Construction of Chimeric Histone Methyltransferase Complexes In Saccharomyces cerevisiae Generate Unique Phenotypes and Clarify the Roles of MLL1 and Set1 Complex Accessory Proteins

Abstract

Genomic rearrangements involving the MLL1 gene are prevalent in human cancers, especially mixed lineage leukemia. This gene encodes an enzyme that methylates histone H3 on lysine 4 (H3K4) as a part of the MLL1 multi‐protein complex. The MLL1 complex regulates the expression of many genes via this epigenetic mark, including homeobox genes. Problematically, higher eukaryotes contain many functionally redundant complexes that complicate the study of MLL1 and associated cancers in a living system. Furthermore, due to the nature of the disease‐causing mutations, namely chromosomal rearrangements, MLL1 is not a promising drug target. However, several accessory proteins within the complex are required for catalytic activity, presenting possible drug targets themselves. Herein we present an in vivo system for the study of the MLL1 complex in Saccharomyces cerevisiae, making use of the homologous Set1/COMPASS complex. We genetically replaced COMPASS members from S. cerevisiae with their human homologs using antibiotic resistance cassettes. We then performed phenotypic characterization of chimeric COMPASS/MLL1 complexes assessing global H3K4 methylation status. Selected chimeric yeast‐human methyltransferase complexes conferred catalytic activity at varying degrees, while others did not confer methyltransferase activity. Notably, we observed H3K4 dimethylation levels comparable to wild type when human Ash2L replaced yeast Bre2 but reduced levels of H3K4 trimethylation with this same chimeric complex. Together, these data represent a proof of concept for simplifying the study of this clinically important protein complex in a tractable in vivo system, and also offer mechanistic insight into the functional role of a catalytically essential accessory protein within the MLL1 complex.Support or Funding InformationThis research was supported by grants from the National Center for Research Resources (5 P20 RR016461) and the National Institute of General Medical Sciences (8 P20 GM103499) from the National Institutes of Health.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.