Genome wide gene expression analysis of two ENU mouse models of major mental illness

Abstract

Major mental illness is now recognised as one of the leading causes of adult morbidity. Of the adult onset psychiatric disorders, the functional psychoses (schizophrenia, bipolar disorder and recurrent major depression) are the most severe and most common in the general population. Evidence suggests that certain genetic factors influence an individual’s susceptibility to developing these disorders when combined with appropriate social and environmental conditions. Several good candidate genes have been identified. Of relevance to this study is Disrupted in Schizophrenia 1 (DISC1) which was identified in a large Scottish family that carried a balanced translocation (t1:11) and had a history of major mental illness. In 2008, two ENU mutant mouse models with missense mutations in exon 2 of Disc1 were characterised and found to have behavioural and neuroanatomical phenotypes consistent with schizophrenia and major depression. The primary aim of this thesis is to further analyse these mouse models by performing whole genome gene expression studies and secondary protein analysis to identify genes involved in the aetiology of schizophrenia and major depression. My initial analysis used Illumina BeadChip microarray technology to identify 368 genes that were differentially expressed in ENU mutant animals under different biological conditions, compared to appropriate control animals. Nine biological groups were compared including one embryonic group at E13, and three groups treated with appropriate anti-psychotic or anti-depressant drugs. Of the 368 genes identified as differentially expressed, 46 were chosen for validation by qRT-PCR based on fold-change, p-value, functional significance, overenrichment of GO terms, pathway analysis and previous implications in major mental illness. NRXN1, NRXN3 and CDH11 were found to be significantly up-regulated in the schizophrenia mouse model with EGR4 significantly down-regulated compared to C57BL/6J wild-type controls. These findings were also replicated in an independent sample using wildtype littermates. The mental retardation gene PAK3 was up-regulated in the schizophrenia mouse model and expression levels were corrected to a level not significantly different to wild-type, when treated with the PDE4 inhibitor Rolipram. Semi-quantitative western blotting also confirmed the disregulation of EGR4 and PAK3 at the protein level in these animals. RNA expression profiles were also characterised for each of the genes above, and DISC1, through development. In summary this thesis describes the striking disregulation of four prominent genetic candidates of major mental illness in an independent animal model. A first functional link between DISC1 and NRXN1 is described suggesting, for the first time, a DISC1- dependant mechanism for regulating neurexin gene expression.