Abstract PO4-28-08: Unraveling the Role of BRCA1-BARD1 E3 Ubiquitin Ligase in DNA Repair: A Promise for Enhanced Chemotherapy Outcomes
Authors: Li Wenjing , Wang Meiling , Tomimatsu Nozomi , Ji Jae-Hoon , Alejo Salvador , Sareddy Gangadhara R. , Burma Sandeep , Klevit Rachel , Zhao Weixing 
Published: May 2024

Abstract
BACKGROUND: BRCA1, a central player in gynecological cancers, collaborates with BARD1 to shape a potent complex pivotal for DNA binding and ubiquitin E3 ligase activities. This complex impacts a myriad of biological pathways. Notably, BRCA1-BARD1’s role in tumor suppression and homology-directed DNA repair (HDR) has been spotlighted. Our innovative approach used RING-domain mutations to craft ligase-dead BRCA1-BARD1 mutants, hypothesizing that these mutants would offer fresh insights into the DNA repair dynamics of BRCA1-BARD1. METHODS: Full length BRCA1-BARD1 or truncated mutants, and histones were purified from E. coli. or insect cells. Nucleosomes were assembled for in vitro ubiquitylation reaction and binding assays. Stable mammalian cell lines HeLa and MDA-MB-436 that express wild type or mutant forms of BRCA1 and BARD1 were established for a host of analyses, including cellular fractionation, foci analysis, Proximity ligation assay (PLA), comet assays and clonogenic survival assays with various DNA damage agents. RESULTS: By systematic biochemical screening and a series of in vitro assays, we generated a truly BRCA1-BARD1 E3 dead mutant, BRCA1I26A, L63A, K65A-BARD1 (BRCA1-E3d), which lacks E3 ligase activity but possesses all other known attributes such as retention of BRCA1-BARD1 heterodimeric structure formation, DNA binding, and intact RAD51-mediated recombinase activity. To our surprise, we discovered that previously described BRCA1-BARD1 RING-domain mutant (BRCA1-I26A) still possessed ubiquitylation activity not found in BRCA1-E3d. To determine the biological significance of these mutants, cells stably expressing BRCA1-E3d were treated with various DNA-damaging agents and shown to more sensitive than WT or previously identified mutants. DNA repair pathway reporter assays determined these cells were deficient in various repair pathways compared to their BRCA1-WT counterparts. Further studies demonstrate that BRCA1-BARD1 E3 ligase is required for DNA resection during HDR, as evidenced in reduced levels of DNA repair-related foci formation such as RPA, RAD51 and CtIP in BRCA1-E3d cells. Furthermore, compared to BRCA1-WT cells, BRCA1-E3d cells were more sensitive to DNA damage reagents after depletion of 53BP1, which indicates BRCA1-BARD1 E3 ligase function also contributes to later stages of DNA repair completion. CONCLUSIONS: Our work dispels prevailing ambiguities surrounding BRCA1-BARD1 E3 ligase functions, underscoring its paramountcy in genome repair. This trailblazing research not only enriches our understanding but also beckons therapeutic interventions targeting tumor suppression. The unveiling of BRCA1-BARD1 E3 ligase's intricate regulatory dynamics combined with our novel mutants paves the way for an exciting new era in cancer therapeutics, hinting at superior treatments to enhance patient recovery.
Citation Format: Wenjing Li, Meiling Wang, Nozomi Tomimatsu, Jae-Hoon Ji, Salvador Alejo, Gangadhara R. Sareddy, Sandeep Burma, Rachel Klevit, Weixing Zhao. Unraveling the Role of BRCA1-BARD1 E3 Ubiquitin Ligase in DNA Repair: A Promise for Enhanced Chemotherapy Outcomes [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-28-08.