Targeted Disruption of the Interaction between WD-40 Repeat Protein 5 (WDR5) and Mixed Lineage Leukemia (MLL)/SET1 Family Proteins Specifically Inhibits MLL1 and SETd1A Methyltransferase Complexes

Abstract

Mixed Lineage Leukemia 1 (MLL1) protein is a member of the SET1 (or MLL) family of histone methyltransferases. In humans, this family consists of six members: MLL1-4, SETd1A, and SETd1B (1-8). The SET1 family catalyzes methylation of histone 3 lysine 4 (H3K4), 3 an epigenetic mark that is associated with active transcription (9 -12). The human SET1 family is composed of large proteins with several well characterized functional domains involved in chromatin binding and protein-protein interactions (13, 14) (Fig. 1A). Although some of these domains differ among family members, all share a C-terminal SET (suppressor of variegation, enhancer of Zeste, trithorax) domain that confers H3K4 methyltransferase activity (15). Like many chromatin-modifying enzymes, the SET1 family works as part of multiprotein complexes that contain binding partners involved in enzymatic regulation and gene targeting. Although the majority of isolated SET1 family SET domains catalyze weak H3K4 monomethylation (H3K4me1), enhanced methylation is observed in the context of a "core complex" (16). The minimal core complex required for enhanced methylation is composed of the SET1/MLL SET domain and a subcomplex called WRAD (WD-40 repeat protein 5 (WDR5), retinoblastoma-binding protein 5 (RbBP5), absent small homeotic 2-like (ASH2L), and dumpy-30 (DPY-30)) (17)(18)(19)(20). Interestingly, SET1 family core complexes preferentially catalyze different levels of H3K4 methylation in a manner that correlates with their evolutionary lineage (16). Whereas SETd1A/B core complexes catalyze mono-, di-, and trimethylation of H3K4 (H3K4me1, H3K4me2, and H3K4me3, respectively), the MLL1 and MLL4 (also known as MLL2) core complexes predominantly catalyze mono-and dimethylation (16). In contrast, MLL2 and MLL3 core complexes catalyze predominantly H3K4 monomethylation (16). In cells, different levels of H3K4 methylation are localized to distinct genomic regions and are associated with distinct functional outcomes (21-23). Assembly of the MLL1 core complex requires a direct interaction between MLL1 and WDR5, whereby WDR5 acts to stabilize the interaction between the MLL1 SET domain and the RbBP5/ASH2L heterodimer (18,24). The MLL1-WDR5 interaction occurs via the conserved Win (WDR5 interaction) * This work was supported, in whole or in part, by the National Institutes of Health Grant R01CA140522 (to M. S. C.). The authors declare that they have no conflicts of interest with the contents of this article. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Displacement of WDR5 from Chromatin by a WIN Site Inhibitor with Picomolar Affinity

“…From a structural perspective, the most obvious route to pharmacologically inhibit WDR5 is via the WIN (WDR5 interaction) site, a well-defined pocket that mediates interaction with an arginine-containing motif (WIN motif; consensus “ARA”) present in multiple WDR5-interaction partners ( Guarnaccia and Tansey, 2018 ). Although the functions of the WIN site are not fully understood, it is clear that the HMT activity of complexes carrying the MLL1 protein, but not other mixed lineage leukemia/Su(var)3–9, Ezh2, Trithorax (MLL/SET) family members, is dependent on WIN site binding by a WIN motif ( Alicea-Velázquez et al, 2016 ), leading to the concept that WIN site inhibitors could alter transcriptional patterns by modulating H3K4 methylation. Consistent with this idea, a moderately potent (K d ~100 nM) small molecule WIN site inhibitor inhibits cancer cells that express mutant forms of CCAAT-enhancer-binding protein α (C/EBPα) ( Grebien et al, 2015 ) and p53 ( Zhu et al, 2015 ).…”

Epigenetic Control of the Tumor Microenvironment

“…Finally, ASH1 knockdown caused a decrease in mRNA levels of these profibrotic genes, suggesting that ASH1 has a significant role in the activation of fibroblasts during fibrosis. It is likely that similar mechanisms are involved in the alterations in gene expression during CAF transdifferentiation, but future studies are necessary to determine the specific roles of histone methyltransferase-mediated gene activation in these fibroblasts, especially since inhibitors of these histone methyltransferases are being developed for use in cancer [65,[67][68].…”

WDR5 positively regulates p53 stability by inhibiting p53 ubiquitination

“…As a key component of Set/MLL complex, WDR5 binds an arginine-bearing motif in MLL, promoting the complex assembly [1]. Disrupting the WDR5-MLL interaction by inhibitors or mutating the MLL WDR5 interaction motif severely inhibits the histone methyltransferase activity [4,5,6,7]. WDR5 represents a potential coactivator of transcription factors by associating transcription factors and promoting histone methylation, resulting in transcription activation [8,9,10,11].…”