Association between PRODH and schizophrenia is not confirmed

Genetics of schizophrenia (Review)

A comprehensive review of the body of genetic studies on schizophrenia seems even more daunting than the battle a psychiatrist wages daily in the office with her archenemy of a thousand faces. The following article reunites some genetic, epigenetic and environmental factors of schizophrenia from revered and vast studies in a chronological and progressive fashion. Twin studies set the basics of heritability and a particular study by Davis and Phelps considers the widely ignored influence of prenatal environment in the development of schizophrenia. Mostly ignited by linkage studies, candidate gene studies explore further by fine-mapping the hypothesized variants [mostly in the forms single nucleotide polymorphisms (SNPs) and less but with greater impact copy number variations (CNVs)] associated with the disease. Genome-wide association studies (GWAS) increase considerably the sample sizes and thus the validity of the results, while the next-generation sequencing (NGS) attain the highest yet unreplicated level of validity results.

Linkage and Association Analyses of Schizophrenia with Genetic Variations on Chromosome 22q11 in Koreans

OBJECTIVE

Chromosome 22q11 has been implicated as a susceptibility locus of schizophrenia. It also contains various candidate genes for which evidence of association with schizophrenia has been reported. To determine whether genetic variations in chromosome 22q11 are associated with schizophrenia in Koreans, we performed a linkage analysis and case-control association study.

METHODS

Three microsatellite markers within a region of 4.35 Mb on 22q11 were genotyped for 47 multiplex schizophrenia families, and a non-parametric linkage analysis was applied. The association analysis was done with 227 unrelated patients and 292 normal controls. For 39 single nucleotide polymorphisms (SNPs) spanning a 1.4 Mb region (33 kb interval) containing four candidate schizophrenia genes (DGCR, COMT, PRODH and ZDHHC8), allele frequencies were estimated in pooled DNA samples.

RESULTS

No significant linkage was found at any of the three microsatellite markers in single and multi-point analyses. Five SNPs showed suggestive evidence of association (p<0.05) and two more SNPs showed a trend for association (p<0.1) in pooled DNA association analysis. Individual genotyping was performed for those seven SNPs and four more intragenic SNPs. In this second analysis, all of the 11 SNPs individually genotyped did not show significant association.

CONCLUSION

The present study suggests that genetic variations on chromosome 22q11 may not play a major role in Korean schizophrenia patients. Inadequate sample size, densities of genetic markers and differences between location of genetic markers of linkage and association can contribute to an explanation of the negative results of this study.

PRODH rs450046 and proline x COMT Val¹⁵⁸ Met interaction effects on intelligence and startle in adults with 22q11 deletion syndrome

RATIONALE

22q11 deletion syndrome (22q11DS) is associated with an increased risk for psychotic disorders, suggesting a relationship between genotypes and the pathophysiology of psychotic disorders. Two genes in the deleted region, catechol-O-methyl-transferase (COMT) and proline dehydrogenase (oxidase) 1 (PRODH), contain polymorphisms associated with neuropsychiatric phenotypes.

OBJECTIVES

Here, we explored the association between polymorphisms and full-scale intelligence (FSIQ), startle reactivity (SR) and prepulse inhibition (PPI) in adults with 22q11DS.

METHODS

Forty-five adults with 22q11DS were genotyped for PRODH rs450046, rs372055 and COMT Val(158)Met. Plasma proline levels, FSIQ, SR and PPI were measured.

RESULTS

Thirty-five percent of the subjects were hyperprolinemic with a median proline value of 456 μmol/L. C allele carriers of PRODH rs450046 had a lower FSIQ compared to T allele carriers, indicating the C allele to be a risk allele (C allele: mean FSIQ 60.2 (sd 8.7); T allele: mean FSIQ 73.7 (sd 11.5); F 1,43 = 7.59; p = 0.009; partial η (2) = 0.15). A significant interaction effect of proline levels and COMT Val(158)Met genotype was found for SR (F 1,16 = 7.9; p = 0.01; partial η (2) = 0.33), but not for PPI and FSIQ. In subjects with hyperprolinemia, the COMT Val(158)Met genotype effect on SR was stronger than in subjects with normal proline levels.

CONCLUSIONS

Overall, these data provide further evidence for the risk effect of elevated proline levels combined with the COMT Met allele and support the possibilities of using 22q11DS as a model to investigate genotype effects on psychiatric disorders.

Association of COMT and PRODH gene variants with intelligence quotient (IQ) and executive functions in 22q11.2DS subjects

The 22q11.2 deletion syndrome (22q11.2DS) carries the highest genetic risk factor for the development of schizophrenia. We investigated the association of genetic variants in two schizophrenia candidate genes with executive function (EF) and IQ in 22q11.2DS individuals. Ninety two individuals with 22q11.2 deletion were studied for the genetic association between COMT and PRODH variants and EF and IQ. Subjects were divided into children (under 12 years old), adolescents (between 12 and 18 years old) and adults (older than 18 years), and genotyped for the COMT Val158Met (rs4680) and PRODH Arg185Trp (rs4819756) polymorphisms. The participants underwent psychiatric evaluation and EF assessment. Our main finding is a significant influence of the COMT Val158Met polymorphism on both IQ and EF performance. Specifically, 22q11.2DS subjects with Met allele displayed higher IQ scores in all age groups compared to Val carriers, reaching significance in both adolescents and adults. The Met allele carriers performed better than Val carriers in EF tasks, being statistically significant in the adult group. PRODH Arg185Trp variant did not affect IQ or EF in our 22q11.2DS cohort. In conclusion, functional COMT variant, but not PRODH, affects IQ and EF in 22q11.2DS subjects during neurodevelopment with a maximal effect at adulthood. Future studies should monitor the cognitive performance of the same individuals from childhood to old age.

PRODH polymorphisms, cortical volumes and thickness in schizophrenia

Schizophrenia is a neurodevelopmental disorder with high heritability. Several lines of evidence indicate that the PRODH gene may be related to the disorder. Therefore, our study investigates the effects of 12 polymorphisms of PRODH on schizophrenia and its phenotypes. To further evaluate the roles of the associated variants in the disorder, we have conducted magnetic resonance imaging (MRI) scans to assess cortical volumes and thicknesses. A total of 192 patients were evaluated using the Structured Clinical Interview for DSM-IV (SCID), Positive and Negative Syndrome Scale (PANSS), Calgary Depression Scale, Global Assessment of Functioning (GAF) and Clinical Global Impression (CGI) instruments. The study included 179 controls paired by age and gender. The samples were genotyped using the real-time polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP)-PCR and Sanger sequencing methods. A sample of 138 patients and 34 healthy controls underwent MRI scans. One polymorphism was associated with schizophrenia (rs2904552), with the G-allele more frequent in patients than in controls. This polymorphism is likely functional, as predicted by PolyPhen and SIFT, but it was not associated with brain morphology in our study. In summary, we report a functional PRODH variant associated with schizophrenia that may have a neurochemical impact, altering brain function, but is not responsible for the cortical reductions found in the disorder.

Schizophrenia two-hit hypothesis in velo-cardio facial syndrome

Deletion of chr22q11 gives rise to velo-cardio facial syndrome (VCFS) and increases schizophrenia risk. The source of this elevated risk although unknown could result from stochastic, environmental, or genetic factors, the latter encompassing a range of complexity from polygenic mechanisms to "second-hit" mutations. For this study we tested the two-hit hypothesis where additional risk is conferred through a second CNV. We identified large (>100 kb) CNVs in 48 VCFS cases (23 with psychosis--25 without) and show in the psychotic VCFS group there is a significant (P = 0.02) increase in the average size of CNVs (354-227 kb). To identify second-hit loci we focused on individuals possessing gene-centric CNVs and through literature mining identified 4 (31%) psychotic VCFS individuals (n = 13) that overlapped loci previously implicated in neuropsychiatric disorders compared to 1 (10%) from the non-psychotic VCFS individuals (n = 10). For replication 17 VCFS patients with schizophrenia from the molecular genetics of schizophrenia dataset were used to identify further CNVs. Thirteen individuals possessing gene-centric CNVs were identified including 3 (23%) individuals possessing a potential second-hit, taking the overall total in the psychotic VCFS group (n = 26) to 7 (27%) potential second-hit loci. Notably a deletion in a psychotic VCFS patient at 2q23.1 hit the gene MBD5 which when deleted gives rise to intellectual disability, epilepsy, and autistic features. Through this study we potentially extend this phenotypic spectrum to include schizophrenia. Our results suggest the two-hit hypothesis may be relevant to a proportion of VCFS patients with psychosis but sample sizes are small and further studies warranted.

Contribution of nonprimate animal models in understanding the etiology of schizophrenia

Schizophrenia is a severe psychiatric disorder that is characterized by positive and negative symptoms and cognitive impairments. The etiology of the disorder is complex, and it is thought to follow a multifactorial threshold model of inheritance with genetic and neurodevelop mental contributions to risk. Human studies are particularly useful in capturing the richness of the phenotype, but they are often limited to the use of correlational approaches. By assessing behavioural abnormalities in both humans and rodents, nonprimate animal models of schizophrenia provide unique insight into the etiology and mechanisms of the disorder. This review discusses the phenomenology and etiology of schizophrenia and the contribution of current nonprimate animal models with an emphasis on how research with models of neuro transmitter dysregulation, environmental risk factors, neurodevelopmental disruption and genetic risk factors can complement the literature on schizophrenia in humans.

A risk PRODH haplotype affects sensorimotor gating, memory, schizotypy, and anxiety in healthy male subjects

BACKGROUND

Significant associations have been shown for haplotypes comprising three PRODH single nucleotide polymorphisms (SNPs; 1945T/C, 1766A/G, 1852G/A) located in the 3' region of the gene, suggesting a role of these variants in the etiopathogenesis of schizophrenia. We assessed the relationship between these high-risk PRODH polymorphisms and schizophrenia-related endophenotypes in a large and highly homogeneous cohort of healthy males.

METHODS

Participants (n = 217) were tested in prepulse inhibition (PPI), verbal and working memory, trait anxiety and schizotypy. The QTPHASE from the UNPHASED package was used for the association analysis of each SNP or haplotype data. This procedure revealed significant phenotypic impact of the risk CGA haplotype. Subjects were then divided in two groups; levels of PPI, anxiety, and schizotypy, verbal and working memory were compared with analysis of variance.

RESULTS

CGA carriers (n = 32) exhibited attenuated PPI (p < .001) and verbal memory (p < .001) and higher anxiety (p < .004) and schizotypy (p < .008) compared with the noncarriers (n = 185). There were no differences in baseline startle, demographics, and working memory. The main significant correlations were schizotypy x PPI [85-dB, 120-msec trials] in the carriers and schizotypy x anxiety in the entire group and the noncarriers but not the carriers group.

CONCLUSIONS

Our results strongly support PPI as a valid schizophrenia endophenotype and highlight the importance of examining the role of risk haplotypes on multiple endophenotypes and have implications for understanding the continuum from normality to psychosis, transitional states, and the genetics of schizophrenia-related traits.

Functional polymorphisms in PRODH are associated with risk and protection for schizophrenia and fronto-striatal structure and function

PRODH, encoding proline oxidase (POX), has been associated with schizophrenia through linkage, association, and the 22q11 deletion syndrome (Velo-Cardio-Facial syndrome). Here, we show in a family-based sample that functional polymorphisms in PRODH are associated with schizophrenia, with protective and risk alleles having opposite effects on POX activity. Using a multimodal imaging genetics approach, we demonstrate that haplotypes constructed from these risk and protective functional polymorphisms have dissociable correlations with structure, function, and connectivity of striatum and prefrontal cortex, impacting critical circuitry implicated in the pathophysiology of schizophrenia. Specifically, the schizophrenia risk haplotype was associated with decreased striatal volume and increased striatal-frontal functional connectivity, while the protective haplotype was associated with decreased striatal-frontal functional connectivity. Our findings suggest a role for functional genetic variation in POX on neostriatal-frontal circuits mediating risk and protection for schizophrenia.

Schizophrenia is a major mental illness affecting 1% of the population. It is known that genetics plays a role in the disease susceptibility, and it is thought that the illness is a complex disorder involving multiple genes. We show that the schizophrenia susceptibility gene, PRODH, conveys its risk through a variation that increases its enzyme activity. We further show that protection is associated with variations that decrease enzyme activity and these protective variations are enriched in their unaffected siblings. We then used brain imaging of structure and memory function to dissect the risk and protective haplotypes differential effects, and found that the schizophrenia risk haplotype was associated with decreased striatal gray matter volume and increased subcortical to frontal lobe functional connectivity, while the schizophrenia protective haplotype was associated with trend-level increase of frontal lobe volume and decreased subcortical to frontal lobe connectivity. These findings indicate a new target for treating schizophrenia and characterize associated structural and functional deficits.

Candidate genes and the behavioral phenotype in 22q11.2 deletion syndrome

There is an overwhelming evidence that children and adults with 22q11.2 deletion syndrome (22q11.2DS) have a characteristic behavioral phenotype. In particular, there is a growing body of evidence that indicates an unequivocal association between 22q11.2DS and schizophrenia, especially in adulthood. Deletion of 22q11.2 is the third highest risk for the development of schizophrenia, with only a greater risk conferred by being the child of two parents with schizophrenia or the monozygotic co-twin of an affected individual. Both linkage and association studies of people with schizophrenia have implicated several susceptibility genes, of which three are in the 22q11.2 region; catechol-o-methyltransferase (COMT), proline dehydrogenase (PRODH), and Gnb1L. In addition, variation in Gnb1L is associated with the presence of psychosis in males with 22q11.2DS. In mouse models of 22q11.2DS, haploinsufficiency of Tbx1 and Gnb1L is associated with reduced prepulse inhibition, a schizophrenia endophenotype. The study of 22q11.2DS provides an attractive model to increase our understanding of the development and pathogenesis of schizophrenia and other psychiatric disorders in 22q11.2DS and in wider population.

PRODH gene is associated with executive function in schizophrenic families

The aim of this study was to investigate the relationship between polymorphisms in the PRODH and COMT genes and selected neurocognitive functions. Six SNPs in PRODH and two SNPs in COMT were genotyped in 167 first-episode schizophrenic families who had been assessed by a set of 14 neuropsychological tests. Neuropsychological measures were selected as quantitative traits for association analysis. The haplotype of SNPs PRODH 1945T/C and PRODH 1852G/A was associated with impaired performance on the Tower of Hanoi, a problem-solving task mainly reflecting planning capacity. There was no significant evidence for association with any other neuropsychological traits for other SNPs or haplotypes of paired SNPs in the two genes. This study takes previous findings of association between PRODH and schizophrenia further by associating variation within the gene with performance on a neurocognitive trait characteristic of the illness. It fails to confirm previous reports of an association between COMT and cognitive function.

Unravelling the genome: a review of molecular genetic research in schizophrenia

BACKGROUND

Schizophrenia is a common and complex mental illness that affects approximately 1% of the population worldwide. Despite intensive research over the years, the aetiology and pathogenesis of schizophrenia is poorly understood. However, it has long been recognised that schizophrenia is highly familial suggesting a possible genetic aetiology.

AIM

To review recent molecular genetic research in schizophrenia.

METHODS

Medline and Embase search.

RESULTS

Over the past decade, with the completion of the Human Genome Project, molecular genetic research has now identified a number of genes that are very likely to predispose to schizophrenia.

CONCLUSION

This article discusses the methodologies that have been used to identify schizophrenia susceptibility genes and provides a review of recently identified genes thought to play a role in the pathogenesis of this illness.

Analysis of TBX1 variation in patients with psychotic and affective disorders

A significant portion of patients with 22q11 deletion syndrome (22q11DS) develop psychiatric disorders, including schizophrenia and other psychotic and affective symptoms, and the responsible gene/s are assumed to also play a significant role in the etiology of nonsyndromic psychiatric disease. The most common psychiatric diagnosis among patients with 22q11DS is schizophrenia, thought to result from neurotransmitter imbalances and also from disturbed brain development. Several genes in the 22q11 region with known or suspected roles in neurotransmitter metabolism have been analyzed in patients with isolated schizophrenia; however, their contribution to the disease remains controversial. Haploinsufficiency of the TBX1 gene has been shown to be sufficient to cause the core physical malformations associated with 22q11DS in mice and humans and via abnormal brain development could contribute to 22q11DS-related and isolated psychiatric disease. 22q11DS populations also have increased rates of psychiatric conditions other than schizophrenia, including mood disorders. We therefore analyzed variations at the TBX1 locus in a cohort of 446 white patients with psychiatric disorders relevant to 22q11DS and 436 ethnically matched controls. The main diagnoses included schizophrenia (n = 226), schizoaffective disorder (n = 67), bipolar disorder (n = 82), and major depressive disorder (n = 29). We genotyped nine tag SNPs in this sample but did not observe significant differences in allele or haplotype frequencies in any of the analyzed groups (all affected, schizophrenia and schizoaffective disorder, schizophrenia alone, and bipolar disorder and major depressive disorder) compared with the control group. Based on these results we conclude that TBX1 variation does not make a strong contribution to the genetic etiology of nonsyndromic forms of psychiatric disorders commonly seen in patients with 22q11DS.

A quantitative association study between schizotypal traits and COMT, PRODH and BDNF genes in a healthy Chinese population

Previous studies have suggested that catechol-O-methyltransferase (COMT), proline dehydrogenase (PRODH), and brain-derived neurotrophic factor (BDNF) genes are possible susceptibility genes for schizophrenia. We hypothesized that these genes are also associated with schizotypal traits, which are heritable and related to schizophrenia. We genotyped five single nucleotide polymorphism (SNPs) from the COMT, PRODH and BDNF genes, and performed a series of association analyses between alleles, genotypes or haplotypes, and quantitative schizotypal trait scores derived from the Schizotypal Personality Questionnaire (SPQ), in 465 Chinese healthy subjects. We found that 'years of education' was a major influence on seven out of nine schizotypal components, three schizotypal factors and the total SPQ scores. Molecular genetic analysis of COMT, PRODH and BDNF genes showed no significant effects of any variants on schizotypal components or factors of SPQ after correction for multiple testing, although there were weak association between COMT Val158Met (rs4680G/A) and the odd speech subscale (allele-wise, P=0.04; genotype-wise, P=0.049), between COMT Val158Met (rs4680G/A) and the suspiciousness subscale (genotype-wise, P=0.024), and between BDNF Val66Met and the Factor 2 interpersonal measure (genotype-wise, P=0.027) before correction. Furthermore, we found SNP Val158Met (rs4680) of the COMT gene significantly influenced the scores of some of schizotypal traits including total SPQ score, the disorganization factor and the constricted affect subscale in male subjects only. However, the effect was in the opposite direction of an earlier association with the SPQ reported by Avramopoulos et al. [Avramopoulos, D., Stefanis, N.C., Hantoumi, I., Smyrnis, N., Evdokimidis, I., Stefanis, C.N., 2002. Higher scores of self reported schizotypy in healthy young males carrying the COMT high activity allele. Molecular Psychiatry 7, 706-711]. We conclude that SNP Val158Met (rs4680) in the COMT gene may be associated with some schizotypal traits in male subjects, but our results are not conclusive.

Analysis of TBX1 variation in patients with psychotic and affective disorders

A significant portion of patients with 22q11 deletion syndrome (22q11DS) develop psychiatric disorders, including schizophrenia and other psychotic and affective symptoms, and the responsible gene/s are assumed to also play a significant role in the etiology of nonsyndromic psychiatric disease. The most common psychiatric diagnosis among patients with 22q11DS is schizophrenia, thought to result from neurotransmitter imbalances and also from disturbed brain development. Several genes in the 22q11 region with known or suspected roles in neurotransmitter metabolism have been analyzed in patients with isolated schizophrenia; however, their contribution to the disease remains controversial. Haploinsufficiency of the TBX1 gene has been shown to be sufficient to cause the core physical malformations associated with 22q11DS in mice and humans and via abnormal brain development could contribute to 22q11DS-related and isolated psychiatric disease. 22q11DS populations also have increased rates of psychiatric conditions other than schizophrenia, including mood disorders. We therefore analyzed variations at the TBX1 locus in a cohort of 446 white patients with psychiatric disorders relevant to 22q11DS and 436 ethnically matched controls. The main diagnoses included schizophrenia (n = 226), schizoaffective disorder (n = 67), bipolar disorder (n = 82), and major depressive disorder (n = 29). We genotyped nine tag SNPs in this sample but did not observe significant differences in allele or haplotype frequencies in any of the analyzed groups (all affected, schizophrenia and schizoaffective disorder, schizophrenia alone, and bipolar disorder and major depressive disorder) compared with the control group. Based on these results we conclude that TBX1 variation does not make a strong contribution to the genetic etiology of nonsyndromic forms of psychiatric disorders commonly seen in patients with 22q11DS.

Genetic variation in COMT and PRODH is associated with brain anatomy in patients with schizophrenia

Haploinsufficiency of 22q11 genes including catechol-O-methyltransferase (COMT) and proline dehydrogenase (PRODH) may result in structural and functional brain abnormalities and increased vulnerability to schizophrenia as observed in patients with microdeletions of 22q11. Thus, COMT and PRODH could be modifier genes for schizophrenia. We examined association of polymorphisms in COMT and PRODH with brain anatomy in young patients with schizophrenia and schizoaffective disorder. We acquired structural magnetic resonance imaging data from 51 male patients and genotyped two single nucleotide polymorphisms (SNPs) in the COMT gene and three in the PRODH gene. Statistical Parametric Mapping software and optimized voxel-based morphometry were used to determine regional gray matter (GM) and white matter (WM) density differences, and total GM and WM volume differences between genotype groups. Two nonsynonymous SNPs in the PRODH gene were associated with bilateral frontal WM density reductions and an SNP in the P2 promoter region of COMT (rs2097603) was associated with GM increase in the right superior temporal gyrus. Furthermore, we found evidence for COMT and PRODH epistasis: in patients with a COMT Val allele (rs4680) and with one or two mutated PRODH alleles, we observed increased WM density in the left inferior frontal lobe. Our results suggest that genetic variation in COMT and PRODH has significant effects on brain regions known to be affected in schizophrenia. Further research is needed to investigate the role of 22q11 genes on brain structure and function and their role in vulnerability for schizophrenia.

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.

Susceptibility genes for schizophrenia: Characterisation of mutant mouse models at the level of phenotypic behaviour

A wealth of evidence indicates that schizophrenia is heritable. However, the genetic mechanisms involved are poorly understood. Furthermore, it may be that genes conferring susceptibility interact with one another and with non-genetic factors to modulate risk status and/or the expression of symptoms. Genome-wide scanning and the mapping of several regions linked with risk for schizophrenia have led to the identification of several putative susceptibility genes including neuregulin-1 (NRG1), dysbindin (DTNBP1), regulator of G-protein signalling 4 (RGS4), catechol-o-methyltransferase (COMT), proline dehydrogenase (PRODH) and disrupted-in-schizophrenia 1 (DISC1). Genetic animal models involving targeted mutation via gene knockout or transgenesis have the potential to inform on the role of a given susceptibility gene on the development and behaviour of the whole organism and on whether disruption of gene function is associated with schizophrenia-related structural and functional deficits. This review focuses on data regarding the behavioural phenotype of mice mutant for schizophrenia susceptibility genes identified by positional candidate analysis and the study of chromosomal abnormalities. We also consider methodological issues that are likely to influence phenotypic effects, as well as the limitations associated with existing molecular techniques.

Analysis of ProDH, COMT and ZDHHC8 risk variants does not support individual or interactive effects on schizophrenia susceptibility

Synergistic interaction between genes on chromosome 22q11 recently has been proposed as a possible mechanism which could confer increased risk for schizophrenia. Based on this hypothesis, our study aimed to explore main, cis- and trans-interacting effects of three candidate genes on 22q11, ProDH, COMT and ZDHHC8. We selected four putative risk variants, residing within these genes, ProDH 1945, ProDH 2026, COMT ValMet and ZDHHC8 rs175174, and studied these in a large family-based schizophrenia association sample of European origin (488 Bulgarian parent-offspring trios). The presence of interaction between the variants was tested by conditional logistic regression analysis based on a case-pseudocontrol design. Our study did not find statistical evidence for allelic (investigation of ProDH markers only), genotypic, haplotypic, or interactive effects between ProDH, COMT and ZDHHC8. Our data do not support the hypothesis that an interaction between these genes influences susceptibility to schizophrenia.

Association study of the G-protein signaling 4 (RGS4) and proline dehydrogenase (PRODH) genes with schizophrenia: a meta-analysis

Schizophrenia is a devastating psychiatric disease that affects up to 1% of the population worldwide. Recent studies suggested that schizophrenia might result from the hypofunction of glutamatergic neurotransmission. Systematic positional, expression and functional studies have implicated the regulator of G-protein signaling 4 (RGS4) and proline dehydrogenase (PRODH) genes as promising and novel candidates for explaining schizophrenia. However, the findings of association studies tend to vary depending on the different populations on which they have been conducted. To reconcile this conflict of evidence, we combined all available population-based and family-based studies up to July 2005 involving eight polymorphisms. However, this meta-analysis did not find statistically significant evidence for association between the two glutamate-related genes and schizophrenia on the basis of either allelic or genotypic analysis. This may be the first systematic meta-analysis study based on RGS4 and PRODH.

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.

Schizophrenia susceptibility genes: emergence of positional candidates and future directions

Schizophrenia is a devastating psychiatric disorder that affects approximately 1% of the population worldwide. It is characterized by so-called 'positive symptoms'--including delusions and hallucinations--'negative symptoms'--including blunted emotions and social isolation--and cognitive deficits--including impairments in attention and working memory. Studies of the inheritance of schizophrenia have revealed that it is a multifactorial disease that is characterized by multiple genetic susceptibility elements, each contributing a modest degree of risk. Linkage studies have identified several potential schizophrenia susceptibility loci, and in recent years major progress has been made in the identification of positional candidate susceptibility genes from these loci. A central goal of future research will be to use this genetic knowledge to generate specific animal models, characterize genetic interactions, investigate the disease pathophysiology and assist drug-discovery efforts.

Schizophrenia genetics: uncovering positional candidate genes

The efforts to decipher the genetic causes of schizophrenia, one of the most devastating mental illnesses, have reached a turning point. Several linkage findings in schizophrenia have been replicated and, in the last few years, have been followed by systematic fine-mapping efforts to identify positional susceptibility genes. Here, we outline the evidence supporting each of the proposed positional candidate genes and identify some general areas of caution in their interpretation. Several of these findings hold considerable promise both for understanding the neuropathology of this brain disorder, the causes of which remain a mystery, but also for development of novel, mechanism-based treatments for the patients.

No association found between 158 Val/Met polymorphism of the COMT gene and schizophrenia with minor physical anomalies

The catechol-O-methyl transferase (COMT) gene has been a promising candidate in genetic research on schizophrenia because of its function in dopamine metabolism and its location on chromosome 22q11.2, which may be implicated in both schizophrenia and velocardiofacial syndrome (VCFS). To explore the possible genetic contribution of COMT to the development of schizophrenia, we focused on the subgroup of patients with schizophrenia characterized by minor physical anomalies as a phenotype and the 158 Val/Met polymorphism as a genotype. Since some physical anomalies are found in both schizophrenia and VCFS, schizophrenia patients with minor physical anomalies could represent the putative subgroup of schizophrenia linked to a disruption in neurodevelopment. Genotyping for the 158 Val/Met (472 G>A) polymorphism in the COMT gene was done for 239 patients with schizophrenia and 248 normal controls. Our analysis did not yield any significant between-group differences in terms of either allele or genotype frequency. We also could not find any association between the COMT gene and the schizophrenia subgroup with minor physical anomalies, although there was a significant difference in Waldrop total scores between the patients with schizophrenia and the normal controls. Analyses of subgroups based on other clinical variables also did not reveal significant differences. Overall, this study does not support the hypothesis that the 158 Val/Met polymorphism in the COMT gene is associated with schizophrenia in Koreans.

No evidence for an association between variants at the proline dehydrogenase locus and schizophrenia or bipolar affective disorder

The proline dehydrogenase locus must be considered as a positional and functional candidate in schizophrenia. It is located in the chromosomal region of the velocardiofacial syndrome on 22q11 that is suspected to contain genes relevant to schizophrenia, and is involved in the metabolism of neurotransmitters. Positive association between single-nucleotide polymorphisms at the proline dehydrogenase locus and schizophrenia further supported the role of proline dehydrogenase in the development of schizophrenia. In order to replicate these findings, we analyzed three single-nucleotide polymorphisms in a sample comprising 299 schizophrenic patients and 300 controls. In addition, we assessed whether proline dehydrogenase also contributes to bipolar affective disorder, because chromosome 22q11 is also implicated in bipolar affective disorder. We therefore included 300 patients with bipolar affective disorder. This is the first study on a potential involvement of the proline dehydrogenase locus in bipolar affective disorder. Neither single marker nor haplotype analysis revealed an association between variants at the proline dehydrogenase locus and schizophrenia or bipolar affective disorder.

Annotation: velo-cardio-facial syndrome

BACKGROUND

Velo-cardio-facial syndrome (VCFS), the most frequent known interstitial deletion identified in man, is associated with chromosomal microdeletions in the q11 band of chromosome 22. Individuals with VCFS are reported to have a characteristic behavioural phenotype with high rates of behavioural, psychiatric, neuropsychological and linguistic disorders.

METHODS

A selective literature review was undertaken.

RESULTS

Children and adults with VCFS have high rates of behavioural, psychiatric and communication disorders. While VCFS children have high rates of ADHD, anxiety and affective disorders, adults have high rates of psychotic disorders, particularly schizophrenia. In addition, the presence of a chromosome 22q11 deletion is associated with specific neuropsychological and neuroanatomical abnormalities.

CONCLUSIONS

People with VCFS have a characteristic behavioural phenotype with high rates of behavioural, psychiatric, neuropsychological and communication disorders. Early diagnosis and treatment within a multidisciplinary framework is of paramount importance for VCFS individuals as this will have a major effect in determining the long-term outcome in affected individuals. Longitudinal studies of VCFS children are currently under way to identify precursor symptoms and areas of dysfunction which precede the later development of major psychiatric disorder. Identification of such prodromal features in VCFS may have enormous implications for the clinical management of major psychiatric disorder in VCFS and in the wider population.

Finding schizophrenia genes

Genetic epidemiological studies suggest that individual variation in susceptibility to schizophrenia is largely genetic, reflecting alleles of moderate to small effect in multiple genes. Molecular genetic studies have identified a number of potential regions of linkage and 2 associated chromosomal abnormalities, and accumulating evidence favors several positional candidate genes. These findings are grounds for optimism that insight into genetic factors associated with schizophrenia will help further our understanding of this disease and contribute to the development of new ways to treat it.

Hyperprolinemia is a risk factor for schizoaffective disorder

DNA sequence variations within the 22q11 DiGeorge chromosomal region are likely to confer susceptibility to psychotic disorders. In a previous report, we identified several heterozygous alterations, including a complete deletion, of the proline dehydrogenase (PRODH) gene, which were associated with moderate hyperprolinemia in a subset of DSM III schizophrenic patients. Our objective was (i) to determine whether hyperprolinemia is associated with increased susceptibility for any of three psychiatric conditions (schizophrenia, schizoaffective disorder and bipolar disorder) and (ii) to establish a correlation between hyperprolinemia and PRODH genotypes. We have conducted a case-control study including 114 control subjects, 188 patients with schizophrenia, 63 with schizoaffective disorder and 69 with bipolar disorder. We report that, taking into account a confounding effect due to valproate treatment, hyperprolinemia is a risk factor for DSM IIIR schizoaffective disorder (P=0.02, Odds ratio=4.6, 95% confidence interval 1.3-16.3). We did not detect 22q11 interstitial deletions associated with the DiGeorge syndrome among the 320 patients of our sample and we found no association between common PRODH polymorphisms and any of the psychotic disorders. In contrast, we found that five rare PRODH alterations (including a complete PRODH deletion and four missense substitutions) were associated with hyperprolinemia. In several cases, two variations were present simultaneously, either in cis or trans in the same subject. A total of 11 from 30 hyperprolinemic subjects bore at least one genetic variation associated with hyperprolinemia. This study demonstrates that moderate hyperprolinemia is an intermediate phenotype associated with certain forms of psychosis.

Functional consequences of PRODH missense mutations

PRODH maps to 22q11 in the region deleted in the velocardiofacial syndrome/DiGeorge syndrome (VCFS/DGS) and encodes proline oxidase (POX), a mitochondrial inner-membrane enzyme that catalyzes the first step in the proline degradation pathway. At least 16 PRODH missense mutations have been identified in studies of type I hyperprolinemia (HPI) and schizophrenia, 10 of which are present at polymorphic frequencies. The functional consequences of these missense mutations have been inferred by evolutionary conservation, but none have been tested directly. Here, we report the effects of these mutations on POX activity. We find that four alleles (R185Q, L289M, A455S, and A472T) result in mild (<30%), six (Q19P, A167V, R185W, D426N, V427M, and R431H) in moderate (30%-70%), and five (P406L, L441P, R453C, T466M, and Q521E) in severe (>70%) reduction in POX activity, whereas one (Q521R) increases POX activity. The POX encoded by one severe allele (T466M) shows in vitro responsiveness to high cofactor (flavin adenine dinucleotide) concentrations. Although there is limited information on plasma proline levels in individuals of known PRODH genotype, extant data suggest that severe hyperprolinemia (>800 microM) occurs in individuals with large deletions and/or PRODH missense mutations with the most-severe effect on function (L441P and R453C), whereas modest hyperprolinemia (300-500 microM) is associated with PRODH alleles with a moderate reduction in activity. Interestingly, three of the four alleles associated with or found in schizophrenia (V427M, L441P, and R453C) resulted in severe reduction of POX activity and hyperprolinemia. These observations plus the high degree of polymorphism at the PRODH locus are consistent with the hypothesis that reduction in POX function is a risk factor for schizophrenia.

Schizophrenia genes, gene expression, and neuropathology: on the matter of their convergence

This review critically summarizes the neuropathology and genetics of schizophrenia, the relationship between them, and speculates on their functional convergence. The morphological correlates of schizophrenia are subtle, and range from a slight reduction in brain size to localized alterations in the morphology and molecular composition of specific neuronal, synaptic, and glial populations in the hippocampus, dorsolateral prefrontal cortex, and dorsal thalamus. These findings have fostered the view of schizophrenia as a disorder of connectivity and of the synapse. Although attractive, such concepts are vague, and differentiating primary events from epiphenomena has been difficult. A way forward is provided by the recent identification of several putative susceptibility genes (including neuregulin, dysbindin, COMT, DISC1, RGS4, GRM3, and G72). We discuss the evidence for these and other genes, along with what is known of their expression profiles and biological roles in brain and how these may be altered in schizophrenia. The evidence for several of the genes is now strong. However, for none, with the likely exception of COMT, has a causative allele or the mechanism by which it predisposes to schizophrenia been identified. Nevertheless, we speculate that the genes may all converge functionally upon schizophrenia risk via an influence upon synaptic plasticity and the development and stabilization of cortical microcircuitry. NMDA receptor-mediated glutamate transmission may be especially implicated, though there are also direct and indirect links to dopamine and GABA signalling. Hence, there is a correspondence between the putative roles of the genes at the molecular and synaptic levels and the existing understanding of the disorder at the neural systems level. Characterization of a core molecular pathway and a 'genetic cytoarchitecture' would be a profound advance in understanding schizophrenia, and may have equally significant therapeutic implications.

Evidence for association between novel polymorphisms in the PRODH gene and schizophrenia in a Chinese population

Haploinsufficiency for or mutation in at least one gene from the velocardiofacial syndrome (VCFS) region at chromosome 22q11 is implicated in psychosis. Linkage disequilibrium mapping of the region in patients identified a segment containing two genes, proline dehydrogenase (PRODH) and DGCR6, as candidates [Liu et al., 2002a] and by analysis of additional polymorphisms the PRODH gene was associated with schizophrenia in adult and early onset patients. In the present study we provide additional evidence in support of genetic association between PRODH and schizophrenia in a Chinese population. We analyzed the PRODH gene in a samples of schizophrenic patients and their families from Sichuan, SW China consisting of 528 family trios and sibling pairs. We genotyped six SNPs, PRODH*1195C-->T, PRODH*1482C-->T, PRODH*1483A-->G, PRODH*1766A-->G, PRODH*1852G-->A PRODH*1945T-->C, two of which (PRODH*1483A-->G and PRODH*1852G-->A) have not been previously reported. We found association with schizophrenia for two haplotypes consisting of PRODH*1945T-->C and PRODH*1852G-->A (Global P = 0.006), and PRODH*1852G-->A and PRODH*1766A-->G (Global P = 0.01) which include one of the newly identified markers. After six-fold Bonferroni correction for multiple testing the PRODH*1945T-C/PRODH*1852G-A haplotypes remained significant. This is a sub-haplotype of the PRODH haplotype previously associated with schizophrenia and it also maps to the 3' region of the gene, indicating that this is the region most likely to contain the underlying risk alleles. Overall this finding supports a role for the PRODH locus in schizophrenia.

Multicenter linkage study of schizophrenia loci on chromosome 22q

The hypothesis of the existence of one or more schizophrenia susceptibility loci on chromosome 22q is supported by reports of genetic linkage and association, meta-analyses of linkage, and the observation of elevated risk for psychosis in people with velocardiofacial syndrome, caused by 22q11 microdeletions. We tested this hypothesis by evaluating 10 microsatellite markers spanning 22q in a multicenter sample of 779 pedigrees. We also incorporated age at onset and sex into the analysis as covariates. No significant evidence for linkage to schizophrenia or for linkage associated with earlier age at onset, gender, or heterogeneity across sites was observed. We interpret these findings to mean that the population-wide effects of putative 22q schizophrenia susceptibility loci are too weak to detect with linkage analysis even in large samples.

Genetic abnormalities of chromosome 22 and the development of psychosis

A microdeletion at chromosome 22q11 is the most frequently known interstitial deletion found in humans, occurring in approximately one of every 4000 live births. Its occurrence is associated with a characteristic facial dysmorphology, a range of congenital abnormalities, and psychiatric problems, especially schizophrenia. The prevalence of psychosis in those with 22q11 deletion syndrome is high (30%), suggesting that haploinsufficiency of a gene or genes in this region may confer a substantially increased risk. In addition, several studies provide evidence for linkage to schizophrenia on 22q, suggesting that a gene in this region could confer susceptibility to schizophrenia in nondeleted cases. Recent studies have provided compelling evidence that haploinsufficiency of TBX1 is likely to be responsible for many of the physical features associated with the deletion. However, although a number of genes have been implicated as possible schizophrenia susceptibility loci, further confirmatory studies are required.

Neuregulin 1 and schizophrenia

We discuss in this review the role of the neuregulin (NRG1) gene in schizophrenia. NRG1 contributes to the genetics of schizophrenia in both Icelandic and Scottish schizophrenia patients. NRG1 participates in glutamatergic signaling by regulating the N-methyl-D-aspartate (NMDA) receptor through the interaction of the NRG1 protein and its receptors. NRG1 plays a central role in neural development and is most likely involved in regulating synaptic plasticity, or how the brain responds or adapts to the environment. The discovery that defects in NRG1 signaling may be involved in some cases of schizophrenia, not only implicates NRG1, but suggests that its biological pathway, active both at developing and mature synapses, is worth inspecting further in a search for other schizophrenia genes possibly in epistasis with NRG1.

The molecular genetics of schizophrenia: new findings promise new insights

The high heritability of schizophrenia has stimulated much work aimed at identifying susceptibility genes using positional genetics. However, difficulties in obtaining clear replicated linkages have led to the scepticism that such approaches would ever be successful. Fortunately, there are now signs of real progress. Several strong and well-established linkages have emerged. Three of the best-supported regions are 6p24-22, 1q21-22 and 13q32-34. In these cases, single studies achieved genome-wide significance at P<0.05 and suggestive positive findings have also been reported in other samples. The other promising regions include 8p21-22, 6q21-25, 22q11-12, 5q21-q33, 10p15-p11 and 1q42. The study of chromosomal abnormalities in schizophrenia has also added to the evidence for susceptibility loci at 22q11 and 1q42. Recently, evidence implicating individual genes within some of the linked regions has been reported and more importantly replicated. The weight of evidence now favours NRG1 and DTNBP1 as susceptibility loci, though work remains before we understand precisely how genetic variation at each locus confers susceptibility and protection. The evidence for catechol-O-methyl transferase, RGS4 and G72 is promising but not yet persuasive. While further replications remain the top priority, the respective contributions of each gene, relationships with aspects of the phenotype, the possibility of epistatic interactions between genes and functional interactions between the gene products will all need investigation. The ability of positional genetics to implicate novel genes and pathways will open up new vistas for neurobiological research, and all the signs are that it is now poised to deliver crucial insights into the nature of schizophrenia.

A systematic genomewide linkage study in 353 sib pairs with schizophrenia

We undertook a genomewide linkage study in a total of 353 affected sib pairs (ASPs) with schizophrenia. Our sample consisted of 179 ASPs from the United Kingdom, 134 from Sweden, and 40 from the United States. We typed 372 microsatellite markers at approximately 10-cM intervals. Our strongest finding was a LOD score of 3.87 on chromosome 10q25.3-q26.3, with positive results being contributed by all three samples and a LOD-1 interval of 15 cM. This finding achieved genomewide significance (P<.05), on the basis of simulation studies. We also found two regions, 17p11.2-q25.1 (maximum LOD score [MLS] = 3.35) and 22q11 (MLS = 2.29), in which the evidence for linkage was highly suggestive. Linkage to all of these regions has been supported by other studies. Moreover, we found strong evidence for linkage (genomewide P<.02) to 17p11.2-q25.1 in a single pedigree with schizophrenia. In our view, the evidence is now sufficiently compelling to undertake detailed mapping studies of these three regions.

Genes for schizophrenia? Recent findings and their pathophysiological implications

CONTEXT

Schizophrenia is highly heritable, but the genes have remained elusive. Identifying the genes is essential if the pathogenesis and pathophysiology of schizophrenia is finally to be understood, and to give the prospect of more effective treatment.

STARTING POINT

H Stefansson and colleagues (Am J Hum Genet 2002; 71: 877-92) showed association of the neuregulin (NRG1) gene with schizophrenia. Other recent papers describe six additional susceptibility genes. Replications are already being reported for some of them. The genes are biologically plausible, and may have convergent effects on glutamatergic and other synapses. We review the evidence for each gene, the possible pathogenic mechanisms, and the implications of the findings. WHERE NEXT? Given earlier failures to replicate apparent breakthroughs, the results should be viewed with caution. Unequivocal replications remain the top priority. The respective contributions of each gene, epistatic effects, and functional interactions between the gene products, all need investigation. Confirmation that any of the genes is a true susceptibility gene for schizophrenia could trigger the same rapid therapeutic progress as has occurred recently in Alzheimer's disease.