Mutant N-RAS induces erythroid lineage dysplasia in human CD34+ cells

Mutant N-ras Induces Myeloproliferative Disorders and Apoptosis in Bone Marrow Repopulated Mice

Mutations that activate the N-ras oncogene are among the most frequently detected genetic alterations in human acute myeloid leukemias (AMLs), Philadelphia chromosome-negative myeloproliferative disorders (MPDs), and myelodysplastic syndromes (MDSs). However, because N-ras has not been shown to induce these disorders in an in vivo model, the role of N-ras in the evolution of myeloid leukemia is unclear. To investigate the potential of N-ras to induce myeloid leukemia, lethally irradiated mice were reconstituted with bone marrow (BM) cells infected with a retroviral vector carrying activated N-ras. Approximately 60% of these mice developed hematopoietic disorders, including severe MPDs resembling human chronic myelogenous leukemia (CML) or AML with differentiation (French-American-British [FAB] classification M2). Other reconstituted mice succumbed to hematopoietic defects that were pathologically similar to human MDSs. The latter disorders appeared to be due to a myeloid impairment that was demonstrated by enumeration of day-12 colony-forming units-spleen (CFU-S) and by in vitro colony assays. A high level of apoptosis associated with thymic atrophy and peripheral blood (PB) lymphopenia was also evident in N-rasreconstituted mice. Our results are consistent with a model in which antiproliferative effects are a primary consequence of N-rasmutations and secondary transforming events are necessary for the development of myeloid leukemia. This is the first report of an in vivo model for N-ras induced MPD and leukemia.

The t(8;21) fusion protein, AML1/ETO, transforms NIH3T3 cells and activates AP-1

The 8;21 translocation is the most common cytogenetic abnormality in human acute myelogenous leukemia, joining the AML1 gene on chromosome 21, to the ETO gene on chromosome 8, forming the AML1/ETO fusion gene. The AMLI/ETO fusion protein has been shown to function mainly as a transcriptional repressor of AML1 target genes and to block AML1 function in vitro and in vivo. However, AML1/ETO can also activate the BCL-2 promoter and cause enhanced hematopoietic progenitor self-renewal in vitro, suggesting gain-of-functions unique to the fusion protein. We used NIH3T3 cells to determine the transforming capacity of AML1/ETO, and to further characterize its mechanism of action. Expression of AML1/ETO in NIH3T3 cells caused cell-type specific cell death, and cellular transformation, characterized by phenotypic changes, anchorage-independent growth, and tumor formation in nude mice. In contrast, neither expression of AML1A, AML1B or ETO altered the normal growth pattern of the cells. To investigate the mechanism of transformation by AML1/ETO, we analysed the levels of activated, phosphorylated c-Jun (ser63) and other constituents of the AP-1 complex, in the presence of various AML1/ETO related proteins. Expression of AML1/ETO increased the level of c-Jun-P (ser63), and activated AP-1 dependent transcription, which was inhibited by expression of a dominant-negative c-Jun protein. Mutational analysis revealed that the runt homology domain (RHD) and a C-terminal transcriptional repression domain in AML1/ETO are required for transformation, activation of c-Jun and increased AP-1 activity. These results establish the transforming potential of the t(8;21) fusion protein and link this gain-of-function property to modulation of AP-1 activity.

Role of Cytokine Signaling Molecules in Erythroid Differentiation of Mouse Fetal Liver Hematopoietic Cells: Functional Analysis of Signaling Molecules by Retrovirus-Mediated Expression

Erythropoietin (EPO) and its cell surface receptor (EPOR) play a central role in proliferation, differentiation, and survival of erythroid progenitors. Signals induced by EPO have been studied extensively by using erythroid as well as nonerythroid cell lines, and various controversial results have been reported as to the role of signaling molecules in erythroid differentiation. Here we describe a novel approach to analyze the EPO signaling by using primary mouse fetal liver hematopoietic cells to avoid possible artifacts due to established cell lines. Our strategy is based on high-titer retrovirus vectors with a bicistronic expression system consisting of an internal ribosome entry site (IRES) and green fluorescent protein (GFP). By placing the cDNA for a signaling molecule in front of IRES-GFP, virus-infected cells can be viably sorted by fluorescence-activated cell sorter, and the effect of expression of the signaling molecule can be assessed. By using this system, expression of cell-survival genes such as Bcl-2 and Bcl-XL was found to enhance erythroid colony formation from colony-forming unit–erythroid (CFU-E) in response to EPO. However, their expression was not sufficient for erythroid colony formation from CFU-E alone, indicating that EPO induces signals for erythroid differentiation. To examine the role of EPOR tyrosine residues in erythroid differentiation, we introduced a chimeric EGFR-EPOR receptor, which has the extracellular domain of the EGF receptor and the intracellular domain of the EPOR, as well as a mutant EGFR-EPOR in which all the cytoplasmic tyrosine residues are replaced with phenylalanine, and found that tyrosine residues of EPOR are essential for erythroid colony formation from CFU-E. We further analyzed the function of the downstream signaling molecules by expressing modified signaling molecules and found that both JAK2/STAT5 and Ras, two major signaling pathways activated by EPOR, are involved in full erythroid differentiation.

Role of Cytokine Signaling Molecules in Erythroid Differentiation of Mouse Fetal Liver Hematopoietic Cells: Functional Analysis of Signaling Molecules by Retrovirus-Mediated Expression

Erythropoietin (EPO) and its cell surface receptor (EPOR) play a central role in proliferation, differentiation, and survival of erythroid progenitors. Signals induced by EPO have been studied extensively by using erythroid as well as nonerythroid cell lines, and various controversial results have been reported as to the role of signaling molecules in erythroid differentiation. Here we describe a novel approach to analyze the EPO signaling by using primary mouse fetal liver hematopoietic cells to avoid possible artifacts due to established cell lines. Our strategy is based on high-titer retrovirus vectors with a bicistronic expression system consisting of an internal ribosome entry site (IRES) and green fluorescent protein (GFP). By placing the cDNA for a signaling molecule in front of IRES-GFP, virus-infected cells can be viably sorted by fluorescence-activated cell sorter, and the effect of expression of the signaling molecule can be assessed. By using this system, expression of cell-survival genes such as Bcl-2 and Bcl-XL was found to enhance erythroid colony formation from colony-forming unit–erythroid (CFU-E) in response to EPO. However, their expression was not sufficient for erythroid colony formation from CFU-E alone, indicating that EPO induces signals for erythroid differentiation. To examine the role of EPOR tyrosine residues in erythroid differentiation, we introduced a chimeric EGFR-EPOR receptor, which has the extracellular domain of the EGF receptor and the intracellular domain of the EPOR, as well as a mutant EGFR-EPOR in which all the cytoplasmic tyrosine residues are replaced with phenylalanine, and found that tyrosine residues of EPOR are essential for erythroid colony formation from CFU-E. We further analyzed the function of the downstream signaling molecules by expressing modified signaling molecules and found that both JAK2/STAT5 and Ras, two major signaling pathways activated by EPOR, are involved in full erythroid differentiation.

Retroviral transduction of TLS-ERG initiates a leukemogenic program in normal human hematopoietic cells

Many chimeric oncogenes have been identified by virtue of the association between chromosomal translocation and specific human leukemias. However, the biological mechanism by which these oncogenes disrupt the developmental program of normal human hematopoietic cells during the initiation of the leukemogenic process is poorly understood due to the absence of an appropriate experimental system to study their function. Here, we report that retroviral transduction of TLS-ERG, a myeloid leukemia-associated fusion gene, to human cord blood cells results in altered myeloid and arrested erythroid differentiation and a dramatic increase in the proliferative and self-renewal capacity of transduced myeloid progenitors. Thus, TLS-ERG expression alone induced a leukemogenic program that exhibited similarities to the human disease associated with this translocation. These results provide an experimental examination of the early stages of the human leukemogenic process induced by a single oncogene and establish a paradigm to functionally assay putative leukemogenic genes in normal human hematopoietic cells.

Involvement of H-ras in erythroid differentiation of TF1 and human umbilical cord blood CD34 cells

To investigate the role of the ras gene in erythroid differentiation, a human erythroleukemic cell line, TF1, was transduced with a selectable retroviral vector carrying a mammalian wild type H-ras gene or a cytoplasmic dominant negative RAS1 gene. Transduction of TF1 cells with the wild type H-ras gene resulted in changes of cell types and up-regulation of erythroid-specific gene expression similar to that seen in differentiating erythroid cells. The number of red blood cell containing colonies derived from TF1 cells transduced with wild type H-ras cDNA was significantly increased and the cells in the colonies were more hemoglobinized as estimated by a deeper red color compared to those colony cells from mock or dominant negative RAS1 gene transduced TF1 cells, suggesting increased erythroid differentiation of TF1 cells after transduction of wild type H-ras in vitro. The mRNA levels of beta- and gamma-, but not alpha-, globin genes were significantly higher in H-ras transduced TF1 cells than those in TF1 cells transduced with mock or dominant negative RAS1 gene. Moreover, a 4kb pre-mRNA of the Erythropoietin receptor (EpoR) was highly expressed only in H-ras transduced TF1 cells. Additionally, human umbilical cord blood (CB) CD34 cells which are highly enriched for hematopoietic stem/progenitor cells were transduced with the same retroviral vectors to evaluate in normal primary cells the activities of H-ras in erythroid differentiation. Increased numbers of erythroid cell containing colonies (BFU-E and CFU-GEMM) were observed in CD34 cells transduced with the H-ras cDNA, compared to that from mock transduced cells. These data suggest a possible role for ras in erythroid differentiation.

Pathogenetic Aspects of Myelodysplastic Syndromes

The myelodysplastic syndromes (MDS) are acquired clonal disorders of the bone marrow. They were clearly defined in morphological terms by the French-American-British (FAB) group in 1982, as five conditions each with their own diagnostic criteria, but with the shared characteristics of ineffective blood cell production in one or more cell line, morphological dysplasia and a variable propensity to evolve into acute myeloid leukaemia (AML). In clinical practice patients typically present in old age with macrocytic anaemia, cytopenias, monocytosis and accumulation of marrow blast cells leading in time to fatal bone marrow failure or AML. To date treatment is unable to alter the natural history of MDS except in those few individuals who are able to undergo allogeneic progenitor cell transplantation.

Oncogenic RAS genes impair erythroid differentiation of erythroleukaemia cells

RAS mutations occur frequently in acute myeloid leukaemia and myelodysplasia, suggesting a functional role for this oncogene in leukaemogenesis. We show here, for the first time, that both N-RAS and H-RAS can impair erythroid differentiation of erythroleukaemia cells induced with hexamethylene bisacetamide. Transformation by RAS allowed extended proliferation in the presence of inducer and also inhibited maturation as measured by impaired haemoglobinization and reduction in cell size. These data provide an interesting counterpoint to the effect of mutant RAS on monocytic cells, where it has a potentiating effect on differentiation and may indicate a causal link between the activation of RAS and erythroid lineage dysplasia in preleukaemia.