Purification of core-binding factor, a protein that binds the conserved core site in murine leukemia virus enhancers

Spectrum of hematological malignancies, clonal evolution and outcomes in 144 Mayo Clinic patients with germline predisposition syndromes

Germline predisposition syndromes (GPS) result from constitutional aberrations in tumor suppressive and homeostatic genes, increasing risk for neoplasia in affected kindred. In this study, we present clinical and genomic data on 144 Mayo Clinic patients with GPS; 59 evaluated prospectively using an algorithm-based diagnostic approach in the setting of a dedicated GPS/ inherited bone marrow failure syndrome (IBMFS) clinic. Seventy-two (50%) patients had IBMFS (telomere biology disorders-32,Fanconi anemia-18, Diamond Blackfan Anemia - 11, congenital neutropenia-5, Schwachman-Diamond Syndrome-5 and Bloom Syndrome-1), 27 (19%) had GPS with antecedent thrombocytopenia (RUNX1-FPD-15, ANKRD26-6, ETV6-2, GATA1-1, MPL-3), 28 (19%) had GPS without antecedent thrombocytopenia (GATA2 haploinsufficiency-16, DDX41-10, CBL-1 and CEBPA-1) and 17 (12%) had general cancer predisposition syndromes (ataxia telangiectasia-7, heterozygous ATM variants-3, CHEK2-2, TP53-2, CDK2NA-1, NF1-1 and Nijmegen Breakage Syndrome-1). Homozygous and heterozygous ATM pathogenic variants were exclusively associated with lymphoproliferative disorders (LPD), while DDX41 GPS was associated with LPD and myeloid neoplasms. The use of somatic NGS-testing identified clonal evolution in GPS patients, with ASXL1, RAS pathway genes, SRSF2 and TET2 being most frequently mutated. Fifty-two (91%) of 59 prospectively identified GPS patients had a change in their management approach, including additional GPS-related screening in 42 (71%), referral for allogenic HSCT workup and screening of related donors in 16 (27%), medication initiation and selection of specific conditioning regimens in 14 (24%), and genetic counseling with specific intent of fertility preservation and preconceptual counseling in 10 (17%) patients; highlighting the importance of dedicated GPS screening, detection and management programs for patients with hematological neoplasms.

Interrogation of enhancer function by enhancer-targeting CRISPR epigenetic editing

Tissue-specific gene expression requires coordinated control of gene-proximal and -distal cis-regulatory elements (CREs), yet functional analysis of gene-distal CREs such as enhancers remains challenging. Here we describe CRISPR/dCas9-based enhancer-targeting epigenetic editing systems, enCRISPRa and enCRISPRi, for efficient analysis of enhancer function in situ and in vivo. Using dual effectors capable of re-writing enhancer-associated chromatin modifications, we show that enCRISPRa and enCRISPRi modulate gene transcription by remodeling local epigenetic landscapes at sgRNA-targeted enhancers and associated genes. Comparing with existing methods, the improved systems display more robust perturbations of enhancer activity and gene transcription with minimal off-targets. Allele-specific targeting of enCRISPRa to oncogenic TAL1 super-enhancer modulates TAL1 expression and cancer progression in xenotransplants. Single or multi-loci perturbations of lineage-specific enhancers using an enCRISPRi knock-in mouse establish in vivo evidence for lineage-restricted essentiality of developmental enhancers during hematopoiesis. Hence, enhancer-targeting CRISPR epigenetic editing provides opportunities for interrogating enhancer function in native biological contexts.

RUNX1 and the endothelial origin of blood

The transcription factor RUNX1 is required in the embryo for formation of the adult hematopoietic system. Here, we describe the seminal findings that led to the discovery of RUNX1 and of its critical role in blood cell formation in the embryo from hemogenic endothelium (HE). We also present RNA-sequencing data demonstrating that HE cells in different anatomic sites, which produce hematopoietic progenitors with dissimilar differentiation potentials, are molecularly distinct. Hemogenic and non-HE cells in the yolk sac are more closely related to each other than either is to hemogenic or non-HE cells in the major arteries. Therefore, a major driver of the different lineage potentials of the committed erythro-myeloid progenitors that emerge in the yolk sac versus hematopoietic stem cells that originate in the major arteries is likely to be the distinct molecular properties of the HE cells from which they are derived. We used bioinformatics analyses to predict signaling pathways active in arterial HE, which include the functionally validated pathways Notch, Wnt, and Hedgehog. We also used a novel bioinformatics approach to assemble transcriptional regulatory networks and predict transcription factors that may be specifically involved in hematopoietic cell formation from arterial HE, which is the origin of the adult hematopoietic system.

Runx transcription factors repress human and murine c-Myc expression in a DNA-binding and C-terminally dependent manner

The transcription factors Runx1 and c-Myc have individually been shown to regulate important gene targets as well as to collaborate in oncogenesis. However, it is unknown whether there is a regulatory relationship between the two genes. In this study, we investigated the transcriptional regulation of endogenous c-Myc by Runx1 in the human T cell line Jurkat and murine primary hematopoietic cells. Endogenous Runx1 binds to multiple sites in the c-Myc locus upstream of the c-Myc transcriptional start site. Cells transduced with a C-terminally truncated Runx1 (Runx1.d190), which lacks important cofactor interaction sites and can block C-terminal-dependent functions of all Runx transcription factors, showed increased transcription of c-Myc. In order to monitor c-Myc expression in response to early and transiently-acting Runx1.d190, we generated a cell membrane-permeable TAT-Runx1.d190 fusion protein. Murine splenocytes treated with TAT-Runx1.d190 showed an increase in the transcription of c-Myc within 2 hours, peaking at 4 hours post-treatment and declining thereafter. This effect is dependent on the ability of Runx1.d190 to bind to DNA. The increase in c-Myc transcripts is correlated with increased c-Myc protein levels. Collectively, these data show that Runx1 directly regulates c-Myc transcription in a C-terminal- and DNA-binding-dependent manner.

Hairless is a cofactor for Runt-dependent transcriptional regulation

Runt is a vital transcriptional regulator in the developmental pathway responsible for segmentation in the Drosophila embryo. Runt activates or represses transcription in a manner that is dependent on both cellular context and the specific downstream target. Here we identify Hairless (H) as a Runt-interacting molecule that functions during segmentation. We find that H is important for maintenance of engrailed (en) repression as was previously demonstrated for Groucho (Gro), Rpd3, and CtBP. H also contributes to the Runt-dependent repression of sloppy-paired-1 (slp1), a role that is not shared with these other corepressors. We further find distinct roles for these different corepressors in the regulation of other Runt targets in the early Drosophila embryo. These findings, coupled with observations on the distinct functional requirements for Runt in regulating these several different targets, indicate that Runt-dependent regulation in the Drosophila blastoderm embryo relies on unique, target-gene-specific molecular interactions.

Post-translational Regulation of Runx2 in Bone and Cartilage

The Runx2 gene product is essential for mammalian bone development. In humans, Runx2 haploinsufficiency results in cleidocranial dysplasia, a skeletal disorder characterized by bone and dental abnormalities. At the molecular level, Runx2 acts as a transcription factor for genes expressed in hypertrophic chondrocytes and osteoblasts. Runx2 gene expression and protein function are regulated on multiple levels, including transcription, translation, and post-translational modification. Furthermore, Runx2 is involved in numerous protein-protein interactions, most of which either activate or repress transcription of target genes. In this review, we discuss expression of Runx2 during development as well as the post-translational regulation of Runx2 through modification by phosphorylation, ubiquitination, and acetylation.

t(8;21)(q22;q22) Fusion proteins preferentially bind to duplicated AML1/RUNX1 DNA-binding sequences to differentially regulate gene expression

Chromosome abnormalities are frequently associated with cancer development. The 8;21(q22;q22) chromosomal translocation is one of the most common chromosome abnormalities identified in leukemia. It generates fusion proteins between AML1 and ETO. Since AML1 is a well-defined DNA-binding protein, AML1-ETO fusion proteins have been recognized as DNA-binding proteins interacting with the same consensus DNA-binding site as AML1. The alteration of AML1 target gene expression due to the presence of AML1-ETO is related to the development of leukemia. Here, using a 25-bp random double-stranded oligonucleotide library and a polymerase chain reaction (PCR)-based DNA-binding site screen, we show that compared with native AML1, AML1-ETO fusion proteins preferentially bind to DNA sequences with duplicated AML1 consensus sites. This finding is further confirmed by both in vitro and in vivo DNA-protein interaction assays. These results suggest that AML1-ETO fusion proteins have a selective preference for certain AML1 target genes that contain multimerized AML1 consensus sites in their regulatory elements. Such selected regulation provides an important molecular mechanism for the dysregulation of gene expression during cancer development.

Development of the hemangioblast defines the onset of hematopoiesis in human ES cell differentiation cultures

The onset of hematopoiesis in the mouse embryo and in the embryonic stem (ES) cell differentiation model is defined by the emergence of the hemangioblast, a progenitor with both hematopoietic and vascular potential. While there is evidence for the existence of a hemangioblast in the mouse, it is unclear if this progenitor develops during the establishment of the human hematopoietic system. In this report, we have mapped hematopoietic development in human ES cell (hESC) differentiation cultures and demonstrated that a comparable hemangioblast population exists. The human hemangioblasts were identified by their capacity to generate blast colonies that display both hematopoietic and vascular potential. These colony-forming cells express the receptor tyrosine kinase KDR (VEGF receptor 2) and represent a transient population that develops in BMP-4-stimulated embryoid bodies (EBs) between 72 and 96 hours of differentiation, prior to the onset of the primitive erythroid program. Two distinct types of hemangioblasts were identified, those that give rise to primitive erythroid cells, macrophages, and endothelial cells and those that generate only the primitive erythroid population and endothelial cells. These findings demonstrate for the first time the existence of the human hemangioblast and in doing so identify the earliest stage of hematopoietic commitment.

AML1 deletion in adult mice causes splenomegaly and lymphomas

AML1 (RUNX1) encodes a DNA-binding subunit of the CBF transcription factor family and is required for the establishment of definitive hematopoiesis. AML1 is one of the most frequently mutated genes associated with human acute leukemia, suggesting that genetic alterations of the gene contribute to leukemogenesis. Here, we report the analysis of mice carrying conditional AML1 knockout alleles that were inactivated using the Cre/loxP system. AML1 was deleted in adult mice by inducing Cre activity to replicate AML1 deletions found in human MDS, familial platelet disorder and rare de novo human AML. At a latency of 2 months after induction, the thymus was reduced in size and frequently populated by immature double negative thymocytes, indicating defective T-lymphocyte maturation, resulting in lymphatic diseases with 50% penetrance, including atypical hyperplasia and thymic lymphoma. Metastatic lymphomas to the liver and the meninges were observed. Mice also developed splenomegaly with an expansion of the myeloid compartment. Increased Howell-Jolly body counts indicated splenic hypofunction. Thrombocytopenia occurred due to immaturity of mini-megakaryocytes in the bone marrow. Together with mild lymphocytopenia in the peripheral blood and increased fractions of immature cells in the bone marrow, AML1 deficient mice display features of a myelodysplastic syndrome, suggesting a preleukemic state.

The Distal Zinc Finger Domain of AML1/MDS1/EVI1 Is an Oligomerization Domain Involved in Induction of Hematopoietic Differentiation Defects in Primary Cells In vitro

AML1/MDS1/EVI1 (AME) is a chimeric transcription factor produced by the (3;21)(q26;q22) translocation. This chromosomal translocation is associated with de novo and therapy-related acute myeloid leukemia and with the blast crisis of chronic myelogenous leukemia. AME is obtained by in-frame fusion of the AML1 and MDS1/EVI1 (ME) genes. The mechanisms by which AME induces a neoplastic transformation in bone marrow cells are unknown. AME interacts with the corepressors CtBP and HDAC1, and it was shown that AME is a repressor in contrast to the parent transcription factors AML1 and ME, which are transcription activators. Studies with murine bone marrow progenitors indicated that the introduction of a point mutation that destroys the CtBP-binding consensus impairs but does not abolish the disruption of cell differentiation and replication associated with AME expression, suggesting that additional events are required. Several chimeric proteins, such as AML1/ETO, BCR/ABL, and PML/RARa, are characterized by the presence of a self-interaction domain critical for transformation. We report that AME is also able to oligomerize and displays a complex pattern of self-interaction that involves at least three oligomerization regions, one of which is the distal zinc finger domain. Although the deletion of this short domain does not preclude the self-interaction of AME, it significantly reduces the differentiation defects caused in vitro by AME in primary murine bone marrow progenitors. The addition of a point mutation that inhibits CtBP binding completely abrogates the effects of AME on differentiation, suggesting that AME induces hematopoietic differentiation defects through at least two separate but cooperating pathways.

Runx1 promotes angiogenesis by downregulation of insulin-like growth factor-binding protein-3

Mouse embryos lacking the Runx1 transcription factor exhibit an angiogenic defect accompanied by the absence of hematopoietic stem cells (HSCs). To ask whether Runx1 plays a direct role in angiogenesis, we established a novel endothelial progenitor cell line, designated AEL-DeltaR1, from the aorta-gonad-mesonephros (AGM) region of Runx1-null mouse. We introduced Runx1 cDNA into AEL-DeltaR1 cells under the doxycycline-inducible promoter. The ability of AEL-DeltaR1 cells to form vascular networks on matrigel was highly enhanced by the restored expression of Runx1. By molecular comparison of mRNAs in AEL-DeltaR1 cells before and after the induction of Runx1, we found that mRNA expression of insulin-like growth factor-binding protein 3 (IGFBP-3) is downregulated by Runx1. Gel retardation and reporter assays revealed that Runx1 binds to the promoter region of mouse IGFBP-3 gene and represses its transcription. When IGFBP-3 was exogenously added in the matrigel assay, the angiogenesis-enhancing activity of Runx1 was suppressed in a dose-dependent manner. These results demonstrate that Runx1 is directly involved in angiogenesis by repression of IGFBP-3 mRNA expression.

The homeobox gene HEX regulates proliferation and differentiation of hemangioblasts and endothelial cells during ES cell differentiation

In this report we have investigated the role of the homeobox gene Hex in the development and differentiation of the blast colony-forming cell (BL-CFC), a progenitor with hemangioblast characteristics generated in embryonic stem (ES) cell-derived embryoid bodies (EBs). Molecular analysis showed that Hex is expressed in mesoderm, in populations that contain BL-CFCs, and in blast cell colonies, the progeny of the BL-CFCs. Hex(-/-) EBs displayed a defect in macrophage development but generated higher numbers of BL-CFCs than did wild-type EBs. In addition to differences in these progenitor populations, we also found that endothelial cells from the Hex(-/-) EBs showed enhanced proliferative potential compared with those from wild-type EBs. Forced expression of Hex at the onset of ES cell differentiation resulted in reduced EB cellularity, fetal liver kinase-1 (Flk-1) expression, and BL-CFC development. Taken together, these findings demonstrate that Hex functions at multiple stages of development within the differentiating EBs and uncover a novel role for this transcription factor as a negative regulator of the hemangioblast and the endothelial lineage.

cAMP response element-binding protein, activating transcription factor-4, and upstream stimulatory factor differentially control hippocampal GABABR1a and GABABR1b subunit gene expression through alternative promoters

Expression of metabotropic GABA(B) receptors is essential for slow inhibitory synaptic transmission in the CNS, and disruption of GABA(B) receptor-mediated responses has been associated with several disorders, including neuropathic pain and epilepsy. The location of GABA(B) receptors in neurons determines their specific role in synaptic transmission, and it is believed that sorting of subunit isoforms, GABA(B)R1a and GABA(B)R1b, to presynaptic or postsynaptic membranes helps to determine this role. GABA(B)R1a and GABA(B)R1b are thought to arise by alternative splicing of heteronuclear RNA. We now demonstrate that alternative promoters, rather than alternative splicing, produce GABA(B)R1a and GABA(B)R1b isoforms. Our data further show that subunit gene expression in hippocampal neurons is mediated by the cAMP response element-binding protein (CREB) by binding to unique cAMP response elements in the alternative promoter regions. Double-stranded oligonucleotide decoys selectively alter levels of endogenous GABA(B)R1a and GABA(B)R1b in primary hippocampal neurons, and CREB knock-out mice show changes in levels of GABA(B)R1a and GABA(B)R1b transcripts, consistent with decoy competition experiments. These results demonstrate a critical role of CREB in transcriptional mechanisms that control GABA(B)R1 subunit levels in vivo. In addition, the CREB-related factor activating transcription factor-4 (ATF4) has been shown to interact directly with GABA(B)R1 in neurons, and we show that ATF4 differentially regulates GABA(B)R1a and GABA(B)R1b promoter activity. These results, together with our finding that the depolarization-sensitive upstream stimulatory factor (USF) binds to a composite CREB/ATF4/USF regulatory element only in the absence of CREB binding, indicate that selective control of alternative GABA(B)R1 promoters by CREB, ATF4, and USF may dynamically regulate expression of their gene products in the nervous system.

Mutation of all Runx (AML1/core) sites in the enhancer of T-lymphomagenic SL3-3 murine leukemia virus unmasks a significant potential for myeloid leukemia induction and favors enhancer evolution toward induction of other disease patterns

SL3-3 murine leukemia virus is a potent inducer of T-lymphomas in mice. Using inbred NMRI mice, it was previously reported that a mutant of SL3-3 with all enhancer Runx (AML1/core) sites disrupted by 3-bp mutations (SL3-3dm) induces predominantly non-T-cell tumors with severely extended latency (S. Ethelberg, J. Lovmand, J. Schmidt, A. Luz, and F. S. Pedersen, J. Virol. 71:7273-7280, 1997). By use of three-color flow cytometry and molecular and histopathological analyses, we have now performed a detailed phenotypic characterization of SL3-3- and SL3-3dm-induced tumors in this mouse strain. All wild-type induced tumors had clonal T-cell receptor beta rearrangements, and the vast majority were CD3(+) CD4(+) CD8(-) T-lymphomas. Such a consistent phenotypic pattern is unusual for murine leukemia virus-induced T-lymphomas. The mutant virus induced malignancies of four distinct hematopoietic lineages: myeloid, T lymphoid, B lymphoid, and erythroid. The most common disease was myeloid leukemia with maturation. Thus, mutation of all Runx motifs in the enhancer of SL3-3 severely impedes viral T-lymphomagenicity and thereby discloses a considerable and formerly unappreciated potential of this virus for myeloid leukemia induction. Proviral enhancers with complex structural alterations (deletions, insertions, and/or duplications) were found in most SL3-3dm-induced T-lymphoid tumors and immature myeloid leukemias but not in any cases of myeloid leukemia with maturation, mature B-lymphoma, or erythroleukemia. Altogether, our results indicate that the SL3-3dm enhancer in itself promotes induction of myeloid leukemia with maturation but that structural changes may arise in vivo and redirect viral disease specificity to induction of T-lymphoid or immature myeloid leukemias, which typically develop with moderately shorter latencies.

Oncogenic potential of the RUNX gene family: 'overview'

Runt-related (RUNX) gene family is composed of three members, RUNX1/AML1, RUNX2 and RUNX3, and encodes the DNA-binding (alpha) subunits of the Runt domain transcription factor polyomavirus enhancer-binding protein 2 (PEBP2)/core-binding factor (CBF), which is a heterodimeric transcription factor. RUNX1 is most frequently involved in human acute leukemia. RUNX2 shows oncogenic potential in mouse experimental system. RUNX3 is a strong candidate as a gastric cancer tumor suppressor. The beta subunit gene of PEBP2/CBF is also frequently involved in chromosome rearrangements associated with human leukemia. In this Overview, I will summarize how this growing field has been formed and what are the challenging new frontiers for better understanding of the oncogenic potential of this gene family.

Expression of alternatively spliced isoforms of human Sp7 in osteoblast-like cells

BACKGROUND

Osteogenic and chondrocytic differentiation involves a cascade of coordinated transcription factor gene expression that regulates proliferation and matrix protein formation in a defined temporo-spatial manner. Bone morphogenetic protein-2 induces expression of the murine Osterix/Specificity protein-7 (Sp7) transcription factor that is required for osteoblast differentiation and bone formation. Regulation of its expression may prove useful for mediating skeletal repair.

RESULTS

Sp7, the human homologue of the mouse Osterix gene, maps to 12q13.13, close to Sp1 and homeobox gene cluster-C. The first two exons of the 3-exon gene are alternatively spliced, encoding a 431-residue long protein isoform and an amino-terminus truncated 413-residue short protein isoform. The human Sp7 protein is a member of the Sp family having 78% identity with Sp1 in the three, Cys2-His2 type, DNA-binding zinc-fingers, but there is little homology elsewhere. The Sp7 mRNA was expressed in human foetal osteoblasts and craniofacial osteoblasts, chondrocytes and the osteosarcoma cell lines HOS and MG63, but was not detected in adult femoral osteoblasts. Generally, the expression of the short (or beta) protein isoform of Sp7 was much higher than the long (or alpha) protein isoform. No expression of either isoform was found in a panel of other cell types. However, in tissues, low levels of Sp7 were detected in testis, heart, brain, placenta, lung, pancreas, ovary and spleen.

CONCLUSIONS

Sp7 expression in humans is largely confined to osteoblasts and chondrocytes, both of which differentiate from the mesenchymal lineage. Of the two protein isoforms, the short isoform is most abundant.

Transcription factor complex formation and chromatin fine structure alterations at the murine c-fms (CSF-1 receptor) locus during maturation of myeloid precursor cells

Expression of the gene for the macrophage colony stimulating factor receptor (CSF-1R), c-fms, has been viewed as a hallmark of the commitment of multipotent precursor cells to macrophages. Lineage-restricted expression of the gene is controlled by conserved elements in the proximal promoter and within the first intron. To investigate the developmental regulation of c-fms at the level of chromatin structure, we developed an in vitro system to examine the maturation of multipotent myeloid precursor cells into mature macrophages. The dynamics of chromatin fine structure alterations and transcription factor occupancy at the c-fms promoter and intronic enhancer was examined by in vivo DMS and UV-footprinting. We show that the c-fms gene is already transcribed at low levels in early myeloid precursors on which no CSF-1R surface expression can be detected. At this stage of myelopoiesis, the formation of transcription factor complexes on the promoter was complete. By contrast, occupancy of the enhancer was acutely regulated during macrophage differentiation. Our data show that cell-intrinsic differentiation decisions at the c-fms locus precede the appearance of c-fms on the cell surface. They also suggest that complex lineage-specific enhancers such as the c-fms intronic enhancer regulate local chromatin structure through the coordinated assembly and disassembly of distinct transcription factor complexes.

Dichotomy of AML1-ETO functions: growth arrest versus block of differentiation

The fusion gene AML1-ETO is the product of t(8;21)(q22;q22), one of the most common chromosomal translocations associated with acute myeloid leukemia. To investigate the impact of AML1-ETO on hematopoiesis, tetracycline-inducible AML1-ETO-expressing cell lines were generated using myeloid cells. AML1-ETO is tightly and strongly induced upon tetracycline withdrawal. The proliferation of AML1-ETO(+) cells was markedly reduced, and most of the cells eventually underwent apoptosis. RNase protection assays revealed that the amount of Bcl-2 mRNA was decreased after AML1-ETO induction. Enforced expression of Bcl-2 was able to significantly delay, but not completely overcome, AML1-ETO-induced apoptosis. Prior to the onset of apoptosis, we also studied the ability of AML1-ETO to modulate differentiation. AML1-ETO expression altered granulocytic differentiation of U937T-A/E cells. More significantly, this change of differentiation was associated with the down-regulation of CCAAT/enhancer binding protein alpha (C/EBPalpha), a key regulator of granulocytic differentiation. These observations suggest a dichotomy in the functions of AML1-ETO: (i) reduction of granulocytic differentiation correlated with decreased expression of C/EBPalpha and (ii) growth arrest leading to apoptosis with decreased expression of CDK4, c-myc, and Bcl-2. We predict that the preleukemic AML1-ETO(+) cells must overcome AML1-ETO-induced growth arrest and apoptosis prior to fulfilling their leukemogenic potential.

Auto-inhibition and partner proteins, core-binding factor beta (CBFbeta) and Ets-1, modulate DNA binding by CBFalpha2 (AML1)

Core-binding factor alpha2 (CBFalpha2; otherwise known as AML1 or PEBP2alphaB) is a DNA-binding subunit in the family of core-binding factors (CBFs), heterodimeric transcription factors that play pivotal roles in multiple developmental processes in mammals, including hematopoiesis and bone development. The Runt domain in CBFalpha2 (amino acids 51 to 178) mediates DNA binding and heterodimerization with the non-DNA-binding CBFbeta subunit. Both the CBFbeta subunit and the DNA-binding protein Ets-1 stimulate DNA binding by the CBFalpha2 protein. Here we quantify and compare the extent of cooperativity between CBFalpha2, CBFbeta, and Ets-1. We also identify auto-inhibitory sequences within CBFalpha2 and sequences that modulate its interactions with CBFbeta and Ets-1. We show that sequences in the CBFalpha2 Runt domain and sequences C terminal to amino acid 214 inhibit DNA binding. Sequences C terminal to amino acid 214 also inhibit heterodimerization with the non-DNA-binding CBFbeta subunit, particularly heterodimerization off DNA. CBFbeta rescinds the intramolecular inhibition of CBFalpha2, stimulating DNA binding approximately 40-fold. In comparison, Ets-1 stimulates CBFalpha2 DNA binding 7- to 10-fold. Although the Runt domain alone is sufficient for heterodimerization with CBFbeta, sequences N terminal to amino acid 41 and between amino acids 190 and 214 are required for cooperative DNA binding with Ets-1. Cooperative DNA binding with Ets-1 is less pronounced with the CBFalpha2-CBFbeta heterodimer than with CBFalpha2 alone. These analyses demonstrate that CBFalpha2 is subject to both negative regulation by intramolecular interactions, and positive regulation by two alternative partnerships.

Tandemization of a subregion of the enhancer sequences from SRS 19-6 murine leukemia virus associated with T-lymphoid but not other leukemias

Most simple retroviruses induce tumors of a single cell type when infected into susceptible hosts. The SRS 19-6 murine leukemia virus (MuLV), which originated in mainland China, induces leukemias of multiple cellular origins. Indeed, infected mice often harbor more than one tumor type. Since the enhancers of many MuLVs are major determinants of tumor specificity, we tested the role of the SRS 19-6 MuLV enhancers in its broad disease specificity. The enhancer elements of the Moloney MuLV (M-MuLV) were replaced by the 170-bp enhancers of SRS 19-6 MuLV, yielding the recombinants DeltaMo+SRS(+) and DeltaMo+SRS(-) M-MuLV. M-MuLV normally induces T-lymphoid tumors in all infected mice. Surprisingly, when neonatal mice were inoculated with DeltaMo+SRS(+) or DeltaMo+SRS(-) M-MuLV, all tumors were of T-lymphoid origin, typical of M-MuLV rather than SRS 19-6 MuLV. Thus, the SRS 19-6 MuLV enhancers did not confer the broad disease specificity of SRS 19-6 MuLV to M-MuLV. However, all tumors contained DeltaMo+SRS M-MuLV proviruses with common enhancer alterations. These alterations consisted of tandem multimerization of a subregion of the SRS 19-6 enhancers, encompassing the conserved LVb and core sites and adjacent sequences. Moreover, when tumors induced by the parental SRS 19-6 MuLV were analyzed, most of the T-lymphoid tumors had similar enhancer alterations in the same region whereas tumors of other lineages retained the parental SRS 19-6 MuLV enhancers. These results emphasize the importance of a subregion of the SRS 19-6 MuLV enhancer in induction of T-cell lymphoma. The relevant sequences were consistent with crucial sequences for T-cell lymphomagenesis identified for other MuLVs such as M-MuLV and SL3-3 MuLV. These results also suggest that other regions of the SRS 19-6 MuLV genome contribute to its broad leukemogenic spectrum.

Core-binding factor influences the disease specificity of Moloney murine leukemia virus

The core site in the Moloney murine leukemia virus (Moloney MLV) enhancer was previously shown to be an important determinant of the T-cell disease specificity of the virus. Mutation of the core site resulted in a significant shift in disease specificity of the Moloney virus from T-cell leukemia to erythroleukemia. We and others have since determined that a protein that binds the core site, one of the core-binding factors (CBF) is highly expressed in thymus and is essential for hematopoiesis. Here we test the hypothesis that CBF plays a critical role in mediating pathogenesis of Moloney MLV in vivo. We measured the affinity of CBF for most core sites found in MLV enhancers, introduced sites with different affinities for CBF into the Moloney MLV genome, and determined the effects of these sites on viral pathogenesis. We found a correlation between CBF affinity and the latent period of disease onset, in that Moloney MLVs with high-affinity CBF binding sites induced leukemia following a shorter latent period than viruses with lower-affinity sites. The T-cell disease specificity of Moloney MLV also appeared to correlate with the affinity of CBF for its binding site. The data support a role for CBF in determining the pathogenic properties of Moloney MLV.

Suppressor mutations within the core binding factor (CBF/AML1) binding site of a T-cell lymphomagenic retrovirus

The transcriptional enhancer of the lymphomagenic mouse retrovirus SL3 contains a binding site for the transcription factor core binding factor (CBF; also called AML1, PEBP2, and SEF1). The SL3 CBF binding site is called the core. It differs from the core of the weakly lymphomagenic mouse retrovirus Akv by one nucleotide (the sequences are TGTGGTTAA and TGTGGTCAA, respectively). A mutant virus called SAA that was identical to SL3 except that its core was mutated to the Akv sequence was only moderately attenuated for lymphomagenicity. In most SAA-infected mice, tumor proviruses contained either reversions of the original mutation or one of two novel core sequences. In 20% of the SAA-infected mice, tumor proviruses retained the original SAA/Akv core mutation but acquired one of two additional mutations (underlined), TGCGGTCAA or TGTGGTCTA, that generated core elements called So and T*, respectively. We tested whether the novel base changes in the So and T* cores were suppressor mutations. SL3 mutants that contained So or T* cores in place of the wild-type sequence were generated. These viruses induced T-cell lymphomas in mice more quickly than SAA. Therefore, the mutations in the So and T* cores are indeed second-site suppressor mutations. The suppressor mutations increased CBF binding in vitro and transcriptional activity of the viral long terminal repeats (LTRs) in T lymphocytes to levels comparable to those of SL3. Thus, CBF binding was increased by any of three different nucleotide changes within the sequence of the SAA core. Increased CBF binding resulted in increased LTR transcriptional activity in T cells and in increased viral lymphomagenicity.

Sequence-specific and/or stereospecific constraints of the U3 enhancer elements of MCF 247-W are important for pathogenicity

The oncogenic potential of many nonacute retroviruses is dependent on the duplication of the enhancer sequences present in the unique 3' (U3) region of the long terminal repeat (LTR). In a molecular clone (MCF 247-W) of the murine leukemia virus MCF 247, a leukemogenic mink cell focus-inducing (MCF) virus, the U3 enhancer sequences are tandemly repeated in the LTR. We mutated the enhancer region of MCF 247-W to test the hypothesis that the duplicated enhancer sequences of this virus have a sequence-specific and/or a stereospecific role in enhancer function required for transformation. In one virus, we inserted 14 nucleotide bp into the novel sequence generated at the junction of the two enhancers to generate an MCF virus with an interrupted enhancer region. In the second virus, only one copy of the enhancer sequences was present. This second virus also lacked the junction sequence present between the two enhancers of MCF 247-W. Both viruses were less leukemogenic and had a longer mean latency period than MCF 247-W. These data indicate that the sequence generated at the junction of the two enhancers and/or the stereospecific arrangement of the two enhancer elements are required for the full oncogenic potential of MCF 247-W. We analyzed proviral LTRs within the c-myc locus in tumor DNAs from mice injected with the MCF virus with the interrupted enhancer region. Some of the proviral LTRs integrated upstream of c-myc contain enhancer regions that are larger than those of the injected virus. These results are consistent with the suggestion that the virus with an interrupted enhancer changes in vivo to perform its role in the transformation of T cells.

The leukemic protein core binding factor beta (CBFbeta)-smooth-muscle myosin heavy chain sequesters CBFalpha2 into cytoskeletal filaments and aggregates

The fusion gene CBFB-MYH11 is generated by the chromosome 16 inversion associated with acute myeloid leukemias. This gene encodes a chimeric protein involving the core binding factor beta (CBFbeta) and the smooth-muscle myosin heavy chain (SMMHC). Mouse model studies suggest that this chimeric protein CBFbeta-SMMHC dominantly suppresses the function of CBF, a heterodimeric transcription factor composed of DNA binding subunits (CBFalpha1 to 3) and a non-DNA binding subunit (CBFbeta). This dominant suppression results in the blockage of hematopoiesis in mice and presumably contributes to leukemogenesis. We used transient-transfection assays, in combination with immunofluorescence and green fluorescent protein-tagged proteins, to monitor subcellular localization of CBFbeta-SMMHC, CBFbeta, and CBFalpha2 (also known as AML1 or PEBP2alphaB). When expressed individually, CBFalpha2 was located in the nuclei of transfected cells, whereas CBFbeta was distributed throughout the cell. On the other hand, CBFbeta-SMMHC formed filament-like structures that colocalized with actin filaments. Upon cotransfection, CBFalpha2 was able to drive localization of CBFbeta into the nucleus in a dose-dependent manner. In contrast, CBFalpha2 colocalized with CBFbeta-SMMHC along the filaments instead of localizing to the nucleus. Deletion of the CBFalpha-interacting domain within CBFbeta-SMMHC abolished this CBFalpha2 sequestration, whereas truncation of the C-terminal-end SMMHC domain led to nuclear localization of CBFbeta-SMMHC when coexpressed with CBFalpha2. CBFalpha2 sequestration by CBFbeta-SMMHC was further confirmed in vivo in a knock-in mouse model. These observations suggest that CBFbeta-SMMHC plays a dominant negative role by sequestering CBFalpha2 into cytoskeletal filaments and aggregates, thereby disrupting CBFalpha2-mediated regulation of gene expression.

Members of the GATA family of transcription factors bind to the U3 region of Cas-Br-E and graffi retroviruses and transactivate their expression

Cas-Br-E and Graffi are two murine viruses that induce myeloid leukemia in mice: while Cas-Br-E induces mostly non-T, non-B leukemia composed of very immature cells, Graffi causes exclusively a granulocytic leukemia (E. Rassart, J. Houde, C. Denicourt, M. Ru, C. Barat, E. Edouard, L. Poliquin, and D. Bergeron, Curr. Top. Microbiol. Immunol. 211:201-210, 1995). In an attempt to understand the basis of the myeloid specificity of these two retroviruses, we used DNase I footprinting analysis and gel mobility shift assays to identify a number of protein binding sites within the Cas-Br-E and Graffi U3 regions. Two protected regions include potential GATA binding sites. Methylation interference analysis with different hematopoietic nuclear extracts showed the importance of the G residues in these GATA sites, and supershift assays clearly identified the binding factors as GATA-1, GATA-2, and GATA-3. Transient assays with long terminal repeat (LTR)-chloramphenicol acetyltransferase constructs showed that these three GATA family members are indeed able to transactivate Cas-Br-E and Graffi LTRs. Thus, the availability and relative abundance of the various members of the GATA family of transcription factors in a given cell type could influence the transcriptional tissue specificity of murine leukemia viruses and hence their disease specificity.

CBF, Myb, and Ets binding sites are important for activity of the core I element of the murine retrovirus SL3-3 in T lymphocytes

Transcriptional enhancers within the long terminal repeats of murine leukemia viruses are major determinants of the pathogenic properties of these viruses. Mutations were introduced into the adjacent binding sites for three transcription factors within the enhancer of the T-cell-lymphomagenic virus SL3-3. The sites that were tested were, in 5'-to-3' order, a binding site for core binding factor (CBF) called core II, a binding site for c-Myb, a site that binds members of the Ets family of factors, and a second CBF binding site called core I. Mutation of each site individually reduced transcriptional activity in T lymphocytes. However, mutation of the Myb and core I binding sites had larger effects than mutation of the Ets or core II site. The relative effects on transcription in T cells paralleled the effects of the same mutations on viral lymphomagenicity, consistent with the idea that the role of these sequences in viral lymphomagenicity is indeed to regulate transcription in T cells. Mutations were also introduced simultaneously into multiple sites in the SL3-3 enhancer. The inhibitory effects of these mutations indicated that the transcription factor in T cells that recognizes the core I element of SL3-3, presumably CBF, needed to synergize with one or more factors bound at the upstream sites to function. This was tested further by generating a multimer construct that contained five tandem core I elements linked to a basal long terminal repeat promoter. This construct was inactive in T cells. However, transcriptional activity was detected with a multimer construct in which the transcription factor binding sites upstream of the core were also present. These results are consistent with the hypothesis that CBF requires heterologous transcription factors bound at nearby sites to function in T cells.

The Saccharomyces cerevisiae Hap5p homolog from fission yeast reveals two conserved domains that are essential for assembly of heterotetrameric CCAAT-binding factor

The CCAAT-binding factor is an evolutionarily conserved heteromeric transcription factor that binds to CCAAT box-containing upstream activation sites within the promoters of numerous eukaryotic genes. The CCAAT-binding factor from Saccharomyces cerevisiae is a heterotetramer that contains the subunits Hap2p, Hap3p, Hap4p, and Hap5p and that functions in the activation of genes involved in respiratory metabolism. Here we describe the isolation of the cDNA encoding the Schizosaccharomyces pombe homolog of Hap5p, designated php5+. We have shown that Php5p is a subunit of the CCAAT-binding factor in fission yeast and is required for transcription of the S. pombe cyc1+ gene. Analysis of the evolutionarily conserved regions of Hap5p, Php5p, and the mammalian homolog CBF-C revealed two essential domains within Hap5p that are required for DNA binding and transcriptional activation. One is an 87-amino-acid core domain that is conserved among Hap5p, Php5p, and CBF-C and that is required for the assembly of the Hap2p-Hap3p-Hap5p heterotrimer both in vitro and in vivo. A second domain that is essential for the recruitment of Hap4p into the CCAAT-binding complex was identified in Hap5p and Php5p.

Groucho-dependent and -independent repression activities of Runt domain proteins

Runt domain proteins are transcriptional regulators that specify cell fates for processes extending from pattern formation in insects to leukemogenesis in humans. Runt domain family members are defined based on the presence of the 128-amino-acid Runt domain, which is necessary and sufficient for sequence-specific DNA binding. We demonstrate an evolutionarily conserved protein-protein interaction between Runt domain proteins and the corepressor Groucho. The interaction, however, is independent of the Runt domain and can be mapped to a 5-amino-acid sequence, VWRPY, present at the C terminus of all Runt domain proteins. Drosophila melanogaster Runt and Groucho interact genetically; the in vivo repression of a subset of Runt-regulated genes is dependent on the interaction with Groucho and is sensitive to Groucho dosage. Runt's repression of one gene, engrailed, is independent of VWRPY and Groucho, thus demonstrating alternative mechanisms for repression by Runt domain proteins. Unlike other transcriptional regulatory proteins that interact with Groucho, Runt domain proteins are known to activate transcription. This suggests that the Runt domain protein-Groucho interaction may be regulated.

Proviral insertions induce the expression of bone-specific isoforms of PEBP2alphaA (CBFA1): evidence for a new myc collaborating oncogene

The til-1 locus was identified as a common retroviral integration site in virus-accelerated lymphomas of CD2-myc transgenic mice. We now show that viral insertions at til-1 lead to transcriptional activation of PEBP2alphaA (CBFA1), a transcription factor related to the Drosophila segmentation gene product, Runt. Insertions are upstream and in the opposite orientation to the gene and appear to activate a variant promoter that is normally silent in T cells. Activity of this promoter was detected in rodent osteogenic sarcoma cells and primary osteoblasts, implicating bone as the normal site of promoter activity. The isoforms encoded by the activated gene all encompass the conserved runt DNA-binding domain and share a novel N terminus different from the previously reported PEBP2alphaA products. Minor products include isoforms with internal deletions due to exon skipping and a novel C-terminal domain unrelated to known runt domain factors. The major isoform expressed from the activated til-1 locus (G1) was found to account for virtually all of the core binding factor activity in nuclear extracts from its corresponding lymphoma cell line. Another member of this gene family, AML1(CBFA2), is well known for its involvement in human hemopoietic tumors. These results provide evidence of a direct oncogenic role for PEBP2alphaA and indicate that the Myc and Runt family genes can cooperate in oncogenesis.

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.

Hematopoiesis in the fetal liver is impaired by targeted mutagenesis of a gene encoding a non-DNA binding subunit of the transcription factor, polyomavirus enhancer binding protein 2/core binding factor

The Pebpb2 gene encodes a non-DNA binding subunit of the heterodimeric transcription factor, polyomavirus enhancer binding protein 2/core binding factor (PEBP2/CBF), and is rearranged in inversion of chromosome 16 associated with human acute myeloid leukemia. To investigate its physiological function, Pebpb2 was mutated by a targeting strategy to generate a null mutant. The homozygous mutation in mice proved lethal in embryos around embryonic day 12.5, apparently due to massive hemorrhaging in the central nervous system. In addition, definitive hematopoiesis in the liver was severely impaired. The observed phenotype was indistinguishable from that reported for homozygous disruption of AML1, which encodes a DNA binding subunit of PEBP2/CBF. Thus, the results indicate that the two subunits function together as a heterodimeric PEBP2/CBF in vivo and that PEBP2/CBF plays an essential role in the development of definitive hematopoiesis.

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.

Absence of fetal liver hematopoiesis in mice deficient in transcriptional coactivator core binding factor beta

Core binding factor beta (CBF beta) is considered to be a transcriptional coactivator that dimerizes with transcription factors core binding factor alpha 1 (CBFA1), -2, and -3, and enhances DNA binding capacity of these transcription factors. CBF beta and CBFA2, which is also called acute myeloid leukemia 1 gene, are frequently involved in chromosomal translocations in human leukemia. To elucidate the function of CBF beta, mice carrying a mutation in the Cbfb locus were generated. Homozygous mutant embryos died between embryonic days 11.5-13.5 due to hemorrhage in the central nervous system. Mutant embryos had primitive erythropoiesis in yolk sac but lacked definitive hematopoiesis in fetal liver. In the yolk sac of mutant embryos, no erythroid or myeloid progenitors of definitive hematopoietic origin were detected, and the expression of flk-2/flt-3, the marker gene for early precursor cells of definitive hematopoiesis, was absent. These data suggest that Cbfb is essential for definitive hematopoiesis in liver, especially for the commitment to early hematopoietic precursor cells.

The extracellular signal-regulated kinase pathway phosphorylates AML1, an acute myeloid leukemia gene product, and potentially regulates its transactivation ability

AML1 (also called PEBP2alphaB, CBFA2, or CBFalpha2) is one of the most frequently disrupted genes in chromosome abnormalities seen in human leukemias. It has been reported that AML1 plays several pivotal roles in myeloid hematopoietic differentiation and other biological phenomena, probably through the transcriptional regulation of various relevant genes. Here, we investigated the mechanism of regulation of AML1 functions through signal transduction pathways. The results showed that AML1 is phosphorylated in vivo on two serine residues within the proline-, serine-, and threonine-rich region, with dependence on the activation of extracellular signal-regulated kinase (ERK) and with interleukin-3 stimulation in a hematopoietic cell line. These in vivo phosphorylation sites of AML1 were phosphorylated directly in vitro by ERK. Although differences between wild-type AML1 and phosphorylation site mutants in DNA-binding affinity were not observed, we have shown that ERK-dependent phosphorylation potentiates the transactivation ability of AML1. Furthermore the phosphorylation site mutations reduced the transforming capacity of AML1 in fibroblast cells. These data indicate that AML1 functions are potentially regulated by ERK, which is activated by cytokine and growth factor stimuli. This study provides some important clues for clarifying unidentified facets of the regulatory mechanism of AML1 function.

An AML-1 consensus sequence binds an osteoblast-specific complex and transcriptionally activates the osteocalcin gene

Tissue and cell-type specific expression of the rat osteocalcin (rOC) gene involves the interplay of multiple transcriptional regulatory factors. In this report we demonstrate that AML-1 (acute myeloid leukemia-1), a DNA-binding protein whose genes are disrupted by chromosomal translocations in several human leukemias, interacts with a sequence essential for enhancing tissue-restricted expression of the rOC gene. Deletion analysis of rOC promoter-chloramphenicol acetyltransferase constructs demonstrates that an AML-1-binding sequence within the proximal promoter (-138 to -130 nt) contributes to 75% of the level of osteocalcin gene expression. The activation potential of the AML-1-binding sequence has been established by overexpressing AML-1 in osteoblastic as well as in nonosseous cell lines. Overexpression not only enhances rOC promoter activity in osteoblasts but also mediates OC promoter activity in a nonosseous human fibroblastic cell line. A probe containing this site forms a sequence specific protein-DNA complex with nuclear extracts from osteoblastic cells but not from nonosseous cells. Antisera supershift experiments indicate the presence of AML-1 and its partner protein core-binding factor beta in this osteoblast-restricted complex. Mutations of the critical AML-1-binding nucleotides abrogate formation of the complex and strongly diminish promoter activity. These results indicate that an AML-1 related protein is functional in cells of the osteoblastic lineage and that the AML-1-binding site is a regulatory element important for osteoblast-specific transcriptional activation of the rOC gene.

Core binding factor beta-smooth muscle myosin heavy chain chimeric protein involved in acute myeloid leukemia forms unusual nuclear rod-like structures in transformed NIH 3T3 cells

Patients with the M4Eo subtype of acute myeloid leukemia almost invariably are found to have an inversion of chromosome 16 in their leukemic cells, which results in a gene fusion between the transcription factor called core binding factor beta (CBFbeta) on 16q and a smooth muscle myosin heavy chain (SMMHC) gene on 16p. Subcellular localizations of the wild-type CBFbeta and the CBFbeta-SMMHC fusion protein were determined by immunofluorescence of NIH 3T3 cells that overexpress wild-type or fusion protein. Normal CBFbeta showed an unexpected perinuclear pattern consistent with primary localization in the Golgi complex. The CBFbeta-SMMHC fusion protein had a very different pattern. Nuclear staining included rod-like crystalline structures as long as 11 microm. The heterodimeric partner of CBFbeta, CBFalpha, formed part of this complex. Cytoplasmic staining included stress fibers that colocalized with actin, probably as a consequence of the myosin heavy chain component of the fusion protein. Deletion of different regions of the CBFbeta portion of the fusion protein showed that binding to CBFalpha was not required for nuclear translocation. However, deletion of parts of the SMMHC domain of the fusion protein involved in myosin-mediated filament formation resulted in proteins that did not form rod-like structures. These observations confirm previous indirect evidence that the CBFbeta-SMMHC fusion protein is capable of forming macromolecular nuclear aggregates and suggests possible models for the mechanism of leukemic transformation.

Conservation and function of the transcriptional regulatory protein Runt

A phylogenetic approach was used to identify conserved regions of the transcriptional regulator Runt. Alignment of the deduced protein sequences from Drosophila melanogaster, Drosophila pseudoobscura, and Drosophila virilis revealed eight blocks of high sequence homology separated by regions with little or no homology. The largest conserved block contains the Runt domain, a DNA and protein binding domain conserved in a small family of mammalian transcription factors. The functional properties of the Runt domain from the D. melanogaster gene and the human AML1 (acute myeloid leukemia 1) gene were compared in vitro and in vivo. Electrophoretic mobility-shift assays with Runt/AML1 chimeras demonstrated that the different DNA binding properties of Runt and AML1 are due to differences within their respective Runt domains. Ectopic expression experiments indicated that proteins containing the AML1 Runt domain function in Drosophila embryos and that sequences outside of this domain are important in vivo.

Transactivation of the Moloney murine leukemia virus and T-cell receptor beta-chain enhancers by cbf and ets requires intact binding sites for both proteins

The Moloney murine leukemia virus (Mo-MLV) enhancer contains binding sites (LVb and LVc) for the ets gene family of proteins and a core site that binds the polyomavirus enhancer-binding protein 2/core-binding factor (cbf) family of proteins. The LVb and core sites in the Mo-MLV enhancer contribute to its constitutive activity in T cells. All three binding sites (LVb, LVc, and core) are required for phorbol ester inducibility of the Mo-MLV enhancer. Adjacent binding sites for the ets and cbf proteins likewise constitute a phorbol ester response element within the human T-cell receptor beta-chain (TCR beta) enhancer and contribute to constitutive transcriptional activity of the TCR beta enhancer in T cells. Here we show that the CBF alpha subunit encoded by the mouse Cbfa2 gene (the murine homolog of human AML1) and three ets proteins, Ets-1, Ets-2, and GA-binding protein (GABP), transactivate both the Mo-MLV and mouse TCR beta enhancer in transient-expression assays. Moreover, we show that transactivation by Cbf alpha 2 requires both intact ets and cbf binding sites. Transactivation by Ets-1, Ets-2, and GABP likewise requires intact binding sites for ets proteins and CBF. Supportive biochemical analyses demonstrate that both proteins can bind simultaneously to a composite enhancer element. These findings suggest that ets and cbf proteins cooperate in vivo to regulate transcription from the Mo-MLV and TCR beta enhancers.

The E47 transcription factor binds to the enhancer sequences of recombinant murine leukemia viruses and influences enhancer function

The genomes of most recombinant murine leukemia viruses (MuLVs) inherit pathogenic U3 region sequences from the endogenous xenotropic provirus Bxv-1. However, the U3 regions of about one-third of recombinant MuLVs from CWD mice, such as CWM-T15, have nonecotropic substitutions that are probably derived from an endogenous polytropic provirus. The CWM-T15 U3 region sequences contain five nucleotide substitutions compared with the less pathogenic sequences of the endogenous ecotropic virus parent, Emv-1. Three of these substitutions are located immediately 3' of the enhancer core, and two form part of an E-box motif that is also found in the Bxv-1 sequence. A series of electromobility shift assays revealed that nuclear extracts from S194 cells and the basic helix-loop-helix transcription factor E47 could distinguish between oligonucleotides that contained the core region sequences of CWM-T15 or Emv-1. The E47 homodimers appeared to bind to the CWM-T15 E-box motif and when expressed at high levels in cells transactivated the CWM-T15 but not the Emv-1 enhancer. Taken together, these results suggest that E47 or related basic helix-loop-helix proteins that are expressed in lymphoid cells bind to and transactivate the CWM-T15 enhancer in vivo. This transactivation may explain why the CWM-T15 and Bxv-1 U3 regions accelerate the onset of lymphoid neoplasms and why related enhancer core region sequences are preferentially incorporated into the genomes of recombinant MuLVs and are found in other leukemogenic mammalian retroviruses.

c-Myb and core-binding factor/PEBP2 display functional synergy but bind independently to adjacent sites in the T-cell receptor delta enhancer

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor delta gene. We have previously shown that a 30-bp element, denoted delta E3, acts as the minimal TCR delta enhancer and that within delta E3, adjacent and precisely spaced binding sites for core-binding factor (CBF/PEBP2) and c-Myb are essential for transcriptional activity. These data suggested that CBF/PEBP2 and c-Myb synergize to mediate transcriptional activity but did not establish the molecular basis for synergy. In this study, we have examined in detail the binding of CBF/PEBP2 and c-Myb to delta E3. We found that CBF/PEBP2 and c-Myb could simultaneously occupy the core site and one of two overlapping Myb sites within delta E3. However, equilibrium binding and kinetic dissociation experiments suggest that the two factors bind to delta E3 independently, rather than cooperatively. This was found to be true by using isoforms of these factors present in extracts of transfected COS-7 cells, as well as the natural factors present in nuclear extracts of the Jurkat T-cell line. We further showed that CBF/PEBP2 and c-Myb provide unique transactivation functions, since the core-Myb combination cannot be substituted by dimerized core or Myb sites. We propose that spatially precise synergy between CBF/PEBP2 and c-Myb may result from the ability of the two factors to form a composite surface that makes unique and stereospecific contacts with one or more additional components of the transcriptional machinery.

The leukemic core binding factor beta-smooth muscle myosin heavy chain (CBF beta-SMMHC) chimeric protein requires both CBF beta and myosin heavy chain domains for transformation of NIH 3T3 cells

An inversion of chromosome 16 associated with the M4Eo subtype of acute myeloid leukemia produces a chimeric protein fusing the beta subunit of the transcription factor core binding factor (CBF beta) to the tail region of smooth muscle myosin heavy chain (SMMHC). We investigated the oncogenic properties of this CBF beta-SMMHC chimeric protein using a 3T3 transformation assay. NIH 3T3 cells expressing CBF beta-SMMHC acquired a transformed phenotype, as indicated by their ability to form foci, grow in soft agarose, and form tumors in nude mice. Cells expressing normal CBF beta or the SMMHC tail domain did not become transformed. Electrophoretic mobility-shift assays showed that extracts from cells transformed by CBF beta-SMMHC no longer formed the normal CBF/DNA complex but instead formed a much larger complex that did not migrate into the gel. Analysis of CBF beta-SMMHC deletion mutants demonstrated that the chimeric protein was transforming only if two domains were both present: (i) CBF beta sequences necessary for association with the CBF alpha subunit, and (ii) SMMHC sequences important for the formation of multimeric filaments. These results are direct evidence that CBF beta-SMMHC can function as an oncoprotein.

Expression of the Runt domain-encoding PEBP2 alpha genes in T cells during thymic development

The PEBP2 alpha A and PEBP2 alpha B genes encode the DNA-binding subunit of a murine transcription factor, PEBP2, which is implicated as a T-cell-specific transcriptional regulator. These two related genes share the evolutionarily conserved region encoding the Runt domain. PEBP2 alpha B is the murine counterpart of human AML1, which is located at the breakpoints of the 8;21 and 3;21 chromosome translocations associated with acute myeloid leukemia. Northern (RNA) blots of various adult mouse tissues revealed that the levels of expression of both genes were most prominent in the thymus. Furthermore, transcripts of PEBP2 alpha A and mouse AML1/PEBP2 alpha B were detected in T lymphocytes in the thymuses from day 16 embryos and newborns, as well as 4-week-old adult mice, by in situ hybridization. The expression of the genes persisted in peripheral lymph nodes of adult mice. The transcripts were detected in all the CD4- CD8-, CD4+ CD8+, CD4+ CD8-, and CD4- CD8+ cell populations. The results indicated that both genes are expressed in T cells throughout their development, supporting the notion that PEBP2 is a T-cell-specific transcription factor. Transcripts of mouse AML1/PEBP2 alpha B were also detected in day 12 fetal hematopoietic liver and in the bone marrow cells of newborn mice. The implication of mouse AML1/PEBP2 alpha B expression in hematopoietic cells other than those of T-cell lineage is discussed in relation to myeloid leukemogenesis.

Long terminal repeat enhancer core sequences in proviruses adjacent to c-myc in T-cell lymphomas induced by a murine retrovirus

The transcriptional enhancer in the long terminal repeat (LTR) of the T-lymphomagenic retrovirus SL3-3 differs from that of the nonleukemogenic virus Akv at several sites, including a single base pair difference in an element termed the enhancer core. Mutation of this T-A base pair to the C-G C-G sequence found in Akv significantly attenuated the leukemogenicity of SL3-3. Thus, this difference is important for viral leukemogenicity. Since Akv is an endogenous virus, this suggests that the C-G in its core is an adaptation to being minimally pathogenic. Most tumors that occurred in mice inoculated with the mutant virus, called SAA, contained proviruses with reversion or potential suppressor mutations in the enhancer core. We also found that the 72-bp tandem repeats constituting the viral enhancer could vary in number. Most tumors contained mixtures of proviruses with various numbers of 72-bp units, usually between one and four. Variation in repeat number was most likely due to recombination events involving template misalignment during viral replication. Thus, two processes during viral replication, misincorporation and recombination, combined to alter LTR enhancer structure and generate more pathogenic variants from the mutant virus. In SAA-induced tumors, enhancers of proviruses adjacent to c-myc had the largest number of core reversion or suppressor mutations of all of the viral enhancers in those tumors. This observation was consistent with the hypothesis that one function of the LTR enhancers in leukemogenesis is to activate proto-oncogenes such as c-myc.

AML1 is expressed in skeletal muscle and is regulated by innervation

Although most skeletal muscle genes are expressed at similar levels in electrically active, innervated muscle and in electrically inactive, denervated muscle, a small number of genes, including those encoding the acetylcholine receptor, N-CAM, and myogenin, are expressed at significantly higher levels in denervated than in innervated muscle. The mechanisms that mediate electrical activity-dependent gene regulation are not understood, but these mechanisms are likely to be responsible, at least in part, for the changes in muscle structure and function that accompany a decrease in myofiber electrical activity. To understand how muscle activity regulates muscle structure and function, we used a subtractive-hybridization and cloning strategy to identify and isolate genes that are expressed preferentially in innervated or denervated muscle. One of the genes which we found to be regulated by electrical activity is the recently discovered acute myeloid leukemia 1 (AML1) gene. Disruption and translocation of the human AML1 gene are responsible for a form of acute myeloid leukemia. AML1 is a DNA-binding protein, but its normal function is not known and its expression and regulation in skeletal muscle were not previously appreciated. Because of its potential role as a transcriptional mediator of electrical activity, we characterized expression of the AML1 gene in innervated, denervated, and developing skeletal muscle. We show that AML1 is expressed at low levels in innervated skeletal muscle and at 50- to 100-fold-higher levels in denervated muscle. Four AML1 transcripts are expressed in denervated muscle, and the abundance of each transcript increases after denervation. We transfected C2 muscle cells with an expression vector encoding AML1, tagged with an epitope from hemagglutinin, and we show that AML1 is a nuclear protein in muscle. AML1 dimerizes with core-binding factor beta (CBF beta), and we show that CGF beta is expressed at high levels in both innervated and denervated skeletal muscle. PEBP2 alpha, which is structurally related to AML1 and which also dimerizes with CBF beta, is expressed at low levels in skeletal muscle and is up-regulated only weakly by denervation. These results are consistent with the idea that AML1 may have a role in regulating gene expression in skeletal muscle.

Identification of a region which directs the monocytic activity of the colony-stimulating factor 1 (macrophage colony-stimulating factor) receptor promoter and binds PEBP2/CBF (AML1)

The receptor for the macrophage colony-stimulating factor (or colony-stimulating factor 1 [CSF-1]) is expressed from different promoters in monocytic cells and placental trophoblasts. We have demonstrated that the monocyte-specific expression of the CSF-1 receptor is regulated at the level of transcription by a tissue-specific promoter whose activity is stimulated by the monocyte/B-cell-specific transcription factor PU.1 (D.-E. Zhang, C.J. Hetherington, H.-M. Chen, and D.G. Tenen, Mol. Cell. Biol. 14:373-381, 1994). Here we report that the tissue specificity of this promoter is also mediated by sequences in a region II (bp -88 to -59), which lies 10 bp upstream from the PU.1-binding site. When analyzed by DNase footprinting, region II was protected preferentially in monocytic cells. Electrophoretic mobility shift assays confirmed that region II interacts specifically with nuclear proteins from monocytic cells. Two gel shift complexes (Mono A and Mono B) were formed with separate sequence elements within this region. Competition and supershift experiments indicate that Mono B contains a member of the polyomavirus enhancer-binding protein 2/core-binding factor (PEBP2/CBF) family, which includes the AML1 gene product, while Mono A is a distinct complex preferentially expressed in monocytic cells. Promoter constructs with mutations in these sequence elements were no longer expressed specifically in monocytes. Furthermore, multimerized region II sequence elements enhanced the activity of a heterologous thymidine kinase promoter in monocytic cells but not other cell types tested. These results indicate that the monocyte/B-cell-specific transcription factor PU.1 and the Mono A and Mono B protein complexes act in concert to regulate monocyte-specific transcription of the CSF-1 receptor.

AML1 is expressed in skeletal muscle and is regulated by innervation

Although most skeletal muscle genes are expressed at similar levels in electrically active, innervated muscle and in electrically inactive, denervated muscle, a small number of genes, including those encoding the acetylcholine receptor, N-CAM, and myogenin, are expressed at significantly higher levels in denervated than in innervated muscle. The mechanisms that mediate electrical activity-dependent gene regulation are not understood, but these mechanisms are likely to be responsible, at least in part, for the changes in muscle structure and function that accompany a decrease in myofiber electrical activity. To understand how muscle activity regulates muscle structure and function, we used a subtractive-hybridization and cloning strategy to identify and isolate genes that are expressed preferentially in innervated or denervated muscle. One of the genes which we found to be regulated by electrical activity is the recently discovered acute myeloid leukemia 1 (AML1) gene. Disruption and translocation of the human AML1 gene are responsible for a form of acute myeloid leukemia. AML1 is a DNA-binding protein, but its normal function is not known and its expression and regulation in skeletal muscle were not previously appreciated. Because of its potential role as a transcriptional mediator of electrical activity, we characterized expression of the AML1 gene in innervated, denervated, and developing skeletal muscle. We show that AML1 is expressed at low levels in innervated skeletal muscle and at 50- to 100-fold-higher levels in denervated muscle. Four AML1 transcripts are expressed in denervated muscle, and the abundance of each transcript increases after denervation. We transfected C2 muscle cells with an expression vector encoding AML1, tagged with an epitope from hemagglutinin, and we show that AML1 is a nuclear protein in muscle. AML1 dimerizes with core-binding factor beta (CBF beta), and we show that CGF beta is expressed at high levels in both innervated and denervated skeletal muscle. PEBP2 alpha, which is structurally related to AML1 and which also dimerizes with CBF beta, is expressed at low levels in skeletal muscle and is up-regulated only weakly by denervation. These results are consistent with the idea that AML1 may have a role in regulating gene expression in skeletal muscle.

Identification of a region which directs the monocytic activity of the colony-stimulating factor 1 (macrophage colony-stimulating factor) receptor promoter and binds PEBP2/CBF (AML1)

The receptor for the macrophage colony-stimulating factor (or colony-stimulating factor 1 [CSF-1]) is expressed from different promoters in monocytic cells and placental trophoblasts. We have demonstrated that the monocyte-specific expression of the CSF-1 receptor is regulated at the level of transcription by a tissue-specific promoter whose activity is stimulated by the monocyte/B-cell-specific transcription factor PU.1 (D.-E. Zhang, C.J. Hetherington, H.-M. Chen, and D.G. Tenen, Mol. Cell. Biol. 14:373-381, 1994). Here we report that the tissue specificity of this promoter is also mediated by sequences in a region II (bp -88 to -59), which lies 10 bp upstream from the PU.1-binding site. When analyzed by DNase footprinting, region II was protected preferentially in monocytic cells. Electrophoretic mobility shift assays confirmed that region II interacts specifically with nuclear proteins from monocytic cells. Two gel shift complexes (Mono A and Mono B) were formed with separate sequence elements within this region. Competition and supershift experiments indicate that Mono B contains a member of the polyomavirus enhancer-binding protein 2/core-binding factor (PEBP2/CBF) family, which includes the AML1 gene product, while Mono A is a distinct complex preferentially expressed in monocytic cells. Promoter constructs with mutations in these sequence elements were no longer expressed specifically in monocytes. Furthermore, multimerized region II sequence elements enhanced the activity of a heterologous thymidine kinase promoter in monocytic cells but not other cell types tested. These results indicate that the monocyte/B-cell-specific transcription factor PU.1 and the Mono A and Mono B protein complexes act in concert to regulate monocyte-specific transcription of the CSF-1 receptor.

PEBP2/CBF, the murine homolog of the human myeloid AML1 and PEBP2 beta/CBF beta proto-oncoproteins, regulates the murine myeloperoxidase and neutrophil elastase genes in immature myeloid cells

The myeloperoxidase (MPO) and neutrophil elastase genes are expressed specifically in immature myeloid cells. The integrity of a polyomavirus enhancer core sequence, 5'-AACCACA-3', is critical to the activity of the murine MPO proximal enhancer. This element binds two species, myeloid nuclear factors 1 alpha and 1 beta (MyNF1 alpha and -beta), present in 32D cl3 myeloid cell nuclear extracts. The levels of the MyNF1s increase during early 32D cl3 cell granulocytic differentiation. Both MyNF1 alpha and -beta supershift with an antiserum raised by using a peptide derived from the N terminus of polyomavirus enhancer-binding protein 2/core-binding factor (PEBP2/CBF) alpha subunit. The specific peptide inhibits these supershifts. In vitro-translated PEBP2/CBF DNA-binding domain binds the murine MPO PEBP2/CBF site. An alternate PEBP2/CBF consensus site, 5'-GACCGCA-3', but not a simian virus 40 enhancer core sequence, 5'-TTCCACA-3', binds the MyNF1s in vitro and activates a minimal murine MPO-thymidine kinase promoter in vivo. The murine neutrophil elastase gene 100-bp 5'-flanking sequences contain several functional elements, including potential binding sites for PU.1, C/EBP, c-Myb, and PEBP2/CBF. The functional element 5'-GGCCACA-3' located at positions -66 to 72 differs from the PEBP2/CBF consensus (5'-PuACCPuCA-3') only by an A-to-G transition at position 2. This DNA element binds MyNF1 alpha and -beta weakly. The N terminis of two PEBP2/CBF alpha subunit family members, PEBP2 alpha A and PEBP2 alpha B (murine AML1), are nearly identical, and 32D c13 cl3 cells contain both corresponding mRNAs. Since t(8;21), t(3;21), and inv(16), associated with myeloid leukemias, disrupt subunits of PEBP2/CBF, we speculate that the resulting oncoproteins, AML1-ETO, AML1-EAP, AML1-Evi1, and CBF beta-MYH11, inhibit early myeloid differentiation.

PEBP2/CBF, the murine homolog of the human myeloid AML1 and PEBP2 beta/CBF beta proto-oncoproteins, regulates the murine myeloperoxidase and neutrophil elastase genes in immature myeloid cells

The myeloperoxidase (MPO) and neutrophil elastase genes are expressed specifically in immature myeloid cells. The integrity of a polyomavirus enhancer core sequence, 5'-AACCACA-3', is critical to the activity of the murine MPO proximal enhancer. This element binds two species, myeloid nuclear factors 1 alpha and 1 beta (MyNF1 alpha and -beta), present in 32D cl3 myeloid cell nuclear extracts. The levels of the MyNF1s increase during early 32D cl3 cell granulocytic differentiation. Both MyNF1 alpha and -beta supershift with an antiserum raised by using a peptide derived from the N terminus of polyomavirus enhancer-binding protein 2/core-binding factor (PEBP2/CBF) alpha subunit. The specific peptide inhibits these supershifts. In vitro-translated PEBP2/CBF DNA-binding domain binds the murine MPO PEBP2/CBF site. An alternate PEBP2/CBF consensus site, 5'-GACCGCA-3', but not a simian virus 40 enhancer core sequence, 5'-TTCCACA-3', binds the MyNF1s in vitro and activates a minimal murine MPO-thymidine kinase promoter in vivo. The murine neutrophil elastase gene 100-bp 5'-flanking sequences contain several functional elements, including potential binding sites for PU.1, C/EBP, c-Myb, and PEBP2/CBF. The functional element 5'-GGCCACA-3' located at positions -66 to 72 differs from the PEBP2/CBF consensus (5'-PuACCPuCA-3') only by an A-to-G transition at position 2. This DNA element binds MyNF1 alpha and -beta weakly. The N terminis of two PEBP2/CBF alpha subunit family members, PEBP2 alpha A and PEBP2 alpha B (murine AML1), are nearly identical, and 32D c13 cl3 cells contain both corresponding mRNAs. Since t(8;21), t(3;21), and inv(16), associated with myeloid leukemias, disrupt subunits of PEBP2/CBF, we speculate that the resulting oncoproteins, AML1-ETO, AML1-EAP, AML1-Evi1, and CBF beta-MYH11, inhibit early myeloid differentiation.

PEBP2 alpha B/mouse AML1 consists of multiple isoforms that possess differential transactivation potentials

A murine transcription factor, PEBP2, is composed of two subunits, alpha and beta. There are two genes in the mouse genome, PEBP2 alpha A and PEBP2 alpha B, which encode the alpha subunit. Two types of the alpha B cDNA clones, alpha B1 and alpha B2, were isolated from mouse fibroblasts and characterized. They were found to represent 3.8- and 7.9-kb transcripts, respectively. The 3.8-kb RNA encodes the previously described alpha B protein referred to as alpha B1, while the 7.9-kb RNA encodes a 387-amino-acid protein, termed alpha B2, which is identical to alpha B1 except that it has an internal deletion of 64 amino acid residues. Both alpha B1 and alpha B2 associate with PEBP2 beta and form a heterodimer. The alpha B2/beta complex binds to the PEBP2 binding site two- to threefold more strongly than the alpha B1/beta complex does. alpha B1 stimulates transcription through the PEBP2 site about 40-fold, while alpha B2 is only about 25 to 45% as active as alpha B1. Transactivation domain is located downstream of the 128-amino-acid runt homology region, referred to as the Runt domain. Mouse chromosome mapping studies revealed that alpha A, alpha B, and beta genes are mapped to chromosomes 17, 16, and 8, respectively. The last two genes are syntenic with the human AML1 on chromosome 21q22 and PEBP2 beta/CBF beta on 16q22 detected at the breakpoints of characteristic chromosome translocations of the two different subtypes of acute myeloid leukemia. These results suggest that previously described chimeric gene products, AML1/MTG8(ETO) and AML1-EAP generated by t(8;21) and t(3;21), respectively, lack the transactivation domain of AML1.