Disrupting the BEX4-Mediated DNA Repair Rheostat with a Novel Inhibitor Suppresses Stemness and Enhances Chemosensitivity in Hepatoblastoma.

Cisplatin resistance in hepatoblastoma (HB) is a major therapeutic barrier. Emerging evidence suggests that cancer cells can activate DNA double-strand break repair pathways to evade chemotherapy-induced apoptosis and sustain stemness; however, the mechanisms underlying cisplatin resistance in HB remain poorly understood. Here, we identified the neuronal protein BEX4 as a key mediator of cisplatin resistance. BEX4 was aberrantly overexpressed in HB cells and maintained a stem-like state. Mechanistically, BEX4 bound to and stabilized the core nonhomologous end-joining (NHEJ) factor X-ray repair cross-complementing protein 5 (XRCC5), thereby promoting NHEJ activity and contributing to cisplatin resistance. DNA damage induced ataxia telangiectasia mutated kinase (ATM)-dependent phosphorylation of BEX4 at Thr107, promoting transient dissociation of the BEX4-XRCC5 complex and facilitating XRCC5 recruitment to DNA damage sites. BEX4-driven NHEJ enhances DNA damage repair, thereby promoting genomic instability, stemness, and chemoresistance. Building on these mechanistic findings, we identified a small-molecule compound, F35-303, which disrupted the BEX4-XRCC5 interaction, suppressed NHEJ activity, and restored cisplatin sensitivity in resistant preclinical models. Our findings identify BEX4 as a key mediator of cisplatin resistance in HB and suggest that targeting the BEX4-XRCC5 interaction with F35-303 may provide a new therapeutic strategy for overcoming chemoresistance in HB and a mechanistic framework for understanding adaptive chemoresistance.