Evidence for coordinate CTCF and histone H3.3 activities in K27M diffuse midline gliomas.

Up to 80% of diffuse midline gliomas (DMGs) are characterized by a lysine to methionine driver mutation (K27M) in the tail of histone variant H3.3, pointing to likely roles for epigenetic mechanisms in K27M-driven tumorigenesis. Understanding the effects of mutant histone H3.3 on the complex patterns of histone modifications and interactions with chromatin structure and modifying enzymes is essential to developing effective combination treatment therapies for K27M DMG such as targeting multiple epigenetic enzymes at once. Here, using a genomics approach, we identified combinatorial patterns of epigenetic modifications that are affected by mutant H3.3 in DMG. We also characterized a strong association between H3.3 and the structural chromatin regulator CTCF, finding that mutant H3.3 leads to ectopic binding of CTCF at many additional sites across the genome in DMG. Notably, a number of these ectopic CTCF binding events occur within the HOX gene loci and are associated with an increase in H3K27me3 levels at bivalent domains and a decrease in HOX gene expression. We also find an association of H3.3 and CTCF at genomic sites adjacent to regions with active or repressive modifications, suggesting a potential role for these two factors in segmenting the chromatin and regulating, perhaps insulating, different types of domains. Together our data suggest that H3.3 K27M both affects epigenetic marks and chromatin organization in part through interaction with CTCF and point to a potentially novel contributory role for CTCF in promoting oncogenesis in DMG. These findings could have potential implications for designing therapy regimens to more effectively target the chromatin changes and genomic instability observed in H3.3K27M glioma cells.