Histone lysine-specific methyltransferase 2 (KMT2A-D) proteins, alternatively called mixed lineage leukemia (MLL1-4) proteins, mediate positive transcriptional memory. Acting as the catalytic subunits of human COMPASS-like complexes, KMT2A-D methylate H3K4 at promoters and enhancers. KMT2A-D contain understudied highly conserved triplets and a quartet of plant homeodomains (PHDs). Here, we show that all clustered (multiple) PHDs localize to the well-defined loci of H3K4me3 and H3 acetylation-rich active promoters and enhancers. Surprisingly, we observe little difference in binding pattern between PHDs from promoter-specific KMT2A-B and enhancer-specific KMT2C-D. Fusion of the KMT2A CXXC domain to the PHDs drastically enhances their preference for promoters over enhancers. Hence, the presence of CXXC domains in KMT2A-B, but not KMT2C-D, may explain the promoter/enhancer preferences of the full-length proteins. Importantly, targets of PHDs overlap with KMT2A targets and are enriched in genes involved in the cancer pathways. We also observe that PHDs of KMT2A-D are mutated in cancer, especially within conserved folding motifs (Cys4HisCys2Cys/His). The mutations cause a domain loss-of-function. Taken together, our data suggest that PHDs of KMT2A-D guide the full-length proteins to active promoters and enhancers, and thus play a role in positive transcriptional memory. Graphical Abstract

Histone lysine methyltransferase (KMT2) proteins form the core of COMPASS and COMPASS-like complexes that mediate transcriptional memory by methylating H3K4 at promoters and enhancers. KMT2A-D proteins, alternatively called mixed lineage leukaemia proteins (MLL1-4), contain highly conserved unique triplet and quartet of plant homeodomains (PHDs). Here, we show that clustered PHDs, expressed in isolation in HeLa cells, localize to well-defined loci of acetylation-rich active promoters and enhancers. Binding sites overlap with targets of full-length KMT2A (MLL1) and the COMPASS-like subunit WDR5, RbBP5 and with cell cycle and cancer-related genes. COSMIC data identify frequent variations in the PHDs of KMT2 proteins, particularly KMT2C, in a wide spectrum of malignancies. Changes are enriched at conserved positions within the PHDs, indicating that they cause loss-of-function mutations. Taken together, the biochemical and cancer data suggest that the PHDs contribute to KMT2A-D targeting to active promoters and enhancers.