Because it is difficult to bridge this gap, few studies are able to provide a mechanism that plausibly explains how aberrant functioning of the identified gene could lead to the onset of schizophrenia. These pitfall was cleverly surmounted by two innovative studies in this issue of Neuron focusing on the biology of the schizophrenia candidate gene DISC1 ( Kang et al., 2011 and Singh et al., 2011). In 1968, a cytogenetic survey of Scottish juvenile delinquents detected a single boy who carried a balanced translocation from chromosome 11 into the long arm of chromosome 1. Later

MLN8237 concentration analysis revealed a major mental illness in roughly half of those family members carrying the t(1;11) translocation, whereas only 1 in 10 cytogenetically normal relatives were so afflicted (St Clair et al., 1990). Three decades after its initial discovery, it was recognized

that this insertion truncates the gene now known as disrupted in schizophrenia 1 (DISC1), a bit of misnomer given that schizophrenia was less prevalent than major mood disorders in the t(1:11) proband ( Millar et al., AG-14699 2000). This association with schizophrenia was first corroborated through linkages studies of the Finnish population and later by using SNP-haplotype (genome-wide) association studies of Caucasian and Asian cohorts. DISC1 is a large scaffolding protein (93 kDa) that is widely expressed throughout the fetal and adult brain, most prominently in the human hippocampus. Initially, yeast two-hybrid screens indentified a host of DISC1 binding partners, including MAP1a, GSK-3β, and PDE4, which bind the N-terminal domain; FEZ1, which binds in the region containing the original t(1,11) disruption; and NDEL1 and LIS1, which bind near the C terminus. Moreover, association studies have linked PDE4, FEZ1, and NDEL1 with disease onset, though none have been rigorously

validated (reviewed by Chubb et al., 2008). Given the wide variety of binding partners, it is not surprising that DISC1 mediates a plethora of different biological functions, both in vitro and in vivo. Some examples include regulating neuroblast migration (Duan et al., 2007 and Ishizuka Dipeptidyl peptidase et al., 2011) or the proliferation of neural progenitors via an interaction with GSK-3β (Mao et al., 2009 and Singh et al., 2010). Signaling through GSK-3β is a key step in the canonical Wnt pathway. This family of pathways is essential for proper development of the fetal forebrain-hippocampus and midbrain dopaminergic systems, the brain regions most frequently implicated in the etiology of schizophrenia and bipolar affective disorder. In fact, one of the first transgenic animal models of schizophrenia was the result of knocking out the Wnt transducing protein disheveled (Lijam et al., 1997). GSK-3β and Wnt signaling also play critical roles in the development and function of neuronal circuits in the adult brain.