Abstract 333: USP1 is required for replication fork stability in BRCA1-deficient tumors

Homologous-recombination (HR) deficient tumors with BRCA1 and BRCA2 mutations exhibit replication fork stability defects. To date, PARP inhibitors are the only targeted therapy available in the clinic against HR deficient tumors, and alternative therapies are needed. In this study, we found a deubiquitinase, USP1, to be significantly upregulated in tumors with mutations in BRCA1. SiRNA mediated silencing or small molecule inhibition of USP1 activity resulted in replication fork destabilization and decreased viability of BRCA1 deficient cells, revealing a synthetic lethal relationship between USP1 and BRCA1. The cofactor of USP1, UAF1, had previously been shown to have DNA-binding activity. We observed that USP1 independently binds to and is stimulated by fork DNA. It is therefore the first known deubiquitinase (DUB) to be directly regulated by DNA binding. A truncated form of USP1, lacking its DNA binding region, was not stimulated by DNA and failed to localize and protect the replication fork. Persistence of monoubiquitinated PCNA at the replication fork was the mechanism of fork destabilization and cell death in the absence of USP1. Loss of monoubiquitinated PCNA, resulting from RAD18 knockdown, rescued the sensitivity and replication fork instability induced by USP1 inhibition. USP1 therefore is the first DUB enzyme exhibiting DNA-mediated activation at the replication fork, and is required for fork protection in BRCA1 deficient cells. We propose small molecule inhibitors against USP1 as a therapeutic option for BRCA1 deficient cancers. Citation Format: Kah Suan Lim, Heng Li, Emma A. Roberts, Emily F. Gaudiano, Connor Clairmont, Karthikeyan Ponnienselvan, Jessica C. Liu, Kalindi Parmar, Ning Zheng, Alan D9Andrea. USP1 is required for replication fork stability in BRCA1-deficient tumors [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 333.