Mediator Kinase Inhibition Further Activates Super-Enhancer Associated Genes in AML

Super-enhancers (SEs), which are composed of large clusters of enhancers densely loaded with the Mediator complex, transcription factors (TFs), and chromatin regulators, drive high expression of genes implicated in cell identity and disease, such as lineage-controlling TFs and oncogenes 1, 2. BRD4 and CDK7 are positive regulators of SE-mediated transcription3,4,5. In contrast, negative regulators of SE-associated genes have not been well described. Here we report that Mediator-associated kinases cyclin-dependent kinase 8 (CDK8) and CDK19 restrain increased activation of key SE-associated genes in acute myeloid leukaemia (AML) cells. We determined that the natural product cortistatin A (CA) selectively inhibited Mediator kinases, had antileukaemic activity in vitro and in vivo, and disproportionately induced upregulation of SE-associated genes in CA-sensitive AML cell lines but not in CA-insensitive cell lines. In AML cells, CA upregulated SE-associated genes with tumour suppressor and lineage-controlling functions, including the TFs CEBPA, IRF8, IRF1 and ETV6 6, 7, 8. The BRD4 inhibitor I-BET151 downregulated these SE-associated genes, yet also has antileukaemic activity. Individually increasing or decreasing expression of these TFs suppressed AML cell growth, providing evidence that leukaemia cells are sensitive to dosage of SE-associated genes. Our results demonstrate that Mediator kinases can negatively regulate SE-associated gene expression in specific cell types and can be pharmacologically targeted as a therapeutic approach to AML

Mediator Kinase Phosphorylation of STAT1 S727 Promotes Growth of Neoplasms With JAK-STAT Activation

Constitutive JAK-STAT signaling drives the proliferation of most myeloproliferative neoplasms (MPN) and a subset of acute myeloid leukemia (AML), but persistence emerges with chronic exposure to JAK inhibitors. MPN and post-MPN AML are dependent on tyrosine phosphorylation of STATs, but the role of serine STAT1 phosphorylation remains unclear. We previously demonstrated that Mediator kinase inhibitor cortistatin A (CA) reduced proliferation of JAK2-mutant AML in vitro and in vivo and also suppressed CDK8-dependent phosphorylation of STAT1 at serine 727. Here we report that phosphorylation of STAT1 S727 promotes the proliferation of AML cells with JAK-STAT pathway activation. Inhibition of serine phosphorylation by CA promotes growth arrest and differentiation, inhibits colony formation in MPN patient samples and reduces allele burden in MPN mouse models. These results reveal that STAT1 pS727 regulates growth and differentiation in JAK-STAT activated neoplasms and suggest that Mediator kinase inhibition represents a therapeutic strategy to regulate JAK-STAT signaling

Incorporation of a nucleoside analog maps genome repair sites in postmitotic human neurons

Neurons are the longest-lived cells in our bodies and lack DNA replication, which makes them reliant on a limited repertoire of DNA repair mechanisms to maintain genome fidelity. These repair mechanisms decline with age, but we have limited knowledge of how genome instability emerges and what strategies neurons and other long-lived cells may have evolved to protect their genomes over the human life span. A targeted sequencing approach in human embryonic stem cell-induced neurons shows that, in neurons, DNA repair is enriched at well-defined hotspots that protect essential genes. These hotspots are enriched with histone H2A isoforms and RNA binding proteins and are associated with evolutionarily conserved elements of the human genome. These findings provide a basis for understanding genome integrity as it relates to aging and disease in the nervous system