Cloning and characterization of subunits of the T-cell receptor and murine leukemia virus enhancer core-binding factor

A large variety of alternatively spliced and differentially expressed mRNAs are encoded by the human acute myeloid leukemia gene AML1

The human chromosome 21 acute myeloid leukemia gene AML1 is frequently rearranged in the leukemia-associated translocations t(8;21) and t(3;21), generating fused proteins containing the amino-terminal part of AML1. In normal blood cells, five size classes (2-8 kb) of AML1 mRNAs have been previously observed. We isolated seven cDNAs corresponding to various AML1 mRNAs. Sequencing revealed that their size differences were mainly due to alternatively spliced 5' and 3' untranslated regions, some of which were vast, exceeding 1.5 kb (5') and 4.3 kb (3'). These untranslated regions contain sequences known to control mRNA translation and stability and seem to modulate AML1 mRNA stability. Further heterogeneity was found in the coding region due to the presence of alternatively spliced stop codon-containing exons. The latter led to production of polypeptides that were smaller than the full-length AML1 protein; they lacked the trans-activation domains but maintained DNA binding and heterodimerization ability. The size of these truncated products was similar to the AML1 segment in the fused t(8;21) and t(3;21) proteins. In thymus, only one mRNA species of 6 kb was detected. Using in situ hybridization, we showed that its expression was confined to the cortical region of the organ. The 6-kb mRNA was also prominent in cultured peripheral blood T cells, and its expression was markedly reduced upon mitogenic activation by phorbol myristate acetate (TPA) plus concanavalin A (ConA). These results and the presence of multiple coding regions flanked by long complex untranslated regions, suggest that AML1 expression is regulated at different levels by several control mechanisms generating the large variety of mRNAs and protein products.

Dosage-sensitive maternal modifiers of the drosophila segmentation gene runt

The protein encoded by the pair-rule gene runt functions as a transcriptional regulator during anterior-posterior patterning of the Drosophila embryo. Results of over-expression experiments as well as parallels drawn from the recent characterization of vertebrate homologues indicate that interactions with other proteins are likely to be central to the function of the Runt protein. To identify factors important for runt activity, we took advantage of an adult visible phenotype observed in animals heterozygous for runt mutations. Using a set of 126 different deficiency chromosomes we screened approximately 65% of the genome for genes that act as dose-sensitive maternal modifiers of runt. Eighteen deficiencies representing 12 putative loci were identified as maternally acting enhancers of runt haplo-insufficiency. Further characterization of two of these regions led to the identification of the interacting loci. Both of these loci affect the spatial regulation of runt transcription and appear genetically complex. Furthermore, the effects of one of these loci, M(1)1B, is indirect and mediated through effects on the transcriptional regulation of posterior gap genes.

Molecular and cytogenetic abnormalities in acute myeloid leukaemia and myelodysplastic syndromes

With the use of molecular techniques it is now possible to define even subtle chromosomal abnormalities and the fusion products resulting from translocations. Defined clinical correlations can now be made and prognostic implications are already found. For instance, patients with AML carrying t(8;21), t(15;17) or inv(16) have a better prognosis for long-term survival. This is also illustrated by Figure 1, which shows data of the Dutch HOVON AML study. The definition of patients with a bad or good prognosis has already resulted in the adjustment of treatment protocols. In the near future, with the use of more defined molecular techniques, we might be able to characterize the chromosomal abnormality of each patient, to individualize his treatment and to recognize very early relapses.

The biological significance of the multidrug resistance gene MRP in inversion 16 leukemias

Multidrug resistance represents an important mechanism by which leukaemic and solid tumour cells escape cell death after exposure to anthracyclines and other natural products. Acute myeloid leukaemia (AML) associated with the inversion chromosome 16: inv(16)(p13q22) has a favourable prognosis and is known to be chemosensitive. The inversion chromosome is seen in a number of FAB subclasses but is most commonly associated with acute myelomonocytic leukaemia with abnormal eosinophils, M4Eo. It results in the creation of a fusion between the myosin heavy chain gene (MYH11) on the short arm and the gene for a transcription factor, core binding factor beta (CBFB) on the long arm. In a subset of these inv(16) AML patients, inversion also results in loss of the gene for the multidrug resistance protein (MRP) at the short arm breakpoint. This gene maps to 16p13.13, centromeric to the primary short arm breakpoint, separated from MYH11 by a distance of approximately 150kb. Deletion of the MRP gene has been demonstrated by in situ hybridisation, gene dosage studies and by loss of heterozygosity of a flanking microsatellite marker (D16S405). Twenty two patients with inv(16) leukaemia were analysed for deletion of the MRP gene. Deletion of the gene was detected in seven patients, fourteen patients showed retention of the gene and in one case the findings were indeterminate. Clinical data from 13 of these patients were analysed revealing deletion of the MRP gene to be significantly associated with longer time from diagnosis until failure (death or relapse from complete remission) in these patients (p = 0.007). From this work and the growing literature concerning MRP, it appears likely that the deletion of an MRP allele, may favourably affect the biology of inv(16) AML and may have important prognostic implications.

AML1, the target of multiple chromosomal translocations in human leukemia, is essential for normal fetal liver hematopoiesis

The AML1-CBF beta transcription factor is the most frequent target of chromosomal rearrangements in human leukemia. To investigate its normal function, we generated mice lacking AML1. Embryos with homozygous mutations in AML1 showed normal morphogenesis and yolk sac-derived erythropoiesis, but lacked fetal liver hematopoiesis and died around E12.5. Sequentially targeted AML1-/-es cell retained their capacity to differentiate into primitive erythroid cells in vitro; however, no myeloid or erythroid progenitors of definitive hematopoietic origin were detected in either the yolk sac or fetal livers of mutant embryos. Moreover, this hematopoietic defect was intrinsic to the stem cells in that AML1-/-ES cells failed to contribute to hematopoiesis in chimeric animals. These results suggest that AML1-regulated target genes are essential for definitive hematopoiesis of all lineages.

Function of PU.1 (Spi-1), C/EBP, and AML1 in early myelopoiesis: regulation of multiple myeloid CSF receptor promoters

Our studies of the promoters of the myeloid CSF receptors (M, GM, and G) in cell lines have led to the findings that the promoters are small, and are all activated by the PU.1 and C/EBP proteins. To date, we have only found evidence for involvement of C/EBP alpha, although further experiments will be needed to exclude the role of C/EBP beta and C/EBP delta in receptor gene expression. These studies suggest a model of hematopoiesis (Fig. 2) in which the lineage commitment decisions of multipotential cells are made by the alternative patterns of expression of certain transcription factors, which then activate growth factor receptors which allow those cells to respond to the appropriate growth factor to proliferate and survive. For example, expression of GATA-1 activates its own expression, as well as that of the erythropoietin receptor, inducing these cells to be capable of responding to erythropoietin. Similarly, expression of PU.1 activates its own promoter, and turns on the three myeloid CSF receptors (M, GM, and G), pushing these cells along the pathway of myeloid differentiation. C/EBP proteins, particularly C/EBP alpha, are also critical for myeloid receptor promoter function, and may also act via autoregulatory mechanisms. Murine C/EBP alpha has a C/EBP binding site in its own promoter. Human C/EBP alpha autoregulates its own expression in adipocytes by activating the USF transcription factor. Myeloid genes expressed later during differentiation, such as CD11b, are also activated by PU.1, which is expressed at highest levels in mature myeloid cells, but not by C/EBP alpha, which is downregulated in a differentiated murine myeloid cell line. Consistent with this model are the findings that overexpression of PU.1 in erythroid cells blocks erythroid differentiation, leading to erythroleukemia, and overexpression of GATA-1 in a myeloid line blocks myeloid differentiation. While these findings have provided some framework for understanding myeloid gene regulation, there are a number of critical questions to be addressed in the near future: What is the pattern of expression of the C/EBP proteins during the course of myeloid differentiation and activation of human CD34+ cells? What is the effect of targeted disruption and other mutations of the C/EBP and AML1 proteins on myeloid development and receptor expression? What are the interactions among these three different types of factors (ets, basic region-zipper, and Runt domain proteins) to activate the promoters? What is the effect of translocations, mutations, and alterations in expression of these factors, particularly in different forms of AML?

Cloning and characterization of a novel insulin-regulated membrane aminopeptidase from Glut4 vesicles

The insulin-regulated glucose transporter isotype GlutT4 expressed only in muscle and adipose cells is sequestered in a specific secretory vesicle. These vesicles harbor another major protein, referred to as vp165 (for vesicle protein of 165 kDa), that like GluT4 redistributes to the plasma membrane in response to insulin. We describe here the cloning of vp165 and show that it is a novel member of the family of zinc-dependent membrane aminopeptidases, with the typical large extracellular catalytic domain and single transmembrane domain but with a unique extended cytoplasmic domain. The latter contains two dileucine motifs, which may be critical for the specific trafficking of vp165, since this has been shown to be the case for this motif in GluT4. However, the tissue distribution of vp165 is much wider than that of GluT4; consequently, vp165 may also function in processes unrelated to insulin action and may serve as a ubiquitous marker for a specialized regulated secretory vesicle.

Cloning, mapping and expression of PEBP2 alpha C, a third gene encoding the mammalian Runt domain

PEBP2/CBF is a heterodimeric transcription factor composed of alpha and beta subunits. Previously, we reported two distinct mouse genes, PEBP2 alpha A and PEBP2 alpha B, which encode the alpha subunit. PEBP2 alpha B is the homologue of human AML1, encoding the acute myeloid leukemia 1 protein. AML1 and human PEBP2/CBF beta were detected independently at the breakpoints of two characteristic chromosome translocations observed frequently in two subtypes of acute myeloid leukemia. The PEBP2 alpha proteins contain a 128-amino-acid (aa) region highly homologous to the Drosophila melanogaster segmentation gene runt. The evolutionarily conserved region, named the Runt domain, harbors DNA-binding and heterodimerizing activities. In this study, we identified the third Runt-domain-encoding gene, PEBP2 alpha C, which maps to 1p36.11-p36.13 in the human chromosome and encodes a 415-aa protein. PEBP2 alpha C forms a heterodimer with PEBP2 beta, binds to the PEBP2 site and transactivates transcription, similar to PEBP2 alpha A and PEBP2 alpha B.

Regulation of granulocyte-macrophage colony-stimulating factor and interleukin 3 expression

Granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin 3 (IL-3) are multilineage acting hematopoietic growth factors which have overlapping but distinct biological properties. Cellular sources of IL-3 are confined to activated T cells, natural killer (NK) cells, mast cells and possibly megakaryocytes, while these cells and activated macrophages, fibroblasts and endothelial cells are important sources of GM-CSF. In vitro studies have implicated both cytokines in the autocrine growth of human myeloid or murine mast cell leukemias. The human GM-CSF and IL-3 genes map to the long arm of chromosome 5, show similar genomic structures, and share several conserved elements in their 5' and 3' flanking regions. The promoters of these genes contain a variety of positive and negative regulatory regions, and the level of expression of these genes is controlled by both transcriptional and post-transcriptional mechanisms.

Transcriptional regulation of the human T cell receptor delta gene

T cell receptor delta gene expression is regulated by a T cell-specific transcriptional enhancer located within the J delta 3-C delta intron. An essential element of the enhancer was localized to a small 30 bp segment denoted delta E3. Two specific factors, CBF/PEBP2 and c-Myb, bind to adjacent sites within delta E3 and cooperate functionally to mediate transcriptional activation. These factors are likely to play essential roles in the developmental activation of the TCR delta gene in vivo.

Abolition of suppressive effect of acute myeloid leukemia cells on normal granulocyte-macrophage colony formation induced by interleukin-5 associated with eosinophilic cell induction

The effects of recombinant interleukin-5 (rIL-5) on leukemic blasts obtained from 22 acute myeloid leukemia (AML) patients were investigated. Recombinant IL-5 stimulated leukemic colony formation in the leukemic blasts of 50% of the patients (11/22), and in 7 of these 11 cell cultures leukemic eosinophilic colonies were induced. Eosinophilic cell induction was associated with cellular proliferation, that is, colony or cluster formation. Leukemic blasts cultured with rIL-5 and forming eosinophilic colonies no longer suppressed normal granulocyte-macrophage colony formation, suggesting that functional differentiation of these leukemic blasts took place. Induction of this type of functional differentiation of leukemic blasts may be of clinical value in promoting normal hematopoiesis.

Hematopoietic lineage commitment: role of transcription factors

This review focuses on the roles of transcription factors in hematopoietic lineage commitment. A brief introduction to lineage commitment and asymmetric cell division is followed by a discussion of several methods used to identify transcription factors important in specifying hematopoietic cell types. Next is presented a discussion of the use of embryonic stem cells in the analysis of hematopoietic gene expression and the use of targeted gene disruption to analyze the role of transcription factors in hematopoiesis. Finally, the status of our current knowledge concerning the roles of transcription factors in the commitment to erythroid, myeloid and lymphoid cell types is summarized.

Transcriptional activity of core binding factor-alpha (AML1) and beta subunits on murine leukemia virus enhancer cores

Core binding factor (CBF), also known as polyomavirus enhancer-binding protein 2 and SL3 enhancer factor 1, is a mammalian transcription factor that binds to an element termed the core within the enhancers of the murine leukemia virus family of retroviruses. The core elements of the SL3 virus are important genetic determinants of the ability of this virus to induce T-cell lymphomas and the transcriptional activity of the viral long terminal repeat in T lymphocytes. CBF consists of two subunits, a DNA binding subunit, CBF alpha, and a second subunit, CBF beta, that stimulates the DNA binding activity of CBF alpha. One of the genes that encodes a CBF alpha subunit is AML1, also called Cbf alpha 2. This locus is rearranged by chromosomal translocations in human myeloproliferative disorders and leukemias. An exogenously expressed Cbf alpha 2-encoded subunit (CBF alpha 2-451) stimulated transcription from the SL3 enhancer in P19 and HeLa cells. Activity was mediated through the core elements. Three different isoforms of CBF beta were also tested for transcriptional activity on the SL3 enhancer. The longest form, CBF beta-187, increased the transcriptional stimulation by CBF alpha 2-451 twofold in HeLa cells, although it had no effect in P19 cells. Transcriptional activation by CBF beta required binding to the CBF alpha subunit, as a form of CBF beta that lacked binding ability, CBF beta-148, failed to increase activity. These results indicated that at least in certain cell types, the maximum activity of CBF required both subunits. They also provided support for the hypothesis that CBF is a factor in T lymphocytes that is responsible for recognition of the SL3 cores. We also examined whether CBF could distinguish a 1-bp difference between the enhancer core of SL3 and the core of the nonleukemogenic virus, Akv. This difference strongly affects transcription in T cells and leukemogenicity of SL3. However, no combination of CBF alpha and CBF beta subunits that we tested was able to distinguish the 1-bp difference in transcription assays. Thus, a complete understanding of how T cells recognize the SL3 core remains to be elucidated.

Granulocyte macrophage-colony stimulating factor-dependent replication of polyoma virus replicon in hematopoietic cells. Analyses of receptor signals for replication and transcription

Granulocyte macrophage-colony stimulating factor (GM-CSF) stimulates proliferation of various hematopoietic cells. Using cytoplasmic deletion mutants of the human GM-CSF receptor (hGMR) beta subunit and tyrosine kinase inhibitors, we previously showed that distinct signaling pathways of hGMR are involved in the induction of c-fos/c-jun mRNAs and of c-myc mRNA/cell proliferation. We used polyoma virus (Py) replicon to analyze the initiation of DNA replication induced by hGM-CSF in mouse BA/F3 pro-B cells expressing hGMR. hGM-CSF efficiently stimulated Py replication in the presence of Py enhancer and Py large T antigen supplied in trans. Analyses of Py enhancer mutants revealed that hGM-CSF promoted Py replication and activated transcription of the Py early promoter through the PEA3/PEBP5 region of Py enhancer. The membrane proximal region of hGMR beta subunit is required for activation of PEA3/PEBP5-dependent replication which is also required for activation of DNA synthesis in the host cells. In contrast, a more distal region which is essential for activation of c-fos and c-jun genes is required for the PEA3/PEBP5-dependent transcription of Py early promoter. These results indicate that distinct signaling pathways of hGMR are required to activate PEA3/PEBP5-dependent replication and transcription although the same enhancer is required for both activities.

Structure of the leukemia-associated human CBFB gene

We have determined the structure of the human CBFB gene, which encodes the beta subunit of the heterodimeric transcription factor core binding factor (CBF). This gene becomes fused to the MYH11 gene encoding smooth muscle myosin heavy chain by an inversion of chromosome 16 that occurs in the M4Eo subtype of acute myeloid leukemia. The CBFB gene contains 6 exons and spans 50 kb. The gene is highly conserved in animal species as distant as Drosophila, and the exon boundaries are in locations identical to those of the murine Cbfb homologue. The CBFB promoter region has typical features of a housekeeping gene, including high G+C content, high frequency of CpG dinucleotides, and lack of canonical TATA and CCAAT boxes. This gene has a single transcriptional start site, 345 nucleotides upstream of the beginning of the coding region. The human and mouse CBFB promoters show conservation of several transcriptional regulatory sequence motifs, including binding sites for Sp1, Ets family members, and Myc, but do not contain any CBF binding sites. The 5' end of the human CBFB gene also contains a highly polymorphic, transcribed CGG repeat that is not present in the murine homologue.

Identification of a new murine runt domain-containing gene, Cbfa3, and localization of the human homolog, CBFA3, to chromosome 1p35-pter

Core binding factor (CBF) is a heterodimeric transcription factor composed of two distinct subunits. The monomeric beta subunit is ubiquitously expressed, whereas expression of the three alpha subunits isolated previously seems to be restricted mainly to hematopoietic tissues. To isolate additional alpha genes, degenerate oligonucleotides derived from the runt domain--a region shared by all alpha genes--were used for screening cDNA libraries. A 228-bp fragment was isolated from a mouse thymus cDNA library, which showed 82 and 76% DNA sequence identity to the previously isolated murine alpha genes, Cbfa1 and Cbfa2. This novel alpha gene was named Cbfa3. The corresponding sequence from the human homolog CBFA3 was obtained by cosmid cloning and sequencing of the appropriate restriction fragment. The corresponding regions of mouse Cbfa3 and human CBFA3 show 91% nucleotide identity and 100% protein identity. In situ hybridization and physical mapping of somatic cell hybrids localized CBFA3 to chromosome 1p35-pter.

Interphase cytogenetics of the t(8;21)(q22;q22) associated with acute myelogenous leukemia by two-color fluorescence in situ hybridization

In the translocation (8;21)(q22;q22) associated with acute myelogenous leukemia (AML), part of the long arm of chromosome 8 is reciprocally translocated onto chromosome 21. At the molecular level the translocation results in the fusion of the 5' region of the AML1 gene on chromosome 21 and almost the entire CDR gene (also ETO or MTG8) on chromosome 8. The translocation can be demonstrated by techniques such as Southern blot analysis of DNA and reverse transcription-polymerase chain reaction (RT-PCR) analysis of mRNA. Neither of these methods demonstrates the translocation in individual cells. To detect the translocation at the single cell level, we used two probes, a cosmid clone containing the first five exons of AML1 and a P1 clone containing the entire CDR gene. Hybridization of the two probes to the distal and proximal side of the translocation breakpoint on chromosome 8 was expected to highlight the 8q-derivative in an interphase cell. To demonstrate the ability to identify the translocation in interphase cells using two-color FISH, these two probes were hybridized simultaneously to the Kasumi-1 cell line containing the 8;21 translocation and to t(8;21)-positive leukemic cells from a patient. Each probe was detected with a different color so that their relationship in the sample could be determined within the same interphase cell. Simultaneous hybridization of the CDR and AML1 probes to interphase cells resulted in one red and one green hybridization signal randomly located in the cell, from the hybridization to the normal chromosomes (8, 21), and one red-green pair of signals from the close hybridization of the two probes to the fusion gene on the derivative 8q-chromosome, indicating the translocation. This technique may be a useful complement for the analysis of the t(8;21), since critical information can be obtained from samples not suited for RT-PCR and conventional cytogenetic techniques. In addition, it may be useful for the assessment of minimal residual disease where RT-PCR is of limited value.

AML1 fusion transcripts in t(3;21) positive leukemia: evidence of molecular heterogeneity and usage of splicing sites frequently involved in the generation of normal AML1 transcripts

The t(3;21)(q26;q22) is associated with chronic myelogenous leukemia in blast crisis (CML-BC), leukemia evolving from (therapy-related) myelodysplasia, and with leukemia following other hematopoietic proliferative diseases. Molecular cytogenetic analysis and cloning of a few t(3;21) cases indicate that the breakpoints are quite heterogeneous even within a specific clinical phenotype. Interestingly some of the (3;21) breakpoints involve the AML1 gene previously found rearranged in the t(8;21) associated with acute myelogenous leukemia. AML1 is related to the Drosophila gene runt and is the human counterpart of the gene for the alpha subunit of the nuclear polyoma enhancer binding protein (PEBP2) also known as the core binding factor (CBF). In the t(3;21) AML1 was found rearranged with EAP, a gene on chromosome 3 encoding a small ribosomal protein, as well as with EV11, another gene on chromosome 3. Here we report our study of six cases of t(3;21). By using fluorescence in situ hybridization (FISH) analysis and AML1 probes we could conclude that at least in two CML-BC cases the breakpoint occurred in the AML1 intron that is disrupted by the t(8;21). An AML1/EAP fusion transcript, different from the one described in a therapy-related myelodysplasia, was detected in both CML-BC cases. This transcript is expected to result in a predicted protein containing the AML1 nuclear binding domain with an attached stretch of 17 amino acids unrelated to the EAP small ribosomal protein. In the other t(3;21) patients we could not detect an AML1/EAP transcript or an AML1/EV11 transcript. This result suggests heterogeneity of the t(3;21) at the molecular level. The AML1 chimeric transcripts identified so far, both in the t(3;21) and in the t(8;21), diverge from the normal transcripts either after exon 5 or exon 6. Here we show that in normal AML1 transcripts different splicing events are seen to occur after AML1 exon 5 as well as exon 6.

Gap gene properties of the pair-rule gene runt during Drosophila segmentation

The Drosophila Runt protein is a member of a new family of transcriptional regulators that have important roles in processes extending from pattern formation in insect embryos to leukemogenesis in humans. We used ectopic expression to investigate runt's function in the pathway of Drosophila segmentation. Transient over-expression of runt under the control of a Drosophila heat-shock promoter caused stripe-specific defects in the expression patterns of the pair-rule genes hairy and even-skipped but had a more uniform effect on the secondary pair-rule gene fushi tarazu. Surprisingly, the expression of the gap segmentation genes, which are upstream of runt in the segmentation hierarchy was also altered in hs/runt embryos. A subset of these effects were interpreted as due to an antagonistic effect of runt on transcriptional activation by the maternal morphogen bicoid. In support of this, expression of synthetic reporter gene constructs containing oligomerized binding sites for the Bicoid protein was reduced in hs/runt embryos. Finally, genetic experiments demonstrated that regulation of gap gene expression by runt is a normal component of the regulatory program that generates the segmented body pattern of the Drosophila embryo.

Transcriptional regulation of T-cell genes during T-cell development

The expression of many T cell specific genes has been shown to be regulated at the transcriptional level. Recent studies of T-cell specific promoters and enhancers have allowed the identification of a number of transcription factors that appear to play distinct but complementary roles in regulating gene expression during T-cell development and activation.

Identification of a T cell-specific transcriptional enhancer 3' of the human T cell receptor gamma locus

Positive and negative transcriptional regulatory mechanisms are thought to play a major role in the expression of T cell antigen receptor (TCR) genes. Since the alpha beta and gamma delta T cell receptor heterodimers are expressed in a mutually exclusive fashion and since TCR genes are sequentially activated during T cell ontogeny, transcriptional activation and repression must at least in part determine T lineage-specific and developmental-specific expression of these genes. We have identified a transcriptional enhancer located 6.5 kb downstream from the human T cell receptor gamma (TRG) locus. The nucleotide sequence of the enhancer core element shows strong sequence homology to the recently identified murine C gamma 1 enhancer. The enhancer demonstrates T cell-specific activity, but not gamma delta sublineage-specificity in combination with either a heterologous or gene-specific promoter. Thus, additional regulatory elements may be required to repress the expression of rearranged TRG genes in non-gamma delta T cells.