Transcriptionally active chromatin recruits homologous recombination at DNA double-strand breaks.

While both Homologous recombination (HR) and Non Homologous End Joining (NHEJ) can repair DNA double Strand Breaks (DSB), the mechanisms by which one or other of these pathways is chosen remain unclear. Here we show that transcriptionally active chromatin is preferentially repaired by HR. Using chromatin immunoprecipitation-sequencing (ChIP-seq), to analyse repair of multiple DSBs induced throughout the human genome, we identify an “HR- prone” subset of DSBs that recruit the HR protein RAD51, undergo resection and rely on RAD51 for efficient repair. These DSBs are located in actively transcribed genes, and targeted to HR repair via the transcription-elongation associated histone mark, histone H3 lysine 36 trimethylation (H3K36me3). In agreement, depletion of SETD2, the main H3K36 tri- methyltransferase, severely impedes HR at such DSBs. Our study thereby demonstrates a primary role of the chromatin context, in which a break occurs, in DSB repair.