[The mechanism of MLL-rearranged leukemogenesis and its targeted therapies]

Leukemia is caused by uncontrolled proliferation of immature hematopoietic progenitors. MLL fusion proteins, generated by chromosomal translocations, activate a broad range of previously transcribed genes to achieve the same expression profile as that of the parent cell in the daughter cells, thereby promoting self-renewal. Normally, replication of the expression profile only occurs in the hematopoietic stem cells (HSCs). A transactivation system comprised of MLL and AF4/ENL/P-TEFb (AEP) complexes promotes it by reactivating CpG-rich promoters. In the normal hematopoietic development, this system is tightly regulated and progressively suppressed during the course of hematopoietic differentiation so that non-HSC hematopoietic cells would not self-renew. Genetic mutations such as fusions of MLL and AEP components generate a constitutively active form of the MLL transcriptional machinery, which aberrantly promotes self-renewal even in non-HSC hematopoietic cells. In this review, I depict a molecular mechanism of MLL fusion-mediated leukemogenesis from a standpoint that leukemogenesis is driven by aberrant self-renewal that is mediated by hyper-active transcriptional machinery, and introduce several molecularly targeted therapies in the making which specifically perturb this transactivation system.