Genetic heterogeneity, modifier genes, and quantitative phenotypes in psychiatric illness: searching for a framework

Familial and sporadic schizophrenia: a comparison of somatic diseases and abuse in patients and their relatives

OBJECTIVE

Comparing schizophrenia patients on the basis of familial and non-familial forms of the illness provides a promising approach to the identification of genes involved in schizophrenia. The aim of this study was to search for somatic factors that discriminate between patients with and without a family history of schizophrenia and between their relatives.

METHODS

Ninety-five schizophrenia patients were structurally interviewed about mental and physical health and alcohol and substance use in themselves and their families. Besides this, complementary information was obtained from the patients' case records. Patients with (41%) and without (59%) a family history were then compared.

RESULTS

The main differences were found in the patients' relatives. Fewer patients with a family history, compared with patients without a family history, had relatives with cancer (p = 0.002). Conversely, there was a tendency towards that more patients with a family history, compared with patients without a family history, had relatives with cardiac infarction (p = 0.05).

CONCLUSION

The genetic risk associated with schizophrenia seems to cosegregate into a factor(s) that protects against cancer and possibly also increases the risk for cardiac infarction.

Fine mapping on chromosome 10q22-q23 implicates Neuregulin 3 in schizophrenia

Linkage studies have implicated 10q22-q23 as a schizophrenia (SZ) susceptibility locus in Ashkenazi Jewish (AJ) and Han Chinese from Taiwan populations. To further explore our previous linkage signal in the AJ population (NPL score: 4.27, empirical p = 2 x 10(-5)), we performed a peakwide association fine mapping study by using 1414 SNPs across approximately 12.5 Mb in 10q22-q23. We genotyped 1515 AJ individuals, including 285 parent-child trios, 173 unrelated cases, and 487 unrelated controls. We analyzed the binary diagnostic phenotype of SZ and 9 heritable quantitative traits derived from a principal components factor analysis of 73 items from our consensus diagnostic ratings and direct assessment interviews. Although no marker withstood multiple test correction for association with the binary SZ phenotype, we found strong evidence of association by using the "delusion" factor as the quantitative trait at three SNPs (rs10883866, rs10748842, and rs6584400) located in a 13 kb interval in intron 1 of Neuregulin 3 (NRG3). Our best p value from family-based association analysis was 7.26 x 10(-7). We replicated this association in the collection of 173 unrelated AJ cases (p = 1.55 x 10(-2)), with a combined p value of 2.30 x 10(-7). After performing 10,000 permutations of each of the phenotypes, we estimated the empirical study-wide significance across all 9 factors (90,000 permutations) to be p = 2.7 x 10(-3). NRG3 is primarily expressed in the central nervous system and is one of three paralogs of NRG1, a gene strongly implicated in SZ. These biological properties together with our linkage and association results strongly support NRG3 as a gene involved in SZ.

Evidence for a role of the NOS1AP (CAPON) gene in schizophrenia and its clinical dimensions: an association study in a South American population isolate

BACKGROUND/AIMS

Recent studies have implicated a region on chromosome 1q21-23, including the NOS1AP gene, in susceptibility to schizophrenia. However, replication studies have been inconsistent, a fact that could partly relate to the marked psychopathological heterogeneity of schizophrenia. The aim of this study is to evaluate association of polymorphisms in the NOS1AP gene region to schizophrenia, in patients from a South American population isolate, and to assess if these variants are associated with specific clinical dimensions of the disorder.

METHODS

We genotyped 24 densely spaced SNPs in the NOS1AP gene region in a schizophrenia trio sample. The transmission disequilibrium test (TDT) was applied to single marker and haplotype data. Association to clinical dimensions (identified by factor analysis) was evaluated using a quantitative transmission disequilibrium test (QTDT).

RESULTS

We found significant association between eight SNPs in the NOS1AP gene region to schizophrenia (minimum p value = 0.004). The QTDT analysis of clinical dimensions revealed an association to a dimension consisting mainly of negative symptoms (minimum p value 0.001).

CONCLUSIONS

Our findings are consistent with a role for NOS1AP in susceptibility to schizophrenia, especially for the 'negative syndrome' of the disorder.

The genetics of symptom-based phenotypes: toward a molecular classification of schizophrenia

OBJECTIVE

Genetic linkage studies in schizophrenia (SZ) have primarily focused on the phenotype of disease susceptibility. A limited number of studies, however, have reported suggestive linkage to specific SZ symptom domains including regions on chromosomes 6, 8, and 20. We examined these chromosomal regions for association to positive, negative, and disorganized symptom clusters, using a dense set of single-nucleotide polymorphisms (SNPs).

METHODS

We ascertained 178 Caucasian patients with SZ for lifetime severity of clinical symptomatology using a structured diagnostic interview. The cohort was genotyped with the Affymetrix 500K microarray, from which we selected, a priori, 4833 intragenic SNPs located within chromosomal regions previously linked to specific SZ symptom clusters. Parametric tests, corrected for multiple testing, were used to compare the effects of allelic variation within these SNPs to the lifetime severity of the specific symptom domain that had been implicated by prior linkage studies.

RESULTS

We were able to extend previous reports of linkage between chromosome 6q and both positive and disorganized symptoms. Lifetime severity of positive symptoms was significantly (P = 2.50 x 10(-5)) associated with a SNP within the origin recognition complex subunit 3-like (ORC3L) gene, a gene implicated in synaptic plasticity. Level of disorganized symptoms was significantly (P < 6.00 x 10(-5)) associated 2 SNPs within the brain-specific angiogenesis inhibitor 3 (BAI3) gene, which is highly expressed in brain during development.

CONCLUSIONS

These data point toward specific candidate genes located within previously implicated linkage peaks for clinical symptomatology. Identification of functional variants within these regions and a characterization of the effect of these risk genotypes on the treatment of specific clinical symptoms are needed.

Twin study refining psychotic symptom dimensions as phenotypes for genetic research

We investigated which psychotic symptom dimensions are likely to be most useful as phenotypes for genetic linkage and association studies. Two hundred twenty-four probandwise twin pairs (106 monozygotic,118 same-sex dizygotic), where probands had psychosis, were ascertained from the Maudsley Twin Register in London. Dimensions were defined as ordinal symptom scores using the OPCRIT checklist, based on previous factor analyses of this and other samples. To qualify as a potentially useful phenotype, dimensions had to show (a) a satisfactory polychoric model fit and significant within-pair correlation in MZ pairs concordant for DSM-III-R psychosis and (b) that they could not be better defined in terms of other combinations of relevant symptoms, single symptoms, or dichotomized dimension scores. Relationships between dimension scores in twin probands and risk of psychosis in co-twins were also investigated. None of the positive or negative dimensions satisfied the phenotypic criteria. The disorganized dimensions showed significant and substantial correlations which maximized for a narrow definition. Combined negative/disorganized dimensions also showed significant and substantial correlations, but did not have advantages over disorganized dimensions. None of the dimensions were significant predictors of psychosis risk in co-twins. We conclude that, of the dimensions analyzed, the narrow disorganized dimension shows most promise as a phenotype for molecular genetic research.

Genomic imprinting in the development and evolution of psychotic spectrum conditions

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

Genetics of personality disorders

This review of the literature on genetic contributions to the etiology of personality disorders broadly follows the DSM classification, and begins by evaluating the current evidence for genetic influences on the DSM axis II disorders. One of the most exciting directions in psychiatric genetics is the rapidly developing field of molecular genetic studies aiming to identify specific genes correlated with psychiatric phenotypes. Personality disorders, like most other psychiatric diagnostic categories, are etiologically complex, which implies that they are influenced by several genes and several environmental factors. The interplay between genes and the environment is a field that is receiving increasing attention and is addressed both in relation to quantitative and molecular methods.

Finding suitable phenotypes for genetic studies of schizophrenia: heritability and segregation analysis

BACKGROUND

Schizophrenia is a highly heritable and complex disorder. Multiple genes are likely to be involved, complicating genetic research into the etiology of this disorder. Intermediate phenotypes or endophenotypes may facilitate genetic research if they display a simpler mode of transmission than schizophrenia itself, i.e., if they reflect more closely the underlying genetic effects.

METHODS

Twenty-five multigenerational families with multiple members affected with schizophrenia (180 subjects) were administered an extensive neuropsychological, psychophysiological, and personality test battery. Familial correlations were calculated to select heritable traits. Subsequent heritability analysis followed by commingling and segregation analysis were performed to unravel the pattern of transmission and to estimate heritability.

RESULTS

Five traits, including sensorimotor gating, openness, verbal fluency, early visual perception, and spatial working memory, showed moderate familial correlations. Heritability estimates for these traits ranged from 37% to 54%. A major gene model resembling dominant transmission was found for both sensorimotor gating and openness. Verbal fluency, early visual perception, and spatial working memory may be accounted for by polygenic, multifactorial, or environmental effects.

CONCLUSIONS

Only 2 of 13 candidate endophenotypes showed a simple mode of transmission useful for successful application in molecular genetic research: sensorimotor gating and openness. To our knowledge, this is the first study to investigate the pattern of transmission for these traits.

Novel linkage to chromosome 20p using latent classes of psychotic illness in 270 Irish high-density families

BACKGROUND

Several lines of evidence suggest that the clinical heterogeneity of schizophrenia is due to genetic heterogeneity. Genetic heterogeneity may decrease the signal-to-noise ratio in linkage and association studies. Therefore, linkage studies of clinically homogeneous classes of psychotic illness may result in greater power to detect at least some loci.

METHODS

Latent class analysis was used to divide psychotic subjects from 270 Irish high-density families (N = 755) into six classes based on the Operational Criteria Checklist for Psychotic Illness. We heuristically call them Bipolar, Schizoaffective, Mania, Schizomania, Deficit Syndrome, and Core Schizophrenia. The latter four had prevalences of greater than .08 and were individually tested for linkage in a 10-cM nonparametric autosomal genomewide scan. Empirical significance was determined using 200 simulated genome scans.

RESULTS

Seven regions achieved empirical criteria for suggestive significance for at least one latent class: 5q23.2-q35.3, 8q13.1-q23.1, 10q23.33-q26.3, 12q21.2-q24.32, 19q13.32-q13.43, 20p13-q22.3, and 21q11.2-q22.3. Five of 200 simulated scans resulted in seven suggestively significant loci (experiment-wide p = .03). Furthermore, at 20p13-p12.2, the Mania and Schizomania classes individually achieved criteria, whereas Deficit Syndrome had a suggestive logarithm of the odds peak 28 cM centromeric to this locus.

CONCLUSIONS

Using empirically derived, clinically homogeneous phenotypes, four chromosomal regions were suggestively linked but provided little evidence of linkage using traditional operationalized criteria. This approach was particularly fruitful on chromosome 20, which had previously yielded little evidence of linkage. Future studies of psychiatric illness may increase their ability to detect linkage or association by using clinically homogeneous phenotypes.

Heterogeneity and hypothesis testing in neuropsychiatric illness

The confounding effects of heterogeneity in biological psychiatry and psychiatric genetics have been widely discussed in the literature. We suggest an approach in which heterogeneity may be put to use in hypothesis testing and may find application in evaluation of the Crespi and Badcock (C&B) imprinting hypothesis. Here we consider three potential sources of etiologic subtypes for analysis.

Genetics of clinical features and subtypes of schizophrenia: a review of the recent literature

Since its earliest descriptions, schizophrenia has been thought to be clinically heterogeneous. Symptomatic features and subtypes tend to aggregate in families, suggesting that genetic factors contribute to individual differences in illness presentation. Over the past 5 years, evidence from genetic linkage and association studies has mounted to suggest that some susceptibility genes are etiologic factors for more or less specific illness subtypes. Furthermore, modifier genes may affect clinical features dimensionally only after a given patient is already affected with the illness. In this paper, we review recent findings supporting the existence of such "modifier" genes. To date, DTNBP1 has provided the greatest evidence of illness modification, as associations with negative and cognitive symptoms and worse outcome have been published in independent samples. Future directions include using whole-genome association studies to search for genetic modifiers of schizophrenia.

Genome-wide association studies: progress and potential for drug discovery and development

Although genetic studies have been critically important for the identification of therapeutic targets in Mendelian disorders, genetic approaches aiming to identify targets for common, complex diseases have traditionally had much more limited success. However, during the past year, a novel genetic approach - genome-wide association (GWA) - has demonstrated its potential to identify common genetic variants associated with complex diseases such as diabetes, inflammatory bowel disease and cancer. Here, we highlight some of these recent successes, and discuss the potential for GWA studies to identify novel therapeutic targets and genetic biomarkers that will be useful for drug discovery, patient selection and stratification in common diseases.

Detecting association using epistatic information

Genetic association studies have been less successful than expected in detecting causal genetic variants, with frequent non-replication when such variants are claimed. Numerous possible reasons have been postulated, including inadequate sample size and possible unobserved stratification. Another possibility, and the focus of this paper, is that of epistasis, or gene-gene interaction. Although unlikely that we may glean information about disease mechanism, based purely upon the data, it may be possible to increase our power to detect an effect by allowing for epistasis within our test statistic. This paper derives an appropriate "omnibus" test for detecting causal loci whist allowing for numerous possible interactions and compares the power of such a test with that of the usual main effects test. This approach differs from that commonly used, for example by Marchini et al. [2005], in that it tests simultaneously for main effects and interactions, rather than interactions alone. The alternative hypothesis being tested by the "omnibus" test is whether a particular locus of interest has an effect on disease status, either marginally or epistatically and is therefore directly comparable to the main effects test at that locus. The paper begins by considering the direct case, in which the putative causal variants are observed and then extends these ideas to the indirect case in which the causal variants are unobserved and we have a set of tag single nucleotide polymorphisms (tag SNPs) representing the regions of interest. In passing, the derivation of the indirect omnibus test statistic leads to a novel "indirect case-only test for interaction".

Catechol-O-methyltransferase gene and obsessive-compulsive symptoms in patients with recent-onset schizophrenia: preliminary results

The catechol-O-methyltransferase (COMT) gene is a candidate gene for schizophrenia because of its role in the breakdown of dopamine in the prefrontal cortex. The COMT gene contains a functional polymorphism changing enzyme activity that has been associated with some neuropsychiatric (endo)phenotypes, e.g. cognitive performance and anxiety. In this study we investigated the association between the COMT Val(158)Met polymorphism and obsessive-compulsive symptoms in patients with schizophrenia. Severity of obsessive-compulsive symptoms in 77 male patients with recent-onset schizophrenia was assessed using the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), and the COMT Val(158)Met polymorphism was genotyped for these patients. We found a significant effect of the COMT genotype on Y-BOCS scores: the Val/Val genotype was associated with the highest Y-BOCS scores, whereas patients with the Met/Met genotype had the lowest Y-BOCS scores. Our data suggest that the COMT high-activity Val allele is associated with more obsessive-compulsive symptoms in young patients with schizophrenia. These results support the hypothesis that the COMT Val(158)Met polymorphism may be a modifier gene for the symptomatology of schizophrenia.

Impact of schizophrenia candidate genes on schizotypy and cognitive endophenotypes at the population level

BACKGROUND

Aspects of cognitive function and schizotypy have been proposed as potential endophenotypes for schizophrenia. It is unknown whether the expression of these endophenotypes at the population level is modulated by the genetic variability of candidate susceptibility genes for schizophrenia.

METHODS

We examined the potential impact of 18 single nucleotide polymorphisms (SNPs) within the DTNBP1, NRG1, DAOA/G32, and DAAO genes, on cognition and self-rated schizotypy, in a representative population of 2243 young male military conscripts. Single SNP and haplotype associations were evaluated.

RESULTS

The DTNBP1 SNPs rs2619522 and rs760761 exhibited several single marker associations, the minor alleles being associated with lower attention capacity but also a decrease in positive and paranoid schizotypy scores. The DTNBP1 haplotype load had borderline associations with nonverbal IQ, paranoid schizotypy, and sustained attention. For individual NRG1 polymorphisms, isolated but weak signals of association were noted with sustained attention and working memory but not schizotypy. The risk allele of functional SNP8NRG243177 was associated with reduced spatial working memory capacity. An isolated effect of DAAO haplotype variability was noted on negative and disorganization schizotypy. No convincing association of DAOA/G32 variability was detected.

CONCLUSIONS

The DTNBP1 and, less so, NRG1 and DAAO variants might exert gene-specific modulating effects on schizophrenia endophenotypes at the population level.

A region of 35 kb containing the trace amine associate receptor 6 (TAAR6) gene is associated with schizophrenia in the Irish study of high-density schizophrenia families

The TAAR6 gene has been previously associated with schizophrenia in 192 pedigrees of European and African ancestry. To replicate these findings we performed an association study of TAAR6 in 265 pedigrees of the Irish Study of High-Density Schizophrenia Families (ISHDSF). Of the 24 genotyped single-nucleotide polymorphisms only rs12189813 and rs9389011 provided single-marker evidence for association (0.0094<or=P<or=0.03). Two-marker haplotypes (rs7772821 and rs12189813; 0.0071<or=P<or=0.0023) and four-marker haplotypes (rs8192622, rs7772821, rs12189813 and rs9389011; 0.0047<or=P<or=0.018) gave strongest evidence for association. The associated high-risk (HR) haplotype in the ISHDSF is defined by the major alleles at rs7772821 and rs12189813 (0.00097<or=P<or=0.023). The associated HR remains positive in a case only test of association by Operational Criteria score analysis in which significant association was observed only with the highest threshold for delusions (P<0.009). When analysis was restricted to affected individuals under the core schizophrenia (D2) diagnosis, the observed associations for HR were most significant in the highest threshold for delusions (P<0.004) and hallucinations (P<0.0004), supporting the family-based association with schizophrenia.

A genome-wide scan for modifier loci in schizophrenia

The purpose of this study was to detect genetic loci that influence clinical features of, but not necessarily susceptibility to, psychotic illness. In the Irish Study of High-Density Schizophrenia Families (n = 270 families, n = 1,408 individuals), subjects with non-affective psychosis were rated using the Operational Criteria Checklist for Psychotic Illness. Factor analysis identified hallucinations, delusions, and negative, manic, and depressive symptom factors. We performed autosomal genome-wide multipoint non-parametric quantitative trait locus linkage analysis, in affected individuals only, using these five factors, as well as age at onset, and course of illness. Determination of empirical significance and correction for multiple testing was implemented using 200 simulated genome scans. We also tested for pleiotropic loci by examining the sums of -log(10)'s of the empirical P values of multiple traits in selected regions. LODs of 2.42 and 2.35 were obtained near D9S934 (9q33.1) and D14S587 (14q24.2), respectively, for course of illness, and of 2.26 between D6S1040-D6S2420 (6q23.1-25.1) and age at onset. No other regions met criteria for suggestive linkage to any one trait. No loci were significant after correction for multiple testing. On 6q, however, the joint linkage of age of onset, course, delusions, and depressive symptoms resulted in a genome-wide P = 0.06. We conclude that genes located near 9q33.1 and 14q24.2 may modify the clinical course and severity of schizophrenia. A gene in 6q may affect several clinical features of illness.

Application of electroencephalography to the study of cognitive and brain functions in schizophrenia

The electroencephalogram (EEG) recorded from the human scalp is widely used to study cognitive and brain functions in schizophrenia. Current research efforts are primarily devoted to the assessment of event-related potentials (ERPs) and event-related oscillations (EROs), extracted from the ongoing EEG, in patients with schizophrenia and in clinically unaffected individuals who, due to their family history and current mental status, are at high risk for developing schizophrenia. In this article, we discuss the potential usefulness of ERPs and EROs as genetic vulnerability markers, as pathophysiological markers, and as markers of possible ongoing progressive cognitive and cortical deterioration in schizophrenia. Our main purpose is to illustrate that these neurophysiological measures can offer valuable quantitative biological markers of basic pathophysiological mechanisms and cognitive dysfunctions in schizophrenia, yet they may not be specific to current psychiatry's diagnosis and classification. These biological markers can provide unique information on the nature and extent of cognitive and brain dysfunction in schizophrenia. Moreover, they can be utilized to gain deeper theoretical insights into illness etiology and pathophysiology and may lead to improvements in early detection and more effective and targeted treatment of schizophrenia. We conclude by addressing several key methodological, conceptual, and interpretative issues involved in this research field and by suggesting future research directions.

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.

Disrupted in schizophrenia 1 genotype and positive symptoms in schizophrenia

BACKGROUND

Converging evidence has demonstrated an association between the Disrupted in Schizophrenia 1 (DISC1) gene and schizophrenia (SZ). Within the DISC1 gene, a single nucleotide polymorphism (SNP), Ser704Cys, has been associated with the structure and function of the hippocampus. Because positive symptoms in SZ have also been associated with hippocampal structure and function, we hypothesized that variation in a DISC1 haplotype containing Ser704Cys would be significantly associated with positive symptomatology in SZ.

METHODS

We tested for an association between variation in a haplotype block within the DISC1 gene containing Ser704Cys and lifetime history of positive symptoms in 199 Caucasian patients with SZ.

RESULTS

We detected significant associations between a DISC1 haplotype containing Ser704Cys and Ser704Cys genotype and lifetime severity of delusions in SZ.

CONCLUSIONS

These data suggest that that the effect of DISC1 genetic variation might be associated with positive symptoms in patients with SZ.

Antibodies to Toxoplasma gondii in patients with schizophrenia: a meta-analysis

Recent studies have linked infectious agents to schizophrenia. The largest number of studies has involved the analysis of Toxoplasma gondii; these studies were subjected to a meta-analysis. Published articles and abstracts were identified by searches of MEDLINE, Ovid, and Google Scholar; by a search of Chinese publications; through letters to researchers; and by visiting China. Published and unpublished controlled studies that used serological methods for measuring T. gondii antibodies to assess inpatients and/or outpatients diagnosed with schizophrenia were selected for analysis, and source documents were translated as needed. Forty-two studies carried out in 17 countries over 5 decades were identified; 23 of these (6 unpublished) met selection criteria. The combined odds ratio (OR) was 2.73 (95% confidence interval, 2.10 to 3.60; chi-square with 1 df 263; P < .000001). Seven studies that included only patients with first-episode schizophrenia (OR 2.54) did not differ significantly from 16 studies that included patients in all clinical phases (OR 2.79). The results suggest that individuals with schizophrenia have an increased prevalence of antibodies to T. gondii. This association is consistent with other epidemiological studies as well as with animal studies. Although the OR of 2.73 is modest, it exceeds that for genetic or other environmental factors identified to date and suggests that Toxoplasma is in some way associated with a large number of cases of schizophrenia. If an etiological association can be proven, it would have implications for the design of measures for the prevention and treatment of this disease.

A resistance gene in disguise for schizophrenia?

We examined the hypothesis that -485 T, a novel polymorphism in the promoter region of the neuropeptide Y gene which was shown to be associated with schizophrenia in our previous paper, confers susceptibility to the disease. For a case-control association study, 331 unrelated Japanese schizophrenics (S(1): milder cases in the previous study, n = 212; and S(2): additional severer cases, n = 119) and 199 unrelated normal controls were recruited. Contribution of -485 T to the risk and the severity of the illness was examined by (1) comparing the risk in each genotype in the general population, (2) testing correlations between the gene-dose of -485 T, and the severity of chronic outcome in S(2) assessed with the Positive and Negative Symptom Scale, and (3) comparing the distribution of age at onset in S(1) + S(2) among the three genotypes. -485 T was significantly associated with schizophrenia in S(1) + S(2). Significant negative correlations were observed between the gene-dose and the symptom assessment scores in all items. The homozygote of -485 T showed a second peak frequency in the late-onset group both in males and females, while the homozygote of the alternative allele showed a single peak in the early-onset group. The higher risk of schizophrenia in the heterozygote than in the homozygote of -485 T in the general population did not support the possibility of linkage disequilibrium with a susceptibility gene. -485 T most likely confers resistance but not susceptibility to schizophrenia. An interaction between a nuclear resistance gene and a presumptive pathogenic gene in the mitochondrial DNA was suggested.

Schizophrenia: a common disease caused by multiple rare alleles

Schizophrenia is widely held to stem from the combined effects of multiple common polymorphisms, each with a small impact on disease risk. We suggest an alternative view: that schizophrenia is highly heterogeneous genetically and that many predisposing mutations are highly penetrant and individually rare, even specific to single cases or families. This "common disease--rare alleles" hypothesis is supported by recent findings in human genomics and by allelic and locus heterogeneity for other complex traits. We review the implications of this model for gene discovery research in schizophrenia.

Schizotypal dimensions: an intermediate phenotype associated with the COMT high activity allele

BACKGROUND

Although catechol-O-methyltransferase (COMT) has long been suggested to be implicated in the pathogenesis of schizophrenia, association studies have generated discrepant results concerning the involvement of the COMT gene in schizophrenia. As several studies have suggested that schizotypal traits might be genetically related to schizophrenia, increased statistical power to detect gene effects could be obtained by using dimensional personality traits in unaffected relatives.

METHODS

We tested the hypothesis that the functional Val158Met COMT polymorphism might contribute to the variance of self-reported schizotypal scores in a sample of 106 unaffected subjects, composed of controls (N = 57), first-degree relatives of schizophrenic (N = 27) and of bipolar (N = 22) probands. We also looked for specific associations between COMT polymorphisms and the three dimensions of schizotypy (positive, negative, disorganized) assessed by the Schizotypal Personality Questionnaire (SPQ).

RESULTS

We found that self-reported SPQ scores are related to COMT genotype (P = 0.01), with individuals homozygous for the high activity allele having the highest scores. This association is primarily due to specific associations with the positive (P = 0.001) and negative (P = 0.04) dimensions.

CONCLUSIONS

Our data support the hypothesis that the functional COMT polymorphism could be involved in different psychotic dimensions. This confirms that studying specific schizotypal dimensions can help to identify the genes involved in the pathogenesis of psychosis.

Partial evidence of an association between epidermal growth factor A61G polymorphism and age at onset in male schizophrenia

Epidermal growth factor (EGF) is a well-known neurotrophic factor regulating the development of various neuronal cells, including dopaminergic neurons, and dysfunction of EGF signals has been demonstrated as a risk factor for schizophrenia. Recently, several researchers have investigated associations including age at onset (AAO) with EGF A61G functional polymorphism, but the results of these studies have been controversial. Thus, we investigated whether A61G plays a role in predisposition to schizophrenia and its effects on AAO. Our subjects included 190 patients with schizophrenia and 347 controls. We assessed three different points of AAO: age at first occurrence of positive psychotic symptoms, medication, and hospitalization as a patient with schizophrenia. We found no differences in allele and genotype frequencies between patients and controls or associations between A61G and AAOs across stratified points in the entire sample and in each gender. However, we found significant gender differences in patients with the AA genotype in all stratified points of AAOs. Subset analyses of G allele distribution between clinical subsets with an AAO cutoff of 20 years revealed that male patients with early onset schizophrenia were more likely to exhibit the common AA homozygote than male patients with adulthood onset schizophrenia. In conclusion, although we were unable to support an association between EGF A61G and schizophrenia, the AA genotype might play a disease-modifying role differentially according to gender.

Applications and limitations of empiric data in provision of recurrence risks for schizophrenia: a practical review for healthcare professionals providing clinical psychiatric genetics consultations

Schizophrenia is a common disorder that may frequently be encountered when taking family histories in the genetics clinic, whether or not the referral is for a psychiatric indication. Like in other common disorders, the provision of recurrence risks for schizophrenia is a complex clinical issue because empiric recurrence risks (while reasonably well established) can rarely be used without individual tailoring. This review seeks to identify and detail some pertinent issues surrounding the clinical utility of empiric recurrence risks for schizophrenia, and to provide an overview of important factors to consider when tailoring empiric risks for individual patients.

Identification of Susceptibility Loci for Alcohol-Related Traits in the Irish Affected Sib Pair Study of Alcohol Dependence

BACKGROUND

Alcoholism is a phenotypically and probably genetically heterogeneous condition. Thus, one strategy for finding genes influencing liability to alcoholism is to study the components of alcoholism, which may be more directly related to the underlying pathophysiology than is clinical diagnosis. The goal of this study was to identify genomic regions containing susceptibility loci for alcohol-related traits.

METHODS

A 4-cM dense whole-genome linkage study was conducted in the Irish Affected Sib Pair Study of Alcohol Dependence. Probands, affected siblings, and parents were evaluated by structured interview. Variance component linkage analysis was applied to data from 485 families for 5 measures: initial sensitivity and tolerance (based on scales from the self-report of the effects of ethanol; maximum drinks within 24 hours, an empirically derived factor score based on withdrawal symptoms, and age at onset of alcohol dependence.

RESULTS

Evidence for linkage (p<0.005) was found on 9 chromosomes. For age at onset, 2 regions were found on chromosome 9 (highest Lod=2.3, p=0.0005). For initial level of response to alcohol, suggestive regions were on chromosomes 1 and 11 (highest Lod=2.9, p=0.0001 on chromosome 11), while those for tolerance signals were on chromosomes 1, 6, and 22. Maximum drinking was associated with regions on chromosomes 12 and 18. For withdrawal symptoms, the highest peak was on chromosome 2 (Lod=2.2, p=0.0007).

CONCLUSIONS

Using quantitative measures of components of alcohol dependence, we identified several regions of the genome that may contain susceptibility loci for specific alcohol-related traits and merit additional study.

Clinical impact of recently detected susceptibility genes for schizophrenia

After years of frustration, the search for genes impacting on schizophrenia is now undergoing some exciting developments. Several proposals of susceptibility genes have been able to be supported by replications. Thus, there are now at least three very strong candidates: the gene for dysbindin (DINBP1), the gene for neuregulin-1 (NRG1), and a less well-understood gene locus, G72/G30, which are likely to influence manifestations of schizophrenia. Other “hot” candidates such as the disrupted-in-schizophrenia 1 gene (DISC1) and the gene coding for protein kinase B (AKT1) might also prove to be susceptibility genes in the next future. The clinical implications of these findings are not yet fully visible. However, some first insights are possible: most of the genetic findings lack diagnostic specificity, and are also reproduced in bipolar disorder. Strong associations are also obtained on a symptomatic level, not only on a diagnostic level. The pathophysiological role of these hot candidate genes is currently under intensive study.

Factor analysis of obsessive-compulsive disorder YBOCS-SC symptoms and association with 5-HTTLPR SERT polymorphism

Symptoms defining obsessive-compulsive disorder (OCD) are diverse. Factor analyses of OCD symptoms have been used to evaluate obsessive-compulsive phenotypes that are thought to be more homogenous than the macro phenotype. There is evidence that three factors (symmetry/ordering, obsessions/checking, and hoarding) are familial and preliminary evidence that repetitive rituals are associated with a functional polymorphism in the promotor region of the serotonin transporter gene (5-HTTLPR). The goal of this study was to examine relationships between OCD symptom dimensions and 5-HTTLPR. We genotyped 153 subjects who met DSM-IV criteria for OCD. We used the Yale-Brown obsessive-compulsive scale symptom checklist (YBOCS-SC) to assess OCD symptoms. Using principle components analysis, we derived four factors from the 13 symptom categories of the YBOCS-SC in this patient sample, which replicated previous factor analyses of this scale. The frequencies of the S allele and the SS genotype were associated with the second factor including obsessions regarding symmetry and compulsions involving repeating, counting, and ordering/arranging. This study may contribute to understanding of molecular genetic features underlying the appearance of symptom clusters in OCD.

Excitement and confusion on chromosome 6q: the challenges of neuropsychiatric genetics in microcosm

The search for genes that are implicated in the pathogenesis of schizophrenia (SCZ), bipolar disorder (BPD) and other complex neuropsychiatric phenotypes has yielded a plethora of positive findings, but has also engendered a substantial degree of confusion. Exciting findings include positive linkage results in a number of chromosomal regions and the identification of several genes that have been associated with SCZ and to a lesser extent with BPD. Confusing aspects include the difference between studies in localization of linkage peaks in the same chromosomal regions, raising the possibility that these regions may harbor more than one gene, the fact that positive linkage findings as well as associated genes appear in several cases to be shared by more than one disorder, and the failure to identify thus far the precise pathogenic variants in associated genes. Recent findings of linkage and association studies on chromosome 6q illustrate the current status of neuropsychiatric genetics in intriguing microcosm. Positive findings from linkage and association studies are reviewed in order to identify approaches that may help to settle apparent contradictions and allow an interpretation of the results that may prove useful in application to findings from other chromosomal regions. Not only SCZ and BPD but also other psychiatric and neurological phenotypes are considered. Taking a topographic approach, we identify five foci of positive findings on chromosome 6q and suggest that each may harbor gene(s) that confer susceptibility to SCZ or BPD or may modify their onset or clinical course. We further suggest that in searching for these genes the possibility that they may be implicated in more than one disorder should be taken into account. We also discuss the potential contribution of rare genetic variants identified in homogeneous, isolated populations to the subsequent identification of common variants in the same gene that contribute to disease susceptibility in outbred populations.