Abstract PURPOSE Malignant peripheral nerve sheath tumors (MPNSTs) are the most common cause of death in neurofibromatosis type 1 (NF-1) and are resistant to radiation therapy (RT), yet the mechanisms underlying RT response are poorly understood. Here, we elucidate mechanisms of RT response in NF-1 associated peripheral nerve sheath tumors through genome-wide CRISPRi screens, genomic analysis of mouse models, and molecular analysis of patient-derived tumor specimens. METHODS Patient derived NF1 mutant plexiform neurofibroma (pNF) cells (NF9511b, NF95.6), and MPNST cells (ST88-14; JH2-002) were used to measure RT responses by cell counts and flow cytometry. Genome-wide CRISPRi screens were used to identify functional modifiers of RT response in ST88-14 and JH2-002 MPNST cells. Nf1-/-; Tp53-/- mouse MPNSTs were subcutaneously implanted in immunocompetent C57/B6 mice, irradiated (2Gyx5), and dissociated for single-cell RNA sequencing (scRNA-seq) using 10x Genomics. Human MPNST tumor specimens (n=45) were analyzed with targeted DNA mutation sequencing and methylation arrays for cell type deconvolution. RESULTS Radiation single-dose response curves revealed MPNST cells (IC50 6.85Gy) were radioresistant compared to pNF cells (IC50 3.13Gy). Genome wide CRISPRi screens in ST88-14 and JH2-002 MPNST cells converged on cell cycle, DNA repair, and immunomodulatory gene sets mediating RT sensitivity. Irradiation of subcutaneous mouse Nf1-/-; Tp53-/- MPNSTs significantly decreased tumor growth (p=0.01, t-test). scRNA-seq of 32,763 cells from 4 irradiated and 3 unirradiated mouse Nf1-/-; Tp53-/- MPNSTs identified 8 tumor clusters and 7 non-tumor clusters. Irradiation demonstrated a pro-inflammatory effect with increased T-cell presence (5.8% versus 3.3%, p=0.04, t-test) and greater activation of immunostimulatory genes (Cxcl10, Stat1, Irf7) in T cells. In human MPNSTs, CDKN2A/B deletion (p=0.04, log-rank test) and decreased immune cell composition (p=0.03, log-rank test) were associated with significantly worse overall survival in response to RT. CONCLUSION MPNST RT responses may depend on immunomodulatory mechanisms that could be leveraged to improve clinical RT response.

RBIO-07. IMMUNOMODULATORY PATHWAYS MEDIATE RADIOTHERAPY RESPONSE IN MALIGNANT PERIPHERAL NERVE SHEATH TUMORS