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The REI Silensing Transcription Factor (REST) is a transcriptional repressor and a canonical regulator of neurogenesis. Its expression is elevated in human sonic hedgehog (SHH) subgroup of medulloblastomas (MBs), where functional studies shown its elevated expression to promote proliferation and block neuronal specification. A role for REST in the control of the MB tumor microenvironment (TME) has not been described previously. Here, we demonstrate that REST also controls the MB-TME, and specifically vascular remodeling. Using our unique RESTTG mouse model, we show that conditional expression of human REST transgene in cerebellar granule neuron progenitors (CGNPs), the cell of origin of a subset of SHH MBs, promoted increased vascular growth. In the context of constitutive activation of SHH signaling, a key driver of SHH-MB development, REST elevation drove tumor progression by altering the tumor vasculature. These findings were validated in mouse orthotopic models of human MB cell lines and through analyses of publicly available transcriptomic database of human MB samples. A strong positive correlation between REST and that of endothelial genes CD31/VEGFR1/ETS1 was seen in samples from patients with SHH-MBs subtypes that are associated with the worst prognosis. Proteomic analyses identified increased secretion of a number of pro-angiogenic factors in the context of upregulated REST expression in MB cells. Unexpectedly, in vitro and in vivo studies showed that MB cells expressed these endothelial markers and co-localized with endothelial cells suggesting that REST elevation may have altered the fate of cells that were destined to become neurons. Finally, ETS1 knockdown in MB cell lines not only downregulated VEGFR1 levels in these cells, and blocked tube formation in vitro, but also caused a reduction in tumor cell co-localization with endothelial cells. Collectively, these data suggest that REST elevation remodels MB vasculature through cell-intrinsic and cell-extrinsic mechanisms.

TAMI-58. A NOVEL ROLE FOR REST IN THE CONTROL OF THE MEDULLOBLASTOMA MICROENVIRONMENT