Treacle and MDC1 coordinate rDNA break repair by homologous recombination

Abstract The repair of DNA double-strand breaks in repetitive sequences is challenging because the abundance of potential templates for homology-directed repair (HDR) exacerbates the risk of ectopic recombination and chromosome rearrangements. Relocalization of repair sites in repetitive sequences to a ‘safe location’ prior to RAD51 loading has been observed in various organisms and is thought to suppress ectopic recombination and chromosomal rearrangements. We characterized this phenomenon in the rDNA repeats that reside within the nucleoli, specialized nuclear compartments where ribosome biosynthesis takes place. DSB induction in the rDNA repeats is associated with large-scale mobilization of the broken rDNA repeats from inside of the nucleoli to the nucleolar periphery where they are repaired by HDR. Here, we show that the two adaptor proteins Treacle and MDC1 are coordinating the sequence of events that ensue in response to rDNA break induction. Recruitment of the HDR machinery to rDNA breaks is dependent on the nucleolar adaptor Treacle, and specifically on its role in rDNA mobilization upon break induction. We demonstrate that following mobilization of the rDNA repeats and subsequent establishment of the γH2AX chromatin domain in the nucleolar periphery, the MDC1-RNF8-RNF168 axis is mediating the recruitment of the BRCA1-PALB2-BRCA2 complex and RAD51 loading. This coordinated sequence of events thus ensures that RAD51 loading is coupled to rDNA break mobilization to the nucleolar periphery, which may prevent ectopic recombination between repeats.