A small UTX stabilization domain of Trr is conserved within mammalian MLL3-4/COMPASS and is sufficient to rescue loss of viability in null animals

Catalytic-inactivating mutations within the Drosophila enhancer H3K4 monomethyltransferase Trr and its mammalian homologs, MLL3/4, cause only minor changes in gene expression compared to whole-gene deletions for these COMPASS members. To identify essential histone methylatransferase-independent functions of Trr, we screened to identify a minimal Trr domain sufficient to rescue Trr-null lethality and demonstrate that this domain binds and stabilizes Utx in vivo. Using the homologous MLL3/MLL4 human sequences, we mapped a short ~80 amino acid UTX-Stabilization-Domain (USD) that promotes UTX stability in the absence of the rest of MLL3/4. Nuclear UTX stability is enhanced when the USD is fused with the MLL4 HMG-box. Thus, COMPASS-dependent UTX stabilization is an essential non-catalytic function of Trr/MLL3/MLL4, suggesting that stabilizing UTX could be a therapeutic strategy for cancers with MLL3/4 loss-of-function mutations. Overall design: ChIP-seq for UTX and H3K27me3 and RNA-seq in MLL4 knockout HCT116 cells with and without doxycyclne-induced expression of a small rescue domain of MLL4.