CNSC-48. DISRUPTED DIFFERENTIATION PROMOTES MEDULLOBLASTOMA INITIATION AND PROGRESSION

Abstract Medulloblastoma (MB) is the most common malignant brain tumor in children. The Sonic Hedgehog (SHH) subgroup of MB predominately originates from cerebellar granule neuron progenitors (GNPs). We previously demonstrated that MB tumor cells retain the capacity to undergo terminal differentiation, and that once differentiated, they irreversibly lose their proliferative and tumorigenic potential. These findings suggest that terminal differentiation serves as a tumor-suppressive mechanism. However, the role of disrupted differentiation in MB tumorigenesis remains poorly understood. NeuroD1, a basic helix-loop-helix (bHLH) transcription factor, is essential for the differentiation of both normal GNPs and MB tumor cells. Using a conditional knockout mouse model, we deleted NeuroD1 in cerebellar GNPs. This resulted in a marked reduction in differentiated neurons and prolonged GNP proliferation, indicative of impaired differentiation. Nevertheless, these mice did not develop tumors, and the sustained proliferation remained dependent on SHH pathway activation. These results indicate that while disrupted differentiation enhances proliferative capacity, it is not sufficient to initiate tumorigenesis. Notably, when NeuroD1-deficient mice were crossed with Ptch1 heterozygous mice—a model predisposed to SHH-driven MB—tumor incidence significantly increased, and latency was markedly shortened. Tumors from these mice exhibited minimal evidence of differentiation, suggesting that NeuroD1 loss also impairs differentiation in tumor cells. These findings indicate that disrupted differentiation promotes both tumor initiation and progression in the context of Ptch1 deficiency. Moreover, RNA sequencing of tumors with and without NeuroD1 deletion revealed no major differences in mRNA expression profiles, suggesting that differentiation blockade does not significantly alter the transcriptional landscape of tumor cells. In summary, our findings reveal that intact differentiation programs are critical for suppressing MB tumorigenesis. Although disrupted differentiation does not independently initiate tumor formation, it creates a permissive environment for oncogenic mutations to promote tumor initiation and accelerate progression.