Abstract
A growing body of evidence suggests that Polycomb group (PcG) proteins, key regulators of lineage specific gene expression, also participate in the repair of DNA double-strand breaks (DSBs) but evidence for direct recruitment of PcG proteins at specific breaks remains limited. Here we explore the association of Polycomb repressive complex 1 (PRC1) components with DSBs generated by inducible expression of the AsiSI restriction enzyme in normal human fibroblasts. Based on immunofluorescent staining, the co-localization of PRC1 proteins with components of the DNA damage response (DDR) in these primary cells is unconvincing. Moreover, using chromatin immunoprecipitation and deep sequencing (ChIP-seq), which detects PRC1 proteins at common sites throughout the genome, we did not find evidence for recruitment of PRC1 components to AsiSI-induced DSBs. In contrast, the S2056 phosphorylated form of DNA-PKcs and other DDR proteins were detected at a subset of AsiSI sites that are predominantly at the 5′ ends of transcriptionally active genes. Our data question the idea that PcG protein recruitment provides a link between DSB repairs and transcriptional repression.
Highlights
The Polycomb group (PcG) proteins are important for establishing the patterns of gene expression in different cell types [1,2,3]
Inducing specific double-strand breaks (DSBs) in normal human fibroblasts To avoid potential anomalies associated with tumor cell lines, we sought a system for inducing specific DSBs in normal, nontransformed human cells
The concept that PcG proteins are involved in the DNA damage response (DDR) has an appealing logic as well as a considerable body of experimental support
Summary
The Polycomb group (PcG) proteins are important for establishing the patterns of gene expression in different cell types [1,2,3]. They operate within multi-component complexes that associate with and post-translationally modify nucleosomal histones. Polycomb repressive complex 1 (PRC1), which in Drosophila comprises equimolar amounts of the Polycomb (Pc), Posterior sex combs (Psc), Polyhomeotic (Ph) and Sex combs extra (Sce) proteins, is credited with the monoubiquitination of histone H2A on lysine 119 (H2AK119ub) [1,2,3]. The Psc and Sce components participate in alternative complexes that contain the RYBP/YAF2 proteins rather than Pc and Ph subunits [5,6,7]. As the Psc-Sce dimer is responsible for the ubiquitin ligase activity [8], the extent to which H2A ubiquitination is performed by the canonical or alternate PRC1 complexes has not been formally established [5,6,7,9,10]
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