A Study Of The Pathogenic Mechanism Of Mixed Lineage Leukemia (MLL)

Mixed lineage leukemia (MLL), caused by MLL fusion protein by11q23chromosome translocation, is one of the acute leukemia with highly malignancy and bad outcome. MLL leukemia is of particular interest due to its unique clinical and biological characteristics. MLL translocations occur in up to70%of infant leukemias, and about10%of adult acute myelogenous leukemias (AML). It is of great significance to understand the pathogenic mechanism of MLL leukemia.We studied MLL leukemia in its fusion protein and its cooperative mutations, respectively. Both RUNX1and MLL1genes encode transcriptional regulators that are essential for normal hematopoietic stem/progenitor cells (HSPC) function. RUNX1/CBFβ is a heterodimeric transcription factor complex that is frequently involved in acute leukemia, it recruits MLL to regulate downstream target genes. Wildtype MLL protein interacts with RUNX1and prevents RUNX1from ubiquitin-mediated degradation, however, the functional consequence of MLL fusions on RUNX1/CBFβ activity has not been fully understood. So we explored the relationship between MLL fusion protein and RUNX1/CBFβ protein complex, we found that the N-terminal MLL portion of MLL fusions downregulate RUNX1and CBFβ protein level via the MLL CXXC domain and flanking regions. Overexpression of RUNX1inhibited the development of AML in Mll-Af9knock-in mice; conversely, further reducing Runxl/Cbfβ levels accelerated MLLAF9-mediated AML in bone marrow transplantation assays.On the other hand, MLL translocation is not sufficient for the fully development of MLL leukemia. To study the coorpatative mutations contribute to MLL leukemia, we used whole-genome sequencing of a pair of monozygotic twins discordant for MLL gene-rearranged leukemia, identified a transforming MLL-NRIP3fusion gene and biallelic mutations in SETD2(encoding a histone H3K.36methyltransferase). Moreover, loss-of-function point mutations in SETD2were recurrent (6.2%) in241patients with acute leukemia and were associated with multiple major chromosomal aberrations. We observed a global loss of H3K36trimethylation (H3K36me3) in leukemic blasts with mutations in SETD2. In the presence of a genetic lesion, downregulation of SETD2contributed to both initiation and progression during leukemia development by promoting the self-renewal potential of leukemia stem cells. RNA-Seq analysis of Setd2knock down cells reveals low expression of Setd2can up-regulate important signaling pathway such as mTOR, Jak-Stat signaling pathway, and Setd2knock down cells are more sensitive to mTOE inhibitors.Therefore, these data reveal a newly defined negative regulation of RUNX1/CBFβ by MLL fusion proteins and suggest that targetingRUNX1/CBFβ levels maybe a potential therapy for MLLs. Our study also provides compelling evidence for SETD2as a new tumor suppressor. Disruption of the SETD2-H3K36me3pathway is a distinct epigenetic mechanism for leukemia development. Overall, our work reveals the pathogenic mechanism of MLL leukemia, and provides new thought for therapy.