ATRX regulates binding of Polycomb repressive complex 2 to Xist RNA and Polycomb targets. Mus musculus

In mammals, dosage compensation entails inactivation of an entire X chromosome in female cells. X-chromosome inactivation (XCI) depends on the long noncoding RNA Xist and its recruitment of Polycomb Repressive Complex 2 (PRC2). How PRC2 specifically interacts with Xist and other Polycomb targets remains unclear. Using XCI as a model, we take an unbiased proteomics approach to search for new Xist and PRC2 regulators. We identify ATRX, a chromatin remodeler associated with mental retardation, alpha thalassemia, and cancer in humans. We show that ATRX is a high-affinity RNA-binding protein. ATRX directly interacts with RepA/Xist RNA and is required for loading of PRC2 onto the RNA in vivo. The Xist gene is a hotspot of ATRX occupancy. Without ATRX, PRC2 cannot load onto Xist RNA nor spread in cis along the X-chromosome. Epigenomic profiling reveals a dependency of PRC2 on ATRX that extends beyond XCI. Depleting ATRX results in genome-wide disruptions to PRC2 localization and trimethylation of histone H3-lysine 27 (H3K27me3). PRC2 is displaced from genes and relocalized to intergenic space, causing reduced H3K27me3 and concomitant changes in expression of Polycomb targets. We conclude that ATRX is a required specificity determinant for PRC2 targeting and function. Overall design: Distribution of ATRX in a transformed female mouse embryonic fibroblast cell line (EY.T4) and female mouse embryonic stem cells and examination of changes in EZH2 and H3K27me3 marks in transformed male embryonic fibroblast cell lines that contain autosomal integrated copies of full length Xist RNA gene upon ATRX knock down.