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

Functional contribution of EEN to leukemogenic transformation by MLL-EEN fusion protein

The EEN (extra eleven nineteen) gene was originally cloned from a case of acute myeloid leukemia M5 subtype with translocation t (11; 19)(q23; p13), in which EEN was fused with MLL. To explore the involvement of EEN in leukemogenesis caused by MLL-EEN, we studied the transformation potential of the MLL-EEN fusion protein. MLL-EEN had oncogenic features, while, as a control, MLLDelta, the truncated form of MLL lacking the EEN moiety, did not show any oncogenic potential. MLL-EEN exerted a dominant-negative effect over wild-type EEN in terms of subcellular localization. Normally, EEN was found in the cytoplasm, but the MLL-EEN fusion protein was located in the nucleus, and EEN could be delocalized by MLL-EEN. This interaction is via a coiled-coil dimerization domain of EEN, which is reserved in the fusion protein. In addition, MLL-EEN might act as a potential transcriptional factor with the MLL part providing the DNA-binding domain and the EEN part providing the transcription activation domain, though EEN seems to have no direct role in transcriptional regulation. As an aberrant transcriptional factor, MLL-EEN could transactivate the promoter of HoxA7, a potential target gene of MLL.

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.

Distinct C/EBP functions are required for eosinophil lineage commitment and maturation

Hematopoietic differentiation involves the commitment of multipotent progenitors to a given lineage, followed by the maturation of the committed cells. To study the transcriptional events controlling these processes, we have investigated the role of C/EBP proteins in lineage choice of multipotent hematopoietic progenitors (MEPs) transformed by the E26 virus. We found that forced expression of either the alpha or beta isoforms of C/EBP in MEPs induced eosinophil differentiation and that in addition, C/EBPbeta could induce myeloid differentiation. Conversely, dominant-negative versions of C/EBPbeta inhibited myeloid differentiation. C/EBP-induced eosinophil differentiation could be separated into two distinct events, lineage commitment and maturation. Thus, eosinophils induced by transactivation-deficient C/EBPbeta alleles were found to be blocked in their maturation, whereas those expressing wild-type C/EBP proteins were not. Likewise, a 1-day activation of a conditional C/EBPbeta allele in multipotent progenitors led to the formation of immature eosinophils, whereas sustained activation produced mature eosinophils. These results show that C/EBP can induce both myeloid and eosinophil lineage commitment and that transactivation independent and dependent C/EBP functions are required during eosinophil lineage commitment and maturation, respectively.

Molecular genetics of childhood leukemias

PURPOSE

This review summarizes the molecular genetics of childhood leukemias, with emphasis on pathogenesis and clinical applications.

DESIGN

We first describe the most common genetic events that occur in pediatric acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and chronic myeloid leukemia (CML). We then illustrate how these molecular alterations may be used to alter therapy.

RESULTS

In childhood ALL, the TEL-AML1 fusion and hyperdiploidy are both associated with excellent treatment outcomes and therefore identify patients who may be candidates for less intensive therapy. In contrast, MLL gene rearrangements and the BCR-ABL fusion confer a poor prognosis; these patients may be best treated by allogeneic bone marrow transplantation in first remission.

CONCLUSIONS

Although clinical features are important prognostic indicators, genetic alterations of leukemic blasts may be better predictors of outcome for acute leukemia patients. We therefore favor risk-adapted therapy based on classification schemes that incorporate both genetic and clinical features.

Superinduction of cyclooxygenase-2 activity in human osteoarthritis-affected cartilage. Influence of nitric oxide

Cartilage specimens from osteoarthritis (OA)-affected patients spontaneously released PGE2 at 48 h in ex vivo culture at levels at least 50-fold higher than in normal cartilage and 18-fold higher than in normal cartilage + cytokines + endotoxin. The superinduction of PGE2 production coincides with the upregulation of cyclooxygenase-2 (COX-2) in OA-affected cartilage. Production of both nitric oxide (NO) and PGE2 by OA cartilage explants is regulated at the level of transcription and translation. Dexamethasone inhibited only the spontaneously released PGE2 production, and not NO, in OA-affected cartilage. The NO synthase inhibitor HN(G)-monomethyl-L-arginine monoacetate inhibited OA cartilage NO production by > 90%, but augmented significantly (twofold) the spontaneous production of PGE2 in the same explants. Similarly, addition of exogenous NO donors to OA cartilage significantly inhibited PGE2 production. Cytokine + endotoxin stimulation of OA explants increased PGE2 production above the spontaneous release. Addition of L-NMMA further augmented cytokine-induced PGE2 production by at least fourfold. Inhibition of PGE2 by COX-2 inhibitors (dexamethasone or indomethacin) or addition of exogenous PGE2 did not significantly affect the spontaneous NO production. These data indicate that human OA-affected cartilage in ex vivo conditions shows (a) superinduction of PGE2 due to upregulation of COX-2, and (b) spontaneous release of NO that acts as an autacoid to attenuate the production of the COX-2 products such as PGE2. These studies, together with others, also suggest that PGE2 may be differentially regulated in normal and OA-affected chondrocytes.

Molecular genetics of acute leukaemia

The outlook for patients with acute promyelocytic leukaemia has improved vastly with the use of all-trans retinoic acid. The development of this therapeutic agent stemmed from the finding that an abnormality of the retinoic acid receptor is involved in this disease. In the search for other molecular abnormalities in the acute leukaemias that might serve as therapeutic targets, the chromosomal translocations associated with this group of disorders have been helpful in indicating where to look for potential cancer genes. Some common signal-transduction pathways through which different such genes act have been identified, and compounds that interfere with these pathways are already being screened for.

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 hybrid PAX3-FKHR fusion protein of alveolar rhabdomyosarcoma transforms fibroblasts in culture

Pediatric alveolar rhabdomyosarcoma is characterized by a chromosomal translocation that fuses parts of the PAX3 and FKHR genes. PAX3 codes for a transcriptional regulator that controls developmental programs, and FKHR codes for a forkhead-winged helix protein, also a likely transcription factor. The PAX3-FKHR fusion product retains the DNA binding domains of the PAX3 protein and the putative activator domain of the FKHR protein. The PAX3-FKHR protein has been shown to function as a transcriptional activator. Using the RCAS retroviral vector, we have introduced the PAX3-FKHR gene into chicken embryo fibroblasts. Expression of the PAX3-FKHR protein in these cells leads to transformation: the cells become enlarged, grow tightly packed and in multiple layers, and acquire the ability for anchorage-independent growth. This cellular transformation in vitro will facilitate studies on the mechanism of PAX3-FKHR-induced oncogenesis.

Runt domain partner proteins enhance DNA binding and transcriptional repression in cultured Drosophila cells

BACKGROUND

The Drosophila gene runt plays multiple roles during embryogenesis, including one as a pair-rule class segmentation gene. The runt protein (Runt) contains an evolutionarily conserved domain (the Runt domain) that is found in several mammalian proteins including the human protein AML1, which is involved in many chromosome translocations associated with leukaemia. Specific DNA binding activity of a mammalian Runt domain is enhanced by a partner protein called PEBP2beta/CBFbeta. DNA binding activity of Drosophila Runt is also stimulated by this protein, suggesting the existence of a similar Runt partner protein in Drosophila.

RESULTS

We report here the cloning of two closely linked Drosophila genes, runt domain partner (rp) beta1 and beta2, that encode homologues of mouse PEBP2beta/CBFbeta. They are highly homologous to each other and to the mammalian counterpart. Either of the rpb proteins is capable of forming a complex with Runt and stimulating its DNA binding activity, but their temporal and spatial distributions are quite dissimilar, suggesting that functional specificity of Runt may be conferred by the interacting partner. Runt represses transcription dominantly when coexpressed with either partner in cultured cells, a function consistent with a direct role for Runt in regulating expression of the even-skipped gene in Drosophila embryos.

CONCLUSIONS

Drosophila Runt can interact with either of two Runt domain partners, and the resulting complex functions as an active repressor of transcription.

Identification of the chimeric protein product of the CBFB-MYH11 fusion gene in inv(16) leukemia cells

An expressed gene formed by fusion between the CBFB transcription factor gene and the smooth muscle myosin heavy chain gene MYH11 is consistently detected by reverse transcription polymerase chain reaction (RT-PCR) in patients who have acute myeloid leukemia (AML) subtype M4Eo with an inversion of chromosome 16. We have previously shown that a CBFB-MYH11 cDNA construct can produce a chimeric protein and transform NIH 3T3 cells. However, the presence of the chimeric protein in patient cells has not been demonstrated previously. Here, we show that such chimeric proteins can be identified in vivo, primarily in the nuclei of the leukemic cells, by use of antibodies against the C-terminus of the smooth muscle myosin heavy chain and the fusion junction peptide. A very high molecular weight protein/DNA complex is generated when nuclear extracts from patient cells are used in electrophoretic mobility shift assays, as seen in NIH 3T3 cells transfected with the CBFB-MYH11 cDNA. Immunofluorescence staining shows that the proteins are organized in vivo into novel structures within cell nuclei. One isoform of the transcript of the CBFB-MYH11 fusion gene, containing the MHC204 C-terminus, was the predominant from in all five cases studied.

Structural alterations in the transcription factor PEBP2/CBF linked to four different types of leukemia

Polyomavirus enhancer binding protein 2 (PEBP2), also called core binding factor (CBF), is a heterodimer composed of the alpha and the beta subunits. Structural alterations of each of the two subunits generated by recurrent chromosome translocations/inversion are associated with acute myeloid leukemia or acute lymphoblastic leukemia. Chimeric proteins containing a part of either the alpha or beta subunits have a potential to affect the transcriptional regulation through the PEBP2/CBF site. Structure and function of PEBP2/CBF and possible mechanisms of leukemogenesis caused by the chimeric proteins are summarized.

Disruption of the Cbfa2 gene causes necrosis and hemorrhaging in the central nervous system and blocks definitive hematopoiesis

The CBFA2 (AML1) gene encodes a DNA-binding subunit of the heterodimeric core-binding factor. The CBFA2 gene is disrupted by the (8;21), (3;21), and (12;21) chromosomal translocations associated with leukemias and myelodysplasias in humans. Mice lacking a CBF alpha 2 protein capable of binding DNA die between embryonic days 11.5 and 12.5 due to hemorrhaging in the central nervous system (CNS), at the nerve/CNS interfaces of cranial and spinal nerves, and in somitic/intersomitic regions along the presumptive spinal cord. Hemorrhaging is preceded by symmetric, bilateral necrosis in these regions. Definitive erythropoiesis and myelopoiesis do not occur in Cbfa2-deficient embryos, and disruption of one copy of the Cbfa2 gene significantly reduces the number of progenitors for erythroid and myeloid cells.

The t(12;21) translocation converts AML-1B from an activator to a repressor of transcription

The t(12;21) translocation is present in up to 30% of childhood B-cell acute lymphoblastic and fuses a potential dimerization motif from the ets-related factor TEL to the N terminus of AML1. The t(12;21) translocation encodes a 93-kDa fusion protein that localizes to a high-salt- and detergent-resistant nuclear compartment. This protein binds the enhancer core motif, TGTGGT, and interacts with the AML-1-binding protein, core-binding factor beta. Although TEL/AML-1B retains the C-terminal domain of AML-1B that is required for transactivation of the T-cell receptor beta enhancer, it fails to activate transcription but rather inhibits the basal activity of this enhancer. TEL/AML-1B efficiently interferes with AML-1B dependent transactivation of the T-cell receptor beta enhancer, and coexpression of wild-type TEL does not reverse this inhibition. The N-terminal TEL helix-loop-helix domain is essential for TEL/AML-1B-mediated repression. Thus, the t(12;21) fusion protein dominantly interferes with AML-1B-dependent transcription, suggesting that the inhibition of expression of AML-1 genes is critical for B-cell leukemogenesis.

Drosophila homologs of the proto-oncogene product PEBP2/CBF beta regulate the DNA-binding properties of Runt

The Drosophila runt gene is the founding member of the Runt domain family of transcriptional regulators. Mammalian Runt domain genes encode the alpha subunit of the heterometric DNA-binding factor PEBP2/CBF. The unrelated PEBP2/CBF beta protein interacts with the Runt domain to increase its affinity for DNA. The conserved ability of the Drosophila Runt protein to respond to the stimulating effect of mammalian PEBP2/CBF beta indicated that flies were likely to have a homologous beta protein. Using the yeast two-hybrid system to isolate cDNAs for Runt-interacting proteins, we identified two Drosophila genes, referred to as Brother and Big-brother, that have substantial sequence homology with PEBP2/CBF beta. Yeast two-hybrid experiments as well as in vitro DNA-binding studies confirmed the functional homology of the Brother, Big-brother, and PEBP2/CBF beta proteins and demonstrated that the conserved regions of the Runt and Brother proteins are required for their heterodimeric interaction. The DNA-bending properties of Runt domain proteins in the presence and absence of their partners were also examined. Our results show that Runt domain proteins bend DNA and that this bending is influenced by Brother protein family members, supporting the idea that heterodimerization is associated with a conformational change in the Runt domain. Analysis of expression patterns in Drosophila embryos revealed that Brother and Big-brother are likely to interact with runt in vivo and further suggested that the activity of these proteins is not restricted to their interaction with Runt.

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.

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.