Transcriptional regulation by the t(8;21) fusion protein, AML-1/ETO.
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Animals↗
Biomedical subjects
S W Hiebert
Publications and source records attributed to S W Hiebert.
CCAAT enhancer-binding protein (C/EBP) and AML1 (CBF alpha2) synergistically activate the macrophage colony-stimulating factor receptor promoter.
Transcription factors play a key role in the development and differentiation of specific lineages from multipotential progenitors. Identification of these regulators and determining the mechanism of how they activate their target genes are important for understanding normal development of monocytes and macrophages and the pathogenesis of a common form of adult acute leukemia, in which the differentiation of monocytic cells is blocked. Our previous work has shown that the monocyte-specific expression of the macrophage colony-stimulating factor (M-CSF) receptor is regulated by three transcription factors interacting with critical regions of the M-CSF receptor promoter, including PU.1 and AML1.PU.1 is essential for myeloid cell development, while the AML1 gene is involved in several common leukemia-related chromosome translocations, although its role in hematopoiesis has not been fully identified. Along with AML1, a third factor, Mono A, interacts with a small region of the promoter which can function as a monocyte-specific enhancer when multimerized and linked to a heterologous basal promoter. Here, we demonstrate by electrophoretic mobility shift assays with monocytic nuclear extracts, COS-7 cell-transfected factors, and specific antibodies that the monocyte-enriched factor Mono A is CCAAT enhancer-binding protein (C/EBP). C/EBP has been shown previously to be an important transcription factor involved in hepatocyte and adipocyte differentiation; in hematopoietic cells, C/EBP is specifically expressed in myeloid cells. In vitro binding analysis reveals a physical interaction between C/EBP and AML1. Further transfection studies show that C/EBP and AML1 in concert with the AML1 heterodimer partner CBF beta synergistically activate M-CSF receptor by more then 60 fold. These results demonstrate that C/EBP and AML1 are important factors for regulating a critical hematopoietic growth factor receptor, the M-CSF receptor, suggesting a mechanism of how the AML1 fusion protein could contribute to acute myeloid leukemia. Furthermore, they demonstrate physical and functional interactions between AML1 and C/EBP transcription factor family members.
Animals↗
The t(12;21) translocation converts AML-1B from an activator to a repressor of transcription.
The t(12;21) translocation is present in up to 30% of childhood B-cell acute lymphoblastic and fuses a potential dimerization motif from the ets-related factor TEL to the N terminus of AML1. The t(12;21) translocation encodes a 93-kDa fusion protein that localizes to a high-salt- and detergent-resistant nuclear compartment. This protein binds the enhancer core motif, TGTGGT, and interacts with the AML-1-binding protein, core-binding factor beta. Although TEL/AML-1B retains the C-terminal domain of AML-1B that is required for transactivation of the T-cell receptor beta enhancer, it fails to activate transcription but rather inhibits the basal activity of this enhancer. TEL/AML-1B efficiently interferes with AML-1B dependent transactivation of the T-cell receptor beta enhancer, and coexpression of wild-type TEL does not reverse this inhibition. The N-terminal TEL helix-loop-helix domain is essential for TEL/AML-1B-mediated repression. Thus, the t(12;21) fusion protein dominantly interferes with AML-1B-dependent transcription, suggesting that the inhibition of expression of AML-1 genes is critical for B-cell leukemogenesis.
Base Sequence↗
The AML1/ETO fusion protein blocks transactivation of the GM-CSF promoter by AML1B.
The t(8;21) translocation, commonly found in acute myelogenous leukemia (AML), generates a fusion protein containing N-terminal AML1 and C-terminal ETO amino acids. The human AML1 gene encodes several related proteins that specifically bind to the sequence TGT/cGGT, located in the promoter regions of a variety of hematopoietic growth factor genes. To examine the abilities of the AML1B protein (which contains 479 amino acids), a shorter AML1A isoform (which contains amino acids 1-250), and the AML1/ETO fusion protein (which contains AML1A amino acids 1-177) to stimulate transcription from the GM-CSF promoter, we performed co-transfection experiments in T cells using a human GM-CSF promoter-CAT reporter gene plasmid and expression vectors that contain the cDNAs for one of the above proteins. Our data demonstrate that AML1B, but not AML1A or AML1/ETO transactivates the GM-CSF promoter, requiring the TGTGGT sequence contained between base pairs -68 and -53. Furthermore, we show that AML1/ETO, but not AML1A, inhibits the ability of AML1B to stimulate CAT expression. Electrophoretic mobility shift assays demonstrated the specific binding of AML1 proteins to the GM-CSF promoter TGTGGT sequence, which does not require GM-CSF sequences immediately upstream of this binding site. Our data support a role for AML1B as a transcriptional activator and establish that the AML1/ETO fusion protein can act as a dominant negative protein on the human GM-CSF promoter. Although AML1/ETO does not stimulate the transcription of GM-CSF, it may function by inhibiting the normal activity of AML1B in AML cells with the t(8;21) translocation.
Base Sequence↗
Functional domains of the t(8;21) fusion protein, AML-1/ETO.
The AML-1/ETO fusion protein is created by the (8;21) translocation, the second most frequent chromosomal abnormality associated with acute myeloid leukemia. In the fusion protein the AML-1 runt homology domain, which is responsible for DNA binding and CBF beta interaction, is linked to ETO, a gene of unknown function. The primary sequences of the runt homology domain indicates no known DNA binding motifs, but is predicted to contain six beta-strands, two alpha-helices and a nucleotide binding motif. Mutagenesis of AML-1/ETO was performed to delimit the functional domains of the chimeric protein. Most mutations in the runt homology domain that resulted in reduced CBF beta binding also inhibited DNA binding, indicating that the DNA and CBF beta binding sequences are tightly linked. However, these activities were separated by a point mutation of residue 144, within the putative ATP binding motif, which nearly eliminated DNA binding, but did not affect CBF beta binding. Random mutagenesis identified the hydrophobic face of the amphipathic fifth beta-strand, adjacent to the putative ATP binding motif, as critical for both DNA and CBF beta binding. C-terminal deletion mutants of AML-1/ETO indicated that ETO sequences are essential for interference with AML-1B-mediated transcriptional activation, and that residue 540 defines the C-terminal boundary of a potential repression domain. Thus, these mutational analyses define the regions of AML-1/ETO which regulate its function and that may be important in promoting leukemia.
Adenosine Triphosphate↗
Heterogeneity in CBF beta/MYH11 fusion messages encoded by the inv(16)(p13q22) and the t(16;16)(p13;q22) in acute myelogenous leukemia.
Inv(16)(p13q22) is one of the most frequent chromosomal rearrangements found in acute myelogenous leukemia (AML), representing approximately 16% of documented karyotypic abnormalities. The inv(16) breakpoints have been cloned and shown to involve the non-DNA binding component of the AML-1 transcription factor complex termed core binding factor beta gene (CBF beta) on 16q and the smooth muscle myosin heavy chain gene (MYH11) on 16p. In this study, we analyzed 37 cases of inv(16)-containing AML and 4 cases with t(16;16)(p13;q22) for expression of the CBF beta/MYH11 chimeric message by reverse transcription-polymerase chain reaction (PCR) analysis. CBF beta/MYH11 chimeric messages were detected in 33 of 37 cases with the inv(16) and in the 4 t(16;16)-containing cases. Sequence analysis of PCR products showed extensive breakpoint heterogeneity in both CBF beta and MYH11. In addition to the previously described breakpoint in CBF beta at nucleotide (nt) 495 (amino acid 165), we have identified a second novel fusion point at nt 399 (amino acid 133) in 7% of the cases. Similarly, a unique breakpoint site was identified in MYH11 at nt 1098, as well as at three previously characterized sites at nts 994, 1201, and 1921. Of the 4 PCR-negative cases, 2 of 3 tested lacked CBF beta rearrangements by Southern blot analysis, suggesting the possible involvement of a different genomic locus in some cases with cytogenetic evidence of inv(16). To assess whether the portions of CBF-beta contained within the CBF beta/MYH11 chimeric products retain the ability to interact with their heterodimeric DNA-binding partner AML-1, we performed in vitro DNA-binding analysis. Recombinant CBF-beta polypeptides consisting of the N-terminal 165 amino acids retained their ability to interact with AML-1, whereas mutants containing only the N-terminal 133 amino acids interacted with AML-1 less efficiently. These data suggest that different CBF beta/MYH11 products may vary subtly in their biologic activities.
Adolescent↗
Fusion of the TEL gene on 12p13 to the AML1 gene on 21q22 in acute lymphoblastic leukemia.
Chromosomal rearrangements involving band 12p13 are found in a wide variety of human leukemias but are particularly common in childhood acute lymphoblastic leukemia. The genes involved in these rearrangements, however, have not been identified. We now report the cloning of a t(12;21) translocation breakpoint involving 12p13 and 21q22 in two cases of childhood pre-B acute lymphoblastic leukemia, in which t(12;21) rearrangements were not initially apparent. The consequence of the translocation is fusion of the helix-loop-helix domain of TEL, an ETS-like putative transcription factor, to the DNA-binding and transactivation domains of the transcription factor AML1. These data show that TEL, previously shown to be fused to the platelet-derived growth factor receptor beta in chronic myelomonocytic leukemia, can be implicated in the pathogenesis of leukemia through its fusion to either a receptor tyrosine kinase or a transcription factor. The TEL-AML1 fusion also indicates that translocations affecting the AML1 gene can be associated with lymphoid, as well as myeloid, malignancy.
Amino Acid Sequence↗
E2F-1:DP-1 induces p53 and overrides survival factors to trigger apoptosis.
The E2F DNA binding activity consists of a heterodimer between E2F and DP family proteins, and these interactions are required for association of E2F proteins with pRb and the pRb-related proteins p107 and p130, which modulate E2F transcriptional activities. E2F-1 expression is sufficient to release fibroblasts from G0 and induce entry into S phase, yet it also initiates apoptosis. To investigate the mechanisms of E2F-induced apoptosis, we utilized interleukin-3 (IL-3)-dependent 32D.3 myeloid cells, a model of hematopoietic progenitor programmed cell death. In the absence of IL-3, E2F-1 alone was sufficient to induce apoptosis, and p53 levels were diminished. DP-1 alone was not sufficient to induce cell cycle progression or alter rates of death following IL-3 withdrawal. However, overexpression of both E2F-1 and DP-1 led to the rapid death of cells even in the presence of survival factors. In the presence of IL-3, levels of endogenous wild-type p53 increased in response to E2F-1, and coexpression of DP-1 further augmented p53 levels. These results provide evidence that E2F is a functional link between the tumor suppressors p53 and pRb. However, induction of p53 alone was not sufficient to trigger apoptosis, suggesting that the ability of E2F to override survival factors involves additional effectors.
Animals↗
The t(8;21) fusion protein interferes with AML-1B-dependent transcriptional activation.
The AML-1/CBF beta transcription factor complex is targeted by both the t(8;21) and the inv(16) chromosomal alterations, which are frequently observed in acute myelogenous leukemia. AML-1 is a site-specific DNA-binding protein that recognizes the enhancer core motif TGTGGT. The t(8;21) translocation fuses the first 177 amino acids of AML-1 to MTG8 (also known as ETO), generating a chimeric protein that retains the DNA-binding domain of AML-1. Analysis of endogenous AML-1 DNA-binding complexes suggested the presence of at least two AML-1 isoforms. Accordingly, we screened a human B-cell cDNA library and isolated a larger, potentially alternatively spliced, form of AML1, termed AML1B. AML-1B is a protein of 53 kDa that binds to a consensus AML-1-binding site and complexes with CBF beta. Subcellular fractionation experiments demonstrated that both AML-1 and AML-1/ETO are efficiently extracted from the nucleus under ionic conditions but that AML-1B is localized to a salt-resistant nuclear compartment. Analysis of the transcriptional activities of AML-1, AML-1B, and AML-1/ETO demonstrated that only AML-1B activates transcription from the T-cell receptor beta enhancer. Mixing experiments indicated that AML-1/ETO can efficiently block AML-1B-dependent transcriptional activation, suggesting that the t(8;21) translocation creates a dominant interfering protein.
Alternative Splicing↗
Indirect and direct disruption of transcriptional regulation in cancer: E2F and AML-1.
The disruption of transcriptional regulatory circuits through the elimination of negative regulatory factors (tumor suppressors), the activation of positive acting factors (oncogenes), or when chimeric proteins result from chromosomal translocations, is likely a key event in multistep tumorigenesis. Here, using the transcription factors E2F and AML-1 as model systems, we discuss the disruption of coordinate transcriptional regulation in oncogenesis. E2F oncogenic signals are released when the pRb tumor suppressor is inactivated, and E2F activation may necessitate the coordinate inactivation of a second tumor suppressor, p53. AML-1 is the target of the (8;21) translocation, found in approximately 15% of acute myeloid leukemia (AML) cases, and the t(12;21), found in up to 30% of childhood B-cell acute lymphoblastic leukemias. The t(8;21) creates a fusion protein between AML-1 and a gene of unknown function, mtg8 (ETO), whereas the t(12;21) fuses the TEL (translocation-ets-leukemia) transcription factor to the N-terminus of AML-1. The inv(16), which is the most frequent anomaly found in AML, also targets AML-1, by fusing the gene that encodes AML-1's heterodimeric partner CBF beta to the smooth muscle myosin heavy chain gene MYHll. Thus, E2F and AML-1 provide excellent models for the disruption of transcriptional regulation in cancer.
Carrier Proteins↗
An E2F dominant negative mutant blocks E1A induced cell cycle progression.
E2F is a cellular transcription factor that is regulated during the cell cycle through interactions with the product of the retinoblastoma susceptibility gene (RB1) and the pRb-like p107 and p130 proteins. Analysis of mutations within both adenovirus E1A and pRb, which affected their ability to regulate cellular proliferation and alter E2F activity, suggested that E2F may play a role in cell cycle progression. Microinjection of a GST-E2F-1 fusion protein into quiescent Balb/c 3T3 cells induced DNA synthesis whereas co-injection of GST-E2F-1 and GST-E2F(95-191) protein, encoding only the DNA binding domain of E2F-1, blocked the induction of S-phase. While E1A likely targets multiple cellular pathways, co-injection of the GST-E2F(95-191) dominant inhibitory protein with 12S E1A protein blocked E1A-mediated induction of DNA synthesis, suggesting that the E2F-dependent pathway is dominant. Analysis of the interval required for microinjected quiescent cells to enter S-phase indicated that E2F-1 acted faster than either E1A or serum.
3T3 Cells↗
Dual control of myc expression through a single DNA binding site targeted by ets family proteins and E2F-1.
NIH3T3 cells expressing a mutant colony-stimulating factor-1 receptor (CSF-1R) containing a phenylalanine for tyrosine substitution in the tyrosine kinase domain at codon 809 exhibit defective myc regulation and do not enter S phase when stimulated by CSF-1. Enforced expression of either ets-1 or ets-2 in these cells restores their mitogenic response, albeit less efficiently than myc itself, suggesting that ets proteins may regulate c-myc expression. Ets-1 transactivates reporter genes driven by the human and mouse c-myc promoters through the binding site for the transcription factor E2F, the latter being required for E1A- and serum-induced c-myc expression. Analysis of E2F-1 sequences identified a minimal DNA binding domain that is related to those of ets proteins. Although E2F and ets proteins interact with similar consensus DNA binding sites, in vitro binding assays revealed that E2F can bind DNA as a homodimer, whereas ets proteins bind these sites as monomers. E2F and ets proteins do not form heterodimers in vitro and do not transactivate c-myc synergistically. Thus, E2F-1 and ets family members may independently regulate c-myc transcription through the same binding site at different times following growth factor stimulation.
Amino Acid Sequence↗
Direct binding of cyclin D to the retinoblastoma gene product (pRb) and pRb phosphorylation by the cyclin D-dependent kinase CDK4.
The product (pRb) of the retinoblastoma gene (RB-1) prevents S-phase entry during the cell cycle, and inactivation of this growth-suppressive function is presumed to result from pRb hyperphosphorylation during late G1 phase. Complexes of the cyclin-dependent kinase, cdk4, and each of three different D-type cyclins, assembled in insect Sf9 cells, phosphorylated a pRb fusion protein in vitro at sites identical to those phosphorylated in human T cells. Only D-type cyclins activated cdk4 enzyme activity, whereas cyclins A, B1, and E did not. When Sf9 cells were coinfected with baculovirus vectors encoding human pRb and murine D-type cyclins, cyclins D2 and D3, but not D1, bound pRb with high stoichiometry in intact cells. Introduction of a vector encoding cdk4, together with those expressing pRb and D-type cyclins, induced pRb hyperphosphorylation and dissociation of cyclins D2 and D3, whereas expression of a kinase-defective cdk4 mutant in lieu of the wild-type catalytic subunit yielded ternary complexes. The transcription factor E2F-1 also bound to pRb in insect cells, and coexpression of cyclin D-cdk4 complexes, but neither subunit alone, triggered pRb phosphorylation and prevented its interaction with E2F-1. The D-type cyclins may play dual roles as cdk4 regulatory subunits and as adaptor proteins that physically target active enzyme complexes to particular substrates.
Adenosine Triphosphate↗
Identification of AML-1 and the (8;21) translocation protein (AML-1/ETO) as sequence-specific DNA-binding proteins: the runt homology domain is required for DNA binding and protein-protein interactions.
The AML1 gene on chromosome 21 is disrupted in the (8;21)(q22;q22) translocation associated with acute myelogenous leukemia and encodes a protein with a central 118-amino-acid domain with 69% homology to the Drosophila pair-rule gene, runt. We demonstrate that AML-1 is a DNA-binding protein which specifically interacts with a sequence belonging to the group of enhancer core motifs, TGT/cGGT. Electrophoretic mobility shift analysis of cell extracts identified two AML-1-containing protein-DNA complexes whose electrophoretic mobilities were slower than those of complexes formed with AML-1 produced in vitro. Mixing of in vitro-produced AML-1 with cell extracts prior to gel mobility shift analysis resulted in the formation of higher-order complexes. Deletion mutagenesis of AML-1 revealed that the runt homology domain mediates both sequence-specific DNA binding and protein-protein interactions. The hybrid product, AML-1/ETO, which results from the (8;21) translocation and retains the runt homology domain, both recognizes the AML-1 consensus sequence and interacts with other cellular proteins.
Animals↗
Regions of the retinoblastoma gene product required for its interaction with the E2F transcription factor are necessary for E2 promoter repression and pRb-mediated growth suppression.
Studies of naturally occurring mutations of the RB1 tumor suppressor gene have indicated that the E1A/T antigen-binding domain is important for pRb function. Mutations engineered within the C-terminal 135 amino acids of pRb also abrogate its growth-suppressive function during the G1 interval of the cell cycle. Both the pRb E1A/T antigen-binding domain and the C-terminal domain are required for interaction with the E2F transcription factor. A series of mutated pRb proteins has been used to define the C-terminal sequences which determine E2F binding, adenovirus E2 promoter inhibition, and negative growth control. Deletion of the C terminus to residue 870 allowed full pRb function, while further deletion to residue 841 inactivated pRb in each assay. Amino acid sequences immediately C-terminal to the E1A/T antigen-binding domain were absolutely required for pRb activity. Mutations which prevented pRb from interacting with E2F also eliminated pRb-mediated E2 promoter repression and inactivated the ability of pRb to suppress cell growth.
Adenovirus E1A Proteins↗
The interaction of RB with E2F coincides with an inhibition of the transcriptional activity of E2F.
Recent experiments have shown that the E2F transcription factor is in a complex with the RB1 gene product. The E2F-pRB complex can be reconstituted in an in vitro assay using a GST-RB fusion protein isolated from Escherichia coli. This interaction is dependent on pRB sequences involved in E1A/T-antigen binding as well as carboxy-terminal pRB sequences that are not necessary for E1A/T binding. Moreover, reconstitution assays reveal a requirement for an accessory factor, in addition to E2F and pRB, for formation of the E2F-pRB complex. Assays of transcription from the adenovirus E2 promoter in transfection experiments demonstrate that formation of the complex containing pRB and E2F coincides with an inhibition of E2F-dependent transcriptional activity. A mutant pRB protein that does not associate with E2F does not inhibit transcription. We conclude that as a consequence of its interaction with E2F, pRB may regulate the transcriptional function of the E2F factor.
Adenovirus Early Proteins↗
Cell cycle regulation of the E2F transcription factor involves an interaction with cyclin A.
We have examined E2F binding activity in extracts of synchronized NIH 3T3 cells. During the G0 to G1 transition, there is a marked increase in the level of active E2F. Subsequently, there are changes in the nature of E2F-containing complexes. A G1-specific complex increases in abundance, disappears, and is then replaced by another complex as S phase begins. Analysis of extracts of thymidine-blocked cells confirms that the complexes are cell cycle regulated. We also show that the cyclin A protein is a component of the S phase complex. Each complex can be dissociated by the adenovirus E1A 12S product, releasing free E2F. The release of E2F from the cyclin A complex coincides with the stimulation of an E2F-dependent promoter. We suggest that these interactions control the activity of E2F and that disruption of the complexes by E1A contributes to a loss of cellular proliferation control.
Adenovirus Early Proteins↗
Role of E2F transcription factor in E1A-mediated trans activation of cellular genes.
Adenovirus E1A-dependent trans activation of the adenovirus E2 gene involves the activation of the cellular transcription factor E2F. E2F binding sites have also been identified in the 5'-flanking region of a number of cellular genes, raising the possibility that such genes are targets for E1A trans activation. We now demonstrate that two genes that possess E2F recognition sites, N-myc and DHFR, are stimulated by E1A, dependent on the E2F sites. We also find that although there are multiple E2F sites in these promoters, a single intact E2F binding site is sufficient for E1A-mediated induction, although not to the full wild-type level. These results thus demonstrate that a variety of cellular genes that possess E2F binding sites are subject to E1A trans activation. Moreover, since the products of most of these genes are likely critical for cellular proliferation, there are obvious consequences of this trans activation for cellular phenotype.
Adenovirus Early Proteins↗