The epigenetic modifier PBRM1 restricts the basal activity of the innate immune system by repressing retinoic acid‐inducible gene‐I‐like receptor signalling and is a potential prognostic biomarker for colon cancer

It has long been known that patients suffering from inflammatory bowel disease (IBD) have an increased risk of developing colorectal cancer (CRC). The innate immune system of host cells provides a first‐line defence against pathogenic infection, whereas an uncontrolled inflammatory response under homeostatic conditions usually leads to pathological consequences, as exemplified by the chronic inflammation of IBD. The key molecules and pathways keeping innate immunity in check are still poorly defined. Here, we report that the chromatin remodeller polybromo‐1 (PBRM1) is a repressor of innate immune signalling mediated by retinoic acid‐inducible gene‐I (RIG‐I)‐like receptors (RLRs). Knockdown of PBRM1 in colon cancer cells increased the expression of two receptor genes (RIG‐I and MDA5) and upregulated interferon (IFN)‐related and inflammation‐related gene signatures. The innate immune signal stimulated by a double‐stranded RNA viral mimic was exaggerated by PBRM1 suppression. PBRM1 cooperated with polycomb protein EZH2 to directly bind the cis‐regulatory elements of RIG‐I and MDA5, thereby suppressing their transcription. Moreover, upregulation of RIG‐I and MDA5 is required for IFN response activation induced by PBRM1 silencing. TRIM25, a protein stimulated by the RLR pathway and IFN production, physically interacted with PBRM1 and induced PBRM1 protein destabilization by promoting its ubiquitination. These findings reveal a PBRM1–RLR regulatory circuit that can keep innate immune activity at a minimal level in resting cells, and also ensure a robust inflammatory response in the case of pathogen invasion. PBRM1 was found to be downregulated in primary tissues from patients with CRC or IBD, and its expression correlated negatively with that of RLR genes and interferon‐stimulated genes in CRC samples. Lower PBRM1 expression was associated with advanced pathological grade and poorer survival of CRC patients, indicating that PBRM1 could serve as a potential prognostic biomarker for CRC. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.