RUNX1 and CBFb-SMMHC transactivate target genes together in abnormal myeloid progenitors for leukemia development

Inversion of chromosome 16 is a consistent finding in patients with acute myeloid leukemia subtype M4 with eosinophilia, which generates a CBFB-MYH11 fusion gene. It is generally considered that CBFb-SMMHC, the fusion protein encoded by CBFB-MYH11, is a dominant negative repressor of RUNX1. However, recent findings challenge the RUNX1-repression model for CBFb-SMMHC–mediated leukemogenesis. To definitively address the role of Runx1 in CBFB-MYH11–induced leukemia, we crossed conditional Runx1 knockout mice (Runx1^f/f) with conditional Cbfb-MYH11 knockin mice (Cbfb1^/56M). On Mx1-Cre activation in hematopoietic cells induced by poly (I:C) injection, all Mx1-CreCbfb1^/56M mice developed leukemia in 5 months, whereas no leukemia developed in Runx1^f/fMx1-CreCbfb1^/56M mice, and this effect was cell autonomous. Importantly, the abnormal myeloid progenitors (AMPs), a leukemia-initiating cell population induced by Cbfb-MYH11 in the bone marrow, decreased and disappeared in Runx1^f/fMx1-CreCbfb1/56M mice. RNA-seq analysis of AMP cells showed that genes associated with proliferation, differentiation blockage, and leukemia initiation were differentially expressed between Mx1-CreCbfb1^/56M and Runx1^f/fMx1-CreCbfb1^/56M mice. In addition, with the chromatin immunocleavage sequencing assay, we observed a significant enrichment of RUNX1/ CBFb-SMMHC target genes in Runx1^f/fMx1-CreCbfb1^/56M cells, especially among downregulated genes, suggesting that RUNX1 and CBFb-SMMHC mainly function together as activators of gene expression through direct target gene binding. These data indicate that Runx1 is indispensable for Cbfb-MYH11–induced leukemogenesis by working together with CBFb-SMMHC to regulate critical genes associated with the generation of a functional AMP population.