A phenotype-based genetic association study reveals the contribution of neuregulin1 gene variants to age of onset and positive symptom severity in schizophrenia

The genetic basis of onset age in schizophrenia: evidence and models

Schizophrenia is a heritable neurocognitive disorder affecting about 1% of the population, and usually has an onset age at around 21-25 in males and 25-30 in females. Recent advances in genetics have helped to identify many common and rare variants for the liability to schizophrenia. Earlier evidence appeared to suggest that younger onset age is associated with higher genetic liability to schizophrenia. Clinical longitudinal research also found that early and very-early onset schizophrenia are associated with poor clinical, neurocognitive, and functional profiles. A recent study reported a heritability of 0.33 for schizophrenia onset age, but the genetic basis of this trait in schizophrenia remains elusive. In the pre-Genome-Wide Association Study (GWAS) era, genetic loci found to be associated with onset age were seldom replicated. In the post-Genome-Wide Association Study era, new conceptual frameworks are needed to clarify the role of onset age in genetic research in schizophrenia, and to identify its genetic basis. In this review, we first discussed the potential of onset age as a characterizing/subtyping feature for psychosis, and as an important phenotypic dimension of schizophrenia. Second, we reviewed the methods, samples, findings and limitations of previous genetic research on onset age in schizophrenia. Third, we discussed a potential conceptual framework for studying the genetic basis of onset age, as well as the concepts of susceptibility, modifier, and "mixed" genes. Fourth, we discussed the limitations of this review. Lastly, we discussed the potential clinical implications for genetic research of onset age of schizophrenia, and how future research can unveil the potential mechanisms for this trait.

NRG1, PIP4K2A, and HTR2C as Potential Candidate Biomarker Genes for Several Clinical Subphenotypes of Depression and Bipolar Disorder

GSK3B, BDNF, NGF, NRG1, HTR2C, and PIP4K2A play important roles in molecular mechanisms of psychiatric disorders. GSK3B occupies a central position in these molecular mechanisms and is also modulated by psychotropic drugs. BDNF regulates a number of key aspects in neurodevelopment and synaptic plasticity. NGF exerts a trophic action and is implicated in cerebral alterations associated with psychiatric disorders. NRG1 is active in neural development, synaptic plasticity, and neurotransmission. HTR2C is another important psychopharmacological target. PIP4K2A catalyzes the phosphorylation of PI5P to form PIP2, the latter being implicated in various aspects of neuronal signal transduction. In the present study, the six genes were sequenced in a cohort of 19 patients with bipolar affective disorder, 41 patients with recurrent depressive disorder, and 55 patients with depressive episode. The study revealed a number of genetic variants associated with antidepressant treatment response, time to recurrence of episodes, and depression severity. Namely, alleles of rs35641374 and rs10508649 (NRG1 and PIP4K2A) may be prognostic biomarkers of time to recurrence of depressive and manic/mixed episodes among patients with bipolar affective disorder. Alleles of NC_000008.11:g.32614509_32614510del, rs61731109, and rs10508649 (also NRG1 and PIP4K2A) seem to be predictive biomarkers of response to pharmacological antidepressant treatment on the 28th day assessed by the HDRS-17 or CGI-I scale. In particular, the allele G of rs10508649 (PIP4K2A) may increase resistance to antidepressant treatment and be at the same time protective against recurrent manic/mixed episodes. These results support previous data indicating a biological link between resistance to antidepressant treatment and mania. Bioinformatic functional annotation of associated variants revealed possible impact for transcriptional regulation of PIP4K2A. In addition, the allele A of rs2248440 (HTR2C) may be a prognostic biomarker of depression severity. This allele decreases expression of the neighboring immune system gene IL13RA2 in the putamen according to the GTEx portal. The variant rs2248440 is near rs6318 (previously associated with depression and effects of psychotropic drugs) that is an eQTL for the same gene and tissue. Finally, the study points to several protein interactions relevant in the pathogenesis of mood disorders. Functional studies using cellular or animal models are warranted to support these results.

BACE1-Dependent Neuregulin-1 Signaling: An Implication for Schizophrenia

Schizophrenia is a chronic psychiatric disorder with a lifetime prevalence of about 1% in the general population. Recent studies have shown that Neuregulin-1 (Nrg1) is a candidate gene for schizophrenia. At least 15 alternative splicing of NRG1 isoforms all contain an extracellular epidermal growth factor (EGF)-like domain, which is sufficient for Nrg1 biological activity including the formation of myelin sheaths and the regulation of synaptic plasticity. It is known that Nrg1 can be cleaved by β-secretase (BACE1) and the resulting N-terminal fragment (Nrg1-ntf) binds to receptor tyrosine kinase ErbB4, which activates Nrg1/ErbB4 signaling. While changes in Nrg1 expression levels in schizophrenia still remain controversial, understanding the BACE1-cleaved Nrg1-ntf and Nrg1/ErbB4 signaling in schizophrenia neuropathogenesis is essential and important. In this review paper, we included three major parts: (1) Nrg1 structure and cleavage pattern by BACE1; (2) BACE1-dependent Nrg1 cleavage associated with schizophrenia in human studies; and (3) Animal studies of Nrg1 and BACE1 mutations with behavioral observations. Our review will provide a better understanding of Nrg1 in schizophrenia and a potential strategy for using BACE1 cleavage of Nrg1 as a unique biomarker for diagnosis, as well as a new therapeutic target, of schizophrenia.

A systematic meta-analysis of the association of Neuregulin 1 (NRG1), D-amino acid oxidase (DAO), and DAO activator (DAOA)/G72 polymorphisms with schizophrenia

The glutamate hypothesis of schizophrenia is related to the proposed dysregulation of D-amino acid oxidase (DAO), DAO activator (DAOA)/G72, and Neuregulin 1 (NRG1) genes. Genetic studies have shown significant associations between DAO, DAOA, NRG1 single-nucleotide polymorphisms (SNPs), and schizophrenia. The systematic literature search yielded 6, 5, and 18 new studies for DAO, DAOA, and NRG1 published after 2011 and not included in the previous SchizophreniaGene (SZGene) meta-analysis. We conducted meta-analyses of 20, 23, and 48 case-control studies, respectively, to comprehensively evaluate the association of 8 DAO, 12 DAOA, and 14 NRG1 SNPs with schizophrenia. The updated meta-analyses resulted in the following findings: the C-allele of DAO rs4623951 was associated with schizophrenia across all pooled studies [Odds ratio (OR) = 0.88, 95% confidence interval (CI) = 0.79-0.98, p = 0.02, N = 3143]; however, no new reports could be included. The G-allele of DAOA rs778293 was associated with schizophrenia in Asian patients (OR = 1.17, 95% CI = 1.08-1.27, p = 0.00008, N = 6117), and the T-allele of DAOA rs3916971 was associated with schizophrenia across all studies (OR = 0.84, 95% CI = 0.73-0.96, p = 0.01, N = 1765). Again, for both SNPs, no new eligible studies were available. After adding new reports, the T-allele of NRG1 SNP8NRG241930 (rs62510682) across all studies (OR = 0.95, 95% CI = 0.91-0.997, p = 0.04, N = 22,898) and in Caucasian samples (OR = 0.95, 95% CI = 0.90-0.99, p = 0.03, N = 16,014), and the C-allele of NRG1 rs10503929 across all studies (OR = 0.89, 95% CI = 0.81-0.97, p = 0.01, N = 6844) and in Caucasian samples (OR = 0.89, 95% CI = 0.81-0.98, p = 0.01, N = 6414) were protective against schizophrenia. Our systematic meta-analysis is the most updated one for the association of DAO, DAOA, and NRG1 SNPs with schizophrenia.

Neuregulin-ERBB signaling in the nervous system and neuropsychiatric diseases

Neuregulins (NRGs) comprise a large family of growth factors that stimulate ERBB receptor tyrosine kinases. NRGs and their receptors, ERBBs, have been identified as susceptibility genes for diseases such as schizophrenia (SZ) and bipolar disorder. Recent studies have revealed complex Nrg/Erbb signaling networks that regulate the assembly of neural circuitry, myelination, neurotransmission, and synaptic plasticity. Evidence indicates there is an optimal level of NRG/ERBB signaling in the brain and deviation from it impairs brain functions. NRGs/ERBBs and downstream signaling pathways may provide therapeutic targets for specific neuropsychiatric symptoms.

Phenotype-Based Genetic Association Studies (PGAS)-Towards Understanding the Contribution of Common Genetic Variants to Schizophrenia Subphenotypes

Neuropsychiatric diseases ranging from schizophrenia to affective disorders and autism are heritable, highly complex and heterogeneous conditions, diagnosed purely clinically, with no supporting biomarkers or neuroimaging criteria. Relying on these "umbrella diagnoses", genetic analyses, including genome-wide association studies (GWAS), were undertaken but failed to provide insight into the biological basis of these disorders. "Risk genotypes" of unknown significance with low odds ratios of mostly <1.2 were extracted and confirmed by including ever increasing numbers of individuals in large multicenter efforts. Facing these results, we have to hypothesize that thousands of genetic constellations in highly variable combinations with environmental co-factors can cause the individual disorder in the sense of a final common pathway. This would explain why the prevalence of mental diseases is so high and why mutations, including copy number variations, with a higher effect size than SNPs, constitute only a small part of variance. Elucidating the contribution of normal genetic variation to (disease) phenotypes, and so re-defining disease entities, will be extremely labor-intense but crucial. We have termed this approach PGAS ("phenotype-based genetic association studies"). Ultimate goal is the definition of biological subgroups of mental diseases. For that purpose, the GRAS (Göttingen Research Association for Schizophrenia) data collection was initiated in 2005. With >3000 phenotypical data points per patient, it comprises the world-wide largest currently available schizophrenia database (N > 1200), combining genome-wide SNP coverage and deep phenotyping under highly standardized conditions. First PGAS results on normal genetic variants, relevant for e.g., cognition or catatonia, demonstrated proof-of-concept. Presently, an autistic subphenotype of schizophrenia is being defined where an unfortunate accumulation of normal genotypes, so-called pro-autistic variants of synaptic genes, explains part of the phenotypical variance. Deep phenotyping and comprehensive clinical data sets, however, are expensive and it may take years before PGAS will complement conventional GWAS approaches in psychiatric genetics.

Common variant at 16p11.2 conferring risk of psychosis

Epidemiological and genetic data support the notion that schizophrenia and bipolar disorder share genetic risk factors. In our previous genome-wide association study, meta-analysis and follow-up (totaling as many as 18 206 cases and 42 536 controls), we identified four loci showing genome-wide significant association with schizophrenia. Here we consider a mixed schizophrenia and bipolar disorder (psychosis) phenotype (addition of 7469 bipolar disorder cases, 1535 schizophrenia cases, 333 other psychosis cases, 808 unaffected family members and 46 160 controls). Combined analysis reveals a novel variant at 16p11.2 showing genome-wide significant association (rs4583255[T]; odds ratio=1.08; P=6.6 × 10(-11)). The new variant is located within a 593-kb region that substantially increases risk of psychosis when duplicated. In line with the association of the duplication with reduced body mass index (BMI), rs4583255[T] is also associated with lower BMI (P=0.0039 in the public GIANT consortium data set; P=0.00047 in 22 651 additional Icelanders).

Schizophrenia-associated HapICE haplotype is associated with increased NRG1 type III expression and high nucleotide diversity

Excitement and controversy have followed neuregulin (NRG1) since its discovery as a putative schizophrenia susceptibility gene; however, the mechanism of action of the associated risk haplotype (HapICE) has not been identified, and specific genetic variations, which may increase risk to schizophrenia have remained elusive. Using a postmortem brain cohort from 37 schizophrenia cases and 37 controls, we resequenced upstream of the type I-IV promoters, and the HapICE repeat regions in intron 1. Relative abundance of seven NRG1 mRNA transcripts in the prefrontal cortex were determined and compared across diagnostic and genotypic groups. We identified 26 novel DNA variants and showed an increased novel variant load in cases compared with controls (χ(2)=7.815; P=0.05). The average nucleotide diversity (θ = 10.0 × 10(-4)) was approximately twofold higher than that previously reported for BDNF, indicating that NRG1 may be particularly prone to genetic change. A greater nucleotide diversity was observed in the HapICE linkage disequilibrium block in schizophrenia cases (θ((case)) = 13.2 × 10(-4); θ((control)) = 10.0 × 10(-4)). The specific HapICE risk haplotype was associated with increased type III mRNA (F = 3.76, P = 0.028), which in turn, was correlated with an earlier age of onset (r = -0.343, P = 0.038). We found a novel intronic five-SNP haplotype ~730 kb upstream of the type I promoter and determined that this region functions as transcriptional enhancer that is suppressed by SRY. We propose that the HapICE risk haplotype increases expression of the most brain-abundant form of NRG1, which in turn, elicits an earlier clinical presentation, thus providing a novel mechanism through which this genetic association may increase risk of schizophrenia.

Environmental risk factors and their impact on the age of onset of schizophrenia: Comparing familial to non-familial schizophrenia

BACKGROUND AND AIMS

Several risk factors for schizophrenia have yet been identified. The aim of our study was to investigate how certain childhood and adolescent risk factors predict the age of onset of psychosis in patients with and without a familial component (i.e. a relative with schizophrenia or schizoaffective disorder).

METHODS

Aside from the age of onset of psychosis, we examined the risk factors for schizophrenia including obstetric complications, birth during winter or spring, behavioral deviances or delayed motor and speech development, exposure to adverse life events and exposure to substance use within a group of 100 patients (45 female, 55 male) with a mean age (± standard deviation) of 35.15 ± 13.21.

RESULTS

Birth complications and cannabis abuse are predictors for an earlier onset of schizophrenia in patients with non-familial schizophrenia. No environmental risk factors for an earlier age of onset in familial schizophrenia have been identified.

CONCLUSIONS

Certain environmental risk factors for schizophrenia seem to have an impact on the age of onset of psychosis in non-familial schizophrenia, they do not seem to have an impact on familial schizophrenia.

Lack of effect of brain derived neurotrophic factor (BDNF) Val66Met polymorphism on early onset schizophrenia in Chinese Han population

Schizophrenia is a chronic psychiatric disorder with high heritability. Schizophrenic patients with early-age onset tend to have a greater genetic component and may be an attractive subpopulation for genetic studies. Brain-derived neurotrophic factor (BDNF) is considered a candidate gene for schizophrenia. A single nucleotide polymorphism (BDNF Val66Met) was reported to be associated with schizophrenia, although discrepancy remains. The aim of this study was to evaluate the association between BDNF Val66Met polymorphism and schizophrenia using an early onset sample in the Chinese Han population. Our sample consisted of 353 schizophrenic patients with onset before age 18 and 394 healthy controls. All subjects were of an ethnically homogenous Han Chinese origin. No significant differences of genotype or allele distribution were identified between the patients and controls. However, the Met allele was significantly associated with an earlier age of onset in male schizophrenic patients (Kaplan-Meier log-rank test P=0.005), but not in females (P=0.289). The BDNF Val66Met polymorphism has an important effect on the age of onset of schizophrenia in a gender-specific manner. This may represent a significant genetic clue for the etiology of schizophrenia and thus, further studies are required to uncover the exact role of BDNF in the development of schizophrenia.