EXTH-24. THERAPEUTIC TARGETING OF MISLOCALIZED NUCLEAR ENVELOPE PROTEINS IN<i>MYC</i>-DRIVEN GROUP 3 MEDULLOBLASTOMA

Abstract Medulloblastoma (MB) is the most common, malignant pediatric brain tumour comprised of four distinct molecular subgroups (WNT, SHH, Group 3, and Group 4). A subset of Group 3 MB tumors harbors focal amplifications of the MYC oncogene (MYC-G3MB) and are particularly prone to tumour recurrence due to their highly proliferative nature linked to its embryonic stem cell-of-origin and programming. We discovered that LBR, an inner nuclear transmembrane protein, while present in all cells at the nuclear envelope (NE), is aberrantly presented to the cell surface in MYC-G3MBs. Transcriptomic profiling of publicly available datasets identified significant NE protein enrichment in MYC-G3MB compared to normal tissue and high expression during fetal development that dampens postnatally and in adult samples. Immunohistochemistry analysis of LBR on patient tumors was able to significantly prognosticate overall and progression free survival. Flow cytometry analysis identified LBR cell surface expression in MYC-G3MBs with low to no expression in human neural stem cells and other MB subgroups. High resolution microscopy of endogenously tagged LBR revealed cell surface presentation to be linked to ER-like vesicles that directly trafficked to the cell surface in MYC-G3MBs. Transcriptomic analysis of LBR cell surface positive and negative cells identified LBR to be linked to mitotic processes and mislocalization was significantly enriched following exposure to chemotherapy and radiation in vitro and in vivo. Therefore, we demonstrate that LBR is an ideal therapeutic target for patients facing treatment-refractory MB and have generated single domain antibodies (sdAb) to develop LBR-CAR T cells. Validation of LBR sdAbs demonstrate on-target binding and comparable affinity and avidity to commercial LBR antibodies. There is an undeniable paucity of targets for MB patients stemming from the lack of neoantigens and genomic aberrations in pediatric tumors. The preclinical development and validation of novel immunotherapeutic targets such as LBR provides alternative therapies for patients otherwise facing palliation.