The NSD family of histone methyltransferases is associated with various malignancies, including aggressive acute leukemia with NUP98-NSD1 translocation. While NSD proteins represent attractive drug targets, their catalytic SET domains exist in autoinhibited conformation, presenting significant challenges for inhibitor development. Here, we employed a fragment-based screening strategy followed by chemical optimization, which resulted in development of the first-in-class irreversible small molecule inhibitors of the NSD1 SET domain. The crystal structure of NSD1 in complex with covalently bound ligand reveals conformational change in the autoinhibitory loop of the SET domain and formation of a channel-like pocket suitable for targeting with small molecules. Our covalent lead, compound BT5, demonstrates on-target activity in NUP98-NSD1 leukemia cells, including inhibition of H3K36 dimethylation and downregulation of target genes, and impairs colony formation in NUP98-NSD1 patient sample. This study will facilitate development of the next generation of potent and selective inhibitors of the NSD histone methyltransferases.

ASH1L (Absent, Small, or Homeotic-like 1) is a histone methyltransferase (HMTase) involved in gene activation that is overexpressed in multiple forms of cancer. Previous studies of ASH1L’s catalytic SET domain identified an autoinhibitory loop that blocks access of histone substrate to the enzyme active site. Here, we used both NMR and X-ray crystallography to identify conformational dynamics in the ASH1L autoinhibitory loop. Using site-directed mutagenesis we found that point mutations in the autoinhibitory loop that perturb the structure of the SET domain result in decreased enzyme activity, indicating that the autoinhibitory loop is not a simple gate to the active site but is rather a key feature critical to ASH1L function. We also identified a second loop in the SET-I subdomain of ASH1L that experiences conformational dynamics, and we trapped two different conformations of this loop using crystallographic studies. Mutation of the SET-I loop led to a large decrease in ASH1L enzymatic activity in addition to a significant conformational change in the SET-I loop, demonstrating the importance of the structure and dynamics of the SET-I loop to ASH1L function. Furthermore, we found that three C-terminal chromatin-interacting domains greatly enhance ASH1L enzymatic activity and that ASH1L requires native nucleosome substrate for robust activity. Our study illuminates the role of concerted conformational dynamics in ASH1L function and identifies structural features important for ASH1L enzymatic activity.

Methylation at histone 3, lysine 36 (H3K36) is a conserved epigenetic mark regulating gene transcription, alternative splicing and DNA repair. Genes encoding H3K36 methyltransferases (KMTases) are commonly overexpressed, mutated or involved in chromosomal translocations in cancer. Molecular biology studies have demonstrated that H3K36 KMTases regulate oncogenic transcriptional programs. Structural studies of the catalytic SET domain of H3K36 KMTases have revealed intriguing opportunities for design of small molecule inhibitors. Nevertheless, potent inhibitors for most H3K36 KMTases have not yet been developed, underlining the challenges associated with this target class. As we now have strong evidence linking H3K36 KMTases to cancer, drug development efforts are predicted to yield novel compounds in the near future. Cellular development and differentiation are controlled by post-translational modifications on DNA and histone proteins, forming an epigenetic (above the genes) regulatory network. Disruption of epigenetic pathways is a nearly universal feature of cancer, causing aberrations in gene expression and genome integrity (reviewed in [1]). A key group of epigenetic enzymes disrupted in cancer is the lysine methyltransferases (KMTases), which install methyl groups on histone proteins. In cancer cells, methylation performed by KMTases drives proliferation and halts differentiation by modifying gene transcription and other DNA-templated processes [2][3][4]. Over the past decade, KMTases have emerged as important drug targets for both industrial and academic research groups. Some progress has been made, most notably by selective inhibitors for the EZH2 and DOT1L KMTases that have reached clinical trials for the treatment of nonHodgkin lymphoma and acute leukemia, respectively [5,6]. Nevertheless, selective and cell permeable inhibitors for many KMTases remain unavailable.Methylation at H3K36 represents a particularly important chromatin mark implicated in diverse forms of cancer. KMTases with specificity toward H3K36 are overexpressed in cancer cells and have been characterized as regulators of cell growth, differentiation, stemness and DNA repair pathways [7][8][9]. However, very few selective and cell-active small molecule inhibitors of H3K36-specfic KMTases have been reported to date. In this article, we provide an overview of cellular pathways involving H3K36 methylation and discuss the diverse functions carried out by the eight different human H3K36-specific KMTases. Then we analyze structural characteristics of the catalytic SET domain of H3K36 KMTases and evaluate prospects for their inhibition by small molecules. Review Rogawski, Grembecka & Cierpicki We conclude with a discussion of the challenges and o pportunities for targeting these proteins. SPECIAL FOCUS y Epigenetic drug discovery H3K36 methylation regulates diverse processes implicated in cancerH3K36 methylation participates in a wide variety of nuclear pathways. Many of these processes, including transcriptional regulation, alternative spli...

We show here that computer game players can build high-quality crystal structures. Introduction of a new feature into the computer game Foldit allows players to build and real-space refine structures into electron density maps. To assess the usefulness of this feature, we held a crystallographic model-building competition between trained crystallographers, undergraduate students, Foldit players and automatic model-building algorithms. After removal of disordered residues, a team of Foldit players achieved the most accurate structure. Analysing the target protein of the competition, YPL067C, uncovered a new family of histidine triad proteins apparently involved in the prevention of amyloid toxicity. From this study, we conclude that crystallographers can utilize crowdsourcing to interpret electron density information and to produce structure solutions of the highest quality.

ASH1L histone methyltransferase plays a crucial role in the pathogenesis of different diseases, including acute leukemia. While ASH1L represents an attractive drug target, developing ASH1L inhibitors is challenging, as the catalytic SET domain adapts an inactive conformation with autoinhibitory loop blocking the access to the active site. Here, by applying fragment-based screening followed by medicinal chemistry and a structure-based design, we developed first-in-class small molecule inhibitors of the ASH1L SET domain. The crystal structures of ASH1L-inhibitor complexes reveal compound binding to the autoinhibitory loop region in the SET domain. When tested in MLL leukemia models, our lead compound, AS-99, blocks cell proliferation, induces apoptosis and differentiation, downregulates MLL fusion target genes, and reduces the leukemia burden in vivo. This work validates the ASH1L SET domain as a druggable target and provides a chemical probe to further study the biological functions of ASH1L as well as to develop therapeutic agents.