The emerging role of Polycomb repressors in the response to DNA damage.

Polycomb group (PcG) genes encode chromatin modifiers that are involved in the maintenance of cell identity and in proliferation, processes that are often deregulated in cancer. Interestingly, besides a role in epigenetic gene silencing, recent studies have begun to uncover a function for PcG proteins in the cellular response to DNA damage. In particular, PcG proteins have been shown to accumulate at sites of DNA double-strand breaks (DSBs). Several signaling pathways contribute to the recruitment of PcG proteins to DSBs, where they catalyze the ubiquitylation of histone H2A. The relevance of these findings is supported by the fact that loss of PcG genes decreases the efficiency of cells to repair DSBs and renders them sensitive to ionizing radiation. The recruitment of PcG proteins to DNA breaks suggests that they have a function in coordinating gene silencing and DNA repair at the chromatin flanking DNA lesions. In this Commentary, we discuss the current knowledge of the mechanisms that allow PcG proteins to exert their positive functions in genome maintenance.