the potential utility of omega-3 fatty acids, specifically DHA, for a spectrum of psychiatric disorders such as stress disorders, bipolar disorder, alcoholism and beyond.

Nominal association between a polymorphism in DGKH and bipolar disorder detected in a meta-analysis of East Asian case-control samples

AIM

Recent genome-wide association studies (GWAS) of bipolar disorder (BD) have detected new candidate genes, including DGKH, DFNB31 and SORCS2. However, the results of these GWAS were not necessarily consistent, indicating the importance of replication studies. In this study, we tested the genetic association of DGKH, DFNB31 and SORCS2 with BD.

METHODS

We genotyped 18 single-nucleotide polymorphisms (SNP) in DGKH, DFNB31 and SORCS2 using Japanese samples (366 cases and 370 controls). We also performed a meta-analysis of four SNP in DGKH, using the previously published allele frequency data of Han-Chinese case-control samples (1139 cases and 1138 controls).

RESULTS

IN the association analysis using Japanese samples, a SNP in SORCS2 (rs10937823) showed nominal genotypic association. However, we could not find any association in an additional analysis of tag SNP around rs10937823. In the meta-analysis of SNP in DGKH, rs9315897, which was not significantly associated with BD in the previous Chinese study, showed nominal association.

CONCLUSION

Although the association was not strong, the result of this study would support the association between DGKH and BD.

Serotonin 1A receptor gene, schizophrenia and bipolar disorder: an association study and meta-analysis

Several investigations have reported associations between serotonin 1A (5-HT1A) receptor and major psychiatric disorders, such as schizophrenia and bipolar disorder (BP), making the 5-HT1A receptor gene (HTR1A) a good candidate gene for the pathophysiology of schizophrenia and BP. To evaluate the association between HTR1A and schizophrenia and BP, we conducted a case-control study of Japanese population samples with two single- nucleotide polymorphisms (SNPs), including rs6295 (C-1019G) in HTR1A. In addition, we conducted a meta-analysis of rs6295, which has been examined in other studies. Using one functional single- nucleotide polymorphism (SNP; rs6295) and one tagging SNP (rs878567), we conducted a genetic association analysis of case-control samples (857 schizophrenic patients, 1028 BP patients and 1810 controls) in the Japanese population. Two association studies for schizophrenia and three association studies for BP, including this study, met our criteria for the meta-analysis of rs6295. We found an association between HTR1A and Japanese BP in a haplotype-wise analysis, the significance of which remained after Bonferroni correction. In addition, we detected an association between rs6295 and BP in the meta-analysis (fixed model: P(Z)=0.000400). However, we did not detect an association between HTR1A and schizophrenia in the allele/genotype-wise, haplotype-wise or meta-analysis. HTR1A may play an important role in the pathophysiology of BP, but not schizophrenia in the Japanese population. In the meta-analysis, rs6295 in HTR1A was associated with BP patients.

Genetics of bipolar disorder

In this chapter, we will attempt to outline the current state of genetic knowledge for bipolar disorder and briefly summarize the main findings from genetic epidemiology studies. We then review the most recent original literature, based largely on genome-wide association study methods. We conclude with some ideas about future directions.

Increased peripheral blood expression of electron transport chain genes in bipolar depression

OBJECTIVE

  To identify specific genetic pathways showing altered expression in peripheral blood of depressed subjects with bipolar disorder (BPD).

METHODS

  Illumina Sentrix BeadChip (Human-6v2) microarrays containing >48,000 transcript probes were used to measure levels of gene expression in peripheral blood from 20 depressed subjects with BPD and in 15 healthy control subjects. Quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) was used to confirm a subset of these differences.

RESULTS

  A total of 1,180 genes were differentially expressed between subjects with BPD and healthy controls (fold-change >1.3, false discovery rate-corrected p < 0.05, covaried for age and sex). Of these, 559 genes were up-regulated in BPD subjects and 621 were down-regulated. Surprisingly, there was no difference between medicated (n = 11) and unmedicated (n = 9) subjects with BPD for any of these genes. Pathway analysis using GeneGo MetaCore software showed that the most significantly affected pathway was the mitochondrial electron transport chain (ETC). Of the 85 objects (genes or proteins) in this pathway, 22 were up-regulated and 2 down-regulated in subjects with BPD. qRT-PCR confirmed up-regulation of nuclear encoded ETC genes in complexes I, III, IV, and V and, in addition, demonstrated up-regulation of mitochondrially encoded genes in each of these complexes.

CONCLUSION

  These results suggest that increased expression of multiple components of the mitochondrial ETC may be a primary deficit in bipolar depression, rather than an effect of medication.

Failure to confirm genetic association of the FXYD6 gene with schizophrenia: the Japanese population and meta-analysis

The FXYD domain-containing ion transport regulator 6 (FXYD6) gene encodes phosphohippolin that regulates cellular ion transport by altering the kinetic properties of Na,K-ATPase. Phosphohippolin is highly expressed in brain regions that are relevant to schizophrenia. The FXYD6 gene is located at chromosome 11q22-24, one of the most established linkage regions for schizophrenia. Therefore, it may be possible that genetic variants in FXYD6, including the regulatory genomic elements could cause abnormal function or expression of phosphohippolin and increase the genetic risk for schizophrenia. A previous study suggested that polymorphisms in FXYD6 are associated with schizophrenia in UK samples. However, conflicting results have been reported in the Japanese population. In this study, we aimed to test the prior genetic association findings using different samples from the ethnically homogeneous Japanese population (1,060 schizophrenic patients and 1,060 age- and sex-matched controls). From the FXYD6 gene, we examined six single nucleotide polymorphisms (rs11216573, rs555577, rs1815774, rs4938445, rs4938446, and rs497768), all of which were previously analyzed for association. We did not detect any significant allelic, genotypic or haplotypic association in our Japanese samples. Meta-analysis incorporating previous and the present studies also showed that the FXYD6 gene is not associated with schizophrenia. We conclude that the FXYD6 gene does not have a major influence on susceptibility to schizophrenia across populations.

Serotonin 6 receptor gene and mood disorders: case-control study and meta-analysis

Several evidence suggests that alterations in serotonin 6 (5-HT6) receptors might be associated with the pathophysiology of mood disorders. Therefore, to evaluate the association between HTR6 and BP and MDD, we conducted a case-control study of Japanese population samples (1007 BP patients, 447 MDD patients and 1753 controls) with five tagging SNPs, including rs1805054 (C267T), in HTR6. In addition, we conducted a meta-analysis of rs1805054, which has been examined in other studies. We selected five tagging SNPs (rs6693503, rs1805054, rs4912138, rs3790757 and rs9659997). Moreover, three association studies for BP and four association studies for MDD, including this study, met our criteria for the meta-analysis of rs1805054. We did not detect an association between tagging SNPs in HTR6 and BP and MDD in the allele/genotype, haplotype analysis or meta-analysis. In conclusion, we found no association involving polymorphism and mood disorder in the Japanese population. However, because changes in expression level or signal transduction of this receptor may be involved in the pathology of these diseases, it will be necessary to conduct the further study about the relationship between this receptor and mood disorders in the future.

Association analysis of group II metabotropic glutamate receptor genes (GRM2 and GRM3) with mood disorders and fluvoxamine response in a Japanese population

BACKGROUND

Several lines of evidence implicate abnormalities in glutamate neural transmission in the pathophysiology of mood disorders, including major depressive disorder (MDD) and bipolar disorder (BP). Preclinical antidepressant effects were also reported for group II metabotropic glutamate receptor (Group II mGluRs) antagonists show dose-dependent antidepressant-like effects in murine models of depression. Also, it has been suggested that abnormalities in the hypothalamic-pituitary-adrenal axis and serotonergic neural transmission are important mechanisms in the pathophysiology of mood disorders. Group II mGluRs play an important role in regulating the function of these mechanisms. From these results, it has been suggested that abnormalities in Group II mGluRs might be involved in the pathophysiology of mood disorders, including MDD) and BP, and may influence the clinical response to treatment with SSRIs in MDD. Therefore, we studied the association between Group II mGluR genes (GRM2 and GRM3) and mood disorders and the efficacy of fluvoxamine treatment in Japanese MDD patients.

MATERIALS AND METHODS

Using three tagging SNPs in GRM2 and an SNP (rs6465084) reported functional variant in GRM3, we conducted a genetic association analysis of case-control samples (325 MDD patients, 155 BP patients and 802 controls) in the Japanese population. In addition, we performed an association analysis of GRM2 and GRM3 and the efficacy of fluvoxamine treatment in 117 Japanese patients with MDD. The MDD patients in this study had scores of 12 or higher on the 17 items of the Structured Interview Guide for Hamilton Rating Scale for Depression (SIGH-D). We defined a clinical response as a decrease of more than 50% in baseline SIGH-D within 8 weeks, and clinical remission as an SIGH-D score of less than 7 at 8 weeks.

RESULTS

We found an association between rs6465084 in GRM3 and MDD in the allele-wise analysis after Bonferroni's correction (P-value=0.0371). However, we did not find any association between GRM3 and BP or the fluvoxamine therapeutic response in MDD in the allele/genotype-wise analysis. We also did not detect any association between GRM2 and MDD, BP or the fluvoxamine therapeutic response in MDD in the allele/genotype-wise or haplotype-wise analysis.

DISCUSSION

We detected an association between only one marker (rs6465084) in GRM3 and Japanese MDD patients. However, because we did not perform an association analysis based on LD and a mutation scan of GRM3, a replication study using a larger sample and based on LD may be required for conclusive results.