Data from Ubiquitinated PCNA Drives USP1 Synthetic Lethality in Cancer

Abstract

<div>Abstract<p>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 <i>BRCA1/2</i> mutant tumors and a subset of <i>BRCA1/2</i> 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 <i>RAD18</i> and <i>UBE2K</i>, which promote PCNA mono- and polyubiquitination respectively, as mediators of <i>USP1</i> 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 <i>PCNA</i> reverses USP1 inhibitor sensitivity. Our results show, for the first time, that <i>USP1</i> dependency hinges on the aberrant processing of mono- and polyubiquitinated PCNA. Moreover, this mechanism of <i>USP1</i> dependency extends beyond <i>BRCA1/2</i> mutant tumors to selected <i>BRCA1/2</i> WT cancer cell lines enriched in ovarian and lung lineages. We further show PARP and USP1 inhibition are strongly synergistic in <i>BRCA1/2</i> mutant tumors. We postulate <i>USP1</i> dependency unveils a previously uncharacterized vulnerability linked to posttranslational modifications of PCNA. Taken together, USP1 inhibition may represent a novel therapeutic strategy for <i>BRCA1/2</i> mutant tumors and a subset of <i>BRCA1/2</i> WT tumors.</p></div>