Diffuse intrinsic pontine gliomas (DIPGs) are aggressive pediatric brain tumors for which there is currently no effective treatment. Some of these tumors combine gain-of-function mutations in ACVR1, PIK3CA, and histone H3-encoding genes. The oncogenic mechanisms of action of ACVR1 mutations are currently unknown. Using mouse models, we demonstrate that Acvr1G328V arrests the differentiation of oligodendroglial lineage cells, and cooperates with Hist1h3bK27M and Pik3caH1047R to generate high-grade diffuse gliomas. Mechanistically, Acvr1G328V upregulates transcription factors which control differentiation and DIPG cell fitness. Furthermore, we characterize E6201 as a dual inhibitor of ACVR1 and MEK1/2, and demonstrate its efficacy toward tumor cells in vivo. Collectively, our results describe an oncogenic mechanism of action for ACVR1 mutations, and suggest therapeutic strategies for DIPGs. Display omitted •Mouse model of the pediatric high-grade diffuse glioma-driving Acvr1G328V mutation•Acvr1G328V causes oligodendroglial lineage differentiation arrest•Combining Acvr1G328V, Hist1h3bK27M, and Pik3caH1047R causes high-grade diffuse gliomas•E6201 is a dual inhibitor of MEK and ACVR1, and shows anti-tumor activity Fortin et al. find that Acvr1G328V upregulates transcription factors to block oligodendroglial cell differentiation. Acvr1G328V cooperates with Hist1h3bK27M and Pik3caH1047R to induce diffuse gliomas in mice. E6201, a covalent MEK1/2 inhibitor, can inhibit ACVR1 and reduce growth of ACVR1 mutant glioma xenografts.