DNA damage-induced sumoylation of Sp1 induces its interaction with RNF4 and degradation in S phase to remove 53BP1 from DSBs and permit HR

The factors involved in DNA damage recognition and repair are tightly regulated to ensure proper repair pathway choice. The mechanism(s) that determines the cell cycle-dependent turnover of these DNA damage repair factors remains unclear. Here, we show that Sp1, which regulates double-strand break (DSB) repair pathway choice through localization of 53BP1, is sumoylated at Lys16 following DNA damage; Sp1 sumoylation is required for its degradation and the removal of both Sp1 and 53BP1 from DSB sites. Induction of DNA DSBs induces Sp1 phosphorylation at DSBs by ATM, which is necessary for the subsequent sumoylation of Sp1. In addition to this damage-induced ATM-dependent phosphorylation and sumoylation, phosphorylation of Sp1 at Ser59 by Cyclin A/cdk2 upon entry into S phase is necessary for recognition, ubiquitination and degradation by the SUMO-targeted E3 ubiquitin ligase, RNF4. Eliminating Sp1 sumoylation by mutation of Sp1 at Lys16 (K16R) precluded removal of both Sp1 and 53BP1 from DSBs in S phase, resulting in decreased BRCA1 recruitment and defective homologous recombination (HR). Like BRCA1 deficient cells, cells expressing Sp1(K16R) are sensitive to PARP inhibition due to failure to degrade Sp1 and recruit BRCA1 resulting in defective HR that is rescued by knockdown of 53BP1. These results reveal the dynamic regulation of Sp1 and its role in the assembly and disassembly of DNA repair factors at DSBs.