The Polycomb Protein Bmi1 is a Key Effector of the H3.3 K27m Oncohistone

Summary Mutations in histones (“oncohistones”) only recently have been recognized as important drivers in various cancers. We have investigated the effects of H3.3-K27M oncohistone expression using primary murine bone marrow cells (mBMCs) as a model. H3.3-K27M blocked senescence and produced aggressively growing immortal cells with early-progenitor immunophenotype and some capability to engraft in immunodeficient mice, confirming functional stemness, but without causing leukemia. RNA-seq identified Bmi1, a stem-cell marker linked to Diffuse Intrinsic Pontine Glioma, among the most significantly early deregulated genes in H3.3-K27Mexpressing BMCs. The Bmi1 inhibitor PTC-209 caused reversible G1/S growth arrest. Forced Bmi1 expression in mBMCs immortalized them with kinetics similar to H3.3-K27M, creating cells of a strikingly similar phenotype, rendering Bmi1 a promising target for treatment of K27M-positive cancers. On the chromatin level, H3.3K27M exhibited heightened occupancy at proximal promoters compared to its wildtype counterpart. Finally, the distribution of H3-K27 trimethylation marks was consistent with failed spreading of heterochromatin in H3.3 K27M-expressing cells.