CNSC-49. ELUCIDATING THERAPEUTIC POTENTIAL OF USP1 INHIBITION IN DIFFUSE MIDLINE GLIOMAS

Abstract Diffuse midline gliomas (DMGs) are aggressive childhood brain tumors with the H3K27M mutation in the H3F3A or H3C2 gene. Despite treatment advances, radiotherapy is the only viable option, but 99% of patients die within 1.5-2 years. Therefore, there is an urgent need to identify potential novel therapeutic targets for the treatment of DMGs. Ubiquitin-specific protease 1 (USP1) is a key deubiquitinating enzyme involved in genome integrity, cell cycle regulation, and cell homeostasis. USP1 is overexpressed in various cancers, including gliomas, is linked to poor prognosis. It promotes glioma stem cell (GSC) self-renewal and radio-resistance by regulating ID1 and CHEK1. Our recent findings show that ID1 is crucial for DMG invasion and migration. Inhibiting USP1 suppresses malignant growth, enhances radiation sensitivity, and improves chemotherapy response, highlighting its potential for combined therapies. However, USP1’s role in DMGs and its feasibility as a therapeutic target remain poorly understood. In this study, we investigated USP1 in DMGs for the first time. RNA sequencing revealed high USP1 expression in DMG patient samples and cell lines, directly linked to the H3K27M mutation. Correcting this mutation reduced USP1 expression in DMG cell lines. Pharmacologic inhibition of USP1 with the specific inhibitor SJB3-019A significantly reduced DMG cell proliferation, triggered apoptosis, and altered cell cycle profile. SJB3-019A-mediated USP1 inhibition decreases migration and invasion in multiple patient-derived DMG cell lines. Additional in vitro experiments revealed that USP1 inhibition leads to a significant decrease in the expression of ID1 and pAKT, suggesting the ID1/AKT pathway as a potential mechanism underlying the observed effects. H3K27M-mediated activation of USP1 in DMG is a promising therapeutic target, with inhibition by SJB3-019A reducing tumor aggressiveness by inhibiting invasion and migration. We will next assess SJB3-019A’s efficacy in vivo using genetically engineered isogenic mouse models (GEMM) with DNp53/PDGFRA/H3.3K27M (PPK) alterations and evaluate USP1 inhibition combined with radiation therapy as a potential radiosensitizer in DMGs.