In all organisms, the fundamental process of transcriptional regulation requires transcription factors, which bind to DNA in response to extra-cellular signals and regulate transcription of target genes. In eukaryotes, this process also involves epigenetic regulation, which includes DNA and histone modifications. Hematopoiesis and leukemia are excellent model systems for studying the higher eukaryotic regulations of gene expression and for identifying important molecules involved in genetic and epigenetic transcriptional regulation. The Mixed-Lineage Leukemia (MLL) protein, a Set1-like H3K4 methyltransferase, and the heterodimeric transcriptional factor AML1/CBFβ are critical for definitive and adult hematopoiesis. They are required for the generation of all hematopoietic lineages and act as tumor suppressors in human leukemia. We have previously shown that the regulation of PU.1 by AML1 is mediated by 3 AML1 binding sites in the PU.1 upstream regulatory element (URE), located −14 kb relative to the transcription start site in mice (Huang et al. Nat Genet. 2008). To understand whether AML1 plays a critical role in regulating the PU.1 locus at the chromatin level, we have utilized this PU.1 regulation system as a model to study the potential interplay between AML1/CBFβ and MLL. We found that MLL binds to the evolutionarily conserved Runt-domain of AML1, enhances the formation of the AML1/CBFβ heterodimer, and blocks the ubiquitin-proteasome mediated degradation of AML1. We also found that AML1/CBFβ is required for maintenance of the H3K4-me3 histone mark at the PU.1 upstream regulatory element (URE) and promoter region, and that MLL is required for AML1 to regulate PU.1 expression. In contrast, we found that several leukemia-associated MLL fusion proteins, including MLL-AF4, MLL-AF9, MLL-ENL, and MLL-AF10, no longer stabilize AML1 but rather induce AML1 degradation and downregulate PU.1 expression. Taken together, our data indicate that MLL and some of its leukemia-associated fusion proteins physically and functionally interact with AML1. Their effects on target genes, particularly PU.1, may be critical for the normal regulation of hematopoiesis and for the aberrant regulation that underlies acute myelogenous leukemia (AML) and acute lymphoblastic leukemia (ALL).

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