Abstract
Medulloblastoma (MB) is the most common pediatric brain malignancy, accounting for 20% of diagnosed brain tumors in children. The Group 3 (G3) MB subtype is found to be especially malignant and is commonly associated with MYC over-expression. Current G3 MB treatment regimens provide for 50% 5-year survival rates and are associated with severe side effects, including hearing loss and neurocognitive deficits. High expression of stress granule (SG) proteins such as G3BP1 and PAPBPC1 correlates with poor survival outcome in G3 MB patients. We propose that formation of SGs, mRNP granules assembled under cellular stress, is integral for MB tumor progression, and suggest that inhibition of SG formation is a novel therapeutic strategy. In order to identify key proteins for SG formation we set up a siRNA screen for 95 genes that have previously been linked to SG formation, and that are highly expressed in MB G3 tumors. The screen has been done in two G3 MB cell lines: MED8A and HDMB03. EIF2AK1, also known as HRI, was identified as one of the hits in the screen; it is an eIF2a kinase responsible for cellular response to oxidative stress. Phosphorylation of eIF2a is essential for canonical SG formation and global translation inhibition. We therefore used ISRIB, a known inhibitor of the integrated stress response, to block the effects of eIF2a phosphorylation, thus maintaining active global translation under stress conditions and preventing SG assembly. MB cells exposed to ISRIB combined with vincristine exhibited significantly decreased proliferation as well as increased apoptosis markers compared to cells exposed to vincristine alone. We show that combining SG inhibition with chemotherapy enhances the effects of the latter, which is increasingly relevant for patients suffering from therapy side effects and high chances of metastasis associated with G3 MB.