Leukemogenesis via aberrant self‐renewal by the MLL/AEP‐mediated transcriptional activation system

Homeostasis of the hematopoietic system is achieved in a hierarchy, with hematopoietic stem cells at the pinnacle. Because only hematopoietic stem cells (HSCs) can self‐renew, the size of the hematopoietic system is strictly controlled. In hematopoietic reconstitution experiments, 1 HSC can reconstitute the entire hematopoietic system, whereas 50 multipotent progenitors cannot. This indicates that only HSCs self‐renew, whereas non‐HSC hematopoietic progenitors are programmed to differentiate or senesce. Oncogenic mutations of the mixed lineage leukemia gene (MLL) overcome this “programmed differentiation” by conferring the self‐renewing ability to non‐HSC hematopoietic progenitors. In leukemia, mutated MLL proteins constitutively activate a broad range of previously transcribed CpG‐rich promoters by an MLL‐mediated transcriptional activation system. This system promotes self‐renewal by replicating an expression profile similar to that of the mother cell in its daughter cells. In this transcriptional activation system, MLL binds to unmethylated CpG‐rich promoters and recruits RNA polymerase II. MLL recruits p300/CBP through its transcriptional activation domain, which acetylates histone H3 at lysines 9, 18, and 27. The AF4 family/ENL family/P‐TEFb complex (AEP) binds to acetylated H3K9/18/27 to activate transcription. Gene rearrangements of MLL with AEP‐ or CBP/p300‐complex components generate constitutively active transcriptional machinery of this transcriptional activation system, which causes aberrant self‐renewal of leukemia stem cells. Inhibitors of the components of this system effectively decrease their leukemogenic potential.