Enhancers control cell type-specific gene expression and are marked by H3K4me1. Active enhancers are further marked by H3K27ac. We identified MLL3/MLL4 as major H3K4me1 methyltransferases and CBP/p300 as the H3K27 acetyltransferases in mammalian cells (EMBO J 2011; eLife 2013). During differentiation of adipocytes, myocytes and ES cells, MLL3/MLL4 co-localize with lineage-determining transcription factors (LDTFs), CBP/p300, Brd4 and MED1 on active enhancers. MLL3/MLL4 are required for enhancer activation, enhancer-promoter interaction, cell type-specific gene expression, and cell differentiation (eLife 2013; PNAS 2016; Nat Comm 2017). MLL3/MLL4 control cell differentiation by orchestrating CBP/p300-mediated enhancer activation (NAR 2017). Ectopic expression of H3.3K4M, an inhibitor of H3K4 methylation, or deletion of the enzymatic SET domain, destabilizes MLL3/MLL4 proteins, prevents enhancer activation, and impairs cell differentiation and tissue development (NAR 2019).
Using proteomic approaches, we found that endogenous MLL4 complex associates with SWI/SNF chromatin remodeling complex components in cells. Using adipogenesis as a model system, we establish an interdependent relationship between SWI/SNF complex BAF and MLL4 in promoting cell type-specific enhancer activation by LDTFs, which rectifies seemingly conflicting results from previous studies (Nat Comm 2021).
We showed that MLL4 protein, rather than MLL4-mediated H3K4me1, controls p300 recruitment to enhancers during ES cell differentiation, suggesting that MLL4 regulates enhancer activation independent of H3K4me1 (PNAS 2016). We have generated enzyme-dead MLL3/MLL4 double knockin ES cells and mice by CRISPR. Initial analyses indicate that MLL3/MLL4 regulate ES cell differentiation and mouse embryonic development through both enzymatic activity-dependent and -independent mechanisms.
Supported by the Intramural Research Program of NIDDK, NIH.
