GENETIC SCREENING REVEALS DEPENDENCY ON FEN1 TO RESIST TEMOZOLOMIDE IN TREATMENT-RESISTANT GLIOBLASTOMA STEM CELLS

BTFC travel award recipient Glioblastoma (GBM) is a lethal brain tumor. Despite intensive standard of care (SoC) consisting of surgical resection followed by radiation therapy (RT) and chemotherapy with temozolomide (TMZ), tumor relapse is inevitable with a median overall survival of 14.6 months. Therapeutic failure is attributable to the presence of GBM stem-like cells (GSCs), which are highly resistant to genotoxic therapies, self-renew and differentiate to perpetuate intra-tumoral heterogeneity, and seed tumor recurrence. However, no clinical advances have been made in overcoming resistance to therapy in GSCs, and the mechanisms underlying this resistance remain largely unknown. With the advent of CRISPR-Cas9 technology, functional genetic screens have been used to identify genes that regulate survival and sensitivity to TMZ in GSCs. However, they have not been applied to SoC and a comprehensive set of modulators governing resistance to combination treatment has yet to be determined. Here, we perform genome-wide CRISPR-Cas9 screening in patient-derived treatment-sensitive and treatment-resistant GSC models under normal growth conditions or while exposed to in vitro chemoradiotherapy. By integrating genetic dependencies and treatment-based conditional genetic interactions modulating sensitivity to SoC, we identify flap endonuclease 1 (FEN1) as a driver of GSC survival and therapy resistance in treatment-resistant GBM. Functionally, FEN1 inhibition demonstrates specific killing and synergy with TMZ in treatment-resistant GSCs while sparing healthy neural stem cells (NSCs) and treatment-sensitive GSCs. At the molecular level, FEN1 inhibition induces DNA damage and repair specifically in treatment-resistant GBM, posing FEN1 as an attractive therapeutic target in this population of GBM patients which SoC currently fails.