Association analyses suggest GPR24 as a shared susceptibility gene for bipolar affective disorder and schizophrenia

Early alterations in the MCH system link aberrant neuronal activity and sleep disturbances in a mouse model of Alzheimer's disease

Early Alzheimer's disease (AD) is associated with hippocampal hyperactivity and decreased sleep quality. Here we show that homeostatic mechanisms transiently counteract the increased excitatory drive to CA1 neurons in App^NL-G-F mice, but that this mechanism fails in older mice. Spatial transcriptomics analysis identifies Pmch as part of the adaptive response in App^NL-G-F mice. Pmch encodes melanin-concentrating hormone (MCH), which is produced in sleep-active lateral hypothalamic neurons that project to CA1 and modulate memory. We show that MCH downregulates synaptic transmission, modulates firing rate homeostasis in hippocampal neurons and reverses the increased excitatory drive to CA1 neurons in App^NL-G-F mice. App^NL-G-F mice spend less time in rapid eye movement (REM) sleep. App^NL-G-F mice and individuals with AD show progressive changes in morphology of CA1-projecting MCH axons. Our findings identify the MCH system as vulnerable in early AD and suggest that impaired MCH-system function contributes to aberrant excitatory drive and sleep defects, which can compromise hippocampus-dependent functions.

The Epigenetics of Psychosis: A Structured Review with Representative Loci

The evidence for an environmental component in chronic psychotic disorders is strong and research on the epigenetic manifestations of these environmental impacts has commenced in earnest. In reviewing this research, the focus is on three genes as models for differential methylation, MCHR1, AKT1 and TDO2, each of which have been investigated for genetic association with psychotic disorders. Environmental factors associated with psychotic disorders, and which interact with these model genes, are explored in depth. The location of transcription factor motifs relative to key methylation sites is evaluated for predicted gene expression results, and for other sites, evidence is presented for methylation directing alternative splicing. Experimental results from key studies show differential methylation: for MCHR1, in psychosis cases versus controls; for AKT1, as a pre-existing methylation pattern influencing brain activation following acute administration of a psychosis-eliciting environmental stimulus; and for TDO2, in a pattern associated with a developmental factor of risk for psychosis, in all cases the predicted expression impact being highly dependent on location. Methylation induced by smoking, a confounding variable, exhibits an intriguing pattern for all three genes. Finally, how differential methylation meshes with Darwinian principles is examined, in particular as it relates to the “flexible stem” theory of evolution.

Melanin Concentrating Hormone Signaling Deficits in Schizophrenia: Association With Memory and Social Impairments and Abnormal Sensorimotor Gating

BACKGROUND

Evidence from anatomical, pharmacological, and genetic studies supports a role for the neuropeptide melanin concentrating hormone system in modulating emotional and cognitive functions. Genome-wide association studies revealed a potential association between the melanin concentrating hormone receptor (MCHR1) gene locus and schizophrenia, and the largest genome-wide association study conducted to date shows a credible genome-wide association.

METHODS

We analyzed MCHR1 and pro-melanin concentrating hormone RNA-Seq expression in the prefrontal cortex in schizophrenia patients and healthy controls. Disruptions in the melanin concentrating hormone system were modeled in the mouse brain by germline deletion of MCHR1 and by conditional ablation of melanin concentrating hormone expressing neurons using a Cre-inducible diphtheria toxin system.

RESULTS

MCHR1 expression is decreased in the prefrontal cortex of schizophrenia samples (false discovery rate (FDR) P < .05, CommonMind and PsychEncode combined datasets, n = 901) while pro-melanin concentrating hormone is below the detection threshold. MCHR1 expression decreased with aging (P = 6.6E-57) in human dorsolateral prefrontal cortex. The deletion of MCHR1 was found to lead to behavioral abnormalities mimicking schizophrenia-like phenotypes: hyperactivity, increased stereotypic and repetitive behavior, social impairment, impaired sensorimotor gating, and disrupted cognitive functions. Conditional ablation of pro-melanin concentrating hormone neurons increased repetitive behavior and produced a deficit in sensorimotor gating.

CONCLUSIONS

Our study indicates that early disruption of the melanin concentrating hormone system interferes with neurodevelopmental processes, which may contribute to the pathogenesis of schizophrenia. Further neurobiological research on the developmental timing and circuits that are affected by melanin concentrating hormone may lead to a therapeutic target for early prevention of schizophrenia.

Melanin concentrating hormone modulates oxytocin-mediated marble burying

Repetitive and perseverative behaviors are common features of a number of neuropsychiatric diseases such as Angelman's syndrome, Tourette's syndrome, obsessive-compulsive disorder, and autism spectrum disorders. The oxytocin system has been linked to the regulation of repetitive behavior in both animal models and humans, but many of its downstream targets have still to be found. We report that the melanin-concentrating hormone (MCH) system is a target of the oxytocin system in regulating one repetitive behavior, marble burying. First we report that nearly 60% of MCH neurons express oxytocin receptors, and demonstrate using rabies mediated tract tracing that MCH neurons receive direct presynaptic input from oxytocin neurons. Then we show that MCH receptor knockout (MCHR1KO) mice and MCH ablated animals display increased marble burying response while central MCH infusion decreases it. Finally, we demonstrate the downstream role of the MCH system on oxytocin mediated marble burying by showing that central infusions of MCH and oxytocin alone or together reduce it while antagonizing the MCH system blocks oxytocin-mediated reduction of this behavior. Our findings reveal a novel role for the MCH system as a mediator of the role of oxytocin in regulating marble-burying behavior in mice.

Increased expression of retinoic acid-induced gene 1 in the dorsolateral prefrontal cortex in schizophrenia, bipolar disorder, and major depression

BACKGROUND

Retinoids regulate gene expression in different cells and tissues at the transcriptional level. Retinoic acid transcriptionally regulates downstream regulatory molecules, including enzymes, transcription factors, cytokines, and cytokine receptors. Animal models indicate an involvement of retinoid signaling pathways in the regulation of synaptic plasticity and learning, especially in the hippocampus. Retinoic acid-inducible or induced gene 1 (RAI-1) is induced during neuronal differentiation, and was associated with the severity of the phenotype and response to medication in schizophrenic patients.

METHODS

In the present study, we used immunohistochemistry to investigate the expression of RAI-1 in 60 brains from the Stanley Neuropathology Consortium (15 cases each from controls and from patients with schizophrenia, bipolar disorder, and major depression). Rating scores for density and intensity were determined in the dorsolateral prefrontal cortex.

RESULTS

All four groups showed high interindividual variation. RAI-1-positive cells were identified as neurons and astrocytes. Significantly increased intensities in cortical neurons were noted in all three major psychiatric groups compared with controls. The density of RAI-1-positive neurons was increased (P=0.06) in schizophrenia and bipolar disorder. In bipolar disorder, RAI-1-positive astrocytes in gray matter showed a significantly increased intensity and compound value. Thus, a significant increase in the parameters measured was found in schizophrenia, bipolar disorder, and major depression.

CONCLUSION

Our study shows a significant increase in expression of RAI-1 in the brains from patients with schizophrenia, bipolar disorder, or major depression. The increased expression might reflect altered signaling pathways, like that for retinoic acid. The underlying mechanisms leading to the increased expression and its functional consequences are so far unknown, and remain to be investigated in future studies.

Evidence for phenotypic plasticity in response to photic cues and the connection with genes of risk in schizophrenia

Numerous environmental factors have been identified as influential in the development of schizophrenia. Some are byproducts of modern life, yet others were present in our evolutionary past and persist to a lesser degree in the current era. The present study brings together published epidemiological data for schizophrenia and data on variables related to photic input for places of residence across geographical regions, using rainfall as an inverse, proxy measure for light levels. Data were gathered from the literature for two countries, the former Yugoslavia and Ireland, during a time in the early 20th century when mobility was relatively limited. The data for Yugoslavia showed a strong correlation between hospital census rates for schizophrenia (by place of birth) and annual rain (r = 0.96, p = 0.008). In Ireland, the hospital census rates and first admissions for schizophrenia (by place of permanent residence) showed a trend for correlation with annual rain, reaching significance for 1st admissions when the rainfall data was weighted by the underlying population distribution (r = 0.71, p = 0.047). In addition, across the years 1921-1945, birth-year variations in a spring quarter season-of-birth effect for schizophrenia in Ireland showed a trend for correlation with January-March rainfall (r = 0.80, p ≤ 0.10). The data are discussed in terms of the effect of photoperiod on the gestation and behavior of offspring in animals, and the premise is put forth that vestigial phenotypic plasticity for such photic cues still exists in humans. Moreover, genetic polymorphisms of risk identified for psychotic disorders include genes modulated by photoperiod and sunlight intensity. Such a relationship between phenotypic plasticity in response to a particular environmental regime and subsequent natural selection for fixed changes in the environmentally responsive genes, has been well studied in animals and should not be discounted when considering human disease.

Neonatal antibodies to infectious agents and risk of bipolar disorder: a population-based case-control study

OBJECTIVE

There is a substantial evidence base linking prenatal exposure to infectious agents and an increased risk of schizophrenia. However, there has been less research examining the potential for these exposures to also contribute to risk for bipolar disorder. The aim of this study was to examine the association between neonatal markers of selected prenatal infections and risk for bipolar disorder.

METHODS

Using population-based Danish registers, we examined 127 individuals with a diagnosis of bipolar disorder, and 127 sex and day-of-birth individually matched controls. Based on neonatal dried blood spots, we measured antibodies to herpes simplex virus type 1 (HSV-1) and 2 (HSV-2), cytomegalovirus (CMV), and Toxoplasma gondii. Relative risks were calculated for the matched pairs when examined for optical density units for antibodies to each of the infectious agents.

RESULTS

There was no association between any of the neonatal markers of prenatal infection and risk of bipolar disorder.

CONCLUSIONS

In contrast with studies of schizophrenia, our analysis does not support maternal infection with HSV-1, HSV-2, CMV, or Toxoplasma gondii as risk factors for bipolar disorder. However, larger study samples are needed, and data on, for example, specific serotypes of Toxoplasma and indicators of the timing of maternal infection are still warranted.

Disease-associated epigenetic changes in monozygotic twins discordant for schizophrenia and bipolar disorder

Studies of the major psychoses, schizophrenia (SZ) and bipolar disorder (BD), have traditionally focused on genetic and environmental risk factors, although more recent work has highlighted an additional role for epigenetic processes in mediating susceptibility. Since monozygotic (MZ) twins share a common DNA sequence, their study represents an ideal design for investigating the contribution of epigenetic factors to disease etiology. We performed a genome-wide analysis of DNA methylation on peripheral blood DNA samples obtained from a unique sample of MZ twin pairs discordant for major psychosis. Numerous loci demonstrated disease-associated DNA methylation differences between twins discordant for SZ and BD individually, and together as a combined major psychosis group. Pathway analysis of our top loci highlighted a significant enrichment of epigenetic changes in biological networks and pathways directly relevant to psychiatric disorder and neurodevelopment. The top psychosis-associated, differentially methylated region, significantly hypomethylated in affected twins, was located in the promoter of ST6GALNAC1 overlapping a previously reported rare genomic duplication observed in SZ. The mean DNA methylation difference at this locus was 6%, but there was considerable heterogeneity between families, with some twin pairs showing a 20% difference in methylation. We subsequently assessed this region in an independent sample of postmortem brain tissue from affected individuals and controls, finding marked hypomethylation (>25%) in a subset of psychosis patients. Overall, our data provide further evidence to support a role for DNA methylation differences in mediating phenotypic differences between MZ twins and in the etiology of both SZ and BD.

A genome-wide study of panic disorder suggests the amiloride-sensitive cation channel 1 as a candidate gene

Panic disorder (PD) is a mental disorder with recurrent panic attacks that occur spontaneously and are not associated to any particular object or situation. There is no consensus on what causes PD. However, it is recognized that PD is influenced by environmental factors, as well as genetic factors. Despite a significant hereditary component, genetic studies have only been modestly successful in identifying genes of importance for the development of PD. In this study, we conducted a genome-wide scan using microsatellite markers and PD patients and control individuals from the isolated population of the Faroe Islands. Subsequently, we conducted a fine mapping, which revealed the amiloride-sensitive cation channel 1 (ACCN1) located on chromosome 17q11.2-q12 as a potential candidate gene for PD. The further analyses of the ACCN1 gene using single-nucleotide polymorphisms (SNPs) revealed significant association with PD in an extended Faroese case-control sample. However, analyses of a larger independent Danish case-control sample yielded no substantial significant association. This suggests that the possible risk alleles associated in the isolated population are not those involved in the development of PD in a larger outbred population.

The melanin-concentrating hormone (MCH) system modulates behaviors associated with psychiatric disorders

Deficits in sensorimotor gating measured by prepulse inhibition (PPI) of the startle have been known as characteristics of patients with schizophrenia and related neuropsychiatric disorders. PPI disruption is thought to rely on the activity of the mesocorticolimbic dopaminergic system and is inhibited by most antipsychotic drugs. These drugs however act also at the nigrostriatal dopaminergic pathway and exert adverse locomotor responses. Finding a way to inhibit the mesocorticolimbic- without affecting the nigrostriatal-dopaminergic pathway may thus be beneficial to antipsychotic therapies. The melanin-concentrating hormone (MCH) system has been shown to modulate dopamine-related responses. Its receptor (MCH1R) is expressed at high levels in the mesocorticolimbic and not in the nigrostriatal dopaminergic pathways. Interestingly a genomic linkage study revealed significant associations between schizophrenia and markers located in the MCH1R gene locus. We hypothesize that the MCH system can selectively modulate the behavior associated with the mesocorticolimbic dopamine pathway. Using mice, we found that central administration of MCH potentiates apomorphine-induced PPI deficits. Using congenic rat lines that differ in their responses to PPI, we found that the rats that are susceptible to apomorphine (APO-SUS rats) and exhibit PPI deficits display higher MCH mRNA expression in the lateral hypothalamic region and that blocking the MCH system reverses their PPI deficits. On the other hand, in mice and rats, activation or inactivation of the MCH system does not affect stereotyped behaviors, dopamine-related responses that depend on the activity of the nigrostriatal pathway. Furthermore MCH does not affect dizocilpine-induced PPI deficit, a glutamate related response. Thus, our data present the MCH system as a regulator of sensorimotor gating, and provide a new rationale to understand the etiologies of schizophrenia and related psychiatric disorders.

Two complex genotypes relevant to the kynurenine pathway and melanotropin function show association with schizophrenia and bipolar disorder

Prior studies of mRNA expression, protein expression, and pathway metabolite levels have implicated dysregulation of the kynurenine pathway in the etiology of schizophrenia and bipolar disorder. Here we investigate whether genes involved in kynurenine pathway regulation might interact with genes that respond to kynurenine metabolites, to enhance risk for these psychiatric phenotypes. Candidate genes were selected from prior studies of genetic association, gene expression profiling and animal models. A single nucleotide polymorphism (SNP) in each of six genes, TDO2, HM74, HM74A, MCHR1, MCHR2 and MC5R, was tested for association with phenotype (475 Caucasians, 88 African Americans with schizophrenia; 97 Caucasians, 3 African Americans with bipolar disorder; 191 Caucasian, 49 African American controls). An A allele in HM74 was significantly associated with schizophrenia and with schizophrenia plus bipolar disorder combined, odds ratios (OR) of 1.48, p=0.011 and 1.50, p=0.007, respectively. Augmentation of disease risk was found for the complex genotype HM74[A,any]+MCHR1[T,any]+MCHR2[C,any] which conferred an OR maximal for the combined diagnostic category of schizophrenia plus bipolar disorder (1.70, p=0.003), carried by 30% of the cases. TDO2[CC]+MC5R[G, any]+MCHR2[GC] conferred an OR maximal for schizophrenia alone (4.84, p=0.005), carried by 8% of schizophrenia cases. The combined risk posed by these related, complex genotypes is greater than any identified single locus and may derive from co-regulation of the kynurenine pathway by interacting genes, a lack of adequate melanotropin-controlled sequestration of the kynurenine-derived pigments, or the production of melanotropin receptor ligands through kynurenine metabolism.

Case-control association study of 65 candidate genes revealed a possible association of a SNP of HTR5A to be a factor susceptible to bipolar disease in Bulgarian population

BACKGROUND

Bipolar affective disorder (BAD) is a psychiatric illness characterized by episodes of mania and depression. Although the etiology is not clear, epidemiological studies suggest it is a result of an interaction of genetic and environmental factors. Despite of enormous efforts and abundant studies conducted, none has yet been identified definitively a gene susceptible to bipolar disorder.

METHODS

Ninety-four Bulgarian patients diagnosed with bipolar disorder and 184 Bulgarian healthy individuals, were used for genotyping of 191 single nucleotide polymorphisms (SNPs) by TaqMan and/or Invader assays. Seventeen SNPs that revealed P value less than 0.05 in the first screening were genotyped using an additional independent set of samples, consisting of 78 BAD cases and 372 controls.

RESULTS

After applying the Bonferonni correction on genotyping results of 172 cases and 556 controls, only one SNP, rs1800883, in the HTR5A gene revealed a significant level of P value (P=0.000097; odds ratio=1.80 (95%CI, 1.27-2.54); corrected P=0.017).

CONCLUSIONS

Our findings suggest that HTR5A gene could play an important role in the pathogenesis of bipolar disorder in our population. However these findings should be viewed with caution and replication studies in other populations are necessary in support of these findings.

Decreased muscarinic receptor binding in the frontal cortex of bipolar disorder and major depressive disorder subjects

BACKGROUND

Dysfunction of the cholinergic muscarinic receptors has been implicated in the pathology of bipolar disorder and major depressive disorder. However, there is conflicting evidence regarding the association between individual muscarinic receptors and the two disorders.

METHODS

We used the muscarinic receptor selective radioligands [3H]pirenzepine, [3H]AFDX-384 and [3H]4-DAMP to measure the levels of muscarinic(1) (CHRM1) and muscarinic(4) (CHRM4) receptors, muscarinic(2) (CHRM2) and muscarinic(4) (CHRM4) receptors and muscarinic(3) (CHRM3) receptor, respectively. Radioligand binding was measured in Brodmann's area (BA) 10 of the rostral prefrontal cortex, BA 46 of the dorsolateral prefrontal cortex and BA 40 of the parietal cortex in the post-mortem CNS from subjects with bipolar disorder or major depressive disorder and control subjects.

RESULTS

[3H]AFDX-384 binding was decreased in BA 46 in both bipolar disorder (p<0.01) and major depressive disorder (p<0.05). [3H]4-DAMP binding was decreased in BA 10 in bipolar disorder (p<0.05) but not major depressive disorder (p>0.05). [3H]AFDX-384 and [3H]4-DAMP binding were unaltered in any other cortical region examined for either disorder (p>0.05). [3H]pirenzepine binding was not significantly altered in either disorder in any cortical region examined (p>0.05).

LIMITATIONS

9 bipolar disorder, 9 major depressive disorder and 19 control subjects were used in the study.

CONCLUSION

Our data is consistent with previously published data implicating a role for CHRM2 receptors in the pathology of bipolar and major depressive disorder. The demonstration of a novel association between decreased CHRM3 receptor expression and bipolar disorder suggests bipolar and major depressive disorder differs in the underlying nature of their cholinergic dysfunction.

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

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

Homozygosity mapping in a family presenting with schizophrenia, epilepsy and hearing impairment

Homozygosity mapping within consanguineous families is a powerful method of localising genes for autosomal recessive disease. We investigated a family from Punjab, Pakistan, a region where consanguineous marriages are frequent. The parents have no detectable clinical disorders. However, five out of six children present with schizophrenia, epilepsy or hearing impairment either alone or in combination. This unusual phenotype in several offspring of first cousins is strongly suggestive of a rare, Mendelian recessive disorder. Two genome-wide scans initially using low-density microsatellites, and subsequently high-density SNP markers were used to map homozygous-by-descent regions in affected individuals. Candidate genes within these loci were subsequently screened for mutations. Homozygosity analysis and inbreeding coefficients were investigated to give an estimate of consanguinity. Two putative disease loci were mapped to 22q12.3-q13.3 and 2p24.3. The candidate locus on chromosome 2p24 overlaps with a deafness locus, DFNB47, linked to autosomal recessive hearing impairment, while positive findings reported for affective psychosis and schizophrenia cluster in a region of 4-5 cM on 22q13.1 within our second candidate locus. Sequence analysis of three candidate genes (KCNF1 (2p); ATF4, CACNG2 (22q)) did not reveal any exonic mutations. Inbreeding coefficients calculated for each family member support a very high degree of ancestral and recent inbreeding. The screening of other candidate genes located within these newly identified disease intervals on Chr2p24.3 and 22q12.3-q13.3 may lead to the discovery of causative variants, and consequent disrupted molecular pathways associated with this rare phenotype.

Evidence for two schizophrenia susceptibility genes on chromosome 22q13

OBJECTIVE

Previous linkage scans and meta-analyses for schizophrenia susceptibility loci failed to include the most distal portion of chromosome 22q. Accordingly, 27 families having individuals affected with schizophrenia and schizophrenia-spectrum disorders were analyzed using a set of highly informative markers covering all of chromosome 22q.

METHODS

Microsatellite and single nucleotide polymorphism markers were evaluated by nonparametric linkage, parametric linkage, and transmission disequilibrium testing of 22q.

RESULTS

The maximum nonparametric logarithm of odd scores were 2.9 (P=0.0016) for schizophrenia and 2.7 (P=0.003) for a broader disease definition that included schizotypal personality disorder-both at 44.5 cM within the Sult4A1 locus. Parametric models assuming dominant modes of inheritance and genetic heterogeneity gave maximum multipoint logarithm of odd scores for the broader disease definition at the Sult4A1 locus of 3.3 (P=0.0006) and single point logarithm of odd scores of 3.1-4.8 for Sult4A1 markers (P=0.000015-0.0005). A distal locus, centered at 61 cM, shows a maximum nonparametric logarithm of odd scores of 1.5 (P=0.072) for the broader disease definition. Transmission disequilibrium testing for three adjacent microsatellite markers located near the distal linkage peak revealed significant values for marker D22s526 for schizophrenia (P=0.0016-0.14) and for broader disease definitions including schizotypal personality disorder (P=0.0002-0.0003), and both schizotypal personality disorder plus schizoaffective disorder (P=0.00001-0.000077).

CONCLUSION

At least two separable, but closely linked, loci within 22q13 influencing susceptibility to schizophrenia-spectrum disorders, might be possible.

MLC1 polymorphisms are specifically associated with periodic catatonia, a subgroup of chronic schizophrenia

BACKGROUND

The MLC1, located on chromosome 22q13.33, has been suggested as a risk gene for schizophrenia, especially the periodic catatonia subtype. An initially identified missense mutation was found to be extremely rare in other patient cohorts; however, a recent report again argued for an association of two intronic MLC1 single nucleotide polymorphisms (SNPs) with schizophrenia and bipolar disorder.

METHODS

A case-control study of these polymorphisms as well as SNPs in the transcriptional control region of MLC1 was conducted in 212 chronic schizophrenic patients, 56 of which suffered from periodic catatonia, 106 bipolar patients, and 284 controls.

RESULTS

Both intronic and promoter polymorphisms were specifically and significantly associated with periodic catatonia but not schizophrenia or bipolar disorder in general. A haplotype constructed from all polymorphisms was also associated with periodic catatonia.

CONCLUSIONS

The MLC1 variation is associated with periodic catatonia; whether it constitutes a susceptibility or a modifier gene has to be determined.