Identification of two transcripts of AML1/ETO-fused gene in t(8;21) leukemic cells and expression of wild-type ETO gene in hematopoietic cells

Fusion transcripts: Unexploited vulnerabilities in cancer?

Gene fusions are an important class of mutations in several cancer types and include genomic rearrangements that fuse regulatory or coding elements from two different genes. Analysis of the genetics of cancers harboring fusion oncogenes and the proteins they encode have enhanced cancer diagnosis and in some cases patient treatment. However, the effect of the complex structure of fusion genes on the biogenesis of the resulting chimeric transcripts they express is not well studied. There are two potential RNA‐related vulnerabilities inherent to fusion‐driven cancers: (a) the processing of the fusion precursor messenger RNA (pre‐mRNA) to the mature mRNA and (b) the mature mRNA. In this study, we discuss the effects that the genetic organization of fusion oncogenes has on the generation of translatable mature RNAs and the diversity of fusion transcripts expressed in different cancer subtypes, which can fundamentally influence both tumorigenesis and treatment. We also discuss functional genomic approaches that can be utilized to identify proteins that mediate the processing of fusion pre‐mRNAs. Furthermore, we assert that an enhanced understanding of fusion transcript biogenesis and the diversity of the chimeric RNAs present in fusion‐driven cancers will increase the likelihood of successful application of RNA‐based therapies in this class of tumors.

This article is categorized under:

RNA Processing > RNA Editing and Modification

RNA Processing > Splicing Regulation/Alternative Splicing

RNA in Disease and Development > RNA in Disease

Decoding of exon splicing patterns in the human RUNX1-RUNX1T1 fusion gene

The t(8;21) translocation is the most widespread genetic defect found in human acute myeloid leukemia. This translocation results in the RUNX1-RUNX1T1 fusion gene that produces a wide variety of alternative transcripts and influences the course of the disease. The rules of combinatorics and splicing of exons in the RUNX1-RUNX1T1 transcripts are not known. To address this issue, we developed an exon graph model of the fusion gene organization and evaluated its local exon combinatorics by the exon combinatorial index (ECI). Here we show that the local exon combinatorics of the RUNX1-RUNX1T1 gene follows a power-law behavior and (i) the vast majority of exons has a low ECI, (ii) only a small part is represented by "exons-hubs" of splicing with very high ECI values, and (iii) it is scale-free and very sensitive to targeted skipping of "exons-hubs". Stochasticity of the splicing machinery and preferred usage of exons in alternative splicing can explain such behavior of the system. Stochasticity may explain up to 12% of the ECI variance and results in a number of non-coding and unproductive transcripts that can be considered as a noise. Half-life of these transcripts is increased due to the deregulation of some key genes of the nonsense-mediated decay system in leukemia cells. On the other hand, preferred usage of exons may explain up to 75% of the ECI variability. Our analysis revealed a set of splicing-related cis-regulatory motifs that can explain "attractiveness" of exons in alternative splicing but only when they are considered together. Cis-regulatory motifs are guides for splicing trans-factors and we observed a leukemia-specific profile of expression of the splicing genes in t(8;21)-positive blasts. Altogether, our results show that alternative splicing of the RUNX1-RUNX1T1 transcripts follows strict rules and that the power-law component of the fusion gene organization confers a high flexibility to this process.

Transcription of the AML1/ETO chimera is guided by the P2 promoter of the AML1 gene in the Kasumi-1 cell line

Chromosomal translocation t (8;21)(q22;22) is one of the most frequent cytogenetic abnormalities found in acute myeloid leukaemia (AML). It generates the AML1/ETO fusion gene, which itself supports human haematopoietic stem cell self-renewal. However, the mechanism guiding transcription of this chimeric gene remains unclear. In our work, we attempted to shed light on this essential issue. We investigated the promoter from which transcription of the AML1/ETO gene is initiated and defined the three-dimensional structure of the whole rearranged locus.

Phf14, a novel regulator of mesenchyme growth via platelet-derived growth factor (PDGF) receptor-α

The regulation of mesenchymal cell growth by signaling molecules plays an important role in maintaining tissue functions. Aberrant mesenchymal cell proliferation caused by disruption of this regulatory process leads to pathogenetic events such as fibrosis. In the current study we have identified a novel nuclear factor, Phf14, which controls the proliferation of mesenchymal cells by regulating PDGFRα expression. Phf14-null mice died just after birth due to respiratory failure. Histological analyses of the lungs of these mice showed interstitial hyperplasia with an increased number of PDGFRα(+) mesenchymal cells. PDGFRα expression was elevated in Phf14-null mesenchymal fibroblasts, resulting in increased proliferation. We demonstrated that Phf14 acts as a transcription factor that directly represses PDGFRα expression. Based on these results, we used an antibody against PDGFRα to successfully treat mouse lung fibrosis. This study shows that Phf14 acts as a negative regulator of PDGFRα expression in mesenchymal cells undergoing normal and abnormal proliferation, and is a potential target for new treatments of lung fibrosis.

Expression of AML/Runx and ETO/MTG family members during hematopoietic differentiation of embryonic stem cells

Runx1/AML1 plays important roles in hematopoiesis, including the commitment of cells to hematopoiesis during embryonic development, and in the maintenance of hematopoietic cell populations. It is also one of the most common genes involved in chromosomal translocations related to leukemia. One such translocation is t(8;21), which fuses the Runx1 gene to the MTG8/ETO gene and generates the Runx1-MTG8 (AML1-ETO) fusion gene. Both Runx1 and MTG8 have two additional family members that are much less studied in hematopoiesis. Here we report the expression of every member of the Runx and MTG families as well as the Runx heterodimerization partner CBFbeta during hematopoietic differentiation of murine embryonic stem cells. We observed substantially increased expression of Runx1, Runx2, and MTG16 during hematopoietic differentiation. Furthermore, the increase in Runx2 expression is delayed relative to Runx1 expression, suggesting their possible sequential contribution to hematopoiesis.

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

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

The Leukemia-associated ETO homologues are differently expressed during hematopoietic differentiation

The Eight twenty-one (ETO) homologues are nuclear repressor proteins including ETO, myeloid-transforming gene-related protein 1 (MTGR1), and myeloid-transforming gene chromosome 16 (MTG16). ETO and MTG16 are both part of fusion proteins resulting from chromosomal translocations associated with acute myeloid leukemia. Expression of these chimeras results in a differentiation block that contributes to the onset of leukemia. In order to elucidate the relation between the ETO homologues and hematopoietic differentiation, we determined the expression of the homologues during differentiation of leukemic and normal hematopoietic cells. Our results showed MTGR1 and MTG16 to be ubiquitously expressed in leukemic cell lines, whereas expression of ETO was observed only in an erythroleukemic cell line. The MTGR1 and MTG16 proteins decreased during all trans-retinoic acid-, but not vitamin D(3)-induced differentiation of leukemic cells. The reduction seemed to reflect a decrease in transcript levels as well as in protein stability. MTGR1 transcripts were ubiquitously expressed in human bone marrow cells. The MTG16 transcripts of CD34(+) progenitor cells were rapidly downregulated by cytokine-induced differentiation into myeloid or erythroid lineages. ETO transcripts, present at very low abundance in CD34(+) progenitor cells, were transiently upregulated during erythroid differentiation. In conclusion, the differential expression of the ETO homologues suggests that they may have a potential role in hematopoietic differentiation.

Deletion of an AML1-ETO C-terminal NcoR/SMRT-interacting region strongly induces leukemia development

Normal blood-cell differentiation is controlled by regulated gene expression and signal transduction. Transcription deregulation due to chromosomal translocation is a common theme in hematopoietic neoplasms. AML1-ETO, which is a fusion protein generated by the 8;21 translocation that is commonly associated with the development of acute myeloid leukemia, fuses the AML1 runx family DNA-binding transcription factor to the ETO corepressor that associates with histone deacetylase complexes. Analyses have demonstrated that AML1-ETO blocks AML1 function and requires additional mutagenic events to promote leukemia. Here, we report that the loss of the molecular events of AML1-ETO C-terminal NCoR/SMRT-interacting domain transforms AML1-ETO into a potent leukemogenic protein. Contrary to full-length AML1-ETO, the truncated form promotes in vitro growth and does not obstruct the cell-cycle machinery. These observations suggest a previously uncharacterized mechanism of tumorigenesis, in which secondary mutation(s) in molecular events disrupting the function of a domain of the oncogene promote the development of malignancy.

Screening and quantification of multiple chromosome translocations in human leukemia

BACKGROUND

Characterization of fusion gene transcripts in leukemia that result from chromosome translocations provides valuable information regarding appropriate treatment and prognosis. However, screening for multiple fusion gene transcripts is difficult with conventional PCR and state-of-the-art real-time PCR and high-density microarrays.

METHODS

We developed a multiplex reverse transcription-PCR (RT-PCR) assay for screening and quantification of fusion gene transcripts in human leukemia cells. Chimeric primers were used that contained gene-specific and universal sequences. PCR amplification of fusion and control gene transcripts was achieved with use of an excess of universal primers to allow the ratio of abundance of fusion gene to endogenous or exogenous controls to be maintained throughout PCR. Multiplex RT-PCR products analyzed by an ABI 310 Genetic Analyzer were consistent with those of duplex RT-PCR (single analytical sample plus control). In addition, multiplex RT-PCR results were analyzed by an assay using an oligonucleotide microarray that contained probes for the splice-junction sequences of various fusion transcripts.

RESULTS

The multiplex RT-PCR assay enabled screening of >10 different fusion gene transcripts in a single reaction. RT-PCR followed by analysis with the ABI Prism 310 Genetic Analyzer consistently detected 1 fusion-transcript-carrying leukemia cell in 100-10 000 cells. The assay covered a 1000-fold range. Preliminary results indicate that multiplex RT-PCR products can also be analyzed by hybridization-based microarray assay.

CONCLUSIONS

The multiplex RT-PCR analyzed by either ABI Prism 310 Genetic Analyzer or microarray provides a sensitive and specific assay for screening of multiple fusion transcripts in leukemia, with the latter an assay that is adaptable to a high-throughput system for clinical screening.

The role of a Runt domain transcription factor AML1/RUNX1 in leukemogenesis and its clinical implications

A Runt domain transcription factor AML1/RUNX1 is essential for generation and differentiation of definitive hematopoietic stem cells. AML1 is the most frequent target of chromosomal translocations in acute leukemias. Several chimeric proteins such as AML1-MTG8 and TEL-AML1 have transdominant properties for wild-type AML1 and acts as transcriptional repressors. The transcriptional repression in AML1 fusion proteins is mediated by recruitment of nuclear corepressor complex that maintains local histone deacetylation. Inhibition of the expression of AML1-responsive genes leads to a block in hematopoietic cell differentiation and consequent leukemic transformation. On the other hand, mutations in the Runt domain of the AML1 are identified in both sporadic acute myeloblastic leukemia (AML) without AML1 translocation and familial platelet disorder with predisposition to AML. These observations indicate that a decrease in AML1 dosage resulting from chromosomal translocations or mutations contributes to leukemogenesis. Furthermore, dysregulated chromatin remodeling and transcriptional control appears to be a common pathway in AML1-associated leukemias that could be an important target for the development of new therapeutic agents.

The myeloid master regulator transcription factor PU.1 is inactivated by AML1-ETO in t(8;21) myeloid leukemia

The transcription factor PU.1 plays a pivotal role in normal myeloid differentiation. PU.1(-/-) mice exhibit a complete block in myeloid differentiation. Heterozygous PU.1 mutations were reported in some patients with acute myeloid leukemia (AML), but not in AML with translocation t(8;21), which gives rise to the fusion gene AML1-ETO. Here we report a negative functional impact of AML1-ETO on the transcriptional activity of PU.1. AML1-ETO physically binds to PU.1 in t(8;21)(+) Kasumi-1 cells. AML1-ETO binds to the beta(3)beta(4) region in the DNA-binding domain of PU.1 and displaces the coactivator c-Jun from PU.1, thus down-regulating the transcriptional activity of PU.1. This physical interaction of AML1-ETO and PU.1 did not abolish the DNA-binding capacity of PU.1. AML1-ETO down-regulates the transactivation capacity of PU.1 in myeloid U937 cells, and the expression levels of PU.1 target genes in AML French-American-British (FAB) subtype M2 patients with t(8;21) were lower than in patients without t(8;21). Conditional expression of AML1-ETO causes proliferation in mouse bone marrow cells and inhibits antiproliferative function of PU.1. Overexpression of PU.1, however, differentiates AML1-ETO-expressing Kasumi-1 cells to the monocytic lineage. Thus, the function of PU.1 is down-regulated by AML1-ETO in t(8;21) myeloid leukemia, whereas overexpression of PU.1 restores normal differentiation.

Isolation and characterization of runxa and runxb, zebrafish members of the runt family of transcriptional regulators

OBJECTIVE

The AML/RUNX family of transcription factors plays important roles in hematopoiesis, neurogenesis, bone development, and segmentation in vertebrate embryos. The aim of this study was to isolate runt-related genes in a genetically and embryologically exploitable system, the zebrafish, and characterize their function during hematopoietic development.

MATERIALS AND METHODS

Two runt-related genes were isolated by degenerate PCR and standard library screening, and a radiation hybrid panel, T51 RH, was used to resolve their chromosomal localization. In situ hybridization demonstrated their expression whereas their transcriptional activity was assessed using an AML1-responsive reporter gene in the MLA 144 T-cell line.

RESULTS

We isolated the zebrafish runxa and runxb cDNAs, which encode proteins highly homologous to the human and murine Runx1 (AML1) and Runx3 (AML2) proteins. In contrast to a recent report, we detected runxa expression in both hematopoietic and neural tissues of the developing zebrafish. runxa transcripts first appear during segmentation in bilateral mesodermal cells that coexpress one of the earliest blood and endothelial cell markers, scl/tal-1. By 24 hours postfertilization (hpf), runxa transcripts are seen in the ventral wall of the dorsal aorta. Hematopoietic runxa expression is lost in cloche mutants, which are defective in blood and endothelial cell formation. runxb transcripts are seen in nonhematopoietic domains. Both Runxa and Runxb transactivate an AML1-responsive human promoter in hematopoietic cells. Genomic localization studies demonstrate that runxa is located on linkage group 1 (LG1), and the runxb gene is located on LG13.

CONCLUSIONS

Our gene expression analysis strongly suggests that both the functional and spatial aorta-gonad-mesonephros (AGM) region has been conserved throughout evolution. Our runxa spatiotemporal expression data shed light on the role of vertebrate Runx1/AML1 in primitive vs definitive hematopoietic development.

The role of the AML1 transcription factor in leukemogenesis

Chromosomal translocations are one of the hallmarks of human leukemias. These structural abnormalities result in the generation of genetic mutations that play a direct role in the transformation of hematopoietic stem cells. Some of the most common targets of these chromosomal rearrangements are the genes that encode the AML1/CBFbeta transcription factor complex. The AML1/CBFbeta complex plays a critical role in normal hematopoiesis, controlling the initiation of a transcriptional cascade required for the formation of definitive hematopoietic stem cells. Understanding how alterations in the normal biologic activity of this transcription factor complex lead to the initiation of leukemia will provide critical insights in the molecular pathogenesis of this disease. These insights in turn are likely to lead to the development of more rational approaches to the treatment of acute leukemia. In this review, we will summarize our current understanding of the mechanisms by which alterations in the activity of AML1/CBFbeta contribute to the development of leukemia.

Induction of MTG8-specific cytotoxic T-cell lines: MTG8 is probably a tumour antigen that is recognized by cytotoxic T cells in AML1-MTG8-fused gene-positive acute myelogenous leukaemia

Several reports have demonstrated the persistent detection of AML1-MTG8 fusion products, representing minimal residual disease (MRD), in patients with t(8;21) acute myelogenous leukaemia (AML) who are in long-term remission. It is probable that immune-mediated mechanisms that are able to suppress the expansion of MRD may result in the continuance of remission. It was previously shown that some t(8;21) AML patients had high anti-MTG8 antibody titres. MTG8 expression in normal adult tissues is limited to the brain or heart in which human leucocyte antigen (HLA) class I cell-surface antigens are either not or are only faintly detectable. We hypothesized that the overexpression of the MTG8 gene in t(8;21) AML cells could act as a possible tumour antigen, which might be able to induce the immune-mediated suppression of the expansion of MRD. We were able to induce HLA-A0201-restricted cytotoxic T-lymphocyte (CTL) lines against an MTG8 peptide (MTG8b amino acids 182-191) using monocyte-derived dendritic cells from a healthy donor. T-cell receptor (TCR)Valpha17, TCRVbeta14 and 15, and TCRJbeta2.1 and 2.3 are predominantly used in these CTL lines. Our data, which suggest that the MTG8 protein could be one of the tumour antigens recognized by CTLs, may be helpful in further investigations of TCR analysis in t(8;21) AML patients with HLA-A0201 who are in long-term remission.

Alterations of the AML1 transcription factor in human leukemia

The identification of clonal chromosomal translocations in human leukemias provided one of the first insights into the underlying pathogenesis of this clinically heterogeneous disease. Over the last decade a large number of these chromosomal rearrangements have been molecularly cloned and the involved genes identified. A surprising finding that has emerged from this work is that many of these chromosomal alterations target the genes encoding the AML1/CBFbeta transcription factor complex, a critical regulator of normal hematopoiesis. In this review, we summarize our present understanding of the mechanisms through which alterations of AML1/CBFbeta contribute to leukemogenesis.

The ETO protein disrupted in t(8;21)-associated acute myeloid leukemia is a corepressor for the promyelocytic leukemia zinc finger protein

The ETO protein was originally identified by its fusion to the AML-1 transcription factor in translocation (8;21) associated with the M2 form of acute myeloid leukemia (AML). The resulting AML-1-ETO fusion is an aberrant transcriptional regulator due to the ability of ETO, which does not bind DNA itself, to recruit the transcriptional corepressors N-CoR, SMRT, and Sin3A and histone deacetylases. The promyelocytic leukemia zinc finger (PLZF) protein is a sequence-specific DNA-binding transcriptional factor fused to retinoic acid receptor alpha in acute promyelocytic leukemia associated with the (11;17)(q23;q21) translocation. PLZF also mediates transcriptional repression through the actions of corepressors and histone deacetylases. We found that ETO is one of the corepressors recruited by PLZF. The PLZF and ETO proteins associate in vivo and in vitro, and ETO can potentiate transcriptional repression by PLZF. The N-terminal portion of ETO forms complexes with PLZF, while the C-terminal region, which was shown to bind to N-CoR and SMRT, is required for the ability of ETO to augment transcriptional repression by PLZF. The second repression domain (RD2) of PLZF, not the POZ/BTB domain, is necessary to bind to ETO. Corepression by ETO was completely abrogated by histone deacetylase inhibitors. This identifies ETO as a cofactor for a sequence-specific transcription factor and indicates that, like other corepressors, it functions through the action of histone deactylase.

Characterization of hematopoietic lineage-specific gene expression by ES cell in vitro differentiation induction system

The continuous generation of mature blood cells from hematopoietic progenitor cells requires a highly complex series of molecular events. To examine lineage-specific gene expression during the differentiation process, we developed a novel method combiningLacZ reporter gene analysis with in vitro hematopoietic differentiation induction from mouse embryonic stem cells. For a model system using this method, we chose the erythroid and megakaryocytic differentiation pathways. Although erythroid and megakaryocytic cells possess distinct functional and morphologic features, these 2 lineages originate from bipotential erythro-megakaryocytic progenitors and share common lineage-restricted transcription factors. A portion of the 5′ flanking region of the human glycoprotein IIb (IIb) integrin gene extending from base −598 to base +33 was examined in detail. As reported previously, this region is sufficient for megakaryocyte-specific gene expression. However, previous reports that used human erythro-megakaryocytic cell lines suggested that one or more negative regulatory regions were necessary for megakaryocyte-specific gene expression. Our data clearly showed that an approximately 200-base enhancer region extending from −598 to −400 was sufficient for megakaryocyte-specific gene expression. This experimental system has advantages over those using erythro-megakaryocytic cell lines because it recapitulates normal hematopoietic cell development and differentiation. Furthermore, this system is more efficient than transgenic analysis and can easily examine gene expression with null mutations of specific genes.

Simple variant t(8;21) acute myeloid leukemias harbor insertions of the AML1 or ETO genes

We report on the molecular characterization of two acute myeloid leukemias (AML), one AML-M1 (patient 1) and one AML-M2 (patient 2) with t(8;21)(p21;q22) and t(8;20)(q22;p13), respectively, at diagnosis. The locations of the breakpoints, 21q22 in patient 1 and 8q22 in patient 2, prompted us to search for a cryptic t(8;21)(q22;q22) and involvement of the AML1 and ETO genes. Dual-color fluorescence in situ hybridization (FISH) using whole chromosome painting probes for chromosomes 8, 20, and 21 confirmed the conventional cytogenetic karyotypes. However, dual-color FISH using appropriate ETO and AML1 probes disclosed an insertion of AML1 into 8q22 on the derivative chromosome 8 in patient 1 and of ETO into 21q22 on one chromosome 21 in patient 2, leading to AML1-ETO fusion signals. Both cases expressed an AML1-ETO transcript, shown by reverse transcriptase polymerase chain reaction and cDNA sequencing. Creation of functional AML1-ETO fusion genes in these two simple variant t(8;21) probably occurred through complex mechanisms, combining translocation and insertion of chromosomal segments.

Association of MTG8 (ETO/CDR), a leukemia-related protein, with serine/threonine protein kinases and heat shock protein HSP90 in human hematopoietic cell lines

A proto-oncogene, MTG8 (ETO/CDR), is disrupted in the t(8;21) translocation associated with acute myeloid leukemia, and the gene product, MTG8, is a phosphoprotein capable of cell transformation in concert with v-H-ras. To obtain insight into functional regulation of MTG8 by phosphorylation, we studied protein kinases that interact with, and phosphorylate, MTG8 in vitro. Recombinant MTG8 protein was first found to be associated with two serine/threonine protein kinases in cell extracts from both HEL cells and a leukemic cell line carrying t(8;21). A cytoplasmic protein kinase of 61 kDa (MTG8N-kinase) phosphorylated the amino-terminal of MTG8, and another of 52 kDa (MTG8C-kinase) phosphorylated the carboxyl-terminal domain. In addition, we demonstrated that heat shock protein 90 (HSP90) specifically binds to the amino-terminal domain of MTG8 in vitro and in vivo. Thus, our results shed new light on post-translational regulation of MTG8, perturbation of which, in AML1-MTG8 protein, probably contributes to leukemogenesis.

Acute myelomonoblastic leukemia carrying the PEBP2beta/MYH11 fusion gene

As recurrent chromosome abnormalities in leukemia are highly associated with particular subtypes, the genetic events of specific chromosome alteration must be associated with leukemogenesis and characteristics of the disease. The chromosomal breakpoints involved in inv(16) and t(16;16) have been shown to generate the fusion gene PEBP2beta(CBFbeta)/MYH11. The PEBP2beta/MYH11 fusion transcripts in all 8 patients with M4Eo, 2 of 18 with M4, and one CML in the blastic phase were detected by using RT-PCR and Southern blotting. We demonstrated the marked expression of CD34 and c-KIT (CD117) antigens in myelomonoblastic leukemia cells from all patients carrying this fusion gene, which was in contrast to the patients with M4 but without the fusion gene. These results indicate that immunophenotypic analysis is useful for detection of leukemia with the fusion gene, and that the PEBP2beta/MYH11 fusion gene is involved in immature cells expressing CD34 and c-KIT antigens.

The capacity of polyomavirus enhancer binding protein 2alphaB (AML1/Cbfa2) to stimulate polyomavirus DNA replication is related to its affinity for the nuclear matrix

The nuclear matrix is thought to play an important role in the DNA replication of eukaryotic cells, although direct evidence for such a role is still lacking. A nuclear matrix-associated transcription factor, polyomavirus (Py) enhancer binding protein 2alphaB1 (PEBP2alphaB1) (AML1/Cbfa2), was found to stimulate Py replication through its cognate binding site. The minimal replication activation domain (RAD) was identified between amino acid (aa) 302 and aa 371 by using a fusion protein containing the GAL4 DNA binding domain (GAL4-RAD). In addition, the region showed affinity for the nuclear matrix and, on the basis of competition studies, binding activity for one or more proteins involved in the initiation of Py DNA replication. A leukemogenic chimeric protein, AML1/ETO(MTG8), which does not contain this region of PEBP2alphaB1/AML1, was also localized in the nuclear matrix fraction and competed for nuclear matrix association with PEBP2alphaB1 and GAL4-RAD. Moreover, AML1/ETO inhibited Py DNA replication stimulated by PEBP2alphaB1 and GAL4-RAD. The inhibition was specific for replication mediated by PEBP2alphaB1 and GAL4-RAD, and proportional to the degree of loss of these activators from the nuclear matrix, suggesting a requirement for nuclear matrix targeting in the stimulation of Py DNA replication by RAD. These results are the first to suggest a molecular link between the initiation of DNA replication and the nuclear matrix compartment.

Tumourigenicity of MTG8, a leukaemia-related gene, in concert with v-Ha-ras gene in BALB/3T3 cells

The MTG8 (ETO) gene has been identified as the translocation partner of AML1 (PEBP2alphaB or CBFalpha2) gene in the AML1/MTG8 (ETO) fused gene caused by t(8;21) translocation in human acute myeloid leukaemia, M2 type. Although AML1/MTG8 chimaeric protein is known to inhibit the functioning of AML1 protein, the precise function of MTG8 gene itself is not known yet. We studied the significance of MTG8 gene in the oncogenicity of AML1/MTG8 fused gene, by introducing full-length MTG8 cDNA into both BALB/3T3 cells containing v-Ha-ras gene (Bhas 42 cells) and BALB/3T3 cells without v-Ha-ras gene. Irrespective of the overexpression of MTG8 gene in both groups of cells, Bhas-MTG8 clones which contained v-Ha-ras gene and expressed the MTG8 gene at a level more than twice that of parental Bhas 42 cells induced cell transformation, whereas BALB-MTG8 clones without v-Ha-ras gene did not. Furthermore, injection of the transformed Bhas-MTG8 clones into the subcutaneous tissue of nude mice induced tumours, whereas that of BALB-MTG8 clones did not. These results suggest that MTG8 gene product, in cooperation with viral Ras protein, resulted in tumour formation. We provide the first evidence that MTG8 gene by itself has a carcinogenic property within the AML1/MTG8 (ETO) fused gene.

Structure and expression of the human MTG8/ETO gene

Translocations involving the putative proto-oncogene MTG8/ETO on 8q22 are frequently found in acute myeloid leukemia. To date, little is known of the genomic organization of this gene. Here, we report that the MTG8 gene consists of 13 exons distributed over 87 kb of genomic DNA. Two polymorphic microsatellite repeats are described, including one in intron 3 (three alleles; heterozygosity 0.34) and another in the 3'UTR (15 alleles; heterozygosity 0.89). Expression of MTG8 was detected in a variety of normal human tissues with the highest mRNA levels occurring in brain and heart. Previously, two mRNA forms produced by the alternative usage of the first exon have been reported. We now describe a novel, abundantly expressed, alternatively spliced transcript resulting from the inclusion of a 155-bp exon (designated 9a) that changes the reading frame and introduces a premature stop codon. Identical alternatively spliced mRNA variants were found to be produced by the highly conserved homologous gene (Cbfa2t1) in the mouse, suggesting an evolutionary significance.

Intrinsic transcriptional activation-inhibition domains of the polyomavirus enhancer binding protein 2/core binding factor alpha subunit revealed in the presence of the beta subunit

A member of the polyomavirus enhancer binding protein 2/core binding factor (PEBP2/CBF) is composed of PEBP2 alphaB1/AML1 (as the alpha subunit) and a beta subunit. It plays an essential role in definitive hematopoiesis and is frequently involved in the chromosomal abnormalities associated with leukemia. In the present study, we report functionally separable modular structures in PEBP2 alphaB1 for DNA binding and for transcriptional activation. DNA binding through the Runt domain of PEBP2 alphaB1 was hindered by the adjacent carboxy-terminal region, and this inhibition was relieved by interaction with the beta subunit. Utilizing a reporter assay system in which both the alpha and beta subunits are required to achieve strong transactivation, we uncovered the presence of transcriptional activation and inhibitory domains in PEBP2 alphaB1 that were only apparent in the presence of the beta subunit. The inhibitory domain keeps the full transactivation potential of full-length PEBP2 alphaB1 below its maximum potential. Fusion of the transactivation domain of PEBP2 alphaB1 to the yeast GAL4 DNA-binding domain conferred transactivation potential, but further addition of the inhibitory domain diminished the activity. These results suggest that the activity of the alpha subunit as a transcriptional activator is regulated intramolecularly as well as by the beta subunit. PEBP2 alphaB1 and the beta subunit were targeted to the nuclear matrix via signals distinct from the nuclear localization signal. Moreover, the transactivation domain by itself was capable of associating with the nuclear matrix, which implies the existence of a relationship between transactivation and nuclear matrix attachment.

Characterization of the ETO and AML1-ETO proteins involved in 8;21 translocation in acute myelogenous leukemia

The AML1 and ETO genes are disrupted by the nonrandom chromosomal translocation t(8;21) in acute myelogenous leukemia (AML). While the AML1 gene encodes a transcription factor indispensable for definitive hematopoiesis, the biological function of ETO is unknown. To understand the role of ETO and AML1-ETO in the pathogenesis of AML, the full length cDNAs of ETO and AML1-ETO were cloned and antibodies against AML1 and ETO proteins have been developed in our laboratory. Western blot analysis showed that ETO and AML1-ETO were identified as 70 kDa and 94 kDa proteins, respectively, and that both proteins, like AML1, were associated with the nuclear matrix. To examine whether the t(8;21)-positive AMLs expressed a 94-kDa AML1-ETO, protein fractions isolated from leukemia blasts of 10 patients with t(8;21)-positive AML and the Kasumi-1 cells were analyzed by Western blotting. The 94 kDa AML1-ETO fusion protein was detected in all samples. However, this fusion protein was not detectable in all 40 patients with t(8;21)-negative AMLs. The biological significance of AML1-ETO was examined in K562 cells, which stably overexpress AML1-ETO. We found that AML1-ETO blocked the erythroid differentiation of K562 cells induced by low doses of Ara-C. Thus, t(8;21)-positive AMLs appear to overexpress the AML1-ETO fusion protein, which may be responsible for differentiation block and leukemogenesis in AML.

Transcriptional regulation during myelopoiesis

The coordinated production of all blood cells from a common stem cell is a highly regulated process involving successive stages of commitment and differentiation. From analyses of mice deficient in transcription factor genes and from the characterizations of chromosome breakpoints in human leukemias, it has become evident that transcription factors are important regulators of hematopoiesis. During myelopoiesis, which includes the development of granulocytic and monocytic lineages, transcription factors from several families are active, including AML1/CBF beta, C/EBP, Ets, c-Myb, HOX, and MZF-1. Few of these factors are expressed exclusively in myeloid cells; instead it appears that they cooperatively regulate transcription of myeloid-specific genes. Here we discuss recent advances in transcriptional regulation during myelopoiesis.