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...

The fusion gene AML1-ETO is the product of t(8;21)(q22;q22), one of the most common chromosomal translocations associated with acute myeloid leukemia. To investigate the impact of AML1-ETO on hematopoiesis, tetracycline-inducible AML1-ETO-expressing cell lines were generated using myeloid cells. AML1-ETO is tightly and strongly induced upon tetracycline withdrawal. The proliferation of AML1-ETO ؉ cells was markedly reduced, and most of the cells eventually underwent apoptosis. RNase protection assays revealed that the amount of Bcl-2 mRNA was decreased after AML1-ETO induction. Enforced expression of Bcl-2 was able to significantly delay, but not completely overcome, AML1-ETO-induced apoptosis. Prior to the onset of apoptosis, we also studied the ability of AML1-ETO to modulate differentiation. AML1-ETO expression altered granulocytic differentiation of U937T-A/E cells. More significantly, this change of differentiation was associated with the down-regulation of CCAAT/enhancer binding protein ␣ (C/EBP␣), a key regulator of granulocytic differentiation. These observations suggest a dichotomy in the functions of AML1-ETO: (i) reduction of granulocytic differentiation correlated with decreased expression of C/EBP␣ and (ii) growth arrest leading to apoptosis with decreased expression of CDK4, c-myc, and Bcl-2. We predict that the preleukemic AML1-ETO ؉ cells must overcome AML1-ETO-induced growth arrest and apoptosis prior to fulfilling their leukemogenic potential.Acute myeloid leukemia (AML) is a common hematopoietic malignancy characterized by the abnormal proliferation and differentiation of myeloid progenitor cells. The fusion gene AML1-ETO is the product of the t(8;21)(q22;q22) chromosomal translocation, which is associated with approximately 40% of cases with the M2 subtype of AML (8,10,30,44). The chimeric protein (AML1-ETO) contains the N terminus of the AML1 (RUNX1, CBF␣, PEBP2␣B) transcription factor and nearly the full-length ETO (MTG8) (39, 40). The AML1 moiety of the AML1-ETO fusion protein has the Drosophila melanogaster protein runt homology domain, which is required for DNA binding and interaction with its heterodimerization partner, CBF␤, but which lacks the transcriptional activation domain of AML1 (35,46,61,68,70). AML1 is a common target for chromosomal translocations and is involved in several other hematopoietic malignancies, such as the TEL-AML1 fusion observed in t(12;21), which is involved in approximately 25% of childhood pre-B-cell acute lymphoblastic leukemias (14, 56, 59); AML1-MTG16 from t(16;21) with AML1 fused to ETO-related gene MTG16 (11); and AML1-EVI1 from t(3;21) in lymphoblast crises evolving from chronic myelogenous leukemia and in therapy-related AML (37, 42, 43). These translocations point to a fundamental role of AML1 in hematopoiesis. Furthermore, analysis of AML1 knockout mice demonstrates that AML1 is a crucial factor for definitive hematopoiesis (48, 67). The ETO moiety of AML1-ETO is able to recruit the nuclear receptor corepressor (N-CoR)-mammalian Sin3 (mSin3)-histone...