Argonaute 1 Promotes Histone H3k9 Trimethylation to Facilitate Homologous Recombination

DNA double-strand breaks (DSBs) pose a severe threat to genome stability in human cells. Two major pathways, the non-homologous end joining (NHEJ) and homologous recombination (HR) pathways, are used to repair most DSBs and are promoted by 53BP1 and BRCA1 1 , respectively. The switch from NHEJ to HR is driven by accumulation of BRCA1 at the site of DSBs to initiate DNA end resection — the first stage of HR 1 . Transcription near DSBs is suppressed to allow repair of DSBs by the NHEJ pathway 2–4 or the HR pathway 5,6 . Chromatin compaction mediated by local H3K9 methylation has been linked to the HR pathway 5,6 , but precise mechanisms of such targeted methylation are still being defined. Paradoxically, nascent transcripts and small RNAs were produced from genomic loci with DSBs 7,8 , raising a question as to why and how transcriptional silencing and transcription both present at DSBs. We report here that nascent transcripts, small RNAs and Ago1 collaborate at transcriptional start sites to silence transcription flanking a DSB site, which in turn is required for the efficient switch from NHEJ to HR. Specifically, Argonaute 1 (Ago1) promotes local H3K9 trimethylation to facilitate robust BRCA1 foci formation. We demonstrate that inhibition of transcription can rescue HR defects in Ago1-depeleted cell, but only if it is triggered post DSB-induction. Our results indicate that transcription associated with DSB formation is required for RNAi-mediated transcriptional silencing in the nucleus of mammalian cells to allow DSB repair.