A transcriptome-wide Mendelian randomization study in isolated human immune cells highlights risk genes involved in viral infections and potential drug repurposing opportunities for schizophrenia

Background Schizophrenia is a neurodevelopmental psychiatric disorder characterised by symptoms of psychosis, thought disorder, and flattened affect. Immune mechanisms are associated with schizophrenia, though the precise nature of this relationship (i.e., causal, correlated, consequential) and the mechanisms involved are not fully understood. Methods To elucidate these mechanisms, we conducted a transcriptome-wide Mendelian randomization study using gene expression exposures from 29 human cis -eQTL datasets encompassing 11 unique immune cell types, all publicly available from the eQTL catalogue. Results These analyses highlighted 196 genes, including 67 located within the human leukocyte antigen (HLA) region. Enrichment analyses indicated an over-representation of immune genes, which was driven by the HLA genes. Stringent validation and replication steps retained 61 candidate genes, 27 of which were the sole causal signals at their respective loci, thereby representing strong candidate effector genes at known risk loci. We highlighted L3HYPDH as a potential novel schizophrenia risk gene and DPYD and MAPK3 as candidate drug repurposing targets. Futhermore, we performed follow-up analyses focused on one of the candidate effectors, interferon regulatory transcription factor 3 ( IRF3 ), which coordinates interferon responses to viral infections. We found evidence of shared genetic aetiology between schizophrenia and autoimmune diseases at the IRF3 locus, and a significant enrichment of IRF3 chromatin binding at known schizophrenia risk loci. Conclusions Our findings highlight a novel schizophrenia risk gene, potential drug repurposing opportunities, and provide support for IRF3 as a schizophrenia hub gene, which may play critical roles in mediating schizophrenia-autoimmune comorbidities and the impact of viral infections on schizophrenia risk.