Abstract 330: BRCA1 and BLM helicase compensate for replication fork defects in the absence of 53BP1 protein

Abstract

Abstract 53BP1 is well-known for its involvement in DNA double strand break (DSB) repair, binding DNA and influencing choice of DSB repair. In S/G2 phases of the cell cycle, 53BP1 accumulates at DSBs, but can be removed by BRCA1, allowing end-resection and homology-directed repair (HDR) to occur. Without BRCA1, HDR is defective unless 53BP1 is also lost, in which case HDR can proceed. Such secondary mutation is relevant as a mechanism of acquired chemoresistance in BRCA1 deficient tumors. BRCA1 is also important for maintaining replication fork stability, particular for blocking end resection by MRE11a at a collapsed fork. We therefore set out to ask whether 53BP1 also plays a role in maintenance of fork stability and how this role relates to the role of BRCA1 at a collapsed fork. 53BP1 has been indicated to co-localize with collapsed forks and indeed we find that in its absence fork stability is compromised in a DNA2, but not MRE11a, dependent manner. With decreased fork stability in the absence of 53BP1 we observe increased HDR, indicating a role for BRCA1 to compensate for the lack of 53BP1 by promoting HDR. However, if both 53BP1 and BRCA1 are both necessary for fork stability, then there must be additional compensating mechanisms providing fork stability when both are absent. In fact, absence of both 53BP1 and BRCA1 results in better replication progression than when either 53BP1 or BRCA1 alone are absent. A third mechanism of fork stabilization was identified involving BLM; a RecQ helicase involved in dissolving homologous recombination intermediates, can facilitate fork restart and reverse fork regression. Additionally, BLM has been shown to interact with both 53BP1 and BRCA1, is found at stalled replication forks, and promotes BRCA1 recruitment to sites of DNA damage. Targeting BLM in cells with impaired 53BP1 and BRCA1 increases chemosensitivity to DNA damaging agents, supporting the redundant nature of these three proteins at stalled replication forks. Our findings indicate that targeting BLM may be an effective treatment for HDR defective cancers that have developed chemoresistance; with applicability to BRCA1 and homologous recombination deficient cancers, such as breast, ovarian, and Ewing sarcoma. Citation Format: Erin H. Sybouts, Sonal S. Tonapi, Alexander J. Bishop. BRCA1 and BLM helicase compensate for replication fork defects in the absence of 53BP1 protein [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 330.