Search

Studies in cell culture and in animals suggest that neuregulin 1 (NRG1), a probable schizophrenia susceptibility gene, regulates the expression of the alpha7 nicotinic acetylcholine receptors (nAChRs). We hypothesized that schizophrenia-associated allelic variations within the NRG1 gene, via their effects on NRG1 isoform expression, would be associated with alterations in nAChR alpha7 receptor levels. We examined the effects of four disease-associated single-nucleotide polymorphisms (SNPs) in the 5' region of the NRG1 gene on nAChR alpha7 mRNA transcript expression in both the dorsolateral prefrontal cortex (DLPFC) and hippocampus of normal controls and patients with schizophrenia using quantitative real-time PCR. NRG1 risk alleles at SNPs SNP8NRG221132 and rs6994992 predicted significantly lower nAChR alpha7 mRNA expression in the DLPFC. Haplotypes containing the risk alleles at the above SNPs were also associated with lower expression of nAChR alpha7 in the DLPFC. The genotype effect for rs6994992 and the haplotype effect were more pronounced within the schizophrenic patient group. To determine whether receptor levels follow that of mRNA expression, we performed receptor binding and autoradiography using [(125)I] alpha-bungarotoxin in the DLPFC. Consistent with the mRNA findings, we found a decrease in binding in risk allele carriers of SNP8NRG221132 as compared with heterozygous individuals. Together, these results suggest that the molecular mechanism of the association between NRG1 risk alleles and schizophrenia may include down-regulation of nAChR alpha7 expression.

Linkage, association and postmortem studies have implicated regulator of G-protein signaling 4 (RGS4), which negatively modulates signal transduction at G-protein-coupled receptors, as a candidate schizophrenia susceptibility gene. We compared RGS4 mRNA expression in the dorsolateral prefrontal cortex (DLPFC), between normal controls and patients with schizophrenia in two independent cohorts (>100 subjects each) (the CBDB/NIMH Collection and the Stanley Array Collection), and in the hippocampus in the CBDB/NIMH Collection. We also examined the effects of the four previously identified putative RGS4 risk SNPs (rs10917670, rs951436, rs951439, rs2661319) on RGS4 expression levels in these cohorts. As dopamine signaling is linked to RGS4 expression and there is evidence for statistical epistasis between COMT Val158Met polymorphism and RGS4 alleles, we also examined relationships between the COMT Val158Met genotype and RGS4 expression in the DLPFC. We did not detect a difference in RGS4 expression levels between schizophrenic patients (or bipolar disorder patients in the Stanley Collection) and controls and found no significant association between any of the RGS4 risk SNPs and RGS4 expression. However, COMT Val158Met genotype was associated with prefrontal and hippocampal RGS4 mRNA expression in an allele dose-dependent manner, with carriers of the COMT Val allele showing significantly lower expression than heterozygous individuals or subjects homozygous for the Met allele. Consistent with these genotype effects, RGS4 mRNA was inversely correlated with the COMT enzyme activity in the DLPFC. These data suggest that RGS4 mRNA expression is associated with cortical dopamine signaling and illustrate the importance of genetic and/or environmental background in gene expression studies in schizophrenia.

DISC1 has been identified as a schizophrenia susceptibility gene based on linkage and SNP association studies and clinical data suggesting that risk SNPs impact on hippocampal structure and function. In cell and animal models, C-terminus-truncated DISC1 disrupts intracellular transport, neural architecture and migration, perhaps because it fails to interact with binding partners involved in neuronal differentiation such as fasciculation and elongation protein zeta-1 (FEZ1), platelet-activating factor acetylhydrolase, isoform Ib, PAFAH1B1 or lissencephaly 1 protein (LIS1) and nuclear distribution element-like (NUDEL). We hypothesized that altered expression of DISC1 and/or its molecular partners may underlie its pathogenic role in schizophrenia and explain its genetic association. We examined the expression of DISC1 and these selected binding partners as well as reelin, a protein in a related signaling pathway, in the hippocampus and dorsolateral prefrontal cortex of postmortem human brain patients with schizophrenia and controls. We found no difference in the expression of DISC1 or reelin mRNA in schizophrenia and no association with previously identified risk DISC1 SNPs. However, the expression of NUDEL, FEZ1 and LIS1 was each significantly reduced in the brain tissue from patients with schizophrenia and expression of each showed association with high-risk DISC1 polymorphisms. Although, many other DISC1 binding partners still need to be investigated, these data implicate genetically linked abnormalities in the DISC1 molecular pathway in the pathophysiology of schizophrenia.