Abstract 2049: Cell-cycle dependent regulation of Sp1 at DSBs promotes repair via NHEJ and sensitizes cells to PARP inhibition

Abstract

Abstract DNA double-strand breaks (DSBs) are life-threatening lesions that must be repaired to preserve chromosomal integrity. These lesions are repaired by two primary pathways: homologous recombination (HR) and non-homologous end-joining (NHEJ). The factors that mediate pathway choice are critical to maintain genomic integrity. While many of these factors, epigenetic changes and cell cycle stages are known, with HR predominating in S and G2, the underlying mechanisms that determine pathway choice are not well understood. Previously, we found that the transcription factor Sp1 is rapidly recruited to DSBs and is necessary for repair. Here, we demonstrate that Sp1 integrates repair pathway choice with cell cycle by promoting NHEJ in G1. Sp1 localizes to DSBs in G1 to recruit the NHEJ repair factors 53BP1 and Ku70. Sp1 is phosphorylated by Cyclin A/cdk2 upon entry into S phase, evicting both Sp1 and 53BP1 from the break site, thereby permitting BRCA1 binding and HR. Mutation of this phosphorylation site (Sp1-S59A) results in persistence of Sp1 and 53BP1 at DSBs in S phase cells, precluding BRCA1 binding and HR. We also demonstrate that Sp1 is sumoylated in S phase in response to damage. This sumoylation in recognized by the E3-ubiquitin ligase, RNF4, and therefore polyubiquitylates Sp1 and targets it for proteasomal degradation. Expression of a sumoylation deficient Sp1 (Sp1-K16R) results in Sp1 and 53BP1 occupancy at DSBs in S phase, inhibiting BRCA1 binding to DSBs and HR. HR proteins are commonly mutated in cancers, resulting in cellular “addiction” to alternative repair pathways. Exploiting this addiction is the basis of synthetic lethality and is applicable to the treatment of many types of malignancies. Similar to BRCA1 deficiency, expression of Sp1-S59A or Sp1-K16R also increases cell sensitivity to PARP inhibition in BRCA1+/+ cells because of the inability to recruit BRCA1. Alternatively, synthetic lethality in BRCA1-/- cells requires 53BP1, whereas loss of 53BP1 confers PARPi resistance in a BRCA1 deficient cell. Consistent with this, depletion of Sp1 also results in PARPi resistance in BRCA1-/- cells. Interestingly, Sp1 is overexpressed in many tumors and is associated with poor prognosis. These data corroborate our finding that Sp1 is required for NHEJ and provide preliminary evidence that Sp1 may be a biomarker for PARPi sensitivity. Citation Format: Michelle Swift, Jane Azizkhan-Clifford. Cell-cycle dependent regulation of Sp1 at DSBs promotes repair via NHEJ and sensitizes cells to PARP inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2049.