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

ABSTRACT: 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.

ORGANISM(S): Mus musculus

PROVIDER: GSE168129 | GEO | 2022/09/10

REPOSITORIES: GEO

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Project description:Background: Mutations in the cone-rod-homeobox protein CRX are typically associated with dominant blinding retinopathies with variable age of onset and severity. Five well-characterized mouse models carrying different Crx mutations show a wide range of disease phenotypes. To determine if the phenotype variability correlates with distinct changes in CRX target gene expression, we perform RNA-seq analyses on three of these models and compare the results with published data. Results: Despite dramatic phenotypic differences between the three models tested, graded expression changes in shared sets of genes are detected. Phenotype severity correlates with the down-regulation of genes encoding key rod and cone phototransduction proteins. Interestingly, in increasingly severe mouse models, the transcription of many rod-enriched genes decreases decrementally, whereas that of cone-enriched genes increases incrementally. Unlike down-regulated genes, which show a high degree of CRX binding and dynamic epigenetic profiles in normal retinas, the up-regulated cone-enriched genes do not correlate with direct activity of CRX, but instead likely reflect a change in rod cell-fate integrity. Furthermore, these analyses describe the impact of minor gene expression changes on the phenotype, as two mutants showed marginally distinguishable expression patterns but huge phenotypic differences, including distinct mechanisms of retinal degeneration. Conclusions: Our results implicate a threshold effect of gene expression level on photoreceptor function and survival, highlight the importance of CRX in photoreceptor subtype development and maintenance, and provide a molecular basis for phenotype variability in CRX-associated retinopathies. All genotypes were analyzed in triplicate. Heterozygous and homozygous mutants were all sequenced at P10, the control for which is the P10 C57BL6/J data. Heterozygous mutants were also analyzed at P21, the control for which is the P21 C57BL6/J data.

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

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