Ubiquitinated PCNA Drives USP1 Synthetic Lethality in Cancer
Antoine Simoneau, Justin L. Engel, Madhavi Bandi, Katherine Lazarides, Shangtao Liu, Samuel R. Meier, Ashley H. Choi, Hongxiang Zhang, Binzhang Shen, Lauren Martires, Deepali Gotur, Truc V. Pham, Fang Li, Lina Gu, Shanzhong Gong, Minjie Zhang, Erik Wilker, Xuewen Pan, Douglas A. Whittington, Scott Throner, John P. Maxwell, Yingnan Chen, Yi Yu, Alan Huang, Jannik N. Andersen, Tianshu Feng
Abstract
CRISPR Cas9-based screening is a powerful approach for identifying and characterizing novel drug targets. Here, we elucidate the synthetic lethal mechanism of deubiquitinating enzyme USP1 in cancers with underlying DNA damage vulnerabilities, specifically BRCA1/2 mutant tumors and a subset of BRCA1/2 wild-type (WT) tumors. In sensitive cells, pharmacologic inhibition of USP1 leads to decreased DNA synthesis concomitant with S-phase-specific DNA damage. Genome-wide CRISPR-Cas9 screens identify RAD18 and UBE2K, which promote PCNA mono- and polyubiquitination respectively, as mediators of USP1 dependency. The accumulation of mono- and polyubiquitinated PCNA following USP1 inhibition is associated with reduced PCNA protein levels. Ectopic expression of WT or ubiquitin-dead K164R PCNA reverses USP1 inhibitor sensitivity. Our results show, for the first time, that USP1 dependency hinges on the aberrant processing of mono- and polyubiquitinated PCNA. Moreover, this mechanism of USP1 dependency extends beyond BRCA1/2 mutant tumors to selected BRCA1/2 WT cancer cell lines enriched in ovarian and lung lineages. We further show PARP and USP1 inhibition are strongly synergistic in BRCA1/2 mutant tumors. We postulate USP1 dependency unveils a previously uncharacterized vulnerability linked to posttranslational modifications of PCNA. Taken together, USP1 inhibition may represent a novel therapeutic strategy for BRCA1/2 mutant tumors and a subset of BRCA1/2 WT tumors.