Investigation of the DAOA/G30 locus in panic disorder

d-Amino Acids and pLG72 in Alzheimer's Disease and Schizophrenia

Numerous studies over the last several years have shown that d-amino acids, especially d-serine, have been related to brain and neurological disorders. Acknowledged neurological functions of d-amino acids include neurotransmission and learning and memory functions through modulating N-methyl-d-aspartate type glutamate receptors (NMDARs). Aberrant d-amino acids level and polymorphisms of genes related to d-amino acids metabolism are associated with neurodegenerative brain conditions. This review summarizes the roles of d-amino acids and pLG72, also known as d-amino acid oxidase activator, on two neurodegenerative disorders, schizophrenia and Alzheimer’s disease (AD). The scope includes the changes in d-amino acids levels, gene polymorphisms of G72 genomics, and the role of pLG72 on NMDARs and mitochondria in schizophrenia and AD. The clinical diagnostic value of d-amino acids and pLG72 and the therapeutic importance are also reviewed.

Panic disorders: The role of genetics and epigenetics

Panic disorder is characterized by symptoms with abrupt surges of fear with palpitations, sweating, trembling, heat sensations. Considering its disease burden on each individual and on society, understanding its etiology is important. Though no one specific etiology has been known, like other psychiatric disorders, multiple factors such as genetic, environmental, neurobiological, psychopathological factors have been suggested. In this article, we reviewed currently known etiologies and related study results, regarding especially genetic and epigenetic aspects of the panic disorder. Early studies, including twin studies, family studies, adoption studies suggested highly familial trait of panic disorder. Linkage studies, either, found panic disorder is not a single gene disorder but confirmed existence of multiple related genes. Chromosome and candidate gene studies found few related genes, NPY, ADORA2A, COMT, IKBKE. Newer method, genome-wide association studies (GWAS) have been searching for newer genes. No genome-wide significant genes, however, were detected, confirming previously known candidate genes, NPY5R on 4q31.3-32, BDKRB2 on 14q32, instead. Epigenetic modification has also been studied on many different psychiatric disorders. Monoamine oxidase A (MAOA) hypomethylation, taken together with negative life events, showed relation with panic disorder. Glutamate decarbodylases 1 (GAD1) hypomethylation was also specific on panic disorder patients. Relation with noradrenaline transporter (NET) gene SLC6a2 promoter methylation has also been studied. In conclusion, no specific gene or epigenetic pattern can fully explain etiology of panic disorder. Few genes and epigenetic patterns, however, showed strong association with panic disorder compared to healthy controls. Considering its multivariable background, further studies with larger populations can confirm current results and clarify etiologies of panic disorder.

Plasma and cerebrospinal fluid G72 protein levels in schizophrenia and major depressive disorder

G72 is a modulator of D-amino acid oxidase, the enzyme that degrades D-serine, an amino acid that plays a critical role in glutamate neurotransmission, and has been implicated in psychiatric disorders. The aim of this study was to examine whether plasma or cerebrospinal fluid (CSF) G72 protein levels were altered in either schizophrenia or major depressive disorder (MDD) and whether any correlation between G72 levels and disease severity existed. Initially, 27 schizophrenic patients, 26 MDD patients, and 27 healthy controls matched for age, sex, and ethnicity were enrolled. Compared to those of controls, plasma or CSF G72 levels were not significantly different in patients with schizophrenia or MDD. Although we found a significant positive correlation between plasma G72 levels and a positive symptoms score on the positive and negative syndrome scale (PANSS), this was not replicated in the second study (40 schizophrenic patients). CSF G72 levels showed no significant correlation with PANSS scores. In MDD, neither plasma nor CSF G72 levels correlated significantly with depression severity. Since severity of our patients were relatively mild, further investigations in a large number of subjects including drug-free patients, younger patients, and more severely affected patients are warranted.

Long noncoding RNAs in psychiatric disorders

Long noncoding RNAs (lncRNAs) are nonprotein coding transcripts longer than 200 nucleotides. Many of these lncRNAs have regulatory functions and have recently emerged as major players in governing fundamental biological processes. Here, we review the definition, distribution, identification, databases, analysis, classification, and functions of lncRNAs. We also discuss the potential roles of lncRNAs in the etiological processes of psychiatric disorders and the implications for clinical diagnosis and treatment.

Candidate genes in panic disorder: meta-analyses of 23 common variants in major anxiogenic pathways

The utilization of molecular genetics approaches in examination of panic disorder (PD) has implicated several variants as potential susceptibility factors for panicogenesis. However, the identification of robust PD susceptibility genes has been complicated by phenotypic diversity, underpowered association studies and ancestry-specific effects. In the present study, we performed a succinct review of case-control association studies published prior to April 2015. Meta-analyses were performed for candidate gene variants examined in at least three studies using the Cochrane Mantel-Haenszel fixed-effect model. Secondary analyses were also performed to assess the influences of sex, agoraphobia co-morbidity and ancestry-specific effects on panicogenesis. Meta-analyses were performed on 23 variants in 20 PD candidate genes. Significant associations after correction for multiple testing were observed for three variants, TMEM132D rs7370927 (T allele: odds ratio (OR)=1.27, 95% confidence interval (CI): 1.15-1.40, P=2.49 × 10(-6)), rs11060369 (CC genotype: OR=0.65, 95% CI: 0.53-0.79, P=1.81 × 10(-5)) and COMT rs4680 (Val (G) allele: OR=1.27, 95% CI: 1.14-1.42, P=2.49 × 10(-5)) in studies with samples of European ancestry. Nominal associations that did not survive correction for multiple testing were observed for NPSR1 rs324891 (T allele: OR=1.22, 95% CI: 1.07-1.38, P=0.002), TPH1 rs1800532 (AA genotype: OR=1.46, 95% CI: 1.14-1.89, P=0.003) and HTR2A rs6313 (T allele: OR=1.19, 95% CI: 1.07-1.33, P=0.002) in studies with samples of European ancestry and for MAOA-uVNTR in female PD (low-active alleles: OR=1.21, 95% CI: 1.07-1.38, P=0.004). No significant associations were observed in the secondary analyses considering sex, agoraphobia co-morbidity and studies with samples of Asian ancestry. Although these findings highlight a few associations, PD likely involves genetic variation in a multitude of biological pathways that is diverse among populations. Future studies must incorporate larger sample sizes and genome-wide approaches to further quantify the observed genetic variation among populations and subphenotypes of PD.

Genetic variation in the G72 gene is associated with increased frontotemporal fiber tract integrity

G72 (syn. DAOA, D-amino acid oxidase activator) is a susceptibility gene for both schizophrenia and bipolar disorder. Diffusion tensor imaging studies hint at changes in fiber tract integrity in both disorders. We aimed to investigate whether a G72 susceptibility haplotype causes changes in fiber tract integrity in young healthy subjects. We compared fractional anisotropy in 47 subjects that were either homozygous for the M23/M24 risk haplotype (n = 20) or homozygous for M23(rs3918342)/M24(rs1421292) wild type (n = 27) using diffusion tensor imaging with 3 T. Tract-based spatial statistics, a method especially developed for diffusion data analysis, was used to delineate the major fiber tracts. We found clusters of increased FA values in homozygous risk haplotype carriers in the right periinsular region and in the right inferior parietal lobe (IPL). We did not find clusters indicating decreased FA values. The insula and the IPL have been implicated in both schizophrenia and bipolar pathophysiology. Increased FA values might reflect changes in dendritic morphology as previously described by in vitro studies. These findings further corroborate the hypothesis that a shared gene pool between schizophrenia and bipolar disorder might lead to neuroanatomic changes that confer an unspecific vulnerability for both disorders.

DISC1-TSNAX and DAOA genes in major depression and citalopram efficacy

BACKGROUND

Major depressive disorder (MDD) is a common disease with high morbidity and still unsatisfying treatment response. Both MDD pathogenesis and antidepressant effect are supposed to be strongly affected by genetic polymorphisms. Among promising candidate genes, distrupted in schizophrenia 1 (DISC1), translin-associated factor X (TSNAX) and D-amino acid oxidase activator (DAOA) were suggested since their regulator role in neurodevelopment, neuroplasticity and neurotransmission, and previous evidence of cross-involvement in major psychiatric diseases.

METHODS

The present paper investigated the role of 13 SNPs within the reported genes in MDD susceptibility through a case-control (n=320 and n=150, respectively) study and in citalopram efficacy (n=157). Measures of citalopram efficacy were response (4th week) and remission (12th week). Pharmacogenetic findings were tested in the STAR(⁎)D genome-wide dataset (n=1892) for replication.

RESULTS

Evidence of association among rs3738401 (DISC1), rs1615409 and rs766288 (TSNAX) and MDD was found (p=0.004, p=0.0019, and p=0.008, respectively). A trend of association between remission and DISC1 rs821616 and DAOA rs778294 was detected, and confirmation was found for rs778294 by repeated-measure ANOVA (p=0.0008). In the STAR(⁎)D a cluster of SNPs from 20 to 40Kbp from DISC1 findings in the original sample was associated with citalopram response, as well as rs778330 (12,325bp from rs778294).

LIMITATIONS

Relatively small size of the original sample and focus on only three candidate genes.

CONCLUSIONS

The present study supported a role of DISC1-TSNAX variants in MDD susceptibility. On the other hand, genetic regions around DAOA rs778294 and DISC1 rs6675281-rs1000731 may influence citalopram efficacy.

Beyond Depression: Towards a Process-Based Approach to Research, Diagnosis, and Treatment

Despite decades of research on the etiology and treatment of depression, a significant proportion of the population is affected by the disorder, fails to respond to treatment and is plagued by relapse. Six prominent scientists, Aaron Beck, Richard Davidson, Fritz Henn, Steven Maier, Helen Mayberg, and Martin Seligman, gathered to discuss the current state of scientific knowledge on depression, and in particular on the basic neurobiological and psychopathological processes at play in the disorder. These general themes were addressed: 1) the relevance of learned helplessness as a basic process involved in the development of depression; 2) the limitations of our current taxonomy of psychological disorders; 3) the need to work towards a psychobiological process-based taxonomy; and 4) the clinical implications of implementing such a process-based taxonomy.

Association of single nucleotide polymorphisms in the diamine oxidase gene with diamine oxidase serum activities

BACKGROUND

Histamine intolerance (HIT) is associated with an excess of histamine because of an impaired function of the histamine-degrading enzyme diamine oxidase (DAO). The genetic background of HIT is unknown yet.

METHODS

Case-control association study of all haplotype tagging and four previously reported DAO SNPs and one HNMT Single nucleotide polymorphism with symptoms of HIT and DAO serum activity in 484 German individuals including 285 patients with clinical symptoms of HIT and 199 controls.

RESULTS

Diamine oxidase serum activity was significantly associated with seven SNPs within the DAO gene. The minor allele at rs2052129, rs2268999, rs10156191 and rs1049742 increased the risk for a reduced DAO activity whereas showing a moderate protective effect at rs2071514, rs1049748 and rs2071517 in the genotypic (P = 2.1 × 10(-8) , 7.6 × 10(-10) , 8.3 × 10(-10) , 0.009, 0.005, 0.00001, 0.006, respectively) and allelic genetic model (P = 2.5 × 10(-11) , 5.4 × 10(-13) , 8.9 × 10(-13) , 0.00002, 0.006, 0.0003, 0.005, respectively). Reporter gene assays at rs2052129 revealed a lower promoter activity (P = 0.016) of the minor allele. DAO mRNA expression in peripheral blood mononuclear cells of homozygous carriers of the minor allele at rs2052129, rs2268999, rs10156191 was lower (P = 0.002) than homozygous carriers of the major allele. Diamine oxidase variants were not associated with the HIT phenotype per se, only with DAO activity alone and the subgroup of HIT patients displaying a reduced DAO activity.

CONCLUSIONS

DAO gene variants strongly influence DAO expression and activity but alone are not sufficient to fully effectuate the potentially associated disease state of HIT, suggesting an interplay of genetic and environmental factors.

Genetic variation in G72 correlates with brain activation in the right middle temporal gyrus in a verbal fluency task in healthy individuals

The D-amino acid oxidase activator gene (G72) has been found associated with several psychiatric disorders such as schizophrenia, major depression, and bipolar disorder. Impaired performance in verbal fluency tasks is an often replicated finding in the mentioned disorders. In functional neuroimaging studies, this dysfunction has been linked to signal changes in prefrontal and lateral temporal areas and could possibly constitute an endophenotype. Therefore, it is of interest whether genes associated with the disorders, such as G72, modulate verbal fluency performance and its neural correlates. Ninety-six healthy individuals performed a semantic verbal fluency task while brain activation was measured with functional MRI. All subjects were genotyped for two single nucleotide polymorphisms (SNP) in the G72 gene, M23 (rs3918342) and M24 (rs1421292), that have previously shown association with the above-mentioned disorders. The effect of genotype on brain activation was assessed with fMRI during a semantic verbal fluency task. Although there were no differences in performance, brain activation in the right middle temporal gyrus (BA 39) and the right precuneus (BA 7) was positively correlated with the number of M24 risk alleles in the G72 gene. G72 genotype does modulate brain activation during language production on a semantic level in key language areas. These findings are in line with structural and functional imaging studies in schizophrenia, which showed alterations in the right middle temporal gyrus.

Advances in molecular genetics of panic disorder

The molecular genetic research on panic disorder (PD) has grown tremendously in the past decade. Although the data from twin and family studies suggest an involvement of genetic factors in the familial transmission of PD with the heritability estimate near 40%, the genetic substrate underlying panicogenesis is not yet understood. The linkage studies so far have suggested that chromosomal regions 13q, 14q, 22q, 4q31-q34, and probably 9q31 are associated with the transmission of PD phenotypes. To date, more than 350 candidate genes have been examined in association studies of PD, but most of these results remain inconsistent, negative, or not clearly replicated. Only Val158Met polymorphism of the catechol-O-methyltransferase gene has been implicated in susceptibility to PD by several studies in independent samples and confirmed in a recent meta-analysis. However, the specific role of this genetic variation in PD requires additional analysis considering its gender- and ethnicity-dependent effect and putative impact on cognitive functions. The recent advantages in bioinformatics and genotyping technologies, including genome-wide association and gene expression methods, provide the means for far more comprehensive discovery in PD. The progress in clinical and neurobiological concepts of PD may further guide genetic research through the current controversies to more definitive findings.

G72/G30 (DAOA) and juvenile-onset mood disorders

The chromosome 13q region has been linked to bipolar disorder in a number of genome scans as well as focused linkage studies. Previously we identified linkage to the 13q32 region in a genome scan of 146 affected sibling pair families from Hungary with juvenile-onset mood disorders. Within this region are the overlapping genes G72/G30, with G72 now officially named as D-amino-acid oxidase activator (DAOA). This locus has been associated with panic disorder, schizophrenia, and bipolar disorder. In this study, we tested for association to 11 markers in these genes and mood disorders in a sample of 646 nuclear families identified with a proband with onset of a mood disorder before 14.9 years of age. We identified evidence for association to three markers within the gene (rs2391191, rs3918341, rs1935062), two of which had been associated with bipolar disorder in previous studies. When corrected for the number of markers tested, the results were no longer significant, however the prior evidence for association of this gene in multiple studies points to this gene as a potential contributor to juvenile-onset mood disorders.

The involvement of the NMDA receptor D-serine/glycine site in the pathophysiology and treatment of schizophrenia

Hypofunction of the N-methyl-D-aspartate receptor (NMDAR) has been implicated in the pathophysiology of schizophrenia. The NMDAR contains a D-serine/glycine site on the NR1 subunit that may be a promising therapeutic target for psychiatric illness. This review outlines the complex regulation of endogenous NMDAR D-serine/glycine site agonists and explores their contribution to schizophrenia pathogenesis and their potential clinical utility. Genetic studies have associated genes influencing NMDAR D-serine/glycine site activation with an increased susceptibility to schizophrenia. Postmortem studies have identified abnormalities in several transcripts affecting D-serine/glycine site activity, consistent with in vivo reports of alterations in levels of endogenous D-serine/glycine site agonists and antagonists. Genetically modified mice with aberrant NMDAR D-serine/glycine site function model certain features of the negative and cognitive symptoms of schizophrenia, and similar behavioral abnormalities have been observed in other candidate genes models. Compounds that directly activate the NMDAR D-serine/glycine site or inhibit glycine transport have demonstrated beneficial effects in preclinical models and clinical trials. Future pharmacological approaches for schizophrenia treatment may involve targeting enzymes that affect D-serine synthesis and metabolism.

Case-control studies show that a non-conservative amino-acid change from a glutamine to arginine in the P2RX7 purinergic receptor protein is associated with both bipolar- and unipolar-affective disorders

Three linkage studies of bipolar disorder have implicated chromosome 12q24.3 with lod scores of over 3.0 and several other linkage studies have found lods between 2 and 3. Fine mapping within the original chromosomal linkage regions has identified several loci that show association with bipolar disorder. One of these is the P2RX7 gene encoding a central nervous system-expressed purinergic receptor. A non-synonymous single nucleotide polymorphism, rs2230912 (P2RX7-E13A, G allele) and a microsatellite marker NBG6 were both previously found to be associated with bipolar disorder (P=0.00071 and 0.008, respectively). rs2230912 has also been found to show association with unipolar depression. The effect of the polymorphism is non-conservative and results in a glutamine to arginine change (Gln460Arg), which is likely to affect P2RX7 dimerization and protein-protein interactions. We have confirmed the allelic associations between bipolar disorder and the markers rs2230912 (P2RX7-E13A, G allele, P=0.043) and NBG6 (P=0.010) in a London-based sample of 604 bipolar cases and 560 controls. When we combined these data with the published case-control studies of P2RX7 and mood disorder (3586 individuals) the association between rs2230912 (Gln460Arg) and affective disorders became more robust (P=0.002). The increase in Gln460Arg was confined to heterozygotes rather than homozygotes suggesting a dominant effect (odds ratio 1.302, CI=1.129-1.503). Although further research is needed to prove that the Gln460Arg change has an aetiological role, it is so far the most convincing mutation to have been found with a role for increasing susceptibility to bipolar and genetically related unipolar disorders.

Association study of the G72 gene with schizophrenia in a Japanese population: a multicenter study

G72 is one of the most widely tested genes for association with schizophrenia. As G72 activates the D-amino acid oxidase (DAO), G72 is termed D-amino acid oxidase activator (DAOA). The aim of this study is to investigate the association between G72 and schizophrenia in a Japanese population, using the largest sample size to date (1774 patients with schizophrenia and 2092 healthy controls). We examined eight single nucleotide polymorphisms (SNPs), which had been associated with schizophrenia in previous studies. We found nominal evidence for association of alleles, M22/rs778293, M23/rs3918342 and M24/rs1421292, and the genotype of M22/rs778293 with schizophrenia, although there was no association of allele or genotype in the other five SNPs. We also found nominal haplotypic association, including M15/rs2391191 and M19/rs778294 with schizophrenia. However, these associations were no longer positive after correction for multiple testing. We conclude that G72 might not play a major role in the risk for schizophrenia in the Japanese population.

Evidence for association and epistasis at the DAOA/G30 and D-amino acid oxidase loci in an Irish schizophrenia sample

The D-amino acid oxidase (DAO) signaling pathway has been implicated in schizophrenia pathogenesis. This may be mediated through modulation of NMDA function by DAO, which is in turn activated by DAO activator (DAOA, formerly G72). Chumakov et al. (2002); PNAS 99: 13675-13680, identifying the novel schizophrenia susceptibility gene DAOA/G30 and a number of independent studies have since reported evidence of association between the DAOA and DAO genes and schizophrenia. However, at least two studies have failed to replicate the epistatic interaction between these loci described in the original report and there have been differences in the associated alleles/haplotypes reported at each locus. In this study, we performed association and epistasis analyses of the DAOA/G30 and DAO loci in a sample of 373 cases with DSM-IV schizophrenia/schizoaffective disorder and 812 controls from the Republic of Ireland. Corrected for the number of tests performed, we found evidence for association between markers at both genes and schizophrenia: DAOA/G30 (P = 0.005, OR = 1.34 (1.09, 1.65)) and DAO (P = 0.003, OR = 1.43 (1.12, 1.84). The data suggest that evidence for association at DAO (marker rs2111902) is more consistent than previously realized, particularly in Caucasian schizophrenia populations. We identified evidence for epistatic interaction between the associated SNPs at DAOA and DAO genes in contributing to schizophrenia risk (OR = 9.3 (1.4, 60.5). Based on these data, more systematic investigation of genes involved in DAO signaling is required.

d-Amino acid oxidase (DAO) genotype and mood symptomatology in schizophrenia

A series of genetic studies have identified the D-amino acid oxidase (DAO) gene as potentially contributing to schizophrenia susceptibility. An interacting gene, D-amino acid oxidase activator (DAOA) has also been implicated and it has been suggested that variation at these genes may influence the efficiency of glutamate gating at N-methyl-D-aspartate-type (NMDA) receptors. However, recent data suggests that DAOA may influence susceptibility to mood episodes across the spectrum of psychotic disorders rather than contributing to a specific psychosis phenotype. The aim of this study was to determine whether risk variation at DAO is similarly associated with affective or other clinical symptoms in psychosis. We have previously reported association between risk variation at DAO and schizophrenia in an Irish case-control sample. In this study we investigated the relationship between a defined genetic risk variant at DAO and PANSS-derived clinical symptom factors in a sample of 249 patients using principal component and Kruskal-Wallis analyses. Carriers of the DAO risk variant scored significantly higher on the 'depression/anxiety' factor than non-carriers (H=9.02, d.f.=2, p=0.01). These data suggest a potential role for DAO in susceptibility to depressive symptoms in schizophrenia, but a more general role for DAO in affective disorders cannot be excluded.

The G72/G30 gene locus in psychiatric disorders: a challenge to diagnostic boundaries?

In follow-up from evidence obtained in linkage studies, systematic linkage disequilibrium mapping within chromosomal region 13q33 has led to the identification of a schizophrenia susceptibility locus which harbors the genes G72 and G30. These association findings have been replicated in several independent schizophrenia samples. Association has also been found between genetic variants at the G72/G30 locus and bipolar affective disorder (BPAD), with replication in independent studies. Results from studies of more detailed psychiatric phenotypes show that association exists with symptom clusters that are common to several disorders as well as with specific psychiatric diagnoses. These findings may indicate that the association lies not with the diagnostic categories per se but with more specific aspects of the phenotype, such as affective symptoms and cognitive effects, which cross traditional psychiatric diagnostic boundaries. At the molecular level, the picture remains far from clear. No putative functional variants have been identified in the coding regions of G72 or G30, and it is therefore likely that disease susceptibility is caused by as yet unidentified variants which alter gene expression or splicing. A further complication is the fact that inconsistencies are evident in the risk alleles and haplotypes observed to be associated across different samples and studies, which may suggest the presence of multiple susceptibility variants at this locus. Functional analyses indicate that the G72 gene product plays a role in the activation of N-methyl-D-aspartate receptors, a molecular pathway implicated in both schizophrenia and BPAD, making it the most plausible candidate gene at this locus.

Panic disorder

Panic disorder is a common mental disorder that affects up to 5% of the population at some point in life. It is often disabling, especially when complicated by agoraphobia, and is associated with substantial functional morbidity and reduced quality of life. The disorder is also costly for individuals and society, as shown by increased use of health care, absenteeism, and reduced workplace productivity. Some physical illnesses (eg, asthma) commonly occur with panic disorder, and certain lifestyle factors (eg, smoking) increase the risk for the disorder, but causal pathways are still unclear. Genetic and early experiential susceptibility factors also exist, but their exact nature and pathophysiological mechanisms remain unknown. Despite an imprecise, although increased, understanding of cause, strong evidence supports the use of several effective treatments (eg, pharmacological, cognitive-behavioural). The adaptation and dissemination of these treatments to the frontlines of medical-care delivery should be urgent goals for the public-health community.

G72/G30 in schizophrenia and bipolar disorder: review and meta-analysis

Association of the G72/G30 locus with schizophrenia and bipolar disorder has now been reported in several studies. The G72/G30 locus may be one of several that account for the evidence of linkage that spans a broad region of chromosome 13q. However, the story of G72/G30 is complex. Our meta-analysis of published association studies shows highly significant evidence of association between nucleotide variations in the G72/G30 region and schizophrenia, along with compelling evidence of association with bipolar disorder. But the associated alleles and haplotypes are not identical across studies, and some strongly associated variants are located approximately 50 kb telomeric of G72. Interestingly, G72 and G30 are transcribed in opposite directions; hence, their transcripts could cross-regulate translation. A functional native protein and functional motifs for G72 or G30 remain to be demonstrated. The interaction of G72 with d-amino acid oxidase, itself of interest as a modulator of N-methyl-d-aspartate receptors through regulation of d-serine levels, has been reported in one study and could be a key functional link that deserves further investigation. The association findings in the G72/G30 region, among the most compelling in psychiatry, may expose an important molecular pathway involved in susceptibility to schizophrenia and bipolar disorder.