ABS 063: Ubiquitin-Specific Protease 1 Inhibitor Impact and Relationship to Poly-ADP-Ribose Glycohydrolase Inhibitors and Base Excision Repair Status

Stefan Leonard ¹ , Wynand Roos ¹ , Robert W. Sobol ¹

¹ Department of Pathology and Laboratory Medicine Warren Alpert Medical School & Legorreta Cancer Center, Brown University

Van Wickle (2025) Volume 1, ABS 063

Introduction: Ubiquitin Specific Protease 1 (USP1) presents an emerging clinical target for Poly-ADP-Ribose Polymerase inhibitor (PARPi) resistant and Breast Cancer Gene 1/2 (BRCA1/2) mutant tumors. USP1 is a member of the deubiquitinating enzyme class (DUBs) responsible for the cleavage of ubiquitin tags on proliferating cell nuclear antigen (PCNA). In the context of DNA repair, USP1 facilitates proper PCNA-mediated polymerase switching from error-prone trans-lesion synthesis (TLS) polymerases to / polymerases on post-lesion sites. USP1 deubiquitinating activity has also been linked to homologous recombination (HR) repair activity. Previous studies have shown that chemical inhibition of USP1 results in decreased nuclear PCNA levels and consequently decreased DNA synthesis and increased basal levels of S-phase-specific DNA damage. Additionally, due to the critical role of USP1 in DNA synthesis and repair, USP1 inhibitors (USP1i) have a synthetic lethal relationship with inhibitors of PARP1 (PARPi). However, limited work has been done to uncover the impact and relationship of USP1i with Poly-ADP-Ribose Glycohydrolase inhibitors (PARGi), an emerging clinical option for PARPi resistant and BRCA1/2 mutant tumors. This work reports a synergistic relationship between ML323, a potent USP1i, and PDD00017273, a PARGi. We show that PARGi combined with USP1i leads to increased levels of mono-ubiquitinated PCNA. Additionally, the combination treatment leads to a significant reduction of poly-ADP-ribose (PAR) foci, suggesting the treatment’s disruption of ubiquitin and PAR signaling pathways. This work may lead to future mechanistic developments of both USP1i
and PARGi-mediated cancer cell death and future progress towards introducing USP1i and related deubiquitinating enzyme inhibitors as clinically useful therapeutical approaches to PARPi resistant and HR deficient tumors.