Comprehensive gene expression analysis detects global reduction of proteasome subunits in schizophrenia

OBJECTIVE: A main challenge in the study of schizophrenia is its high heterogeneity. While it is generally accepted that there exist several biological mechanisms that may define distinct schizophrenia subtypes, they have not been identified yet. We applied comprehensive gene expression analysis, searching for molecular signals that differentiate patients with schizophrenia from healthy controls, and examined whether the identified signal characterizes a particular subgroup of the patients. METHODS: We performed transcriptome sequencing of 14 superior temporal gyrus (STG) samples of relatively young (mean age: 44) subjects with schizophrenia and 15 matched controls from the Stanley Medical Research Institute. Analyses of differential expression and pathway enrichment were applied and the results were compared with those obtained from an independent cohort of elderly (mean age: 74) patients. Replicability was then tested on six additional independent datasets of various brain regions. RESULTS: The two STG cohorts of relatively young and elderly subjects showed high replicability. Pathway enrichment analysis of the down-regulated genes pointed to proteasome-related pathways. Meta-analysis of differential expression identified down-regulation of 12 of 39 proteasome subunits in schizophrenia. Down-regulation of multiple proteasome subunits was replicated in six additional datasets (overall 8 cohorts, with 267 schizophrenia and 266 control samples, from 5 brain regions, were studied). This signal was concentrated in a subgroup of the patients. CONCLUSIONS: We detect global down-regulation of proteasome subunits in a subgroup of the patients with schizophrenia. The proteasome is a major intracellular protein degradation system, where ubiquitinated proteins (proteins bound by the small protein called ubiquitin) are targeted for degradation. We hypothesize that the down-regulation we detect leads to proteasome dysfunction that causes accumulation of ubiquitinated proteins. Such accumulation has recently been identified, also in a subgroup of the studied patients with schizophrenia. Thus, down-regulation of proteasome subunits might define a biological subtype of schizophrenia.