MDB-25. REST-USP37 axis controls GLI1 levels in sonic hedgehog medulloblastoma cells.

Medulloblastoma (MB), the most common pediatric cerebellar tumor, has a 5-year survival rate of approximately 70-80%. About 30% of MBs are driven by constitutive activation of Sonic Hedgehog (SHH) signaling exhibiting driver mutations in PTCH1, SMO, and SUFU. Thus, there exist other contributors to SHH-MB pathology. We propose that the transcriptional repressor RE1-Silencing Transcription factor (REST) plays an important role in SHH-MB progression. We previously discovered that REST elevation in SHH-MBs is associated with poor patient outcomes. We also showed that REST controls SHH pathway activity by causing epigenetic loss of Ptch1 haploinsufficiency. Here, we describe the role of REST in controlling levels of terminal effectors of the SHH signaling pathway, GLI1 and GLI2. Using single-cell transcriptomic dataset of high- and low-REST SHH-MB patient samples, we found an increase in cycling cerebellar granule progenitor cells (GNPs) in the context of REST elevation, which was associated with a decrease and an increase in the number of cells expressing GLI1 and GLI2, respectively. Interestingly, we found that USP37-a REST target gene product and a deubiquitylase, controls GLI1 stability. USP37 decline in REST-high SHH-MBs reduces GLI1 levels, although cells are maintained in a proliferative state. GLI1 is also known to activate its own gene expression. So, we attribute the maintenance of cell proliferation under high REST conditions to the loss of stabilization of p27, a cyclin-dependent kinase inhibitor that is required for terminal cell cycle exit and initiation of terminal neurogenesis. A potential role for a compensatory increase in GLI2 levels in maintaining SHH pathway activity and cell proliferation cannot be excluded at this time. Consistent with this effect of REST on SHH signaling activity, GLI inhibitors reduce the growth of SHH-MB cells in vitro. In summary, our work has uncovered a novel REST-USP37-GLI1 axis in controlling SHH pathway activity in SHH-MBs. Our findings may have therapeutic implications for patients with SHH-MBs with REST elevation.