Medulloblastoma-associated mutations in the RNA helicase DDX3X/DED1 cause defects in the translational response to TORC1 inhibition.

Medulloblastoma is the most common pediatric brain cancer, but current treatments are largely nonspecific, often causing developmental side effects. Genomic sequencing identified the RNA helicase DDX3X as one of the most frequently mutated genes in this cancer and a potential treatment target, yet its role in tumor progression remains elusive. Prior studies have indicated that the mutations cause specific defects in translation; however, both DDX3X and its yeast ortholog Ded1 have also been associated with cellular stress responses, suggesting that the contribution of the DDX3X mutations to medulloblastoma might result from defects in the translational response to stress. Building on our prior study that replicated the DDX3X mutations in yeast DED1 (ded1-medulloblastoma-associated-mutants [ded1-mam]), we examined the mutants' effects following Target of Rapamycin (TOR) pathway inactivation. First, we demonstrated that ded1-mam displayed substantial rapamycin-resistant growth compared to WT cells. In addition, similar to other ded1 mutants, the ded1-mam had increased protein abundance of Ded1 and the translation factor eIF4G1 under TOR inactivation. Notably, these differences did not result in increased bulk translation following rapamycin; rather, the growth phenotypes appeared to be driven by translation of specific mRNAs. Reporter assays demonstrated enhanced translation of mRNA with an unstructured 5' UTR in ded1-mam following TOR inhibition and a decrease in a structured reporter. Furthermore, known Ded1 target genes with relatively unstructured 5' UTRs showed upregulated protein levels in rapamycin. We hypothesize that mutant DDX3X selectively upregulates translation of unstructured, progrowth transcripts while downregulating other structured transcripts, allowing tumor cells to bypass stress-induced growth controls and promoting medulloblastoma progression.