Roles of the SUMO-related enzymes, PIAS1, PIAS4, and RNF4, in DNA double-strand break repair by homologous recombination.

Post-translational modification of proteins by small ubiquitin-like modifier (SUMO) is known to be involved in a variety of cellular events. This modification, called SUMOylation, is carried out by the E1 activating enzyme, the E2 conjugating enzyme, and multiple E3 ligases. Previous studies have demonstrated that the SUMO E3 ligases, protein inhibitors of activated STAT 1 (PIAS1) and 4 (PIAS4), and the SUMO-targeted ubiquitin ligase, RING finger protein 4 (RNF4), play important roles in the repair of DNA double-strand breaks (DSBs). However, the mechanism by which these SUMO-related enzymes promote DSB repair is still poorly understood. In the present study, we focused on homologous recombination (HR), the most accurate DSB repair pathway, and aimed to elucidate the mechanism by which PIAS1, PIAS4, and RNF4 promote HR. In γ-ray-irradiated normal human fibroblasts, DSB end resection and RAD51 loading, the two essential steps of HR, were significantly impaired by small interfering RNA (siRNA)-mediated depletion of PIAS1, PIAS4, or RNF4. The recruitment of BRCA1, a major HR factor, to DSB sites was reduced in cells depleted of these SUMO-related enzymes. Consistent with the role of BRCA1 in counteracting the p53-binding protein 1 (53BP1)-mediated resection blockade, 53BP1 depletion rescued the reduced resection and RAD51 loading in the cells depleted of PIAS1, PIAS4, or RNF4. Moreover, Rap1-interacting factor 1 (RIF1), a resection inhibitor downstream of 53BP1, became more abundant at DSBs when PIAS1, PIAS4, RNF4, or BRCA1 was depleted. Importantly, the concomitant depletion of BRCA1 with either one of the SUMO-related enzymes did not further increase RIF1 at DSBs, when compared to single depletion of BRCA1. Collectively, these results suggest that PIAS1, PIAS4, RNF4, and BRCA1 work epistatically to counteract 53BP1/RIF1-mediated resection blockade, thereby promoting resection.