Although linkage disequilibrium with other genetic alterations cannot be excluded, the CHRNA7 core promoter variants, found in this study, may contribute to a common pathophysiologic feature of schizophrenia.

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Neuronal nicotinic acetylcholine receptors are expressed in the human central nervous system. A specific subtype of this receptor family, the alpha7 nicotinic acetylcholine receptor, is thought to be the principal alpha-bungarotoxin (alphaBTX)-binding protein in mammalian brain. Although the expression of this receptor subtype has been characterized in rat, no study has specifically compared the expression of both the alpha7 gene and the localization of BTX binding sites in human brain. Expression of alpha7 mRNA and receptor protein in human postmortem brain tissue was examined by in situ hybridization and [125I]-alpha-bungarotoxin autoradiography, respectively, with particular emphasis on regions associated with sensory processing. Regions with high levels of both alpha7 gene expression and [125I]-alphaBTX binding include the nucleus reticularis of the thalamus, the lateral and medial geniculate bodies, the basilar pontine nucleus, the horizontal limb of the diagonal band of Broca, the nucleus basalis of Meynert, and the inferior olivary nucleus. High-to-moderate levels of alpha7 probe hybridization were also seen in the hippocampus and the cerebral cortex; however, there was a reduced or variable degree of [125I]-alphaBTX binding in these regions compared with the level of probe hybridization. In most brain regions, [125I]-alphaBTX binding was localized to neuronal cell bodies similar in morphology to those that exhibited alpha7 hybridization, suggesting that the high-affinity [125I]-alphaBTX binding sites in the human brain are likely to be principally composed of alpha7 receptor subtypes.

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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.

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 α7 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.

The α7 neuronal nicotinic receptor gene (CHRNA7) has been implicated in the pathophysiology of schizophrenia by genetic and pharmacological studies. Expression of the α7* receptor, as measured by [ 125 I]α-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 α7* receptor, seen in the autoradiographic studies, suggests aberrant assembly or trafficking of the receptor.

Background-The α7 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 multipletesting 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.