Linkage disequilibrium analyses in the Costa Rican population suggests discrete gene loci for schizophrenia at 8p23.1 and 8q13.3

Systematic exploration of a decade of publications on psychiatric genetics in Latin America

Psychiatric disorders have a great impact in terms of mortality, morbidity, and disability across the lifespan. Considerable effort has been devoted to understanding their complex and heterogeneous genetic architecture, including diverse ancestry populations. Our aim was to review the psychiatric genetics research published with Latin American populations from 2010 to 2019, and classify it according to country of origin, type of analysis, source of funding, and other variables. We found that most publications came from Brazil, Mexico, and Colombia. Also, local funds are generally not large enough for genome-wide studies in Latin America, with the exception of Brazil and Mexico; larger studies are often done in collaboration with international partners, mostly funded by US agencies. In most of the larger studies, the participants are individuals of Latin American ancestry living in the United States, which limits the potential for exploring the complex gene-environment interaction. Family studies, traditionally strong in Latin America, represent about 30% of the total research publications. Scarce local resources for research in Latin America have probably been an important limitation for conducting bigger and more complex studies, contributing to the reduced representation of these populations in global psychiatric genetics studies. Increasing diversity must be a goal to improve generalizability and applicability in clinical settings.

Convergent genomic and pharmacological evidence of PI3K/GSK3 signaling alterations in neurons from schizophrenia patients

Human-induced pluripotent stem cells (hiPSCs) allow for the establishment of brain cellular models of psychiatric disorders that account for a patient's genetic background. Here, we conducted an RNA-sequencing profiling study of hiPSC-derived cell lines from schizophrenia (SCZ) subjects, most of which are from a multiplex family, from the population isolate of the Central Valley of Costa Rica. hiPSCs, neural precursor cells, and cortical neurons derived from six healthy controls and seven SCZ subjects were generated using standard methodology. Transcriptome from these cells was obtained using Illumina HiSeq 2500, and differential expression analyses were performed using DESeq2 (|fold change|>1.5 and false discovery rate < 0.3), in patients compared to controls. We identified 454 differentially expressed genes in hiPSC-derived neurons, enriched in pathways including phosphoinositide 3-kinase/glycogen synthase kinase 3 (PI3K/GSK3) signaling, with serum-glucocorticoid kinase 1 (SGK1), an inhibitor of glycogen synthase kinase 3β, as part of this pathway. We further found that pharmacological inhibition of downstream effectors of the PI3K/GSK3 pathway, SGK1 and GSK3, induced alterations in levels of neurite markers βIII tubulin and fibroblast growth factor 12, with differential effects in patients compared to controls. While demonstrating the utility of hiPSCs derived from multiplex families to identify significant cell-specific gene network alterations in SCZ, these studies support a role for disruption of PI3K/GSK3 signaling as a risk factor for SCZ.

Decreased plasma levels of neureglin-1 in drug naïve patients and chronic patients with schizophrenia

Although the neuregulin-1 (NRG1) gene is one of the susceptibility genes for schizophrenia and various other psychiatric diseases, it remains unclear how individual psychiatric diseases affect the expression of the NRG1 protein in patients. A previous study reported a schizophrenia-linked decrease in serum NRG1 levels. The present study aimed to replicate this initial finding and to assess its disease specificity for schizophrenia. We collected plasma samples from drug-naïve patients with first-episode schizophrenia (n=80), patients with chronic schizophrenia (n=86), patients with bipolar I disorder (n=60), patients with bipolar II disorder (n=60) and patients with major depressive disorder (n=60), we measured the plasma levels of NRG1β1 and compared the levels with those of age- and sex-matched healthy volunteers (n=82). One-way ANOVA and post hoc analyses detected specific NRG1β1 decreases in the participants with first-episode and chronic schizophrenia but not in those with bipolar I disorder, bipolar II disorder or major depressive disorder. The mean plasma levels of NRG1β1 immunoreactivity were 4.27±0.71 ng/mL in the participants with first-episode schizophrenia, 4.08±0.64 ng/mL in the participants with chronic schizophrenia and 7.21±0.91 ng/mL in the healthy controls. Although we analyzed the pathological correlations of NRG1β1 immunoreactivity in terms of the clinical parameters of the sample, we observed only weak positive correlations with the age of the participants with chronic schizophrenia and the disease onset times of the participants with bipolar II disorder. We failed to identify correlations between other clinical parameters and plasma NRG1β1 immunoreactivity among all patient subjects. These findings suggest that NRG1 may serve as a relatively specific disease marker for schizophrenia. However, the pathological role of this decrease must be explored further.

Neuropsychological and dimensional behavioral trait profiles in Costa Rican ADHD sib pairs: Potential intermediate phenotypes for genetic studies

Attention deficit hyperactivity disorder (ADHD) is associated with substantial functional impairment in children and in adults. Many individuals with ADHD have clear neurocognitive deficits, including problems with visual attention, processing speed, and set shifting. ADHD is etiologically complex, and although genetic factors play a role in its development, much of the genetic contribution to ADHD remains unidentified. We conducted clinical and neuropsychological assessments of 294 individuals (269 with ADHD) from 163 families (48 multigenerational families created using genealogical reconstruction, 78 affected sib pair families, and 37 trios) from the Central Valley of Costa Rica (CVCR). We used principal components analysis (PCA) to group neurocognitive and behavioral variables using the subscales of the Child Behavior Checklist (CBCL) and 15 neuropsychological measures, and created quantitative traits for heritability analyses. We identified seven cognitive and two behavioral domains. Individuals with ADHD were significantly more impaired than their unaffected siblings on most behavioral and cognitive domains. The verbal IQ domain had the highest heritability (92%), followed by auditory attention (87%), visual processing speed and problem solving (85%), and externalizing symptoms (81%). The quantitative traits identified here have high heritabilities, similar to the reported heritability of ADHD (70-90%), and may represent appropriate alternative phenotypes for genetic studies. The use of multigenerational families from a genetically isolated population may facilitate the identification of ADHD risk genes in the face of phenotypic and genetic heterogeneity.

Evidence of allelic imbalance in the schizophrenia susceptibility gene ZNF804A in human dorsolateral prefrontal cortex

The rs1344706, an intronic SNP within the zinc-finger protein 804A gene (ZNF804A), was identified as one of the most compelling risk SNPs for schizophrenia (SZ) and bipolar disorder (BD). It is however not clear by which molecular mechanisms ZNF804A increases disease risk. We evaluated the role of ZNF804A in SZ and BD by genotyping the originally associated rs1344706 SNP and an exonic SNP (rs12476147) located in exon four of ZNF804A in a sample of 422 SZ, 382 BD, and 507 controls from the isolated population of the Costa Rica Central Valley. We also investigated the rs1344706 SNP for allelic specific expression (ASE) imbalance in the dorsolateral prefrontal cortex (DLPFC) of 46 heterozygous postmortem brains. While no significant association between rs1344706 and SZ or BD was observed in the Costa Rica sample, we observed an increased risk of SZ for the minor allele (A) of the exonic rs12476147 SNP (p=0.026). Our ASE assay detected a significant over-expression of the rs12476147 A allele in DLPFC of rs1344706 heterozygous subjects. Interestingly, cDNA allele ratios were significantly different according to the intronic rs1344706 genotypes (p-value=0.03), with the rs1344706 A allele associated with increased ZNF804A rs12476147 A allele expression (average 1.06, p-value=0.02, for heterozygous subjects vs. genomic DNA). In conclusion, we have demonstrated a significant association of rs12476147 with SZ, and using a powerful within-subject design, an allelic expression imbalance of ZNF804A exonic SNP rs12476147 in the DLPFC. Although this data does not preclude the possibility of other functional variants in ZNF804A, it provides evidence that the rs1344706 SZ risk allele is the cis-regulatory variant directly responsible for this allelic expression imbalance in adult cortex.

Genome-wide association analysis of age at onset in schizophrenia in a European-American sample

We performed a genome-wide association analysis to identify genetic variants influencing age at onset (AAO) and examine gene × gender interactions for AAO in schizophrenia (SCZ) using a European-American sample (1,162 cases). Linear regression model in PLINK was used to test for associations with AAO while the GxE option was chosen to test for the influence of gene × gender interactions. The most significant association with AAO was observed with SNP rs7819815 (P = 3.10×10(-7)) at 8q24.22. The next best signal was at 4q25 in COL25A1 gene (rs17039583, P = 4.30×10(-6)) and the third region was at 4p16.1 (rs17407555, P = 4.56×10(-6) , near RAF1P1, and rs4697924, P = 1.23×10(-5) within WDR1 gene). Conditional analysis on chromosome 4 indicated that 4p16.1 and 4q25 loci were independent. Furthermore, 2 SNPs (rs16834822 and rs16834824) at 1q43 in RYR2 showed strong associations in the female sample (P = 2.10×10(-6) and 2.33×10(-6) , respectively) and strong gene × gender interactions in influencing AAO (P = 9.23×10(-7) and 1.15×10(-6) , respectively) while the second best region showing gene × gender interaction was at 7q22.3 (rs179863, P = 2.33×10(-6) ). Using an independent sample of 1,068 cases, we could not replicate the associations for above top SNPs; however, we found nominal significance associations for their flanking SNPs (P < 0.05). These findings provide evidence of several genetic variants influencing AAO of SCZ.

Lack of association to a NRG1 missense polymorphism in schizophrenia or bipolar disorder in a Costa Rican population

A missense polymorphism in the NRG1 gene, Val>Leu in exon 11, was reported to increase the risk of schizophrenia in selected families from the Central Valley region of Costa Rica (CVCR). The present study investigated the relationship between three NRG1 genetic variants, rs6994992, rs3924999, and Val>Leu missense polymorphism in exon 11, in cases and selected controls from an isolated population from the CVCR. Isolated populations can have less genetic heterogeneity and increase power to detect risk variants in candidate genes. Subjects with bipolar disorder (BD, n=358), schizophrenia (SZ, n=273), or unrelated controls (CO, n=479) were genotyped for three NRG1 variants. The NRG1 promoter polymorphism (rs6994992) was related to altered expression of NRG1 Type IV in other studies. The expression of NRG1 type IV in the dorsolateral prefrontal cortex (DLPFC) and the effect of the rs6994992 genotype on expression were explored in a postmortem cohort of BD, SZ, major depressive disorder (MDD) cases, and controls. The missense polymorphism Val>Leu in exon 11 was not significantly associated with schizophrenia as previously reported in a family sample from this population, the minor allele frequency is 4%, thus our sample size is not large enough to detect an association. We observed however an association of rs6994992 with NRG1 type IV expression in DLPFC and a significantly decreased expression in MDD compared to controls. The present results while negative do not rule out a genetic association of these SNPs with BD and SZ in CVCR, perhaps due to small risk effects that we were unable to detect and potential intergenic epistasis. The previous genetic relationship between expression of a putative brain-specific isoform of NRG1 type IV and SNP variation was replicated in postmortem samples in our preliminary study.

A genome-wide association study for quantitative traits in schizophrenia in China

Few genome-wide association studies (GWAS) of schizophrenia have included Chinese populations, and verification of positive genetic findings from other ethnic groups is rare in Chinese groups. We used fluid intelligence as the quantitative trait reflecting schizophrenia dysfunction in Chinese populations, and determined the impact of genetic variation on fluid intelligence phenotypic patterns to identify genetic influences in schizophrenia. The study sample comprised 98 patients with schizophrenia and 60 healthy controls. The general fluid intelligence of participants was assessed with Cattell's Culture-Free Intelligence Test (CCFIT). Subjects were genotyped using the Illumina HumanHap 660 beadchip. We identified the methionine sulfoxide reductase A (MSRA) gene on chromosome 8 as having an association with fluid intelligence. However, only CCFIT subtest 1 (series score) demonstrated a significant result for the interaction term using the criteria of the quantitative trait (QT) analysis of 10(-5) for at least three SNPs. There were 15 haplotype blocks of MSRA gene SNPs identified using Haploview 4.2 with solid spine D' > 0.80. The strongest QT interaction was noted in Block 3, with the most common haplotypes being AAACAGCAG and CGCAGAAGA. In conclusion, we report data from a GWAS with quantitative traits design from Chinese first-episode schizophrenia patients and matched controls. Although the gene identified requires confirmation in an independent sample, the MSRA gene located on chromosome 8 was found to be associated with the phenotype of schizophrenia.

Genome-scan for bipolar disorder with sib-pair families in the Sardinian population: a new susceptibility locus on chromosome 1p22-p21?

The discovery of the genetic factors implicated in the predisposition to complex diseases may greatly profit from genetic studies in isolated populations. In this perspective, we performed a genome-wide scan using 507 microsatellite markers, with an average interval size of 7.6 cM, on a sample of 88 nuclear families with at least two affected sibs with bipolar disorder recruited in the Sardinian population. An initial analysis yielded non-parametric linkage exceeding 3.4 with P-values <0.0003 at two adjacent markers, D1S206 and D1S435 in the 1p22-p21 chromosomal region. Moreover, positive linkage ranging between 2.0 and 3.0 was obtained for other loci in several cases in regions that have already been linked to predisposition to bipolar disorder, such as 5p15.33, 8q24.13, and 11q14.3. A subsequent analysis of the 1p22-p21 region using the same set of families and a dense panel of 20 new microsatellite markers, spaced at 1.2 cM on average, reinforced the finding of suggestive linkage for this region. Interestingly, NPL values above 2.1 and P-values <0.02 were obtained for a cluster of 10 markers comprising D1S435. Thus, this study suggests that the 1p22-p21 region may contain a new locus participating to the genetic susceptibility to bipolar disorder and reproduces positive linkage for several other loci already implicated in this pathology. Since the Sardinian population presents a peculiar genetic homogeneity, these results may pave the way to further studies for replication in this population contributing to the rapid discovery of the genetic factors predisposing to bipolar disorder.

Copy number variations associated with idiopathic autism identified by whole-genome microarray-based comparative genomic hybridization

OBJECTIVES

Autism spectrum disorder (ASD) has been thought to have strong genetic background, but major contributing genes or associated molecular-genetic pathways are yet to be identified. To explore the idiopathic ASD-associated copy number variations (CNVs), we conducted case-control study using whole-genome copy number analysis.

METHODS

Whole-genome microarray-based comparative genomic hybridization was carried out on 28 children (24 boys and four girls) diagnosed as ASD and 62 Korean adults (45 males and 17 females) without any signs of abnormalities and family history of genetic disorders as normal controls. Fluorescence in-situ hybridization and capillary electrophoresis-single-strand conformational polymorphism were used for quantitative verification of the ASD-associated CNVs.

RESULTS

Thirty-eight CNVs were identified. Among them, the distributions of copy number loss CNVs on 8p23.1 (odds ratio: 5.1, 95% confidence interval: 1.7-14.5, P=0.003) and on 17p11.2 (odds ratio: uncalculable because of zero cell, P=0.008) were found to be significantly different between ASD and control groups. DEFENSIN family occurs in a cluster at 8p23.1 region. Fluorescence in-situ hybridization and capillary electrophoresis-single-strand conformational polymorphism coherently showed reduced copy number of DEFENSIN in cases with 8p23.1 copy number loss CNV, which validated microarray-based comparative genomic hybridization results; but there are no known coding genes in the CNV on 17p11.2.

CONCLUSION

Our approach as well as results can help to elucidate the genetic mechanism of idiopathic ASD.

Chromosome 8p as a potential hub for developmental neuropsychiatric disorders: implications for schizophrenia, autism and cancer

Defects in genetic and developmental processes are thought to contribute susceptibility to autism and schizophrenia. Presumably, owing to etiological complexity identifying susceptibility genes and abnormalities in the development has been difficult. However, the importance of genes within chromosomal 8p region for neuropsychiatric disorders and cancer is well established. There are 484 annotated genes located on 8p; many are most likely oncogenes and tumor-suppressor genes. Molecular genetics and developmental studies have identified 21 genes in this region (ADRA1A, ARHGEF10, CHRNA2, CHRNA6, CHRNB3, DKK4, DPYSL2, EGR3, FGF17, FGF20, FGFR1, FZD3, LDL, NAT2, NEF3, NRG1, PCM1, PLAT, PPP3CC, SFRP1 and VMAT1/SLC18A1) that are most likely to contribute to neuropsychiatric disorders (schizophrenia, autism, bipolar disorder and depression), neurodegenerative disorders (Parkinson's and Alzheimer's disease) and cancer. Furthermore, at least seven nonprotein-coding RNAs (microRNAs) are located at 8p. Structural variants on 8p, such as copy number variants, microdeletions or microduplications, might also contribute to autism, schizophrenia and other human diseases including cancer. In this review, we consider the current state of evidence from cytogenetic, linkage, association, gene expression and endophenotyping studies for the role of these 8p genes in neuropsychiatric disease. We also describe how a mutation in an 8p gene (Fgf17) results in a mouse with deficits in specific components of social behavior and a reduction in its dorsomedial prefrontal cortex. We finish by discussing the biological connections of 8p with respect to neuropsychiatric disorders and cancer, despite the shortcomings of this evidence.

Linkage analysis of schizophrenia in African-American families

While many studies have sought a window into the genetics of schizophrenia, few have focused on African-American families. An exception is the Project among African-Americans to Explore Risks for Schizophrenia (PAARTNERS), which seeks to identify novel and known risk variation for schizophrenia by genetic analyses of African-American families. We report a linkage study of diagnostic status in 217 African-American families using the Illumina Linkage Panel. Due to assumed incomplete and time-dependent penetrance, we performed linkage analysis using two different treatments of diagnosis: (1) treating both affected and unaffected individuals as informative for linkage (using the program SIBPAL) and (2) treating only affected individuals as informative (using the program MERLIN). We also explore three definitions of affected status: narrowly defined schizophrenia; one broadened to include schizoaffective disorder; and another including all diagnoses indicating psychosis. Several regions show a decrease in the evidence for linkage as the definition broadens 8q22.1 (rs911, 99.26 cM; SIBPAL p-value [p] goes from 0.006 to 0.02), 16q24.3 (rs1006547, 130.48 cM; p from 0.00095 to 0.0085), and 20q13.2 (rs1022689, 81.73 cM; p from 0.00015 to 0.032). One region shows a substantial increase in evidence for linkage, 11p15.2 (rs722317, 24.27 cM; p from 0.0022 to 0.0000003); MERLIN results support the significance of the SIBPAL results (p=0.00001). Our linkage results overlap two broad, previously-reported linkage regions: 8p23.3-p12 found in studies sampling largely families of European ancestry; and 11p11.2-q22.3 reported by a study of African-American families. These results should prove quite useful for uncovering loci affecting risk for schizophrenia.

Methionine sulfoxide reductase: a novel schizophrenia candidate gene

Methionine sulfoxide reductase (MSRA) is an antioxidant enzyme implicated in protection against oxidative stress and protein maintenance. We have previously reported the association of marker D8S542, located within the MSRA gene, with schizophrenia in the Central Valley of Costa Rica (CVCR). By performing fine mapping analysis, we have now identified a potential three-marker at risk haplotype within MSRA in the same CVCR sample, with a global P-value slightly above nominal significance (P = 0.0526). By sequencing the MSRA gene in individuals carrying this haplotype, we identified a novel 4-base pair deletion 1,792 bases upstream of the MSRA transcription start site. This deletion was significantly under-transmitted to schizophrenia patients in the CVCR sample (P = 0.0292) using FBAT, and this was replicated in a large independent sample of 321 schizophrenia families from the Hispanic population (P = 0.0367). These findings suggest a protective effect of the deletion against schizophrenia. Further, MSRA mRNA levels were significantly lower in lymphoblastoid cell lines of individuals homozygous for the deletion compared to carriers of the normal allele (P = 0.0135), although significance was only evident when genotypes were collapsed. This suggests that the deleted sequence may play a role in regulating MSRA expression. In conclusion, this work points towards MSRA as a novel schizophrenia candidate gene. Further studies into the mechanisms by which MSRA is involved in schizophrenia pathophysiology may shed light into the biological underpinnings of this disorder.

A reanalysis of 409 European-Ancestry and African American schizophrenia pedigrees reveals significant linkage to 8p23.3 with evidence of locus heterogeneity

The detection and replication of schizophrenia risk loci can require substantial sample sizes, which has prompted various collaborative efforts for combining multiple samples. However, pooled samples may comprise sub-samples with substantial population genetic differences, including allele frequency differences. We investigated the impact of population differences via linkage reanalysis of Molecular Genetics of Schizophrenia 1 (MGS1) affected sibling-pair data, comprising two samples of distinct ancestral origin: European (EA: 263 pedigrees) and African-American (AA: 146 pedigrees). To exploit the linkage information contained within these distinct continental samples, we performed separate analyses of the individual samples, allowing for within-sample locus heterogeneity, and the pooled sample, allowing for both within-sample and between-sample heterogeneity. Significance levels, corrected for the multiple tests, were determined empirically. For all suggestive peaks, stronger linkage evidence was obtained in either the EA or AA sample than the combined sample, regardless of how heterogeneity was modeled for the latter. Notably, we report genomewide significant linkage of schizophrenia to 8p23.3 and evidence for a second, independent susceptibility locus, reaching suggestive linkage, 29 cM away on 8p21.3. We also detected suggestive linkage on chromosomes 5p13.3 and 7q36.2. Many regions showed pronounced differences in the extent of linkage between the EA and AA samples. This reanalysis highlights the potential impact of population differences upon linkage evidence in pooled data and demonstrates a useful approach for the analysis of samples drawn from distinct continental groups.

Genomic imprinting in the development and evolution of psychotic spectrum conditions

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

The genetics of bipolar disorder: genome 'hot regions,' genes, new potential candidates and future directions

Bipolar disorder (BP) is a complex disorder caused by a number of liability genes interacting with the environment. In recent years, a large number of linkage and association studies have been conducted producing an extremely large number of findings often not replicated or partially replicated. Further, results from linkage and association studies are not always easily comparable. Unfortunately, at present a comprehensive coverage of available evidence is still lacking. In the present paper, we summarized results obtained from both linkage and association studies in BP. Further, we indicated new potential interesting genes, located in genome 'hot regions' for BP and being expressed in the brain. We reviewed published studies on the subject till December 2007. We precisely localized regions where positive linkage has been found, by the NCBI Map viewer (http://www.ncbi.nlm.nih.gov/mapview/); further, we identified genes located in interesting areas and expressed in the brain, by the Entrez gene, Unigene databases (http://www.ncbi.nlm.nih.gov/entrez/) and Human Protein Reference Database (http://www.hprd.org); these genes could be of interest in future investigations. The review of association studies gave interesting results, as a number of genes seem to be definitively involved in BP, such as SLC6A4, TPH2, DRD4, SLC6A3, DAOA, DTNBP1, NRG1, DISC1 and BDNF. A number of promising genes, which received independent confirmations, and genes that have to be further investigated in BP, have been also systematically listed. In conclusion, the combination of linkage and association approaches provided a number of liability genes. Nevertheless, other approaches are required to disentangle conflicting findings, such as gene interaction analyses, interaction with psychosocial and environmental factors and, finally, endophenotype investigations.

The myelin-pathogenesis puzzle in schizophrenia: a literature review

Schizophrenia is a serious and disabling mental disorder with symptoms such as auditory hallucinations, disordered thinking and delusions, avolition, anhedonia, blunted affect and apathy. In this review article we seek to present the current scientific findings from linkage studies and susceptible genes and the pathophysiology of white matter in schizophrenia. The article has been reviewed in two parts. The first part deals with the linkage studies and susceptible genes in schizophrenia in order to have a clear-cut picture of the involvement of chromosomes and their genes in schizophrenia. The genetic linkage results seem to be replicated in some cases but in others are not. From these results, we cannot draw a fine map to a single locus or gene, leading to the conclusion that schizophrenia is not caused by a single factor/gene. In the second part of the article we present the oligodendrocyte-related genes that are associated with schizophrenia, as we hypothesize a potential role of oligodendrocyte-related genes in the pathology of the disorder.