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10 September 2001, Volume 20, Number 40, Pages 5660-5679
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AML1 and the AML1-ETO fusion protein in the pathogenesis of t(8;21) AML
Jonathan D Licht

Derald H. Ruttenberg Cancer Center and Department of Medicine, Mount Sinai School of Medicine, Box 1130, One Gustave L. Levy Place, New York, NY 10029, USA

Correspondence to: J D Licht, Derald H. Ruttenberg Cancer Center and Department of Medicine, Mount Sinai School of Medicine, Box 1130, One Gustave L. Levy Place, New York, NY 10029, USA. E-mail: jonathan.licht@mssm.edu

Abstract

Oncogene (2001) 20, 5660-5679.

Keywords

AML1; RUNX; transcription ETO; leukemia; translocation

Introduction

The past decade has yielded immense progress in the understanding of the pathogenesis of acute leukemia. The cloning of consistent chromosomal breakpoints associated with particular subtypes of leukemia has led to the identification of fusion proteins largely responsible for disease pathogenesis. The AML1 gene is frequently affected in acute myeloid leukemia (Look, 1997; Mitelman and Heim, 1992). The t(8;21) (q21;q22) translocation associated with the M2 form of AML, is among the commonest rearrangements found in AML. The importance of the AML1 (now known as Runx1) gene and its heterodimeric partner CBFbeta in normal and malignant hematopoiesis was apparent by the identification of these factors as critical regulators of normal myeloid-specific transcription, the involvement of both genes in leukemic translocations and the failure of hematopoiesis upon deletion of these genes. The role of AML1/RUNX in normal hematopoiesis has been extensively studied. Much less is known about the Eight-Twenty One oncoprotein (ETO)/MTG8 protein, which was unknown prior to its identification as a fusion partner. In this overview the roles of the AML1 and ETO proteins will be reviewed individually. The way in which the function of AML1 might be subverted by the leukemogenic fusion protein and its therapeutic implications will be explored.

The AML1/CBF/RUNX complex

The AML1 transcription factor was originally characterized by the study of the murine polyoma virus enhancer and the identification of similar cis-acting sequences in the promoters of hematopoietic genes. The DNA binding factor that recognizes these sequences was called polyoma enhancer binding protein 2 (PEBP2) and was found to be composed of two subunits (Kamachi et al., 1990). The activity was also known as core binding factor (CBF) for the ability of this complex to bind to a core enhancer sequence of the Moloney leukemia virus LTR (Golemis et al., 1990). The first molecular cloning of the CBF components came with the characterization of the fusion gene of the M2 form of AML associated with t(8;21). This gene was named AML1 (Miyoshi et al., 1991) and was found fused to a novel gene called Eight-Twenty One oncoprotein (ETO) (Erickson et al., 1992) or Myeloid translocation gene on chromosome 8 (MTG8) (Miyoshi et al., 1993). With subsequent cloning of the murine PEBP2/CBF genes it was realized that AML1 was the human homologue of the DNA binding subunit of PEBP2 (Ogawa et al., 1993b). The resulting predicted AML1 proteins were found to be highly homologous to a Drosophila transcriptional regulator known as runt (Kania et al., 1990). Also within a short period of time in 1993 the non-DNA binding subunit of PEBP2 was cloned and called PEBP2beta or CBFbeta (core binding factor beta). This subunit too was found involved in leukemia, specifically in M4 AML associated with inversion 16 (Liu et al., 1993). In this case, the beta subunit of the core complex was found fused to smooth muscle myosin heavy chain. These findings quickly focused a great deal of attention on the AML1/PEBP2/CBF transcription complex and have yielded competing and potentially confusing nomenclature. Table 1 indicates the names of the subunits involved in the normal and oncogenic forms of the AML1 complex and the HUGO recognized names.

AML1 gene and protein structure

The AML1 gene, now officially known as RUNX1, is located on chromosome 22q22.3 and is a member of a family of genes all with great similarity to the Drosophila runt gene (Levanon et al., 1994) (Table 1). RUNX2, formerly known as AML3, is the gene mutated in the human genetic disease of cleidocranial dysostosis/dysplasia characterized by multiple bony anomalies and is required for the generation of osteoblasts in the mouse (Komori et al., 1997). This gene was also identified as a critical regulator of bone-specific gene expression (Ducy et al., 1997). RUNX3, formerly called AML2, has not yet been associated with any disease but appears important for immunoglobulin class switching (Shi and Stavnezer, 1998). The pattern of expression of the three genes has only partial overlap.

AML1/RUNX1 encodes multiple four transcripts ranging in size from 2-8 Kb (Bae et al., 1993; Erickson et al., 1992; Levanon et al., 1996; Miyoshi et al., 1993; Nisson et al., 1992) that yield alternative forms of the AML protein. AML1/RUNX1 is expressed in hematopoietic cell lines such as HL60 and K562 and is up-regulated in U937 cells treated with the differentiation inducer TPA (Levanon et al., 1994; Tanaka et al., 1995a). It is also expressed in T and B lymphoid lines and is highly expressed in lymphoid tissues (Bae et al., 1993; Meyers et al., 1996; Ogawa et al., 1993b). However, among human tissues, AML1 is expressed in virtually all tissues except the heart and brain (Miyoshi et al., 1995). During murine development AML1 is expressed in fetal liver cells representing myeloid and erythroid progenitors as well as T and B cell precursors (Corsetti and Calabi, 1997; Satake et al., 1995a).

The AML1 gene consists of 12 exons spread over >260 kb of DNA. Alternative splicing and differential use of 3' polyadenylation sequences leads to the formation of forms of AML1 with different amounts of C-terminal sequence (Levanon et al., 2001; Miyoshi et al., 1995). Alternative promoter usage can lead to the formation of a N-terminal extended form of the protein (Ghozi et al., 1996; Levanon et al., 1996). The two AML1 promoters are subjected to differential regulation and yield different size 5' UTRs which modulate the ability of the protein to be translated (Pozner et al., 2000). The promoters are not tissue specific indicating that more distant elements are responsible for the expression of AML1 in specific tissues. The distal promoter yields a protein with a N-terminal extension that is efficiently translated while the proximal promoter yields a transcript whose translation is tightly controlled through an internal ribosomal binding site. This may allow subtle regulation of the AML1/RUNX protein during development and differentiation.

CBFbeta is a ubiquitously expressed gene that yields polypeptides of 187, 182 and 155 amino acids through alternative splicing (Ogawa et al., 1993a; Wang et al., 1993). CBFbeta is homologous to the Drosophila Big Brother and Brother proteins (Golling et al., 1996). It augments the ability of AML1 to bind to its cognate sequence, overcoming the adjacent segments of AML1 that inhibit DNA binding, but does not appear to bind to DNA on its own nor interact with any important co-factors. However the three isoforms of the protein do display some differences in their ability to augment transcription (Ogawa et al., 1993a; Zaiman et al., 1995). This may be due to some difference in their ability to shield the AML1/RUNX protein from ubiquitination and degradation (Huang et al., 2001).

Structure and function of the AML1 protein

The general architecture of the AML1 protein is diagramed in Figure 1. The N-terminal amino acids inhibit the intrinsic ability of the 128 amino acid runt homology domain (RHD) (92% identity with Drosophila runt) to bind to DNA. The sequence bound by PEPB2/CBF in the polyoma enhancer is PuACCPuCA (Kamachi et al., 1990), while by site selection the human AML protein was determined to bind a similar consensus TG(T/C)GGT (Meyers et al., 1993). The RHD is a target of mutation in familial platelet disorder (FPD) associated with a predisposition to leukemia (Song et al., 1999) and is mutated in sporadic cases of AML and MDS without chromosomal translocations (Imai et al., 2000; Osato et al., 1999; Preudhomme et al., 2000). The RHD of RUNX1 has an immunoglobulin-type fold also seen in STAT transcription factors and NF-kappaB (Berardi et al., 1999; Huang et al., 1999; Nagata et al., 1999; Warren et al., 2000). The residues mutated in FPD and AML are on the DNA binding face of the protein. The structure of CBFbeta was also solved (Goger et al., 1999) and it binds to the Runt domain in relatively close proximity to the RHD residues involved in DNA binding. Upon CBFbeta binding, the conformation of these critical DNA binding residues changes with the resulting increase in DNA binding affinity of the heterodimer (Tahirov et al., 2001). The C-terminal portion of the RHD also contains a potential ATP binding site whose function is uncertain.

AML1 is localized to the nucleus in part through sequences within the RHD (Lu et al., 1995). However, AML1 is specifically targeted to the nuclear matrix through a specific targeting sequence in the C-terminus of the protein (Zeng et al., 1997) that forms a distinct loop structure (Tang et al., 1999). By confocal microscopy a subset of nuclear AML1 can be found in foci co-localizing with nascent RNA transcripts and RNA polymerase II (Zeng et al., 1998) suggesting that the specific compartmentalization of the protein is important for its transcriptional effects. In addition matrix localization also appears important for the ability of AML1 to stimulate DNA replication (Chen et al., 1998). CBFbeta in contrast is generally a cytoplasmic protein and is brought into the nucleus through interaction with the RHD (Lu et al., 1995).

The C-terminal domain of AML1 is critical for its ability to influence gene transcription. A shorter variant of the protein lacking the C-terminal 220 amino acids was unable to activate transcription from the T-cell receptor beta chain enhancer while the extended form could (Meyers et al., 1995). This shorter protein has increased affinity for DNA compared to the extended protein and co-expression of this shorter splice variant with the full-length protein inhibited transcriptional activation by the longer protein (Tanaka et al., 1995b). Another splice variant truncates the N-terminus of the protein including a portion of the RHD. This protein was defective for trans-activation and DNA binding yet could inhibit activation by the full-length protein, potentially by competing for the CBFbeta subunit (Zhang et al., 1997). These naturally occurring variants may play regulatory roles to limit the extent of transcriptional activity by AML1. As described below the leukemic fusion proteins of AML inhibit normal AML1 activity in an exaggerated manner.

Immediately C-terminal to the RHD are proline, serine and threonine rich sequences that can be phosphorylated though the action of the MAP kinase cascade and binding of the ERK protein. Co-expression of ERK stimulated the transcriptional action of AML1 (Tanaka et al., 1996). Deletion mutations and Gal4 tethering experiments defined the transcriptional activation domains of AML1/RUNX1 (Kanno et al., 1998a; Figure 1). Three C-terminal activation domains were defined between aa 243-291, 291-331 and 331-371, one of which overlaps with the nuclear matrix targeting signal. In addition domains that mask or inhibit the activation potential of the AML1 protein were noted. The extreme C-terminus of the protein contains the sequence VWRPY that is also conserved in the Drosophila runt protein (Aronson et al., 1997). This is a recognition motif for the Groucho/TLE family of transcriptional co-repressors that interact with AML1 and other RUNX proteins.

AML1 target genes and transcriptional mechanisms

The AML1 protein has been shown to be able to regulate a number of genes relevant to hematopoietic differentiation including those of the T-cell receptor beta chain, cytokines such as IL3 and GM-CSF and granulocyte proteins such as myeloperoxidase, neutrophil elastase and granzyme B (Table 2). Studies of these promoters as well as those of viruses indicate that in general AML1 is ineffective as an activator in the absence of cooperating factors bound to adjacent promoter sites. For example on the TCR promoter AML1 synergistically activates transcription through the binding and recruitment of Ets-1 into a ternary DNA-protein complex (Giese et al., 1995). The formation of such a complex is facilitated by the presence of the DNA bending protein TCF/LEF. On the M-CSF receptor promoter AML1 synergistically activates in combination with C/EBPalpha and PU.1. In the case of C/EBPalpha the two proteins bind to each other and cooperatively bind to DNA. In contrast PU.1 and AML1 also synergistically activate transcription but bind to each other weakly and do not bind to DNA in a cooperative manner (Petrovick et al., 1998). In this case the two proteins might cooperate to recruit co-activators such as CBP/p300 to the promoter. On the myeloperoxidase promoter AML1 is an effective transactivator only when co-expressed with c-myb (Britos-Bray and Friedman, 1997). On the TCRbeta promoter AML1 and Ets-1 bind to DNA cooperatively and direct binding of Ets to AML1 augments DNA binding by AML1. In a reciprocal manner, binding of AML1 to Ets activates DNA binding by that protein due to intermolecular interaction between inhibitory domains within the two proteins that usually act in an intramolecular manner to inhibit their transcriptional activity (Goetz et al., 2000; Gu et al., 2000; Kim et al., 1999b). Similarly AML1 and the AML1-ETO protein bind to a newly described myeloid ets factor (MEF) and this factor can cooperate with AML1 to activate the IL3 promoter (Mao et al., 1999). In addition an interaction between AML1 and SMAD protein was described implicating AML1 in the TGF signaling pathway (Jakubowiak et al., 2000; Pardali et al., 2000; Zhang and Derynck, 2000). On the basis of these and other studies, a general model for activation by AML1 proteins has been constructed in which AML1 is among an array of proteins bound to the proximal promoters of hematopoietic-specific genes. In concert these factors bend DNA and recruit co-activators and the basal transcriptional machinery to the promoter. Many interactions between AML1 and other transcription factors are mediated by the RHD and therefore are expected to be preserved in the AML1-ETO fusion protein generated by t(8:21).

Further insight into the mode of action of AML1 has come from the identification of partner proteins. The p300 and CBP proteins can be co-immunoprecipitated with AML1 with the interaction mapping to the N-terminal portion of p300 that is also required for interaction with factors such as jun and c-myb. The C-terminal activation domain of AML1 was required for interaction with the co-activator (Kitabayashi et al., 1998b). p300 or CBP may serve as integrator proteins, binding to AML1 as well as other trans-activators such as c-myb, c/EBPalpha and PU.1 that activate hematopoietic promoters in concert. Two other co-activators have been found to bind to the C-terminal activation domain of AML1. ALY is a ubiquitously expressed nuclear protein which can bind to the C-terminal activation domain of AML1 (Bruhn et al., 1997). ALY does not have an intrinsic activation domain but can form multimers and may act to bridge interactions between AML1 and other transcription factors arrayed along hematopoietic promoters such as TCR/LEF. YAP (Yes associated protein) a WW domain protein (Chen and Sudol, 1995) binds to a PPPY motif found in the C-terminal activation domain of AML1 (Yagi et al., 1999; Figure 1). YAP1 itself contains a strong intrinsic activation domain and may act to convert AML1 into a stronger activator. An adjacent inhibitory domain of AML1 acts to limit the interaction between YAP and AML1. YAP was originally isolated as a c-Yes tyrosine kinase associated protein, suggesting that tyrosine kinase signal transduction cascades could modify the transcriptional activity of the AML1 protein.

The interaction between AML1 and all three of these co-activators is expected to be lost in the AML1-ETO fusion protein of M2 AML. Furthermore the nuclear matrix targeting signal of AML1 is lost in the fusion protein. These facts may account in part for the aberrant transcriptional activities of this leukemic fusion protein.

In some studies transfection of AML1 with a target promoter can also yield transcriptional repression. This may be attributable to different transcription activities of the various members of the AML1/RUNX families. Competition for AML1 binding sites between the endogenous protein present in the cell and a form of the protein transfected into the cells that represents a weaker activator can be read-out as repression (Takahashi et al., 1995). However the Drosophila runt proteins clearly inhibit the repression of some target genes and can repress transcription in co-transfection experiments (Aronson et al., 1997). This is due to the ability of the protein to recruit the groucho/TLE family of transcriptional repressors through a VWRPY motif at the very C-terminus of the protein. This amino acid sequence is conserved in AML1 and other mammalian RUNX proteins and AML1 can interact with Drosophila groucho (Aronson et al., 1997) as well as mammalian TLE proteins (Levanon et al., 1998). The TLE proteins do not bind DNA but contain the WD repeat involved in protein-protein interactions. As AML1 and LEF can bind the same co-activator, ALY, they can both bind the TLE1 co-repressor. Whether the two co-factors act together to actively repress target genes or whether the interaction with the TLE co-repressor simply serves to limit transcriptional activation by AML1 is not yet certain. Consistent with the latter idea, co-expression of TLE with AML1 can inhibit the ability of AML1 to activate the promoters of the M-CSF and neutrophil elastase gene (Imai et al., 1998). The situation may be even more complex. Recent studies showed that the b-HLH HES protein, a transcriptional repressor, can bind to AML1 and RUNX2 and augment activation by such proteins (McLarren et al., 2000). AML1 in turn can bind to HES and prevented HES-TLE interaction, acting to de-repress transcription (McLarren et al., 2001). Therefore a complex interplay between AML1, TLE and third factors may serve to integrate transcriptional signals and add an extra level of transcriptional control. Still another negative regulator of AML1 transcription was identified by the yeast two-hybrid system (Ahn et al., 1998). The ear-2 protein bound to AML1 in the region immediately C-terminal to the RHD of AML1 and inhibited its ability to transactivate. Whether this might be in part due to inhibition of DNA binding is not yet certain. Of note ear-2 can be detected in myeloid 32D cells and is down-regulated with G-CSF treatment, suggesting that AML1 function could be de-inhibited by G-CSF treatment to promote the transcription of genes critical for differentiation (Ahn et al., 1998).

AML1 may also repress though other mechanisms involving histone deacetylase (HDAC) complexes. In co-transfection experiments, AML1 could inhibit the transcription of the p21Waf1/Cip1 promoter, in a binding site-dependent manner, with the VWRPY groucho/TLE interaction domain dispensable for this effect (Lutterbach et al., 2000). AML1 could be coprecipitated with the msin3A co-repressor. A more central portion of the AML1 protein proximal to the activation domains (172-275) was required for this interaction. Repression of the p21 promoter was reversed by trichostatin A, implicating a HDAC mechanism. In summary, these data indicate the potential for AML1 to activate and repress target genes, depending on the promoter context and possibly the cellular background. In the AML1-ETO fusion protein all of the regions responsible for physiological interaction with co-repressors are lost and the C-terminus of AML1 is replaced with ETO yielding a novel constitutive repression activity to the AML1 protein.

AML1 in hematopoietic development

The essential role for the AML1 complex in blood development was determined by a series of gene targeting experiments in mice. Several different groups created AML1 null mice and obtained very similar results (Okuda et al., 1996; Wang et al., 1996a). The homozygous disruption of the gene led to fetal death at E11.5-12.5 likely due to severe CNS bleeding and damage. In addition the AML1 null animals were completely deficient in fetal hematopoiesis, whereas only yolk-sac derived primative hematopoiesis was intact (Okuda et al., 1996). Yolk sacs harbor the precursor cells of definitive hematopoiesis that migrate to the liver and no such precursors could be detected in the null animals. Double null AML1 ES cells could differentiate into primitive erythrocytes but not into definitive blood elements such as macrophages and in chimeric analysis these ES cells did not contribute to the hematopoietic lineage. The defect in ES cell differentiation could be rescued by replacement of the targeted gene with a AML1 cDNA by homologous recombination. This allowed for a structure/function analysis that indicated that the shorter AML1a protein could not induce hematopoietic differentiation; the C-terminal activation domain of the longer AML1 isoforms was required for differentiation and the C-terminal VWRPY/TLE interaction domain was not required for hematopoietic differentiation. This strongly suggests that transcriptional activation by AML1 is required for its ability to induce differentiation (Okuda et al., 2000).

Knockout of the non-DNA binding CBFbeta protein yielded a virtually identical phenotype (Niki et al., 1997; Wang et al., 1996b; Sasaki et al., 1996). This might be explained by the recent observation that CBFbeta protects AML1 from proteolytic degradation. Indeed in CBFbeta null embryos AML1 mRNA is readily detected but AML1 protein is not (Huang et al., 2001). More recent studies have refined the role of AML1 in murine blood development. Using a beta-gal knock-in approach AML1 expressing cells were tracked during development. AML1/Runx1 was expressed in the yolk sac in emerging primitive erythocytes and was found in extraembyonic endothelial cells. With further development expression was noted in the fetal liver in hematopoietic precursor cells and in clusters in the aorta/gonodal/mesodermal area, another source of hematopoietic stem cells. When this knock-in mouse was crossed with another mouse heterozygous for another defective AML1 allele, the fate of AML1 positive cells was determined. The defect was tracked to a failure of intra-aortic hematopoietic clusters to form. Defects in endothelial cells of the AGM region were also noted. Together the information suggests that AML1 is required of the generation of blood cells through a hemogenic endothelium (North et al., 1999). In one study heterozygous null mice had a quantitative defect in hematopoiesis (Wang et al., 1996a). This effect was confirmed and it was found that AML1 was critical for the generation of transplantable hematopoietic stem cells from the AGM region. With the loss of AML1 no stem cells were recovered and with the loss of one allele of AML1, there was a quantitative decrease and a qualitative change in the generation of stem cells. In the heterozygous animals, definitive stem cells were unexpectedly found a day earlier than expected in yolk sac rather than in the AGM. Recovery of stem cells from the AGM was severely decreased and this was associated with defects in the AGM hemogenic cluster region. The dose of AML1 also seems critical for development of the lymphoid lineage. Loss of one dose of AML1 or insertion of a dominant interfering form of AML1 into the lymphoid lineage affected the fate of T cells (Hayashi et al., 2000). Specifically the transition of double positive T cells into cells positive for CD4 or CD8 was impaired as was the proliferative response to TCR stimulation. Overexpression of AML1 in the T lineage caused a skewing of T cell subsets towards CD8 expression and induced the formation of lymphomas (Vaillant et al., 1999). Together these data indicate a widespread role for AML1 in the induction of cell differentiation and proliferation. Changes in the level of AML1 may have significant implications for blood development.

AML1 null animal models were used to determine potential targets of the protein. For example, in AML1 null animals expression of hematopoietic genes such as PU.1, M-CSF (previously identified as an AML1 target), vav, c-myb, G-CSF receptor and flk2 and flk3 kinases could not be detected in the AGM or fetal liver. Other genes such as GATA1, GATA2 and SCL could be detected. In contrast some of the genes absent in the AML1 null embyros genes could be detected in c-myb null animals suggesting that AML1 may regulate a specific subset of critical hematopoietic genes (Okada et al., 1998).

The finding of pervasive CNS hemorrhage in AML1 null animals also pointed to a role for the protein in angiogenesis. The exact role appeared complex as ex-vivo culture of the AGM region from AML1 null animals failed to yield hematopoietic cells but still could yield endothelial like cells (Mukouyama et al., 2000). AML1 null mice were analysed specifically in regard to the vascular system and were found to have fewer capillaries with less branches in many organs (Takakura et al., 2000). Using specialized culture conditions, the formation of endothelial cell networks from splanchnopleural mesoderm of AML1 null mice was defective. Addition of wild-type hematopoietic stem cells to AML1 null mesoderm culture rescued the formation of endothelial networks indicating that stem cells actively promote angiogenesis. This was due to the expression of angiogenic factors on the surface of stem cells that actively promote migration and sprouting of vascular endothelial cells. In all of these studies the critical role of AML1 in generating the definitive hematopoietic precursor is clear, however the fate of the cells destined to become the progenitors is not. Are these cells resident in the endothelium of the AGM region waiting for critical gene activation to differentiate and proliferate? What are the critical genes to push the differentiation of these cells? Use of AML1 null ES cells may offer a solution to this question. Such cells are blocked in definitive hematopoietic differentiation in an embryoid body model. A screen for genes that can overcome this blockage might reveal genes that need to be activated by AML1 in hematopoietic development. One group has begun to use the AML1 null mice to detect target genes. Using RDA comparing wild type and AML1 null ES cells a novel RING finger gene related to PML was discovered. HERF1 was expressed only in AML1 replete cells. The expression of this gene was up-regulated with erythroid differentiation and enforced expression of the gene augmented differentiation while an antisense expression construct inhibited differentiation. The mechanism of action of the HERF1 protein is unknown (Harada et al., 1999).

t(8;21) and the disruption of AML in leukemia

The (8;21) translocation is associated with about 40% of cases of M2 AML with karyotypic abnormalities (Bitter et al., 1987) and represents the most frequent chromosomal anomaly in leukemia (18-20%) (Look, 1997; Mitelman and Heim, 1992). The cloning of the breakpoint and discovery of the AML1 gene helped to spur the great focus on this transcription factor in normal hematopoiesis as documented above. As the result of this fusion the AML1 gene on chromsome 21 is fused to the ETO/MTG8 gene on chromosome 8. The breakpoint within the AML1 locus is between exons 5 and 6 and topoisomerase II cleavage sites are found near the breakpoint region (de Greef et al., 1995; Levanon et al., 2001; Nucifora and Rowley, 1995; Tighe et al., 1993). In all cases the resulting fusion protein yields a fusion transcript that encodes the initial 177 amino acids of AML1 linked to ETO (eight-twenty one oncogene) sequences (Erickson et al., 1992; Miyoshi et al., 1991, 1993; Nisson et al., 1992) (Figure 2). ETO is subject to alternative splicing, so that different forms of AML1-ETO have been isolated with varying amounts of ETO sequence (Kozu et al., 1993). In addition a transcript encoding a truncated form of AML1 was cloned from these patients, representing an alternatively spliced form of the AML1-ETO gene (Era et al., 1995; Tighe and Calabi, 1994; van de Locht et al., 1994). An AML1-ETO fusion transcript can be detected by reverse-transcription/PCR and can be used for the diagnosis of the syndrome (Downing et al., 1993; Kozu et al., 1993; Maruyama et al., 1994; Nucifora et al., 1993a). A reciprocal ETO-AML transcript has not been found. A large amount of literature has been devoted to the use of PCR for the diagnosis and monitoring of t(8;21)-associated AML. From this several generalizations can be made.

  1. Qualitative PCR using one or two rounds can make a definitive diagnosis in the absence of obvious karyotypic abnormalities. The sensitivity of this technique is about 1/105-1/106 leukemic cells. In fact in one series of patients entered into clinical trials the (8;21) translocation was detected in 8% of cases and the AML1-ETO fusion transcript was detected in an additional 5% without karyotypic abnormalities (Langabeer et al., 1997). This suggests that screening for this abnormality should be considered in the assessment of AML without cytogenetic changes.
  2. The presence of the AML1-ETO transcript correlates with some distinct phenotypic characteristics of the leukemic blasts including a single Auer rod, abnormal cytoplsmic granules high-level expression of CD34 and CD19 and low levels of CD33 (Andrieu et al., 1996; Nucifora et al., 1994). This suggests that the AML1-ETO fusion inhibits differentiation at a particular stage of myeloid development.
  3. AML1-ETO can be detected in the peripheral blood and marrow of patients with long-term complete remissions after treatment with chemotherapy or stem cell transplantation (Kusec et al., 1994; Miyamoto et al., 1995; Nucifora et al., 1993b; Saunders et al., 1994, 1996). AML1-ETO positive, multipotent progenitor cells that yield normal blood cell colonies, including B cells can be demonstrated in these patients along with normal progenitors (Guerrasio et al., 1995; Miyamoto et al., 1996, 2000; Saunders et al., 1997). This strongly suggests that the AML1-ETO fusion event occurs in an early stem cell or progenitor cell and that additional events are required for the cell to become fully transformed.
  4. With more standardization and use of quantitative techniques such as real-time PCR, determination of the level of AML1-ETO transcript in the peripheral blood and marrow of patients may be a useful tool for prediction of remission. After long-term remission and after stem cell transplant patients in some studies were found to become PCR negative (Morschhauser et al., 2000; Preudhomme et al., 1996; Satake et al., 1995b; Sakata et al., 1997). Some of the inter-study difference in the detection of the AML1-ETO transcript in remission patients may be due to the fact that some groups used two rounds of nested PCR which may have increased sensitivity and that different primer sets may have been used. An international working group has recently advocated a standardized and validated set of primers to use for a variety of hematological malignancies in future clinical studies (van Dongen et al., 1999). Given that some patients can remain in CR with readily detectable transcripts, qualitative measurement of AML1-ETO levels without standardization cannot be recommended for patient care. A number of studies have followed patients serially using competitive (Muto et al., 1996; Tobal et al., 2000; Tobal and Yin, 1996) or closed tube real-time PCR (Kondo et al., 2000; Marcucci et al., 1998; Sugimoto et al., 2000) and in general have found a trend indicating that AML1-ETO transcript levels tend to drop with repeated courses of chemotherapy and stem cell transplant. A rise in fusion transcript levels in asymptomatic patients using these quantitative techniques does appear to herald a relapse (Krauter et al., 1999; Marcucci et al., 1998; Tobal et al., 2000).

The ETO gene family

ETO (MTG8) was unknown prior to its identification as the fusion partner of AML1 in t(8;21) (Erickson et al., 1992; Miyoshi et al., 1993; Nisson et al., 1992). Subsequently two other members of this family have been cloned, one by its ability to heterodimerize with the ETO protein (EHT/MTGR1) and through data base searches (Fracchiolla et al., 1998; Kitabayashi et al., 1998a). A second ETO family member, ETO-2 was identified by low stringency screening of murine and human cDNA libraries (Calabi and Cilli, 1998; Davis et al., 1999) (Figure 3). Remarkably the human homologue of ETO-2, MTG16, was identified as a fusion partner with AML1 in rare cases of AML and myelodysplastic syndrome (Gamou et al., 1998; Salomon-Nguyen et al., 2000). As in the case of the AML1/RUNX proteins, the nomenclature has varied in the literature and this is displayed in Table 1 along with the HUGO approved names (CBFAT1-3) that are not yet in common use. ETO is also evolutionarily conserved. The mouse and human proteins are 99% identical and genes related to ETO can be detected readily by low stringency Southern blotting in a variety of mammals and chicken with weaker similarity to Xenopus sequences (Niwa-Kawakita et al., 1995). ETO is the mammalian homologue of the Drosophila nervy gene (Feinstein et al., 1995), a target of the ultra bithorax gene. Nervy appears to be involved in the development of the CNS of the fly.

The ETO/MTG8 gene is located at chromosome 8q22 and consists of 13 exons spread over 87 kB of DNA (Wolford and Prochazka, 1998). Alternative promoters and first exons yield transcripts of ~5.5 kB yielding proteins of 577 and 604 amino acids (Erickson et al., 1994). Alternative splicing can yield transcripts with premature stop codons as well. In the (8;21) translocation the breakpoint within the ETO gene occurs in the introns between the first two alternative exons of ETO (Tighe and Calabi, 1995; Tighe et al., 1993). As a result the AML1-ETO fusion contains almost the entire open reading frame of ETO linked C-terminal to the first 177 amino acids of AML1. Dinucleotide repeats found downstream of the third exon and the last non-coding exon are polymorphic in the population and can be used to screen for loss of the ETO allele in other forms of cancer or for genetic associations (Wolford et al., 1998; Wolford and Prochazka, 1998). ETO is expressed in a variety of tissues but is most abundant in the heart, brain, lung and testis (Erickson et al., 1994; Wolford and Prochazka, 1998). ETO is expressed, as is AML1, in CD34+ progenitor cells but is not present in more differentiated leukocytes (Erickson et al., 1996). Among hematopoietic cell lines, ETO is found in B cells like Raji and Nalm6, the erythroid HEL cell line and the AML1-ETO harboring Kasumi-1 cell line (Era et al., 1995). The MTGR1 gene is more widely expressed than ETO with its expression virtually ubiquitous in tissues (Calabi and Cilli, 1998; Morohoshi et al., 2000). Similarly MTG16/MTGR2/ETO-2 is expressed in a wide number of tissues and in myeloid cell lines such as FDCP mix and the erythroid cell line MEL (Davis et al., 1999). While ETO expression is found in the marrow it is absent in peripheral leukocytes and the spleen. In contrast MTGR1 is present in spleen and leukocytes. Together this information suggests that ETO might be important in the development of blood elements but is dispensable in more differentiated cells. ETO-related proteins might have a continuing role. The (8;21) translocation, by generating high levels of a particular ETO species in the cell, might upset the balance among the ETO family members, potentially contributing to the leukemic phenotype.

The AML1-ETO and ETO protein

The ETO gene yields proteins of 577 and 604 amino acids. In the t(8;21) almost the entire open reading frame of ETO is linked to AML1. Thus the understanding of the function of the fusion protein and the wild-type ETO protein are intimately linked. The initial inspection of the ETO protein sequence gave few clues as to its function, but comparison of its sequence to that of Drosophila nervy showed the presence of four conserved regions NHR1-4 (Figure 3). The first region shares similarity with the TAF110 and related TAF proteins. The second, NHR2 has a predicted coiled structure with a heptad repeat of hydrophobic amino acids. The third region has notable homology with nervy and the other ETO family members but no other similarities to offer clues to its function. The fourth region has also been termed the MYND domain (MTG8, Nervy, Deformed). This region has a two non-classical zinc fingers, the first of the form CxxC-CxxC, and the second CxxxCHxxxC (Gross and McGinnis, 1996). Drosophila Deformed is a transcription factor but the MYND domain of the protein is believed to be a protein-protein interaction motif and no sequence-specific DNA binding activity has been noted for the ETO protein (S Hiebert-personal communication). The ETO protein has a high content of serine and threonine sequences and is phosphorylated on these residues (Erickson et al., 1996). Proline, serine and threonine rich sequences are found in the N and C-terminus of the protein that could affect protein stability. Specific kinases that act upon ETO can be isolated from cell extracts but their identity and physiological relevance is not yet known (Komori et al., 1999).

Cells from human bone marrow can be stained with antibodies against the protein but expression is much lower in peripheral blood leukocytes, in agreement with RNA expression data. ETO can be immunologically detected in the cell nucleus in a punctate pattern but has also been detected in the cytoplasm of neurons, particularly in the synaptosome, suggesting that the protein might have multiple roles in the cell (Sacchi et al., 1996). ETO is directed into the nucleus by a non-canonical, lysine and arginine-rich nuclear localization sequence aa 250-280 and can interact in vitro with importins (Odaka et al., 2000). The TAF110/NHRI domain is required for localization of ETO into distinct speckles, as deletion of the domain yields a nuclear diffuse pattern. Though forming distinct speckles, the ETO expression domains do not overlap with SC35 bodies that harbor splicing factors (Odaka et al., 2000). ETO nuclear speckles also do not overlap with the PML nuclear body (reviewed in Lamond and Earnshaw, 1998; Melnick and Licht, 1999; Ruggero et al., 2000) although overexpression of ETO or AML1-ETO changes the number and location of the PML nuclear bodies. Furthermore ETO can be found in the nuclear matrix fraction of the nucleus, as can PML, suggesting an interrelationship among components of these nuclear dot complexes (McNeil et al., 2000; Wood et al., 2000).

Transcriptional function of AML1-ETO and ETO

The t(8;21) fusion generated in AML maintains the RHD of the AML1 protein and the N-terminal region whose importance is not clear and fuses it to virtually all of ETO (Figure 4). This yields a protein of 752 amino acids which can be detected as a ~95 kDa band in t(8;21) cells and in the nucleus by immunofluorescnce (Sacchi et al., 1996; Tanaka et al., 1998). From the description above it is apparent that several important portions of AML1 are lost in the fusion protein including (i) The C-terminal activation domains which interact with specific co-activators; as well as (ii) interaction sites for the sin3 and TLE co-repressors. Furthermore (iii) the nuclear localization signal outside of the RHD is lost, as is the nuclear matrix targeting signal. It would be predicted that the chimeric protein would have very different properties from the wild-type protein, a fact borne out in many experiments. These experiments have led to the definition of the leukemogenic mechanism of the AML1-ETO protein and have given insight into the possible normal role of ETO and related proteins in cellular metabolism.

Initial work with the AML1-ETO fusion indicated that it could bind to the cognate AML1 binding site. The AML1-ETO fusion could be detected as a DNA protein complex in EMSA experiments using t(8;21) leukemic cells and could heterodimerize with CFBbeta (Meyers et al., 1993) even more efficiently than wild-type AML1 (Tanaka et al., 1998), perhaps sequestering this critical component away from wild-type AML1. AML1-ETO was localized to the nucleus, indicating that sequences within ETO could direct the protein to the nucleus in the absence of the AML1 nuclear localization sequence. AML1-ETO failed to activate the TCRbeta reporter and actually blocked the ability of wild-type AML1 to activate the reporter. This occurred even when 25-fold less AML1-ETO was co-transfected with the AML1 vector. Subsequent experiments on the GM-CSF promoter showed that AML1-ETO not only blocked the ability of AML1 to activate the promoter but also decreased the expression of the promoter below baseline levels. Mutagenesis of the AML1-ETO protein indicated that both the RHD and sequences within the C-terminus of ETO were required for the ability of AML1-ETO to repress activation by AML1. These data strongly suggested that AML1-ETO was a dominant repressive form of AML1 that did not simply work by competing for the AML1 binding site but actively repressed transcription of AML1 target genes. Furthermore AML1-ETO could also inhibit trans-activation by the AML2/RUNX3 protein and likely blocks the activity of RUNX2 as well (Meyers et al., 1996). Hence the AML1-ETO fusion might broadly inhibit the genetic effects of the entire family of RUNX proteins that can be co-expressed in hematopoietic cells.

The mechanism by which ETO actively represses AML1-mediated transcription became apparent with the finding that the ETO protein can interact with specific domains of the highly related N-Cor and SMRT co-repressors as well as mSin3A (Gelmetti et al., 1998; Lutterbach et al., 1998b; Wang et al., 1998). ETO itself was a potent transcriptional repressor when fused to the GAL4 DNA binding domain and this effect was suppressed by HDAC inhibitors (Wang et al., 1999). Furthermore ETO and AML1-ETO could be co-immunoprecipitated with histone deacteylase activity and can complex with HDAC1 and HDAC2. This indicates that the AML1-ETO fusion is an active repressor that recruits a multi-protein complex including HDACs to AML1 target genes. This would replace the AML1 complex that contains co-activators including the p300/CBP histone acetyl transferases. This is very similar to the leukemogenic mechanism in acute promyelocytic leukemia where fusion of novel proteins to the RAR leads to increased affinity for co-repressors and active repression of RAR target genes (reviewed in Melnick and Licht, 1999). The MYND/Zinc finger motifs of ETO were critical for interaction with N-COR and for repression of the MDR1 promoter (Lutterbach et al., 1998a,b; Wang et al., 1998) although NH3 appeared to assist in the binding of the related SMRT repressor (Zhang et al., 2001). On other promoters the MYND domain appeared dispensable for repression suggesting the presence of additional co-repressor sites in the ETO protein that could be utilized in certain contexts. Deletion of NHR3 and particularly the NHR2/heptad repeat inhibited the ability of AML1-ETO to repress transcription also indicating other important contacts for co-repressors. The ability of AML1-ETO to interact with co-repressors was shown to be critical for its biological function. The ability of AML1-ETO to repress a target promoter was blocked by HDAC inhibitors (Lutterbach et al., 1998b). U937 cells transduced with wild-type AML1-ETO were blocked in their ability to differentiate with vitamin D3 and TGFbeta. Deletion of the C-terminal N-Cor/SMRT interaction domain yielded a protein no longer able to block differentiation (Gelmetti et al., 1998).

Recent studies with the PML-RARalpha and PLZF-RARalpha fusion proteins suggested that an aberrant tendency for homodimerization and formation of high molecular weight complexes contributed to the ability of these proteins to repress RAR targets and induce leukemia (Lin and Evans, 2000; Minucci et al., 2000). ETO can be isolated by sucrose sedimentation as a high molecular weight complex of up to 600 kDa (Hildebrand et al., 2001; Lutterbach et al., 1998b). This may represent homo-multimers or heteromers of ETO bound to the related MTGR1 or MTGR2 proteins (Davis et al., 1999; Kitabayashi et al., 1998a). N-Cor, mSin3A and HDAC1 can also co-sediment with ETO. AML1-ETO can also be purified under native conditions as a high molecular weight complex (Minucci et al., 2000). Deletion of the NHR/heptads repeat prevented the formation of this high molecular weight complex and such proteins were defective in transcriptional repression and inhibition of myeloid cell differentiation. This was at first attributed to a lack of homodimerization of the ETO moiety of AML1-ETO (Zhang et al., 2001) protein but more recent studies indicate that the NHR2 site overlaps with a binding site for mSin3A on ETO (Hildebrand et al., 2001) and S Hiebert-personal communication). In addition replacement of the HHR/NHR2 region of ETO with the GCN4 homodimerization motifs yielded an AML1-ETO protein defective in repression (Lutterbach et al., 1998a). Such a protein would be unable to heterodimerize with ETO family members. The full repression complex of ETO or AML1-ETO may indeed be a high molecular weight multimer, but it appears that mSin3a is a critical component of the complex and ubiquitously expressed ETO family members may be part of the complex as well. Multimerization per se may not be as important for the repression activity of AML1-ETO as its ability to recruit an array of co-repressors to the promoter. Through interaction with mSin3A and N-Cor as well as by more direct interaction (S Hiebert-personal communication), ETO recruits a number of HDACs to its targets.

AML1-ETO as well as AML1 was also reported to activate the bcl2 and M-CSF promoters (Klampfer et al., 1996; Rhoades et al., 1996). This effect required both the MYND and NHR2 domains. Given the strong association of the ETO protein with a co-repressor complex and tendency of AML-1 itself to bind to sin3A it seems unlikely that AML1-ETO can recruit co-activators to promoter sites. Activation of the M-CSF receptor promoter was only observed when wild-type AML1 and AML1-ETO are co-expressed (Rhoades et al., 1996). This suggests that AML1-ETO may be removing mSin3A from AML1 bound to the promoter, tipping the balance of wild-type AML1 action more towards activation. Such a mechanism could be occurring in vivo as well as in co-transfection experiments since there was a tendency towards increased expression of the M-CSFR in M2 AML associated with t(8;21). The case of the bcl2 promoter is harder to explain since AML1-ETO on its own could activate this promoter in an AML1 binding site dependent manner. However it is possible that AML1-ETO removed co-repressors from other RUNX family protein bound to the promoter site and present in the cells prior to transfection. The physiological relevance of the bcl2 gene as a target of AML1-ETO is uncertain, since two studies found that in leukemic specimens from t(8;21) patients, bcl2 expression was actually lower than in other forms of leukemia (Banker et al., 1998; Shikami et al., 1999). This indicates the potential problems in extrapolating from promoter studies to the actual genetic targets of the fusion transcription factor in vivo.

AML1-ETO can block activation by other transcription factors as well. While AML1 cooperates with the c/EBPalpha protein to activate the NP3/defensin promoter, AML1-ETO blocks activation of the promoter by either AML1 or C/EBPalpha. AML1-ETO binds to c/EBP and repression of c/EBP action required the co-repressor binding moieties of ETO (Westendorf et al., 1998). Similarly AML1-ETO binds to the MEF Ets domain protein through a C-terminal portion of the RHD of AML1 and inhibits transcription through MEF DNA binding sites (Mao et al., 1999). AML1-ETO may exert a dominant effect on transcriptional activators arrayed on a promoter. For example TFGbeta activation of the immunoglobulin alpha-chain promoter mediated by SMAD binding sites can be completely suppressed by AML1-ETO (Jakubowiak et al., 2000). Hence widespread disruption of gene activation pathways might be expected in t(8;21)-associated AML.

AML1-ETO and ETO also have important effects on transcriptional repressors. We found that ETO and the PLZF transcriptional repressor rearranged in t(11;17) associated APL can be found as a complex in vivo and that ETO can augment transcriptional repression by PLZF in a HDAC-dependent manner (Melnick et al., 2000c). While the N-Cor and SMRT proteins interact with PLZF through the BTB/POZ domain (Ahmad et al., 1998; Melnick et al., 2000a; Wong and Privalsky, 1998), ETO interacted with a second repression domain within PLZF. Subsequently we found ETO complexed with other BTB/POZ transcriptional repressors, suggesting that ETO and potentially other ETO family members may serve a general purpose as a co-repressor that helps recruit HDACs to promoters. Recently ETO was discovered to form a complex with atrophin-1, a protein affected in dentate-rubral and pallido-luysian atrophy, a neurodegenerative disorder associated with the expansion of CAG repeats and protein polyglutamine tracts (Wood et al., 2000). Atrophin and ETO co-localized in cells along with sin3A and atrophin itself was a transcriptional repressor when tethered to the GAL4 protein. This further implicates ETO as part of a complex set of co-repressor molecules.

Like ETO itself the AML1-ETO fusion protein present in t(8;21)-harboring SKNO-1 cells formed a stable complex with PLZF. However, in co-transfection experiments, while ETO enhanced repression by PLZF, AML1-ETO completely blocked the ability of PLZF to repress a target promoter (Melnick et al., 2000b). This could not be overcome by transfection of additional ETO. We found PLZF to be tightly associated with nuclear matrix, but in the presence of AML1-ETO this association was completely lost. In addition AML1-ETO inhibited the ability of PLZF to bind DNA. As a consequence of this interaction, we theorize that in addition to inhibiting AML1 target genes required for differentiation, AML1-ETO might block the ability of PLZF and other factors to repress genes critical for cell cycle control such as cyclin A (Yeyati et al., 1999).

This information also suggests that AML1-ETO could affect other transcriptional pathways by binding to and mis-targeting transcription factors within nuclear sub-compartments. ETO and AML1-ETO can both be found in the nuclear matrix fraction but this is a rather gross measure of the compartmentalization of these proteins. Indeed the loss of the AML1 nuclear matrix targeting signal in AML1-ETO fusion and replacement by ETO sequences leads to a complete redirection of the protein to a subnuclear compartment that does not overlap with the transcriptionally active foci of wild-type AML1 (McNeil et al., 1999). This indicates the presence of different sites within the nuclear matrix, some of which may be associated with transcriptional activation and others with repression. It is possible that AML1-ETO forces PLZF away from a compartment where it can complex with co-repressors. AML1-ETO also appears to force AML1 itself out of its usual matrix attachment compartment, potentially compounding its negative effect of AML1 target genes (Chen et al., 1998).

In summary, the substitution of the ETO motif for the C-terminal activation and matrix targeting regions of AML1 leads to pleiotrophic and complex effects on transcription. From the above information its seems quite possible that the direct targets of AML1 as well as other transcriptional regulators will be aberrantly regulated in t(8;21) AML. The key targets affected to stimulate leukemogenesis are not yet certain. The analysis of cellular and animal models described below has started the identification of these genes.

Cellular models of AML1, ETO and AML1-ETO function

Cell models have given some insights into the action of these proteins. Enforced expression of AML1b, the full-length protein did not affect erythroid or megakaryocytic differentiation of eythroid K562 cells, while the shorter AML1a isoform, lacking the C-terminal activation domains and inactive in transcription (Meyers et al., 1993; Miyoshi et al., 1991) blocked erythroid differentiation and encouraged megakaryocytic differentiation. Intriguingly the erythroid response of these cells to butyrate was blocked but not the response to hemin, indicating the involvement of AML1 in specific gene pathways (Niitsu et al., 1997). In the myeloid 32Dcl3 system both the C-terminal truncated variant of AML1 as well as an N-terminal version devoid of DNA binding activity blocked differentiation of these cells in response to G-CSF and allowed the continued proliferation of the cells in the presence of the cytokine (Tanaka et al., 1995b; Zhang et al., 1997). Co-expression of extended, full-length AML1b protein with this variant rescued the differentiation effect. This indicates that the interplay among the naturally occurring forms of AML1 might modulate the differentiation of hematopoietic cells. In a clinical correlate, the levels of shorter AML1a isoform were elevated in leukemic patients without the t(8:21) translocation, perhaps contributing to the differentiation block characteristic of leukemia (Tanaka et al., 1995b).

AML1 has been implicated in angiogenesis as well as hematopoiesis. The effect on angiogenesis was though to be indirect by stimulating the expression of angiogenic factor on the surface of hematopoietic stem cells that make contact with the endothelial cells (Takakura et al., 2000). However AML1 expression is induced in endothelial cell lines in the presence of VEGF and expression of dominant negative forms of AML1 in these cells inhibited the formation of vascular tube structures (Namba et al., 2000), suggesting a cell autonomous role for AML1 in vascular development.

Overexpression of AML1b itself in NIH3T3 cells can generate transformed foci and cells that are tumorigenic in nude mice. This phenomenon is dependent on the DNA binding and transcriptional activation domains of the protein (Kurokawa et al., 1996; Tanaka et al., 1996). In this regard overexpression of AML1 can up-regulate the early growth response gene c-fos as well as bind and trans-activate the c-fos promoter (Hwang et al., 1999). Although expression of full-length AML1b in 32D cells does not block differentiation it does shorten the length of the G1 phase of the cell cycle and activates the expression of cell cycle activators such as CDK2 and cyclin D2 and can activate the cyclin D2 promoter (Strom et al., 2000). AML1 expression may also control cell life decisions. Engineered expression of AML1 in a T cell hybridoma was associated with the down-regulation of the fas ligand, a key inducer of T cell apoptosis, up-regulation of BCL2 and increased expression of the IL2 receptor. Upon ligation of the T cell receptor these changes in gene expression prevented apoptosis and shifted the cells to a proliferative program (Fujii et al., 1998). This information, along with the observed absence of hematopoietic stem cells in AML1 null mice, suggests that by regulation of a specific set of genes including cell cycle regulators and modulators of apoptosis, AML1 promotes cell life and proliferation.

The normal role of the ETO protein in cell proliferation or differentiation is not yet certain but stable transfection of ETO into NIH3T3 cells led to growth to a higher cell density, colony formation in soft agar and tumor formation in nude mice (Wang et al., 1997). Another group, using BALB/c 3T3 cells, found that ETO could only transform cells in combination with the ras oncogene (Sueoka et al., 1998). This might represent the ability of ETO to influence the expression of genes involved in cell cycle regulation, potentially by sequestering co-repressor factors and de-repressing key targets. This could be a potential mechanism of AML1-ETO action in addition to suppression of AML1 targets. The true role of ETO and family members could be clarified by the generation of knockout animals for these genes.

AML1-ETO has significant effects in model cellular systems. When expressed in NIH3T3 cells, one group found that AML1-ETO could form transformed foci and generate tumors in nude mice, although unlike others they did not find this effect with AML1 or ETO on their own (Frank et al., 1999). Full activity required the C-terminal domains of ETO (NHR2 and MYND) involved in co-repressor binding. Over-expression of AML1-ETO was associated with increased levels of phosphorylated c-jun and ATF-2 proteins and AML1-ETO could activate the expression of the collagenase promoter through the action of AP1. Though the mechanism by which AML1-ETO activated the transcription factors is not yet understood, these experiments further indicate that AML1-ETO may have pleiotropic effects in the cell in addition to simple inhibition of the action of AML1.

When expressed in myeloid L-G (Ahn et al., 1998; Hwang et al., 1999; Kitabayashi et al., 1998a; Kohzaki et al., 1999) or 32Dcl3 cells, AML1-ETO prevents the usual differentiation of these cells into mature myeloid forms when they are switched from IL-3 to G-CSF. Instead, the cells continue to proliferate in the presence of G-CSF. This was associated with up-regulation of bcl2 in the cell line model although, as noted above, the relevance, of this to leukemic pathophysiology is uncertain. Nevertheless forced expression of bcl2 in 32D cells did not block differentiation, implicating other genetics changes in the aberrant response of the AML1-ETO-harboring cells to G-CSF (Kohzaki et al., 1999). Identifying such genes could be key for understanding the pathophysiology of AML1-ETO-associated leukemia. One such gene might be ear-2, a negative regulator of AML1 function. Enforced expression of ear-2 blocked myeloid differentiation of 32D cells and engineered expression of AML1-ETO in these cells induces ear-2 expression. Whereas ear-2 levels usually decline in myeloid cells induced to differentiate, they remain elevated in AML1-ETO expressing cells. This suggests that in addition to directly inhibiting AML1 targets, AML1-ETO expression might lead to the inactivation of the function of the remaining allele of AML1 (Ahn et al., 1998). Furthermore the ear-2 protein might bind to other critical activators of myeloid differentiation contributing the differentiation block and continued proliferation characteristic of leukemia.

An even more compelling target of the AML1-ETO fusion is the G-CSF receptor. Enforced expression of AML1-ETO up-regulated the expression of this protein, explaining the increased sensitivity of the cells to the cytokine (Shimizu et al., 2000). AML1-ETO induced expression of the G-CSFR promoter by an indirect effect, inducing the expression of C/EBPepsilon by an unknown mechanism. Elevated levels of the G-CSF receptor were found in fresh t(8;21) leukemic specimens and cell lines, highlighting the likely importance of this finding. Nevertheless only part of the effect of AML1-ETO could be explained by up-regulation of the G-CSFR. Overexpression of C/EBPepsilon or G-CSF receptor in myeloid cell lines could prolong the proliferative response to G-CSF but eventually the cells did slow in growth and mature. In contrast AML1-ETO expressing cells continuously proliferated without differentiation. To identify other events responsible for the differentiation block, novel genes affected by AML1-ETO were isolated from these cells by differential display (Shimada et al., 2000). A surprising number of genes were up-regulated by AML1-ETO and a significant number of these genes were not known to be regulated by AML1, did not have AML1 binding sites in their promoters and were not modulated by overexpression of AML1 itself. Deletion of the NH2 and adjacent sin3A binding site prevented modulation of most of these genes implicating the multi-protein complex assembled on the ETO moiety as critical for regulation of these genes. One of the induced genes, TIS11b, encoding a potential RNA binding protein, stimulated proliferation and delayed differentiation of myeloid cells in response to G-CSF. The expression of this gene was elevated in fresh t(8;21) specimens suggesting that it represented a pathophysiologically important target of the AML1-ETO fusion.

In addition to the AML1-ETO fusion, the (8;21) translocation generates a truncated form of AML1 consisting of the first 177 amino acids of the protein including the RHD. This protein has been detected in immunoblots of Kasumi-1 cells in some studies although it is difficult to determine whether the observed protein species might have represented a degradation product of the wild-type AML1 or AML1-ETO protein (Sacchi et al., 1996). Nevertheless the effect of this truncated protein in myeloid cells was tested (Britos-Bray et al., 1996). Although this protein could inhibit DNA binding by AML1, stable expression of the protein in 32D cells did not induce IL-3 independent growth nor blocked differentiation of 32D cells in response to G-CSF. This suggests that the AML1-ETO protein is indeed the predominant oncogene generated in the translocation although the truncated form may play an accessory role.

There are two extant model cells of the t(8;21) leukemia, Kasumi-1 (Asou et al., 1991) and SKNO-1 (Matozaki et al., 1995). Treatment of these cells with antisense oligonucleotides (Sakakura et al., 1994) or hammerhead ribozymes (Matsushita et al., 1995) directed against the AML1-ETO fusion transcript inhibited their growth. In addition the antisense oligonucleotides induced differentiation of Kasumi cells. In combination with the data from engineered expression of AML1-ETO into model cell lines it is apparent that the AML1-ETO fusion protein should be the key target of any novel therapeutic strategies. In this regard it was noted that the Kasumi and SKNO cell line were unique among a panel of myeloid leukemia cells in that they underwent apoptosis in response to dexamethasone treatment (Miyoshi et al., 1997). This was associated with increased expression of the glucocorticoid receptor in these cells lines. In contrast these cells were resistant to retinoic acid and other differentiation agents. However these cells could be induced to differentiate with phenyl butyrate, a known inhibitor of histone deacetylases (Candido et al., 1978; Wang et al., 1999). An additive effect on differentiation was found with concomitant treatment with GM-CSF, and butyrate also increased apoptosis due to dexamethasone. This suggested that like APL, t(8;21) AML might be treated with modulators of transcription and has led to the formulation of a clinical trial of GM-CSF, dexamethasone and butyrate for relapsed t(8;21)-associated AML.

Animal models of t(8;21) AML

Recent success in diseases such as APL (He et al., 1999) and CML-(Deininger et al., 2000), indicate that the mouse offers highly faithful models of human leukemia and offers great opportunities to understand disease mechanisms and test new therapeutic maneuvers such as ATRA and STI571. Generation of an animal model of t(8;21) leukmemia has been fraught with difficulty. Introduction of the AML1-ETO cDNA into the AML1 locus by homologous recombination led to embryonic lethality in heterozygous animals (Yergeau et al., 1997; Okuda et al., 1998). This was associated with a phenotype very similar to the deletion of the AML1 gene or CBFbeta genes and characterized by the loss of definitive fetal liver hematopoiesis and CNS hemorrhage. This same phenotype was also observed in mice heterozygous for a knock-in of the CBFbeta-MYH11 fusion gene. (Castilla et al., 1996). These genetic data indicate that the AML1 and CBFbeta fusion genes act in a dominant negative manner on AML1 since they yield virtual phenocopies of the AML1 deletion. Culture of the yolk-sac of these mice strains yielded dysplastic monocytic colonies (Yergeau et al., 1997) while in a similar knock-in mouse, aberrant myelomonocytic colonies were derived from fetal liver (Okuda et al., 1998). When these liver-derived progenitors were cultured with multiple cytokines they could be grown in vitro indefinitely, while wild-type progenitor culture ceased growing after six passages. In addition, infection of murine marrow with a retrovirus harboring the AML1-ETO fusion yielded primitive cells with increased self-renewal potential in vitro. These experiments are consistent with those in cell lines suggesting that AML1-ETO can alter the differentiation program and encourage uncontrolled cell growth but do not offer an animal model for the human leukemia. The knock-in model was used to try to clone genes differentially regulated by AML1-ETO. One such gene was UBP43, a ubiquitin-specific protease, up-regulated in the knock-in mice. This gene is transiently up-regulated during monocytic differentiation, however enforced expression of the gene blocked differentiation without affecting proliferation. This may be one of the genetic anomalies responsible for the block of myeloid maturation in t(8;21)-AML (Liu et al., 1999).

In an attempt to bypass the lethal effects of the AML1-ETO allele, one group created a tetracycline inducible transgenic model for t(8;21) (Rhoades et al., 2000). Although the mice clearly expressed AML1-ETO in bone marrow and in macrophages, the mice did not exhibit abnormal hematopoiesis or any tendency to develop leukemia. However when progenitor cells from these mice were grown in vitro they again showed increased self-renewal as well as decreased differentiation. Removal of the AML1-ETO fusion protein led to increased differentiation as well as an increase in proliferation. This suggests that although the AML1-ETO containing progenitor cell itself may have a potential to grow in isolation, it has a proliferative disadvantage in vivo compared to normal stem cells. This notion is further supported by a conditional knock-in of the AML1-ETO cDNA that expressed the protein in the marrow of adult mice (Higuchi et al., 2000). Again progenitors from these mice can be serially passaged in vivo, but the mice did not develop leukemia during nearly a year of observation. Treatment of these mice with the a mutagen induced leukemia and granulocytic sarcoma similar to that seen in patients with t(8;21). This indicates that AML1-ETO is necessary but not sufficient for the development of leukemia and secondary mutations are required for the development of overt disease. The exact nature of these genetic changes is not yet certain, but it is now thought that the p53 system must be overcome to develop any malignancy (Sherr, 2000). The SKNO-1 and Kasumi-1 cell lines have mutated p53 genes but 0/28 patients with t(8;21) had such a mutation (Banker et al., 1998). However t(8;21) patients were found to have hypermethyaltion of the p15 and adjacent p16 gene (Wong et al., 2000). This may suppress expression of these cyclin dependent kinase inhibitors as well as the overlapping ARF gene that governs the function of MDM2, all of which could modulate p53 levels or efficacy in the cell. Future studies focusing on such changes in the murine model or interbreeding the AML1-ETO mice with mice deleted for some of these tumor suppressors should be informative as to the other steps required for the pathogenesis of t(8;21)-AML.

The AML1-ETO model and other disorders affecting AML1

In this review I have focused on the AML1-ETO fusion protein and AML1. However Table 3 indicates the variety of ways the AML1/CBFbeta complex is affected in multiple forms of leukemia. There remain several probable AML1 fusion genes that have not yet been characterized and could serve to identify genes, which like ETO, could have impotent regulatory roles in the cell.

The study of AML1-ETO has served as a paradigm for understanding the other abnormalities of the RUNX complex in AML. For example, the TEL-AML1 fusion found in ALL fuses the N-terminal dimerization and repression domain of the ets protein TEL to AML1 protein. As a result TEL-AML represses AML1 target genes by the recruitment of co-repressors and histone deacetylases to promoters (Chakrabarti and Nucifora, 1999; Fenrick et al., 1999; Guidez et al., 2000). Fusion of CBFbeta to MYH11 can sequester some AML1 in the cytoplasm (Adya et al., 1998; Kanno et al., 1998b) but the fusion protein can also be found in the nucleus. The MYH11 moiety contains a cryptic repression domain (Lutterbach et al., 1999). In t(3;21), the AML1 gene is fused to EVI1 which encodes a transcriptional repressor. The C-terminal activation domain of AML1 is replaced with the DNA binding and transcriptional effector domains of EVI1. The resulting protein represses target promoters (Sood et al., 1999). In all of these forms of leukemia, aberrant repression of AML1 target genes as well as activation of other targets by the pleiotropic actions of overexpresion of an aberrant repressor may largely explain the leukemic effect. This suggests that use of HDAC inhibitors might be considered in all of these disorders. The way in which point mutations of the AML1 gene may lead to the development of leukemia is more difficult to explain. Potentially a reduced gene dosage could lead to reduced levels of genes that inhibit the cell cycle or promote differentiation and could lead to a subpopulation of cells with increased proliferative potential. Accumulation of mutations in AML1 target genes or silencing of AML1 target genes already expressed at a reduced level by methylation might render the hematopoietic cell functionally null for a critical growth regulator and lead to leukemia.

Conclusion and future directions

The past decade has sent the range of tools available to the modern molecular biologist from structural biology to homologous recombination to attack the problem of M2 AML. This has led to a substantial increase in the understanding of the role of AML1 in normal and malignant blood development. Key for the full understanding of the t(8;21) translocation is the establishment of a faithful animal model of the disease. With this in hand the model of aberrant transcriptional repression and modulation of transcription can be tested with the increasing number of histone deacetylase inhibitors in development (Saunders et al., 1999). It is clear that the AML1-ETO fusion alone does not cause leukemia and a hunt for the secondary changes involved in the development of full-blown disease will be crucial. The use of replication-competent retroviruses to infect AML1-ETO harboring animals and induce leukemia offers the prospect of tagging and identifying these genes in the mouse. With the availability of the human genome sequence, the mutation and expression status of such genes in human leukemia patients could be rapidly determined. With the help of new technologies such as cDNA and protein arrays the next 5 years should also see the identification of the bona fide targets of AML1 and the identification of those AML1 genes de-regulated in t(8;21) AML. This offers the hope that targeted therapy of this form of leukemia akin to the use of ATRA in APL and STI571 in CML might one day be achieved.

Acknowledgements

Supported by NIH Grant CA59936 and ACS DHP 160. I would like to thank Scott Hiebert for his advice and for sharing unpublished results and Melanie McConnell and Dong-Er Zhang for critical reading of the manuscript.

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Figures

Figure 1 Structure of AML-1 and binding sites for partner proteins

Figure 2 Domain structure of AMLI, ETO and fusion proteins

Figure 3 Similarity of ETO family members

Figure 4 AMLI-ETO interacts with co-repressors

Tables

Table 1 The RUNX and ETO family genes

Table 2 Targets of AML1

Table 3 Abnormalities of AML1 in leukemia

10 September 2001, Volume 20, Number 40, Pages 5660-5679
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