The transcription factor CCAAT/enhancer binding protein alpha, or C/EBPalpha, encoded by the CEBPA gene, is crucial for the differentiation of granulocytes. Conditional expression of C/EBPalpha triggers neutrophilic differentiation, and Cebpa knockout mice exhibit an early block in maturation. Dominant-negative mutations of CEBPA have been found in some patients with acute myeloid leukemia (AML), but not in AML with the t(8;21) translocation which gives rise to the fusion gene RUNX1-CBF2T1 (also known as AML1-ETO) encoding the AML1-ETO fusion protein. RUNX1-CBF2T1 positive-AML blasts had eight-fold lower CEBPA RNA levels and undetectable C/EBPalpha protein levels compared with other subgroups of AML patients. Conditional expression of RUNX1-CBF2T1 in U937 cells downregulated CEBPA mRNA, protein and DNA binding activity. AML1-ETO appears to suppress C/EBPalpha expression indirectly by inhibiting positive autoregulation of the CEBPA promoter. Conditional expression of C/EBPalpha in AML1-ETO-positive Kasumi-1 cells results in neutrophilic differentiation. We suggest that restoring C/EBPalpha expression will have therapeutic implications in RUNX1-CBF2T1-positive leukemias.

The t(8;21) is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). The translocation, which involves the AML1 gene on chromosome 21 and the ETO gene on chromosome 8, generates an AML1-ETO fusion transcription factor. To examine the effect of the AML1-ETO fusion protein on leukemogenesis, we made transgenic mice in which expression of AML1-ETO is under the control of the human MRP8 promoter (hMRP8-AML1-ETO). AML1-ETO is specifically expressed in myeloid cells, including common myeloid progenitors of hMRP8-AML1-ETO transgenic mice. The transgenic mice were healthy during their life spans, suggesting that AML1-ETO alone is not sufficient for leukemogenesis. However, after treatment of newborn hMRP8-AML1-ETO transgenic mice and their wild-type littermates with a strong DNA-alkylating mutagen, N-ethyl-N-nitrosourea, 55% of transgenic mice developed AML and the other 45% of transgenic mice and all of the wild-type littermates developed acute T lymphoblastic leukemia. Our results provide direct evidence that AML1-ETO is critical for causing myeloid leukemia, but one or more additional mutations are required for leukemogenesis. The hMRP8-AML1-ETO-transgenic mice provide an excellent model that can be used to isolate additional genetic events and to further understand the molecular pathogenesis of AML1-ETO-related leukemia.T he acute myeloid leukemia (AML)-1 gene (AML1, also known as CBFA2, PEBP2␣B, and RUNX1) was initially identified as a target of chromosomal translocation in t(8;21), which is associated with Ϸ15% of AML (1-3). This translocation involves the AML1 gene on chromosome 21 and the ETO (MTG8) gene on chromosome 8, and generates an AML1-ETO fusion transcription factor (4). This fusion protein consists of the N terminus of AML1 fused to a nearly full-length ETO protein (4). Native AML1 is able to form a heterodimer with CBF␤ (PEBP2␤) and regulate the transcription of target genes by binding to the DNA sequence TGT͞cGGT through its runt homology domain (5-7). Subsequently, AML1 was also found to be disrupted by several other translocations, including AML1-Evi1 from t(3;21) in blast crises of chronic myeloid leukemia and in therapy-related AML (8, 9); TEL-AML1 from t(12;21), which is involved in Ϸ25% of childhood pre-B cell acute lymphoblastic leukemia (10); AML1-MTG16 from t(16;21) in rare cases of AML (11); and in radiation-associated AML (12). Furthermore, the function of AML1 is disrupted indirectly by the inv(16) that is found in 12-15% of AML cases (13). The inv(16) fuses MYH11, a smooth muscle myosin heavy chain gene, to the gene that encodes core-binding factor ␤ (CBF␤), an AML1 heterodimeric partner. Thus, translocations targeting the AML1͞ CBF␤ transcription factor complex are among the most frequent mutations in human acute leukemia.Although in vitro studies have revealed the oncogenic potential of the AML1-ETO fusion gene, they do not fully represent the molecular pathogenesis of AML. Therefore, we and other groups have developed mouse models with the AML...