Point mutations in histone variant H3.3 (H3.3K27M, H3.3G34R) and the H3.3-specific ATRX/DAXX chaperone complex are frequent events in paediatric gliomas. These H3.3 point mutations affect many chromatin modifications but it is unclear how these changes drive oncogenesis. Histone H3.3 is known to localise to nuclear compartments known as PML nuclear bodies (PML-NBs), which are frequently mutated and confirmed as oncogenic drivers in acute promyelocytic leukaemia.
We find that the glioma-associated H3.3 point mutations disrupt the formation of PML-NBs, leading to a high degree of variability in size and number of PML-NBs in both mouse ES cells and human glioma cell lines. The H3.3 K27M mutated gliomas are unable to differentiate down glial lineages and this phenotype is recapitulated in PML knockdown cells, demonstrating that PML is essential for glial differentiation. It is therefore plausible that H3.3 mutations drive oncogenesis by interfering with the formation of functional PML-NBs.
Acute promyelocytic leukaemias with mutations in PML can be cured with arsenic trioxide, an agent which targets PML-NBs. We find that the defects in PML-NBs caused by H3.3 K27M renders these gliomas hyper-sensitive to arsenic trioxide and this response can be partially rescued by overexpression of PML. Furthermore, we found that cells with IDH1/2 point mutations, which are common in both glial and myeloid malignancies, also fail to form normal PML-NBs and are similarly sensitive to treatment with arsenic trioxide. This demonstrates that disparate mutations can converge on PML and reveals an unexpected commonality between paediatric gliomas and myeloid leukaemias.
These findings demonstrate that H3.3 mutations interfere with the formation of functional PML-NBs which are required for glial differentiation, and targeting PML-NBs could potentially be effective in the treatment of H3.3 mutated gliomas.