Abstract P3-05-18: Unravelling the clinicopathological and functional significance of Replication Protein A (RPA) heterotrimeric complex in sporadic breast cancers
Authors: Algethami Mashael A. , Toss Michael , Brownlie Juliette , Woodcock Corinne L. , Jaipal Chandar , SHOQAFI AHMED , Mesquita Katia , Alblihy Adel , Mongan Nigel , Rakha Emad , Jeyapalan Jennie N. , Madhusudan Srinivasan , Green Andrew R. 
Published: March 2023

Abstract
Unravelling the clinicopathological and functional significance of Replication Protein A (RPA) heterotrimeric complex in sporadic breast cancers Mashael Algethami1, Michael S Toss 1,4, Juliette Brownlie 1, Corinne L Woodcock 1,2, Chandar Jaipal1, Ahmed Shoqafi 1, Katia A Mesquita1, Adel Alblihy1,3, Andrew R Green1, Nigel P Mongan 1,5, Emad A Rakha 1,4, Jennie N Jeyapalan 1,2 and Srinivasan Madhusudan1,6 1 Nottingham Biodiscovery Institute, Academic Unit for Translational Medical Sciences, School of Medicine, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 2 Faculty of Medicine and Health Sciences, Centre for Cancer Sciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire LE12 5RD, UK 3 Medical Center, King Fahad Security College (KFSC), Riyadh 11461, Saudi Arabia. 4 Department of Pathology, Nottingham University Hospital, City Campus, Hucknall Road, Nottingham NG5 1PB, UK. 5 Department of Pharmacology, Weill Cornell Medicine, New York, NY, 10065, USA 6 Department of Oncology, Nottingham University Hospitals, Nottingham NG5 1PB, UK.   Introduction: Replication Protein A (RPA) is a critical single-stranded DNA (ssDNA)-binding protein that coats and protects exposed ssDNA from endogenous nucleases. RPA is a heterotrimeric complex consisting of RPA1 (70kDa), RPA2 (32kDa), and RPA3 (14kDa) subunits. RPA provides a platform for recruitment of factors required during replication, checkpoint regulation, DNA repair [including homologous recombination (HR), nucleotide excision repair (NER), base excision repair (BER) and mismatch repair (MMR)], telomere maintenance and retro-transposition. To understand the role of RPA in breast cancer pathogenesis we conducted a comprehensive genomic, transcriptomic, bioinformatic, proteomic and preclinical study. Patients and methods: RPA1, RPA2 and RPA3 protein expression was evaluated in 4221 primary invasive breast carcinomas and 776 breast ductal carcinoma in situ (DCIS). Transcriptomic investigation was completed in the METABRIC cohort (n=1980). Detailed bioinformatics was conducted in the TCGA cohort (n=1090). RPA deficient breast cancer cell lines were tested for Cisplatin, Palbociclib and Olaparib sensitivity. Results: RPA1, RPA2 and RPA3 loss is frequent in DCIS and linked to aggressive phenotype (including high grade, ER and PR negativity) (all p values &lt; 0.01). DCIS with low RPA1 was also associated with poor local recurrence-free interval (p&lt; 0.00001). In invasive breast cancers, low RPA1, low RPA2 and low RPA3 were all associated with larger size, lympho-vascular invasion, higher histological grade, high stage, ER negativity and poor breast cancer specific survival. Transcriptomic alterations in low RPA tumors included those genes involved in steroid hormone biosynthesis, chemical carcinogenesis, and drug metabolism. Pre-clinically, RPA deficient breast cancer cells were sensitive to Cisplatin and Palbociclib (CDK4/6 inhibitor) therapy compared to controls. Additionally, the PARP1 inhibitor Olaparib was synthetically lethal in RPA1 and RPA2 deficient cells compared to controls. Increased Olaparib sensitivity was associated with double strand breaks, S-phase cell cycle arrest and increased apoptosis. Conclusions: We provide the first comprehensive evidence that RPA loss is an early event during breast cancer pathogenesis and promotes aggressive phenotypes. Pre-clinically RPA deficient breast cancer cells were selectively toxic to Cisplatin, Palbociclib and Olaparib.
Citation Format: Mashael A. Algethami, Michael Toss, Juliette Brownlie, Corinne L. Woodcock, Chandar Jaipal, AHMED SHOQAFI, Katia Mesquita, Adel Alblihy, Nigel Mongan, Emad Rakha, Jennie N. Jeyapalan, Srinivasan Madhusudan, Andrew R. Green. Unravelling the clinicopathological and functional significance of Replication Protein A (RPA) heterotrimeric complex in sporadic breast cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-05-18.