Mutations in BCOR, a novel co-repressor of OTX2/CRX, cause pediatric inherited retinal degeneration

Many transcription factors regulating the production, survival and function of photoreceptor cells in the retina have been identified, but little is known about transcriptional co-regulators in retinal health and disease. Here we show that BCL6 co-repressor (BCOR), a polycomb repressive complex I factor mutated in various cancers, negatively regulates photoreceptor gene expression. Using proteomics and promoter assays, we find that BCOR interacts with the photoreceptor transcription factors OTX2 and CRX, and reduces their ability to activate the promoters of photoreceptor-specific genes, such as Rhodopsin and Nrl. CUT&RUN sequencing further shows that BCOR shares genome-wide binding profiles with CRX/OTX2, consistent with a general co-repression activity. Finally, we identify missense variants in the human BCOR gene in six families with early-onset X-linked inherited retinal degeneration (IRD). Together, this work uncovers BCOR as a co-repressor of OTX2/CRX essential for photoreceptor cell biology and survival. RNA-seq of BCOR knockdown in mouse retina/CUT&RUN sequencing of BCOR and Otx2/Crx during retinal development and adult stages.