Transcriptional activation by MLL fusion proteins in leukemogenesis.

Chromosomal translocations involving the mixed lineage leukemia (MLL) gene cause aggressive leukemia. Fusion proteins of MLL and a component of the AF4 family/ENL family/P-TEFb complex (AEP) are responsible for two-thirds of MLL-associated leukemia cases. MLL-AEP fusion proteins trigger aberrant self-renewal of hematopoietic progenitors by constitutively activating self-renewal-related genes. MLL-AEP fusion proteins activate transcription initiation by loading the TATA-binding protein (TBP) to the TATA element via selectivity factor 1. Although AEP retains transcription elongation and mediator recruiting activities, the rate-limiting step activated by MLL-AEP fusion proteins appears to be the TBP-loading step. This is contrary to prevailing views, in which the recruitment of transcription elongation activities are emphasized. Here, I review recent advances towards elucidating the mechanisms underlying gene activation by MLL-AEP fusion proteins in leukemogenesis.

Yokoyama A 《-》

TBP loading by AF4 through SL1 is the major rate-limiting step in MLL fusion-dependent transcription.

Okuda H ，Takahashi S ，Takaori-Kondo A ，Yokoyama A ... -  《-》

Molecular mechanisms of MLL-associated leukemia.

Yokoyama A 《-》

AF4 uses the SL1 components of RNAP1 machinery to initiate MLL fusion- and AEP-dependent transcription.

Okuda H ，Kanai A ，Ito S ，Matsui H ，Yokoyama A ... -  《Nature Communications》

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.

Yokoyama A 《-》