Catechol-o-methyltransferase, cognition, and psychosis: Val158Met and beyond

Dopamine genes and schizophrenia: case closed or evidence pending?

The dopamine hypothesis of schizophrenia (SZ) has motivated a large number of genetic association studies but few if any dopaminergic (DA) polymorphisms are accepted as credible risk factors at present. To evaluate whether dopamine-related genes have been investigated adequately, we surveyed public genetic databases and published SZ association studies with regard to 14 conventional DA genes and 7 selected dopamine-interacting proteins. We estimate that 325 polymorphisms would be required to evaluate the impact of common variation on SZ risk among Caucasian samples. To date, 98 polymorphisms have been analyzed in published association studies. We estimate that only 19 of these variations have been evaluated in samples with at least 50% power to detect an association of the effect size commonly found in genetically complex disorders. While it is possible that DA genes do not harbor genetic risk factors for SZ, our review suggests that satisfactory conclusions for most genes cannot be drawn at present. Whole-genome association studies have begun to fill this void, but additional analyses are likely to be needed. Recommendations for future association studies include analysis of adequately powered samples, judiciously selected polymorphisms, multiple ethnic groups, and concurrent evaluation of function at associated single-nucleotide polymorphisms.

Differential effect of catechol-O-methyltransferase Val158Met genotype on emotional recognition abilities in healthy men and women

The catechol-O-methyltransferase (COMT) Val158Met polymorphism modulates executive functions and working memory and recent neuroimaging studies implicate an association with emotional processing. We examined the relationship between the COMT Val158Met polymorphism and facial emotion recognition and differentiation in 100 healthy individuals. Compared to Met homozygosity, Val homozygosity was associated with better and faster recognition of negative facial expressions such as anger and sad. Our study provides evidence for a possible influence of the COMT polymorphism on emotion recognition abilities in healthy subjects. Additional research is needed to further define the neurocognitive phenotypes associated with COMT polymorphisms.

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

Epistasis between catechol-O-methyltransferase and type II metabotropic glutamate receptor 3 genes on working memory brain function

Dopaminergic and glutamatergic systems are critical components responsible for prefrontal signal-to-noise tuning in working memory. Recent functional MRI (fMRI) studies of genetic variation in these systems in catechol-O-methyltransferase (COMT) and in metabotropic glutamate receptor mgluR3 (GRM3), respectively, suggest that these genes influence prefrontal physiological signal-to-noise in humans. Here, using fMRI, we extend these individual gene findings to examine the combined effects of COMT and GRM3 on dissociable components of the frontoparietal working memory network. We observed an apparent epistatic interaction of these two genes on the engagement of prefrontal cortex during working memory. Specifically, the GRM3 genotype putatively associated with suboptimal glutamatergic signaling was significantly associated with inefficient prefrontal engagement and altered prefrontal-parietal coupling on the background of COMT Val-homozygous genotype. Conversely, COMT Met-homozygous background mediated against the effect of GRM3 genotype. These findings extend putative brain dopaminergic and glutamatergic relationships indexed by COMT and GRM3 to a systems-level interaction in human cortical circuits implicated in working memory dysfunction such as in schizophrenia.

COMT genotype increases risk for bipolar I disorder and influences neurocognitive performance

OBJECTIVES

Bipolar disorder (BPD) is an affective disorder characterized by episodes of depression and mania. Cognitive impairment in patients with BPD is common and recent data suggest that it may represent a trait-like feature of the illness, as it persists during periods of wellness and co-segregates in families of bipolar patients. This suggests an underlying genetic predisposition towards cognitive impairment in patients with BPD; however, there have been few studies investigating the effects of candidate genes on this symptom domain. The catechol-O-methyl transferase (COMT) gene is located on chromosome 22q11 and is involved in the metabolism of dopamine and norepinephrine. A large body of evidence suggests that COMT is associated with cognitive performance in patients with schizophrenia and in healthy volunteers but there have been no reports of its relationship to cognition in BPD.

METHODS

We genotyped 52 Caucasian bipolar I probands and 102 Caucasian healthy controls across four single nucleotide polymorphisms (SNPs) within the COMT gene and administered a cognitive screening battery. We first assessed the relationship between COMT genotype and diagnosis and then tested for effects on cognition.

RESULTS

We observed a modest but significant association between SNP rs165599 and BPD I, with the g allele being over-represented in cases versus controls (odds ratio, OR = 2.41; allelic p = 0.02; genotypic p = 0.04). Further, we found a relationship between the risk allele at this SNP and poorer performance on measures of verbal memory [California Verbal Learning Test (CVLT) Trials 1-5; p = 0.005, eta(2) = 0.07] particularly with regard to prefrontal aspects of learning (CVLT semantic cluster; p = 0.037, eta(2) = 0.04) in patients with BPD and in healthy controls. The common Val158Met polymorphism was not associated with diagnosis (p = 0.32) or cognition in our sample.

CONCLUSIONS

These data suggest that COMT genetic variation at SNP rs165599 is associated with BPD I and influences prefrontal aspects of verbal memory in bipolar patients and healthy controls.

Glutamate and dopamine dysregulation in schizophrenia--a synthesis and selective review

The dopamine hypothesis of schizophrenia is the principal explanatory model of antipsychotic drug action. Recent discoveries extend our understanding of the neurochemistry of schizophrenia, with increasing evidence of dysfunction in glutamate and GABA as well as dopamine systems. In this review, we study the evidence for dopaminergic dysfunction in schizophrenia, drawing data from neurochemical imaging studies. We also review the NMDA receptor hypofunction hypothesis of schizophrenia as a supplementary explanatory model for the illness. We examine predictions made by the NMDA receptor hypofunction hypothesis and consider how they fit with current neurochemical findings in patients and animal models. We consider the case for glutamatergic excitotoxicity as a key process in the development and progression of schizophrenia, and suggest ways in which glutamate and dopamine dysregulation may interact in the condition.

The met(158) allele of catechol-O-methyltransferase (COMT) is associated with obsessive-compulsive disorder in men: case-control study and meta-analysis

Existing data suggest a genetic association between the met(158) allele of catechol-O-methyltransferase (COMT) and obsessive-compulsive disorder (OCD). However, the results are inconclusive and complicated by possible gender differences. We sought to resolve the question in two ways. First, we carried out a new case-control study in 87 adults with OCD and 327 healthy comparison subjects. The study replicated reports of an increased met(158) allele frequency in men with OCD (odds ratio (OR)=1.91, 95% confidence interval (CI) 1.07-3.40, P=0.026), with no effect in women (OR=1.13, 95% CI 0.74-1.72, P=0.56). Second, we performed a meta-analysis of all published case-control data (n=1908 subjects). This revealed an association of COMT met(158) with OCD (OR=1.23, 95% CI 1.06-1.42, P=0.005) and an interaction with gender (z=4.27, P<0.0001). The association between COMT met(158) and OCD was present in men (OR=1.88, 95% CI 1.45-2.44, P<0.001) but not in women (OR=0.98, 95% CI 0.78-1.22, P=0.83). We conclude that COMT may play a role in the genetic aetiology of OCD in men. The biological plausibility of the association is increased by the functionality of the val(158)met polymorphism in terms of its effect on COMT enzyme activity, and by the role of COMT in cortical dopamine signalling and information processing. The finding also extends the evidence for sexual dimorphism in COMT and in OCD.

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.

Gene-gene interaction associated with neural reward sensitivity

Reward processing depends on dopaminergic neurotransmission and is modulated by factors affecting dopamine (DA) reuptake and degradation. We used fMRI and a guessing task sensitive to reward-related activation in the prefrontal cortex and ventral striatum to study how individual variation in genes contributing to DA reuptake [DA transporter (DAT)] and degradation [catechol-o-methyltransferase (COMT)] influences reward processing. Prefrontal activity, evoked by anticipation of reward irrespective of reward probability and magnitude, was COMT genotype-dependent. Volunteers homozygous for the Met allele, associated with lower enzyme activity and presumably greater DA availability, showed larger responses compared with volunteers homozygous for the Val allele. A similar COMT effect was observed in the ventral striatum. As reported previously, the ventral striatum was also found to code gain-related expected value, i.e., the product of reward magnitude and gain probability. Individual differences in ventral striatal sensitivity for value were in part explained by an epistatic gene-gene interaction between COMT and DAT. Although most genotype combinations exhibited the expected activity increase with more likely and larger rewards, two genotype combinations (COMT Met/Met DAT 10R and COMT Val/Val 9R) were associated with blunted ventral striatal responses. In view of a consistent relationship between reduced reward sensitivity and addiction, our findings point to a potential genetic basis for vulnerability to addiction.

Dopamine: 50 years in perspective

The discovery of dopamine as a neurotransmitter in brain by Arvid Carlsson approximately 50 years ago, and the subsequent insight provided by Paul Greengard into the cellular signalling mechanisms triggered by dopamine, gained these researchers the Nobel Prize for Medicine in 2000. Dopamine research has had a greater impact on the development of biological psychiatry and psychopharmacology than work on any other neurotransmitter. Neuropsychological views of the role of dopamine in the CNS have evolved from that of a simple reward signal to a more complex situation in which dopamine encodes the importance or 'salience' of events in the external world. Hypofunctional dopamine states underlie Parkinson's disease and attention deficit hyperactivity disorder, and there is increasing evidence for dopamine hyperactivity in schizophrenia. Some of the medicines that are most widely used in psychiatry, such as L-DOPA, methylphenidate and neuroleptic drugs, act on dopaminergic mechanisms.

Convergent genetic modulation of the endocrine stress response involves polymorphic variations of 5-HTT, COMT and MAOA

Highly prevalent stress-related disorders such as major depression (MD) are characterised by a dysregulation of the neuroendocrine system. Although heritability for these disorders is high, the role of genes in the underlying pathophysiology is poorly understood. Here, we show that polymorphic variations in genes coding for serotonin transporter (5-HTT), catechol-O-methyl transferase (COMT) and monoamine oxidase A (MAOA) as well as sex differences influence the regulation of hypothalamic-pituitary-adrenal (HPA)-axis response to acute psychological and endocrine challenges. In our sample, the effects of COMT on the release of adrenocorticotrophin hormone (ACTH) depend on the presence of the low-expression MAOA variant in the same individual. By including individuals varying in their degree of susceptibility to MD, we showed evidence of interactions between 5-HTT and MD susceptibility in baseline cortisol, and between MAOA and MD susceptibility in baseline ACTH measures, indicating a role for these genotypes in stable-state endocrine regulation. Collectively, these results indicate that the simultaneous investigation of multiple monoaminergic genes in interaction with gender have to be measured to understand the endocrine regulation of stress. These findings point towards a genetic susceptibility to stress-related disorders.

Association between the DTNBP1 gene and intelligence: a case-control study in young patients with schizophrenia and related disorders and unaffected siblings

BACKGROUND

The dystrobrevin-binding protein 1 (DTNBP1) gene is a susceptibility gene for schizophrenia. There is growing evidence that DTNPB1 contributes to intelligence and cognition. In this study, we investigated association between single nucleotide polymorphisms (SNPs) in the DTNBP1 gene and intellectual functioning in patients with a first episode of schizophrenia or related psychotic disorder (first-episode psychosis, FEP), their healthy siblings, and unrelated controls.

METHODS

From all subjects IQ measurements were obtained (verbal IQ [VIQ], performance IQ [PIQ], and full scale IQ [FSIQ]). Seven SNPs in the DTNBP1 gene were genotyped using single base primer extension and analyzed by matrix-assisted laser deionization mass spectrometry (MALDI-TOF).

RESULTS

Mean VIQ, PIQ, and FSIQ scores differed significantly (p < 0.001) between patients, siblings, and controls. Using a family-based and a case-control design, several single SNPs were significantly associated with IQ scores in patients, siblings, and controls.

CONCLUSION

Although preliminary, our results provide evidence for association between the DTNBP1 gene and intelligence in patients with FEP and their unaffected siblings. Genetic variation in the DTNBP1 gene may increase schizophrenia susceptibility by affecting intellectual functioning.

Effect of schizotypy on cognitive performance and its tuning by COMT val158 met genotype variations in a large population of young men

BACKGROUND

Mirroring schizophrenia, specific dimensions of schizotypy are related to cognitive dysfunction. The relation of schizotypy and state psychopathology to cognitive performance and its link to catechol-O-methyltransferase (COMT) val(158) met genotype variations was studied in a large sample of young men.

METHODS

State psychopathology and schizotypy were assessed with self-rated questionnaires. Cognitive performance was assessed with tests of reasoning ability, sustained attention, and verbal and spatial working memory. Subjects were genotyped for the val(158) met polymorphism of the gene for COMT (low enzymatic activity met/met, intermediate met/val, and high val/val).

RESULTS

The val/val group had higher scores in measures of state psychopathology as well as negative and disorganized schizotypy dimensions, whereas there was no effect of COMT genotype on cognitive performance measures. Structural equation modeling showed that cognitive performance accuracy but not speed decreased with increasing negative schizotypy, increased with increasing paranoid schizotypy, and was not affected by state psychopathology. Increasing val loading resulted in a dose-dependent increase in the factor loading for the relation between negative schizotypy and cognitive performance accuracy.

CONCLUSIONS

Different schizotypal phenotypes had opposing relations to cognitive performance in the population. COMT genotype modulated the relation between the negative schizotypal phenotype and cognitive performance.

Clinical involvement of catechol-O-methyltransferase polymorphisms in schizophrenia spectrum disorders: influence on the severity of psychotic symptoms and on the response to neuroleptic treatment

Genetic variation in the catechol-O-methyltransferase (COMT) gene may influence the susceptibility to schizophrenia and the response to neuroleptic treatment. The authors tested for an association between a COMT haplotype and schizophrenia-spectrum disorders and for an eventual influence of a specific COMT genotype in the clinical outcome and in the response to treatment. The genotypes for single nucleotide polymorphisms rs737865, rs4633, rs6267, rs4680 (Val 158 Met) and rs165599 were determined in 207 patients with schizophrenia-spectrum disorders and 204 paired controls. Statistical tests for linkage disequilibrium and for case-control differences in haplotype frequencies were performed using log-linear modelling embedded within the expectation-maximization algorithm. P-values based on permutations were calculated using the software UNPHASED, and odds ratios were estimated using the SHEsis platform. The response to neuroleptic treatment was assessed by the Global Assessment of Functioning scale and the severity of psychotic symptoms by the positive and negative syndrome scale (PANSS) scale. The overall disease status was significantly associated with the T-G (Val) diplotype for rs4633-rs4680 (P=0.0049). A significant association was observed between schizophrenia, but not other related disorders, and genotypes GG (Val/Val) for rs4680 and TT for rs4633. Val/Val patients with schizophrenia showed a higher severity of the psychotic symptoms and a worse response to the neuroleptic treatment. COMT genetic variation seems to be involved in the psychotic symptomatology of the schizophrenia-spectrum disorders and specifically in the narrow schizophrenia phenotype. Our results show an influence of the Val 158 Met polymorphism on the severity of psychotic symptoms and on the response to treatment.

Impact of catechol-O-methyltransferase on prefrontal brain functioning in schizophrenia spectrum disorders

The enzyme catechol-O-methyltransferase (COMT) has attracted increasing interest regarding a genetic disposition towards schizophrenias and as a modulator of prefrontal brain function. A common SNP in the COMT gene causes a Val to Met transition at AA158/AA108 (Val158Met), resulting in reduced COMT activity in Met allele carriers. An impact of COMT genotype on cognition has been well established; however, the exact nature of this influence has yet to be elucidated. The aim of this study was to determine whether COMT genotype affects an electrophysiological marker of prefrontal activation and neuropsychological frontal lobe measures in schizophrenia. To this end, 56 acutely psychotic in-patients with schizophrenia spectrum disorders were investigated. Patients with the COMT 1947AA (Met/Met) genotype (n=13) were compared to a carefully matched sample of patients with a G1947A (Val/Met) genotype (n=15); matching criteria included patients' age, handedness, gender distribution, diagnosis, and medication status. A small group of six homozygous Val allele carriers was additionally included to allow an assessment of possible gene-dosage effects. P300 amplitudes and latencies, as well as an electrophysiological marker of prefrontal brain function (NoGo-Anteriorization/NGA) and neuropsychological measures (Stroop Test, Verbal Fluency, Trail Making Test) were regarded. Homozygous Met allele carriers had significantly increased NGA values and fronto-central Nogo amplitudes compared to patients with at least one Val allele. They also tended to perform better in the Stroop task, as compared to the matched group of Val/Met patients. These results indicate that COMT genotype exerts a strong impact on prefrontal functioning and executive control in schizophrenia spectrum disorders.

Neurobiology of schizophrenia

With its hallucinations, delusions, thought disorder, and cognitive deficits, schizophrenia affects the most basic human processes of perception, emotion, and judgment. Evidence increasingly suggests that schizophrenia is a subtle disorder of brain development and plasticity. Genetic studies are beginning to identify proteins of candidate genetic risk factors for schizophrenia, including dysbindin, neuregulin 1, DAOA, COMT, and DISC1, and neurobiological studies of the normal and variant forms of these genes are now well justified. We suggest that DISC1 may offer especially valuable insights. Mechanistic studies of the properties of these candidate genes and their protein products should clarify the molecular, cellular, and systems-level pathogenesis of schizophrenia. This can help redefine the schizophrenia phenotype and shed light on the relationship between schizophrenia and other major psychiatric disorders. Understanding these basic pathologic processes may yield novel targets for the development of more effective treatments.

Intermediate phenotypes and genetic mechanisms of psychiatric disorders

Genes are major contributors to many psychiatric diseases, but their mechanisms of action have long seemed elusive. The intermediate phenotype concept represents a strategy for characterizing the neural systems affected by risk gene variants to elucidate quantitative, mechanistic aspects of brain function implicated in psychiatric disease. Using imaging genetics as an example, we illustrate recent advances, challenges and implications of linking genes to structural and functional variation in brain systems related to cognition and emotion.

Warriors versus worriers: the role of COMT gene variants

Behavioral phenotypes are generally complex, reflecting the action of multiple different genes. Nevertheless, there is growing evidence that key gene variants can alter activity within specific neuronal circuits and, therefore, influence particular cognitive-affective phenomena. One example is the catechol-O-methyltransferase (COMT) gene, which has a common variant at codon 158. Those with valine (Val158) alleles have increased greater COMT activity and lower prefrontal extracellular dopamine compared with those with the methionine (Met158) substitution. Val158 alleles may be associated with an advantage in the processing of aversive stimuli (warrior strategy), while Met158 alleles may be associated with an advantage in memory and attention tasks (worrier strategy). Under conditions of increased dopamine release (eg, stress), individuals with Val158 alleles may have improved dopaminergic transmission and better performance, while individuals with Met158 alleles may have less efficient neurotransmission and worse performance. Some evidence suggests that Val158 alleles are associated with schizophrenia, while Met158 alleles are associated with anxiety.

Evidence for statistical epistasis between catechol-O-methyltransferase (COMT) and polymorphisms in RGS4, G72 (DAOA), GRM3, and DISC1: influence on risk of schizophrenia

Catechol-O-methyltransferase (COMT) regulates dopamine degradation and is located in a genomic region that is deleted in a syndrome associated with psychosis, making it a promising candidate gene for schizophrenia. COMT also has been shown to influence prefrontal cortex processing efficiency. Prefrontal processing dysfunction is a common finding in schizophrenia, and a background of inefficient processing may modulate the effect of other candidate genes. Using the NIMH sibling study (SS), a non-independent case-control set, and an independent German (G) case-control set, we performed conditional/unconditional logistic regression to test for epistasis between SNPs in COMT (rs2097603, Val158Met (rs4680), rs165599) and polymorphisms in other schizophrenia susceptibility genes. Evidence for interaction was evaluated using a likelihood ratio test (LRT) between nested models. SNPs in RGS4, G72, GRM3, and DISC1 showed evidence for significant statistical epistasis with COMT. A striking result was found in RGS4: three of five SNPs showed a significant increase in risk [LRT P-values: 90387 = 0.05 (SS); SNP4 = 0.02 (SS), 0.02 (G); SNP18 = 0.04 (SS), 0.008 (G)] in interaction with COMT; main effects for RGS4 SNPs were null. Significant results for SNP4 and SNP18 were also found in the German study. We were able to detect statistical interaction between COMT and polymorphisms in candidate genes for schizophrenia, many of which had no significant main effect. In addition, we were able to replicate other studies, including allelic directionality. The use of epistatic models may improve replication of psychiatric candidate gene studies.

Hypomethylation of MB-COMT promoter is a major risk factor for schizophrenia and bipolar disorder

The variability in phenotypic presentations and the lack of consistency of genetic associations in mental illnesses remain a major challenge in molecular psychiatry. Recently, it has become increasingly clear that altered promoter DNA methylation could play a critical role in mediating differential regulation of genes and in facilitating short-term adaptation in response to the environment. Here, we report the investigation of the differential activity of membrane-bound catechol-O-methyltransferase (MB-COMT) due to altered promoter methylation and the nature of the contribution of COMT Val158Met polymorphism as risk factors for schizophrenia and bipolar disorder by analyzing 115 post-mortem brain samples from the frontal lobe. These studies are the first to reveal that the MB-COMT promoter DNA is frequently hypomethylated in schizophrenia and bipolar disorder patients, compared with the controls (methylation rate: 26 and 29 versus 60%; P=0.004 and 0.008, respectively), particularly in the left frontal lobes (methylation rate: 29 and 30 versus 81%; P=0.003 and 0.002, respectively). Quantitative gene-expression analyses showed a corresponding increase in transcript levels of MB-COMT in schizophrenia and bipolar disorder patients compared with the controls (P=0.02) with an accompanying inverse correlation between MB-COMT and DRD1 expression. Furthermore, there was a tendency for the enrichment of the Val allele of the COMT Val158Met polymorphism with MB-COMT hypomethylation in the patients. These findings suggest that MB-COMT over-expression due to promoter hypomethylation and/or hyperactive allele of COMT may increase dopamine degradation in the frontal lobe providing a molecular basis for the shared symptoms of schizophrenia and bipolar disorder.

Endophenotype approach to developmental psychopathology: implications for autism research

This paper discusses the utility of the endophenotype approach in the study of developmental psychopathology. It is argued that endophenotype research holds considerable promise for the study of gene-brain/cognition-behaviour pathways for developmental disorders. This paper outlines the criteria for determining useful endophenotypes. Possible endophenotypes for autism are discussed as an example of an area where endophenotype research on developmental disorders may be fruitful. It is concluded that although the endophenotype approach holds promise for the study of gene-brain/cognition-behaviour pathways, much work remains to be done in order to validate endophenotype measures. It is also noted that the changing nature of any developmental psychopathology poses a particular challenge to this type of research.

RGS4 mRNA expression in postmortem human cortex is associated with COMT Val158Met genotype and COMT enzyme activity

Linkage, association and postmortem studies have implicated regulator of G-protein signaling 4 (RGS4), which negatively modulates signal transduction at G-protein-coupled receptors, as a candidate schizophrenia susceptibility gene. We compared RGS4 mRNA expression in the dorsolateral prefrontal cortex (DLPFC), between normal controls and patients with schizophrenia in two independent cohorts (>100 subjects each) (the CBDB/NIMH Collection and the Stanley Array Collection), and in the hippocampus in the CBDB/NIMH Collection. We also examined the effects of the four previously identified putative RGS4 risk SNPs (rs10917670, rs951436, rs951439, rs2661319) on RGS4 expression levels in these cohorts. As dopamine signaling is linked to RGS4 expression and there is evidence for statistical epistasis between COMT Val158Met polymorphism and RGS4 alleles, we also examined relationships between the COMT Val158Met genotype and RGS4 expression in the DLPFC. We did not detect a difference in RGS4 expression levels between schizophrenic patients (or bipolar disorder patients in the Stanley Collection) and controls and found no significant association between any of the RGS4 risk SNPs and RGS4 expression. However, COMT Val158Met genotype was associated with prefrontal and hippocampal RGS4 mRNA expression in an allele dose-dependent manner, with carriers of the COMT Val allele showing significantly lower expression than heterozygous individuals or subjects homozygous for the Met allele. Consistent with these genotype effects, RGS4 mRNA was inversely correlated with the COMT enzyme activity in the DLPFC. These data suggest that RGS4 mRNA expression is associated with cortical dopamine signaling and illustrate the importance of genetic and/or environmental background in gene expression studies in schizophrenia.

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

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

Catechol-o-methyltransferase enzyme activity and protein expression in human prefrontal cortex across the postnatal lifespan

The prefrontal cortex (PFC) dopamine system, which is critical for modulating PFC function, undergoes remodeling until at least young adulthood in primates. Catechol-o-methyltransferase (COMT) alters extracellular dopamine levels in PFC, and its gene contains a functional polymorphism (Val(158)Met) that has been associated with variation in PFC function. We examined COMT enzyme activity and protein immunoreactivity in the PFC during human postnatal development. Protein was extracted from PFC of normal individuals from 6 age groups: neonates (1-4 months), infants (5-11 months), teens (14-18 years), young adults (20-24 years), adults (31-43 years), and aged individuals (68-86 years; n = 5-8 per group). There was a significant 2-fold increase in COMT enzyme activity from neonate to adulthood, paralleled by increases in COMT protein immunoreactivity. Furthermore, COMT protein immunoreactivity was related to Val(158)Met genotype, as has been previously demonstrated. The significant increase in COMT activity from neonate to adulthood complements previous findings of protracted postnatal changes in the PFC dopamine system and may reflect an increasing importance of COMT for PFC dopamine regulation during maturation.

Lack of influence of COMT and NET genes variants on executive functions in schizophrenic and bipolar patients, their first-degree relatives and controls

Abnormal dopaminergic function in the prefrontal cortex (PFC) may be a key factor in the etiopathogeny of schizophrenia and bipolar disorder. Both schizophrenic and bipolar subjects have executive functions (EF) deficits, thought to reflect abnormal PFC function. The main inactivation pathways for dopamine in the PFC are enzymatic cleavage by the Carboxy-O-Methyl-Transferase (COMT) and reuptake by the nor-epinephrine transporter (NET). Our aim in this study was to replicate previous studies that investigated influence of the COMT genotype on EF in schizophrenic subjects, their relatives and controls and extend their scope by including bipolar patients, and their relatives and by exploring NET gene polymorphisms influence on executive performances. We investigated one functional polymorphism of the COMT gene and two polymorphisms of the NET gene. EF were assessed by means of the Trail Making Test (TMT) and the Wisconsin Card Sorting Test (WCST). We assessed the effect of each of the three genotypes on EF for the whole sample (N = 318) and separately in schizophrenic (N = 66), bipolar (N = 94) and healthy subjects (i.e., relatives and controls N = 158). Separate analyses were performed because of the presence, in patients samples, of potentially confounding factors, especially medication. Genotype had no significant effect on the cognitive measures in any of the analyses (for the two EF measures, the three polymorphisms, and the four groups). In our sample we found no evidence in favor of a major effect of COMT or NET polymorphisms on the two tests of EF.