Heritability and number of quantitative trait loci of neurocognitive functions in families with schizophrenia

Heritable anisotropy associated with cognitive impairments among patients with schizophrenia and their non-psychotic relatives in multiplex families

BACKGROUND

To test the functional implications of impaired white matter (WM) connectivity among patients with schizophrenia and their relatives, we examined the heritability of fractional anisotropy (FA) measured on diffusion tensor imaging data acquired in Pittsburgh and Philadelphia, and its association with cognitive performance in a unique sample of 175 multigenerational non-psychotic relatives of 23 multiplex schizophrenia families and 240 unrelated controls (total = 438).

METHODS

We examined polygenic inheritance (h2r) of FA in 24 WM tracts bilaterally, and also pleiotropy to test whether heritability of FA in multiple WM tracts is secondary to genetic correlation among tracts using the Sequential Oligogenic Linkage Analysis Routines. Partial correlation tests examined the correlation of FA with performance on eight cognitive domains on the Penn Computerized Neurocognitive Battery, controlling for age, sex, site and mother's education, followed by multiple comparison corrections.

RESULTS

Significant total additive genetic heritability of FA was observed in all three-categories of WM tracts (association, commissural and projection fibers), in total 33/48 tracts. There were significant genetic correlations in 40% of tracts. Diagnostic group main effects were observed only in tracts with significantly heritable FA. Correlation of FA with neurocognitive impairments was observed mainly in heritable tracts.

CONCLUSIONS

Our data show significant heritability of all three-types of tracts among relatives of schizophrenia. Significant heritability of FA of multiple tracts was not entirely due to genetic correlations among the tracts. Diagnostic group main effect and correlation with neurocognitive performance were mainly restricted to tracts with heritable FA suggesting shared genetic effects on these traits.

Variants in regulatory elements of PDE4D associate with major mental illness in the Finnish population

We have previously reported a replicable association between variants at the PDE4D gene and familial schizophrenia in a Finnish cohort. In order to identify the potential functional mutations underlying these previous findings, we sequenced 1.5 Mb of the PDE4D genomic locus in 20 families (consisting of 96 individuals and 79 independent chromosomes), followed by two stages of genotyping across 6668 individuals from multiple Finnish cohorts for major mental illnesses. We identified 4570 SNPs across the PDE4D gene, with 380 associated to schizophrenia (p ≤ 0.05). Importantly, two of these variants, rs35278 and rs165940, are located at transcription factor-binding sites, and displayed replicable association in the two-stage enlargement of the familial schizophrenia cohort (combined statistics for rs35278 p = 0.0012; OR = 1.18, 95% CI: 1.06-1.32; and rs165940 p = 0.0016; OR = 1.27, 95% CI: 1.13-1.41). Further analysis using additional cohorts and endophenotypes revealed that rs165940 principally associates within the psychosis (p = 0.025, OR = 1.18, 95% CI: 1.07-1.30) and cognitive domains of major mental illnesses (g-score p = 0.044, β = -0.033). Specifically, the cognitive domains represented verbal learning and memory (p = 0.0091, β = -0.044) and verbal working memory (p = 0.0062, β = -0.036). Moreover, expression data from the GTEx database demonstrated that rs165940 significantly correlates with the mRNA expression levels of PDE4D in the cerebellum (p-value = 0.04; m-value = 0.9), demonstrating a potential functional consequence for this variant. Thus, rs165940 represents the most likely functional variant for major mental illness at the PDE4D locus in the Finnish population, increasing risk broadly to psychotic disorders.

Brain function during stages of working memory in schizophrenia and psychotic bipolar disorder

Working memory (WM) is impaired in psychotic disorders and linked to functional outcome. Most neurobiological models emphasize prefrontal cortex (PFC) dysfunction in the etiology of WM impairment. However, WM is composed of multiple processes, including encoding and maintenance, and the delineation of the neurobiology of these sub-processes has not been well characterized in schizophrenia and psychotic bipolar disorder. Functional MRI was obtained during an event-related spatial delayed match-to-sample task from 58 healthy individuals, 72 individuals with schizophrenia and 41 people with bipolar I disorder with psychotic features in order to: 1) characterize neural responses during encoding, maintenance and retrieval stages of WM using complementary region-of-interest and whole brain approaches; 2) determine whether schizophrenia and psychotic bipolar disorder exhibit similar abnormalities in WM-related brain function; and 3) elucidate the associations between WM-related brain function, task performance, and neuropsychological functioning. Both schizophrenia and psychotic bipolar disorder groups showed encoding- and maintenance-related impairments in the posterior parietal cortex (PPC) and frontal eye fields (FEF). BOLD response in the PPC and FEF, during encoding and maintenance respectively, was associated with task performance independent of group. Additionally, encoding-related activation in the PPC correlated with general neuropsychological functioning independent of group. Only encoding-related activation in the right ventral striatum differed between schizophrenia and psychotic bipolar disorder; individuals with schizophrenia showed significantly lower activation than both psychotic bipolar disorder and healthy groups. Our results are consistent with emerging evidence implicating PPC dysfunction in WM impairment and suggest interventions targeting neural activation in PPC may improve WM and neuropsychological functioning across psychotic disorders.

Gene expression changes related to immune processes associate with cognitive endophenotypes of schizophrenia

Schizophrenia is a heterogeneous disorder characterized by a spectrum of symptoms and many different underlying causes. Thus, instead of using the broad diagnosis, intermediate phenotypes can be used to possibly decrease the underlying complexity of the disorder. Alongside the classical symptoms of delusions and hallucinations, cognitive deficits are a core feature of schizophrenia. To increase our understanding of the biological processes related to these cognitive deficits, we performed a genome-wide gene expression analysis. A battery of 14 neuropsychological tests was administered to 844 individuals from a Finnish familial schizophrenia cohort. We grouped the applied neuropsychological tests into five factors for further analysis. Cognitive endophenotypes, whole blood mRNA, genotype, and medication use data were studied from 47 individuals. Expression level of several RNA probes were significantly associated with cognitive performance. The factor representing Verbal Working Memory was associated with altered expression levels of 11 probes, of which one probe was also associated with a specific sub-measure of this factor (WMS-R Digit span backward). While, the factor Processing speed was related to one probe, which additionally associated among 55 probes with a specific sub-measure of this factor (WAIS-R Digit symbol). Two probes were associated with the measure recognition memory performance. Enrichment analysis of these differentially expressed probes highlighted immunological processes. Our findings are in line with genome-wide genetic discoveries made in schizophrenia, suggesting that immunological processes may be of biological interest for future drug design towards schizophrenia and the cognitive dysfunctions that underlie it.

Intranasal Oxytocin May Improve High-Level Social Cognition in Schizophrenia, But Not Social Cognition or Neurocognition in General: A Multilevel Bayesian Meta-analysis

While there is growing interest in the potential for intranasal oxytocin (IN-OT) to improve social cognition and neurocognition (ie, nonsocial cognition) in schizophrenia, the extant literature has been mixed. Here, we perform a Bayesian meta-analysis of the efficacy of IN-OT to improve areas of social and neurocognition in schizophrenia. A systematic search of original research publications identified randomized controlled trials (RCTs) of IN-OT as a treatment for social and neurocognitive deficits in schizophrenia for inclusion. Standardized mean differences (SMD) and corresponding variances were used in multilevel Bayesian models to obtain meta-analytic effect-size estimates. Across a total of 12 studies (N = 273), IN-OT did not improve social cognition (SMD = 0.07, 95% credible interval [CI] = [-0.06, 0.17]) or neurocognition (SMD = 0.12, 95% CI = [-0.12, 0.34]). There was moderate between study heterogeneity for social cognition outcomes (τs= 0.12). Moderator analyses revealed that IN-OT had a significantly larger effect on high-level social cognition (ie, mentalizing and theory of mind) compared to low-level social cognition (ie, social cue perception) (b = 0.19, 95% CI = [0.05, 0.33]). When restricting our analysis to outcomes for high-level social cognition, there was a significant effect of IN-OT (SMD = 0.20, 95 % CI = [0.05, 0.33]) but the effect was not robust to sensitivity analyses. The present analysis indicates that IN-OT may have selective effects on high-level social cognition, which provides a more focused target for future studies of IN-OT.

Shared and distinct neurocognitive endophenotypes of schizophrenia and psychotic bipolar disorder

Objective

Schizophrenia and bipolar disorder are characterized by the presence of neurocognitive impairments on the psychosis continuum. The present study aimed to explore the shared and distinct endophenotypes between these disorders.

Methods

The study included 34 probands with remitted schizophrenia and 34 probands with euthymic bipolar disorder who had a history of psychotic symptoms that met the Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV) criteria, unaffected first-degree relatives of probands (31 relatives of probands with schizophrenia and 29 relatives of probands with bipolar disorder), and 34 healthy controls. Cognitive assessments were performed using the digit span, continuous performance, Rey auditory and visual learning, complex figure, verbal fluency, Wisconsin card sorting, and finger tapping tests.

Results

Probands with schizophrenia showed the most generalized and severe cognitive deficits across cognitive domains (working memory, verbal learning and memory, visual memory, verbal fluency, and executive function). Some domains of cognitive function (working memory, verbal learning, and memory) were also impaired in probands with bipolar disorder, but to a lesser degree than in probands with schizophrenia. All probands and relatives showed a common deficit in working memory compared to healthy controls. Relatives of probands with schizophrenia also showed verbal fluency dysfunction. Cognitive performance of all relatives was intermediate to the performance of both patients and healthy controls.

Conclusion

These findings suggest that a deficit in working memory could be a shared endophenotype of genetic vulnerability to schizophrenia and psychotic bipolar disorder, and verbal fluency could be a candidate endophenotype for schizophrenia specifically.

Genetic complexity of episodic memory: a twin approach to studies of aging

Episodic memory change is a central issue in cognitive aging, and understanding that process will require elucidation of its genetic underpinnings. A key limiting factor in genetically informed research on memory has been lack of attention to genetic and phenotypic complexity, as if "memory is memory" and all well-validated assessments are essentially equivalent. Here we applied multivariate twin models to data from late-middle-aged participants in the Vietnam Era Twin Study of Aging to examine the genetic architecture of 6 measures from 3 standard neuropsychological tests: the California Verbal Learning Test-2, and Wechsler Memory Scale-III Logical Memory (LM) and Visual Reproductions (VR). An advantage of the twin method is that it can estimate the extent to which latent genetic influences are shared or independent across different measures before knowing which specific genes are involved. The best-fitting model was a higher order common pathways model with a heritable higher order general episodic memory factor and three test-specific subfactors. More importantly, substantial genetic variance was accounted for by genetic influences that were specific to the latent LM and VR subfactors (28% and 30%, respectively) and independent of the general factor. Such unique genetic influences could partially account for replication failures. Moreover, if different genes influence different memory phenotypes, they could well have different age-related trajectories. This approach represents an important step toward providing critical information for all types of genetically informative studies of aging and memory.

Genome-Wide Analyses of Working-Memory Ability: A Review

Working memory, a theoretical construct from the field of cognitive psychology, is crucial to everyday life. It refers to the ability to temporarily store and manipulate task-relevant information. The identification of genes for working memory might shed light on the molecular mechanisms of this important cognitive ability and-given the genetic overlap between, for example, schizophrenia risk and working-memory ability-might also reveal important candidate genes for psychiatric illness. A number of genome-wide searches for genes that influence working memory have been conducted in recent years. Interestingly, the results of those searches converge on the mediating role of neuronal excitability in working-memory performance, such that the role of each gene highlighted by genome-wide methods plays a part in ion channel formation and/or dopaminergic signaling in the brain, with either direct or indirect influence on dopamine levels in the prefrontal cortex. This result dovetails with animal models of working memory that highlight the role of dynamic network connectivity, as mediated by dopaminergic signaling, in the dorsolateral prefrontal cortex. Future work, which aims to characterize functional variants influencing working-memory ability, might choose to focus on those genes highlighted in the present review and also those networks in which the genes fall. Confirming gene associations and highlighting functional characterization of those associations might have implications for the understanding of normal variation in working-memory ability and also for the development of drugs for mental illness.

WORKING MEMORY IMPAIRMENT AS AN ENDOPHENOTYPIC MARKER OF A SCHIZOPHRENIA DIATHESIS

This chapter focuses on the viability of working memory impairment as an endophenotypic marker of a schizophrenia diathesis. It begins with an introduction of the construct of working memory. It follows with a review of the operational criteria for defining an endophenotype. Research findings regarding the working memory performance of schizophrenia and schizophrenia-spectrum patients, first-degree relatives of schizophrenia patients and healthy controls, are reviewed in terms of the criteria for being considered an endophenotypic marker. Special attention is paid to specific components of the working memory deficit (namely, encoding, maintenance, and manipulation), in terms of which aspects are likely to be the best candidates for endophenotypes. We consider the extant literature regarding working memory performance in bipolar disorder and major depression in order to address the issue of relative specificity to schizophrenia. Despite some unresolved issues, it appears that working memory impairment is a very promising candidate for an endophenotypic marker of a schizophrenia diathesis but not for mood disorders. Throughout this chapter, we identify future directions for research in this exciting and dynamic area of research and evaluate the contribution of working memory research to our understanding of schizophrenia.

Does originating from a genetic isolate affect the level of cognitive impairments in schizophrenia families?

Earlier studies have detected differences in the prevalence, symptomatology and genetic risk variants of schizophrenia between a north-eastern Finnish genetic isolate and the rest of Finland. This study compared a population-based isolate sample (145 persons with schizophrenia, 304 first-degree relatives and 32 controls) with a rest of Finland sample (73 persons with schizophrenia, 100 first-degree relatives and 80 controls) in cognitive functioning. Persons from the isolate outperformed persons in the rest of Finland sample in verbal learning, verbal ability and cognitive flexibility in the schizophrenia groups and in verbal learning, speeded processing and attentional control in the relatives groups. The differences between the subsamples remained significant after taking into account an intragenic Reelin STR allele, previously associated with cognitive impairments and almost absent from the isolate, in addition to disorder characteristics and familial loading. In control groups, we observed no differences between the isolate and the rest of Finland. In conclusion, cognitive impairments were milder in schizophrenia patients and their first-degree relatives within than outside the isolate. An absence of differences between the control samples suggests that the differences in schizophrenia families may relate to genetic background, possibly to partly distinct variants affecting the liability inside and outside the isolate.

Discrimination within Recognition Memory in Schizophrenia

Episodic memory is one of the most affected cognitive domains in schizophrenia. First-degree biological relatives of individuals with schizophrenia also have been found to exhibit a similar, but milder, episodic memory deficit. Unlike most studies that focus on the percent of previously presented items recognized, the current investigation sought to further elucidate the nature of memory dysfunction associated with schizophrenia by examining the discrimination of old and new material during recognition (measured by d') to consider false recognition of new items. Using the Recurring Figures Test and the California Verbal Learning Test (CVLT), we studied a sample of schizophrenia probands and the first-degree biological relatives of patients with schizophrenia, as well as probands with bipolar disorder and first-degree biological relatives to assess the specificity of recognition memory dysfunction to schizophrenia. The schizophrenia sample had poorer recognition discrimination in both nonverbal and verbal modalities; no such deficits were identified in first-degree biological relatives or bipolar disorder probands. Discrimination in schizophrenia and bipolar probands failed to benefit from the geometric structure in the designs in the manner that controls did on the nonverbal test. Females performed better than males in recognition of geometric designs. Episodic memory dysfunction in schizophrenia is present for a variety of stimulus domains and reflects poor use of item content to increase discrimination of old and new items.

Sex differences in familiality effects on neurocognitive performance in schizophrenia

BACKGROUND

Numerous studies have documented that patients with schizophrenia show neurocognitive impairments, which are also heritable in schizophrenia families. In view of these findings, the current investigation tested the hypothesis that neurocognitive performance of schizophrenia probands can predict the neurocognitive performance of their unaffected family members.

METHODS

Participants (n=1967; schizophrenia=369; first-degree relatives=1072; community comparison subjects=526) in the Consortium on the Genetics of Schizophrenia were administered the Penn Computerized Neurocognitive Battery.

RESULTS

Consistent with prior work, probands showed significant neurocognitive impairment, and neurocognitive ability was significantly heritable across domains. On average, unaffected relatives did not differ from community comparison subjects in their neurocognitive performance. However, in six of seven domains, proband scores predicted the performance of their unaffected siblings. Male, but not female, proband performance was predictive of their unaffected relatives' (siblings and mothers) performance, most consistently in face memory and spatial processing.

CONCLUSIONS

Using a novel approach in which individual probands are paired with their respective unaffected relatives within each family, we found that male proband performance predicted both sister and brother performance, an effect that was most powerfully observed for face memory and spatial processing. Results suggest that the familial transmission of sexually dimorphic neurocognitive domains, in which a particular sex tends to show a performance advantage over the other, may not itself be sex specific in schizophrenia families.

Pearls and pitfalls in genetic studies of migraine

Purpose of review

Migraine is a prevalent neurovascular brain disorder with a strong genetic component, and different methodological approaches have been implemented to identify the genes involved. This review focuses on pearls and pitfalls of these approaches and genetic findings in migraine.

Summary

Common forms of migraine (i.e. migraine with and without aura) are thought to have a polygenic make-up, whereas rare familial hemiplegic migraine (FHM) presents with a monogenic pattern of inheritance. Until a few years ago only studies in FHM yielded causal genes, which were identified by a classical linkage analysis approach. Functional analyses of FHM gene mutations in cellular and transgenic animal models suggest abnormal glutamatergic neurotransmission as a possible key disease mechanism. Recently, a number of genes were discovered for the common forms of migraine using a genome-wide association (GWA) approach, which sheds first light on the pathophysiological mechanisms involved.

Conclusions

Novel technological strategies such as next-generation sequencing, which can be implemented in future genetic migraine research, may aid the identification of novel FHM genes and promote the search for the missing heritability of common migraine.

[Schizophrenia and/or bipolar disorder: the neurocognitive endophenotypes]

Although Kraepelinian dichotomous conceptualization of psychosis was historically beneficial, modern studies do not support the existence of a sub-typing of psychotic illnesses into schizophrenic and affective psychoses. Years of intensive investigation on the genetic bases of schizophrenia and bipolar disorder suggest that these disorders, rather than being wholly distinct disorders, share common genetic risks. However, one of the most serious difficulties for genetic research in these illnesses is their enormous phenotypic heterogeneity. A response to this problem is the use of neurocognitive functions as endophenotypes or intermediate phenotypes. A review of the literature suggests that in both schizophrenia and bipolar disorder, neurocognitive functions are influenced by genetic factors and that there exists neuropsychological deficits in the nonaffected relatives of probands. However, it is unclear whether or not patterns of performance on neurocognitive tasks across probands as well as unaffected family members offer potential for identifying shared and illness-specific neurocognitive phenotypes for schizophrenia and bipolar disorder. Overlapping and unique neurocognitive endophenotypic signatures of the two psychoses are comprehensively described.

An examination of relationship between neurological soft signs and neurocognition

OBJECTIVE

Neurological soft signs (NSS) and cognitive function had been examined in schizophrenia, but their relationship has remained elusive for several years. We examined the relationship between NSS and cognitive functions in the present study.

METHOD

A cross sectional study was carried out. Subjects were drawn from first degree relatives of schizophrenia patients, admitted as inpatient or attending as an outpatient. Controls were recruited by word of mouth from hospital staff and visitors of hospitalized patients. Those subjects who satisfied the screening process were subjected to Cambridge Neurological Inventory for soft sign assessment and digit span test, paired associate learning test (PALT) and visuo-spatial working memory matrix (VSWMM) for cognitive function assessment. Correlation analysis and structural equation modeling (SEM) was used for analysis.

RESULT

Significant negative correlation of primitive reflexes with PALT; of motor coordination with VSWMM, working memory (WM) and cognitive index; of total NSS with WM and cognitive index among first degree relatives. SEM showed that motor soft signs have important negative influence over WM.

CONCLUSION

The current findings indicate that NSS have significant negative effect on cognitive functioning.

Genetic influence on the working memory circuitry: behavior, structure, function and extensions to illness

Working memory is a highly heritable complex cognitive trait that is critical for a number of higher-level functions. However, the neural substrates of this behavioral phenotype are intricate and it is unknown through what precise biological mechanism variation in working memory is transmitted. In this review we explore different functional and structural components of the working memory circuitry, and the degree to which each of them is contributed to by genetic factors. Specifically, we consider dopaminergic function, glutamatergic function, white matter integrity and gray matter structure all of which provide potential mechanisms for the inheritance of working memory deficits. In addition to discussing the overall heritability of these measures we also address specific genes that may play a role. Each of these heritable components has the potential to uniquely contribute to the working memory deficits observed in genetic disorders, including 22q deletion syndrome, fragile X syndrome, phenylketonuria (PKU), and schizophrenia. By observing the individual contributions of disruptions in different components of the working memory circuitry to behavioral performance, we highlight the concept that there may be many routes to a working memory deficit; even though the same cognitive measure may be a valid endophenotype across different disorders, the underlying cause of, and treatment for, the deficit may differ. This has implications for our understanding of the transmission of working memory deficits in both healthy and disordered populations.

Genetic overlap between episodic memory deficits and schizophrenia: results from the Maudsley Twin Study

BACKGROUND

Visual and verbal episodic memory deficits are putative endophenotypes for schizophrenia; however, the extent of any genetic overlap of these with schizophrenia is unclear. In this study, we set out to quantify the genetic and environmental contributions to variance in visual and verbal memory performance, and to quantify their genetic relationship with schizophrenia.

METHOD

We applied bivariate genetic modelling to 280 twins in a classic twin study design, including monozygotic (MZ) and dizygotic (DZ) pairs concordant and discordant for schizophrenia, and healthy control twins. We assessed episodic memory using subtests of the Wechsler Memory Scale - Revised (WMS-R).

RESULTS

Genetic influences (i.e. heritability) contributed significantly to variance in immediate recall of both verbal memory and visual learning, and the delayed recall of verbal and visual memory. Liability to schizophrenia was associated with memory impairment, with evidence of significant phenotypic correlations between all episodic memory measures and schizophrenia. Genetic factors were the main source of the phenotypic correlations for immediate recall of visual learning material; both immediate and delayed recall of verbal memory; and delayed recall of visual memory that, for example, shared genetic variance with schizophrenia, which accounted for 88% of the phenotypic correlation (rph=0.41) between the two.

CONCLUSIONS

Verbal memory and visual learning and memory are moderately heritable, share a genetic overlap with schizophrenia and are valid endophenotypes for the condition. The inclusion of these endophenotypes in genetic association studies may improve the power to detect susceptibility genes for schizophrenia.

Associations of anhedonia and cognition in persons with schizophrenia spectrum disorders, their siblings, and controls

The aim of the current study was to investigate the levels of social and physical anhedonia, as measured with the Chapman Scales for social and physical anhedonia in groups of patients with schizophrenia spectrum psychosis (n = 91), their unaffected siblings (n = 105), and control subjects drawn from a general population (n = 67). The second aim was to explore the effect of physical and social anhedonia on neuropsychological variables. Subjects with schizophrenia spectrum disorder had significantly more anhedonia than population controls, but the unaffected siblings did not differ from controls. Subjects with schizophrenia spectrum disorders had generalized cognitive deficits. Unaffected sibling status predicted impairments in executive and performance speed measures. Elevated physical anhedonia associated with deficits in verbal functions, but this was not related to genetic liability to schizophrenia. In conclusion, social and physical anhedonia did not seem to mediate neuropsychological deficits of schizophrenia family members.

Cognition, dopamine and bioactive lipids in schizophrenia

Schizophrenia is a remarkably complex disorder with a multitude of behavioral and biological perturbations. Cognitive deficits are a core feature of this disorder, and involve abnormalities across multiple domains, including memory, attention, and perception. The complexity of this debilitating illness has led to a view that the key to unraveling its pathophysiology lies in deconstructing the clinically-defined syndrome into pathophysiologically distinct intermediate phenotypes. Accumulating evidence suggests that one of these intermediate phenotypes may involve phospholipid signaling abnormalities, particularly in relation to arachidonic acid (AA). Our data show relationships between levels of AA and performance on tests of cognition for schizophrenia patients, with defects in AA signaling associated with deficits in cognition. Moreover, dopamine may moderate these relationships between AA and cognition. Taken together, cognitive deficits, dopaminergic neurotransmission, and bioactive lipids have emerged as related features of schizophrenia. Existing treatment options for cognitive deficits in schizophrenia do not specifically target lipid-derived signaling pathways; understanding these processes could inform efforts to identify novel targets for treatment innovation.

Prospective memory in non-psychotic first-degree relatives of patients with schizophrenia

Although a number of studies have found prospective memory (PM) impairment in patients with schizophrenia, very little is known about the PM performance in non-psychotic relatives of these patients. The current study aimed to explore the PM performance in non-psychotic first-degree relatives of these patients. Two groups of participants (26 non-psychotic first-degree relatives of schizophrenia patients and 26 healthy comparison participants) were administered three PM tasks (time-, event-, and activity-based) and a set of neurocognitive tests. Results showed that the relatives performed significantly worse than the comparisons on most indices of the PM tasks, with a similar pattern of impairment found in other neurocognitive measures. Together with findings from previous studies, results of the current study suggest that PM may be a potential endophenotype for schizophrenia.

RGS4 polymorphisms associated with variability of cognitive performance in a family-based schizophrenia sample

Polymorphisms of the gene encoding the regulator of G protein signaling, subtype 4 (RGS4), may be associated with schizophrenia. Among first-episode schizophrenia patients, they are also associated with dorsolateral prefrontal cortex (DLPFC) volume. The DLPFC is a key region that regulates heritable cognitive functions implicated in schizophrenia pathogenesis. To further understand the relationship of RGS4 variants to schizophrenia, we examined their associations with cognitive functions among schizophrenia patients and their relatives. We analyzed 31 multiplex, multigenerational Caucasian families with schizophrenia recruited on the basis of 2 affected first-degree relatives. All participants underwent a computerized neurocognitive battery that evaluates accuracy and speed (response time) of performance on abstraction/mental flexibility; attention; verbal, spatial, and face memory; and spatial ability. "Tag" single-nucleotide polymorphisms (SNPs) representing common polymorphisms were genotyped. Measured genotype analyses accounting for family relationships were performed using Sequential Oligogenic Linkage Analysis Routines. SNPs rs10917670 ("SNP1") and rs951439 ("SNP7") were associated with face memory speed (P = .0003) at a significance level that survived Bonferroni correction (P = .039). The same SNPs have earlier been reported to be associated with schizophrenia. There also were uncorrected associations with rs10917670 ("SNP1") and rs951439 ("SNP7") on face memory efficiency (P = .03) and verbal memory efficiency (P = 0.02), rs28757217 on abstraction/mental flexibility speed (P = .02) and verbal memory efficiency (P = .03), SNP18 (rs2661319) on spatial memory accuracy (P = 0.02) and face memory speed (P = .03). RGS4 polymorphisms are associated with variations in cognitive functions and contribute a small but statistically significant proportion of variance in a family-based sample.

Replication of association between working memory and Reelin, a potential modifier gene in schizophrenia

BACKGROUND

The challenges in gene identification for psychiatric disorders have awakened interest toward quantitative traits and endophenotypes that are potentially more closely related to the underlying biology and provide more power in the linkage and association analyses. Previously, we successfully replicated schizophrenia linkage on chromosome 7q21-32 in Finnish families and demonstrated that an intragenic short tandem repeat (STR) allele of the regional Reelin (RELN) gene is associated with multiple cognitive traits representing central cognitive functions regarded as valid endophenotypes for schizophrenia.

METHODS

Here, we used an extended sample of 290 Finnish families with schizophrenia and 375 control subjects in an association analysis between 96 SNPs and three STRs in RELN and diagnostic categories, clinical disorder features, as well as central cognitive functions impaired in schizophrenia.

RESULTS

We replicated the original association between RELN intragenic STR allele and working memory in individuals (n = 342) not overlapping with the previous study. This risk allele remained central in the whole study sample by being associated with impaired cognitive functioning and more severe positive and negative symptoms of schizophrenia (p = .0005-.00002). Additionally, multiple SNPs indicated association with the severity of positive symptoms of schizophrenia and together showed potential additive effect on the severity of the symptoms (p = .0000001). However, no significant associations with clinical diagnostic categories emerged.

CONCLUSIONS

The strongest effects on cognitive functions were detected among the affected individuals. We thus propose a particular role for RELN as a modifier gene of the pathogenesis of schizophrenia.

Genetics and intermediate phenotypes of the schizophrenia--bipolar disorder boundary

Categorization of psychotic illnesses into schizophrenic and affective psychoses remains an ongoing controversy. Although Kraepelinian subtyping of psychosis was historically beneficial, modern genetic and neurophysiological studies do not support dichotomous conceptualization of psychosis. Evidence suggests that schizophrenia and bipolar disorder rather present a clinical continuum with partially overlapping symptom dimensions, neurophysiology, genetics and treatment responses. Recent large scale genetic studies have produced inconsistent findings and exposed an urgent need for re-thinking phenomenology-based approach in psychiatric research. Epidemiological, linkage and molecular genetic studies, as well as studies in intermediate phenotypes (neurocognitive, neurophysiological and anatomical imaging) in schizophrenia and bipolar disorders are reviewed in order to support a dimensional conceptualization of psychosis. Overlapping and unique genetic and intermediate phenotypic signatures of the two psychoses are comprehensively recapitulated. Alternative strategies which may be implicated into genetic research are discussed.

Genome-wide approaches to schizophrenia

Schizophrenia (SZ) is a common and severe psychiatric disorder with both environmental and genetic risk factors, and a high heritability. After over 20 years of molecular genetics research, new molecular strategies, primarily genome-wide association studies (GWAS), have generated major tangible progress. This new data provides evidence for: (1) a number of chromosomal regions with common polymorphisms showing genome-wide association with SZ (the major histocompatibility complex, MHC, region at 6p22-p21; 18q21.2; and 2q32.1). The associated alleles present small odds ratios (the odds of a risk variant being present in cases vs. controls) and suggest causative involvement of gene regulatory mechanisms in SZ. (2) Polygenic inheritance. (3) Involvement of rare (<1%) and large (>100kb) copy number variants (CNVs). (4) A genetic overlap of SZ with autism and with bipolar disorder (BP) challenging the classical clinical classifications. Most new SZ findings (chromosomal regions and genes) have generated new biological leads. These new findings, however, still need to be translated into a better understanding of the underlying biology and into causal mechanisms. Furthermore, a considerable amount of heritability still remains unexplained (missing heritability). Deep resequencing for rare variants and system biology approaches (e.g., integrating DNA sequence and functional data) are expected to further improve our understanding of the genetic architecture of SZ and its underlying biology.

Association between a synaptosomal protein (SNAP-25) gene polymorphism and verbal memory and attention in patients with endogenous psychoses and mentally healthy subjects

Synaptosomal protein SNAP-25 is involved in the process of transmitting nerve spikes in the CNS and in the consolidation of memory traces in the hippocampus. Two independent studies have demonstrated associations between SNAP-25 gene polymorphisms and intellectual functions in a group of mentally healthy subjects and patients with schizophrenia. The aim of the present work was to perform a comparative study of the association between the MnlI polymorphism of SNAP-25 and cognitive functions (verbal memory, attention/executive functions) in 66 patients with endogenous psychoses, 75 of their mentally healthy relatives, and 136 healthy control subjects. Statistical analysis showed that the effectiveness of performing cognitive tests was significantly affected by group assignment (p = 0.00001) and genotype (p = 0.012). The interaction between genotype and group assignment also had an influence (p = 0.02). In all groups, carriers of the TT genotype had worse measures than carriers of other genotypes. The similar nature of the influences of the MnlI polymorphism on variations in measures in all groups indicates that this gene is related to overall intellect.

Cognitive profiles of healthy siblings of schizophrenia patients: application of the cognitive domains of the MATRICS consensus battery

Even though a large body of data suggests the presence of various types of cognitive deficits in the unaffected relatives of schizophrenia patients, more study is needed to clarify the comparative sensitivities of specific cognitive measures for relative-control differences. In this study, the authors compared the cognitive profiles of unaffected siblings of schizophrenia patients and those of patients and normal controls, and attempted to identify cognitive markers that might be associated with genetic liability to schizophrenia. Eighty-eight clinically stable schizophrenia patients, 44 healthy patient siblings, and 100 normal controls were evaluated using comprehensive neuropsychological tests. The domain structure of the MATRICS consensus cognitive battery was adopted, and both domain scores and individual test scores were used in the analysis. Performances of the sibling group were intermediate between those of patients and controls on most measures. A significant difference between the sibling and control groups was observed only in the Category Fluency Test. This cognitive deficit might be caused by familial predisposition to schizophrenia and could be a candidate of endophenotype for schizophrenia.

Heritability of Trail Making Test performance in multiplex schizophrenia families: implications for the search for an endophenotype

The impairment of the Trail Making Test (TMT) performance as a measure of executive function deficits has been found both in patients with schizophrenia and in their unaffected first-degree relatives, suggesting that it might be considered as a familial vulnerability marker, but its heritability estimates are not well known. This study investigated the genetic heritability of impairments in TMT performance using a sample of 80 schizophrenia patients, 145 unaffected first-degree relatives and 127 healthy controls from families with multiple members with schizophrenia. Consistent with previous reports in the literature, relatives performed in between healthy controls and schizophrenia patients. Based on these results, a variance component-analysis provided small, but significant additive heritability estimates for performance indices relating performance in TMT-version A to TMT-version B. These results showed that this significant but small evidence of heritability on the one hand suggests an association with genetic predisposition to schizophrenia, but that TMT performance is also associated with epigenetic or environmental factors.

Heritability of neurocognitive traits in familial schizophrenia

Neurocognitive deficits are considered promising endophenotypes for gene discovery in schizophrenia. Understanding the heritability and genetic inter-relationships of neurocognitive traits could support their use as alternatives to diagnosis. Participants were 85 adults from 17 multiplex Canadian families with familial schizophrenia linked to 1q23 who had neurocognitive testing results available. Heritability of 13 standard measures assessing motor skills, processing speed, verbal, and visuospatial memory, attention/working memory, executive functioning, and IQ was estimated using variance component models and SOLAR software. We then investigated bivariate relationships between those variables found to be heritable. IQ showed the highest heritability (h(2) = 0.64-0.74) and seven other neurocognitive measures, reflecting immediate and delayed verbal memory, attention/working memory, delayed visual memory, processing speed and motor skills, showed significant heritability (h(2) = 0.31-0.62) under one or more of the models assessed. A schizophrenia diagnostic covariate was significant (P < 0.0001) for all heritable variables. Bivariate analyses suggested that memory-IQ and visuomotor-processing speed formed two groups of heritable traits. The results provide further evidence of the heritability of selected neurocognitive measures, and their relationship to schizophrenia and underlying genetic architecture. Composite measures of memory or processing speed may be heritable phenotypes useful for studies of neurocognition.

Association of AKT1 with verbal learning, verbal memory, and regional cortical gray matter density in twins

AKT1, encoding the protein kinase B, has been associated with the genetic etiology of schizophrenia and bipolar disorder. However, minuscule data exist on the role of different alleles of AKT1 in measurable quantitative endophenotypes, such as cognitive abilities and neuroanatomical features, showing deviations in schizophrenia and bipolar disorder. We evaluated the contribution of AKT1 to quantitative cognitive traits and 3D high-resolution neuroanatomical images in a Finnish twin sample consisting of 298 twins: 61 pairs with schizophrenia (8 concordant), 31 pairs with bipolar disorder (5 concordant) and 65 control pairs matched for age, sex and demographics. An AKT1 allele defined by the SNP rs1130214 located in the UTR of the gene revealed association with cognitive traits related to verbal learning and memory (P = 0.0005 for a composite index). This association was further fortified by a higher degree of resemblance of verbal memory capacity in pairs sharing the rs1130214 genotype compared to pairs not sharing the genotype. Furthermore, the same allele was also associated with decreased gray matter density in medial and dorsolateral prefrontal cortex (P < 0.05). Our findings support the role of AKT1 in the genetic background of cognitive and anatomical features, known to be affected by psychotic disorders. The established association of the same allelic variant of AKT1 with both cognitive and neuroanatomical aberrations could suggest that AKT1 exerts its effect on verbal learning and memory via neural networks involving prefrontal cortex.

Cognitive functioning of bipolar I patients and relatives from families with or without schizophrenia or schizoaffective disorder

BACKGROUND

Bipolar I disorder patients show cognitive impairments, and genetic vulnerability to other psychotic disorders may modify these impairments. We set out to assess cognitive functions and estimate their heritability in bipolar I disorder patients (bipolar families) and unaffected relatives in a group of families with bipolar I disorder only and in another group of families with both bipolar I disorder and schizophrenia or schizoaffective disorder (mixed families).

METHODS

A neuropsychological test battery was administered to 20 bipolar patients and 36 relatives from bipolar families, 19 bipolar patients and 28 relatives from mixed families and 55 controls, all representing population-based samples.

RESULTS

Irrespective of the family group, patients and relatives were impaired in psychomotor processing speed. Both patient groups were impaired in executive functioning, but the deficit was more severe in patients from mixed families. Patients from bipolar families scored lower than controls in nearly all measures of verbal memory. All relatives were slightly impaired in executive functioning. The heritability of cognitive functions was generally similar irrespective of psychopathology in the family. However, there were greater genetic effects in several cognitive tasks in mixed families.

LIMITATIONS

The small sample size and familial type of bipolar disorder could limit the generalizability of the results.

CONCLUSION

Impaired psychomotor processing speed and executive functions may represent markers of susceptibility to bipolar I disorder irrespective of psychopathology within the family. Generalized impairment in verbal memory, in turn, may associate more with bipolar disorder than to vulnerability to other psychotic disorders.

Association between genes of Disrupted in schizophrenia 1 (DISC1) interactors and schizophrenia supports the role of the DISC1 pathway in the etiology of major mental illnesses

BACKGROUND

Disrupted in Schizophrenia 1 (DISC1) is currently one of the most interesting candidate genes for major mental illness, having been demonstrated to associate with schizophrenia, bipolar disorder, major depression, autism, and Asperger's syndrome. We have previously reported a DISC1 haplotype, HEP3, and an NDE1 spanning tag haplotype to associate to schizophrenia in Finnish schizophrenia families. Because both DISC1 and NDE1 display association in our study sample, we hypothesized that other genes interacting with DISC1 might also have a role in the etiology of schizophrenia.

METHODS

We selected 11 additional genes encoding components of the "DISC1 pathway" and studied these in our study sample of 476 families including 1857 genotyped individuals. We performed single nucleotide polymorphism (SNP) and haplotype association analyses in two independent sets of families. For markers and haplotypes found to be consistently associated in both sets, the overall significance was tested with the combined set of families.

RESULTS

We identified three SNPs to be associated with schizophrenia in PDE4D (rs1120303, p = .021), PDE4B (rs7412571, p = .018), and NDEL1 (rs17806986, p = .0038). Greater significance was observed with allelic haplotypes of PDE4D (p = .00084), PDE4B (p = .0022 and p = .029), and NDEL1 (p = .0027) that increased or decreased schizophrenia susceptibility.

CONCLUSIONS

Our findings with other converging lines of evidence support the underlying importance of DISC1-related molecular pathways in the etiology of schizophrenia and other major mental illnesses.

Molecular genetics of migraine

Migraine is an episodic neurovascular disorder that is clinically divided into two main subtypes that are based on the absence or presence of an aura: migraine without aura (MO) and migraine with aura (MA). Current molecular genetic insight into the pathophysiology of migraine predominantly comes from studies of a rare monogenic subtype of migraine with aura called familial hemiplegic migraine (FHM). Three FHM genes have been identified, which all encode ion transporters, suggesting that disturbances in ion and neurotransmitter balances in the brain are responsible for this migraine type, and possibly the common forms of migraine. Cellular and animal models expressing FHM mutations hint toward neuronal hyperexcitability as the likely underlying disease mechanism. Additional molecular insight into the pathophysiology of migraine may come from other monogenic syndromes (for instance cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, which is caused by NOTCH3 mutations), in which migraine is prominent. Investigating patients with common forms of migraine has had limited successes. Except for 5',10'-methylenetetrahydrolate reductase, an enzyme in folate metabolism, the large majority of reported genetic associations with candidate migraine genes have not been convincingly replicated. Genetic linkage studies using migraine subtypes as an end diagnosis did not yield gene variants thus far. Clinical heterogeneity in migraine diagnosis may have hampered the identification of such variants. Therefore, the recent introduction of more refined methods of phenotyping, such as latent-class analysis and trait component analysis, may be certainly helpful. Combining the new phenotyping methods with genome-wide association studies may be a successful strategy toward identification of migraine susceptibility genes. Likely the identification of reliable biomarkers for migraine diagnosing will make these efforts even more successful.

Genetic influences on hippocampal structure and function in recombinant inbred mice

Previously, we identified separate genetic influences on ventral versus dorsal hippocampal volume in BXD recombinant inbred mice [Martin MV, Dong HX, Vallera D, Lu L, Williams RW, Rosen GD, et al. Independent quantitative trait loci influence ventral and dorsal hippocampal volume in recombinant inbred strains of mice. Genes Brain Behav 2006;5:614-23]. Based on genotype at genetic markers associated with ventral hippocampal volume, we evaluated BXD mouse strains with relatively small versus large ventral hippocampal volumes using numerous behavioral paradigms known to rely upon hippocampal function and several other tasks that tap into behaviors analogous to those often impaired in schizophrenia. We observed a relationship between genotype at markers known to influence ventral hippocampal volume and working memory at an intermediate memory load. There was no association between genotype at markers known to influence ventral hippocampal volume and spatial reference memory, prepulse inhibition, or elevated plus maze performance. The relevance of these findings for understanding the pathophysiology of schizophrenia are discussed, including the possibility that genetic predisposition toward anterior hippocampal volume reductions and working memory deficits in schizophrenia may be related through a shared genetic locus.

The evolution of drug development in schizophrenia: past issues and future opportunities

Schizophrenia is a disease syndrome with major public health implications. The primary advance in pharmacotherapeutics was in 1952 with the introduction of antipsychotic medications (ie, chlorpromazine, dopamine D2 antagonism). Barriers to progress have been substantial, but many will be subject to rapid change based on current knowledge. There are attractive psychopathology indications for drug discovery (eg, impaired cognition and negative symptoms), and drugs with efficacy in these domains may have application across a number of disease classes. These pathologies are observed prior to psychosis raising the possibility of very early intervention and secondary prevention. Success in drug discovery for cognition and negative symptom pathologies may bring forth issues in ethics as the potential for enhancing normal function is explored.

Verbal working memory impairments in individuals with schizophrenia and their first-degree relatives: findings from the Consortium on the Genetics of Schizophrenia

Working memory (WM) impairment is a promising candidate endophenotype for schizophrenia that could facilitate the identification of susceptibility genes for this disorder. The validity of this putative endophenotype was assessed by determining whether 149 probands with schizophrenia and 337 of their first-degree relatives demonstrated WM impairment as compared to 190 unaffected community comparison subjects. Subjects were participants in the Consortium on the Genetics of Schizophrenia (COGS) project, a seven-site research network that was established to investigate the genetic architecture of endophenotypes for schizophrenia. Participants received comprehensive clinical assessments and completed two verbal WM tasks, one requiring transient on-line storage and another requiring maintenance plus complex manipulation of information by reordering the stimuli. Schizophrenia probands performed worse than the other groups on both tasks, with larger deficits found for the more challenging reordering WM task. The probands' relatives performed more poorly than community comparison subjects on both tasks, but the difference was significant only for the more challenging maintenance plus complex manipulation WM task. This WM impairment was not attributable to diagnoses of schizophrenia spectrum disorder, mood disorders, or substance use disorders in the relatives. In conjunction with evidence that WM abilities are substantially heritable, the current results support the validity and usefulness of verbal WM impairments in manipulation of information as endophenotypes for schizophrenia in large-scale genetic linkage and association studies.

Association between BDNF Val66Met polymorphism and trait depression is mediated via resting EEG alpha band activity

A functional polymorphism of the brain-derived neurotrophic factor, BDNF Val66Met, is associated with risk for major depression alongside impairments in memory and selective attention. This study aims to identify the mediating neural mechanisms in links between BDNF and depression using highly heritable electroencephalographic (EEG) recordings. In 305 healthy subjects, BDNF Val66Met genotypes were compared in terms of trait depression, neural function (EEG during a resting state) and cognitive performance. The mediating effects of the EEG brain imaging endophenotypes were also examined using structural equation (path) modeling. A genotype-endophenotype-phenotype path model showed that Met homozygosity predicted elevated working memory commission errors and altered EEG activity; that is elevated relative theta and delta power coupled with reduced alpha power. In turn, reduced EEG alpha activity mediated the relationship between the Met/Met genotype and trait depression. These findings demonstrate the utility of an integrative endophenotype approach. They suggest that the BDNF Met/Met homozygote has a direct impact on memory systems, but impacts trait depression via the secondary effects of neural changes.

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.

Neurocognitive allied phenotypes for schizophrenia and bipolar disorder

Psychiatric disorders are genetically complex and represent the end product of multiple biological and social factors. Links between genes and disorder-related abnormalities can be effectively captured via assessment of phenotypes that are both associated with genetic effects and potentially contributory to behavioral abnormalities. Identifying intermediate or allied phenotypes as a strategy for clarifying genetic contributions to disorders has been successful in other areas of medicine and is a promising strategy for identifying susceptibility genes in complex psychiatric disorders. There is growing evidence that schizophrenia and bipolar disorder, rather than being wholly distinct disorders, share genetic risk at several loci. Further, there is growing evidence of similarity in the pattern of cognitive and neurobiological deficits in these groups, which may be the result of the effects of these common genetic factors. This review was undertaken to identify patterns of performance on neurocognitive and affective tasks across probands with schizophrenia and bipolar disorder as well as unaffected family members, which warrant further investigation as potential intermediate trait markers. Available evidence indicates that measures of attention regulation, working memory, episodic memory, and emotion processing offer potential for identifying shared and illness-specific allied neurocognitive phenotypes for schizophrenia and bipolar disorder. However, very few studies have evaluated neurocognitive dimensions in bipolar probands or their unaffected relatives, and much work in this area is needed.

Replication of linkage on chromosome 7q22 and association of the regional Reelin gene with working memory in schizophrenia families

Schizophrenia is a common and complex mental disorder. Hereditary factors are important for its etiology, but despite linkage signals reported to several chromosomal regions in different populations, final identification of predisposing genes has remained a challenge. Utilizing a large family-based schizophrenia study sample from Finland, we have identified several linked loci: 1q32.2-q42, 2q, 4q31, 5q and 7q22. In this study, an independent sample of 352 nuclear schizophrenia families (n=1626) allowed replication of linkage on 7q21-32. In a sample of 245 nuclear families (n=1074) originating from the same geographical region as the families revealing the linkage, SNP and microsatellite association analyses of the four regional candidate genes, GRM3, RELN, SEMA3A and VGF, revealed no significant association to the clinical diagnosis of schizophrenia. Instead, quantifiable trait component analyses with neuropsychological endophenotypes available from 186 nuclear families (n=861) of the sample showed significant association to RELN variants for traits related to verbal (P=0.000003) and visual working memory (P=0.002), memory (P=0.002) and executive functioning (P=0.002). Trait-associated allele-positive subjects scored lower in the tests measuring working memory (P=0.0004-0.0000000004), memory (P=0.02-0.0001) and executive functioning (P=0.001). Our findings suggest that allelic variants of RELN contribute to the endophenotypes of schizophrenia.

Dimensions of impulsivity are associated with poor spatial working memory performance in monkeys

Impulsive behavior and novelty seeking are dimensions of temperament that are behavioral determinants of risk for attention deficit/hyperactivity disorder and its neurocognitive endophenotypes, and variation in the dopamine D4 receptor gene (DRD4) explains at least a portion of the variance in the traits. To further characterize the dimensional phenotype associated with impulsiveness, adolescent male monkeys were evaluated using ecologically valid tests of impulsive approach and aggression in response to social or nonsocial stimuli; subsequently, a delayed response task was implemented to assess spatial working memory performance. Subjects were selected into this study based on their response to the social challenge task or by DRD4 genotype, resulting in three groups: low-impulsivity/common DRD4 allele, high-impulsivity/common DRD4 allele, or rare DRD4 allele. All animals acquired the delayed response task and could perform at near ceiling levels when a approximately 0 s delay version was imposed, but as delays were lengthened, high-impulsive animals, regardless of DRD4 genotype, made fewer correct responses than did low-impulsive subjects; an inverse relationship existed for working memory and impulsivity. Notably, impulsive behavior evoked by social and nonsocial stimuli explained overlapping and independent portions of the variance in working memory performance. CSF levels of monoamine metabolites did not significantly differentiate the high- and low-impulsive animals, although monkeys carrying the DRD4 rare allele tended to exhibit higher monoamine turnover. These data indicate that dimensions of impulsivity may impact on working memory performance in qualitatively similar ways but through different mechanisms.

Heritability of cognitive functions in families with bipolar disorder

Bipolar disorder is highly heritable. Cognitive dysfunctions often observed in bipolar patients and their unaffected relatives implicate that these impairments may be associated with genetic predisposition to bipolar disorder and thus fulfill the criteria of a valid endophenotype for the disorder. However, the most fundamental criterion, their heritability, has not been directly studied in any bipolar population. This population-based study estimated the heritability of cognitive functions in bipolar disorder. A comprehensive neuropsychological test battery and the Structured Clinical Interview for DSM-IV were administered to a population-based sample of 110 individuals from 52 families with bipolar disorder. Heritability of cognitive functions as assessed with neuropsychological test scores were estimated using the Solar package. Significant additive heritabilities were found in verbal ability, executive functioning, and psychomotor processing speed. Genetic contribution was low to verbal learning functions. High heritability, in executive functioning and psychomotor processing speed suggest that these may be valid endophenotypic traits for genetic studies of bipolar disorder.

Impaired executive performance in healthy siblings of schizophrenia patients in a population-based study

There is increasing evidence that healthy siblings of schizophrenia patients have similar, although milder, neuropsychological deficits than their affected family members. However, the interpretation of these findings has been complicated by methodological differences, for example the selection of relatives studied and the sensitivity of tests used. We studied neuropsychological functioning in schizophrenia families in representative, population-based samples of schizophrenia patients (n=81) and healthy siblings (n=78) from 58 families, and control subjects (n=70). We found that the healthy sibling group was impaired in tests measuring performance speed and executive functions. The patients were significantly impaired in all neuropsychological variables studied when compared with the control subjects, and also when compared with the healthy siblings. The effects of age, sex and education were controlled for. In conclusion, in a study of representative, population-based sample the healthy siblings of schizophrenia patients demonstrated deficits in processing speed and executive functions.

Adjudicating neurocognitive endophenotypes for schizophrenia

Although genetic influences on schizophrenia are well established, localization of the genes responsible for this illness has proven extremely difficult. Given evidence that genes predisposing to schizophrenia may be transmitted without expression of the clinical phenotype, efforts have focused on developing endophenotypes. While several neuropsychological measures have been proposed to be endophenotypes, few studies have systematically assessed batteries of neurocognitive tests to determine which tests are most sensitive to liability for the illness. Two hundred sixty-nine Latino individuals were administered a standard neuropsychological battery. Two hundred fourteen of these were members of families with at least two siblings diagnosed with schizophrenia or schizoaffective disorder. The remaining were community controls without history of major psychiatric illness. Neurocognitive measures found to be heritable were entered into analyses designed to determine which tests covary with the degree of genetic relationship to affected individuals. Although five measures were found to uniquely model genetic liability for schizophrenia, digit symbol coding was the most sensitive. To assess the specificity of these endophenotypes, performance on these measures were compared to family members with bipolar and unipolar affective disorders. These markers clearly distinguished between individuals with psychotic illnesses and those with major depression. As measures contributed uniquely to discriminate individuals at varying risk for schizophrenia, our findings imply multiple independently inherited elements to the liability for the illness. We present a practical model for adjudicating endophenotypes and determining which measures are best suited for use in linkage analyses.

The endophenotype concept in psychiatric genetics

The idea that some phenotypes bear a closer relationship to the biological processes that give rise to psychiatric illness than diagnostic categories has attracted considerable interest. Much effort has been devoted to finding such endophenotypes, partly because it is believed that the genetic basis of endophenotypes will be easier to analyse than that of psychiatric disease. This belief depends in part on the assumption that the effect sizes of genetic loci contributing to endophenotypes are larger than those contributing to disease susceptibility, hence increasing the chance that genetic linkage and association tests will detect them. We examine this assumption by applying meta-analytical techniques to genetic association studies of endophenotypes. We find that the genetic effect sizes of the loci examined to date are no larger than those reported for other phenotypes. A review of the genetic architecture of traits in model organisms also provides no support for the view that the effect sizes of loci contributing to phenotypes closer to the biological basis of disease is any larger than those contributing to disease itself. While endophenotype measures may afford greater reliability, it should not be assumed that they will also demonstrate simpler genetic architecture.

The Consortium on the Genetics of Schizophrenia: neurocognitive endophenotypes

The Consortium on the Genetics of Schizophrenia (COGS) is a 7-site collaboration that examines the genetic architecture of quantitative endophenotypes in families with schizophrenia. Here we review the background and rationale for selecting neurocognitive tasks as endophenotypic measures in genetic studies. Criteria are outlined for the potential of measures as endophenotypic vulnerability markers. These include association with illness, state independence (ie, adequate test-retest stability, adequate between-site reliability, impairments in patients not due to medications, impairments observed regardless of illness state), heritability, findings of higher rates in relatives of probands than in the general population, and cosegregation within families. The COGS required that, in addition, the measures be "neurocognitive" and thus linked to neurobiology and that they be feasible in multisite studies. The COGS neurocognitive assessment includes measures of attention, verbal memory, working memory, and a computerized neurocognitive battery that also includes facial processing tasks. Here we describe data demonstrating that these neurobehavioral measures meet criteria for endophenotypic candidacy. We conclude that quantitative neurocognitive endophenotypes need further evidence for efficacy in identifying genetic effects but have the potential of providing unprecedented insight into gene-environment interaction related to dimensions of brain and behavior in health and disease.

Families with the risk allele of DISC1 reveal a link between schizophrenia and another component of the same molecular pathway, NDE1

We have previously reported a robust association between an allelic haplotype of 'Disrupted in Schizophrenia 1' (DISC1) and schizophrenia in a nationwide collection of Finnish schizophrenia families. This specific DISC1 allele was later identified to associate with visual working memory, selectively in males. DISC1 association to schizophrenia has since been replicated in multiple independent study samples from different populations. In this study, we conditioned our sample of Finnish families for the presence of the Finnish tentative risk allele for DISC1 and re-analyzed our genome-wide scan data of 443 markers on the basis of this stratification. Two additional loci displayed an evidence of linkage (LOD > 3) and included a locus on 16p13, proximal to the gene encoding NDE1, which has been shown to biologically interact with DISC1. Although none of the observed linkages remained significant after multiple test correction through simulation, further analysis of NDE1 revealed an association between a tag-haplotype and schizophrenia (P = 0.00046) specific to females, which proved to be significant (P = 0.011) after multiple test correction. Our finding would support the concept that initial gene findings in multifactorial diseases will assist in the identification of other components of complex genetic etiology. Notably, this and other converging lines of evidence underline the importance of DISC1-related functional pathways in the etiology of schizophrenia.

Spatial working memory function in twins with schizophrenia and bipolar disorder

BACKGROUND

Family studies are in conflict as to whether schizophrenia and bipolar disorder have independent genetic etiologies. Given the relatively low prevalence (approximately 1%) of these disorders, the use of quantitative endophenotypic markers of genetic liability might provide a more sensitive strategy for evaluating their genetic overlap. We have previously demonstrated that spatial working memory deficits increase in a dose-dependent fashion with increasing genetic proximity to a proband among the unaffected co-twins of schizophrenic patients. Here, we evaluated whether such deficits might also mark genetic susceptibility to bipolar disorder.

METHODS

The Wechsler Memory Scale-Revised Visual Memory Span and Digit Span subtests were administered to 46 schizophrenic patients, 32 of their unaffected co-twins, 22 bipolar patients, 16 of their unaffected co-twins, and 100 control twins, representing unselectively nationwide twin samples.

RESULTS

Schizophrenic patients and their unaffected co-twins performed significantly worse than control subjects on the spatial working memory task, whereas only the schizophrenic patients performed significantly below the control subjects on the verbal working memory task. Neither bipolar patients nor their unaffected co-twins differed from control subjects on these measures.

CONCLUSIONS

Our findings support the hypothesis that impairment in spatial working memory might effectively reflect an expression of genetic liability to schizophrenia but less clearly to bipolar disorder.