Enhancer regulation by H3K4 methyltransferases MLL3/MLL4

Enhancers control cell type‐specific gene expression and are marked by H3K4me1. Active enhancers are further marked by H3K27ac. We previously identified CBP/p300 as major H3K27 acetyltransferases and MLL3/4 as major enhancer H3K4me1 methyltransferases ( EMBO J 2011; eLife 2013 ). During differentiation of adipocytes, myocytes and ES cells, MLL3/4 co‐localizes with lineage‐determining TFs (LDTFs) on active enhancers and are required for enhancer activation, cell type‐specific gene expression, and cell differentiation ( eLife 2013; PNAS 2016 ). Further, we show that while enhancer‐priming by MLL3/4 is dispensable for cell identity maintenance, it controls cell fate transition by orchestrating CBP/p300‐mediated enhancer activation. MLL4 protein, rather than MLL4‐mediated H3K4me1, controls p300 recruitment to enhancers, suggesting that MLL4 may regulate enhancer activation independent of its enzymatic activity ( PNAS 2016; NAR 2017) . Our recent findings identify Brd4 as an enhancer epigenomic reader that links active enhancers with cell identity gene induction in differentiation. Our data suggest a model of sequential actions of epigenomic regulators on enhancers: 1) pioneer TFs and LDTFs recruit MLL3/4 to prime enhancer regions and label them with H3K4me1; 2) MLL3/4 facilitate the binding of CBP/p300, which activate enhancers and label them with H3K27ac; 3) H3K27ac and acetylated TFs are recognized by Brd4, which recruits Mediator and RNA Pol II to establish enhancerpromoter interactions and activate cell type‐specific gene expression ( Nat Comm, 2017 ). To investigate the role of MLL3/4‐mediated H3K4me1 in enhancer activation and cell differentiation, we have generated two mouse lines: one expressing H3.3K4M, a lysine‐4‐to‐methionine (K4M) mutation of histone H3.3 that inhibits H3K4 methylation, and the other carrying conditional double knockout of MLL3/4 enzymatic SET domain. Unexpectedly, expression of H3.3K4M, or deletion of the SET domain, in lineage‐specific precursor cells destabilizes MLL3/4, prevents enhancer activation, and impairs cell differentiation and tissue development ( NAR 2018) . We are using CRISPR to generate enzyme‐dead knockin mutants of MLL3/4 in mice. The phenotypes of these mice will be reported at the meeting. By immunoprecipitation followed by mass spectrometry analysis, we have found that endogenous MLL3/4 complex interacts with SWI/SNF chromatin remodeling complex in cells. The biological significance of such physical interactions will be reported at the meeting. Support or Funding Information Supported by the Intramural Research Program of NIDDK, NIH. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .