Several types of pediatric cancers reportedly contain high frequency missense mutations in histone H3, yet the underlying oncogenic mechanism remains poorly characterized. Here, we report that the H3 lysine 36 to methionine (H3K36M) mutation impairs the differentiation of mesenchymal progenitor cells and generates undifferentiated sarcoma in vivo. H3K36M mutant nucleosomes inhibit the enzymatic activities of several H3K36 methyltransferases. Depleting H3K36 methyltransferases, or expressing an H3K36I mutant that similarly inhibits H3K36 methylation, is sufficient to phenocopy the H3K36M mutation. Following the loss of H3K36 methylation, a genome-wide gain in H3K27 methylation leads to a redistribution of Polycomb Repressive Complex 1 and de-repression of its target genes known to block mesenchymal differentiation. Our findings are mirrored in human undifferentiated sarcomas where novel K36M/I mutations in H3.1 are identified.

RNA performs important cellular functions in contemporary life forms. Its ability to act both as a catalyst and a storage mechanism for genetic information is also an important part of the RNA world hypothesis. Compartmentalization within modern cells allows the local concentration of RNA to be controlled and it has been suggested that this was also important in early life forms. Here, we mimic intracellular compartmentalization and macromolecular crowding by partitioning RNA in an aqueous two-phase system (ATPS). We show that the concentration of RNA is enriched by up to 3,000-fold in the dextran-rich phase of a polyethylene glycol/dextran ATPS and demonstrate that this can lead to approximately 70-fold increase in the rate of ribozyme cleavage. This rate enhancement can be tuned by the relative volumes of the two phases in the ATPS. Our observations support the importance of compartmentalization in the attainment of function in an RNA World as well as in modern biology.

Viral proteins mimic host protein structure and function to redirect cellular processes and subvert innate defenses1. Small basic proteins compact and regulate both viral and cellular DNA genomes. Nucleosomes are the repeating units of cellular chromatin and play an important role in innate immune responses2. Viral encoded core basic proteins compact viral genomes but their impact on host chromatin structure and function remains unexplored. Adenoviruses encode a highly basic protein called protein VII that resembles cellular histones3. Although protein VII binds viral DNA and is incorporated with viral genomes into virus particles4,5, it is unknown whether protein VII impacts cellular chromatin. Our observation that protein VII alters cellular chromatin led us to hypothesize that this impacts antiviral responses during adenovirus infection. We found that protein VII forms complexes with nucleosomes and limits DNA accessibility. We identified post-translational modifications on protein VII that are responsible for chromatin localization. Furthermore, proteomic analysis demonstrated that protein VII is sufficient to alter protein composition of host chromatin. We found that protein VII is necessary and sufficient for retention in chromatin of members of the high-mobility group protein B family (HMGB1, HMGB2, and HMGB3). HMGB1 is actively released in response to inflammatory stimuli and functions as a danger signal to activate immune responses6,7. We showed that protein VII can directly bind HMGB1 in vitro and further demonstrated that protein VII expression in mouse lungs is sufficient to decrease inflammation-induced HMGB1 content and neutrophil recruitment in the bronchoalveolar lavage fluid. Together our in vitro and in vivo results show that protein VII sequesters HMGB1 and can prevent its release. This study uncovers a viral strategy in which nucleosome binding is exploited to control extracellular immune signaling.