Indistinguishable nuclear factor binding to functional core sites of the T-cell receptor delta and murine leukemia virus enhancers

ClinGen Myeloid Malignancy Variant Curation Expert Panel recommendations for germline RUNX1 variants

Standardized variant curation is essential for clinical care recommendations for patients with inherited disorders. Clinical Genome Resource (ClinGen) variant curation expert panels are developing disease-associated gene specifications using the 2015 American College of Medical Genetics and Genomics (ACMG) and Association for Molecular Pathology (AMP) guidelines to reduce curation discrepancies. The ClinGen Myeloid Malignancy Variant Curation Expert Panel (MM-VCEP) was created collaboratively between the American Society of Hematology and ClinGen to perform gene- and disease-specific modifications for inherited myeloid malignancies. The MM-VCEP began optimizing ACMG/AMP rules for RUNX1 because many germline variants have been described in patients with familial platelet disorder with a predisposition to acute myeloid leukemia, characterized by thrombocytopenia, platelet functional/ultrastructural defects, and a predisposition to hematologic malignancies. The 28 ACMG/AMP codes were tailored for RUNX1 variants by modifying gene/disease specifications, incorporating strength adjustments of existing rules, or both. Key specifications included calculation of minor allele frequency thresholds, formulating a semi-quantitative approach to counting multiple independent variant occurrences, identifying functional domains and mutational hotspots, establishing functional assay thresholds, and characterizing phenotype-specific guidelines. Preliminary rules were tested by using a pilot set of 52 variants; among these, 50 were previously classified as benign/likely benign, pathogenic/likely pathogenic, variant of unknown significance (VUS), or conflicting interpretations (CONF) in ClinVar. The application of RUNX1-specific criteria resulted in a reduction in CONF and VUS variants by 33%, emphasizing the benefit of gene-specific criteria and sharing internal laboratory data.

AML1 is overexpressed in patients with myeloproliferative neoplasms and mediates JAK2V617F-independent overexpression of NF-E2

The transcription factor NF-E2 is overexpressed in the majority of patients with polycythemia vera (PV). Concomitantly, 95% of these patients carry the JAK2(V617F) mutation. Although NF-E2 levels correlate with JAK2(V671F) allele burden in some PV cohorts, the molecular mechanism causing aberrant NF-E2 expression has not been described. Here we show that NF-E2 expression is also increased in patients with essential thrombocythemia and primary myelofibrosis independent of the presence of the JAK2(V617F) mutation. Characterization of the NF-E2 promoter revealed multiple functional binding sites for AML1/RUNX-1. Chromatin immunoprecipitation demonstrated AML1 binding to the NF-E2 promoter in vivo. Moreover, AML1 binding to the NF-E2 promoter was significantly increased in granulocytes from PV patients compared with healthy controls. AML1 mRNA expression was elevated in patients with PV, essential thrombocythemia, and primary myelofibrosis both in the presence and absence of JAK2(V617F). In addition, AML1 and NF-E2 expression were highly correlated. RNAi-mediated suppression of either AML1 or of its binding partner CBF-beta significantly decreased NF-E2 expression. Moreover, expression of the leukemic fusion protein AML/ETO drastically decreased NF-E2 protein levels. Our data identify NF-E2 as a novel AML1 target gene and delineate a role for aberrant AML1 expression in mediating elevated NF-E2 expression in MPN patients.

Co-activator activator (CoAA) prevents the transcriptional activity of Runt domain transcription factors

Runx proteins are essential for a number of developmental processes and are aberrantly expressed in many human cancers. Runx factors bind DNA and co-factors to activate or repress genes crucial for bone formation, hematopoiesis, and neuronal development. Co-activator activator (CoAA) is a nuclear protein that regulates gene expression, RNA splicing and is overexpressed in many human tumors. In this study, we identified CoAA as a Runx2 binding protein. CoAA repressed Runx factor-dependent activation of reporter genes in a histone deacetylase-independent manner. CoAA also blocked Runx2-mediated repression of the Axin2 promoter, a novel Runx target gene. The carboxy-terminus of CoAA is essential for binding the Runt domains of Runx1 and Runx2. In electophoretic mobility shift assays, CoAA inhibited Runx2 interactions with DNA. These data indicate that CoAA is an inhibitor of Runx factors and can negate Runx factor regulation of gene expression. CoAA is expressed at high levels in human fetal osteoblasts and osteosarcoma cell lines. Suppression of CoAA expression by RNA interference reduced osteosarcoma cell viability in vitro, suggesting that it contributes to the proliferation and/or survival of osteoblast lineage cells.

Cell cycle and developmental control of hematopoiesis by Runx1

Runx1 binds DNA in cooperation with CBFbeta to activate or repress transcription, dependent upon cellular context and interaction with a variety of co-activators and co-repressors. Runx1 is required for emergence of adult hematopoietic stem cells (HSC) during embryonic development and for lymphoid, myeloid, and megakaryocyte lineage maturation from HSC in adult marrow. Runx1 levels vary during the cell cycle, and Runx1 regulates G1 to S cell cycle progression. Both Cdk and ERK phosphorylate Runx1 to influence its interaction with co-repressors, and the Wnt effector LEF-1/TCF also modulates Runx1 activities. These links likely allow cytokines and signals from adjacent cells to influence HSC proliferation versus quiescence and the rate of progenitor expansion, in response to developmental or environmental demands. J. Cell. Physiol. 219: 520-524, 2009. (c) 2009 Wiley-Liss, Inc.

Proleukemic RUNX1 and CBFbeta mutations in the pathogenesis of acute leukemia

The existence of non-random mutations in critical regulators of cell growth and differentiation is a recurring theme in cancer pathogenesis and provides the basis for our modern, molecular approach to the study and treatment of malignant diseases. Nowhere is this more true than in the study of leukemogenesis, where research has converged upon a critical group of genes involved in hematopoietic stem and progenitor cell self-renewal and fate specification. Prominent among these is the heterodimeric transcriptional regulator, RUNX1/CBFbeta. RUNX1 is a site-specific DNA-binding protein whose consensus response element is found in the promoters of many hematopoietically relevant genes. CBFbeta interacts with RUNX1, stabilizing its interaction with DNA to promote the actions of RUNX1/CBFbeta in transcriptional control. Both the RUNX1 and the CBFbeta genes participate in proleukemic chromosomal alterations. Together they contribute to approximately one-third of acute myelogenous leukemia (AML) and one-quarter of acute lymphoblastic leukemia (ALL) cases, making RUNX1 and CBFbeta the most frequently affected genes known in the pathogenesis of acute leukemia. Investigating the mechanisms by which RUNX1, CBFbeta, and their proleukemic fusion proteins influence leukemogenesis has contributed greatly to our understanding of both normal and malignant hematopoiesis. Here we present an overview of the structural features of RUNX1/CBFbeta and their derivatives, their roles in transcriptional control, and their contributions to normal and malignant hematopoiesis.

Cyclin-dependent kinase phosphorylation of RUNX1/AML1 on 3 sites increases transactivation potency and stimulates cell proliferation

RUNX1/AML1 regulates lineage-specific genes during hematopoiesis and stimulates G1 cell-cycle progression. Within RUNX1, S48, S303, and S424 fit the cyclin-dependent kinase (cdk) phosphorylation consensus, (S/T)PX(R/K). Phosphorylation of RUNX1 by cdks on serine 303 was shown to mediate destabilization of RUNX1 in G2/M. We now use an in vitro kinase assay, phosphopeptide-specific antiserum, and the cdk inhibitor roscovitine to demonstrate that S48 and S424 are also phosphorylated by cdk1 or cdk6 in hematopoietic cells. S48 phosphorylation of RUNX1 paralleled total RUNX1 levels during cell-cycle progression, S303 was more effectively phosphorylated in G2/M, and S424 in G1. Single, double, and triple mutation of the cdk sites to the partially phosphomimetic aspartic acid mildly reduced DNA affinity while progressively increasing transactivation of a model reporter. Mutation to alanine increased DNA affinity, suggesting that in other gene or cellular contexts phosphorylation of RUNX1 by cdks may reduce transactivation. The tripleD RUNX1 mutant rescued Ba/F3 cells from inhibition of proliferation by CBFbeta-SMMHC more effectively than the tripleA mutant. Together these findings indicate that cdk phosphorylation of RUNX1 potentially couples stem/progenitor proliferation and lineage progression.

Two novel translocations disrupt the RUNX1 gene in acute myeloid leukemia

Translocations involving 21q22 are commonly observed in both de novo and therapy-related acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). They often result in the disruption of RUNX1 and give rise to fusion genes consisting of RUNX1 and different partner genes, which contribute to leukemogenesis. To date, at least 21 such translocations are known from the literature. Here we report two novel translocations involving the RUNX1 gene: t(1;21)(q12;q22) in a 53-year-old woman with AML-M5b and t(11;21)(q13;q22) in a 65-year-old man with AML-M2. The abnormalities revealed by R-banding karyotypic analysis were confirmed with interphase and metaphase fluorescence in situ hybridization (FISH), chromosome painting, and M-FISH.

Regulation of the murine Ddelta2 promoter by upstream stimulatory factor 1, Runx1, and c-Myb

Accessibility control of V(D)J recombination at Ag receptor loci depends on the coordinate activities of transcriptional enhancers and germline promoters. Recombination of murine Tcrd gene segments is known to be regulated, at least in part, by the Tcrd enhancer (Edelta) situated in the Jdelta2-Cdelta intron. However, there has been little characterization of promoters and other cis-acting elements that are activated by or collaborate with Edelta and that might function to regulate Tcrd gene recombination events. We now describe a strong promoter that is tightly associated with the murine Ddelta2 gene segment. EMSAs reveal that upstream stimulatory factor 1, Runx1, c-Myb, lymphoid enhancer binding factor 1, NF1, and E47 all interact with this promoter in vitro. Of these, upstream stimulatory factor 1, Runx1, and c-Myb appear necessary for full promoter activity in transiently transfected cells. Moreover, the same three factors were found to interact with the promoter in vivo by chromatin immunoprecipitation. We suggest that these factors play important roles as Edelta-dependent regulators of Ddelta2 accessibility in vivo. Consistent with the established roles of c-Myb and Runx factors in Edelta function, we detected low level, enhancer-independent activity of the Ddelta2 promoter in transient transfection experiments. We speculate that the Ddelta2 promoter may play a role as a weak, enhancer-independent regulator in vivo, and might contribute to residual Tcrd rearrangement in Edelta(-/-) mice.

Pathogenesis of acute myeloid leukaemia and inv(16)(p13;q22): a paradigm for understanding leukaemogenesis?

Acute myeloid leukaemia (AML) has been proposed to arise from the collaboration between two classes of mutation, a class I, or proliferative, mutation and a class II, or blocking, mutation. A limitation of this so-called 'two-hit' hypothesis has been the lack of identifiable proliferative and blocking mutations in most AML cases. However, it is now known that the CBFbeta-MYH11 fusion gene in AML and inv(16), by disrupting the normal transcription factor activity of core binding factor (CBF), functions as a class II mutation. In addition, nearly 70% of patients with AML and inv(16) are known to possess mutually exclusive mutations of the receptor tyrosine kinases (RTKs), c-KIT and FLT3, as well as RAS genes, that provide a class I, or proliferative, signal. AML and inv(16), therefore, is one of the best understood of the acute leukaemias at the genetic level and so provides a paradigm for the 'two-hit' hypothesis of leukaemogenesis. This paper reviews the recent advances in the molecular pathology of AML and inv(16) and discusses possible therapeutic implications of the current pathogenetic model.

Histone deacetylase 3 interacts with runx2 to repress the osteocalcin promoter and regulate osteoblast differentiation

The Runt domain transcription factor Runx2 (AML-3, and Cbfa1) is essential for osteoblast development, differentiation, and bone formation. Runx2 positively or negatively regulates osteoblast gene expression by interacting with a variety of transcription cofactor complexes. In this study, we identified a trichostatin A-sensitive autonomous repression domain in the amino terminus of Runx2. Using a candidate approach, we found that histone deacetylase (HDAC) 3 interacts with the amino terminus of Runx2. In transient transfection assays, HDAC3 repressed Runx2-mediated activation of the osteocalcin promoter. HDAC inhibitors and HDAC3-specific short hairpin RNAs reversed this repression. In vivo, Runx2 and HDAC3 associated with the osteocalcin promoter. These data indicate that HDAC3 regulates Runx2-mediated transcription of osteoblast genes. Suppression of HDAC3 in MC3T3 preosteoblasts by RNA interference accelerated the expression of Runx2 target genes, osteocalcin, osteopontin, and bone sialoprotein but did not significantly alter Runx2 levels. Matrix mineralization also occurred earlier in HDAC3-suppressed cells, but alkaline phosphatase expression was not affected. Thus, HDAC3 regulates osteoblast differentiation and bone formation. Although HDAC3 is likely to affect the activity of multiple proteins in osteoblasts, our data show that it actively regulates the transcriptional activity of the osteoblast master protein, Runx2.

Identification of novel genes of the bone-specific transcription factor Runx2

INTRODUCTION

The transcription factor Runx2 is a key regulator of osteoblast development and plays a role in chondrocyte maturation. The identification of transcriptional target genes of Runx2 may yield insight into how osteoblastic differentiation is achieved on a molecular level.

MATERIALS AND METHODS

Using a differential hybridization technique (selective amplification through biotin and restriction-mediated enrichment [SABRE]) and cDNA microarray analysis, 15 differentially expressed genes were identified using mRNA from C3H 10Tl/2 cells with constitutive and inducible overexpression of Runx2.

RESULTS AND CONCLUSIONS

Among the 15 genes identified, 4 encode the extracellular matrix proteins Ecml, Mgp, Fbn5, and Osf-2, three represent the transcription factors Esxl, Osrl, and Sox9, whereas others were Ptn, Npdc-1, Higl, and Tem l. The gene for Pttg1ip was upregulated in Runx2-expressing cells. Pttg1ip is widely expressed during development, but at highest levels in limbs and gonads. The Pttg1ip promoter binds Runx2 in a sequence specific manner, and Runx2 is able to transactivate the Pttg lip promoter in MC3T3-El cells. Therefore, Pttg1ip is likely tobe a novel direct transcriptional target gene of Runx2. In conclusion, the genes identified in this study are important candidates for mediating Runx2 induced cellular differentiation.

A novel gene, FGA7, is fused to RUNX1/AML1 in a t(4;21)(q28;q22) in a patient with T-cell acute lymphoblastic leukemia

AML1 is among the most frequent targets of chromosomal rearrangements in human leukemias. We report here the molecular analysis of a t(4;21)(q28;q22) that has disrupted AML1 in a patient with de novo T-cell acute lymphoblastic leukemia. By using 3'-RACE analysis, we show that this rearrangement results in the fusion of a novel gene immediately downstream of exon 5 or exon 6 of AML1, indicating that the AML1 breakpoint lies in intron 6 and that alternative fusion splice variants are generated. The sequence of the novel gene, located at 4q28, does not have any significant homology with any of the known genes in the human GenBank DNA database. However, the first 118 bases are identical to a part of a human ovarian EST. Also, its high homology with mouse and rat sequences suggests that this sequence most probably represents a part of a novel gene, which we named FGA7 (Fused Gene 7 to AML1). Following the AML1 open reading frame, the FGA7 sequence encodes an unknown protein of 27 amino acids. We isolated three bacterial artificial chromosome (BAC) clones that contain the FGA7 sequence and confirmed the breakpoint of the gene on the patient's metaphase spreads by fluorescence in situ hybridization using these BACs as probes. RT-PCR and Northern blot analyses revealed that FGA7 is expressed in ovarian and skeletal muscle tissues. The predicted AML1-FGA7 chimeric proteins contained a limited number of residues fused to AML1 in a situation similar to that reported for the AML1-EAP fusion that is a product of t(3;21). It is possible that the expression of a constitutively shortened AML1 could compete with full-length AML1 and act as a dominant negative inhibitor of the promoters that the core binding factor activates.

The phosphorylation state of an autoregulatory domain controls PACS-1-directed protein traffic

PACS-1 is a cytosolic sorting protein that directs the localization of membrane proteins in the trans-Golgi network (TGN)/endosomal system. PACS-1 connects the clathrin adaptor AP-1 to acidic cluster sorting motifs contained in the cytoplasmic domain of cargo proteins such as furin, the cation-independent mannose-6-phosphate receptor and in viral proteins such as human immunodeficiency virus type 1 Nef. Here we show that an acidic cluster on PACS-1, which is highly similar to acidic cluster sorting motifs on cargo molecules, acts as an autoregulatory domain that controls PACS-1-directed sorting. Biochemical studies show that Ser278 adjacent to the acidic cluster is phosphorylated by CK2 and dephosphorylated by PP2A. Phosphorylation of Ser278 by CK2 or a Ser278-->Asp mutation increased the interaction between PACS-1 and cargo, whereas a Ser278-->Ala substitution decreased this interaction. Moreover, the Ser278-->Ala mutation yields a dominant-negative PACS-1 molecule that selectively blocks retrieval of PACS-1-regulated cargo molecules to the TGN. These results suggest that coordinated signaling events regulate transport within the TGN/endosomal system through the phosphorylation state of both cargo and the sorting machinery.

AML1/RUNX1 increases during G1 to S cell cycle progression independent of cytokine-dependent phosphorylation and induces cyclin D3 gene expression

AML1/RUNX1, a member of the core binding factor (CBF) family stimulates myelopoiesis and lymphopoiesis by activating lineage-specific genes. In addition, AML1 induces S phase entry in 32Dcl3 myeloid or Ba/F3 lymphoid cells via transactivation. We now found that AML1 levels are regulated during the cell cycle. 32Dcl3 and Ba/F3 cell cycle fractions were prepared using elutriation. Western blotting and a gel shift/supershift assay demonstrated that endogenous CBF DNA binding and AML1 levels were increased 2-4-fold in S and G(2)/M phase cells compared with G(1) cells. In addition, G(1) arrest induced by mimosine reduced AML1 protein levels. In contrast, AML1 RNA did not vary during cell cycle progression relative to actin RNA. Analysis of exogenous Myc-AML1 or AML1-ER demonstrated a significant reduction in G(1) phase cells, whereas levels of exogenous DNA binding domain alone were constant, lending support to the conclusion that regulation of AML1 protein stability contributes to cell cycle variation in endogenous AML1. However, cytokine-dependent AML1 phosphorylation was independent of cell cycle phase, and an AML1 mutant lacking two ERK phosphorylation sites was still cell cycle-regulated. Inhibition of AML1 activity with the CBFbeta-SMMHC or AML1-ETO oncoproteins reduced cyclin D3 RNA expression, and AML1 bound and activated the cyclin D3 promoter. Signals stimulating G(1) to S cell cycle progression or entry into the cell cycle in immature hematopoietic cells might do so in part by inducing AML1 expression, and mutations altering pathways regulating variation in AML1 stability potentially contribute to leukemic transformation.

Role of RUNX1 in adult hematopoiesis: analysis of RUNX1-IRES-GFP knock-in mice reveals differential lineage expression

The Runx1/core binding factor-beta (CBFbeta) transcriptional complex is required for the establishment of hematopoiesis during development. Despite its critical role during development, a detailed analysis of Runx1 expression within specific lineages and developmental stages of the adult hematopoietic system is lacking. To address this, we have developed a Runx1-green fluorescent protein (GFP) knock-in mouse. We show that Runx1 is expressed in several hematopoietic lineages, including myeloid, B-lymphoid, and T-lymphoid cells. By contrast, Runx1 is weakly expressed in early erythroid cells, and its expression is rapidly extinguished during later stages of erythropoiesis. Runx1 expression is induced during early B-cell development and is expressed at a uniform level during all subsequent stages of B-cell development. Within the thymus, Runx1 is expressed at the highest level in CD4-CD8- double-negative thymocytes. In peripheral T cells, Runx1 is differentially expressed, with CD4+ T cells expressing 2- to 3-fold higher levels of Runx1 than CD8+ cells. Taken together, these findings indicate that although widely expressed in the hematopoietic system, the expression of Runx1 is regulated in a cell type- and maturation stage-specific manner. In addition, the Runx1-IRES-GFP knock-in mouse strain should prove valuable for investigation of Runx1 function in adult hematopoiesis.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

Lymphoid enhancer factor-1 and beta-catenin inhibit Runx2-dependent transcriptional activation of the osteocalcin promoter

Functional control of the transcription factor Runx2 is crucial for normal bone formation. Runx2 is detectable throughout osteoblast development and maturation and temporally regulates several bone-specific genes. In this study, we identified a novel post-translational mechanism regulating Runx2-dependent activation of the osteocalcin promoter. A functional binding site for the high mobility group protein lymphoid enhancer-binding factor 1 (LEF1) was found adjacent to the proximal Runx2-binding site in the osteocalcin promoter. In transcription assays, LEF1 repressed Runx2-induced activation of the mouse osteocalcin 2 promoter in several osteoblast lineage cell lines. Mutations in the LEF1-binding site increased the basal activity of the osteocalcin promoter; however, the LEF1 recognition site in the osteocalcin promoter was surprisingly not required for LEF1 repression. A novel interaction between the DNA-binding domains of Runx2 and LEF1 was identified and found crucial for LEF1-mediated repression of Runx2. LEF1 is a nuclear effector of the Wnt/LRP5/beta-catenin signaling pathway, which is also essential for osteoblast proliferation and normal skeletal development. A constitutively active beta-catenin enhanced LEF1-dependent repression of Runx2. These data identify a novel mechanism of regulating Runx2 activity in osteoblasts and link Runx2 transcriptional activity to beta-catenin signaling.

Runx1, c-Myb, and C/EBPalpha couple differentiation to proliferation or growth arrest during hematopoiesis

Immature hematopoietic precursors proliferate as they differentiate, whereas terminal differentiation is associated with cell cycle arrest. Stem cell lineage commitment and subseqent maturation is regulated predominantly by transcription factors. Runx1 and c-Myb act in early stage hematopoietic cells to both stimulate proliferation and differentiation, whereas C/EBPalpha, and perhaps other C/EBP family members, block progression from G1 to S and induce terminal maturation. Coupling of differentiation to either proliferation or growth arrest by transcription factors is likely an important regulatory mechanism in multiple developmental systems.

Identification of RUNX1/AML1 as a classical tumor suppressor gene

Based on our previous results indicating the presence of a tumor suppressor gene (TSG), chromosome 21 was analysed for loss of heterozygosity (LOH) in 18 patients with acute myeloid leukemia (17, AML-M0; one, AML-M1). Allelotyping at polymorphic loci was performed on purified material, allowing unequivocal detection of allelic loss and homozygous deletions. Six AML-M0 patients shared a common region of LOH harboring a single gene: RUNX1 (AML1), the most frequent site of translocations in acute leukemia and a well-known fusion oncogene. Fluorescence in situ hybridization allowed the identification of deletions with breakpoints within RUNX1 in two patients as the cause of LOH. In the four others the LOH pattern and the presence of two karyotypically normal chromosomes 21 were in line with mitotic recombination. Further molecular and cytogenetic analyses showed that this caused homozygosity of primary RUNX1 mutations: two point mutations, a partial deletion and, most significantly, a complete deletion of RUNX1. These findings identify RUNX1 as a classical TSG: both alleles are mutated or absent in cancer cells from four of the 17 AML-M0 patients examined. In contrast to AML-M0, the AML-M1 patient was trisomic for chromosome 21 and has two mutated and one normal RUNX1 allele, suggesting that the order of mutagenic events leading to leukemia may influence the predominant tumor type.

Transcription factor fusions in acute leukemia: variations on a theme

The leukemia-associated fusion proteins share several structural or functional similarities, suggesting that they may impart a leukemic phenotype through common modes of transcriptional dysregulation. The fusion proteins generated by these translocations usually contain a DNA-binding domain, domains responsible for homo- or hetero-dimerization, and domains that interact with proteins involved in chromatin remodeling (e.g., co-repressor molecules or co-activator molecules). It is these shared features that constitute the 'variations on the theme' that underling the aberrant growth and differentiation that is the hallmark of acute leukemia cells.

AML1 stimulates G1 to S progression via its transactivation domain

Inhibition of AML1-mediated transactivation potently slows G1 to S cell cycle progression. In Ba/F3 cells, activation of exogenous AML1 (RUNX1)-ER with 4-hydroxytamoxifen prevents inhibition of G1 progression mediated by CBFbeta-SMMHC, a CBF oncoprotein. We expressed three AML1-ER variants with CBFbeta-SMMHC in Ba/F3 cells. In these lines, CBFbeta-SMMHC expression is regulated by the zinc-responsive metallothionein promoter. Deletion of 72 AML1 C-terminal residues, which includes a transrepression domain, did not alter the activity of AML1-ER, whereas further deletion of 98 residues, removing the most potent AML1 transactivation domain (TAD), prevented rescue of cell cycle inhibition. Notably, the two variants which did not stimulate G1 exacerbated CBFbeta-SMMHC-mediated cell cycle arrest, suggesting that they dominantly inhibit AML1 activities. In addition, the two variants which stimulated G1 also induced apoptosis in 5-15% of the cells, an effect consistent with excessive G1 stimulation. These observations indicate that AML1 activates transcription of one or more genes critical for the G1 to S transition via its C-terminal transactivation domain. Inactivation of AML in acute leukemia is expected to slow proliferation unless additional genetic alterations co-exist which accelerate G1.

Potential involvement of the AML1-MTG8 fusion protein in the granulocytic maturation characteristic of the t(8;21) acute myelogenous leukemia revealed by microarray analysis

The AML1 (RUNX1)-MTG8 (ETO) fusion transcription factor generated by the t(8;21) translocation is believed to deregulate the expression of genes that are crucial for normal differentiation and proliferation of hematopoietic progenitors, resulting in acute myelogenous leukemia. To elucidate the role of AML1-MTG8 in leukemogenesis, we used oligonucleotide microarrays to detect alterations in gene expression caused by ectopic expression of AML1-MTG8 in a murine myeloid progenitor cell line, L-G. Microarray analysis of approximately 6500 genes identified 32 candidate genes under the downstream control of AML1-MTG8. Among the 32 genes, 23 were not known to be regulated by AML1-MTG8. These included many granule protein genes and several cell surface antigen genes. Interestingly, AML1-MTG8 enhanced the expression of several genes that are usually induced during granulocytic differentiation, particularly those encoding azurophil granule proteins, including cathepsin G, myeloperoxidase and lysozyme. This indicates that AML1-MTG8 induces partial differentiation of myeloid progenitor cells into promyelocytes in the absence of the usual differentiation signals, while it inhibits terminal differentiation into mature granulocytes. Thus, AML1-MTG8 itself may play a crucial role in defining a unique cytologic type with abnormal maturation, characteristic of t(8;21) acute myelogenous leukemia.

Runx1/AML1 in leukemia: disrupted association with diverse protein partners

Runx1/AML1, a chromosome 21q22 hematopoietic regulator, is frequently translocated in leukemia. Its protein product, a relatively weak transcriptional activator, becomes an effective transcriptional enhancer or repressor, when co-operating with transcriptional co-activators or co-repressors. Runx1/AML1 association with its partners is disrupted in leukemia. For example, Runx1/AML1 mutations and translocations (e.g. t(8;21), t(12;21) and t(3;21)) impair binding of Runx1/AML1-CBFbeta complexes to Runt motifs in myelopoietically active promoters, preventing normal hematopoiesis. However, Runx1/AML1-associated translocations are not leukemogenic in animal models, suggesting the involvement of yet unidentified regulatory proteins. New candidates are cholinesterases, inhibition of which increases leukemic risk in a manner potentially associated with Runx1/AML1.

Clusterin gene in rat sertoli cells is regulated by a core-enhancer element

Clusterin is a ubiquitous glycoprotein that is promiscuously expressed at a low basal level but can be highly induced by a variety of stress conditions. In contrast, in some secretory cells associated with tissue-fluid interfaces such as the Sertoli cells in the testis, clusterin demonstrates high constitutive expression. In this study, we address the mechanisms that regulate the constitutive expression of the clusterin gene by using primary cultures of immature rat Sertoli cells. We have identified a region of the rat clusterin gene promoter that activated transcription only in Sertoli cells and that mapped between positions -426 and -311. Sequence analysis of this region revealed a high concentration of potential regulatory elements. Using gel-shift assays combined with hydroxyl radical footprinting, we identified the elements recognized by the Sertoli cell nuclear factors. Comparison of the interactions with this region of the nuclear factors from different cell types demonstrated that recognition of the core-enhancer element is specific for the Sertoli cells, and in vitro, the core region was recognized by the transcription factor CBF. Transient transfections showed that a core enhancer is responsible for more than a half of the total promoter activity and is an essential element for the cell-specific activity of the Sertoli-specific region. In addition to the core enhancer, tandem Sp1 sites are also required for maximal activity of this region.

Analysis of genes under the downstream control of the t(8;21) fusion protein AML1-MTG8: overexpression of the TIS11b(ERF-1, cMG1) gene induces myeloid cell proliferation in response to G-CSF

The AML1-MTG8 fusion transcription factor generated by t(8;21) translocation is thought to dysregulate genes that are crucial for normal differentiation and proliferation of hematopoietic progenitors to cause acute myelogenous leukemia (AML). Although AML1-MTG8 has been shown to repress the transcription of AML1 targets, none of the known targets of AML1 are probably responsible for AML1-MTG8-mediated leukemogenesis. In this study, 24 genes under the downstream control of AML1-MTG8 were isolated by using a differential display technique. Analysis with deletion mutants of AML1-MTG8 demonstrated that the regulation of the majority of these genes requires the region of 51 residues (488-538) containing the Nervy homology region 2 (NHR2), through which AML1-MTG8 interacts with MTGR1. Among the 24 genes identified, 10 were considered to be genes under the control of AML1, because their expression was altered by AML1b or AML1a or both. However, the other 14 genes were not affected by either AML1b or AML1a, suggesting the possibility that AML1-MTG8 regulates a number of specific target genes that are not normally regulated by AML1. Furthermore, an up-regulated gene, TIS11b (ERF-1,cMG1), was highly expressed in t(8;21) leukemic cells, and the overexpression of TIS11b induced myeloid cell proliferation in response to granulocyte colony-stimulating factor. These results suggest that the high-level expression of TIS11b contributes to AML1-MTG8-mediated leukemogenesis.

Analysis of genes under the downstream control of the t(8;21) fusion protein AML1-MTG8: overexpression of the TIS11b(ERF-1, cMG1) gene induces myeloid cell proliferation in response to G-CSF

The AML1-MTG8 fusion transcription factor generated by t(8;21) translocation is thought to dysregulate genes that are crucial for normal differentiation and proliferation of hematopoietic progenitors to cause acute myelogenous leukemia (AML). Although AML1-MTG8 has been shown to repress the transcription of AML1 targets, none of the known targets of AML1 are probably responsible for AML1-MTG8-mediated leukemogenesis. In this study, 24 genes under the downstream control of AML1-MTG8 were isolated by using a differential display technique. Analysis with deletion mutants of AML1-MTG8 demonstrated that the regulation of the majority of these genes requires the region of 51 residues (488-538) containing the Nervy homology region 2 (NHR2), through which AML1-MTG8 interacts with MTGR1. Among the 24 genes identified, 10 were considered to be genes under the control of AML1, because their expression was altered by AML1b or AML1a or both. However, the other 14 genes were not affected by either AML1b or AML1a, suggesting the possibility that AML1-MTG8 regulates a number of specific target genes that are not normally regulated by AML1. Furthermore, an up-regulated gene, TIS11b (ERF-1,cMG1), was highly expressed in t(8;21) leukemic cells, and the overexpression of TIS11b induced myeloid cell proliferation in response to granulocyte colony-stimulating factor. These results suggest that the high-level expression of TIS11b contributes to AML1-MTG8-mediated leukemogenesis.

Exogenous cdk4 overcomes reduced cdk4 RNA and inhibition of G1 progression in hematopoietic cells expressing a dominant-negative CBF - a model for overcoming inhibition of proliferation by CBF oncoproteins

Core Binding Factor (CBF) is required for the development of definitive hematopoiesis, and the CBF oncoproteins AML1-ETO, TEL-AML1, and CBFbeta-SMMHC are commonly expressed in subsets of acute leukemia. CBFbeta-SMMHC slows the G1 to S cell cycle transition in hematopoietic cells, but the mechanism of this effect is uncertain. We have sought to determine whether inhibition of CBF-mediated trans-activation is sufficient to slow proliferation. We demonstrate that activation of KRAB-AML1-ER, a protein containing the AML1 DNA-binding domain, the KRAB repression domain, and the Estrogen receptor ligand binding domain, also slows G1, if its DNA-binding domain is intact. Also, exogenous AML1 overcame CBFbeta-SMMHC-induced inhibition of proliferation. Representational difference analysis (RDA) identified cdk4 RNA expression as an early target of KRAB-AML1 activation. Inhibition of CBF activities by KRAB-AML1-ER or CBFbeta-SMMHC rapidly reduced endogenous cdk4 mRNA levels, even in cells proliferating at or near control rates as a result of exogenous cdk4 expression. Over-expression of cdk4, especially a variant which cannot bind p16INK4a, overcame cell cycle inhibition resulting from activation of KRAB-AML1-ER, although cdk4 did not accelerate proliferation when expressed alone. These findings indicate that mutations which alter the expression of G1 regulatory proteins can overcome inhibition of proliferation by CBF oncoproteins. Oncogene (2000).

Identification of the regions involved in DNA binding by the mouse PEBP2alpha protein

The polyomavirus enhancer binding protein 2alpha (PEBP2alpha) is a DNA binding transcriptional regulatory protein that binds conserved sites in the polyomavirus enhancer, mammalian type C retroviral enhancers and T-cell receptor gene enhancers. Binding of PEBP2alpha and homologous proteins to the consensus DNA sequence TGPyGGTPy is mediated through a protein domain known as the runt domain. Although recent NMR studies of DNA-bound forms of the runt domain have shown an immunoglobulin-like (Ig) fold, the identification of residues of the protein that are involved in DNA binding has been obscured by the low solubility of the runt domain. Constructs of the mouse PEBP2alphaA1 gene were generated with N- and C-terminal extensions beyond the runt homology region. The construct containing residues Asp90 to Lys225 of the sequence (PEBP2alpha90-225) yielded soluble protein. The residues that participate in DNA binding were determined by comparing the NMR spectra of free and DNA-bound PEBP2alpha90-225. Analysis of the changes in the NMR spectra of the two forms of the protein by chemical shift deviation mapping allowed the unambiguous determination of the regions that are responsible for specific DNA recognition by PEBP2alpha. Five regions in PEBP2alpha90-225 that are localized at one end of the beta-barrel were found to interact with DNA, similar to the DNA binding interactions of other Ig fold proteins.

Role of the transcription factor AML-1 in acute leukemia and hematopoietic differentiation

Chromosomal translocations affecting the AML-1 gene are among the most frequent aberrations found in acute leukemia. Because the AML-1 transcription factor is a critical regulator of hematopoeitic cell development, normal homeostasis is disrupted in cells containing these translocations. In this review we describe the mechanisms of transcriptional activation and repression by AML-1 and how this transcriptional control is disrupted by the chromosomal translocations that affect AML-1. Finally, we discuss how the mechanism of transcriptional repression by these chromosomal translocation fusion proteins is a possible target of therapeutic intervention in acute leukemia.

AML1, the target of chromosomal rearrangements in human leukemia, regulates the expression of human complement receptor type 1 (CR1) gene

The human CR1 gene is expressed specifically in hematopoietic cells. It is suggested that some cell-type specific factors which involve in gene-specific activation or repression exist in cells according to the result that the gene expression varies differently depend on differentiation stage. Here, we demonstrate that the integrity of a polyomavirus enhancer core sequence, 5'-TGTGGT-3', is critical to the human CR1 promoter activity. AML1 is a site-specific DNA-binding protein that recognizes the enhancer core motif TGTGGT. We show that the AML1 binds specifically to this site and activates the human CR1 promoter. Furthermore, we demonstrate that the Ets binding site (GGAA) located 2 bp upstream of the AML1 site is also involved in the regulation of the human CR1 promoter activity. Point mutations of either the AML1 or the Ets binding site that abolish the binding of the respective factors result in significant decreases of the human CR1 promoter activity. These results suggest that AML1 and Ets proteins direct the expression of the human CR1 promoter.

Regulation of c-fos gene transcription and myeloid cell differentiation by acute myeloid leukemia 1 and acute myeloid leukemia-MTG8, a chimeric leukemogenic derivative of acute myeloid leukemia 1

Both acute myeloid leukemia 1 and c-Fos are regulatory factors of hematopoietic cell differentiation. We identified that the c-fos promoter contains an acute myeloid leukemia 1 binding site at nucleotide positions -6-+14. c-fos promoter activity was induced by transient overexpression of acute myeloid leukemia 1 in Jurkat T-cells, but not by that of the short form of acute myeloid leukemia 1-MTG8, a chimeric acute myeloid leukemia 1 protein. In 32Dcl3 myeloid cells, stable overexpression of acute myeloid leukemia 1-MTG8 blocked the c-fos gene transcription and cell differentiation, but that of acute myeloid leukemia did not. These data suggest that acute myeloid leukemia 1 and acute myeloid leukemia 1-MTG8 reciprocally regulate the myeloid cell differentiation, possibly by the way of regulating c-fos gene transcription.

Nuclear factors that bind to the U3 region of two murine myeloid leukemia-inducing retroviruses, Cas-Br-E and Graffi

Cas-Br-E and Graffi are two myeloid leukemia-inducing murine viruses. Cas-Br-E induces, in NIH-Swiss mice, mostly non-T, non-B leukemia composed of very immature cells with no specific characteristics (Bergeron et al. (1993). Leukemia 7, 954-962). The Graffi murine leukemia virus causes exclusively myeloid leukemia, but the tumor cells are clearly of granulocytic nature (Ru et al. (1993). J. Virol. 67, 4722). We were interested to understand the role of the long terminal repeat (LTR) U3 region in the myeloid specificity of these two retroviruses. We used DNase I footprinting and gel mobility shift assays to identify a number of protein binding sites within Cas-Br-E and Graffi U3 regions. The pattern of protected regions is highly similar for the two viruses. Some factors identified in other murine leukemia viruses, like the core binding factor, also bind to Cas-Br-E and Graffi LTR; however, other binding sites seem specific for these two viruses. Only one difference between them was noted, at the 5' end of the U3 region. Transcriptional activity of both LTRs was also analyzed in various cell lines and compared with other murine leukemia viruses. The results show a slight myeloid specificity for the two LTRs, and indicate that the Graffi enhancer is quite strong in a broad range of cell types.

The core binding factor (CBF) alpha interaction domain and the smooth muscle myosin heavy chain (SMMHC) segment of CBFbeta-SMMHC are both required to slow cell proliferation

We have expressed several variants of core binding factor beta (CBFbeta)-smooth muscle myosin heavy chain (SMMHC) from the metallothionein promoter in Ba/F3 cells. Deletion of amino acids 2-11 from the CBFbeta segment, required for interaction with CBFalpha, prevented CBFbeta-SMMHC from inhibiting CBF DNA binding and cell cycle progression. Deletion of 283 carboxyl-terminal residues from the SMMHC domain, required for multimerization, also inactivated CBFbeta-SMMHC. Nuclear expression of CBFbeta(Delta2-11)-SMMHC was decreased relative to CBFbeta-SMMHC. CBFbeta(Delta2-11)-SMMHC linked to a nuclear localization signal still did not slow cell growth. The ability of each CBFbeta-SMMHC variant to inhibit CBF DNA binding and cell proliferation correlated with its ability to inhibit transactivation by an AML1-VP16 fusion protein. Thus, CBFbeta-SMMHC slows cell cycle progression from G1 to S phase by inhibiting CBF DNA binding and transactivation.

Developmental regulation of V(D)J recombination at the TCR alpha/delta locus

The T-cell receptor (TCR) alpha/delta locus includes a large number of V, D, J and C gene segments that are used to produce functional TCR delta and TCR alpha chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T-lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis-acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR alpha/delta locus imparts developmental control to VDJ delta and VJ alpha rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR alpha/delta locus, highly localized and region-specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

The Partner Gene of AML1 in t(16;21) Myeloid Malignancies Is a Novel Member of the MTG8(ETO) Family

The t(16;21)(q24;q22) translocation is a rare but recurrent chromosomal abnormality associated with therapy-related myeloid malignancies and a variant of the t(8;21) translocation in which theAML1 gene on chromosome 21 is rearranged. Here we report the molecular definition of this chromosomal aberration in four patients. We cloned cDNAs from the leukemic cells of a patient carrying t(16;21) by the reverse transcription polymerase chain reaction using anAML1-specific primer. The structural analysis of the cDNAs showed that AML1 was fused to a novel gene named MTG16(Myeloid Translocation Gene on chromosome16) which shows high homology to MTG8(ETO/CDR) and MTGR1. Northern blot analysis usingMTG16 probes mainly detected 4.5 kb and 4.2 kb RNAs, along with several other minor RNAs in various human tissues. As in t(8;21), the t(16;21) breakpoints occurred between the exons 5 and 6 ofAML1, and between the exons 1 and 2 or the exons 3 and 4 ofMTG16. The two genes are fused in-frame, resulting in the characteristic chimeric transcripts of this translocation. Although the reciprocal chimeric product, MTG16-AML1, was also detected in one of the t(16;21) patients, its protein product was predicted to be truncated. Thus, the AML1-MTG16 gene fusion in t(16;21) leukemia results in the production of a protein that is very similar to the AML1-MTG8 chimeric protein.

The Partner Gene of AML1 in t(16;21) Myeloid Malignancies Is a Novel Member of the MTG8(ETO) Family

The t(16;21)(q24;q22) translocation is a rare but recurrent chromosomal abnormality associated with therapy-related myeloid malignancies and a variant of the t(8;21) translocation in which theAML1 gene on chromosome 21 is rearranged. Here we report the molecular definition of this chromosomal aberration in four patients. We cloned cDNAs from the leukemic cells of a patient carrying t(16;21) by the reverse transcription polymerase chain reaction using anAML1-specific primer. The structural analysis of the cDNAs showed that AML1 was fused to a novel gene named MTG16(Myeloid Translocation Gene on chromosome16) which shows high homology to MTG8(ETO/CDR) and MTGR1. Northern blot analysis usingMTG16 probes mainly detected 4.5 kb and 4.2 kb RNAs, along with several other minor RNAs in various human tissues. As in t(8;21), the t(16;21) breakpoints occurred between the exons 5 and 6 ofAML1, and between the exons 1 and 2 or the exons 3 and 4 ofMTG16. The two genes are fused in-frame, resulting in the characteristic chimeric transcripts of this translocation. Although the reciprocal chimeric product, MTG16-AML1, was also detected in one of the t(16;21) patients, its protein product was predicted to be truncated. Thus, the AML1-MTG16 gene fusion in t(16;21) leukemia results in the production of a protein that is very similar to the AML1-MTG8 chimeric protein.

Targeted disruption of Cbfa1 results in a complete lack of bone formation owing to maturational arrest of osteoblasts

A transcription factor, Cbfa1, which belongs to the runt-domain gene family, is expressed restrictively in fetal development. To elucidate the function of Cbfa1, we generated mice with a mutated Cbfa1 locus. Mice with a homozygous mutation in Cbfa1 died just after birth without breathing. Examination of their skeletal systems showed a complete lack of ossification. Although immature osteoblasts, which expressed alkaline phophatase weakly but not Osteopontin and Osteocalcin, and a few immature osteoclasts appeared at the perichondrial region, neither vascular nor mesenchymal cell invasion was observed in the cartilage. Therefore, our data suggest that both intramembranous and endochondral ossification were completely blocked, owing to the maturational arrest of osteoblasts in the mutant mice, and demonstrate that Cbfa1 plays an essential role in osteogenesis.

Regulation of T cell receptor delta gene rearrangement by CBF/PEBP2

We have analyzed transgenic mice carrying versions of a human T cell receptor (TCR)-delta gene minilocus to study the developmental control of VDJ (variable/diversity/joining) recombination. Previous data indicated that a 1.4-kb DNA fragment carrying the TCR-delta enhancer (E(delta)) efficiently activates minilocus VDJ recombination in vivo. We tested whether the transcription factor CBF/PEBP2 plays an important role in the ability of E(delta) to activate VDJ recombination by analyzing VDJ recombination in mice carrying a minilocus in which the deltaE3 element of E(delta) includes a mutated CBF/PEBP2 binding site. The enhancer-dependent VD to J step of minilocus rearrangement was dramatically inhibited in three of four transgenic lines, arguing that the binding of CBF/PEBP2 plays a role in modulating local accessibility to the VDJ recombinase in vivo. Because mutation of the deltaE3 binding site for the transcription factor c-Myb had previously established a similar role for c-Myb, and because a 60-bp fragment of E(delta) carrying deltaE3 and deltaE4 binding sites for CBF/PEBP2, c-Myb, and GATA-3 displays significant enhancer activity in transient transfection experiments, we tested whether this fragment of E(delta) is sufficient to activate VDJ recombination in vivo. This fragment failed to efficiently activate the enhancer-dependent VD to J step of minilocus rearrangement in all three transgenic lines examined, indicating that the binding of CBF/PEBP2 and c-Myb to their cognate sites within E(delta), although necessary, is not sufficient for the activation of VDJ recombination by E(delta). These results imply that CBF/PEBP2 and c-Myb collaborate with additional factors that bind elsewhere within E(delta) to modulate local accessibility to the VDJ recombinase in vivo.

Functional dissection of the alpha and beta subunits of transcription factor PEBP2 and the redox susceptibility of its DNA binding activity

The mouse transcription factor PEBP2 is a heterodimer of two subunits: a DNA binding subunit alpha and its partner subunit beta. The alpha subunit shares a region of high homology, termed the Runt domain, with the products of the Drosophila melanogaster segmentation gene runt and the human acute myeloid leukemia-related gene AML1. To study the molecular basis for the DNA binding and heterodimerization functions of this factor, we constructed series of deletions of the alpha and beta subunits and examined their activities by electrophoretic mobility shift and affinity column assays. The minimal functional region of the alpha subunit for DNA binding and dimerization was shown to coincide with the Runt domain. On the other hand, the region of the beta subunit required for heterodimerization was localized to the N-terminal 135 amino acids. Furthermore, it was found that the DNA binding activity of the Runt domain is regulated by a reduction/oxidization (redox) mechanism and that its reductively activated state, which is extremely labile, is stabilized by the beta subunit. These findings add a new layer to the mechanism and significance of the regulatory interplay between the two subunits of PEBP2.

Cooperation between core binding factor and adjacent promoter elements contributes to the tissue-specific expression of interleukin-3

Tissue-specific expression of interleukin-3 (IL-3) is mediated via cis-acting elements located within 315 base pairs of the transcription start. This is achieved in part through the positive activities of the AP-1 and Elf-1 sites in the IL-3 promoter. The contribution to T cell-specific expression by other promoter sites was assessed in a transient expression assay with IL-3 promoter constructs linked to a luciferase gene, focusing initially on the core binding factor (CBF) site, which is footprinted in vivo upon T cell activation. Activity of the CBF site is shown to be critically dependent on the adjacent activator site Act-1. Together the Act-1 and CBF sites form a functional unit (AC unit) with dual activity. The AC unit is demonstrated to enhance basal activity of promoters both in fibroblasts and T cells. This activity is further inducible in activated T cells, but not in fibroblasts. In addition to the already identified NIP repressor site, evidence is presented for a second repressor region that restricts promoter activity in fibroblasts. Finally, a novel positive regulatory element has been mapped in the IL-3 promoter between nucleotide -180 and -210 that leads to increased expression in T cells. Together these results demonstrate that T cell expression of IL-3 is not specified by the activity of a single tissue-specific element, but instead involves multiple interacting elements that provide both specific positive regulation in T cells and specific negative regulation in fibroblasts.

Structural and functional characterization of the human CD36 gene promoter: identification of a proximal PEBP2/CBF site

CD36 is a cell surface glycoprotein composed of a single polypeptide chain, which interacts with thrombospondin, collagens type I and IV, oxidized low density lipoprotein, fatty acids, anionic phospholipids, and erythrocytes parasitized with Plasmodium falciparum. Its expression is restricted to a few cell types, including monocyte/macrophages. In these cells, CD36 is involved in phagocytosis of apoptotic cells, and foam cell formation by uptake of oxidized low density lipoprotein. To study the molecular mechanisms that control the transcription of the CD36 gene in monocytic cells we have isolated and analyzed the CD36 promoter. Transient expression experiments of 5'-deletion fragments of the CD36 promoter coupled to luciferase demonstrated that as few as 158 base pairs upstream from the transcription initiation site were sufficient to direct the monocyte-specific transcription of the reporter gene. Within the above region, the fragment spanning nucleotides -158 to -90 was required for optimal transcription in monocytic cells. Biochemical analysis of the region -158/-90 revealed a binding site for transcription factors of the polyomavirus enhancer-binding protein 2/core-binding factor (PEBP2/CBF) family at position -103. Disruption of the PEBP2/CBF site markedly diminished the role of the PEBP2/CBF factors in the constitutive transcription of the CD36 gene. The involvement of members of the PEBP2/CBF family in chromosome translocations associated with acute myeloid leukemia, and in the transcriptional regulation of the myeloid-specific genes encoding for myeloperoxidase, elastase, and the colony-stimulating factor receptor, highlights the relevance of the regulation of the CD36 gene promoter in monocytic cells by members of the PEBP2/CBF family.

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.

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.

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.

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.