Abstract PR05: Distinct BRCA1- and BRCA2-specific functions at stalled replication forks: Clinical implications for differences between BRCA1 and BRCA2 mutation-driven cancer
Authors: Duan Haohui , Reed Rachel , Garber Judy , Pathania Shailja 
Published: April 2017

Abstract
BRCA1 and BRCA2 are tumor suppressor genes, and germ line mutations in these two genes increase the risk of breast cancer. Both BRCA1 and BRCA2 are required for stabilization and repair of stalled replication forks. Stalled forks, when not resolved, lead to mutations, or collapse into double strand breaks (DSBs). Both outcomes result in what is commonly referred to as replication stress (RS), which, when chronic, is a driving force behind cancer development. However, it is not clear what are the differences, if any, between BRCA1 and BRCA2 dependent stabilization and repair of stalled replication fork. Getting a better understanding of how these two proteins help suppress RS will not only help reveal how they contribute to breast cancer suppression, but will also give us an insight into the differences in the spectrum of cancers in BRCA1 and BRCA2 mutation carriers. Such knowledge is also critical in designing better therapeutic options for BRCA1 and BRCA2 specific mutation carriers.
Here we report a novel role for BRCA2 in suppressing RS by regulating RPA (replication protein A) accumulation on ssDNA (single stranded DNA) at stalled forks. More specifically, we show that BRCA2 is required for efficient turnover of RPA, and in absence of BRCA2 there is “persistent” accumulation of mostly un-phosphorylated RPA (at S33, T21 and S4/S8) at stalled forks. Furthermore, inefficient turnover of RPA in absence of BRCA2 not only results in fork collapse, as marked by increased accumulation of both 53BP1 and Mre11 at the site of stalling, but also leads to suppression of nucleotide excision repair (NER). No such persistent RPA coated ssDNA is observed in BRCA1 depleted cells, suggesting that this intermediate is exclusive to BRCA2 deficiency. We also show that generation of this intermediate was not dependent on Mre11 implying that accumulation of RPA coated ssDNA was not a result of excessive DNA processing by Mre11 exonuclease in absence of BRCA2.
To determine whether stalled fork repair (SFR) is defective in normal/healthy breast cells from BRCA2 mutation carriers (BRCA1mut/+), we generated 12 primary mammary epithelial cell (MECs) strains from prophylactic mastectomies performed on BRCA2 mutation carrying women. BRCA2+/+ control MECs were derived from tissue collected during reduction mammoplasties. Our current data shows that BRCA2mut/+ strains are defective in SFR. Importantly, we also show that BRCA2mut/+ strains, and not BRCA1mut/+ strains, are haploinsufficient for NER. Inefficient NER in BRCA2mut/+ strains, and not in BRCA1mut/+ strains, suggests an increased risk in BRCA2 mutation carriers for accumulating DNA lesions that require NER for repair. Such a defect is known to contribute towards increased risk for melanoma, and could partly explain why women with BRCA2 mutation, and not BRCA1 mutation, are prone to increased risk for skin lesions including melanoma. This mechanistic difference between BRCA1 and BRCA2 response at stalled replication forks, and subsequently a crosstalk with NER pathway, might also contribute towards other clinical differences observed in BRCA1 and BRCA2 mutation driven breast cancer.
This abstract is also being presented as Poster A35.
Citation Format: Haohui Duan, Rachel Reed, Judy Garber, Shailja Pathania. Distinct BRCA1- and BRCA2-specific functions at stalled replication forks: Clinical implications for differences between BRCA1 and BRCA2 mutation-driven cancer [abstract]. In: Proceedings of the AACR Special Conference on DNA Repair: Tumor Development and Therapeutic Response; 2016 Nov 2-5; Montreal, QC, Canada. Philadelphia (PA): AACR; Mol Cancer Res 2017;15(4_Suppl):Abstract nr PR05.