Editor's Note: On Sunday, 1 April 2007, at the International Congress on Schizophrenia Research in Colorado Springs, Joel Kleinman, of the Clinical Brain Disorders Branch at NIMH, chaired the session “Pathways to Schizophrenia: Susceptibility genes and their signaling cascades.” Young Investigator travel awardee Eric Epping of the University of Iowa was there to report for you.
6 June 2007. The presentations during the session focused on several important schizophrenia candidate genes and the effect of variation in these genes on pathways that may impact the biology of schizophrenia.
Results from a detailed analysis of the dysbindin gene were presented by Brien Riley from Virginia Commonwealth University. Previous work has shown that this gene, which is found on chromosome 6p, contains a haplotype associated with a high risk for developing schizophrenia. An analysis of the dysbindin haplotype and a region on chromosome 8 with evidence of linkage to schizophrenia was performed to determine if these haplotypes may be epistatic; however, no linkage was found to the chromosome 8 region in families who had the high-risk dysbindin haplotype. A comparative analysis of human and primate alleles was performed to characterize variants that may have arisen during the course of evolution. In the group’s Irish sample, a total of 26 variants were found in dysbindin that were specific to the Irish population high-risk haplotype, with one variant producing a non-synonymous coding change. However, this variant was not associated with schizophrenia. As dysbindin and Akt, a seronine/threonine kinase involved in neuronal growth and survival, may be involved in the same cellular pathways, expression of 21 genes in the Akt cell survival pathway were measured to compare expression according to dysbindin genotype from samples in the Stanley brain collection. No differences in gene expression were found by dysbindin genotype. Further work will include assessment of dysbindin transcript variants, allele-specific expression, and expression in other cell types.
Joel Kleinman discussed work to date on catechol-O-methyl transferase (COMT) in his talk entitled “Complex genetics in the human brain: lessons from COMT.” He provided a thorough summary of previous COMT work from multiple labs that have focused on the differential effects of the COMT Val/Met polymorphism, with the Val form producing increasing COMT-mediated dopamine breakdown. Kleinman cited results indicating that other alleles in COMT itself and other genes likely interact genetically with the Val/Met variant. One example was the work from Nackley and colleagues which identified differences in COMT mRNA stability according to haplotype (Nackley et al., 2006), of which the Val/Met polymorphism is only one variant contained in COMT haplotypes. Nicodemus and colleagues from the NIMH group identified epistasis between COMT and several genes, including GAD1, DAOA, DISC1, GRM3, and RGS4 (Nicodemus et al., 2007). While Lipska and colleagues found no differences in RGS4 expression in postmortem samples when comparing cases with controls, differences in RGS4 expression were found by COMT genotype in both the dorsolateral prefrontal cortex and in the hippocampus (Lipska et al., 2006). Caspi and colleagues also identified a gene-environment interaction between COMT genotype and marijuana use before age 15, with COMT Val homozygotes who used cannabis having the highest risk of developing psychosis by age 26 (Caspi et al., 2005). Kleinman also presented data showing a relationship between COMT and cannabinoid receptor CNR1 expression, with differential decreases in CNR1 expression by COMT genotype as individuals age. It is hypothesized that these differences in COMT-associated CNR1 expression influence susceptibility to psychosis secondary to cannabis use. Based on the results presented, Kleinman’s conclusion was, “Don’t close down the [COMT] mine just yet.”
Amanda Law from the University of Oxford, and in collaboration with researchers at NIMH, presented data on the function of the NRG1/ErbB4 pathway in schizophrenia. Neuregulin 1 (NRG1) was previously identified by genetic linkage and association studies with schizophrenia, and its various isoforms are involved in neuronal growth, synaptic plasticity, and cell migration. ErbB4 is a tyrosine kinase membrane receptor activated by NRG1, and it is also a schizophrenia susceptibility gene. Results from experiments on NRG1 expression in postmortem human hippocampus samples were presented, with specific SNPs at the 5’ end of the gene associated with schizophrenia having effects on NRG1 expression. One SNP and the previously identified high-risk haplotype were found to be associated with increased Type IV NRG1 expression in the human brain in schizophrenia. Type I expression was also increased in subjects with schizophrenia, but increased expression in both control and schizophrenia groups was associated with a SNP in the high-risk haplotype and specifically with type IV (Law et al., 2006; see SRF related news story). Type IV NRG1 expression was also examined in fetal brain tissue and a range of other human tissues. Full-length Type IV was cloned and characterized in the adult human brain, and novel variants in the NRG1 gene were identified during human brain development. Data on ErbB4 expression was also presented, with increased expression found in the dorsolateral prefrontal cortex, as well as association of schizophrenia risk variants with alteration in ErbB4 expression (Law et al., 2007). In order to determine if peripheral blood B lymphoblasts could be used as a model for genetic studies in the NRG1 pathway, ErbB4 expression was examined. Increased expression of the ErbB4 variant found to be altered in the brain in schizophrenia, and in relation to genetic risk in the gene, was replicated in B lymphoblasts. Individuals homozygous for the schizophrenia risk alleles had significantly higher levels of ErbB4. NRG1 expression was not found in this cell type. Downstream factors in this pathway were investigated, with one phosphatidylinositol-3 kinase (PI3K) complex found to have increased expression in B lymphoblasts associated with the ErbB4 high-risk SNP and disease. The same expression differences were found in the hippocampus. As the NRG1 pathway induces signaling processes via the production of the molecule PIP3, the effect of NRG1 on PIP3 levels in schizophrenia was measured in B lymphoblasts. No differences between subjects with schizophrenia or controls were found; however, significant differences were observed in relation to genetic risk variants. Law suggested that alterations in the NRG1/ErbB4 signaling pathway in relation to genetic risk for schizophrenia may mediate their effects by the PI3K pathway.
Barbara Lipska, also from NIMH, presented data from her group’s work on characterization of gene expression in the DISC1 pathway. DISC1 (disrupted in schizophrenia) was identified in a region on chromosome 1 associated with not only schizophrenia, but also depression and bipolar affective disorder when a translocation with this region occurs with chromosome 11. Other genetic associations with DISC1 and schizophrenia, cognitive deficits, as well as altered hippocampal structure and function have been reported. The DISC1 protein is involved in cell division, neuronal migration, neurite outgrowth, and synapse formation. It specifically functions as a scaffold protein (Camargo et al., 2007), and proven or putative interactions with over 100 different proteins have been identified. LIS1, FEZ1, and NUDEL are of particular interest, and their expression was examined in the studies presented (see also Lipska et al., 2006a; Lipska et al, 2006b). Expression of DISC1 in the human brain was measured from postmortem tissues obtained from a range of ages at the time of death (including fetal tissues), with the highest expression of DISC1 found during infancy. LIS1, FEZ1, and NUDEL were found to have significantly increased expression during the second trimester of gestation, but this was not seen with DISC1. The location of DISC1 expression in the brain was also studied, with expression highest in the superior temporal gyrus (STG), but also higher than average in the neocortex, amygdala, hippocampus, entorhinal cortex, and gray matter of the dorsolateral prefrontal cortex (DLPFC). Differential expression of FEZ1, LIS1, and NUDEL was also identified, with FEZ1 expression lower than average in the amygdala, DLPFC, and STG, but higher in the neocortex (which is also where LIS1 expression was found to be higher), but it was lower than average in the amygdala and hippocampus. NUDEL expression was highest in the DLPFC, STG, and neocortex.
DISC1 mRNA expression in controls and schizophrenia patients was found to be similar, and mRNA levels did not vary by DISC1 genotype. However, increased DISC1 protein was found in hippocampal regions of schizophrenia patients by Western blotting, and high-risk alleles were found to be associated with increased protein levels. DISC1 is alternatively spliced into different isoforms, with a novel transcript identified in hippocampal tissue that was expressed in association with schizophrenia risk alleles. The presence of high-risk DISC1 alleles was also associated with decreased FEZ1, NUDEL, and LIS1 expression in the hippocampus and DLPFC of schizophrenia patients. A high-risk allele identified in LIS1 was also found to be associated with differential expression of LIS1. This work has shown variable expression of genes in the DISC1 pathway in schizophrenia in brain regions implicated in the pathophysiology of the disorder, and this expression is modulated by alleles that confer increased risk of developing the disorder.—Eric Epping.