Compartments in medulloblastoma with extensive nodularity are connected through differentiation along the granular precursor lineage

Medulloblastomas with extensive nodularity are cerebellar tumors characterized by two distinct compartments and variable disease progression. The mechanisms governing the balance between proliferation and differentiation in MBEN remain poorly understood. Here, we employ a multi-modal single cell transcriptome analysis to dissect this process. In the internodular compartment, we identify proliferating cerebellar granular neuronal precursor-like malignant cells, along with stromal, vascular, and immune cells. In contrast, the nodular compartment comprises postmitotic, neuronally differentiated malignant cells. Both compartments are connected through an intermediate cell stage resembling actively migrating CGNPs. Notably, we also discover astrocytic-like malignant cells, found in proximity to migrating and differentiated cells at the transition zone between the two compartments. Our study sheds light on the spatial tissue organization and its link to the developmental trajectory, resulting in a more benign tumor phenotype. This integrative approach holds promise to explore intercompartmental interactions in other cancers with varying histology.

MEDB-60. Medulloblastoma with extensive nodularity mimics cerebellar development and differentiates along the granular precursor lineage

BACKGROUND: Medulloblastoma with extensive nodularity (MBEN) represents a rare type of cerebellar tumors of infancy comprising two histologically distinct components that differ in cell differentiation and mitotic activity. Whereas some children suffering from MBEN experience disease recurrence, MBEN can also spontaneously differentiate and discontinue to grow. The underlying mechanisms of this variable biological behavior may offer insight into how embryonal tumors develop. METHODS: Fresh frozen and FFPE tumor tissue from nine MBEN-patients was subjected to multi-omics characterization including bulk sequencing, microdissection followed by RNA sequencing, single nucleus RNA-sequencing using the 10X Genomics- and SMART Seq. V2-protocols and spatial transcriptomics via RNAscope. RESULTS: All cases were molecularly classified as Sonic Hedgehog (SHH)-MB, and harbored somatic mutations within the SHH-pathway. After quality control, a total of ~30.000 cells were subjected to downstream analysis. Several non-malignant cell types, such as glial cells, were identified. In accordance with previous studies, we found only sparse immune infiltration. Unsupervised clustering identified cell clusters that differed in differentiation state and represented a continuum from embryonal-like cells with SHH-upregulation over intermediate cell states, to neuronal-like, postmitotic cells. Mapping to a single nucleus sequencing atlas of cerebellar development indicated that tumor cells reflected various stages of normally developing cerebellar granular precursors. Interestingly, one cluster of malignant cells with tumor-specific copy number alterations showed both transcriptomic features of astrocytes and embryonal cells. Using spatial transcriptomics, we were able to correlate different clusters of MBEN cells with distinct histologic MBEN compartments, with astrocyte-like tumor cells being located in the internodular compartment and in close proximity to mitotically active cancer cells. CONCLUSION: MBEN is formed by a continuum of malignant cell differentiation along the granular precursor lineage, with a subset of cells developing into cells that may represent tumor astrocytes. This differentiation process is reflected in the bicompartmental structure of MBEN.