Examination of genetic linkage of chromosome 15 to schizophrenia in a large Veterans Affairs Cooperative Study sample

The Human-Restricted Isoform of the α7 nAChR, CHRFAM7A: A Double-Edged Sword in Neurological and Inflammatory Disorders

CHRFAM7A is a relatively recent and exclusively human gene arising from the partial duplication of exons 5 to 10 of the α7 neuronal nicotinic acetylcholine receptor subunit (α7 nAChR) encoding gene, CHRNA7. CHRNA7 is related to several disorders that involve cognitive deficits, including neuropsychiatric, neurodegenerative, and inflammatory disorders. In extra-neuronal tissues, α7nAChR plays an important role in proliferation, differentiation, migration, adhesion, cell contact, apoptosis, angiogenesis, and tumor progression, as well as in the modulation of the inflammatory response through the “cholinergic anti-inflammatory pathway”. CHRFAM7A translates the dupα7 protein in a multitude of cell lines and heterologous systems, while maintaining processing and trafficking that are very similar to the full-length form. It does not form functional ion channel receptors alone. In the presence of CHRNA7 gene products, dupα7 can assemble and form heteromeric receptors that, in order to be functional, should include at least two α7 subunits to form the agonist binding site. When incorporated into the receptor, in vitro and in vivo data showed that dupα7 negatively modulated α7 activity, probably due to a reduction in the number of ACh binding sites. Very recent data in the literature report that the presence of the duplicated gene may be responsible for the translational gap in several human diseases. Here, we will review the studies that have been conducted on CHRFAM7A in different pathologies, with the intent of providing evidence regarding when and how the expression of this duplicated gene may be beneficial or detrimental in the pathogenesis, and eventually in the therapeutic response, to CHRNA7-related neurological and non-neurological diseases.

A novel knockout mouse for the small EDRK-rich factor 2 (Serf2) showing developmental and other deficits

The small EDRK-rich factor 2 (SERF2) is a highly conserved protein that modifies amyloid fibre assembly in vitro and promotes protein misfolding. However, the role of SERF2 in regulating age-related proteotoxicity remains largely unexplored due to a lack of in vivo models. Here, we report the generation of Serf2 knockout mice using an ES cell targeting approach, with Serf2 knockout alleles being bred onto different defined genetic backgrounds. We highlight phenotyping data from heterozygous Serf2^+/− mice, including unexpected male-specific phenotypes in startle response and pre-pulse inhibition. We report embryonic lethality in Serf2^−/− null animals when bred onto a C57BL/6 N background. However, homozygous null animals were viable on a mixed genetic background and, remarkably, developed without obvious abnormalities. The Serf2 knockout mice provide a powerful tool to further investigate the role of SERF2 protein in previously unexplored pathophysiological pathways in the context of a whole organism.

Alpha7 Nicotinic Receptors as Therapeutic Targets in Schizophrenia

While current treatments for schizophrenia often provide much relief for positive symptoms such as hallucinations, other symptoms, particularly cognitive deficits, persist and contribute to substantial suffering and reduced quality of life for patients. In searching for novel therapeutic avenues to treat cognitive deficits in schizophrenia, recent work is exploring nicotinic receptor neurobiology. Supported by a large body of evidence, with contributions from studies of smoking behaviors, genetics, receptor distribution and function, animal models and nicotinic effects on illness symptoms, the alpha7 nicotinic receptor has emerged as a potential therapeutic target. Despite promise in early clinical trials, however, no drug targeting nicotinic systems has succeeded in larger phase 3 trials. Following a brief review of nicotinic receptor biology and the evidence that has led to pursuit of alpha7 nicotinic agonism as a therapeutic strategy, this review will provide an update on the status of recent trials, discuss potential issues that may have contributed to negative outcomes, and point to new directions and promising advances in developing alpha7 nicotinic receptor-based treatment for cognitive symptoms in schizophrenia.

IMPLICATIONS

By examining alpha7 nicotinic receptor biology and recent efforts to target the receptor in clinical trials, it is hoped that investigators will be motivated to explore novel, promising directions focusing on the receptor as a strategy to treat cognitive symptoms in schizophrenia.

Mechanistic insights into the genetics of affective psychosis from Prader-Willi syndrome

Schizophrenia and bipolar disorder are common, severe, and disabling psychotic disorders, which are difficult to research. We argue that the genetically determined neurodevelopmental disorder Prader-Willi syndrome (PWS), which is associated with a high risk of affective psychotic illness, can provide a window into genetic mechanisms and associated neural pathways. People with PWS can all show non-psychotic psychopathology and problem behaviours, but the prevalence of psychotic illness differs markedly by genetic subtype; people with PWS due to chromosome 15 maternal uniparental disomy have higher prevalence of psychotic illness compared with patients with PWS due to 15q11-13 deletions of paternal origin. On the basis of this observation and the neural differences between genetic subtypes, we hypothesise that the combined effects of the absent expression of specific maternally imprinted genes at 15q11-13, and excess maternally imprinted or paternally expressed genes on chromosome 15, affect the γ-aminobutyric acid-glutamatergic pathways and associated neural networks that underpin mood regulation and sensory processing, resulting in psychotic illness. We propose a model of potential mechanisms of psychosis in PWS, which might be relevant in the general population, and should inform future research.

The human CHRNA7 and CHRFAM7A genes: A review of the genetics, regulation, and function

The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is ubiquitously expressed in both the central nervous system and in the periphery. CHRNA7 is genetically linked to multiple disorders with cognitive deficits, including schizophrenia, bipolar disorder, ADHD, epilepsy, Alzheimer's disease, and Rett syndrome. The regulation of CHRNA7 is complex; more than a dozen mechanisms are known, one of which is a partial duplication of the parent gene. Exons 5-10 of CHRNA7 on chromosome 15 were duplicated and inserted 1.6 Mb upstream of CHRNA7, interrupting an earlier partial duplication of two other genes. The chimeric CHRFAM7A gene product, dupα7, assembles with α7 subunits, resulting in a dominant negative regulation of function. The duplication is human specific, occurring neither in primates nor in rodents. The duplicated α7 sequence in exons 5-10 of CHRFAM7A is almost identical to CHRNA7, and thus is not completely queried in high throughput genetic studies (GWAS). Further, pre-clinical animal models of the α7nAChR utilized in drug development research do not have CHRFAM7A (dupα7) and cannot fully model human drug responses. The wide expression of CHRNA7, its multiple functions and modes of regulation present challenges for study of this gene in disease. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Meta-analyses of 10 polymorphisms associated with the risk of schizophrenia

Schizophrenia (SCZ) is a severe complex psychiatric disorder that generates problems for the associated family and society and causes disability with regards to work for patients. The aim of the present study was to assess the contribution of 10 genetic polymorphisms to SCZ susceptibility. Meta-analyses were conducted using the data without a limitation for time or language. A total of 27 studies with 7 genes and 10 polymorphisms were selected for the meta-analyses. Two polymorphisms were found to be significantly associated with SCZ. SNAP25 rs3746544 was shown to increase the SCZ risk by 18% [P=0.01; odds ratio (OR), 1.18; 95% confidence interval (CI), 1.05-1.34] and GRIK3 rs6691840 was found to increase the risk by 30% (P=0.008; OR, 1.30; 95% CI, 1.07-1.58). Significant results were found under the dominant (P=0.001; OR, 1.36; 95% CI, 1.13-1.65) and additive (P=0.02; OR, 1.45; 95% CI, 1.06-1.98) model for the SNAP25 rs3746544 polymorphism and under the additive model for the GRIK3 rs6691840 polymorphism (P=0.03; OR, 1.73; 95% CI, 1.04-2.85). There were no significant results observed for the other eight polymorphisms, which were CCKAR rs1800857, CHRNA7 rs904952, CHRNA7 rs6494223, CHRNA7 rs2337506, DBH Ins>Del, FEZ1 rs559668, FEZ1 rs597570 and GCLM rs2301022. In conclusion, the present meta-analyses indicated that the SNAP25 rs3746544 and GRIK3 rs6691840 polymorphisms were risk factors of SCZ, which may provide valuable information for the clinical diagnosis of SCZ.

Evaluating the role of the alpha-7 nicotinic acetylcholine receptor in the pathophysiology and treatment of schizophrenia

The group of schizophrenia disorders affects approximately 1% of the population and has both genetic and environmental etiologies. Sufferers report various behavioral abnormalities including hallucinations and delusions (positive symptoms), reduced joy and amotivation (negative symptoms), plus inattention and poor learning (cognitive deficits). Despite the heterogeneous symptoms experienced, most patients smoke. The self-medication hypothesis posits that patients smoke to alleviate symptoms, consistent with evidence for nicotine-induced enhancement of cognition. While nicotine acts on multiple nicotinic acetylcholine receptors (nAChRs), the primary target of research is often the homomeric α7 nAChR. Given genetic linkages between schizophrenia and this receptor, its association with P50 sensory gating deficits, and its reduced expression in post-mortem brains, many have attempted to develop α7 nAChR ligands for treating schizophrenia. Recent evidence that ligands can be orthosteric agonists or positive allosteric modulators (PAMs) has revitalized the hope for treatment discovery. Herein, we present evidence regarding: (1) pathophysiological alterations of α7 nAChRs that might occur in patients; (2) mechanistic evidence for the normal action of α7 nAChRs; (3) preclinical studies using α7 nAChR orthosteric agonists and type I/II PAMs; and (4) where successful translational testing has occurred for particular compounds, detailing what is still required. We report that the accumulating evidence is positive, but that greater work is required using positron emission tomography to understand current alterations in α7 nAChR expression and their relationship to symptoms. Finally, cross-species behavioral tasks should be used more regularly to determine the predictive efficacy of treatments.

Exploring the relationships between tobacco smoking and schizophrenia in first-degree relatives

Up to 90% of individuals with schizophrenia suffer from nicotine dependence. Both schizophrenia and nicotine consumption have strong genetic components, which may overlap. The relationship between schizophrenia and nicotine dependence remains unclear, due in part to confounding factors. Studies of the relationship between nicotine consumption and milder schizophrenia-related phenotypes, such as schizotypy, in first-degree relatives of individuals with schizophrenia could help to better understand the relationship between smoking and schizophrenia while avoiding such confounders. We assessed the proportion of smokers, their level of nicotine dependence and their level of schizotypy in a sample of 98 first-degree relatives of schizophrenic subjects and 110 healthy controls. Partial correlation analysis was used to assess the relationship between schizotypal dimensions and smoking dependence. The prevalence of smoking and nicotine dependence levels were higher in the relatives than in the healthy control group. We found no relationship between nicotine dependence and the magnitude of schizotypal features in either group. Our results support the hypothesis that the relationship between schizophrenia and smoking is largely mediated by common familial factors, which may be genetic.

Nicotinic mechanisms in the treatment of psychotic disorders: a focus on the α7 nicotinic receptor

Nicotine is heavily abused by persons with schizophrenia. Nicotine better enables people with schizophrenia to filter out extraneous auditory stimuli. Nicotine also improves prepulse inhibition when compared to placebo. Nicotine similarly increases the amplitude of patients' duration mismatch negativity. The 15q13-14 region of the genome coding for the α7 nicotinic receptor is linked to schizophrenia. Multiple single nucleotide polymorphisms have been identified in this 15q13-14 gene promoter region that are more frequently present in people with schizophrenia than in normal controls. Abnormalities in expression and regulation of central nicotinic cholinoceptors with decreased α7 binding in multiple brain regions are also present. Nicotine enhances cognition in schizophrenia. Alternative agents that activate the nicotinic receptor have been tested including 3-[2,4-dimethoxybenzylidene]anabaseine (DMXB-A). This compound improved attention, working memory, and negative symptoms in an add-on study in nonsmoking patients with schizophrenia. There are multiple other nicotinic agents, including positive allosteric modulators, in the preclinical stages of development. Finally, the effects of varenicline and clozapine and their relation to smoking cessation are discussed.

The chimeric gene CHRFAM7A, a partial duplication of the CHRNA7 gene, is a dominant negative regulator of α7*nAChR function

The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is a candidate gene for schizophrenia and an important drug target for cognitive deficits in the disorder. Activation of the α7*nAChR, results in opening of the channel and entry of mono- and divalent cations, including Ca(2+), that presynaptically participates to neurotransmitter release and postsynaptically to down-stream changes in gene expression. Schizophrenic patients have low levels of α7*nAChR, as measured by binding of the ligand [(125)I]-α-bungarotoxin (I-BTX). The structure of the gene, CHRNA7, is complex. During evolution, CHRNA7 was partially duplicated as a chimeric gene (CHRFAM7A), which is expressed in the human brain and elsewhere in the body. The association between a 2bp deletion in CHRFAM7A and schizophrenia suggested that this duplicate gene might contribute to cognitive impairment. To examine the putative contribution of CHRFAM7A on receptor function, co-expression of α7 and the duplicate genes was carried out in cell lines and Xenopus oocytes. Expression of the duplicate alone yielded protein expression but no functional receptor and co-expression with α7 caused a significant reduction of the amplitude of the ACh-evoked currents. Reduced current amplitude was not correlated with a reduction of I-BTX binding, suggesting the presence of non-functional (ACh-silent) receptors. This hypothesis is supported by a larger increase of the ACh-evoked current by the allosteric modulator 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea (PNU-120596) in cells expressing the duplicate than in the control. These results suggest that CHRFAM7A acts as a dominant negative modulator of CHRNA7 function and is critical for receptor regulation in humans.

Transcriptional repression of the α7 nicotinic acetylcholine receptor subunit gene (CHRNA7) by activating protein-2α (AP-2α)

The CHRNA7 gene, which encodes the α7 nicotinic acetylcholine receptor (α7*nAChR), has been implicated as a candidate gene in schizophrenia. Expression of the α7*nAChR mRNA and protein are reduced in multiple regions of post-mortem brain from patients diagnosed with schizophrenia. Transcriptional regulation may therefore be an important mechanism for the regulation of this gene. A 230-bp proximal promoter fragment, necessary for transcription in cultured neuroblastoma cells, was used to study a putative AP-2α binding site. Mutation of the site indicates that AP-2α plays a negative role in regulating CHRNA7 transcription. This was confirmed through knockdown and overexpression of AP-2α. Electrophoretic mobility shift assays (EMSAs) identified positive DNA-protein interaction at this same site, and supershift assays indicate that the complex includes AP-2α. The interaction was confirmed in cells using chromatin immunoprecipitation (ChIP). DNA methylation was discovered as an anomalous mechanism for CHRNA7 regulation in one cell line. These studies suggest a role for AP-2α regulation of CHRNA7 mRNA expression in multiple tissues during development.

Evidence for association of the non-duplicated region of CHRNA7 gene with bipolar disorder but not with Schizophrenia

OBJECTIVE

Biological evidence in both human and animal studies suggests α7 neuronal nicotinic acetylcholine receptor subunit gene (CHRNA7) as a suitable functional candidate for genetic studies in psychiatric populations. This gene maps to chromosome 15q13-14, a major linkage hotspot for schizophrenia (SCH) and bipolar disorder (BD). In this study we examine the role of CHRNA7 in influencing the risk of SCH and BD.

METHODS

In the present investigation four SNPs of the non-duplicated region of CHRNA7 were genotyped: -86C/T variant, located in the 5'-upstream regulatory region; and three intronic polymorphisms (rs883473, rs6494223 and rs904952). Genetic analysis was performed on 510 patients diagnosed with SCH, 245 with BD and on 793 unrelated healthy controls.

RESULTS

SNP analysis suggested a significant difference in -86C/T allele (P=0.025) and genotype (P=0.03) frequencies between BD and control groups, although significance was lost after correction for multiple testing. Besides, the nucleotide change (T) in rs6494223 had a protective effect against BD [odds ratio (OR)=0.70 (0.57-0.87); P=0.001]. Genotype frequencies also showed significant association (P=0.001) [CT genotype OR=0.71 (0.5-0.96); TT genotype OR=0.47 (0.29-0.77)]. Haplotypic analysis revealed a positive association of the gene with BD (global-stat=24.18, P value=0.007) with a maximum effect in the region that covered introns 3 and 4. In contrast, no evidence of risk variants was found in the analysis of the SCH sample.

CONCLUSION

Our data support the non-duplicated region of CHRNA7 gene as a susceptibility region for BD but not for SCH. Further genotyping of this region may help to delimit the causal polymorphism.

The α7 nicotinic acetylcholine receptor and the acute stress response: maternal genotype determines offspring phenotype

α7 Nicotinic acetylcholine receptors (α7nAchRs) modulate immune activation by suppressing production of pro-inflammatory cytokines in peripheral immune cells. α7nAchRs also modulate inhibitory output in the hippocampus, which provides input to key circuits of the HPA axis. Therefore, the α7 nicotinic acetylcholine receptor gene (CHRNA7) may be associated with cortisol stress response. Polymorphisms in the CHRNA7 promoter decrease its expression and may dampen the cholinergic response, leading to an increase in inflammation. Increased inflammation may change the intrauterine environment, altering neuroendocrine development in the offspring. Maternal CHRNA7 genotype could affect an offspring's HPA regulation via reprogramming in utero. Patients with allergic disorders have a differential cortisol response to stress. This study utilized samples collected from a cohort of 198 adolescents in a previous study of atopic disorders, who demonstrated a disturbance in HPA response associated with atopy. Salivary cortisol samples collected from the adolescents after a series of laboratory procedures and DNA samples collected from the adolescents and their parents were used for further analysis. DNA samples were genotyped for allelic variation in the CHRNA7 promoter. Genetic association analyses with the cortisol levels were performed in the adolescents. Maternal genotype influences were investigated for the CHRNA7 gene. We also included maternal and child atopy diagnosis as covariates in determining cortisol levels and tested for association of CHRNA7 to atopy. Polymorphisms in the CHRNA7 promoter were associated with lower cortisol levels after a small laboratory stress. Our findings also show that although the child's CHRNA7 genotype affects stress response, the maternal genotype has a stronger influence on cortisol release after stress in male offspring. These effects were independent of atopy status.

Research review: Cholinergic mechanisms, early brain development, and risk for schizophrenia

The onset of diagnostic symptomology for neuropsychiatric diseases is often the end result of a decades-long process of aberrant brain development. Identification of novel treatment strategies aimed at normalizing early brain development and preventing mental illness should be a major therapeutic goal. However, there are few models for how this goal might be achieved. This review uses the development of a psychophysiological correlate of attentional deficits in schizophrenia to propose a developmental model with translational primary prevention implications. Review of genetic and neurobiological studies suggests that an early interaction between alpha7 nicotinic receptor density and choline availability may contribute to the development of schizophrenia-associated attentional deficits. Therapeutic implications, including perinatal dietary choline supplementation, are discussed.

Nature and nurture in neuropsychiatric genetics: where do we stand?

Both genetic and nongenetic risk factors, as well as interactions and correlations between them, are thought to contribute to the etiology of psychiatric and behavioral phenotypes. Genetic epidemiology consistently supports the involvement of genes in liability. Molecular genetic studies have been less successful in identifying liability genes, but recent progress suggests that a number of specific genes contributing to risk have been identified. Collectively, the results are complex and inconsistent, with a single common DNA variant in any gene influencing risk across human populations. Few specific genetic variants influencing risk have been unambiguously identified. Contemporary approaches, however, hold great promise to further elucidate liability genes and variants, as well as their potential inter-relationships with each other and with the environment. We will review the fields of genetic epidemiology and molecular genetics, providing examples from the literature to illustrate the key concepts emerging from this work.

Studies on the hippocampal formation: From basic development to clinical applications: Studies on schizophrenia

The hippocampal formation plays a critical role in cognitive function. The developmental events that shape the hippocampal formation are continuing to be elucidated and their implications for brain function are emerging as well as applying those advances to interventions that have important possibilities for the treatment of brain dysfunction. The story told in this chapter is about the use of the in oculo transplant method to illuminate intrinsic and extrinsic features that underlie the development of the dentate gyrus and adjacent hippocampus and the role of one molecule in the hippocampus and schizophrenia. Schizophrenia, originally conceptualized as a dysfunction in dopaminergic neurotransmission, is now known to involve multiple neuronal systems. Dysfunction of hippocampal neurons is emerging as one of its signature pathological features. Basic insights into the development and function of hippocampal interneurons form the basis of a new treatment initiative for this illness. Evidence for the role of the alpha 7-nicotinic acetylcholine receptor in the development and function of these neurons in rodents has led to human trials of nicotinic agonists for cognitive dysfunction in schizophrenia and the possibility of improving hippocampal development in children at risk for schizophrenia by perinatal supplementation with choline, which can act as an alpha 7-nicotinic acetylcholine receptor agonist.

Differential regulation of alpha7 nicotinic receptor gene (CHRNA7) expression in schizophrenic smokers

The alpha7 neuronal nicotinic receptor gene (CHRNA7) has been implicated in the pathophysiology of schizophrenia by genetic and pharmacological studies. Expression of the alpha7* receptor, as measured by [(125)I]alpha-bungarotoxin autoradiography, is decreased in postmortem brain of schizophrenic subjects compared to non-mentally ill controls. Most schizophrenic patients are heavy smokers, with high levels of serum cotinine. Smoking changes the expression of multiple genes and differentially regulates gene expression in schizophrenic hippocampus. We examined the effects of smoking on CHRNA7 expression in the same tissue and find that smoking differentially regulates expression of both mRNA and protein for this gene. CHRNA7 mRNA and protein levels are significantly lower in schizophrenic nonsmokers compared to control nonsmokers and are brought to control levels in schizophrenic smokers. Sufficient protein but low surface expression of the alpha7* receptor, seen in the autoradiographic studies, suggests aberrant assembly or trafficking of the receptor.

A 2-base pair deletion polymorphism in the partial duplication of the alpha7 nicotinic acetylcholine gene (CHRFAM7A) on chromosome 15q14 is associated with schizophrenia

Multiple genetic linkage studies support the hypothesis that the 15q13-14 chromosomal region contributes to the etiology of schizophrenia. Among the putative candidate genes in this area are the alpha7 nicotinic acetylcholine receptor gene (CHRNA7) and its partial duplication, CHRFAM7A. A large chromosomal segment including the CHRFAM7A gene locus, but not the CHRNA7 locus, is deleted in some individuals. The CHRFAM7A gene contains a polymorphism consisting of a 2 base pair (2 bp) deletion at position 497-498 bp of exon 6. We employed PCR-based methods to quantify the copy number of CHRFAM7A and the presence of the 2 bp polymorphism in a large, multi-ethnic population. The 2 bp polymorphism was associated with schizophrenia in African Americans (genotype p=0.005, allele p=0.015), and in Caucasians (genotype p=0.015, allele p=0.009). We conclude that the presence of the 2 bp polymorphism at the CHRFAM7A locus may have a functional significance in schizophrenia.

Association of the 5'-upstream regulatory region of the alpha7 nicotinic acetylcholine receptor subunit gene (CHRNA7) with schizophrenia

BACKGROUND

The alpha7 neuronal nicotinic acetylcholine receptor subunit gene (CHRNA7) is localized in a chromosomal region (15q14) linked to schizophrenia in multiple independent studies. CHRNA7 was selected as the best candidate gene in the region for a well-documented endophenotype of schizophrenia, the P50 sensory processing deficit, by genetic linkage and biochemical studies.

METHODS

Subjects included Caucasian-Non Hispanic and African-American case-control subjects collected in Denver, and schizophrenic subjects from families in the NIMH Genetics Initiative on Schizophrenia. Thirty-five single nucleotide polymorphisms (SNPs) in the 5'-upstream regulatory region of CHRNA7 were genotyped for association with schizophrenia, and for smoking in schizophrenia.

RESULTS

The rs3087454 SNP, located at position -1831 bp in the upstream regulatory region of CHRNA7, was significantly associated with schizophrenia in the case-control samples after multiple-testing correction (P=0.0009, African American; P=0.013, Caucasian-Non Hispanic); the association was supported in family members. There was nominal association of this SNP with smoking in schizophrenia.

CONCLUSIONS

The data support association of regulatory region polymorphisms in the CHRNA7 gene with schizophrenia.

Association study of a (TG)n dinucleotide repeat at chromosome 15q13.3 and schizophrenia in the Chinese population

Linkage studies have suggested that chromosome 15q13-q14 may harbor a susceptibility locus for schizophrenia. In the current study, the association between a (TG)n dinucleotide repeat polymorphism at D15S976 and schizophrenia was investigated using two independent samples from the Han Chinese population. In a population-based study, no significant difference was found between the genotype and allele frequency distributions in schizophrenia patients and control subjects. In a family-based study, no significant transmission disequilibrium from heterozygous parents to affected offspring was observed. Further analysis of the parent-of-origin effect found nominally significant allele-wise transmission disequilibrium through maternal transmissions, while 157bp and 159bp alleles showed significant individual allelic transmission disequilibrium from heterozygous mothers to affected offspring. Our results did not support the hypothesis that the (TG)n dinucleotide repeat polymorphism plays a major role in schizophrenia susceptibility in the Chinese population. Further studies are needed to elucidate the putative parent-of-origin effect and its role in schizophrenia susceptibility.

Smoking, Genetics and Schizophrenia: Evidence for Self Medication

Schizophrenia is a common mental illness with a high prevalence of smoking. More than 80% of schizophrenics smoke compared to 25% of the general population. Both schizophrenia and tobacco use have strong genetic components, which may overlap. It has been suggested that smoking in schizophrenia may be a form of self-medication in an attempt to treat an underlying biological pathology. Smoking normalizes auditory evoked potential and eye tracking deficits in schizophrenia, as well as improving cognitive function. Nicotine acts through a family of nicotinic receptors with either high or low affinity for nicotine. The loci for several of these receptors have been genetically linked to both smoking and to schizophrenia. Smoking changes gene expression for more than 200 genes in human hippocampus, and differentially normalizes aberrant gene expression in schizophrenia. The α7* nicotinic receptor, linked to schizophrenia and smoking, has been implicated in sensory processing deficits and is important for cognition and protection from neurotoxicity. Nicotine, however, has multiple health risks and desensitizes the receptor. A Phase I trial of DMXB-A, an α7* agonist, shows improvement in both P50 gating and in cognition, suggesting that further development of nicotinic cholinergic drugs is a promising direction in schizophrenia research.

Treating schizophrenia symptoms with an alpha7 nicotinic agonist, from mice to men

Current antipsychotic treatments fail to fully address the range of symptoms of schizophrenia, particularly with respect to social and occupational dysfunctions. Recent work has highlighted the role of nicotinie in both cognitive and attentional deficits as well as deficient processing of repetitive sensory information. The predilection for schizophrenia patients to be extremely heavy cigarette smokers may be related to their attempt to compensate for a reduction in hippocampal alpha7 nicotinic cholinergic receptors by delivering exogenous ligand to the remaining receptors. Studies in rodent models of both learning and memory deficits and deficits in sensory inhibition have confirmed a role for the alpha7 subtype of the nicotinic cholinergic receptor in these processes. Rodent studies also demonstrated the efficacy of a selective partial alpha7 nicotinic agonist, DMXBA, to improve these deficits. Subsequent human clinical trials demonstrated improved sensory inhibition in 12 schizophrenia patients and showed improvement in several subtests of the RBANS learning and memory assessment instrument. These data suggest that therapeutic agents selected for alpha7 nicotinic activity may have utility in treating certain symptoms of schizophrenia.

Sensory gating and alpha-7 nicotinic receptor gene allelic variants in schizoaffective disorder, bipolar type

OBJECTIVES

Single nucleotide allelic variants in the promoter region of the chromosome 15 alpha-7 acetylcholine nicotinic receptor gene (CHRNA7) are associated with both schizophrenia and the P50 auditory evoked potential sensory gating deficit. The purpose of this study was to determine if CHRNA7 promoter allelic variants are also associated with abnormal P50 ratios in persons with schizoaffective disorder, bipolar type.

METHODS

P50 auditory evoked potentials were recorded in a paired stimulus paradigm in 17 subjects with schizoaffective disorder, bipolar type. The P50 test to conditioning ratio was used as the measure of sensory gating. Mutation screening of the CHRNA7 promoter region was performed on the subjects' DNA samples. Comparisons to previously obtained data from persons with schizophrenia and controls were made.

RESULTS

Subjects with schizophrenia, regardless of allele status, had an abnormal mean P50 ratio. Subjects with schizoaffective disorder, bipolar type and a variant allele had an abnormal mean P50 ratio, whereas those schizoaffective subjects with the common alleles had a normal mean P50 ratio. Normal control subjects had a normal mean ratio, but controls with variant alleles had higher P50 ratios.

CONCLUSIONS

In persons with bipolar type schizoaffective disorder, CHRNA7 promoter region allelic variants are linked to the capacity to inhibit the P50 auditory evoked potential and thus are associated with a type of illness genetically and biologically more similar to schizophrenia.

Schizophrenia and the alpha7 nicotinic acetylcholine receptor

In addition to the devastating symptoms of psychosis, many people with schizophrenia also suffer from cognitive impairment. These cognitive symptoms lead to marked dysfunction and can impact employability, treatment adherence, and social skills. Deficits in P50 auditory gating are associated with attentional impairment and may contribute to cognitive symptoms and perceptual disturbances. This nicotinic cholinergic-mediated inhibitory process represents a potential new target for therapeutic intervention in schizophrenia. This chapter will review evidence implicating the nicotinic cholinergic, and specifically, the alpha7 nicotinic receptor system in the pathology of schizophrenia. Impaired auditory sensory gating has been linked to the alpha7 nicotinic receptor gene on the chromosome 15q14 locus. A majority of persons with schizophrenia are heavy smokers. Although nicotine can acutely reverse diminished auditory sensory gating in people with schizophrenia, this effect is lost on a chronic basis due to receptor desensitization. The alpha7 nicotinic agonist 3-(2,4 dimethoxy)benzylidene-anabaseine (DMXBA) can also enhance auditory sensory gating in animal models. DMXBA is well tolerated in humans and a new study in persons with schizophrenia has found that DMXBA enhances both P50 auditory gating and cognition. alpha7 Nicotinic acetylcholine receptor agonists appear to be viable candidates for the treatment of cognitive disturbances in schizophrenia.

Linkage disequilibrium analysis of the CHRNA7 gene and its partially duplicated region in schizophrenia

Several previous studies have reported a significant linkage between markers in the alpha 7 nicotinic cholinergic receptor subunit (CHRNA7) gene and either schizophrenia or the P50 sensory gating deficit, a schizophrenia endophenotype. However, CHRFAM7A, a partially duplicated gene 1.6Mb upstream of the CHRNA7 gene, has complicated further genetic analysis. We genotyped 14 polymorphic markers throughout the full-length CHRNA7 gene and the duplicated region in 188 unrelated Han Chinese patients with schizophrenia and 188 controls. The duplicated regions were assessed by genotyping up- and down-stream polymorphic markers in the vicinity of each region and analyzing the linkage disequilibrium (LD) between each pair of markers. No evidence of risk variants for schizophrenia in either the CHRNA7 gene or the partially duplicated region was found in the LD analysis. A significant deviation from the Hardy-Weinberg equilibrium (HWE) was found only in the genotypic distribution of SNP9 (IVS4-1912) in patients (p=0.00829), but not in controls. In conclusion, our LD analysis did not reveal any association between schizophrenia in our Han Chinese population and the CHRNA7 gene or its partially duplicated region. However, we could not exclude the possibility of a weak genetic effect due to the small sample size. Analyses of larger samples and higher-density markers, particularly around SNP9 (IVS4-1912), are still needed.

Genetics of chromosome 15q13-q14 in schizophrenia

Positive genetic linkage to the 15q13-q14 region has been found in 11 studies, and several association reports support this locus as a candidate region for schizophrenia. The locus is unusual in that it was first linked to an endophenotype found in schizophrenia, the P50 deficit, and subsequently to schizophrenia. There is also biological data showing that a candidate gene in the region, the alpha7 nicotinic receptor CHRNA7, plays a seminal role in the linked endophenotype, and is decreased in expression in the patient population. The 15q13-q14 region is complicated by a partial duplication of the CHRNA7 gene that includes exons 5-10 and considerable sequence downstream. Evidence from multiple studies supports a broad region of genetic linkage around the marker D15S1360.

Population-based and family-based association studies of an (AC)n dinucleotide repeat in alpha-7 nicotinic receptor subunit gene and schizophrenia

The human alpha-7 neuronal nicotinic receptor subunit (CHRNA7) gene, located at chromosome 15q13.2, represents a strong candidate gene for schizophrenia. We have examined an (AC)n dinucleotide repeat in intron 2 of the CHRNA7 gene, which was previously shown to be strongly linked with schizophrenia, using both population-based and family-based association studies. In the population-based study, no significant differences between the genotype and allele frequency distributions in schizophrenia patients and control subjects were observed after correction for multiple testing, although a nominally significant association between the most common allele and schizophrenia was observed (P = 0.023, uncorrected for multiple testing). In the family-based study, there is no significant over-transmission (Transmitted/Non-transmitted: 61/50) of the same allele in 160 family trios. Overall, our results do not support a major role for the (AC)n dinucleotide repeat in schizophrenia susceptibility in Han Chinese. Further large-scale genetic studies based on a set of single nucleotide polymorphisms (SNPs) that fully characterize the linkage disequilibrium patterns at the CHRNA7 gene are necessary to determine the relevance of this gene as a risk factor for schizophrenia susceptibility.

Genetic interaction between alpha4 and beta2 subunits of high affinity nicotinic receptor: analysis in schizophrenia

Cholinergic dysfunction is one of the hypotheses for the cognitive deficits of schizophrenia. Neurocognitive deficits, which are well-described clinical features of schizophrenia, may be remediated by nicotine; therefore investigations of nicotinic receptor subtypes is of considerable clinical interest. We typed polymorphisms in CHRNA4 and CHRNB2 genes controlling the expression of neuronal high-affinity nicotinic receptors in 117 Canadian families having at least one schizophrenic patient. Using a family-based association strategy, we performed allele, haplotype and interaction analysis of these two loci. In the families tested, the two cholinergic genes interact to affect schizophrenia in combination (P=0.010), while neither was sufficient alone to confer susceptibility. Our present study provided the first line of direct evidence suggesting that the CHRNA4 gene combined with CHRNB2 receptor gene may be linked to schizophrenia.

Characterization of allelic variants at chromosome 15q14 in schizophrenia

Evidence of genetic linkage for schizophrenia at chromosome 15q14 has been reported in nine independent studies, but the molecular variants responsible for transmission of genetic risk are unknown. National Institute of Mental Health Schizophrenia Genetics Initiative families were genotyped for single nucleotide polymorphisms (SNPs) and dinucleotide repeat markers in the 15q14 linkage region and analyzed based on the presence of particular alleles of the dinucleotide repeat marker D15S165 in the 15q14 region. Two alleles showed both familial transmission disequilibrium and population-wide association with schizophrenia. The two groups identified by these two D15S165 alleles differ in age of onset, number of hospitalizations and intensity of nicotine abuse, as well as in predominant ethnicity. Variations in the frequency of SNPs in CHRNA7, the alpha-7-nicotinic acetylcholine receptor subunit gene at 15q14, were found in each group. Further sequencing in these two groups may yield more definitive identification of the molecular pathology.

Screening of PIP5K2A promoter region for mutations in bipolar disorder and schizophrenia

OBJECTIVE

To analyze the promoter region of PIP5K2A, a phosphatidylinositol 4-phosphate 5-kinase that maps to 10p in a region linked to both bipolar disorder and schizophrenia.

METHODS

The promoter region was screened by single-strand conformation polymorphism analysis and DNA sequencing. Allele frequencies were determined in a case-control study. Functional significance of a promoter variant was determined by electromobility gel shift assays.

RESULTS

Homozygosity for a rare putative promoter variant, -1007C-->T, was found in only two patients with schizophrenia and in no controls or bipolar patients. The variant forms a 7/8 base match for the binding site of Oct-1, a member of the POU homeodomain family. Electromobility gel shift assays revealed increased binding of a brain-specific nuclear protein to the -1007T allele compared with -1007C.

CONCLUSION

The data suggest that homozygosity for -1007T could be a rare genetic factor in the development of schizophrenia.

Genome scan of schizophrenia families in a large Veterans Affairs Cooperative Study sample: evidence for linkage to 18p11.32 and for racial heterogeneity on chromosomes 6 and 14

Genome-wide linkage analyses of schizophrenia have identified several regions that may harbor schizophrenia susceptibility genes but, given the complex etiology of the disorder, it is unlikely that all susceptibility regions have been detected. We report results from a genome scan of 166 schizophrenia families collected through the Department of Veterans Affairs Cooperative Studies Program. Our definition of affection status included schizophrenia and schizoaffective disorder, depressed type and we defined families as European American (EA) and African American (AA) based on the probands' and parents' races based on data collected by interviewing the probands. We also assessed evidence for racial heterogeneity in the regions most suggestive of linkage. The maximum LOD score across the genome was 2.96 for chromosome 18, at 0.5 cM in the combined race sample. Both racial groups showed LOD scores greater than 1.0 for chromosome 18. The empirical P-value associated with that LOD score is 0.04 assuming a single genome scan for the combined sample with race narrowly defined, and 0.06 for the combined sample allowing for broad and narrow definitions of race. The empirical P-value of observing a LOD score as large as 2.96 in the combined sample, and of at least 1.0 in each racial group, allowing for narrow and broad racial definitions, is 0.04. Evidence for the second and third largest linkage signals come solely from the AA sample on chromosomes 6 (LOD = 2.11 at 33.2 cM) and 14 (LOD = 2.13 at 51.0). The linkage evidence differed between the AA and EA samples (chromosome 6 P-value = 0.007 and chromosome 14 P-value = 0.004).

Decreased amphetamine-induced locomotion and improved latent inhibition in mice mutant for the M5 muscarinic receptor gene found in the human 15q schizophrenia region

M5 muscarinic receptors are coexpressed with D2 dopamine receptors in the ventral tegmentum and striatum, and are important for reward in rodents. Previously, we reported that disruption of the M5 receptor gene in mice reduced dopamine release in the nucleus accumbens. In this study, we established a polymerase chain reaction (PCR) genotyping method for M5 mutant mice, and, using RT-PCR, found that M5 mRNA expression was highest in the ventral tegmentum, striatum, and thalamus in wild-type mice. In the M5 mutant mice, D2 mRNA expression was increased in several brain structures, including the striatum. Genome mapping studies showed the M5 gene is localized to chromosome 2E4 in mice, and to 15q13 in humans in the region that has been linked to schizophrenia. Amphetamine-induced locomotion, but not baseline locomotion or motor functions, decreased in M5 mutant mice, consistent with lower accumbal dopamine release. Previous reports found latent inhibition improvement in rats following nucleus accumbens lesions, or blockade of dopamine D2 receptors with neuroleptic drugs. Here, latent inhibition was significantly increased in M5 mutant mice as compared with controls, consistent with reduced dopamine function in the nucleus accumbens. In summary, our results showed that M5 gene disruption in mice decreased amphetamine-induced locomotion and increased latent inhibition, suggesting that increased M5 mesolimbic function may be relevant to schizophrenia.

Identification of molecular variants at the promoter region of the human α7 neuronal nicotinic acetylcholine receptor subunit gene but lack of association with schizophrenia

The human alpha7 neuronal nicotinic receptor subunit gene has been considered as a candidate gene for P50 sensory gating deficit in schizophrenic patients. Because P50 sensory gating deficit is a common neurophysiological dysfunction in patients with schizophrenia and schizophrenia spectrum disorders, it is conceivable to hypothesize that the human alpha7 neuronal nicotinic receptor subunit gene might be a susceptible gene for schizophrenia. Researchers have reported that mutations in the protein-coding sequences of the human alpha7 neuronal nicotinic receptor subunit gene are very rare. Therefore, we searched for mutations at the promoter region of the human alpha7 neuronal nicotinic receptor subunit gene and performed a genetic association study in 249 unrelated Han Chinese schizophrenic patients and 273 non-psychotic subjects from Taiwan. Two molecular variants were identified and designated g.-213G>A and g.-324A>G, respectively. The g.-213G>A variant was found to obliterate a putative NF-1 transcription factor binding site using computer analysis. One out of 249 patients was detected to be a heterozygote for this variant, but none of 273 control subjects was. The g.-324A>G variant was also very rare in both patients and control subjects, only one heterozygote of this variant was identified in 249 patients and 273 control subjects, respectively. Hence, in this study, we did not find mutations in the human alpha7 neuronal nicotinic receptor subunit gene that are associated with schizophrenia in our population.

Phencyclidine-induced changes in rat cortical gene expression identified by microarray analysis: implications for schizophrenia

Acute phencyclidine induces schizophrenia-like symptoms in healthy humans and psychotic episodes in schizophrenics. Although phencyclidine is known as a N-methyl d-aspartate receptor antagonist (NMDA-R), the molecular events underlying the behavioral symptoms remain largely unknown. Statistical analysis of oligonucleotide microarray data was used to identify phencyclidine-induced alterations in rat cortical gene expression. Acute phencyclidine produced a statistically significant change in 477 genes in rat prefrontal cortex (PFC), a brain area associated with cognitive dysfunction in schizophrenics. Real-time quantitative PCR (RTQ-PCR) confirmed a subset of these changes ranging from -59% to 255% (smallest confirmation: -19%). Subsequent time-course and dose-response studies using RTQ-PCR confirmed and extended the original microarray results. At the molecular level, genes altered by phencyclidine are related to diverse biological processes including stress, inflammatory response, growth and development, neural plasticity and signal transduction. Further analysis, aimed at assessing the relevance of our results to schizophrenia, revealed dysregulation of genes related to: (i) thalamocortical projections, (ii) neurotransmission and neuromodulation, (iii) thyroid hormone activity, (iv) oligodendrocyte linage, (v) brain lipid metabolism, (vi) sleep architecture and (viii) the velocardiofacial syndrome.

Nicotinic receptors and schizophrenia

The incidence of smoking is very high in non-schizophrenic subjects presenting various psychiatric disorders (35 to 54%). However, the incidence of smoking is extremely high in schizophrenic patients: 80% to 90%, versus 25% to 30% of the general population. Various studies have demonstrated that the use of tobacco transiently restores the schizophrenic patient's cognitive and sensory deficits. Smoking cessation also appears to exacerbate the symptoms of the disease. Post-mortem binding studies have revealed a disturbance of nicotinic receptor expression, affecting the alpha(7) and alpha(4)beta(2) subunits, in various cerebral areas. Genetic linkage studies have also shown that the alpha(7) subunit is involved in schizophrenia. This review assesses the involvement of the nicotinic system in schizophrenia and suggests ways in which this system may participate in the pathophysiology of this disease.

Identification of PIK3C3 promoter variant associated with bipolar disorder and schizophrenia

BACKGROUND

Genes involved in phosphoinositide (PI) lipid metabolism are excellent candidates to consider in the pathogenesis of bipolar disorder (BD) and schizophrenia (SZ). One is PIK3C3, a member of the phosphatidylinositide 3-kinase family that maps closely to markers on 18q linked to both BD and SZ in a few studies.

METHODS

The promoter region of PIK3C3 was analyzed for mutations by single-strand conformation polymorphism analysis and sequencing. A case-control association study was conducted to determine the distribution of variant alleles in unrelated patients from three cohorts. Electromobility gel shift assays (EMSA) were performed to assess the functional significance of variants.

RESULTS

Two polymorphisms in complete linked disequilibrium with each other were identified, -432C- > T and a "C" insert at position -86. The -432T allele occurs within an octamer containing an ATTT motif resembling members of the POU family of transcription factors. In each population analyzed, an increase in -432T was found in patients. EMSAs showed that a -432T containing oligonucleotide binds to brain proteins that do not recognize -432C.

CONCLUSIONS

A promoter mutation in a PI regulator affecting the binding of a POU-type transcription factor may be involved in BD and SZ in a subset of patients.

Linkage studies of schizophrenia

The study of schizophrenia genetics has revealed much about the disease but none of the essential secrets of its etiology, so far, for numerous reasons. First, schizophrenia is a complex trait, influenced by both genes and environment. Second, it appears to be a highly heterogeneous disease, with locus and allelic heterogeneity both between and within families likely. Third, since it is common, it is likely that the genetic liability variants are common, and so are found with relatively high frequency in the general population. Fourth, linkage methods, which deliver rapid coverage of the genome, have great power to identify single genes causing Mendelian disorders but are poorly suited to the genetic architecture of complex traits. Although association methods are undeniably more powerful in such situations, affordable technologies to deliver the much higher density whole genome coverage required are not yet available and candidate gene studies of schizophrenia have not produced robust and replicable results. In spite of these limitations, there are now sufficient data to support several conclusions. Numerous regions of the human genome give consistent, though by no means unanimous, support for linkage. The precise nature of these signals is not yet understood, and power to position the effects is poor, but metanalyses show the co-occurrence is unlikely to be due to chance. Combined approaches utilizing linkage for rapid genome coverage and association for fine-scale follow-up have identified several promising candidate genes. Although the definition of replication in a complex trait is itself complex, a number of these candidates have been supported by numerous studies. These converging lines of evidence suggest that the genetics of schizophrenia, long considered a most intractable problem, are at last beginning to be unraveled.

Dementia rating and nicotinic receptor expression in the prefrontal cortex in schizophrenia

BACKGROUND

The etiology of dementia that occurs in patients with schizophrenia is not well understood. Nicotinic acetylcholine receptors have been implicated in cognitive function, and deficits in these receptors have been reported in schizophrenia.

METHODS

The present study investigates possible associations of nicotinic receptor subunit expression in the dorsal lateral prefrontal cortex, an area known to be affected in schizophrenia, and dementia rating.

RESULTS

alpha7 immunoreactivity was reduced by 20% to 28% and [(3)H]epibatidine binding was increased twofold in groups of patients with schizophrenia compared to normal control subjects matched for age, postmortem delay, and low levels of brain nicotine and cotinine. In contrast, no significant differences in alpha4, alpha3, or beta2 immunoreactivity or alpha7 messenger RNA expression were observed in schizophrenia patients compared with control subject values. Clinical dementia ratings in patients with schizophrenia were correlated with neither [(3)H]epibatidine binding nor nicotinic receptor subunit expression.

CONCLUSIONS

These data indicate no relationship between the trend for reduced neocortical alpha7 subunit protein expression in schizophrenia and dementia. Further investigations are required to establish whether the reduction in alpha7 protein in the dorsal lateral prefrontal cortex is associated with clinical features other than dementia in schizophrenia.

Polymorphism screening of PIP5K2A: a candidate gene for chromosome 10p-linked psychiatric disorders

Lithium is a potent noncompetitive inhibitor of inositol monophosphatases, enzymes involved in phosphoinositide (PI) and inositol phosphate metabolism. A critical component of the PI pathway is phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), which is hydrolyzed to second messengers and has a direct role in synaptic vesicle function. Interestingly, a number of genes involved in the synthesis and dephosphorylation of PtdIns(4,5)P(2) are found in regions of the genome previously mapped in bipolar disorder (BD) including 10p12, 21q22, and 22q11, among others. Some of these regions overlap with loci mapped in schizophrenia (SZ). One gene involved in PI metabolism that maps to a region of interest is 10p12-linked PIP5K2A, a member of the phosphatidylinositol 4-phosphate 5-kinase family. Polymorphism screening revealed the existence of an imperfect CT repeat polymorphism located near the exon 9-intron 9 splice donor site. A modest difference was found in the distribution of alleles from this highly polymorphic variant when bipolar and schizophrenic subjects were compared with controls; relatively rare short repeat variants were found more commonly in patients and homozygosity for a common long repeat variant was found more commonly in controls. These data suggest that the imperfect CT repeat in PIP5K2A intron 9 should be further investigated as a possible candidate allele for 10p12-linked psychiatric disorders.

Comparison of polymorphisms in the alpha7 nicotinic receptor gene and its partial duplication in schizophrenic and control subjects

The hypothesis that the 15q13-15 region of chromosome 15 contains a gene that contributes to the etiology of schizophrenia is supported by multiple genetic linkage studies. The alpha7 neuronal nicotinic acetylcholine receptor (CHRNA7) gene was selected as the best candidate gene in this region for molecular investigation, based on these linkage findings and biological evidence in both human and rodent models. CHRNA7 receptors are decreased in expression in postmortem brain of schizophrenic subjects. A dinucleotide marker, D15S1360, in intron two of the CHRNA7 gene is genetically linked to an auditory gating deficit found in schizophrenics and half of the first-degree relatives of patients. Single strand conformation polymorphism (SSCP) and sequence analyses of DNA from schizophrenic and control individuals identified 33 variants in the coding region and intron/exon borders of the CHRNA7 gene and its partial duplication, dupCHRNA7; common polymorphisms were mapped. Twenty-one variants were found in the exons, but non-synonymous changes were rare. Although the expression of CHRNA7 is decreased in schizophrenia, the general structure of the remaining receptors is likely to be normal.

Genome scan meta-analysis of schizophrenia and bipolar disorder, part II: Schizophrenia

Schizophrenia is a common disorder with high heritability and a 10-fold increase in risk to siblings of probands. Replication has been inconsistent for reports of significant genetic linkage. To assess evidence for linkage across studies, rank-based genome scan meta-analysis (GSMA) was applied to data from 20 schizophrenia genome scans. Each marker for each scan was assigned to 1 of 120 30-cM bins, with the bins ranked by linkage scores (1 = most significant) and the ranks averaged across studies (R(avg)) and then weighted for sample size (N(sqrt)[affected casess]). A permutation test was used to compute the probability of observing, by chance, each bin's average rank (P(AvgRnk)) or of observing it for a bin with the same place (first, second, etc.) in the order of average ranks in each permutation (P(ord)). The GSMA produced significant genomewide evidence for linkage on chromosome 2q (PAvgRnk<.000417). Two aggregate criteria for linkage were also met (clusters of nominally significant P values that did not occur in 1,000 replicates of the entire data set with no linkage present): 12 consecutive bins with both P(AvgRnk) and P(ord)<.05, including regions of chromosomes 5q, 3p, 11q, 6p, 1q, 22q, 8p, 20q, and 14p, and 19 consecutive bins with P(ord)<.05, additionally including regions of chromosomes 16q, 18q, 10p, 15q, 6q, and 17q. There is greater consistency of linkage results across studies than has been previously recognized. The results suggest that some or all of these regions contain loci that increase susceptibility to schizophrenia in diverse populations.

The genetics of sensory gating deficits in schizophrenia

Sensory gating abnormalities are an early clinical symptom of schizophrenia, and are characterized by a decrease in the brain's normal ability to inhibit the response to unimportant stimuli. Patients appear hypervigilant and have difficulty focusing their attention. A neurobiologic mechanism involved in these difficulties is nicotinic cholinergic modulation of inhibitory neuronal activity in the hippocampus. One measure of sensory gating abnormalities, diminished inhibition of the P50 evoked response to repeated auditory stimuli, has been linked to the chromosome 15q14 locus of the alpha-7-nicotinic receptor gene. This site is one of several that have shown evidence for linkage to schizophrenia, as well as to bipolar disorder, across several studies. Polymorphisms in the core promoter of the gene are associated with schizophrenia and also with diminished inhibition of the P50 response. These genetic data may identify a new pathophysiologic target for drug discovery.

Modest evidence for linkage and possible confirmation of association between NOTCH4 and schizophrenia in a large Veterans Affairs Cooperative Study sample

Wei and Hemmings [2000: Nat Genet 25:376-377], using 80 British parent-offspring trios, identified a number of NOTCH4 variants and haplotypes that showed statistically significant evidence of association to schizophrenia. Specifically, the 10 repeat allele of a (CTG)(n) marker and the 8 repeat allele of a (TAA)(n) marker demonstrated excess transmission to affected individuals; SNP21 and haplotypes SNP2-(CTG)(n) and SNP12-SNP2-(CTG)(n) also showed significant associations. In an attempt to replicate these findings, we tested for linkage and association between the same five markers used by Wei and Hemmings in 166 families collected from a multi-center study conducted by the Department of Veterans Affairs (DVA) Cooperative Study Program (CSP). The families include 392 affected subjects (schizophrenia or schizoaffective disorder, depressed) and 216 affected sibling pairs. The families represent a mix of European Americans (n = 62, 37%), African Americans (n = 60, 36%), and racially mixed or other races (n = 44, 27%). We identified moderate evidence for linkage in the pooled race sample (LOD = 1.25) and found excess transmission of the 8 (P = 0.06) and 13 (P = 0.04) repeat alleles of the (TAA)(n) marker to African American schizophrenic subjects. The 8 and 13 repeat alleles were previously identified to be positively associated with schizophrenia by Wei and Hemmings [2000: Nat Genet 25:376-377] and Sklar et al. [2001: Nat Genet 28:126-128], respectively.

Smoking and mental illness

Patients with mental illness have a higher incidence of smoking than the general population and are the major consumers of tobacco products. This population includes subjects with schizophrenia, manic depression, depression, posttraumatic stress disorder (PTSD), attention-deficit disorder (ADD), and several other less common diseases. Smoking cessation treatment in this group of patients is difficult, often leading to profound depression. Several recent findings suggest that increased smoking in the mentally ill may have an underlying biological etiology. The mental illness schizophrenia has been most thoroughly studied in this regard. Nicotine administration normalizes several sensory-processing deficits seen in this disease. Animal models of sensory deficits have been used to identify specific nicotinic receptor subunits that are involved in these brain pathways, indicating that the alpha 7 nicotinic receptor subunit may play a role. Genetic linkage in schizophrenic families also supports a role for the alpha 7 subunit with linkage at the alpha 7 locus on chromosome 15. Bipolar disorder has some phenotypes in common with schizophrenia and also exhibits genetic linkage to the alpha 7 locus, suggesting that these two disorders may share a gene defect. The alpha 7 receptor is decreased in expression in schizophrenia. [(3)H]-Nicotine binding studies in postmortem brain indicate that high-affinity nicotinic receptors may also be affected in schizophrenia.