RUNX1/EVI1 That Blocks Myeloid Differentiation Inhibits C/EBPα Function.

RUNX1/EVI1 chimeric transcription factor produced by t(3;21) causes leukemic transformation in hematopoietic stem cell tumors such as chronic myelogenous leukemia (CML) in blastic crisis and myelodysplastic syndrome (MDS) in leukemic transformation, possibly through differentiation block of malignant myeloid progenitors. We have recently reported that Runx1/EVI1 knock-in heterozygous mice show defective hematopoiesis in the fetal liver similar to Runx1 knock-out mice, but possess dysplastic hematopoietic progenitors with high self-renewal capacity. Notably, Runx1/EVI1 knock-in chimeric mice developed acute megakaryoblastic leukemia. The molecular characterization of RUNX1/EVI1 points two major functions; one is dominant-suppressive function over wild-type RUNX1 and the other is EVI1’s own function; blockade of TGFb-mediated signal, inhibition of JNK and stimulation of AP-1 activity. C/EBPa is a key transcriptional regulator that induces the granulocytic differentiation of myeloid progenitors and several lines of evidence suggest that disturbance in C/EBPa signaling is one of the major molecular events in myeloid malignancies. In this study, we investigated whether RUNX1/EVI1 affects the expression and function of C/EBPa. We introduced RUNX1/EVI1 cDNA into LG-3 cells that differentiate along the myeloid lineage upon granulocyte colony-stimulating factor exposure, and confirmed that RUNX1/EVI1 suppressed the differentiation. To further investigate the molecular mechanisms of RUNX1/EVI1-mediated differentiation block, we analyzed RUNX1/EVI1’s effect on the functions of C/EBPa. RUNX1/EVI1 was found to associate with C/EBPa. By using the reporter assay with the CEBPA promoter, we observed a dominant-negative effect of RUNX1/EVI1 over C/EBPa-mediated transcriptional activation via the CtBP-binding site in the EVI1 portion. In the gel-shift assay, RUNX1/EVI1 down-regulated DNA-binding activity of C/EBPa. Therefore, recruitment of histone deacetylase via CtBP and disruption of DNA binding could be likely scenarios for the RUNX1/EVI1-induced dominant repression on C/EBPa. Importantly, co-expression of C/EBPa restored differentiation ability of the RUNX1/EVI1-expressing LG-3 cells. All these data argue that inhibition of C/EBPa function may be causatively related to the RUNX1/EVI1’s leukemogenic potential.