Bromodomain inhibitors revisited: Bromodomain and extraterminal domain (BET) proteins contribute to the pathogenesis of cancer and immune diseases through their effects on transcriptional regulation. BET proteins contain two nearly identical bromodomains, BD1 and BD2, structural modules that have attracted great interest as targets for drug development. First-generation drugs that inhibited both BD1 and BD2 showed promising therapeutic activity in preclinical models but proved to be less efficacious in clinical trials. Gilan et al. took a different approach and designed drugs that selectively inhibited BD1 or BD2 (see the Perspective by Filippakopoulos and Knapp). They found that BD1 and BD2 inhibitors altered gene expression in different ways and that BD2 inhibitors had greater therapeutic activity than BD1 inhibitors in preclinical models of inflammation and autoimmune disease. Science , this issue p. 387 ; see also p. 367

Angiogenesis underlies development, physiology and pathogenesis of cancer, eye and cardiovascular diseases. Inhibiting aberrant angiogenesis using anti-angiogenic therapy (AAT) has been successful in the clinical treatment of cancer and eye diseases. However, resistance to AAT inevitably occurs and its molecular basis remains poorly understood. Here, we uncover molecular modifiers of the blood endothelial cell (EC) response to a widely used AAT bevacizumab by performing a pooled genetic screen using three-dimensional microcarrier-based cell culture and CRISPR–Cas9. Functional inhibition of the epigenetic reader BET family of proteins BRD2/3/4 shows unexpected mitigating effects on EC survival and/or proliferation upon VEGFA blockade. Moreover, transcriptomic and pathway analyses reveal an interaction between epigenetic regulation and anti-angiogenesis, which may affect chromosomal structure and activity in ECs via the cell cycle regulator CDC25B phosphatase. Collectively, our findings provide insight into epigenetic regulation of the EC response to VEGFA blockade and may facilitate development of quality biomarkers and strategies for overcoming resistance to AAT.

DOT1L and Menin are essential cofactors for the oncogenic activity of MLL fusion proteins (MLL-FPs) in leukaemia. However, the mechanisms underpinning the therapeutic effects of their inhibitors remain unclear. Here we identify a critical role for the non-canonical Polycomb repressive complex 1.1 (PRC1.1) in mediating the cellular responses to DOT1L and Menin inhibitors. Menin inhibition induces PRC1.1-dependent deposition of H2AK119ub to silence a subset of MLL-FP targets, whereas DOT1L inhibition results in a genome-wide increase in H2AK119ub. We show that enhanced PRC1.1 activity arises specifically from the progressive loss of DOT1L-mediated H3K79 methylation, independent of MLL-FP displacement or transcriptional repression. This regulatory crosstalk is conserved across cell types and is driven by direct biochemical antagonism between H3K79 methylation and PRC1 activity. Together, our findings establish DOT1L as a component of transcriptional memory co-opted in leukaemia and suggest it serves as the missing link balancing the opposing forces of the MLL–Polycomb axis.

Summary: The regulation of all chromatin-templated processes involves the selective recruitment of chromatin factors to facilitate DNA repair, replication, and transcription. Chromatin immunoprecipitation (ChIP) is a critical experimental method used to provide spatiotemporal evidence for the coordination of these chromatin-based events including the dynamic regulation of chromatin modifications at cis-regulatory elements. However, obtaining a global appreciation of all the factors that influence a specific chromatin event has remained challenging. Here, as a proof of concept we demonstrate the utility of coupling unbiased functional genomics with ChIP to identify the factors associated with active transcription. Specifically, we use this method to identify the major chromatin factors associated with the catalysis of two evolutionarily conserved histone modifications; H3K4me3 present at the transcriptional start site and H3K79me2 present through the gene body of actively transcribed genes. With CRISPR-ChIP, we identify all the non-redundant COMPASS complex members required for H3K4me3 and demonstrate that RNA polymerase II is dispensable for the maintenance of H3K4me3. As H3K79me2 has a putative oncogenic function in leukaemia cells driven by MLL-translocations, using CRISPR-ChIP we reveal a functional partitioning of H3K79 methylation into two distinct regulatory units. An oncogenic DOT1L complex, where the malignant driver directs the catalytic activity of DOT1L at MLL-Fusion target genes and a separate endogenous DOT1L complex, where catalytic activity is directed by MLLT10. This functional demarcation provides an explanation for the observed synergy with Menin and DOT1L inhibitors and why Menin inhibition surprisingly controls methylation of H3K79 at a critical subset of genes that sustain MLL-fusion leukaemia. Overall, CRISPR-ChIP provides a powerful tool for the unbiased interrogation of the mechanisms underpinning chromatin regulation.