Microglia in diffuse midline glioma contribute to extracellular matrix remodelling and cancer cell invasion

Diffuse midline glioma, H3K27-altered (DMG), is an aggressive and uniformly fatal paediatric brain tumour arising in midline structures and characterised by substantial microglial infiltration. We investigated whether microglia adopt a reactive state in response to DMG cells that functionally contributes to tumour progression. Transcriptomic profiling of microglia exposed to DMG, H3K27M cells, together with analysis of tumour associated myeloid cells isolated from DMG patient biopsies, revealed a pronounced upregulation of extracellular matrix (ECM) components, including fibronectin. Single cell transcriptomic analysis further identified microglia as the primary fibronectin expressing cell population within human DMG, H3K27M tumours. Functional invasion assays using a panel of patient-derived DMG, H3K27M cells, revealed that microglia-derived fibronectin significantly enhances tumour cell invasiveness, while its chemical inhibition with RGDS peptide or Avapritinib or its genetic silencing using small-interfering RNAs effectively suppresses invasion. Across independent patient cohorts (Kids First, PNOC, and CBTTC), and in archival tissues, DMG tumours were found to exhibit elevated expression of ECM components, and high fibronectin expression that correlated with poor prognosis. These findings suggest that microglia actively contribute to DMG invasiveness through ECM component production, identifying fibronectin as a potential therapeutic target in this lethal paediatric cancer.