Development and Use of Assay Conditions Suited to Screening for and Profiling of SET-Domain-Targeted Inhibitors of the MLL/SET1 Family of Lysine Methyltransferases.

Methylation of histone H3 lysine-4 (H3K4) is an important, regulatory, epigenetic post-translational modification associated with actively transcribed genes. In humans, the principal mediators of this modification are part of the MLL/SET1 family of methyltransferases, which comprises six members, MLLs1-4 and SET1A/SET1B. Aberrations in the structure, expression, and regulation of these enzymes are implicated in various disease states, making them important potential targets for drug discovery, particularly for oncology indications. The MLL/SET1 family members are most enzymatically active when part of a "core complex," the catalytic SET-domain-containing subunits bound to a subcomplex consisting of the proteins WDR5, RbBP5, Ash2L and a homodimer of DPY-30 (WRAD2). The necessity of MLL/SET1 members to bind WRAD2 for full activity is the basis of a particular drug development strategy, which seeks to disrupt the interaction between the MLL/SET1 subunits and WDR5. This strategy is not without its theoretical and practical drawbacks, some of which relate to the ease with which complexes of Escherichia coli-expressed MLL/SET1 and WRAD2 fall apart. As an alternative strategy, we explore ways to stabilize the complex, focusing on the use of an excess of WRAD2 to drive the binding equilibria toward complex formation while maintaining low concentrations of the catalytic subunits. The purpose of this approach is to seek inhibitors that bind the SET domain, an approach proven successful with the related, but inherently more stable, enhancer of zeste homolog 2 (EZH2) complex.