RUNX1 mutations promote leukemogenesis of myeloid malignancies in ASXL1-mutated leukemia

BACKGROUND: Additional sex combs-like 1 (ASXL1) mutations have been described in all forms of myeloid neoplasms including chronic myelomonocytic leukemia (CMML) and associated with inferior outcomes, yet the molecular pathogenesis of ASXL1 mutations (ASXL1-MT) remains poorly understood. Transformation of CMML to secondary AML (sAML) is one of the leading causes of death in CMML patients. Previously, we observed that transcription factor RUNX1 mutations (RUNX1-MT) coexisted with ASXL1-MT in CMML and at myeloid blast phase of chronic myeloid leukemia. The contribution of RUNX1 mutations in the pathogenesis of myeloid transformation in ASXL1-mutated leukemia, however, remains unclear. METHODS: To evaluate the leukemogenic role of RUNX1-MT in ASXL1-mutated cells, we co-expressed RUNX1-MT (R135T) and ASXL1-MT (R693X) in different cell lines and performed immunoblot, co-immunoprecipitation, gene expression microarray, quantitative RT-PCR, cell proliferation, differentiation, and clonogenic assays for in vitro functional analyses. The in vivo effect was investigated using the C57BL/6 mouse bone marrow transplantation (BMT) model. RESULTS: Co-expression of two mutant genes increased myeloid stem cells in animal model, suggesting that cooperation of RUNX1 and ASXL1 mutations played a critical role in leukemia transformation. The expression of RUNX1 mutant in ASXL1-mutated myeloid cells augmented proliferation, blocked differentiation, and increased self-renewal activity. At 9 months post-BMT, mice harboring combined RUNX1 and ASXL1 mutations developed disease characterized by marked splenomegaly, hepatomegaly, and leukocytosis with a shorter latency. Mice transduced with both ASXL1 and RUNX1 mutations enhanced inhibitor of DNA binding 1 (ID1) expression in the spleen, liver, and bone marrow cells. Bone marrow samples from CMML showed that ID1 overexpressed in coexisted mutations of RUNX1 and ASXL1 compared to normal control and either RUNX1-MT or ASXL1-MT samples. Moreover, the RUNX1 mutant protein was more stable than WT and increased HIF1-α and its target ID1 gene expression in ASXL1 mutant cells. CONCLUSION: The present study demonstrated the biological and functional evidence for the critical role of RUNX1-MT in ASXL1-mutated leukemia in the pathogenesis of myeloid malignancies.