MDB-21. THE OTX2-REGULATED ALTERNATIVE SPLICING LANDSCAPE CONTROLS DIFFERENTIATION AND RECONSTITUTES THE DEVELOPMENTAL ORIGINS OF GROUP 3 MEDULLOBLASTOMA

Medulloblastoma (MB) is the most common malignant primary pediatric brain tumor with Group 3 MB exhibiting the worst prognosis. Overexpression/amplification of OTX2 is a molecular hallmark of Group 3 MB and is primarily known to control tumor growth through regulation of cell cycle. Here, we utilized mass spectrometry and RNA-sequencing to report a non-canonical role for OTX2 in regulating alternative splicing. RNA-sequencing conducted on two OTX2-silenced Group 3 MB tumorsphere models identified 48 differentially spliced genes (DSG) associated with RNA processing, splicing and neurodevelopment that were common between both cell lines. Further interrogation of the OTX2-regulated DSGs revealed that 15 are also significantly correlated with OTX2 expression in a set of Group 3/Group 4 MB patient samples. Pseudotime mapping of the developmental trajectories of the DSGs onto the developing rhombic lip (RL) revealed significant overlap with RL lineages, including the majority of the 15 shared OTX2-regulated DSGs. We then functionally validated specific alternatively spliced exons of two DSGs, PPHLN1 and MADD which represent “early” and “late” RL developmental stages, respectively. PPHLN1 and MADD splice-blocking morpholinos were utilized to drive exon skipping and recapitulate splicing events identified in OTX2-silenced or OTX2-wildtype tumorspheres. Exon skipping altered tumorigenic properties of Group 3 MB cells in vitro, including tumorsphere size and number. Additionally, intracerebellar injections of Group 3 MB cells treated with PPHLN1 morpholino impaired tumor initiation and growth in vivo. Further breakdown of Group 3/Group 4 MBs by PPHLN1 and MADD percent spliced in (PSI) revealed that the PPHLN1 high/MADD low PSI signature observed in our OTX2 wildtype tumorspheres is associated with the most aggressive and primitive Group 3g subtype. Together, our findings reveal a previously unrecognized regulatory role for OTX2 in controlling Group 3 MB AS and offer fresh opportunities to exploit MB-specific AS events therapeutically to abrogate tumor progression.