A previously unidentified alternatively spliced isoform of t(8;21) transcript promotes leukemogenesis

Complementing mutations in core binding factor leukemias: from mouse models to clinical applications

A great proportion of acute myeloid leukemias (AMLs) display cytogenetic abnormalities including chromosomal aberrations and/or submicroscopic mutations. These abnormalities significantly influence the prognosis of the disease. Hence, a thorough genetic work-up is an essential constituent of standard diagnostic procedures. Core binding factor (CBF) leukemias denote AMLs with chromosomal aberrations disrupting one of the CBF transcription factor genes; the most common examples are translocation t(8;21) and inversion inv(16), which result in the generation of the AML1-ETO and CBFbeta-MYH11 fusion proteins, respectively. However, in murine models, these alterations alone do not suffice to generate full-blown leukemia, but rather, complementary events are required. In fact, a substantial proportion of primary CBF leukemias display additional activating mutations, mostly of the receptor tyrosine kinase (RTK) c-KIT. The awareness of the impact and prognostic relevance of these 'second hits' is increasing with a wider range of mutations tested in clinical trials. Furthermore, novel agents targeting RTKs are emanating rapidly and entering therapeutic regimens. Here, we present a concise review on complementing mutations in CBF leukemias including pathophysiology, mouse models, and clinical implications.

t(8;21)(q22;q22) Fusion proteins preferentially bind to duplicated AML1/RUNX1 DNA-binding sequences to differentially regulate gene expression

Chromosome abnormalities are frequently associated with cancer development. The 8;21(q22;q22) chromosomal translocation is one of the most common chromosome abnormalities identified in leukemia. It generates fusion proteins between AML1 and ETO. Since AML1 is a well-defined DNA-binding protein, AML1-ETO fusion proteins have been recognized as DNA-binding proteins interacting with the same consensus DNA-binding site as AML1. The alteration of AML1 target gene expression due to the presence of AML1-ETO is related to the development of leukemia. Here, using a 25-bp random double-stranded oligonucleotide library and a polymerase chain reaction (PCR)-based DNA-binding site screen, we show that compared with native AML1, AML1-ETO fusion proteins preferentially bind to DNA sequences with duplicated AML1 consensus sites. This finding is further confirmed by both in vitro and in vivo DNA-protein interaction assays. These results suggest that AML1-ETO fusion proteins have a selective preference for certain AML1 target genes that contain multimerized AML1 consensus sites in their regulatory elements. Such selected regulation provides an important molecular mechanism for the dysregulation of gene expression during cancer development.

Identification and characterization of novel AML1-ETO fusion transcripts in pediatric t(8;21) acute myeloid leukemia: a report from the Children's Oncology Group

t(8;21)(q22;q22) results in the AML1-ETO (A1E) fusion gene and is a common cytogenetic abnormality in acute myeloid leukemia (AML). Although insertions at the breakpoint region of the A1E fusion transcripts have been reported, additional structural alterations are largely uncharacterized. By RT-PCR amplifications and DNA sequencing, numerous in-frame and out-of-frame AML1b-ETO and AML1c-ETO transcripts were identified in 13 pediatric t(8;21) AMLs, likely resulting from alternate splicing, internal deletions and/or breakpoint region insertions involving both the AML1 (RUNX1) and ETO regions. The in-frame A1E fusion transcript forms represented minor forms. These structure alterations were found in AML1c-ETO but not AML1b-ETO transcripts in two adult t(8;21) AMLs. Although no analogous alterations were detected in native AML1b transcripts, identical alterations in native ETO transcripts were identified. When transfected into HeLa cells, only AML1b, and not the in-frame A1E forms, transactivated the GM-CSF promoter. In co-transfection experiments, the effects of A1E proteins on GM-CSF transactivation by AML1b ranged from repressive to activating. Our results demonstrate a remarkable and unprecedented heterogeneity in A1E fusion transcripts in t(8;21) myeloblasts and suggest that synthesis of alternate A1E transcript and protein forms can significantly impact the regulation of AML1 responsive genes.

ETO, but not leukemogenic fusion protein AML1/ETO, augments RBP-Jkappa/SHARP-mediated repression of notch target genes

Notch is a transmembrane receptor that determines cell fates and pattern formation in all animal species. After specific ligand binding, the intracellular part of Notch is cleaved off and translocates to the nucleus, where it targets the DNA binding protein RBP-Jkappa. In the absence of Notch, RBP-Jkappa represses Notch target genes by recruiting a corepressor complex. We and others have previously identified SHARP as one component of this complex. Here, we show that the corepressor ETO as well as the leukemogenic fusion protein AML1/ETO directly interacts with SHARP, that ETO is part of the endogenous RBP-Jkappa-containing corepressor complex, and that ETO is found at Notch target gene promoters. In functional assays, corepressor ETO, but not AML1/ETO, augments SHARP-mediated repression in an histone deacetylase-dependent manner. Furthermore, either the knockdown of ETO or the overexpression of AML1/ETO activates Notch target genes. Therefore, we propose that AML1/ETO can disturb the normal, repressive function of ETO at Notch target genes. This activating (or derepressing) effect of AML1/ETO may contribute to its oncogenic potential in myeloid leukemia.

Review: genetic models of acute myeloid leukaemia

The use of genetically engineered mice (GEM) have been critical in understanding disease states such as cancer, and none more so than acute myelogenous leukaemia (AML), a disease characterized by over 100 distinct chromosomal translocations. A substantial proportion of cases exhibiting recurrent reciprocal translocations at diagnosis, such as t(8;21) or t(15;17) have been exhaustively studied and are currently employed in clinical diagnosis. However, a definitive conclusion regarding the leukaemogenic potential of defined transgenes for this disease remains elusive. While it is increasingly apparent that a number of cooperating mutations are necessary to develop a leukaemic phenotype, the number of models reflecting these synergisms remains few. Furthermore, little emphasis has been paid to the effect of chromosomal translocations other than recurrent genetic abnormalities, with no models reflecting the multiple abnormalities observed in high-risk cases of AML accounting for 8-10% of adult AML. Here we review the differing technologies employed in generation of GEM of AML. We discuss the relevance of GEM AML from embryonic stem cell-mediated (for example retinoic acid receptor-alpha fusions and AML1/ETO) models; through to the valuable retroviral-mediated gene transfer models. The latter have been used to great effect in defining the transforming properties of chromosomal translocation products such as MLL (found in 5-6% of all AML cases) and NUP98 (denoting poor prognosis in therapy-related disease) and particularly when co-transduced with bad prognostic factors such as Flt3 mutations. Finally, we comment on the emergence of newer transduction technologies, which can regulate the level of expression to defined cell lineages in both primary murine and human xenografts, and discuss how combining multiple genetic modalities, more relevant models of this complex disease are being generated.

Loss of TLE1 and TLE4 from the del(9q) commonly deleted region in AML cooperates with AML1-ETO to affect myeloid cell proliferation and survival

Deletions on chromosome 9q are seen in a subset of acute myeloid leukemia (AML) cases and are specifically associated with t(8;21) AML. We previously defined the commonly deleted region in del(9q) AML and characterized the genes in this interval. To determine the critical lost gene(s) that might cooperate with the AML1-ETO fusion gene produced by t(8;21), we developed a set of shRNAs directed against each gene in this region. Within this library, shRNAs to TLE1 and TLE4 were the only shRNAs capable of rescuing AML1-ETO expressing U937T-A/E cells from AML1-ETO-induced cell-cycle arrest and apoptosis. Knockdown of TLE1 or TLE4 levels increased the rate of cell division of the AML1-ETO-expressing Kasumi-1 cell line, whereas forced expression of either TLE1 or TLE4 caused apoptosis and cell death. Knockdown of Gro3, a TLE homolog in zebrafish, cooperated with AML1-ETO to cause an accumulation of noncirculating hematopoietic blast cells. Our data are consistent with a model in which haploinsufficiency of these TLEs overcomes the negative survival and antiproliferative effects of AML1-ETO on myeloid progenitors, allowing preleukemic stem cells to expand into AML. This study is the first to implicate the TLEs as potential tumor suppressor genes in myeloid leukemia.

Mechanism for ubiquitylation of the leukemia fusion proteins AML1-ETO and PML-RARalpha

The chromosomal translocation products AML1-ETO and PML-RARalpha contribute to the pathogenesis of leukemias. Here, we demonstrate that both AML1-ETO and PML-RARalpha are degraded by the ubiquitin-proteasome system and that their turnover critically depends on the E2-conjugase UbcH8 and the E3-ligase SIAH-1. Contrary to its role in HDAC2 degradation, the E3-ligase RLIM does not target AML1-ETO and PML-RARalpha for ubiquitin-dependent elimination. RLIM rather is a substrate of SIAH-1, which indicates that these E3-ligases operate in a hierarchical order. Remarkably, proteasomal degradation of leukemia fusion proteins, in addition to the block of histone deacetylase (HDAC) enzymatic activity is a consequence of HDAC-inhibitor treatment. The former requires the induction of UbcH8 expression and each of these processes might be beneficial for leukemia treatment. Our observations shed light on the mechanism determining the interplay between E2-conjugases, E3-ligases, and their substrates and suggest a strategy for utilizing the ubiquitylation machinery in a therapeutic setting.

Multi-sites cleavage of leukemogenic AML1-ETO fusion protein by caspase-3 and its contribution to increased apoptotic sensitivity

Leukemia-associated fusion protein AML1-ETO is a product of the chromosome translocation (8;21) frequently occurred in acute myeloid leukemia (AML). The fusion oncoprotein blocks leukemic cell differentiation, and it also induces growth arrest with increased sensitivity to apoptosis induction. Such dichotomous functions make it difficult to clarify the role of AML1-ETO in leukemogenesis. Here, we systematically showed that constitutively and overexpressed AML1-ETO protein was cleaved to four fragments of 70, 49, 40 and 25 kDa by activated caspase-3 during apoptosis induction by extrinsic mitochondrial and death receptor signaling pathways. The in vitro proteolytic system combined with MALDI-TOF/TOF mass spectrometer confirmed that AML1-ETO and wild-type ETO but not RUNX1 (AML1) proteins were direct substrates of apoptosis executioner caspase-3. Site-directed mutagenesis analyses identified two nonclassical aspartates (TMPD188 and LLLD368) as caspase-3-targeted sites in the AML1-ETO sequence. When these two aspartates were mutated into alanines, more intriguingly, the apoptosis-amplified action of AML1-ETO induction completely disappeared, while inducible expression of the caspase-3-cleaved 70 kDa fragment of AML1-ETO after tetracycline removal is sufficient to enhance apoptotic sensitivity. Further investigations on the potential in vivo effects of such a cleavage and its possible role in leukemogenesis would provide new insights for understanding the biology and treatment of AML1-ETO-associated leukemia.

Gatekeeper function of the RUNX1 transcription factor in acute leukemia

The RUNX1 gene encodes the alpha subunit of the core binding factor (CBF) and is a common target of genetic mutations in acute leukemia. We propose that RUNX1 is a gatekeeper gene, the disruption of which leads to the exodus of a subset of hematopoietic progenitors with increased self-renewal potential from the normal environmental controls of homeostasis. This pool of "escaped" cells is the target of secondary mutations, accumulating over time to induce the aggressive manifestation of acute leukemia. Evidence from patient and animal studies supports the concept that RUNX1 mutations are the initiating event in different leukemia subtypes, but also suggests that diverse mechanisms are used to subvert RUNX1 function. One common result is the inhibition of differentiation-but its effect impinges on different lineages and stages of differentiation, depending on the mutation or fusion partner. A number of different approaches have led to the identification of secondary events that lead to the overt acute phase; however, the majority is unknown. Finally, the concept of the "leukemia stem cell" and its therapeutic importance is discussed in light of the RUNX1 gatekeeper function.

From dissection of disease pathogenesis to elucidation of mechanisms of targeted therapies: leukemia research in the genomic era

Leukemia is a group of heterozygous diseases of hematopoietic stem/progenitor cells that involves dynamic change in the genome. Dissection of genetic abnormalities critical to leukemia initiation provides insights into the elusive leukemogenesis, identifies distinct subsets of leukemia and predicts prognosis individually, and can also provide rational therapeutic targets for curative approaches. The past three decades have seen tremendous advances in the analysis of genotype-phenotype connection of leukemia, and in the identification of molecular biomarkers for leukemia subtypes. Intriguingly, differentiation therapy, targeted therapy and chemotherapy have turned several subtypes of leukemia from highly fatal to highly curable. The use of all-trans retinoic acid and arsenic trioxide, which trigger degradation of PML-RARalpha, the causative fusion protein generated by t (15;17) translocation in acute promyelocytic leukemia (APL), has led to a dramatic improvement of APL clinical outcome. Imatinib mesylate/ Gleevec/STI571, which inhibits the tyrosine kinase activity of BCR-ABL oncoprotein, has now become the new gold standard for the treatment of chronic myeloid leukemia. Optimal use of chemotherapeutic agents together with a stringent application of prognostic factors for risk-directed therapy in clinical trials has resulted in a steady improvement in the treatment outcome of acute lymphoblastic leukemia. Hence, the pace of progress extrapolates to a prediction of leukemia control in the twenty-first century.

The multi-functional cellular adhesion molecule CD44 is regulated by the 8;21 chromosomal translocation

The 8;21 translocation is a common chromosomal abnormality in acute myeloid leukemia (AML). We recently identified a naturally occurring leukemogenic splice variant, AML1-ETO9a (acute myeloid leukemia-1 transcription factor and the eight-twenty-one corepressor-9a), of t(8;21). To understand the leukemic potential of AML1-ETO9a, we performed microarray analysis with the murine multipotential hematopoietic FDCP-mix A4 cell line. We identified changes in expression of various genes including CD44. CD44 is a type I transmembrane protein and functions as the major cellular adhesion molecule for hyaluronic acid, a component of the extracellular matrix. CD44 is expressed in most human cell types and is implicated in myeloid leukemia pathogenesis. We show that the presence of AML1-ETO9a significantly increased the expression of CD44 at both RNA and protein levels. Furthermore, the CD44 promoter is bound by AML1-ETO9a and AML1-ETO at the chromatin level. In addition, in the AML1-ETO9a leukemia mouse model CD44 is regulated in a cell context-dependent manner. Thus, our observations suggest that AML1-ETO and its splice variant AML1-ETO9a are able to regulate the expression of the CD44 gene, linking the 8;21 translocation to the regulation of a cell adhesion molecule that is involved in the growth and maintenance of the AML blast/stem cells.

Cell type dependent regulation of multidrug resistance-1 gene expression by AML1-ETO

The AML1-ETO fusion protein is generated from the 8;21 chromosome translocation that is commonly identified in acute myeloid leukemia. AML1-ETO is a DNA binding transcription factor and has been demonstrated to play a critical role in promoting leukemogenesis. Therefore, it is important to define the molecular mechanism of AML1-ETO in the regulation of gene expression. Here, we report that the effect of AML1-ETO on the promoter of multidrug resistance-1 (MDR1) gene, a known AML1-ETO target, is highly cell type specific. Besides observing repression of the MDR1 promoter in C33A and CV-1 cells as reported previously, AML1-ETO strongly activated the promoter in K562 and B210 cells. More importantly, this activation required both the AML1 and ETO portions of the fusion protein, but did not depend on the AML1 binding site in MDR1 promoter. Furthermore, results from promoter deletion analysis and chromatin immunoprecipitation assays suggested that this activation effect was likely through the influence of the general transcription machinery rather than promoter-specific factors. Based on these data, we propose that AML1-ETO may have opposing effects on gene expression depending on the various conditions of the cellular environment.

Leukemogenic AML1-ETO fusion protein increases carcinogen-DNA adduct formation with upregulated expression of cytochrome P450-1A1 gene

OBJECTIVE

AML1-ETO fusion protein is a product of chromosome translocation t(8;21) frequently occurred in acute myeloid leukemia (AML), but its sole expression appears to fail to cause overt leukemia in vivo. In this study, we investigated whether AML1-ETO expression impinged on action of chemical carcinogens-DNA adduct formation.

MATERIALS AND METHODS

AML1-ETO fusion protein was conditionally induced in engineered U937-A/E 9/14/18 cells. The formation of polycyclic aromatic hydrocarbon (PAH)-DNA adducts and the expression of PAH-metabolizing enzymes cytochrome P450 (CYP) 1A1 and arylhydrocarbon receptor (AhR) were detected by Western blot and/or quantitative RT-PCR. Luciferase reporter system was used to detect the regulation of AML1-ETO on CYP1A1 transcription.

RESULTS

Our results showed that AML1-ETO induction significantly increased the formation of carcinogen benzopyrene-DNA adducts in leukemic cells. In line with the effect, we also found that AML1-ETO induction upregulated CYP1A1 expression, which was dependent on AML1-binding motif in the promotor of CYP1A1 gene. Additionally, AML1-ETO protein also increased AhR expression, a ligand-activated transcription factor that mediates PAHs-induced CYP1A1 gene expression.

CONCLUSION

These data, combined with its inhibitory effect on DNA repair as reported previously, propose that the presence of AML1-ETO increases the susceptibility of cells to chemical carcinogens, which favors the development of additional genetic alterations.

Structural basis for recognition of SMRT/N-CoR by the MYND domain and its contribution to AML1/ETO's activity

AML1/ETO results from the t(8;21) associated with 12%-15% of acute myeloid leukemia. The AML1/ETO MYND domain mediates interactions with the corepressors SMRT and N-CoR and contributes to AML1/ETO's ability to repress proliferation and differentiation of primary bone marrow cells as well as to enhance their self renewal in vitro. We solved the solution structure of the MYND domain and show it to be structurally homologous to the PHD and RING finger families of proteins. We also determined the solution structure of an MYND-SMRT peptide complex. We demonstrated that a single amino acid substitution that disrupts the interaction between the MYND domain and the SMRT peptide attenuated AML1/ETO's effects on proliferation, differentiation, and gene expression.

Gene-expression profiling identifies distinct subclasses of core binding factor acute myeloid leukemia

Core binding factor (CBF) leukemias, characterized by either inv(16)/t(16;16) or t(8;21), constitute acute myeloid leukemia (AML) subgroups with favorable prognosis. However, there exists substantial biologic and clinical heterogeneity within these cytogenetic groups that is not fully reflected by the current classification system. To improve the molecular characterization we profiled gene expression in a large series (n = 93) of AML patients with CBF leukemia [(inv (16), n = 55; t(8;21), n = 38)]. By unsupervised hierarchical clustering we were able to define a subgroup of CBF cases (n = 35) characterized by shorter overall survival times (P = .03). While there was no obvious correlation with fusion gene transcript levels, FLT3 tyrosine kinase domain, KIT, and NRAS mutations, the newly defined inv(16)/t(8;21) subgroup was associated with elevated white blood cell counts and FLT3 internal tandem duplications (P = .011 and P = .026, respectively). Supervised analyses of gene expression suggested alternative cooperating pathways leading to transformation. In the "favorable" CBF leukemias, antiapoptotic mechanisms and deregulated mTOR signaling and, in the newly defined "unfavorable" subgroup, aberrant MAPK signaling and chemotherapy-resistance mechanisms might play a role. While the leukemogenic relevance of these signatures remains to be validated, their existence nevertheless supports a prognostically relevant biologic basis for the heterogeneity observed in CBF leukemia.

Oridonin, a diterpenoid extracted from medicinal herbs, targets AML1-ETO fusion protein and shows potent antitumor activity with low adverse effects on t(8;21) leukemia in vitro and in vivo

Studies have documented the potential antitumor activities of oridonin, a compound extracted from medicinal herbs. However, whether oridonin can be used in the selected setting of hematology/oncology remains obscure. Here, we reported that oridonin induced apoptosis of t(8;21) acute myeloid leukemic (AML) cells. Intriguingly, the t(8;21) product AML1-ETO (AE) fusion protein, which plays a critical role in leukemogenesis, was degraded with generation of a catabolic fragment, while the expression pattern of AE target genes investigated could be reprogrammed. The ectopic expression of AE enhanced the apoptotic effect of oridonin in U937 cells. Preincubation with caspase inhibitors blocked oridonin-triggered cleavage of AE, while substitution of Ala for Asp at residues 188 in ETO moiety of the fusion abrogated AE degradation. Furthermore, oridonin prolonged lifespan of C57 mice bearing truncated AE-expressing leukemic cells without suppression of bone marrow or reduction of body weight of animals, and exerted synergic effects while combined with cytosine arabinoside. Oridonin also inhibited tumor growth in nude mice inoculated with t(8;21)-harboring Kasumi-1 cells. These results suggest that oridonin may be a potential antileukemia agent that targets AE oncoprotein at residue D188 with low adverse effect, and may be helpful for the treatment of patients with t(8;21) AML.

Acute myeloid leukemia with the 8q22;21q22 translocation: secondary mutational events and alternative t(8;21) transcripts

Nonrandom and somatically acquired chromosomal translocations can be identified in nearly 50% of human acute myeloid leukemias. One common chromosomal translocation in this disease is the 8q22;21q22 translocation. It involves the AML1 (RUNX1) gene on chromosome 21 and the ETO (MTG8, RUNX1T1) gene on chromosome 8 generating the AML1-ETO fusion proteins. In this review, we survey recent advances made involving secondary mutational events and alternative t(8;21) transcripts in relation to understanding AML1-ETO leukemogenesis.

Targeting the Oligomerization Domain of ETO Interferes with RUNX1/ETO Oncogenic Activity in t(8;21)-Positive Leukemic Cells

About 12% of all de novo acute myeloid leukemias are characterized by the translocation t(8;21), which generates the oncogenic fusion protein RUNX1/ETO. RUNX1/ETO has a modular structure and contains several docking sites for heterologous proteins, including transcriptional co-repressors like N-CoR, SMART, and mSIN3A. RUNX1/ETO is found in high molecular weight complexes, which are crucial for the block in myeloid differentiation observed in RUNX1/ETO-transformed cells. Essential for high molecular weight complex formation is the nervy homology region 2 (NHR2) within ETO, which serves as interacting surface for oligomerization as well as association with members of the ETO protein family. Here, we show that the expression of a fusion peptide consisting of 128 amino acids (NC128), including the entire NHR2 domain of ETO, disrupts the stability of the RUNX1/ETO high molecular weight complexes, restores transcription of RUNX1/ETO target genes, and reverts the differentiation block induced by RUNX1/ETO in myeloid cells. In the presence of NC128, RUNX1/ETO-transformed cells lose their progenitor cell characteristics, are arrested in cell cycle progression, and undergo cell death. Our results indicate that selective interference with the oligomerization domain of ETO could provide a promising strategy to inhibit the oncogenic properties of the leukemia-associated fusion protein RUNX1/ETO.

Implication of the molecular characterization of acute myeloid leukemia

The identification of molecular genetic alterations such as gene mutations or deregulated gene expression in acute myeloid leukemia (AML) has greatly advanced our understanding of leukemogenesis. These markers now allow us to unravel the enormous heterogeneity seen within cytogenetically defined subgroups of AML. Furthermore, the molecular alterations are providing targets for molecular therapies. In this article, major molecular findings of prognostic and predictive significance are reviewed, with an emphasis on the discussion of gene mutations found in two major AML subgroups, cytogenetically normal and core-binding factor AML.

A leukemia fusion protein attenuates the spindle checkpoint and promotes aneuploidy

The 8;21 chromosomal translocation occurs in 15% to 40% of patients with the FAB M2 subtype of acute myeloid leukemia (AML). This chromosomal abnormality fuses part of the AML1/RUNX1 gene to the ETO/MTG8 gene and generates the AML1-ETO protein. We previously identified a C-terminal truncated AML1-ETO protein (AEtr) in a mouse leukemia model. AEtr is almost identical to the AML1-ETO exon 9a isoform expressed in leukemia patients. Here, we describe a novel function of AEtr in the development of aneuploidy through spindle checkpoint attenuation. AEtr cells had a reduced mitotic index following nocodazole treatment, suggesting a failure in a subset of cells to arrest in mitosis with a functional spindle checkpoint. Additionally, primary leukemia cells and cell lines expressing AEtr were aneuploid. Moreover, AEtr cells had reduced levels of several spindle checkpoint proteins including BubR1 and securin following treatment with the spindle poison nocodazole. These results suggest that inactivation of the spindle checkpoint may contribute to the development of aneuploidy described in t(8;21) leukemia patients.

Oncogenic pathways of AML1-ETO in acute myeloid leukemia: multifaceted manipulation of marrow maturation

The leukemic fusion protein AML1-ETO occurs frequently in human acute myeloid leukemia (AML) and has received much attention over the past decade. An initial model for its pathogenetic effects emphasized the conversion of a hematopoietic transcriptional activator, RUNX1 (or AML1), into a leukemogenic repressor which blocked myeloid differentiation at the level of target gene regulation. This view has been absorbed into a larger picture of AML1-ETO pathogenesis, encompassing dysregulation of hematopoietic stem cell homeostasis at several mechanistic levels. Recent reports have highlighted a multifaceted capacity of AML1-ETO directly to inhibit key hematopoietic transcription factors that function as tumor suppressors at several nodal points during hematopoietic differentiation. A new model is presented in which AML1-ETO coordinates expansion of the stem cell compartment with diminished lineage commitment and with genome instability.