the expression of the suicidal phenotype. Whole genome scans of depressed and schizophrenic patients have revealed areas on the human genome containing genes that contribute to the depressive and/or schizophrenic phenotype. In the past for example, candidate genes implicated by our understanding of the pathophysiology of these diseased states or by their location in regions identified by whole genome scans have been investigated. So far, this has been a tedious and relatively unproductive approach in that researchers have only been able to evaluate one gene at a time. Previous technical limitations have severely hindered efforts to study the level and pattern of expression of many brain genes simultaneously in different regions. However, recent technological developments have dramatically altered the scientific landscape and made it possible, for the first time, to use microarray technology in postmortem brain tissue to screen 10,000 to 15,000 genes in one experiment.
The microarray technology involves the extraction of mRNA from brain regions of interest, construction of mRNA expression arrays following PCR amplification and purification of DNA clones, hybridization of labeled cDNA tissue to the array, and the quantification of the arrays. The arrays, which are an inch or so square and contain elements identifying up to 15,000 genes, can be robotically printed or obtained commercially. These arrays can be labeled with fluor dyes and the differential expression of the genes between patient and control tissue can be detected with lasers. A large number of genes can be screened for differential expressions between patient and control tissue, thus identifying new candidate genes.
A research strategy involves (1) identifying regions of interest in postmortem brain tissue and individually matched controls, (2) performing high density microarray analysis comparing the relative levels of gene transcripts in patients and controls, (3) analyzing gene expression profiling with cluster analysis and data mining tools, (4) evaluating selected mRNAs with real time quantitative polymerase chain reaction (PCR) and in situ hybridization mapping, and (5) repeating analyses of additional cohorts for each disease or condition. In the future, evidence from three bodies of data may converge to signal the relevance of specific identified genes. A gene of major interest could have (1) been identified as over- or underexpressed in suicidal patients versus controls, (2) occurred in a region of the genome identified as a “hotspot” in suicidal patients by whole genome scans; (3) occurred in brain regions and cell types compatible with our understanding of the pathophysiology of suicide. Possible experiments to attempt to identify a set of genes unique to suicide might involve scientists investigating brain tissue from a cohort of major depressive disorder patients with and without suicidal behavior, bipolar affective disorder patients with and without suicidal behavior, and schizo-