Polymorphisms within 5' end of the Neuregulin 1 gene are genetically associated with schizophrenia in the Chinese population

Nrg1 intracellular signaling regulates the development of interhemispheric callosal axons in mice

Schizophrenia is associated with altered cortical circuitry. Although the schizophrenia risk gene NRG1 is known to affect the wiring of inhibitory interneurons, its role in excitatory neurons and axonal development is unclear. Here, we investigated the role of Nrg1 in the development of the corpus callosum, the major interhemispheric connection formed by cortical excitatory neurons. We found that deletion of Nrg1 impaired callosal axon development in vivo. Experiments in vitro and in vivo demonstrated that Nrg1 is cell-autonomously required for axonal outgrowth and that intracellular signaling of Nrg1 is sufficient to promote axonal development in cortical neurons and specifically in callosal axons. Furthermore, our data suggest that Nrg1 signaling regulates the expression of Growth Associated Protein 43, a key regulator of axonal growth. In conclusion, our study demonstrates that NRG1 is involved in the formation of interhemispheric callosal connections and provides a novel perspective on the relevance of NRG1 in excitatory neurons and in the etiology of schizophrenia.

Meta-analysis reveals associations between genetic variation in the 5' and 3' regions of Neuregulin-1 and schizophrenia

Genetic, post-mortem and neuroimaging studies repeatedly implicate neuregulin-1 (NRG1) as a critical component in the pathophysiology of schizophrenia. Although a number of risk haplotypes along with several genetic polymorphisms in the 5' and 3' regions of NRG1 have been linked with schizophrenia, results have been mixed. To reconcile these conflicting findings, we conducted a meta-analysis examining 22 polymorphisms and two haplotypes in NRG1 among 16 720 cases, 20 449 controls and 2157 family trios. We found significant associations for three polymorphisms (rs62510682, rs35753505 and 478B14-848) at the 5'-end and two (rs2954041 and rs10503929) near the 3'-end of NRG1. Population stratification effects were found for the rs35753505 and 478B14-848(4) polymorphisms. There was evidence of heterogeneity for all significant markers and the findings were robust to publication bias. No significant haplotype associations were found. Our results suggest genetic variation at the 5' and 3' ends of NRG1 are associated with schizophrenia and provide renewed justification for further investigation of NRG1's role in the pathophysiology of schizophrenia.

Genetic association between NRG1 and schizophrenia, major depressive disorder, bipolar disorder in Han Chinese population

Schizophrenia, major depressive disorder, and bipolar disorder are three major psychiatric disorders affecting around 0.66%, 3.3%, and 1.5% of the Han Chinese population respectively. Several genetic linkage analyses and genome wide association studies identified NRG1 as a susceptibility gene of schizophrenia, which was validated by its role in neurodevelopment, glutamate, and other neurotransmitter receptor expression regulation. To further investigate whether NRG1 is a shared risk gene for major depressive disorder, bipolar disorder as well as schizophrenia, we performed an association study among 1,248 schizophrenia cases, 1,056 major depression cases, 1,344 bipolar disorder cases, and 1,248 controls. Totally 15 tag SNPs were genotyped and analyzed, and no population stratification was found in our sample set. Among the sites, rs4236710 (corrected Pgenotye  = 0.015) and rs4512342 (Pallele  = 0.03, Pgenotye  = 0.045 after correction) were associated with schizophrenia, and rs2919375 (corrected Pgenotye  = 0.004) was associated with major depressive disorder. The haplotype rs4512342-rs6982890 showed association with schizophrenia (P = 0.03 for haplotype "TC" after correction), and haplotype rs4531002-rs11989919 proved to be a shared risk factor for both major depressive disorder ("CC": corrected P = 0.009) and bipolar disorder ("CT": corrected P = 0.003). Our results confirmed that NRG1 was a shared common susceptibility gene for major mental disorders in Han Chinese population.

Neurological dysfunctions associated with altered BACE1-dependent Neuregulin-1 signaling

Inhibition of BACE1 is being pursued as a therapeutic target to treat patients suffering from Alzheimer's disease because BACE1 is the sole β-secretase that generates β-amyloid peptide. Knowledge regarding other cellular functions of BACE1 is therefore critical for the safe use of BACE1 inhibitors in human patients. Neuregulin-1 (Nrg1) is a BACE1 substrate and BACE1 cleavage of Nrg1 is critical for signaling functions in myelination, remyelination, synaptic plasticity, normal psychiatric behaviors, and maintenance of muscle spindles. This review summarizes the most recent discoveries associated with BACE1-dependent Nrg1 signaling in these areas. This body of knowledge will help to provide guidance for preventing unwanted Nrg1-based side effects following BACE1 inhibition in humans. To initiate its signaling cascade, membrane anchored Neuregulin (Nrg), mainly type I and III β1 Nrg1 isoforms and Nrg3, requires ectodomain shedding. BACE1 is one of such indispensable sheddases to release the functional Nrg signaling fragment. The dependence of Nrg on the cleavage by BACE1 is best manifested by disrupting the critical role of Nrg in the control of axonal myelination, schizophrenic behaviors as well as the formation and maintenance of muscle spindles.

Schizophrenia-risk variant rs6994992 in the neuregulin-1 gene on brain developmental trajectories in typically developing children

The neuregulin-1 (NRG1) gene is one of the best-validated risk genes for schizophrenia, and psychotic and bipolar disorders. The rs6994992 variant in the NRG1 promoter (SNP8NRG243177) is associated with altered frontal and temporal brain macrostructures and/or altered white matter density and integrity in schizophrenic adults, as well as healthy adults and neonates. However, the ages when these changes begin and whether neuroimaging phenotypes are associated with cognitive performance are not fully understood. Therefore, we investigated the association of the rs6994992 variant on developmental trajectories of brain macro- and microstructures, and their relationship with cognitive performance. A total of 972 healthy children aged 3-20 years had the genotype available for the NRG1-rs6994992 variant, and were evaluated with magnetic resonance imaging (MRI) and neuropsychological tests. Age-by-NRG1-rs6994992 interactions and genotype effects were assessed using a general additive model regression methodology, covaried for scanner type, socioeconomic status, sex and genetic ancestry factors. Compared with the C-carriers, children with the TT-risk-alleles had subtle microscopic and macroscopic changes in brain development that emerge or reverse during adolescence, a period when many psychiatric disorders are manifested. TT-children at late adolescence showed a lower age-dependent forniceal volume and lower fractional anisotropy; however, both measures were associated with better episodic memory performance. To our knowledge, we provide the first multimodal imaging evidence that genetic variation in NRG1 is associated with age-related changes on brain development during typical childhood and adolescence, and delineated the altered patterns of development in multiple brain regions in children with the T-risk allele(s).

Analysis of association between common SNPs in ErbB4 and bipolar affective disorder, major depressive disorder and schizophrenia in the Han Chinese population

Neuregulin-1 (NRG1) is associated with schizophrenia. As one of the receptors of NRG1, v-erb-a erythroblastic leukemia viral oncogene homolog 4 (ErbB4) has also been reported to be associated with schizophrenia. Since there can be shared genetic variants among bipolar affective disorder, major depressive disorder and schizophrenia, we tested the association between ErbB4 and these three major psychiatric disorders in the Han Chinese population. Five single nucleotide polymorphisms (SNPs) were selected based on previous positive reports and linkage disequilibrium information of the HapMap Han Chinese individuals from Beijing (CHB)+individuals from Tokyo, Japan (JPT) population. These SNPs were genotyped in 1140 bipolar affective disorder (BPAD) patients, 1140 schizophrenia (SCZ) patients, 1139 major depressive disorder (MDD) patients and 1140 normal controls. Two SNPs (rs707284 and rs839523) showed nominal significance in the BPAD patients but this was eliminated after permutation. No significant association between ErbB4 and the two other psychiatric disorders was observed, nor did haplotype analysis reveal any positive signal.

Concise review: the promise of human induced pluripotent stem cell-based studies of schizophrenia

Schizophrenia (SCZD) is a heritable developmental disorder. Although the molecular mechanism of disease remains unclear, insights into the disorder have been made through a vast array of experimental techniques. Together, magnetic resonance brain imaging, pharmacological, and post-mortem pathological studies have observed decreased brain volume, aberrant neurotransmitter signaling, reduced dendritic arborization, and impaired myelination in SCZD. Genome-wide association studies (GWAS) have identified common single nucleotide polymorphisms as well as rare copy number variants that contribute to SCZD, while mouse models of candidate SCZD genes show behavioral abnormalities and anatomical perturbations consistent with human disease. The advent of human induced pluripotent stem cells (hiPSCs) makes it possible to study SCZD using live human neurons with a genetic predisposition toward SCZD, even without knowledge of the genes interacting to produce the disease state. SCZD hiPSC neurons show cellular defects comparable to those identified in post-mortem human and mouse studies, and gene expression changes are consistent with predictions made by GWAS. SCZD hiPSC neurons represent a new tool to look beyond phenotype and begin to dissect the molecular mechanisms of SCZD.

White matter abnormalities and animal models examining a putative role of altered white matter in schizophrenia

Schizophrenia is a severe mental disorder affecting about 1% of the population worldwide. Although the dopamine (DA) hypothesis is still keeping a dominant position in schizophrenia research, new advances have been emerging in recent years, which suggest the implication of white matter abnormalities in schizophrenia. In this paper, we will briefly review some of recent human studies showing white matter abnormalities in schizophrenic brains and altered oligodendrocyte-(OL-) and myelin-related genes in patients with schizophrenia and will consider abnormal behaviors reported in patients with white matter diseases. Following these, we will selectively introduce some animal models examining a putative role of white matter abnormalities in schizophrenia. The emphasis will be put on the cuprizone (CPZ) model. CPZ-fed mice show demyelination and OLs loss, display schizophrenia-related behaviors, and have higher DA levels in the prefrontal cortex. These features suggest that the CPZ model is a novel animal model of schizophrenia.

Familial dyslexia in a large Swedish family: a whole genome linkage scan

There is a compelling body of evidence that developmental dyslexia runs in families and seems to be highly inheritable. Several investigations during the last two decades have shown possible locations of genes that might be involved in dyslexia, including regions of chromosomes 1, 2, 3, 6, 11, 13, 15 and 18. In addition, six candidate genes (KIAA0319, DYX1C1, DCDC2, ROBO1, MRPL19 and C2ORF3) seem to be related to dyslexia. The present study carried out a whole genome scan in a six-generation pedigree. In addition to literacy skills the assessment included cognitive skills and records concerning the history of reading and writing ability. Thirty-five percent were regarded as dyslexic in the family. A linkage analysis using both a quantitative and a qualitative approach has been performed. No evidence was obtained to support the hypothesis that the transmission of dyslexia in this pedigree is due to a highly penetrant major gene, and previous linkage findings were not replicated; however, power in this small study was not adequate to confirm linkage of genes with small to moderate effects. The results were discussed in relation to diagnostic procedures and sample characteristics.

Cytokine hypothesis of schizophrenia pathogenesis: evidence from human studies and animal models

The pathogenesis of schizophrenia has yet to be fully characterized. Gene-environment interactions have been found to play a crucial role in the vulnerability to this disease. Among various environmental factors, inflammatory immune processes have been most clearly implicated in the etiology and pathology of schizophrenia. Cytokines, regulators of immune/inflammatory reactions and brain development, emerge as part of a common pathway of genetic and environmental components of schizophrenia. Maternal infection, obstetric complications, neonatal hypoxia and brain injury all recruit cytokines to mediate inflammatory processes. Abnormal expression levels of specific cytokines such as epidermal growth factor, interleukins (IL) and neuregulin-1 are found both in the brain and peripheral blood of patients with schizophrenia. Accordingly, cytokines have been proposed to transmit peripheral immune/inflammatory signals to immature brain tissue through the developing blood-brain barrier, perturbing structural and phenotypic development of the brain. This cytokine hypothesis of schizophrenia is also supported by modeling experiments in animals. Animals treated with specific cytokines of epidermal growth factor, IL-1, IL-6, and neuregulin-1 as embryos or neonates exhibit schizophrenia-like behavioral abnormalities after puberty, some of which are ameliorated by treatment with antipsychotics. In this review, we discuss the neurobiological mechanisms underlying schizophrenia and novel antipsychotic candidates based on the cytokine hypothesis.

Association analysis of Neuregulin 1 candidate regions in schizophrenia and bipolar disorder

Schizophrenia (SCZ) and bipolar disorder (BPD) are severe heritable psychiatric disorders involving a complex genetic aetiology. Neuregulin 1 (NRG1) is a leading candidate gene for SCZ, and has recently been implicated in BPD. We previously reported association of two NRG1 haplotypes with SCZ and BPD in a Scottish case-control sample. One haplotype is located at the 5' end of the gene (region A), and the other is located at the 3' end (region B). Here, association to haplotypes within regions A and B was assessed in patients with SCZ and BPD in a second Scottish case-control sample and in the two Scottish samples combined. Association to region B was also assessed in patients with SCZ and BPD in a German case-control sample, and in all three samples combined. No evidence was found for association in the new samples when analysed individually; however, in the joint analysis of the two Scottish samples, a region B haplotype comprising two SNPs (rs6988339 and rs3757930) was associated with SCZ and the combined case group (SCZ: p=0.0037, OR=1.3, 95% CI: 1.1-1.6; BPD+SCZ: p=0.0080, OR=1.2, 95% CI: 1.1-1.5), with these associations withstanding multiple testing correction at the single-test level (SCZ: p(st)=0.022; BPD+SCZ: p(st)=0.044). This study supports the involvement of NRG1 variants in the less well studied 3' region in conferring susceptibility to SCZ and BPD in the Scottish population.

Nature and nurture in neuropsychiatric genetics: where do we stand?

Both genetic and nongenetic risk factors, as well as interactions and correlations between them, are thought to contribute to the etiology of psychiatric and behavioral phenotypes. Genetic epidemiology consistently supports the involvement of genes in liability. Molecular genetic studies have been less successful in identifying liability genes, but recent progress suggests that a number of specific genes contributing to risk have been identified. Collectively, the results are complex and inconsistent, with a single common DNA variant in any gene influencing risk across human populations. Few specific genetic variants influencing risk have been unambiguously identified. Contemporary approaches, however, hold great promise to further elucidate liability genes and variants, as well as their potential inter-relationships with each other and with the environment. We will review the fields of genetic epidemiology and molecular genetics, providing examples from the literature to illustrate the key concepts emerging from this work.

Epigenetics and biomarkers in the staging of neuropsychiatric disorders

Epigenetics, or alterations in the phenotype or gene expression due to mechanisms other than changes in the underlying DNA sequence, reflects the sensitivity and responsiveness of human and animal brains in constantly varying circumstances regulating gene expression profiles that define the biomarkers and present the ultimate phenotypical outcomes, such as cognition and emotion. Epigenetics is associated with functionally relevant alterations to the genome in such a fashion that under the particular conditions of early, adolescent, and adult life, environmental signals may activate intracellular pathways that remodel the "epigenome," triggering changes in gene expression and neural function. Thus, genetic influences in neuropsychiatric disorders that are subject to clinical staging, epigenetics in schizophrenia, epigenetic considerations in the expression of sensorimotor gating resulting from disease conditions, biomarkers of drug use and addiction, current notions on the role of dopamine in schizophrenia spectrum disorders, and the discrete interactions of biomarkers in persistent memory were to greater or lesser extents reflected upon. The relative contributions of endophenotypes and epistasis for mediating epigenetic phenomena and the outcomes as observed in the analysis of biomarkers appear to offer a multitude of interactive combinations to further complicate the labyrinthine machinations of diagnosis, intervention, and prognosis.

Case-control study of association between the functional candidate gene ERBB3 and schizophrenia in Caucasian population

Schizophrenia is a common psychiatric disorder with a complex genetic aetiology. Evidence shows that the oligodendrocyte and myelin-related genes including ERBB3 are closely related to schizophrenia. Two recent studies (Kanazawa et al. (Am J Med Genet B Neuropsychiatr Genet 2007;144:113)) and Watanabe et al. (Neurosci Res 2007;57:574) reported there was no association between ERBB3 and schizophrenia in Japanese population. We investigated the ERBB3 gene given the putative functional nature of the gene and population heterogeneity between Asian and Caucasian. Scottish case and control samples were sequenced with four SNPs (rs705708 at intron 15, rs2271189, rs773123, rs2271188 at exon 27). We detected rs773123, which is a nonsynonymous Ser/Cys polymorphism located seven bases downstream of rs2271189, with P=0.034. The subgroups of male patients and patients with age at onset <45 showed evidence of significant association with P values of 0.0046 and 0.0055, respectively. To our knowledge, this is the first association study between ERBB3 and schizophrenia in the Caucasian population. Further investigation with large sample size should be helpful to clarify the nature of the gene.

Neuregulin-1 genotypes and eye movements in schizophrenia

Neuregulin-1 (NRG-1) is a putative susceptibility gene for schizophrenia but the neurocognitive processes that may involve NRG-1 in schizophrenia are unknown. Deficits in antisaccade (AS) and smooth pursuit eye movements (SPEM) are promising endophenotypes, which may be associated with brain dysfunctions underlying the pathophysiology of schizophrenia. The aim of this study was to investigate the associations of NRG-1 genotypes with AS and SPEM in schizophrenia patients and healthy controls. Patients (N = 113) and controls (N = 106) were genotyped for two NRG-1 single nucleotide polymorphisms (SNPs); SNP8NRG222662, a surrogate marker for the originally described Icelandic NRG-1 risk haplotype, and SNP8NRG243177, which has recently been associated with individual differences in brain function. Subjects underwent infrared oculographic assessment of AS and SPEM. The study replicates previous findings of impaired AS and SPEM performance in schizophrenia patients (all P < 0.005; all d = 0.5-1.5). SNP8NRG243177 risk allele carriers had marginally increased variability of AS spatial error (P = 0.050, d = 0.3), but there were no significant genotype effects on other eye movement variables and no significant diagnosis-by-genotype interactions. Generally, risk allele carriers (G allele for SNP8NRG222662 and T allele for SNP8NRG243177) had numerically worse performance than non-carriers on most AS and SPEM variables. The results do not suggest that NRG-1 genotype significantly affects AS and SPEM task performance. However, the power of the sample to identify small effects is limited and the possibility of a type II error must be kept in mind. Larger samples may be needed to reliably investigate such gene effects on oculomotor endophenotypes.

Identification of neuroglycan C and interacting partners as potential susceptibility genes for schizophrenia in a Southern Chinese population

Chromosome 3p was reported by previous studies as one of the regions showing strong evidence of linkage with schizophrenia. We performed a fine-mapping association study of a 6-Mb high-LD and gene-rich region on 3p in a Southern Chinese sample of 489 schizophrenia patients and 519 controls to search for susceptibility genes. In the initial screen, 4 SNPs out of the 144 tag SNPs genotyped were nominally significant (P < 0.05). One of the most significant SNPs (rs3732530, P = 0.0048) was a non-synonymous SNP in the neuroglycan C (NGC, also known as CSPG5) gene, which belongs to the neuregulin family. The gene prioritization program Endeavor ranked NGC 8th out of the 129 genes in the 6-Mb region and the highest among the genes within the same LD block. Further genotyping of NGC revealed 3 more SNPs to be nominally associated with schizophrenia. Three other genes (NRG1, ErbB3, ErbB4) involved in the neuregulin pathways were subsequently genotyped. Interaction analysis by multifactor dimensionality reduction (MDR) revealed a significant two-SNP interaction between NGC and NRG1 (P = 0.015) and three-SNP interactions between NRG1 and ErbB4 (P = 0.009). The gene NGC is exclusively expressed in the brain. It is implicated in neurodevelopment in rats and was previously shown to promote neurite outgrowth. Methamphetamine, a drug that may induce psychotic symptoms, was reported to alter the expression of NGC. Taken together, these results suggest that NGC may be a novel candidate gene, and neuregulin signaling pathways may play an important role in schizophrenia.

NRG1 gene in recurrent major depression: no association in a large-scale case-control association study

The Neuregulin 1 (NRG1) gene was initially implicated in schizophrenia (SZ) and has recently been associated with bipolar disorder (BPD) in two studies. An association with major depressive disorder (MDD) has not yet been investigated but is warranted in view of the genetic overlap between MDD and BPD. We have performed a large-scale case-control study investigating the association between NRG1 polymorphisms and MDD, genotyping a selection of 14 single nucleotide polymorphisms (SNPs) spanning the NRG1 gene in a sample of 1,398 patients of White European ancestry with a diagnosis of MDD and 1,304 ethnically matched controls from three clinical sites in the UK. We found no single marker or haplotype associations that withstood correction for multiple testing. Our findings do not provide evidence that NRG1 plays a role in MDD or that this gene explains part of the genetic overlap with BPD.

Family-based association study of Neuregulin 1 with psychotic bipolar disorder

The Neuregulin 1 gene (NRG1) has been associated with schizophrenia, and, to a lesser extent, with bipolar disorder (BP). We investigated the association of NRG1 with BP in a large family sample, and then performed analyses according to the presence of psychotic features or mood-incongruent psychotic features. We genotyped 116 tagSNPs and four Icelandic "core" SNPs in 1,199 subjects from 314 nuclear families. Of 515 BP offspring, 341 had psychotic features, and 103 had mood-incongruent psychotic features. In single-marker and sliding window haplotype analyses using FBAT, there was little association using the standard BP or mood-incongruent psychotic BP phenotypes, but stronger signals were seen in the psychotic BP phenotype. The most significant associations with psychotic BP were in haplotypes within the 5' "core" region. The strongest global P-value was across three SNPs: NRG241930-NRG243177-rs7819063 (P = 0.0016), with an undertransmitted haplotype showing an individual P = 0.0007. The most significant individual haplotype was an undertransmitted two-allele subset of the above (NRG243177-rs7819063, P = 0.0004). Additional associations with psychotic BP were found across six SNPs in a 270 kb central region of the gene. The most 3' of these, rs7005606 (P = 0.0029), is located approximately 4 kb from the type I NRG1 isoform promoter. In sum, our study suggests that NRG1 may be specifically associated with the psychotic subset of BP; however, our results should be interpreted cautiously since they do not meet correction for multiple testing and await independent replication.

Impact of neuregulin-1 on the pathophysiology of schizophrenia in human post-mortem studies

To a large extend schizophrenia has been shown to be heritable, with neuregulin-1 (NRG1) one of the candidate genes considered to play a role in the pathophysiology of the disorder. While several polymorphisms within this gene have been reported to be associated with schizophrenia, the impact of NRG1 risk genotypes on disturbed brain function and symptoms of the disease is unknown and might be elucidated using post-mortem studies. Neuregulins are signalling proteins and the NRG1 family encodes at least 15 different splice variants, classified into four isoforms. They play an important role in cell differentiation, migration, myelination and proliferation of oligodendrocytes and neurons. Dysfunction in these processes may be related to neurodevelopmental disturbances in schizophrenia. NRG1 isoforms are differentially expressed in relevant brain regions of schizophrenia patients such as the prefrontal cortex and hippocampus and may contribute to pathophysiological processes. Different NRG1 genotypes have been shown to influence gene expression of isoforms and the risk-associated variants are in primarily non-coding and promoter regions, probably operating by altering gene expression or splicing. In addition, NRG1 regulates the expression of the nicotinic acetylcholine receptor, and expression of the gamma-aminobutyric acid (GABA(A)) and N-methyl-D: -aspartate receptor in the brain. However, the contribution of NRG1 risk genotypes to expression of isoforms and cognitive or psychotic symptoms in patients remain to be investigated in prospective post-mortem studies. In animal models of ischemia/hypoxia, NRG1 has been shown to act as a therapeutic, neuroprotective agent and should be investigated in more detail in transgenic animal models.

Neuregulin-1 haplotype HAP(ICE) is associated with lower hippocampal volumes in schizophrenic patients and in non-affected family members

The neuregulin-1 (NRG1) gene on chromosome 8p has been suggested as a potential susceptibility gene for schizophrenia. The exact way in which genetic variation in NRG1 might impact on this susceptibility for the disorder is a focus of current research. The present study aimed at investigating the possible relationship between a putative NRG1 at-risk haplotype (HAP(ICE)) and hippocampal volumes in schizophrenic patients and their healthy first-degree relatives. We genotyped 30 schizophrenic patients and 52 non-affected family members with regard to the presence or absence of the NRG1 haplotype HAP(ICE). Structural magnetic resonance imaging was used to determine hippocampal brain volumes in the same subjects. Patients and relatives carrying haplotype HAP(ICE) both had smaller relative hippocampal volumes as compared to patients or relatives who did not carry this haplotype. These findings provide first direct evidence for a link between NRG1 genetic variation and hippocampal volume reductions in schizophrenic patients and non-affected relatives. This preliminary evidence may help to guide further research into the pathophysiological pathways that underlie this genetic susceptibility for schizophrenia.

Support for neuregulin 1 as a susceptibility gene for bipolar disorder and schizophrenia

BACKGROUND

There is support that Neuregulin 1 (NRG1) plays a role in susceptibility to schizophrenia but limited evidence for its involvement in bipolar disorder. We wished to investigate further the involvement of NRG1 in schizophrenia and bipolar disorder.

METHODS

We used hierarchical association analysis in parent-offspring trios, 634 with schizophrenia/schizoaffective disorder (SZ/SA) and 243 with bipolar 1 disorder (BP1). The primary analysis was the markers defining the "core Icelandic haplotype" (HAP(ICE)). We undertook polymorphism discovery, additional genotyping, and also explored phenotypic associations, as a secondary analysis aimed at refining the signal.

RESULTS

The initial global haplotype test yielded significant evidence for association (p = .01) with SZ/SA and BP1 (p = .004), although HAP(ICE) was not overtransmitted. The marker showing strongest evidence for association in the deCODE studies, SNP8NRG221533, was associated with SZ/SA (p(corrected) = .039) and with BP1 (p(corrected) = .039), with BP1 showing association to the opposite allele as SZ/SA. The pattern of transmission at SNP8NRG221533 was significantly different in SZ/SA than in BP1 (p = .0004). Secondary analyses of markers and phenotypes provided no additional evidence for association to SZ/SA. However, a new marker, rs7014762, was associated with an a priori defined "typical" bipolar phenotype characterized by excellent recovery between episodes and no mood incongruent features (p(corrected) = .003).

CONCLUSIONS

Our data provide significant levels of support for NRG1 as a susceptibility gene for both major forms of psychosis, and this cannot be interpreted as being due to population stratification. More tentatively, they also might indicate the presence of multiple alleles that influence the psychosis phenotype.

Neuregulin 1 in neural development, synaptic plasticity and schizophrenia

Schizophrenia is a highly debilitating mental disorder that affects approximately 1% of the general population, yet it continues to be poorly understood. Recent studies have identified variations in several genes that are associated with this disorder in diverse populations, including those that encode neuregulin 1 (NRG1) and its receptor ErbB4. The past few years have witnessed exciting progress in our knowledge of NRG1 and ErbB4 functions and the biological basis of the increased risk for schizophrenia that is potentially conferred by polymorphisms in the two genes. An improved understanding of the mechanisms by which altered function of NRG1 and ErbB4 contributes to schizophrenia might eventually lead to the development of more effective therapeutics.

Age-related changes in the expression of schizophrenia susceptibility genes in the human prefrontal cortex

The molecular basis of complex neuropsychiatric disorders most likely involves many genes. In recent years, specific genetic variations influencing risk for schizophrenia and other neuropsychiatric disorders have been reported. We have used custom DNA microarrays and qPCR to investigate the expression of putative schizophrenia susceptibility genes and related genes of interest in the normal human brain. Expression of 31 genes was measured in Brodmann's area 10 (BA10) in the prefrontal cortex of 72 postmortem brain samples spanning half a century of human aging (18-67 years), each without history of neuropsychiatric illness, neurological disease, or drug abuse. Examination of expression across age allowed the identification of genes whose expression patterns correlate with age, as well as genes that share common expression patterns and that possibly participate in common cellular mechanisms related to the emergence of schizophrenia in early adult life. The expression of GRM3 and RGS4 decreased across the entire age range surveyed, while that of PRODH and DARPP-32 was shown to increase with age. NRG1, ERBB3, and NGFR show expression changes during the years of greatest risk for the development of schizophrenia. Expression of FEZ1, GAD1, and RGS4 showed especially high correlation with one another, in addition to the strongest mean levels of absolute correlation with all other genes studied here. All microarray data are available at NCBI's Gene Expression Omnibus: GEO Series accession number GSE11546 (http://www.ncbi.nlm.nih.gov/geo) [corrected]

Genomic structural variation and schizophrenia

It has recently been demonstrated that a large amount of structural variation exists in the human genome. Since 2004, when two landmark studies reported polymorphic levels of copy number variation in phenotypically normal individuals, our understanding of genome-wide levels of copy number variation has grown. This has inspired hypotheses about this class of variation's contribution to complex genetic phenotypes, including the specific hypothesis that structural variation is associated with psychiatric illness. The technology to accurately and efficiently detect polymorphic structural variants is still largely under development, but some examples of genomic imbalance contributing to schizophrenia and bipolar disorder already have been identified. Although much optimism surrounds this burgeoning field, the technical challenges in reliably identifying structural variation mean recent literature should be approached with caution.

Association of schizophrenia with DTNBP1 but not with DAO, DAOA, NRG1 and RGS4 nor their genetic interaction

Recent reports indicate that DAO, DAOA, DTNBP1, NRG1 and RGS4 are some of the most-replicated genes implicated in susceptibility to schizophrenia. Also, the functions of these genes could converge in a common pathway of glutamate metabolism. The aim of this study was to evaluate if each of these genes, or their interaction, was associated with schizophrenia. A case-control study was conducted in 589 Spanish patients having a diagnosis of schizophrenia, and compared with 617 equivalent control subjects. Several single nucleotide polymorphisms (SNPs) in each gene were determined in all individuals. SNP and haplotype frequencies were compared between cases and controls. The interaction between different SNPs at the same, or at different gene, loci was analyzed by the multifactor dimensionality reduction (MDR) method. We found a new schizophrenia risk and protective haplotypes in intron VII of DTNBP1; one of the most important candidate genes for this disorder, to-date. However, no association was found between DAO, DAOA, NRG1 and RGS4 and schizophrenia. The hypothesis that gene-gene interaction in these five genes could increase the risk for the disorder was not confirmed in the present study. In summary, these results may provide further support for an association between the dysbindin gene (DTNBP1) and schizophrenia, but not between the disease and DAO, DAOA, NRG1 and RGS4 or with the interaction of these genes. In the light of recent data, these results need to be interpreted with caution and future analyses with dense genetic maps are awaited.

Increased levels of serotonin 2A receptors and serotonin transporter in the CNS of neuregulin 1 hypomorphic/mutant mice

Changes in neuregulin 1 expression have been reported in the CNS from subjects with schizophrenia. As neuregulin 1 is important in cortical development we postulated that changes in neuregulin 1 expression may contribute towards changes in cholinergic, glutamatergic and serotonergic markers that are well documented in the CNS of subjects with that disorder. To begin to test this hypothesis, we used in situ radioligand binding to measure levels of muscarinic M1/M4 receptors, the kainate receptor, the NMDA receptor, the serotonin 2A receptor, the serotonin 1A receptor and the serotonin transporter in the CNS from heterozygous transmembrane domain neuregulin 1 mutant mice. The major outcomes from these studies was the demonstration of an overall increase in levels of the serotonin 2A receptor (F=11.3, d.f.=3,1,72, p=0.0012) and serotonin transporter (F=5.00, d.f.=1,3,72, p<0.05) in the mutant mice. Levels of the other receptors did not vary in the mutant mice compared to their wild type-like litter mates. These data are the first evidence to suggest that NRG1 gene expression may be involved in regulating the development of the serotonergic system in the mammalian CNS.

Evidence of missense mutations on the neuregulin 1 gene affecting function of prepulse inhibition

BACKGROUND

Neuregulin 1 (NRG1) is one of the leading candidate genes in schizophrenia. Rodents with NRG1 knock-out showed significantly impaired prepulse inhibition (PPI) in the original report linking NRG1 to schizophrenia. A widely used surrogate measure of psychosis in animal models, PPI is considered a schizophrenia endophenotype. We hypothesized that if NRG1 influences PPI in rodents, then it should have a similar effect on PPI in humans.

METHODS

We examined the potential neurophysiological effects of two nonsynonymous single nucleotide polymorphisms located on NRG1 (rs3924999 and rs10503929) on PPI. Genotyping was completed in 430 unrelated individuals, including 244 schizophrenia cases and 186 controls. PPI was available in a subgroup of 113 cases and 63 controls.

RESULTS

Rs3924999 genotype was significantly associated with PPI (p = .003): PPI was lowest in the subjects who were homozygous for the minor allele A/A carriers, intermediate in A/G carriers, and highest in homozygous major alleles G/G carriers. The associations persisted within cases (p = .02) and controls (p = .02) analyzed separately. An additive model suggested that rs3924999 alone contributes to 7.9% of the PPI variance. In contrast, rs10503929 genotype was not associated with PPI (p = .85). Schizophrenia patients had reduced PPI compared to control subjects (p = .04). Neither single nucleotide polymorphism was associated with schizophrenia (all p > .37). However, schizophrenia patients with abnormal PPI may be associated with rs3924999 (p = .05).

CONCLUSIONS

A missense mutation on rs3924999 of the neuregulin 1 gene may have a functional effect on prepulse inhibition in both schizophrenia and healthy control populations.

Family-based association study of neuregulin-1 gene and psychosis in a Spanish sample

Neuregulin 1 (NRG1) is one of the most exciting candidate genes for schizophrenia since its first association with the disorder in an Icelandic population. Since then, many studies have analyzed allele and haplotype frequencies in European and Asian populations in cases and controls yielding varying results. We investigated the association of NRG1 with psychosis in a total sample set of 575 individuals from 151 Spanish nuclear families. We tested eight SNPs across 1.2 Mb along NRG1 including regions previously associated to schizophrenia in association studies. After correction for multiple testing, the TDT analysis for each marker did not show a significant over-transmission of alleles from the parents to the affected offspring for any of the markers (P > 0.05). The haplotypic analysis with TRANSMIT and PDT did not show preferential transmission for any of the haplotypes analyzed in our sample. These results do not seem to suggest that the investigated NRG1 markers play a role in schizophrenia in the Spanish population, although the finding of a trend for association with one SNP in the 3'of the gene warrants further investigation.

New genes associated with schizophrenia in neurite formation: a review of cell culture experiments

New genes consistently associated with schizophrenia include NRG1, Akt, DISC-1 and dysbindin-1. Since these genes participate in neurotransmission, neuroplasticity and neurodevelopment it has not been easy to elucidate which of these roles are abnormal in patients with schizophrenia. Neurite formation is identified as a crucial stage in development, and it is proposed that a defect in neurite formation originating from abnormally encoded proteins by these new genes could be at least an in vitro marker that reflects the most consistent molecular and neuroanatomical findings in schizophrenia. A systematic review of the literature linking the process of neurite formation to genes with replicated evidence that supported their association with schizophrenia was conducted. In addition, an outline of the process of neurite formation was included. Neurite formation was shown to be induced by neuregulins, the product of the gene NRG1. The activation of Akt, a serine/threonine kinase, promoted neurite formation in six independent studies. Conversely, two studies found that Akt inhibits neurite outgrowth. Stronger evidence supporting an association with the new genes related to schizophrenia and neurite formation comes from DISC-1. Defects in DISC-1 protein were shown to directly alter the process of neurite formation. Dysbindin-1 has not yet been directly implicated in neurite outgrowth. These findings suggest that the proteins encoded by NRG1, Akt and DISC-1 are implicated in the process of neurite formation in cellular models as well as, at least in part, animal models during development. Abnormalities in this process could have potential etiologic implications for schizophrenia. Direct evidence, however, of abnormal neurite formation in patients with schizophrenia is still missing. Limitations to this model are identified.

Systematic study of association of four GABAergic genes: glutamic acid decarboxylase 1 gene, glutamic acid decarboxylase 2 gene, GABA(B) receptor 1 gene and GABA(A) receptor subunit beta2 gene, with schizophrenia using a universal DNA microarray

Several studies have suggested the dysfunction of the GABAergic system as a risk factor in the pathogenesis of schizophrenia. In the present study, case-control association analysis was conducted in four GABAergic genes: two glutamic acid decarboxylase genes (GAD1 and GAD2), a GABA(A) receptor subunit beta2 gene (GABRB2) and a GABA(B) receptor 1 gene (GABBR1). Using a universal DNA microarray procedure we genotyped a total of 20 SNPs on the above four genes in a study involving 292 patients and 286 controls of Chinese descent. Statistically significant differences were observed in the allelic frequencies of the rs187269C/T polymorphism in the GABRB2 gene (P=0.0450, chi(2)=12.40, OR=1.65) and the -292A/C polymorphism in the GAD1 gene (P=0.0450, chi(2)=14.64 OR=1.77). In addition, using an electrophoretic mobility shift assay (EMSA), we discovered differences in the U251 nuclear protein binding to oligonucleotides representing the -292 SNP on the GAD1 gene, which suggests that the -292C allele has reduced transcription factor binding efficiency compared with the 292A allele. Using the multifactor-dimensionality reduction method (MDR), we found that the interactions among the rs187269C/T polymorphism in the GABRB2 gene, the -243A/G polymorphism in the GAD2 gene and the 27379C/T and 661C/T polymorphisms in the GAD1 gene revealed a significant association with schizophrenia (P<0.001). These findings suggest that the GABRB2 and GAD1 genes alone and the combined effects of the polymorphisms in the four GABAergic system genes may confer susceptibility to the development of schizophrenia in the Chinese population.

Heterozygous neuregulin 1 mice are more sensitive to the behavioural effects of Delta9-tetrahydrocannabinol

RATIONALE

Cannabis use may precipitate schizophrenia especially if the individual has a genetic vulnerability to this mental disorder. Human and animal research indicates that neuregulin 1 (Nrg1) is a susceptibility gene for schizophrenia.

OBJECTIVES

The aim of this study was to investigate whether dysfunction in the Nrg1 gene modulates the behavioural effects of Delta(9)-tetrahydrocannabinol (THC), the major psychotropic component of cannabis.

MATERIALS AND METHODS

Heterozygous Nrg1 transmembrane-domain knockout mice (Nrg1 HET) were treated with acute THC (0, 5 or 10 mg/kg i.p.) 30 min before being tested using open field (OF), hole board (HB), light-dark (LD), elevated plus maze (EPM), social interaction (SI) and prepulse inhibition (PPI) tests.

RESULTS

Nrg1 HET mice showed differences in baseline behaviour with regard to locomotor activity, exploration and anxiety. More importantly, they were more sensitive to the locomotor suppressant actions of THC compared to wild type-like (WT) mice. In addition, Nrg1 HET mice expressed a greater THC-induced enhancement in % PPI than WT mice. The effects of THC on anxiety-related behaviour were task-dependent, with Nrg1 HET mice being more susceptible than WT mice to the anxiogenic effects of THC in LD, but not in the EPM, SI and OF tests.

CONCLUSIONS

Nrg1 HET mice were more sensitive to the acute effects of THC in an array of different behaviours including those that model symptoms of schizophrenia. It appears that variation in the schizophrenia-related neuregulin 1 gene alters the sensitivity to the behavioural effects of cannabinoids.

Activation of the canonical Wnt pathway by the antipsychotics haloperidol and clozapine involves dishevelled-3

Protein kinase B (Akt), glycogen synthase kinase-3 (GSK-3) and members of the Wnt signal transduction pathway were recently found to be altered in schizophrenia and targeted by antipsychotic drugs. In the current study, selected Wnt signalling proteins were investigated to determine if they are altered by the antipsychotics clozapine or haloperidol in the rat prefrontal cortex. Pheochromocytoma (PC12) and neuroblastoma (SH-SY5Y) cells were also used to elucidate how antipsychotics generated the pattern of changes observed in vivo. Western blotting (WB) revealed that treatment with haloperidol or clozapine caused an up-regulation of Wnt-5a, dishevelled-3, Axin, total and phosphorylated GSK-3 and beta-catenin protein levels. Treatment of PC12 and SH-SY5Y cells with a variety of pharmacological agents as well as the over-expression of several Wnt related proteins failed to mimic the pattern observed in vivo following antipsychotic treatment. However, the over-expression of dishevelled-3 nearly perfectly duplicated the changes observed in vivo. Immunoprecipitations (IP) conducted using protein isolated from the rat prefrontal cortex indicated that dishevelled-3 is associated with the D2 dopamine receptor thereby suggesting that antipsychotics may act on dishevelled-3 via D2 dopamine receptors to initiate a cascade of downstream changes involving Axin, GSK-3 and beta-catenin that may help to alleviate psychosis in schizophrenic patients.

The role of DTNBP1, NRG1, and AKT1 in the genetics of schizophrenia in Finland

Several putative schizophrenia susceptibility genes have recently been identified. Significant associations between schizophrenia and neuregulin 1 (NRG1) and dysbindin (DTNBP1) were first reported in 2002 and studies in several populations have since independently reported positive associations to these gene regions. Further, both tentative functional and genetic data have implicated the role of AKT1 in the genetic background of this disorder. However, findings have not been consistent in all populations. We investigated the allelic diversity of these three genes NRG1, DTNBP1 and AKT1 in a representative nation-wide study sample of 441 Finnish schizophrenia families consisting of 865 affected individuals, in order to assess their role in one of the largest population-based study samples. DTNBP1 and AKT1 failed to show evidence of association, whereas two SNPs in the 3' region of the NRG1 gene yielded suggestive evidence of association (p=0.012 and p=0.048) in family-based association analyses. Thus, our study does not indicate that AKT1 or DTNBP1 play a role in the etiology of schizophrenia in the Finnish population. Furthermore, results do not support a major role for NRG1, but we cannot completely exclude a minor role of this gene in the Finnish population.

Are some genetic risk factors common to schizophrenia, bipolar disorder and depression? evidence fromDISC1, GRIK4 andNRG1

Depression is common in patients with schizophrenia and it is well established from family studies that rates of depression are increased among relatives of probands with schizophrenia, making it likely that the phenotypes described under the categories of affective and non-affective psychoses share some genetic risk factors. Family linkage studies have identified several chromosomal regions likely to contain risk genes for schizophrenia and bipolar disorder, suggesting common susceptibility loci. Candidate gene association studies have provided further evidence to suggest that some genes including two of the most studied candidates, Disrupted in Schizophrenia 1 (DISC1) and Neuregulin 1 (NRG1) may be involved in both types of psychosis. We have recently identified another strong candidate for a role in both schizophrenia and affective disorders, GRIK4 a glutamate receptor mapped to chromosome 11q23 [Glutamate Receptor, Ionotropic, Kainate, type 4]. This gene is disrupted by a translocation breakpoint in a patient with schizophrenia, and case control studies show significant association of GRIK4 with both schizophrenia and bipolar disorder. Identifying genes implicated in the psychoses may eventually provide the basis for classification based on biology rather than symptoms, and suggest novel treatment strategies for these complex brain disorders.

No association between the ERBB3 gene and schizophrenia in a Japanese population

There is cumulative evidence that neuregulin 1 (NRG1) is a susceptibility gene for schizophrenia. Postmortem studies on brains from schizophrenia patients have revealed changes in the mRNA expression levels of v-erb-b2 erythroblastic leukemia viral oncogene homolog 3 (ERBB3), one of the NRG1 receptor genes. These observations suggest that NRG1-ERBB signaling is involved in the pathogenesis of schizophrenia. To assess whether the ERBB3 gene could be implicated in vulnerability to schizophrenia, we conducted a case-control (399 patients and 438 controls) association study in Japanese subjects. There were no significant association between the polymorphisms or haplotypes of ERBB3 and schizophrenia. The present study shows that ERBB3 does not play a major role in conferring susceptibility to schizophrenia in the Japanese population.

Schizophrenia susceptibility genes: in search of a molecular logic and novel drug targets for a devastating disorder

Schizophrenia is a devastating psychiatric disorder that affects approximately one percent of the population worldwide. We argue that the efforts to decipher the genetic causes of schizophrenia have reached another turning point and describe evidence supporting some of the major recent genetic findings in the field. In addition, we identify some general areas of caution in the interpretation of these findings and addresses the promise this recently acquired knowledge holds for the generation of reliable animal models, characterization of genetic interactions, dissection of the disease pathophysiology and development of novel, mechanism-based treatments for the patients.

Association of Neuregulin 1 with schizophrenia and bipolar disorder in a second cohort from the Scottish population

Neuregulin 1 (NRG1) is a strong candidate for involvement in the aetiology of schizophrenia. A haplotype, initially identified as showing association in the Icelandic and Scottish populations, has shown a consistent effect size in multiple European populations. Additionally, NRG1 has been implicated in susceptibility to bipolar disorder. In this first study to select markers systematically on the basis of linkage disequilibrium across the entire NRG1 gene, we used haplotype-tagging single-nucleotide polymorphisms to identify single markers and haplotypes associated with schizophrenia and bipolar disorder in an independently ascertained Scottish population. Haplotypes in two regions met an experiment-wide significance threshold of P=0.0016 (Nyholt's SpD) and were permuted to correct for multiple testing. Region A overlaps with the Icelandic haplotype and shows nominal association with schizophrenia (P=0.00032), bipolar disorder (P=0.0011), and the combined case group (P=0.0017). This region includes the 5' exon of the NRG1 GGF2 isoform and overlaps the expressed sequence tag (EST) cluster Hs.97362. However, no haplotype in Region A remains significant after permutation analysis (P>0.05). Region B contains a haplotype associated with both schizophrenia (P=0.00014), and the combined case group (P=0.000062), although it does not meet Nyholt's threshold in bipolar disorder alone (P=0.0022). This haplotype remained significant after permutation analysis in both the schizophrenia and combined case groups (P=0.024 and P=0.016, respectively). It spans a approximately 136 kb region that includes the coding sequence of the sensory and motor neuron derived factor (SMDF) isoform and 3' exons of all other known NRG1 isoforms. Our study identifies a new of NRG1 region involved in schizophrenia and bipolar disorder in the Scottish population.

Involvement of neuropeptide systems in schizophrenia: human studies

Neuropeptides are heterogeneously distributed throughout the digestive, circulatory, and nervous systems and serve as neurotransmitters, neuromodulators, and hormones. Neuropeptides are phylogenetically conserved and have been demonstrated to regulate numerous behaviors. They have been hypothesized to be pathologically involved in several psychiatric disorders, including schizophrenia. On the basis of preclinical data, numerous studies have sought to examine the role of neuropeptide systems in schizophrenia. This chapter reviews the clinical data, linking alterations in neuropeptide systems to the etiology, pathophysiology, and treatment of schizophrenia. Data for the following neuropeptide systems are included: arginine-vasopressin, cholecystokinin (CCK), corticotropin-releasing factor (CRF), interleukins, neuregulin 1 (NRG1), neurotensin (NT), neuropeptide Y (NPY), opioids, secretin, somatostatin, tachykinins, thyrotropin-releasing hormone (TRH), and vasoactive intestinal peptide (VIP). Data from cerebrospinal fluid (CSF), postmortem and genetic studies, as well as clinical trials are described. Despite the inherent difficulties associated with human studies (including small sample size, variable duration of illness, medication status, the presence of comorbid psychiatric disorders, and diagnostic heterogeneity), several findings are noteworthy. Postmortem studies support disease-related alterations in several neuropeptide systems in the frontal and temporal cortices. The strongest genetic evidence supporting a role for neuropeptides in schizophrenia are those studies linking polymorphisms in NRG1 and the CCKA receptor with schizophrenia. Finally, the only compounds that act directly on neuropeptide systems that have demonstrated therapeutic efficacy in schizophrenia are neurokinin receptor antagonists. Clearly, additional investigation into the role of neuropeptide systems in the etiology, pathophysiology, and treatment of schizophrenia is warranted.

Variations in the vesicular monoamine transporter 1 gene (VMAT1/SLC18A1) are associated with bipolar i disorder

The vesicular monoamine transporter 1 gene (VMAT1/SLC18A1) maps to the shared bipolar disorder (BPD)/schizophrenia (SZ) susceptibility locus on chromosome 8p21. Vesicular monoamine transporters are involved in transport of monoamine neurotransmitters which have been postulated to play a relevant role in the etiology of BPD and/or SZ. Variations in the VMAT1 gene might affect transporter function and/or expression and might be involved in the etiology of BPD and/or SZ. Genotypes of 585 patients with BPD type I and 563 control subjects were obtained for three missense single nucleotide polymorphisms (SNPs) (Thr4Pro, Thr98Ser, Thr136Ile) and four non-coding SNPs (rs988713, rs2279709, rs3735835, rs1497020). All cases and controls were of European descent. Allele frequencies differed significantly for the potential functional polymorphism Thr136Ser between BPD patients and controls (p=0.003; df=1; OR=1.34; 95% CI: 1.11-1.62). Polymorphisms in the promoter region (rs988713: p=0.005, df=1; OR=1.31; 95% CI: 1.09-1.59) and intron 8 (rs2279709: p=0.039, df=1; OR=0.84; 95% CI: 0.71-0.99) were also associated with disease. Expression analysis confirmed that VMAT1 is expressed in human brain at the mRNA and protein level. Results suggest that variations in the VMAT1 gene may confer susceptibility to BPD in patients of European descent. Additional studies are necessary to confirm this effect and to elucidate the role of VMAT1 in central nervous system physiology.

Early brain development disruption from NMDA receptor hypofunction: relevance to schizophrenia

Disruption to brain development at an early stage can potentially alter chemically coded neural networks and can affect behavior in later life. During early brain development antagonism of glutamate NMDA receptors, which play an important role in neuronal outgrowth and survival, leads to neuronal damage in several brain regions and causes behavioral alterations in rodents that mimic schizophrenia symptoms and endophenotypes. There are several lines of evidence implicating involvement of a dysfunctional glutamate system in schizophrenia. In normal subjects, NMDA receptor antagonists produce behavioral and neurochemical changes that mimic schizophrenia symptoms better than any other psychotomimetic drug. Moreover, these drugs worsen symptoms in schizophrenia patients and can trigger a recrudescence of the acute psychotic state in patients with stable chronic schizophrenia. In addition, genes consistently reported as being altered in schizophrenia play roles in development, neuroplasticity and glutamate/GABAergic neurotransmission. Perinatal NMDA receptor antagonist treatment is a useful model for studying the neurodevelopmental and NMDA receptor hypofunction hypotheses of schizophrenia because neurochemical and behavioral changes, reminiscent of those seen in schizophrenia, are present long after cessation of drug administration, which suggests that a permanent change in brain structure and organization has occurred during brain development.

Neuregulin 1 and schizophrenia: genetics, gene expression, and neurobiology

Neuregulin 1 (NRG1) is a leading schizophrenia susceptibility gene. The NRG1 locus on chromosome 8p shows linkage to the disorder, and genetic association has been found between schizophrenia and various non-coding polymorphisms and haplotypes, especially at the 5' end of the NRG1 gene, in many but not all case-control and family studies. NRG1 is a pleiotropic growth factor, important in nervous system development and functioning; roles include the modulation of neuronal migration, synaptogenesis, gliogenesis, neuron-glia communication, myelination, and neurotransmission. Understanding the neurobiology of NRG1 and its involvement in schizophrenia is challenged by the complexity of the gene, which gives rise to multiple functionally distinct isoforms, including six "types" of NRG1 defined by 5' exon usage. Type IV and type I NRG1 may be particularly relevant to schizophrenia, with initial data showing altered expression of these isoforms in the disorder or in association with NRG1 risk alleles. We review the structure and functions of NRG1, consider the evidence for and against it being a schizophrenia susceptibility gene, and discuss mechanisms that might underlie the contribution of NRG1 to disease pathophysiology.

Evaluation of a susceptibility gene for schizophrenia: genotype based meta-analysis of RGS4 polymorphisms from thirteen independent samples

BACKGROUND

Associations between schizophrenia (SCZ) and polymorphisms at the regulator of G-protein signaling 4 (RGS4) gene have been reported (single nucleotide polymorphisms [SNPs] 1, 4, 7, and 18). Yet, similar to other SCZ candidate genes, studies have been inconsistent with respect to the associated alleles.

METHODS

In an effort to resolve the role for RGS4 in SCZ susceptibility, we undertook a genotype-based meta-analysis using both published and unpublished family-based and case-control samples (total n = 13,807).

RESULTS

The family-based dataset consisted of 10 samples (2160 families). Significant associations with individual SNPs/haplotypes were not observed. In contrast, global analysis revealed significant transmission distortion (p = .0009). Specifically, analyses suggested overtransmission of two common haplotypes that account for the vast majority of all haplotypes. Separate analyses of 3486 cases and 3755 control samples (eight samples) detected a significant association with SNP 4 (p = .01). Individual haplotype analyses were not significant, but evaluation of test statistics from individual samples suggested significant associations.

CONCLUSIONS

Our collaborative meta-analysis represents one of the largest SCZ association studies to date. No individual risk factor arose from our analyses, but interpretation of these results is not straightforward. Our analyses suggest risk due to at least two common haplotypes in the presence of heterogeneity. Similar analysis for other putative susceptibility genes is warranted.

Association of the NRG1 gene and schizophrenia: a meta-analysis

We investigated the association of the NRG1 gene and schizophrenia using meta-analytic techniques, combining all published data while restricting our analysis to studies investigating the most commonly reported single marker (SNP8NRG221533). We also investigated whether ancestry (European vs East Asian) and study design (family-based vs case-control) moderated any association. We found no evidence for an association of SNP8NRG221533 with schizophrenia, and significant between-study heterogeneity, which persisted when family-based studies were combined separately. However, when haplotype-based P-values were combined, there was evidence in support of an association of NRG1 with schizophrenia, and no evidence of between-study heterogeneity. Our meta-analysis provides support for the association of NRG1 with schizophrenia, but indicates that firmly establishing the role of NRG1 gene in schizophrenia by genetic association requires much larger sample sizes than have hitherto been reported. Association analyses and replications should take place at the level of the gene, rather than at the level of SNP, haplotype, or functional variant. Meta-analysis would then be carried out on the basis of the combination of P-values.

Meta-analysis shows strong positive association of the neuregulin 1 (NRG1) gene with schizophrenia

Chromosome 8p22-p11 has been identified as a locus for schizophrenia in several genome-wide scans and confirmed by meta-analysis of published linkage data. Systematic fine mapping using extended Icelandic pedigrees identified an associated haplotype in the gene neuregulin 1 (NRG1), also known as heuregulin, glial growth factor, NDF43 and ARIA. A 290 kb core at risk haplotype at the 5' end of the gene (HAP(ICE)), defined by five SNPs and two microsatellite polymorphisms was found to be associated with schizophrenia in the Icelandic and Scottish populations. A number of subsequent independent studies have attempted to replicate the association, and while some have been successful, the associated haplotype is not always HAP(ICE). Furthermore, no obviously functional or pathogenic variants have been identified, and the relationship between the gene and schizophrenia has remained inconclusive. To reconcile these conflicting findings and to give a comprehensive picture of the genetic architecture of this important gene, we performed a meta-analysis of 13 published population-based and family-based association studies up to November 2005. We analysed data from the SNP markers SNP8NRG241930, SNP8NRG243177, SNP8NRG221132 and SNP8NRG221533, and the microsatellite markers 478B14-848, 420M9-1395. Across these studies, strong positive association was found for all six polymorphisms. The haplotype analysis also showed significant association in the pooled international populations (OR=1.22, 95% CI 1.15-1.3, P=8 x 10(-10)). In Asian populations, the risk haplotype was focused around the two microsatellite markers, 478B14-848, 420M9-1395 (haplotype block B), and in Caucasian populations with the remaining four SNP markers (haplotype block A). This meta-analysis supports the involvement of NRG1 in the pathogenesis of schizophrenia, but with association between two different but adjacent haplotypes blocks in the Caucasian and Asian populations.

Neuregulin 1 transcripts are differentially expressed in schizophrenia and regulated by 5' SNPs associated with the disease

Genetic variation in neuregulin 1 (NRG1) is associated with schizophrenia. The disease-associated SNPs are noncoding, and their functional implications remain unknown. We hypothesized that differential expression of the NRG1 gene explains its association to the disease. We examined four of the disease-associated SNPs that make up the original risk haplotype in the 5' upstream region of the gene for their effects on mRNA abundance of NRG1 types I-IV in human postmortem hippocampus. Diagnostic comparisons revealed a 34% increase in type I mRNA in schizophrenia and an interaction of diagnosis and genotype (SNP8NRG221132) on this transcript. Of potentially greater interest, a single SNP within the risk haplotype (SNP8NRG243177) and a 22-kb block of this core haplotype are associated with mRNA expression for the novel type IV isoform in patients and controls. Bioinformatic promoter analyses indicate that both SNPs lead to a gain/loss of putative binding sites for three transcription factors, serum response factor, myelin transcription factor-1, and High Mobility Group Box Protein-1. These data implicate variation in isoform expression as a molecular mechanism for the genetic association of NRG1 with schizophrenia.

Linkage and association of schizophrenia with genetic variations in the locus of neuregulin 1 in Korean population

Chromosome 8p21-12 has been reported to be a susceptibility locus for schizophrenia based on genome-wide linkage scans. After neuregulin 1 (NRG1) was identified as a positional candidate gene for schizophrenia in this locus, several independent association studies have reported controversial results. To determine whether genetic variations in this locus are associated with schizophrenia in the Korean population, we investigated multiplex families and unrelated patients using linkage and association analyses. Seven microsatellite markers in 8p21-12 were genotyped for 40 families with schizophrenia, and a non-parametric linkage analysis was applied. The association study was performed with 242 unrelated schizophrenia patients and the same number of normal controls for three single nucleotide polymorphisms (SNPs), two microsatellite markers and their haplotypes. A significant linkage signal was observed on D8S1769, which is located 352 kb upstream of the 5' end of the first exon of NRG1 for two ("narrow" and "narrow with auditory hallucination (AH)") of the three adopted phenotype classes. In the association study, the G allele of SNP8NRG241930 was significantly in excess in the subgroup of patients with AHs. We also found haplotypes which were associated with schizophrenia with a protective effect. This study provides additional suggestive evidence for both the linkage and association of genetic variations on 8p12, a locus of NRG1, with schizophrenia. NRG1 might either play a role in the predisposition to schizophrenia or be in linkage disequilibrium (LD) with a causal locus of this illness.

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.