Non-canonical regulation of homologous recombination DNA repair by the USP9X deubiquitinase

In order to prevent the deleterious effects of genotoxic agents, cells have developed complex surveillance mechanisms and DNA repair pathways that allow them to maintain genome integrity. The Ubiquitin Specific Protease 9X (USP9X) contributes to genome stability during DNA replication and chromosome segregation. Depletion of USP9X leads to DNA double strand breaks, some of which are triggered by replication fork collapse. Here we identify USP9X as a novel regulator of homologous recombination (HR) DNA repair in human cells. Using a cellular HR reporter assay, irradiation induced focus formation assays, and colony formation assays we show that USP9X is required for efficient HR. Mechanistically, we show USP9X is important to sustain the expression levels of key HR factors, namely BRCA1 and RAD51 through a non-canonical regulation of their mRNA abundance. Intriguingly, we find that USP9X contribution to BRCA1 and RAD51 expression is independent of its known catalytic activity. Thus this work identifies USP9X as a regulator of HR, demonstrates a novel mechanism by which USP9X can regulate protein levels and provides insights in to the regulation of BRCA1 and RAD51 mRNA.