The RNA-binding protein IGF2BP3 is critical for MLL-AF4-mediated leukemogenesis

Despite recent advances in therapeutic approaches, patients with MLL-rearranged leukemia still have poor outcomes. Here, we find that the RNA-binding protein IGF2BP3, which is overexpressed in MLL-translocated leukemia, strongly amplifies MLL-Af4-mediated leukemogenesis. Deletion of Igf2bp3 significantly increases the survival of mice with MLL-Af4-driven leukemia and greatly attenuates disease, with a minimal impact on baseline hematopoiesis. At the cellular level, MLL-Af4 leukemia-initiating cells require Igf2bp3 for their function in leukemogenesis. At the molecular level, IGF2BP3 regulates a complex posttranscriptional operon governing leukemia cell survival and proliferation. IGF2BP3-targeted mRNA transcripts include important MLL-Af4-induced genes, such as those in the Hoxa locus, and the Ras signaling pathway. Targeting of transcripts by IGF2BP3 regulates both steady-state mRNA levels and, unexpectedly, pre-mRNA splicing. Together, our findings show that IGF2BP3 represents an attractive therapeutic target in this disease, providing important insights into mechanisms of posttranscriptional regulation in leukemia.

A high-throughput screen indicates gemcitabine and JAK inhibitors may be useful for treating pediatric AML

Improvement in survival has been achieved for children and adolescents with AML but is largely attributed to enhanced supportive care as opposed to the development of better treatment regimens. High risk subtypes continue to have poor outcomes with event free survival rates <40% despite the use of high intensity chemotherapy in combination with hematopoietic stem cell transplant. Here we combine high-throughput screening, intracellular accumulation assays, and in vivo efficacy studies to identify therapeutic strategies for pediatric AML. We report therapeutics not currently used to treat AML, gemcitabine and cabazitaxel, have broad anti-leukemic activity across subtypes and are more effective relative to the AML standard of care, cytarabine, both in vitro and in vivo. JAK inhibitors are selective for acute megakaryoblastic leukemia and significantly prolong survival in multiple preclinical models. Our approach provides advances in the development of treatment strategies for pediatric AML.

Attempts to transform murine hemopoietic cells by Rauscher leukemia virus

Primary Rauscher leukemia virus (RLV)-induced myeloid leukemias can produce many small clones in agar in the absence of a factor needed for the proliferation of normal myeloid cells. It seems that leukemic cells can more efficiently utilize the small amount of colony-stimulating factor (CSF) that is produced by them. At optimal stimulation by exogenous CSF, leukemic cells exhibit a poorer rate of proliferation than normal bone marrow cells. Hemopoietic cells can replicate the virus after infection in vitro. In some experiments, infection of normal bone marrow cells leads to the production of some cells with the same growth pattern as cells from primary leukemias.

Mouse fetal antigens in Rauscher leukemia

Recently, various reports on fetal antigens associated with the plasma membranes of virus-induced tumor cells have appeared. Hanna et al. also suggested the presence of a fetal antigen on the cell membrane of Rauscher virus-induced leukemia cells. We were able to demonstrate the presence of cross-reacting antibodies against embryonic cells and Rauscher leukemia cells in the sera collected from C57BL/6 mice immunized with Rauscher leukemia cells, employing the indirect immunofluorescence test. Such cross-reacting antigenicity was also found in AKR lymphoma cells by the absorption test. The antibody was completely absorbed with embryonic cells, but not with adult spleen cells or adult C57BL/6 thymocytes. The absorption test showed that this antigen was different from Gross and Friend-Moloney-Rauscher antigens. The antigen detectable by the serum first appeared in 12-day-old embryos. It was found abundantly in the cells from the digestive tract and lungs, but not in the cells from the liver or brain. A natural antibody against the fetal antigen was also found to exist in the sera from a few aged C57BL/6 mice. The presence of the natural antibody, however, could not be related to the history of pregnancy of the mouse.

In vivo interactions between murine leukemia and sarcoma viruses

Experiments have been performed with the aim of elucidating the nature and the extent of the in vivo interactions between murine leukemia viruses (MuLVs) and murine sarcoma virus (MSV). BALB/c and CBA mice, injected neonatally with Graffi or passage A Gross viruses (MuLV-Gi, MuLV-G), have been inoculated as young adults with murine sarcoma virus, Moloney strain (MSV-M). A higher percentage of nonregressing sarcomas appeared in these animals, sometimes accompanied simultaneously by leukemia. The immune reactivity of mice receiving MuLV-Gi at birth was found to be significantly depressed when evaluated by the hemolytic palque-forming cell (PFC) technique. However, in mice infected with MuLV-Gi and MSV-M the number of PFC ranged within the control values or slightly increased. The potentiation of MSV-M oncogenicity following infection with MuLV was studied in a more natural situation. Adult AKR mice, known to release endogenous MuLV continuously, were injected with MSV-M. The incidence of induced sarcomas was similar to that observed in control BALB/c mice inoculated with MSV-M. Moreover, tumors developed with a very long latent period. On the other hand, the great majority of tumors showed no regression and ultimately killed the host. Additional experiments, making use of immunologic manipulation of the host and Fl hybrids, suggest that the relative resistance to MSV-M oncogenesis in AKR mice is influenced by genetic and immunologic factors. MSV recovered from MSV-M-induced tumors in AKR and C58 mice was typed by highly specific mouse antisera. The results clearly showed that formation of a new MSV pseudotype occurred in vivo, the endogenous Gross virus acting as helper.

Differentiation of Friend virus-induced leukemia cells

A long-term cultured Friend leukemia cell is able to differentiate along the erythrocytic series following treatment with some substances and that can be demonstrated by a conjugated erythrocyte membrane-specific antibody technique. Differentiation is induced by inhibition of DNA synthesis and by dimethylsulfoxide (DMSO), erythropoietin (EP) and Vitamin B12 (B12), and other agents. The effective substances were divided into 2 groups by the mode of differentiation of Friend cells. A differentiated state continues for several days when cells are re-cultured in the medium without substances. The decreased tumorigenecity of differentiated cells is proved by back transplantation to mice.

Depletion of cytotoxic T-cells does not protect NUP98-HOXD13 mice from myelodysplastic syndrome but reveals a modest tumor immunosurveillance effect

Myelodysplastic syndrome (MDS) and aplastic anemia (AA) patients both present with symptoms of bone marrow failure. In many AA patients, these features are thought to result from an oligoclonal expansion of cytotoxic T-cells that destroy haematopoietic stem or progenitor cells. This notion is supported by the observation that AA patients respond to immunosuppressive therapy. A fraction of MDS patients also respond well to immunosuppressive therapy suggesting a similar role for cytotoxic T-cells in the etiology of MDS, however the role of cytotoxic T-cells in MDS remains unclear. Mice that express a NUP98-HOXD13 (NHD13) transgene develop a MDS that closely mimics the human condition in terms of dysplasia, ineffective hematopoiesis, and transformation to acute myeloid leukemia (AML). We followed a cohort of NHD13 mice lacking the Rag1 protein (NHD13/Rag1KO) to determine if the absence of lymphocytes might 1) delay the onset and/or diminish the severity of the MDS, or 2) effect malignant transformation and survival of the NHD13 mice. No difference was seen in the onset or severity of MDS between the NHD13 and NHD13/Rag1KO mice. However, NHD13/Rag1KO mice had decreased survival and showed a trend toward increased incidence of transformation to AML compared to the NHD13 mice, suggesting protection from AML transformation by a modest immuno-surveillance effect. In the absence of functional Tcrb signaling in the NHD13/Rag1KO T-cell tumors, Pak7 was identified as a potential Tcrb surrogate survival signal.

Cathepsin L is required for ecotropic murine leukemia virus infection in NIH3T3 cells

Recently it has been reported that a cathepsin B inhibitor, CA-074Me, attenuates ecotropic murine leukemia virus (Eco-MLV) infection in NIH3T3 cells, suggesting that cathepsin B is required for the Eco-MLV infection. However, cathepsin B activity was negative or extremely low in NIH3T3 cells. How did CA-074Me attenuate the Eco-MLV infection? The CA-074Me treatment of NIH3T3 cells inhibited cathepsin L activity, and a cathepsin L specific inhibitor, CLIK148, attenuated the Eco-MLV vector infection. These results indicate that the suppression of cathepsin L activity by CA-074Me induces the inhibition of Eco-MLV infection, suggesting that cathepsin L is required for the Eco-MLV infection in NIH3T3 cells. The CA-074Me treatment inhibited the Eco-MLV infection in human cells expressing the exogenous mouse ecotropic receptor and endogenous cathepsins B and L, but the CLIK148 treatment did not, showing that only the cathepsin L suppression by CLIK148 is not enough to prevent the Eco-MLV infection in cells expressing both of cathepsins B and L, and CA-074Me inhibits the Eco-MLV infection by suppressing both of cathepsins B and L. These results suggest that either cathepsin B or L is sufficient for the Eco-MLV infection.

Murine leukemias with retroviral insertions at Lmo2 are predictive of the leukemias induced in SCID-X1 patients following retroviral gene therapy

Five X-linked severe combined immunodeficiency patients (SCID-X1) successfully treated with autologous bone marrow stem cells infected ex vivo with an IL2RG-containing retrovirus subsequently developed T-cell leukemia and four contained insertional mutations at LMO2. Genetic evidence also suggests a role for IL2RG in tumor formation, although this remains controversial. Here, we show that the genes and signaling pathways deregulated in murine leukemias with retroviral insertions at Lmo2 are similar to those deregulated in human leukemias with high LMO2 expression and are highly predictive of the leukemias induced in SCID-X1 patients. We also provide additional evidence supporting the notion that IL2RG and LMO2 cooperate in leukemia induction but are not sufficient and require additional cooperating mutations. The highly concordant nature of the genetic events giving rise to mouse and human leukemias with mutations at Lmo2 are an encouraging sign to those wanting to use mice to model human cancer and may help in designing safer methods for retroviral gene therapy.

Twenty patients with X-linked severe combined immunodeficiency (SCID-X1) have been successfully treated by gene therapy. Unfortunately, five of these patients have developed T-cell leukemia two or more years after receiving the therapeutic gene IL2RG on a retroviral vector. The leukemias developed because the vector inserted itself near cancer-causing genes and disrupted their normal regulation. Remarkably, in four patients, the vector inserted near a known T-cell oncogene, LMO2. We have found that in mice, similar retroviruses cause T-cell leukemias by inserting near Lmo2. We have found two leukemias that have retroviral insertions near Lmo2 and Il2rg in the same cell. The probability of these insertions happening by chance is exceedingly small and these results imply that these two genes are deregulated together to induce leukemia. Our data show that Lmo2 and Il2rg cooperate but may not be sufficient for leukemia development and additional mutations contribute to leukemia development. We have also found cooperating retroviral insertions in genes that are abnormally expressed in human T-cell leukemias. The mouse models provide unique insight into the pathogenesis of T-cell leukemia, and they are highly predictive of the leukemias caused by SCID-X1 gene therapy.

Contributions of autocrine and non-autocrine mechanisms to tumorigenicity in a murine model for leukaemia

We have surveyed the possible mechanisms by which factor-dependent FDC-P1 cells can be rendered leukaemogenic by exposure of cells to the chemical mutagen, ethyl methane sulphonate. Cell lines established on the basis of an ability to proliferate in the absence of exogenous colony-stimulating factors (CSFs) fall into two classes; those that are maximally stimulated and show no evidence of production of CSFs and others that grow in a density-dependent manner and express granulocyte-macrophage CSF (GM-CSF). That the growth of this latter class can be suppressed by the inclusion of antisense GM-CSF oligonucleotides in the growth medium indicates that the basis for their in vitro proliferation, and probably their ability to initiate the formation of transplantable leukaemias, is autocrine stimulation by GM-CSF. The ability of low levels of CSF to sustain autocrine stimulation, as we have shown, raises the possibility of an autocrine basis for the proliferation of certain human leukaemic cells. The ability to detect low concentrations of CSFs and develop in vitro assays that closely mimic the conditions that exist in vivo will be important aids in the classification of human leukaemias.

Searching for leukemia stem cells--not yet the end of the road?

Defining the characteristics of leukemia stem cells is critical in order to better understand both the genesis of leukemic disease and strategies by which such cells may be eradicated. In this issue of Cancer Cell, Somervaille and Cleary describe studies in which the properties of malignant stem cells are elucidated in a mouse model of leukemia induced by expression of the MLL-AF9 translocation. Biological features of leukemia stem cells in this system challenge previous thinking in several ways and suggest an unexpected degree of heterogeneity among stem cells in various forms of leukemia.

Identification and characterization of leukemia stem cells in murine MLL-AF9 acute myeloid leukemia

Using a mouse model of human acute myeloid leukemia (AML) induced by the MLL-AF9 oncogene, we demonstrate that colony-forming cells (CFCs) in the bone marrow and spleen of leukemic mice are also leukemia stem cells (LSCs). These self-renewing cells (1) are frequent, accounting for 25%-30% of myeloid lineage cells at late-stage disease; (2) generate a phenotypic, morphologic, and functional leukemia cell hierarchy; (3) express mature myeloid lineage-specific antigens; and (4) exhibit altered microenvironmental interactions by comparison with the oncogene-immortalized CFCs that initiated the disease. Therefore, the LSCs responsible for sustaining, expanding, and regenerating MLL-AF9 AML are downstream myeloid lineage cells, which have acquired an aberrant Hox-associated self-renewal program as well as other biologic features of hematopoietic stem cells.

Chronic lymphocytic leukemia: molecular genetics and animal models

Chronic lymphocytic leukemia accounts for almost 30% of all adult leukemia cases in the United States and Western Europe. Although several common genomic abnormalities in CLL have been identified, mutational and functional analysis of corresponding genes so far have not proved their involvement in CLL. Our latest studies demonstrated functional involvement of Tcl1 oncoprotein and microRNA genes in the pathogenesis of CLL. Deregulated expression of Tcl1 in transgenic mice resulted in CLL. These CLL tumors showed abnormalities in expression of murine microRNA genes mmu-mir-15a and mmu-mir-16-1. Interestingly, human homologs of these genes, mir-15a and mir-16-1, located at the chromosome 13q14 are also deleted in human CLL samples. In this review we summarize and discuss these new developments. These recently emerged insights into the molecular mechanisms of CLL will allow for the development of new approaches to treat this disease.

A Phase I/II Clinical Trial of β-Globin Gene Therapy for β-Thalassemia

Recent success in the long-term correction of mouse models of human beta-thalassemia and sickle cell anemia by lentiviral vectors and evidence of high gene transfer and expression in transduced human hematopoietic cells have led to a first clinical trial of gene therapy for the disease. A LentiGlobin vector containing a beta-globin gene (beta(A-T87Q)) that produces a hemoglobin (Hbbeta(A-T87Q)) that can be distinguished from normal hemoglobin will be used. The LentiGlobin vector is self-inactivating and contains large elements of the beta-globin locus control region as well as chromatin insulators and other features that should prevent untoward events. The study will be done in Paris with Eliane Gluckman as the principal investigator and Philippe Leboulch as scientific director.

Role of erythropoietin receptor signaling in Friend virus-induced erythroblastosis and polycythemia

Friend virus is an acutely oncogenic retrovirus that causes erythroblastosis and polycythemia in mice. Previous studies suggested that the Friend virus oncoprotein, gp55, constitutively activates the erythropoietin receptor (EPOR), causing uncontrolled erythroid proliferation. Those studies showed that gp55 confers growth factor independence on an interleukin-3 (IL-3)-dependent cell line (Ba/F3) when the EPOR is coexpressed. Subsequently, we showed that a truncated form of the stem-cell kinase receptor (sf-STK) is required for susceptibility to Friend disease. Given the requirement for sf-STK, we sought to establish the in vivo significance of gp55-mediated activation of the EPOR. We found that the cytoplasmic tyrosine residues of the EPOR, and signal transducer and activator of transcription-5 (STAT5), which acts through these sites, are not required for Friend virus-induced erythroblastosis. The EPOR itself was required for the development of erythroblastosis but not for gp55-mediated erythroid proliferation. Interestingly, the murine EPOR, which is required for gp55-mediated Ba/F3-cell proliferation, was dispensable for erythroblastosis in vivo. Finally, gp55-mediated activation of the EPOR and STAT5 are required for Friend virus-induced polycythemia. These results suggest that Friend virus activates both sf-STK and the EPOR to cause deregulated erythroid proliferation and differentiation.

Directing oncogenic fusion genes into stem cells via an SCL enhancer

TEL-TRKC is a fusion gene generated by chromosomal translocation and encodes an activated tyrosine kinase. Uniquely, it is found in both solid tumors and leukemia. However, a single exon difference (in TEL) in TEL-TRKC fusions is associated with the two sets of cancer phenotypes. We expressed the two TEL-TRKC variants in vivo by using the 3' regulatory element of SCL that is selectively active in a subset of mesodermal cell lineages, including endothelial and hematopoietic stem cells and progenitors. The leukemia form of TEL-TRKC (-exon 5 of TEL) enhanced hematopoietic stem cell renewal and initiated leukemia. In contrast, the TEL-TRKC solid tumor variant (+ TEL exon 5) elicited an embryonic lethal phenotype with impairment of both angiogenesis and hematopoiesis indicative of an effect at the level of the hemangioblasts. The ability of TEL-TRKC to repress expression of Flk1, a critical regulator of early endothelial and hematopoietic cells, depended on TEL exon 5. These data indicate that related oncogenic fusion proteins similarly expressed in a hierarchy of early stem cells can have selective, cell type-specific developmental impacts.

Preinfection treatment of resistant mice with CpG oligodeoxynucleotides renders them susceptible to friend retrovirus-induced leukemia

We recently reported that immunostimulatory oligodeoxynucleotides (CpG oligodeoxynucleotides [CpG-ODN]) were effective in postexposure treatment of retrovirus-induced disease (A. R. M. Olbrich et al., J. Virol. 76:11397-11404, 2002). We now show that the timing of treatment is a critical factor in treatment efficacy. In stark contrast to the success of postexposure treatments, we found that CpG treatment of susceptible mice prior to Friend retrovirus infection accelerated the development of virus-induced erythroleukemia. Furthermore, 70.8% of mice that were resistant to Friend virus-induced leukemia developed disease after inoculation of CpG-ODN before infection. The CpG pretreatment of these mice enhanced viral loads in their spleens and blood compared to controls that received ODN without CpG motifs. The main target cells of Friend virus, erythroid precursor cells and B cells, proliferated after CpG-ODN inoculation and provided an enlarged target cell population for viral infection. Our present findings together with our previous report demonstrate that CpG-ODN treatment of viral infections may be a double-edged sword that can result in an effective therapy but also in an acceleration of disease progression depending on the time point of treatment.

Prenatal and postnatal myeloid cells demonstrate stepwise progression in the pathogenesis of MLL fusion gene leukemia

The steps to leukemia following an in utero fusion of MLL (HRX, ALL-1) to a partner gene in humans are not known. Introduction of the Mll-AF9 fusion gene into embryonic stem cells results in leukemia in mice with cell-type specificity similar to humans. In this study we used myeloid colony assays, immunophenotyping, and transplantation to evaluate myelopoiesis in Mll-AF9 mice. Colony assays demonstrated that both prenatal and postnatal Mll-AF9 tissues have significantly increased numbers of CD11b(+)/CD117(+)/Gr-1(+/-) myeloid cells, often in compact clusters. The self-renewal capacity of prenatal myeloid progenitors was found to decrease following serial replating of colony-forming cells. In contrast, early postnatal myeloid progenitors increased following replating; however, the enhanced self-renewal of early postnatal myeloid progenitor cells was limited and did not result in long-term cell lines or leukemia in vivo. Unlimited replating, long-term CD11b/Gr-1(+) myeloid cell lines, and the ability to produce early leukemia in vivo in transplantation experiments, were found only in mice with overt leukemia. Prenatal Mll-AF9 tissues had reduced total (mature and progenitor) CD11b/Gr-1(+) cells compared with wild-type tissues. Colony replating, immunophenotyping, and cytochemistry suggest that any perturbation of cellular differentiation from the prenatal stage onward is partial and largely reversible. We describe a novel informative in vitro and in vivo model system that permits study of the stages in the pathogenesis of Mll fusion gene leukemia, beginning in prenatal myeloid cells, progressing to a second stage in the postnatal period and, finally, resulting in overt leukemia in adult animals.

Novel role of CD8(+) T cells and major histocompatibility complex class I genes in the generation of protective CD4(+) Th1 responses during retrovirus infection in mice

CD4(+) Th1 responses to virus infections are often necessary for the development and maintenance of virus-specific CD8(+) T-cell responses. However, in the present study with Friend murine retrovirus (FV), the reverse was also found to be true. In the absence of a responder H-2(b) allele at major histocompatibility complex (MHC) class II loci, a single H-2D(b) MHC class I allele was sufficient for the development of a CD4(+) Th1 response to FV. This effect of H-2D(b) on CD4(+) T-cell responses was dependent on CD8(+) T cells, as demonstrated by depletion studies. A direct effect of CD8(+) T-cell help in the development of CD4(+) Th1 responses to FV was also shown in vaccine studies. Vaccination of nonresponder H-2(a/a) mice induced FV-specific responses of H-2D(d)-restricted CD8(+) cytotoxic T lymphocytes (CTL). Adoptive transfer of vaccine-primed CD8(+) T cells to naive H-2(a/a) mice prior to infection resulted in the generation of FV-specific CD4(+) Th1 responses. This novel helper effect of CD8(+) T cells could be an important mechanism in the development of CD4(+) Th1 responses following vaccinations that induce CD8(+) CTL responses. The ability of MHC class I genes to facilitate CD4(+) Th1 development could also be considerable evolutionary advantage by allowing a wider variety of MHC genotypes to generate protective immune responses against intracellular pathogens.

NF-kappaB1 can inhibit v-Abl-induced lymphoid transformation by functioning as a negative regulator of cyclin D1 expression

Mounting evidence implicates deregulated Rel/NF-kappaB signaling as a common feature of lymphoid malignancies. Despite the fact that they promote the survival and proliferation of normal lymphocytes, the underlying mechanisms by which various Rel/NF-kappaB proteins with different transcriptional regulatory capacities might facilitate transformation remain to be established. Here we show that the proliferation and tumorigenicity of Abelson murine leukemia virus (A-MuLV)-transformed pre-B cells are enhanced in the absence of NF-kappaB1 and that this coincides with elevated levels of cyclin D1. Support for a link between cyclin D1 expression and v-Abl transformation came from the finding that proliferation of transformed pre-B cells was reduced in the absence of cyclin D1, while enforced cyclin D1 expression increased the proliferation and tumorigenicity of wild-type transformants. A reduction in endogenous cyclin D1 levels that coincided with NF-kappaB1 transgene reversal of enhanced nfkb1(-/-) pre-B-cell transformation, coupled with NF-kappaB1 inhibition of v-Abl-induced kappaB-dependent murine cyclin D1 transcription, lends support to a model in which v-Abl-induced cyclin D1 transcription in transformed pre-B cells is controlled by Rel/NF-kappaB dimers with different activities.

Leukemic predisposition of pSca-1/Cb2 transgenic mice

OBJECTIVE

The gene encoding the peripheral cannabinoid receptor Cb2 is located in the common virus integration site Evi11 and is associated with hematopoietic malignancies in mice. To determine the effect of Cb2 overexpression on hematopoietic development in vivo, Cb2 transgenic mice were generated.

MATERIALS AND METHODS

A Cb2 expression vector was constructed containing a Cb2 cDNA fragment cloned into the 14kb Sca-1 (Ly-6E.1) gene. Two transgenic lines in which Cb2 expression is controlled by the Sca-1 promoter were generated, and the effect on hematopoietic development was studied. Expression of Cb2 mRNA or protein was studied by RNase protection analysis and ligand binding assays, respectively. Leukemic predisposition was investigated by injecting newborn transgenic as well as control animals with Cas-Br-M murine leukemia virus (Cas-Br-M MuLV).

RESULTS

Although increased expression of the Cb2 gene was observed in hematopoietic tissues, follow-up of more than 1 year did not reveal any hematologic defect. Interestingly, infection of newborn pSca-1/Cb2 transgenic mice with Cas-Br-M MuLV revealed that significantly more transgenic mice developed leukemia than virus-treated control littermates. Because these studies provide evidence for the cooperative potential of Cb2 in leukemia progression, we wished to identify genes that may collaborate with Cb2 in leukemic transformation. Our study suggests that Evi1, another common target for proviral integration in mouse leukemias, may be overexpressed in virus-induced leukemias in pSca-1/Cb2 transgenic mice.

CONCLUSIONS

The data indicate that hematopoietic precursor cells that express high levels of Cb2 possess increased susceptibility for leukemia development and that Cb2 and Evi1 might collaborate in leukemogenesis.

Immunophenotypic analysis of the TCR-Vbeta repertoire in 98 persistent expansions of CD3(+)/TCR-alphabeta(+) large granular lymphocytes: utility in assessing clonality and insights into the pathogenesis of the disease

At present, a major challenge in the initial diagnosis of leukemia of large granular lymphocytes (LGLs) is to establish the clonal nature of the expanded population. In the present study we have analyzed by flow cytometry immunophenotyping the TCR-Vbeta repertoire of 98 consecutive cases of persistent expansions of CD4(+) or CD8(+bright) CD3(+)/TCR-alphabeta(+) LGLs and compared the results with those obtained in molecular studies of TCR-beta gene rearrangements. Fifty-eight cases were considered to be monoclonal in molecular studies whereas in the remaining 40 cases there was no evidence for monoclonality (11 cases were considered oligoclonal and 29 polyclonal). The TCR-Vbeta repertoire was biased to the preferential use of one or more TCR-Vbeta families in 96% of cases, a total of 124 TCR-Vbeta expansions being diagnosed: one TCR-Vbeta expansion in 71 cases and two or more TCR-Vbeta expansions in 23 cases. The highest TCR-Vbeta expansion observed in each case was higher among monoclonal (74 +/- 19%) as compared to nonmonoclonal cases (24 +/- 14%) (P = 0.001), as did the fraction of LGLs that exhibited a TCR-Vbeta-restricted pattern (86 +/- 16% and 42 +/- 23%, respectively; P = 0.0001); by contrast, the proportion of cases displaying more than one TCR-Vbeta expansion was higher in the latter group: 7% versus 48%, respectively (P = 0.001). Results obtained in oligoclonal cases were intermediate between those obtained in polyclonal and monoclonal cases and similar results were observed for CD4(+) as for CD8(+bright) T-cell expansions. TCR-Vbeta families expressed in CD8(+bright) T-cell-LGL proliferations showed a pattern of distribution that mimics the frequency at which the individual TCR-Vbeta families are represented in normal peripheral blood T cells. Assuming that a given proliferation of LGLs is monoclonal whenever there is an expansion of a given TCR-Vbeta family of at least 40% of the total CD4(+) or CD8(+bright) T-cell compartment, we were able to predict clonality with a sensitivity of 93% and a specificity of 80%. By increasing the cut-off value to 60%, sensitivity and specificity were of 81% and 100%. In summary, our results suggest that flow cytometry immunophenotypic analysis of the TCR-Vbeta repertoire is a powerful screening tool for the assessment of T-cell clonality in persistent expansions of TCR-alphabeta(+) LGLs.

Retroviral integration at the Epi1 locus cooperates with Nf1 gene loss in the progression to acute myeloid leukemia

Juvenile myelomonocytic leukemia (JMML) is a disease that occurs in young children and is associated with a high mortality rate. In most patients, JMML has a progressive course leading to death by virtue of infection, bleeding, or progression to acute myeloid leukemia (AML). As it is known that children with neurofibromatosis type 1 syndrome have a markedly increased risk of developing JMML, we have previously developed a mouse model of JMML through reconstitution of lethally irradiated mice with hematopoietic stem cells homozygous for a loss-of-function mutation in the Nf1 gene (D. L. Largaespada, C. I. Brannan, N. A. Jenkins, and N. G. Copeland, Nat. Genet. 12:137-143, 1996). In the course of these experiments, we found that all these genetically identical reconstituted mice developed a JMML-like disorder, but only a subset went on to develop more acute disease. This result strongly suggests that additional genetic lesions are responsible for disease progression to AML. Here, we describe the production of a unique tumor panel, created using the BXH-2 genetic background, for identification of these additional genetic lesions. Using this tumor panel, we have identified a locus, Epi1, which maps 30 to 40 kb downstream of the Myb gene and appears to be the most common site of somatic viral integration in BXH-2 mice. Our findings suggest that proviral integrations at Epi1 cooperate with loss of Nf1 to cause AML.

Notch signaling in leukemia

Mammalian Notch homologs were first identified from the involvement of Notch1 in a recurrent chromosomal translocation in a subset of human T-cell leukemias. The effect of the translocation was twofold: Notch expression was placed under the control of a T-cell-specific element, and Notch was truncated, resulting in a constitutively active protein. Subsequent work has shown that Notch1 is required for T cell commitment and is exclusively oncotropic for T cells. During the past year, several murine models have been used to dissect the function of Notch signaling in lymphoid development and leukemia. These models show that Notch1 drives the earliest stages of T cell commitment and that Notch signaling must be downregulated by the double positive stage for proper T cell development to occur. Constitutive Notch signaling mediated by Notch1, Notch2, or Notch3 predisposes to T-cell leukemia. Future studies are expected to elucidate the mechanisms by which Notch leads to transformation. Identification of the transcriptional targets of Notch signaling is likely to yield important insights.

Mechanisms of retrovirus-induced oncogenesis

Retroviruses are implicated in a series of human and animal tumours such as leukaemias, mammary tumours or skin cancer. The mechanism that they use to induce tumour formation varies. Insertional mutagenesis is a common mechanism in rodent, feline and avian retroviruses, where the retrovirus integrates into the host genome and affects the transcription of the neighbouring genes. Cloning of these affected genes led to identification of a series of oncogenes that play a significant role in the induction of human neoplasms. Retrovirus insertion also serves as a model to identify collaborating oncogenes. Human retroviruses use different, more complex mechanisms contributing to oncogenesis. Studies of the propagation and induction mechanisms used by retroviruses have given insight to the understanding of oncogenesis.

Role of lymphoid cells in age-related change of susceptibility to Friend leukemia virus-induced leukemia

Susceptibility for Friend leukemia virus (FLV)-induced leukemogenesis was examined in the C3H/He (C3H)-->C57BL/6 (B6) radiation bone marrow chimeras of various age groups, and the effect of aging of host mice on the susceptibility was determined. The bone marrow chimera system provided the various age of FLV-resistant host mice (B6) possessing the same age of FLV-susceptible target cells from C3H mice. Using this system, we could determine the aging effect on the host resistancy against FLV without an influence of the aging effect on target cells. First, the young C3H-->young B6 chimeras and young C3H-->old B6 chimeras were compared. The young-->old chimeras were more susceptible to FLV-induced acute disease than the young-->young chimeras. The spleen CD4+ as well as CD8+ T cells were reduced in young-->old chimeras compared with young-->young chimeras. Similarly, the old C3H-->old B6 chimeras were more susceptible than old-->young chimeras and revealed the lower CD4+ T cell ratio in the spleen. Discussion was made on the possible implication of these findings on the role of T cells in age-related change of resistance to FLV-induced leukemogenesis.

NUP98-HOXA9 expression in hemopoietic stem cells induces chronic and acute myeloid leukemias in mice

Here we describe hemopoietic chimeras serving as a mouse model for NUP98-HOXA9-induced leukemia, which reproduced several of the phenotypes observed in human disease. Mice transplanted with bone marrow cells expressing NUP98-HOXA9 through retroviral transduction acquire a myeloproliferative disease (MPD) and eventually succumb to acute myeloid leukemia (AML). The NUP98 portion of the fusion protein was shown to be responsible for transforming a clinically silent pre-leukemic phase observed for Hoxa9 into a chronic, stem cell-derived MPD. The co-expression of NUP98-HOXA9 and Meis1 accelerated the transformation of MPD to AML, identifying a genetic interaction previously observed for Hoxa9 and Meis1. Our findings demonstrate the presence of overlapping yet distinct molecular mechanisms for MPD versus AML, illustrating the complexity of leukemic transformation.

Fatal leukemia in interleukin 15 transgenic mice follows early expansions in natural killer and memory phenotype CD8+ T cells

Inflammation likely has a role in the early genesis of certain malignancies. Interleukin (IL)-15, a proinflammatory cytokine and growth factor, is required for lymphocyte homeostasis. Intriguingly, the expression of IL-15 protein is tightly controlled by multiple posttranscriptional mechanisms. Here, we engineered a transgenic mouse to overexpress IL-15 by eliminating these posttranscriptional checkpoints. IL-15 transgenic mice have early expansions in natural killer (NK) and CD8+ T lymphocytes. Later, these mice develop fatal lymphocytic leukemia with a T-NK phenotype. These data provide novel evidence that leukemia, like certain other cancers, can arise as the result of chronic stimulation by a proinflammatory cytokine.

The src homology 2 domain of Bcr/Abl is required for efficient induction of chronic myeloid leukemia-like disease in mice but not for lymphoid leukemogenesis or activation of phosphatidylinositol 3-kinase

The effect of mutations in the Src homology 2 (SH2) domain of the BCR/ABL oncogene on leukemogenesis was tested in a quantitative murine bone marrow transduction/transplantation assay that accurately models human Philadelphia-positive B-lymphoid leukemia and chronic myeloid leukemia (CML). The SH2 domain was not required for induction of B-lymphoid leukemia in mice by BCR/ABL. Under conditions where the p190 and p210 forms of BCR/ABL induce fatal CML-like myeloproliferative disease within 4 weeks, p210 SH2 mutants induced CML-like disease in some mice only after a significant delay, with other recipients succumbing to B-lymphoid leukemia instead. In contrast, p190 BCR/ABL SH2 point and deletion mutants rapidly induced CML-like disease. These results provide the first direct evidence of significant differences in cell signaling by the Bcr/Abl tyrosine kinase between these distinct leukemias. Contrary to previous observations, high levels of phosphatidylinositol 3-kinase (PI 3-kinase) activity in primary malignant lymphoblasts and myeloid cells from recipients of marrow transduced with the BCR/ABL SH2 mutants were found. Hence, the decreased induction of CML-like disease by the p210 BCR/ABL SH2 mutants is not due to impaired activation of PI 3-kinase.

Bcl-2 expression restores the leukemogenic potential of a BCR/ABL mutant defective in transformation

Growth factor-dependent hematopoietic cell lines expressing the BCR/ABL oncoprotein of the Ph chromosome show growth factor-independent proliferation and resistance to apoptosis. Apoptosis resistance of BCR/ABL-expressing cells may depend on enhanced expression of anti-apoptotic proteins as well as reduced expression and/or inactivation of pro-apoptotic proteins. Compared to myeloid precursor 32Dcl3 cells expressing wild type BCR/ABL, cells expressing a BCR/ABL mutant lacking amino acids 176-426 in the BCR domain (p185 delta BCR) are susceptible to apoptosis induced by interleukin-3 (IL-3) deprivation. These cells exhibited the hypophosphorylated apoptotic BAD and markedly reduced levels of Bcl-2. Upon ectopic expression of Bcl-2, these cells showed no changes in BAD phosphorylation, but they became apoptosis-resistant and proliferated in the absence of IL-3, albeit more slowly than cells expressing wild type BCR/ABL. Moreover, the p185 delta BCR/Bcl-2 double transfectants were leukemogenic when injected into immunodeficient mice, but Bcl-2 expression did not restore the leukemia-inducing effects of p185 delta BCR to the levels of wild type BCR/ABL. Leukemic cells recovered from the spleen of mice injected with p185 delta BCR/Bcl-2 cells did not show rearrangements in the Bcl-2 genomic locus, but they exhibited enhanced proliferation in culture and induced a rapidly fatal disease process when inoculated in secondary recipient mice. Together, these data support the importance of anti-apoptotic pathways for BCR/ABL-dependent leukemogenesis and suggest that Bcl-2 expression promotes secondary changes leading to a more aggressive tumor phenotype. (Blood. 2000;96:3915-3921)

Erythroid defects and increased retrovirally-induced tumor formation in Evi1 transgenic mice

Aberrant expression of the Evi1 (ecotropic virus integration site 1) proto-oncogene has been associated with hematopoietic malignancies in both mice and man. To determine the effect of enforced expression of Evi1 in vivo, we developed a transgenic mouse model utilizing the murine Sca-1 (Ly-6E.1) promoter. Here, we describe the generation and analysis of three independent lines of Evi1 transgenic mice. Transgenic animals of two founder lines developed normally. These mice did not show any obvious hematological abnormalities but showed a significant reduction in the number of bone marrow colony-forming unit erythroid (CFU-E)-derived colonies. This implies a defect of normal erythroid hematopoiesis affecting relatively late erythroid progenitor cells. We also show that when newborn Evi1 transgenic mice of these two lines were infected with Cas-Br-M MuLV, tumor incidence was greatly enhanced in comparison with nontransgenic littermates, indicating an increased susceptibility for leukemia development. Interestingly, analysis of a third founder line revealed that all male progeny consistently displayed severely impaired erythropoiesis with major defects in the bone marrow, spleen and peripheral blood. Taken together, our results present the first evidence of Evi1 disturbing normal erythropoiesis in vivo and provides evidence for cooperative potential of Evi1 in tumor progression.

At least four non-env factors that reside in the LTR, in the 5'-non-coding region, in gag and in part of pol affect neuropathogenicity of PVC-441 murine leukemia virus (MuLV)

PVC-441 murine leukemia virus (MuLV) is neuropathogenic in F344 rats. Recently, an infectious DNA clone was isolated and its nucleotide sequence was determined (J. Virol. 72: 3423-3426. 1998). To identify the viral determinants of neuropathogenicity of the molecularly cloned PVC-441 MuLV, chimeras were constructed between PVC-441 MuLV and F-MuLV clones at appropriate restriction enzyme sites that divide the viral genome approximately in LTR-non-coding, gag-, pol-, and env-gene regions. Results indicated that the LTR-non-coding and the gag-gene regions of PVC-441 MuLV affected independently the neuropathogenicity in combination with the env gene region as evidenced clinically and pathologically. Studies on the distribution of vacuolar degeneration suggested that the pons and cervical spinal cord areas were the primary targets and the large brain was the latest target of PVC-441 MuLV. Further studies with chimeric viruses that were formed in the LTR-non-coding and the gag gene regions revealed that at least four factors affected the neuropathogenicity of PVC-441 MuLV. Two factors were found in the U3, and R-U5-5'-non-coding regions, and at least two factors in the gag gene region that contained the N-terminal part of the pol gene. Among these factors, at least two factors seemed to be 'cis-acting' from each other

Absence of the effects of 50 Hz magnetic fields on the progression of acute myeloid leukaemia in rats

PURPOSE

As the most recent epidemiological studies provide no definite conclusions about the effects of 50/60 Hz magnetic fields (MFs) on the incidence of leukaemia in humans, animal models in a well-controlled environment are useful for evaluating the possibility of an association between MFs and leukaemia. The present study was designed to determine whether 50 Hz magnetic fields can alter the progression of leukaemia.

MATERIALS AND METHODS

A well-characterized model of transplantable acute myeloid leukaemia in rats was used for the first time. This model is closely related to human acute myeloid leukaemia, the type most frequently reported in epidemiological studies of adults. After leukaemic cell implantation, rats were exposed to a sinusoidal 50 Hz MF of 100 microT for 18 h a day, 7 days a week, throughout leukaemia progression. The parameters investigated were: survival time, body weight, haematologic parameters, infiltration of blood, bone marrow, spleen and liver by leukaemic cells.

RESULTS

The results showed no significant changes (p > 0.05) in leukaemic MF-exposed versus unexposed rats for any of the parameters involved in leukaemia progression.

CONCLUSION

These data do not support the hypothesis that 50 Hz magnetic fields influence leukaemia progression in humans.

Myeloproliferative disorder and leukaemia in mice induced by different classes of constitutive mutants of the human IL-3/IL-5/GM-CSF receptor common beta subunit

Several constitutively active mutant forms of the common beta subunit of the human IL-3, IL-5 and GM-CSF receptors (hbetac), which enable it to signal in the absence of ligand, have recently been described. Two of these, V449E and I374N, are amino acid substitutions in the transmembrane and extracellular regions of hbetac, respectively. A third, FIDelta, contains a 37 amino acid duplication in the extracellular domain. We have shown previously that when expressed in primary murine haemopoietic cells, the extracellular mutants confer factor-independence on cells of the neutrophil and monocyte lineages only, whereas V449E does so on all cell types of the myeloid and erythroid compartments. To study the in vivo effects and leukaemic potential of these mutants, we have expressed all three in mice by bone marrow reconstitution using retrovirally infected donor cells. Expression of the extracellular mutants leads to an early onset, chronic myeloproliferative disorder marked by elevations in the neutrophil, monocyte, erythrocyte and platelet lineages. In contrast, expression of V449E leads to an acute leukaemia-like syndrome of anaemia, thrombocytopaenia and blast cell expansion. These data support the possibility that activating mutations in hbetac are involved in haemopoietic disorders in man.

A novel ubiquitin-specific protease, UBP43, cloned from leukemia fusion protein AML1-ETO-expressing mice, functions in hematopoietic cell differentiation

Using PCR-coupled subtractive screening-representational difference analysis, we have cloned a novel gene from AML1-ETO knockin mice. This gene is highly expressed in the yolk sac and fetal liver of the knockin mice. Nucleotide sequence analysis indicates that its cDNA contains an 1,107-bp open reading frame encoding a 368-amino-acid polypeptide. Further protein sequence and protein translation analysis shows that it belongs to a family of ubiquitin-specific proteases (UBP), and its molecular mass is 43 kDa. Therefore, we have named this gene UBP43. Like other ubiquitin proteases, the UBP43 protein has deubiquitinating enzyme activity. Protein ubiquitination has been implicated in many important cellular events. In wild-type adult mice, UBP43 is highly expressed in the thymus and in peritoneal macrophages. Among nine different murine hematopoietic cell lines analyzed, UBP43 expression is detectable only in cell lines related to the monocytic lineage. Furthermore, its expression is regulated during cytokine-induced monocytic cell differentiation. We have investigated its function in the hematopoietic myeloid cell line M1. UBP43 was introduced into M1 cells by retroviral gene transfer, and several high-expressing UBP43 clones were obtained for further study. Morphologic and cell surface marker examination of UBP43/M1 cells reveals that overexpression of UBP43 blocks cytokine-induced terminal differentiation of monocytic cells. These data suggest that UBP43 plays an important role in hematopoiesis by modulating either the ubiquitin-dependent proteolytic pathway or the ubiquitination state of another regulatory factor(s) during myeloid cell differentiation.

Evidence that stress and surgical interventions promote tumor development by suppressing natural killer cell activity

Stress and surgery have been suggested to compromise host resistance to infectious and malignant diseases in experimental and clinical settings. Because stress affects numerous physiological systems, the role of the immune system in mediating such effects is unclear. In the current study, we assessed the degree to which stress-induced alterations in natural killer (NK) cell activity underlie increased susceptibility to tumor development in F344 rats. Two stress paradigms were used: forced swim and abdominal surgery. Host resistance to tumor development was studied using 3 tumor models syngeneic to inbred F344 rats: CRNK-16 leukemia and the MADB106 mammary adenocarcinoma, both sensitive to NK activity, and the NK-insensitive C4047 colon cancer. Swim stress increased CRNK-16-associated mortality and metastatic development of MADB106 but not metastasis of C4047 cells. In both stress paradigms, stress suppressed NK activity (NKA) for a duration that paralleled its metastasis-enhancing effects on the MADB106 tumor. In vivo depletion of large granular lymphocyte/NK cells abolished the metastasis-enhancing effects of swim stress but not of surgical stress. Our findings indicate that stress-induced suppression of NKA is sufficient to cause enhanced tumor development. Under certain stressful conditions, suppression of NKA is the primary mediator of the tumor-enhancing effects of stress, while under other conditions, additional factors play a significant role. Clinical circumstances in which surgical stress may induce enhanced metastatic growth are discussed.

Advantages of the CBA mouse in leukemogenesis research

The objectives of this review are to: (a) demonstrate that the male CBA/Ca mouse has several characteristics that make it an excellent animal for the study of leukemogenesis, (b) show that several of the genetic abnormalities observed in the male CBA/Ca mouse during the development of radiation induced acute myeloid leukemia (AML) are syntenic with those frequently detected in patients with myeloid disorders such as myelodysplastic syndrome and AML, (c) illustrate that leukemia-related chromosomal lesions are the indicators for high risk individuals.

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.