Therapeutic targeting of preleukemia cells in a mouse model of NPM1 mutant acute myeloid leukemia

The initiating mutations that contribute to cancer development are sometimes present in premalignant cells. Whether therapies targeting these mutations can eradicate premalignant cells is unclear. Acute myeloid leukemia (AML) is an attractive system for investigating the effect of preventative treatment because this disease is often preceded by a premalignant state (clonal hematopoiesis or myelodysplastic syndrome). In Npm1c/Dnmt3a mutant knock-in mice, a model of AML development, leukemia is preceded by a period of extended myeloid progenitor cell proliferation and self-renewal. We found that this self-renewal can be reversed by oral administration of a small molecule (VTP-50469) that targets the MLL1-Menin chromatin complex. These preclinical results support the hypothesis that individuals at high risk of developing AML might benefit from targeted epigenetic therapy in a preventative setting.

Novel ETV6-RUNX1 Mouse Model to Study the Role of ELF-MF in Childhood B-Acute Lymphoblastic Leukemia: a Pilot Study

Exposure to extremely low-frequency magnetic fields (ELF-MFs) has been classified by the International Agency for Research on Cancer (IARC) as "possibly carcinogenic to humans," based on limited scientific evidence concerning childhood leukemia. This assessment emphasized the lack of appropriate animal models recapitulating the natural history of this disease. Childhood B-cell acute lymphoblastic leukemia (B-ALL) is the result of complex interactions between genetic susceptibility and exposure to exogenous agents. The most common chromosomal alteration is the ETV6-RUNX1 fusion gene, which confers a low risk of developing the malignancy by originating a preleukemic clone requiring secondary hits for full-blown disease to appear. To develop potential prophylactic interventions, we need to identify the environmental triggers of the second hit. Recently, we generated a B-ALL mouse model of the human ETV6-RUNX1+ preleukemic state. Here, we present the results from the ARIMMORA pilot study, obtained by exposing 34 Sca1-ETV6-RUNX1 mice (vs. 27 unexposed) to a 50 Hz magnetic field of 1.5 mT with both fundamental and harmonic content, with an on/off cycle of 10 min/5 min, for 20 h/day, from conception until 3 months of age. Mice were monitored until 2 years of age and peripheral blood was periodically analyzed by flow cytometry. One of the exposed mice developed B-ALL while none of the non-exposed did. Although the results are statistically non-significant due to the limited number of mice used in this pilot experiment, overall, the results show that the newly developed Sca1-ETV6-RUNX1 mouse can be successfully used for ELF-MF exposure studies about the etiology of childhood B-ALL. Bioelectromagnetics. 2019;40:343-353. © 2019 Bioelectromagnetics Society.

Ets1 Plays a Critical Role in MLL/EB1-Mediated Leukemic Transformation in a Mouse Bone Marrow Transplantation Model

Leukemogenic potential of MLL fusion with the coiled-coil domain-containing partner genes and the downstream target genes of this type of MLL fusion have not been clearly investigated. In this study, we demonstrated that the coiled-coil–four-helix bundle structure of EB1 that participated in the MLL/EB1 was required for immortalizing mouse bone marrow (BM) cells and producing myeloid, but not lymphoid, cell lines. Compared to MLL/AF10, MLL/EB1 had low leukemogenic ability. The MLL/EB1 cells grew more slowly owing to increased apoptosis in vitro and induced acute monocytic leukemia with an incomplete penetrance and longer survival in vivo. A comparative analysis of transcriptome profiling between MLL/EB1 and MLL/AF10 cell lines revealed that there was an at least two-fold difference in the induction of 318 genes; overall, 51.3% (163/318) of the genes were known to be bound by MLL, while 15.4% (49/318) were bound by both MLL and MLL/AF9. Analysis of the 318 genes using Gene Ontology–PANTHER overrepresentation test revealed significant differences in several biological processes, including cell differentiation, proliferation/programmed cell death, and cell homing/recruitment. The Ets1 gene, bound by MLL and MLL/AF9, was involved in several biological processes. We demonstrated that Ets1 was selectively upregulated by MLL/EB1. Short hairpin RNA knockdown of Ets1 in MLL/EB1 cells reduced the expression of CD115, apoptosis rate, competitive engraftment to BM and spleen, and incidence of leukemia and prolonged the survival of the diseased mice. Our results demonstrated that MLL/EB1 upregulated Ets1, which controlled the balance of leukemia cells between apoptosis and BM engraftment/clonal expansion.

Novelty and impact of this study

The leukemogenic potential of MLL fusion with cytoplasmic proteins containing coiled-coil dimerization domains and the downstream target genes of this type of MLL fusion remain largely unknown. Using a retroviral transduction/transplantation mouse model, we demonstrated that MLL fusion with the coiled-coil–four-helix bundle structure of EB1 has low leukemogenic ability; Ets1, which is upregulated by MLL/EB1, plays a critical role in leukemic transformation by balance between apoptosis and BM engraftment/clonal expansion.

IKZF2 Drives Leukemia Stem Cell Self-Renewal and Inhibits Myeloid Differentiation

Leukemias exhibit a dysregulated developmental program mediated through both genetic and epigenetic mechanisms. Although IKZF2 is expressed in hematopoietic stem cells (HSCs), we found that it is dispensable for mouse and human HSC function. In contrast to its role as a tumor suppressor in hypodiploid B-acute lymphoblastic leukemia, we found that IKZF2 is required for myeloid leukemia. IKZF2 is highly expressed in leukemic stem cells (LSCs), and its deficiency results in defective LSC function. IKZF2 depletion in acute myeloid leukemia (AML) cells reduced colony formation, increased differentiation and apoptosis, and delayed leukemogenesis. Gene expression, chromatin accessibility, and direct IKZF2 binding in MLL-AF9 LSCs demonstrate that IKZF2 regulates a HOXA9 self-renewal gene expression program and inhibits a C/EBP-driven differentiation program. Ectopic HOXA9 expression and CEBPE depletion rescued the effects of IKZF2 depletion. Thus, our study shows that IKZF2 regulates the AML LSC program and provides a rationale to therapeutically target IKZF2 in myeloid leukemia.

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.

LEDGF/p75 is dispensable for hematopoiesis but essential for MLL-rearranged leukemogenesis

Mixed lineage leukemia (MLL) represents a genetically distinct and aggressive subset of human acute leukemia carrying chromosomal translocations of the MLL gene. These translocations result in oncogenic fusions that mediate aberrant recruitment of the transcription machinery to MLL target genes. The N-terminus of MLL and MLL-fusions form a complex with lens epithelium-derived growth factor (LEDGF/p75; encoded by the PSIP1 gene) and MENIN. This complex contributes to the association of MLL and MLL-fusion multiprotein complexes with the chromatin. Several studies have shown that both MENIN and LEDGF/p75 are required for efficient MLL-fusion-mediated transformation and for the expression of downstream MLL-regulated genes such as HOXA9 and MEIS1 In light of developing a therapeutic strategy targeting this complex, understanding the function of LEDGF/p75 in normal hematopoiesis is crucial. We generated a conditional Psip1 knockout mouse model in the hematopoietic compartment and examined the effects of LEDGF/p75 depletion in postnatal hematopoiesis and the initiation of MLL leukemogenesis. Psip1 knockout mice were viable but showed several defects in hematopoiesis, reduced colony-forming activity in vitro, decreased expression of Hox genes in the hematopoietic stem cells, and decreased MLL occupancy at MLL target genes. Finally, in vitro and in vivo experiments showed that LEDGF/p75 is dispensable for steady-state hematopoiesis but essential for the initiation of MLL-mediated leukemia. These data corroborate the MLL-LEDGF/p75 interaction as novel target for the treatment of MLL-rearranged leukemia.

Normal ABL1 is a tumor suppressor and therapeutic target in human and mouse leukemias expressing oncogenic ABL1 kinases

Leukemias expressing constitutively activated mutants of ABL1 tyrosine kinase (BCR-ABL1, TEL-ABL1, NUP214-ABL1) usually contain at least 1 normal ABL1 allele. Because oncogenic and normal ABL1 kinases may exert opposite effects on cell behavior, we examined the role of normal ABL1 in leukemias induced by oncogenic ABL1 kinases. BCR-ABL1-Abl1(-/-) cells generated highly aggressive chronic myeloid leukemia (CML)-blast phase-like disease in mice compared with less malignant CML-chronic phase-like disease from BCR-ABL1-Abl1(+/+) cells. Additionally, loss of ABL1 stimulated proliferation and expansion of BCR-ABL1 murine leukemia stem cells, arrested myeloid differentiation, inhibited genotoxic stress-induced apoptosis, and facilitated accumulation of chromosomal aberrations. Conversely, allosteric stimulation of ABL1 kinase activity enhanced the antileukemia effect of ABL1 tyrosine kinase inhibitors (imatinib and ponatinib) in human and murine leukemias expressing BCR-ABL1, TEL-ABL1, and NUP214-ABL1. Therefore, we postulate that normal ABL1 kinase behaves like a tumor suppressor and therapeutic target in leukemias expressing oncogenic forms of the kinase.

A novel mouse model identifies cooperating mutations and therapeutic targets critical for chronic myeloid leukemia progression

The introduction of highly selective ABL-tyrosine kinase inhibitors (TKIs) has revolutionized therapy for chronic myeloid leukemia (CML). However, TKIs are only efficacious in the chronic phase of the disease and effective therapies for TKI-refractory CML, or after progression to blast crisis (BC), are lacking. Whereas the chronic phase of CML is dependent on BCR-ABL, additional mutations are required for progression to BC. However, the identity of these mutations and the pathways they affect are poorly understood, hampering our ability to identify therapeutic targets and improve outcomes. Here, we describe a novel mouse model that allows identification of mechanisms of BC progression in an unbiased and tractable manner, using transposon-based insertional mutagenesis on the background of chronic phase CML. Our BC model is the first to faithfully recapitulate the phenotype, cellular and molecular biology of human CML progression. We report a heterogeneous and unique pattern of insertions identifying known and novel candidate genes and demonstrate that these pathways drive disease progression and provide potential targets for novel therapeutic strategies. Our model greatly informs the biology of CML progression and provides a potent resource for the development of candidate therapies to improve the dismal outcomes in this highly aggressive disease.

Advances in HLA genetics

The major histocompatibility complex (MHC) is a genetic system of over 70 known genes that occupies the midportion of the short arm of the sixth chromosome (C6p) and spans about 4 million base pairs of DNA. The high-resolution typing of class I and class II MHC genes and the identification of genes between and near them has increased the definition of the genetic basis of immune responses and diseases of unknown etiology such as autoimmune diseases in man. Although there are many more genetic systems that participate in the rejection of tissues and in the immune response, the MHC plays a central role in tissue compatibility and immune response against cancer and infectious diseases. In this paper, the authors review evidence about the role of HLA polymorphism in the pathogenesis and development of cancer, infectious diseases, autoimmune diseases and transplantation.

Two decades of leukemia oncoprotein epistasis: the MLL1 paradigm for epigenetic deregulation in leukemia

MLL1, located on human chromosome 11, is disrupted in distinct recurrent chromosomal translocations in several leukemia subsets. Studying the MLL1 gene and its oncogenic variants has provided a paradigm for understanding cancer initiation and maintenance through aberrant epigenetic gene regulation. Here we review the historical development of model systems to recapitulate oncogenic MLL1-rearrangement (MLL-r) alleles encoding mixed-lineage leukemia fusion proteins (MLL-FPs) or internal gene rearrangement products. These largely mouse and human cell/xenograft systems have been generated and used to understand how MLL-r alleles affect diverse pathways to result in a highly penetrant, drug-resistant leukemia. The particular features of the animal models influenced the conclusions of mechanisms of transformation. We discuss significant downstream enablers, inhibitors, effectors, and collaborators of MLL-r leukemia, including molecules that directly interact with MLL-FPs and endogenous mixed-lineage leukemia protein, direct target genes of MLL-FPs, and other pathways that have proven to be influential in supporting or suppressing the leukemogenic activity of MLL-FPs. The use of animal models has been complemented with patient sample, genome-wide analyses to delineate the important genomic and epigenomic changes that occur in distinct subsets of MLL-r leukemia. Collectively, these studies have resulted in rapid progress toward developing new strategies for targeting MLL-r leukemia and general cell-biological principles that may broadly inform targeting aberrant epigenetic regulators in other cancers.

Restoring specific lactobacilli levels decreases inflammation and muscle atrophy markers in an acute leukemia mouse model

The gut microbiota has recently been proposed as a novel component in the regulation of host homeostasis and immunity. We have assessed for the first time the role of the gut microbiota in a mouse model of leukemia (transplantation of BaF3 cells containing ectopic expression of Bcr-Abl), characterized at the final stage by a loss of fat mass, muscle atrophy, anorexia and inflammation. The gut microbial 16S rDNA analysis, using PCR-Denaturating Gradient Gel Electrophoresis and quantitative PCR, reveals a dysbiosis and a selective modulation of Lactobacillus spp. (decrease of L. reuteri and L. johnsonii/gasseri in favor of L. murinus/animalis) in the BaF3 mice compared to the controls. The restoration of Lactobacillus species by oral supplementation with L. reuteri 100-23 and L. gasseri 311476 reduced the expression of atrophy markers (Atrogin-1, MuRF1, LC3, Cathepsin L) in the gastrocnemius and in the tibialis, a phenomenon correlated with a decrease of inflammatory cytokines (interleukin-6, monocyte chemoattractant protein-1, interleukin-4, granulocyte colony-stimulating factor, quantified by multiplex immuno-assay). These positive effects are strain- and/or species-specific since L. acidophilus NCFM supplementation does not impact on muscle atrophy markers and systemic inflammation. Altogether, these results suggest that the gut microbiota could constitute a novel therapeutic target in the management of leukemia-associated inflammation and related disorders in the muscle.

[Therapeutic effect of focal adhesion kinase gene silence on leukemia]

This study was aimed to investigate the effects of focal adhesion kinase (FAK) gene silence on leukemia cell growth, leukemogenesis and efficacy of chemotherapy drug. Vector containing lentiviral-FAK-shRNA was constructed and transfected into BCR/ABL-BaF3 leukemic cells, the cell growth and apoptosis were detected in vitro. The effect of FAK shRNA on leukemogenesis was studied in a murine model with leukemia. The apoptosis of leukemia cells and survival of leukemic mice treated by FAK shRNA combined with drug STI571 were monitored. The results showed that FAK gene expression was knocked down by lentiviral-FAK-shRNA. FAK gene silencing inhibited leukemia cell growth in vitro. The apoptosis test results showed that the percentages of Annexin V(+) cells in vector control group and FAK shRNA group were (3.46 ± 0.56)% and (7.3 ± 0.79)%, respectively, and the difference was statistically significant (p < 0.05). The mice in vector control group died at day 21 to 27, while the mice in FAK shRNA group died between day 52 and 60, and the difference was statistically significant (p < 0.05). Moreover, FAK gene silence combined with drug STI571 could enhance the apoptosis of leukemia cells and prolong survival time of leukemic mice. It is concluded that FAK gene silence inhibits leukemogenesis and promotes efficacy of chemotherapy drug on leukemia cells, indicating FAK gene silence may be considered as a new therapeutic strategy for leukemia.

CD4(+) T cells contribute to the remodeling of the microenvironment required for sustained tumor regression upon oncogene inactivation

Oncogene addiction is thought to occur cell autonomously. Immune effectors are implicated in the initiation and restraint of tumorigenesis, but their role in oncogene inactivation-mediated tumor regression is unclear. Here, we show that an intact immune system, specifically CD4(+) T cells, is required for the induction of cellular senescence, shutdown of angiogenesis, and chemokine expression resulting in sustained tumor regression upon inactivation of the MYC or BCR-ABL oncogenes in mouse models of T cell acute lymphoblastic lymphoma and pro-B cell leukemia, respectively. Moreover, immune effectors knocked out for thrombospondins failed to induce sustained tumor regression. Hence, CD4(+) T cells are required for the remodeling of the tumor microenvironment through the expression of chemokines, such as thrombospondins, in order to elicit oncogene addiction.

Mechanism of T cell tolerance induced by myeloid-derived suppressor cells

Ag-specific T cell tolerance plays a critical role in tumor escape. Recent studies implicated myeloid-derived suppressor cells (MDSCs) in the induction of CD8(+) T cell tolerance in tumor-bearing hosts. However, the mechanism of this phenomenon remained unclear. We have found that incubation of Ag-specific CD8(+) T cells, with peptide-loaded MDSCs, did not induce signaling downstream of TCR. However, it prevented subsequent signaling from peptide-loaded dendritic cells. Using double TCR transgenic CD8(+) T cells, we have demonstrated that MDSC induced tolerance to only the peptide, which was presented by MDSCs. T cell response to the peptide specific to the other TCR was not affected. Incubation of MDSCs with Ag-specific CD8(+) T cells caused nitration of the molecules on the surface of CD8(+) T cells, localized to the site of physical interaction between MDSC and T cells, which involves preferentially only TCR specific for the peptide presented by MDSCs. Postincubation with MDSCs, only nitrotyrosine-positive CD8(+) T cells demonstrated profound nonresponsiveness to the specific peptide, whereas nitrotyrosine-negative CD8(+) T cells responded normally to that stimulation. MDSCs caused dissociation between TCR and CD3zeta molecules, disrupting TCR complexes on T cells. Thus, these data describe a novel mechanism of Ag-specific CD8(+) T cell tolerance in cancer.

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.

[Characteristics of the morphofunctional condition of cell populations by the modal cell class in response to transplantation of lymphocytic leukemia p-388]

On the model of transplanted leukemia p-388, cytophotometry has shown that tumors' impact on the body includes two stages: direct affection of the target organ, indirect affection through changes in functional relations with cell populations of other organs due to the impact of transformed cells of the damaged target organ. Moreover, the progress of tumor growth alters functional relations between the organs.

Gene expression profiling of leukemia stem cells

Characterization of gene expression programs and pathways important for normal and cancer stem cells has become an active area of investigation. Microarray analysis of various cell populations provides an opportunity to assess genomewide expression programs to define cellular identity and to potentially identify pathways activated in various stem cells. Here we describe methods to isolate a leukemia stem cell population, amplify RNA, and perform microarray analyses.

Murine APOBEC1 is a powerful mutator of retroviral and cellular RNA in vitro and in vivo

Mammalian APOBEC molecules comprise a large family of cytidine deaminases with specificity for RNA and single-stranded DNA (ssDNA). APOBEC1s are invariably highly specific and edit a single residue in a cellular mRNA, while the cellular targets for APOBEC3s are not clearly established, although they may curtail the transposition of some retrotransposons. Two of the seven member human APOBEC3 enzymes strongly restrict human immunodeficiency virus type 1 in vitro and in vivo. We show here that ssDNA hyperediting of an infectious exogenous gammaretrovirus, the Friend-murine leukemia virus, by murine APOBEC1 and APOBEC3 deaminases occurs in vitro. Murine APOBEC1 was able to hyperdeaminate cytidine residues in murine leukemia virus genomic RNA as well. Analysis of the edited sites shows that the deamination in vivo was due to mouse APOBEC1 rather than APOBEC3. Furthermore, murine APOBEC1 is able to hyperedit its primary substrate in vivo, the apolipoprotein B mRNA, and a variety of heterologous RNAs. In short, murine APOBEC1 is a hypermutator of both RNA and ssDNA in vivo, which could exert occasional side effects upon overexpression.

[Biphasic effect of TIMP-2 on the growth of leukemic SHI-1 cells in nude mice]

OBJECTIVE

To investigate the influence of tissue inhibitor of metalloproteinase 2 (TIMP-2) on the infiltrative patterns of human monocytic leukemic cell line SHI-1 in nude mice.

METHODS

1) 1 x 10(7) TIMP-2 gene transduced SHI-1 (SHI-1-TIMP-2) and SHI-1 transduced MSCV gene (SHI-1-MSCV) cells were inoculated via tail vein into 6-week nude mice, which pretreated by splenectomy, cytoxan intraperitoneal injection, and sublethal irradiation(referred as SCI nude mice). 30 days after inoculation, half of the mice were sacrificed, and the infiltration patterns were investigated by histological exam and human CD45 immunohistochemistry, other mice were observed for survival time. 2) Leukemic cells inoculated subcutaneously into the axillary area of mice without any pre-treatment. On day 23 and 30, mice were sacrificed to measure the volume of neoplasm. TIMP-2 protein expression and the micro vein density were detected by immunohistochemistry.

RESULTS

In SCI nude mice inoculated via caudal vein with SHI-1-TIMP-2 cells, the survival time was shorter and infiltration (including in central nervous system) was higher than that in those inoculated with SHI-1-MSCV cells. However, in inoculated subcutaneously group, the neoplasm though grew rapidly at first, over expression of TIMP-2 limited the tumor growth and angiogenesis.

CONCLUSION

The functions of TIMP-2 are diversity; the role of TIMP-2 in tumor infiltration and metastasis was worthy of further investigation.

Inhibition of progression of erythroleukemia induced by Friend virus in BALB/c mice by natural products--berberine, curcumin and picroliv

The infection with Friend murine leukemia virus (FMuLv) is being used as a retrovirus infection model for searching the potential anti-viral medicinal preparations or establishing new treatment strategies. In the present study we have evaluated the inhibitory effect of three non-toxic antiviral natural compounds namely berberine, curcumin or picroliv against FMuLv induced erythroleukemia in BALB/c mice. To understand the effect of these compounds in the initiation and progression of leukemia we did a series of analysis, which include hematological and biochemical parameters, histopathological evaluations of the liver and the spleen and expression analysis of selected genes such as Bcl-2, p53, p45NFE2, Raf-1, Erk-1, IFNgamma receptor and erythropoietin in spleen. The treatment with berberine, curcumin or picroliv were found to (a) elevate the life span of leukemia harboring animals by more than 60 days; (b) decreased the anemic condition which was highly prevalent in FMuLv alone treated group; (c) histopathological evaluations showed that the compounds tested here inhibited the massive leukemic cell infiltrations to sinusoidal spaces in spleen; (d) decrease the expression of Bcl-2, Raf-1, Erk-1 IFNgamma receptor and erythropoietin; (e) induce the expression of p53. Overall, our results suggest that berberine, curcumin and picroliv were able to suppress the progression of leukemia induced by FMuLv and further support their chemopreventive potential against virally induced cancers.

Runx1 protects hematopoietic stem/progenitor cells from oncogenic insult

The RUNX1/AML1 gene encodes a transcription factor essential for the generation of hematopoietic stem cells and is frequently targeted in human leukemia. In human RUNX1-related leukemias, the RAS pathway is often concurrently mutated, but the mechanism of the synergism remains elusive. Here, we found that inactivation of Runx1 in mouse bone marrow cells results in an increase in the stem/progenitor cell fraction due to suppression of apoptosis and elevated expression of the polycomb gene Bmi-1, which is important for stem cell self-renewal. Introduction of oncogenic N-RAS into wild-type cells, in contrast, reduced the stem/progenitor cell fraction because of senescence, apoptosis, and differentiation. Such detrimental events presumably occurred because of the cellular fail-safe program, although hyperproliferation was initially induced by an oncogenic stimulus. Runx1 insufficiency appears to impair such a fail-safe mechanism, particularly in the stem/progenitor cells, thereby supporting the clonal maintenance of leukemia-initiating cells expressing an activated oncogene. Disclosure of potential conflicts of interest is found at the end of this article.

The Wilms’ tumor gene WT1-GFP knock-in mouse reveals the dynamic regulation of WT1 expression in normal and leukemic hematopoiesis

The Wilms' tumor gene WT1 is overexpressed in most of human leukemias regardless of disease subtypes. To characterize the expression pattern of WT1 during normal and neoplastic hematopoiesis, we generated a knock-in reporter green fluorescent protein (GFP) mouse (WT1(GFP/+)) and assayed for WT1 expression in normal and leukemic hematopoietic cells. In normal hematopoietic cells, WT1 was expressed in none of the long-term (LT) hematopoietic stem cells (HSC) and very few (<1%) of the multipotent progenitor cells. In contrast, in murine leukemias induced by acute myeloid leukemia 1 (AML1)/ETO+TEL/PDGFbetaR or BCR/ABL, WT1 was expressed in 40.5 or 38.9% of immature c-kit(+)lin(-)Sca-1(+) (KLS) cells, which contained a subset, but not all, of transplantable leukemic stem cells (LSCs). WT1 expression was minimal in normal fetal liver HSCs and mobilized HSCs, both of which are stimulated for proliferation. In addition, overexpression of WT1 in HSCs did not result in proliferation or expansion of HSCs and their progeny in vivo. Thus, the mechanism by which expansion of WT1-expressing cells occurs in leukemia remains unclear. Nevertheless, our results demonstrate that the WT1(GFP/+) mouse is a powerful tool for analyzing WT1-expressing cells, and they highlight the potential of WT1, as a specific therapeutic target that is expressed in LSCs but not in normal HSCs.

Vector-based RNAi approach to isoform-specific downregulation of vascular endothelial growth factor (VEGF)165 expression in human leukemia cells

Vascular endothelial growth factor (VEGF) plays a critical role during normal embryonic angiogenesis and also in the pathological angiogenesis that occurs in a number of diseases, including cancer. K562 human leukemia cells overexpress VEGF, with a shift in isoform production from membrane-bound VEGF189 to the more soluble VEGF165. In the present study, three 19 bp reverse repeated motifs targeting exons 5 and 7 boundary of VEGF165 gene sequence with 9 bp spacer were synthesized and cloned into eukaryotic expression plasmid pGenesil-1 containing U6 shRNA promoter and termination signal of RNA polymerase. The recombinant plasmids pGenesil-VR1, pGenesil-VR2, pGenesil-VR3 and pGenesil-con (plasmid containing random DNA fragment) were transfected into K562 cells, respectively, through lipofectamine reagent. A vector-based small interfering RNA(SiRNA) inhibited VEGF165 mRNA expression by 72% and protein production by 67% in K562 cells. Human microvascular endothelial cell migration induced by conditioned medium from VEGFsi-transfected K562 cells was significantly less than that induced by conditioned medium from K562 cells and control vector-transfected K562 cells. Furthermore, the VEGF shRNA dramatically suppressed tumor angiogenesis and tumor growth in a K562 s.c. xenograft model. Vessel density as assessed by vWF immunohistochemical analysis was also decreased. This strategy provides a novel tool to study the function of various VEGF isoforms and may contribute to VEGF-specific treatment in cancer.

Inhibition of heat shock protein 90 prolongs survival of mice with BCR-ABL-T315I-induced leukemia and suppresses leukemic stem cells

Development of kinase domain mutations is a major drug-resistance mechanism for tyrosine kinase inhibitors (TKIs) in cancer therapy. A particularly challenging example is found in Philadelphia chromosome-positive chronic myelogenous leukemia (CML) where all available kinase inhibitors in clinic are ineffective against the BCR-ABL mutant, T315I. As an alternative approach to kinase inhibition, an orally administered heat shock protein 90 (Hsp90) inhibitor, IPI-504, was evaluated in a murine model of CML. Treatment with IPI-504 resulted in BCR-ABL protein degradation, decreased numbers of leukemia stem cells, and prolonged survival of leukemic mice bearing the T315I mutation. Hsp90 inhibition more potently suppressed T315I-expressing leukemia clones relative to the wild-type (WT) clones in mice. Combination treatment with IPI-504 and imatinib was more effective than either treatment alone in prolonging survival of mice simultaneously bearing both WT and T315I leukemic cells. These results provide a rationale for use of an Hsp90 inhibitor as a first-line treatment in CML by inhibiting leukemia stem cells and preventing the emergence of imatinib-resistant clones in patients. Rather than inhibiting kinase activity, elimination of mutant kinases provides a new therapeutic strategy for treating BCR-ABL-induced leukemia as well as other cancers resistant to treatment with tyrosine kinase inhibitors.

Leukemic transformation of hematopoietic cells in mice internally exposed to depleted uranium

Depleted uranium (DU) is a dense heavy metal used in military applications. During military conflicts, US military personnel have been wounded by DU shrapnel. The health effects of embedded DU are unknown. Published data from our laboratory demonstrated that DU exposure in vitro can transform immortalized human osteoblast cells (HOS) to the tumorigenic phenotype. Results from our laboratory have also shown that DU is genotoxic and mutagenic in cultured human cells. Internalized DU could be a carcinogenic risk and concurrent alpha particle and heavy metal toxic effects complicate this potential risk. Anecdotal reports have suggested that DU can cause leukemia. To better assess this risk, we have developed an in vivo leukemogenesis model. This model involves using murine hematopoietic cells (FDC-P1) that are dependent on stimulation by granulocyte-macrophage colony stimulating factor (GM-CSF) or interleukin 3 (IL-3) and injected into mice to produce myeloid leukemia. Although immortalized, these cells are not tumorigenic on subcutaneous inoculation in mice. Intravenous injection of FDC-P1 cells into DU-implanted DBA/2 mice was followed by the development of leukemias in 76% of all mice implanted with DU pellets. In contrast, only 12% of control mice developed leukemia. Karyotypic analysis confirmed that the leukemias originated from FDC-P1 cells. The growth properties of leukemic cells from bone marrow, spleen, and lymph node were assessed and indicate that the FDC-P1 cells had become transformed in vivo. The kidney, spleen, bone marrow, muscle, and urine showed significant elevations in tissue uranium levels prior to induction of leukemia. These results demonstrated that a DU altered in vivo environment may be involved in the pathogenesis of DU induced leukemia in an animal model.

JunB is a gatekeeper for B-lymphoid leukemia

Loss of JunB has been observed in human leukemia and lymphoma, but it remains unknown, whether this loss is relevant to disease progression. Here, we investigated the consequences of JunB deficiency using Abelson-induced B-lymphoid leukemia as a model system. Mice deficient in JunB expression succumbed to Abelson-induced leukemia with increased incidence and significantly reduced latency. Similarly, bcr/abl p185-transformed JunB-deficient (junB(Delta/Delta)) cells induced leukemia in RAG2(-/-) mice displaying a more malignant phenotype. These observations indicated that cell intrinsic effects within the junB(Delta/Delta) tumor cells accounted for the accelerated leukemia development. Indeed, explantated bcr/abl p185 transformed junB(Delta/Delta) cells proliferated faster than the control cells. The proliferative advantage emerged slowly after the initial transformation process and was associated with increased expression levels of the cell cycle kinase cdk6 and with decreased levels of the cell cycle inhibitor p16(INK4a). These alterations were due to irreversible reprogramming of the cell, because - once established - accelerated disease induced by junB(Delta/Delta) cells was not reverted by re-introducing JunB. Consistent with this observation, we found that the p16 promoter was methylated. Thus, JunB functions as a gatekeeper during tumor evolution. In its absence, transformed leukemic cells acquire an enhanced proliferative capacity, which presages a more malignant disease.

Targeted degradation of the AML1/MDS1/EVI1 oncoprotein by arsenic trioxide

Arsenic trioxide (ATO) has been found to be an effective treatment for acute promyelocytic leukemia patients and is being tested for treating other hematologic malignancies. We have previously shown that AML1/MDS1/EVI1 (AME), a fusion gene generated by a t(3;21)(q26;q22) translocation found in patients with chronic myelogenous leukemia during blast phase, myelodysplastic syndrome, or acute myelogenous leukemia (AML), impairs hematopoiesis and eventually induces an AML in mice. Both fusion partners of AME, AML1 and MDS1/EVI1, encode transcription factors and are also targets of a variety of genetic abnormalities in human hematologic malignancies. In addition, aberrant expression of ectopic viral integration site 1 (EVI1) has also been found in solid tumors, such as ovarian and colon cancers. In this study, we examined whether ATO could target AME and related oncoproteins. We found that ATO used at therapeutic levels degrades AME. The ATO treatment induces differentiation and apoptosis in AME leukemic cells in vitro as well as reduces tumor load and increases the survival of mice transplanted with these cells. We further found that ATO targets AME via both myelodysplastic syndrome 1 (MDS1) and EVI1 moieties and degrades EVI1 via the ubiquitin-proteasome pathway and MDS1 in a proteasome-independent manner. Our results suggest that ATO could be used as a part of targeted therapy for AME-, AML1/MDS1-, MDS1/EVI1-, and EVI1-positive human cancers.

Cdx4 and menin co-regulate Hoxa9 expression in hematopoietic cells

Background

Transcription factor Cdx4 and transcriptional coregulator menin are essential for Hoxa9 expression and normal hematopoiesis. However, the precise mechanism underlying Hoxa9 regulation is not clear.

Methods and Findings

Here, we show that the expression level of Hoxa9 is correlated with the location of increased trimethylated histone 3 lysine 4 (H3K4M3). The active and repressive histone modifications co-exist along the Hoxa9 regulatory region. We further demonstrate that both Cdx4 and menin bind to the same regulatory region at the Hoxa9 locus in vivo, and co-activate the reporter gene driven by the Hoxa9 cis-elements that contain Cdx4 binding sites. Ablation of menin abrogates Cdx4 access to the chromatin target and significantly reduces both active and repressive histone H3 modifications in the Hoxa9 locus.

Conclusion

These results suggest a functional link among Cdx4, menin and histone modifications in Hoxa9 regulation in hematopoietic cells.

Targeting the NF-kappaB signaling pathway in Notch1-induced T-cell leukemia

T-cell acute lymphoblastic leukemia (T-ALL), unlike other ALL types, is only infrequently associated with chromosomal aberrations, but it was recently shown that most individuals with T-ALL carry activating mutations in the NOTCH1 gene. However, the signaling pathways and target genes responsible for Notch1-induced neoplastic transformation remain undefined. We report here that constitutively active Notch1 activates the NF-kappaB pathway transcriptionally and via the IkappaB kinase (IKK) complex, thereby causing increased expression of several well characterized target genes of NF-kappaB in bone marrow hematopoietic stem cells and progenitors. Our observations demonstrate that the NF-kappaB pathway is highly active in established human T-ALL and that inhibition of the pathway can efficiently restrict tumor growth both in vitro and in vivo. These findings identify NF-kappaB as one of the major mediators of Notch1-induced transformation and suggest that the NF-kappaB pathway is a potential target of future therapies of T-ALL.

Activation of an oncogenic microRNA cistron by provirus integration

Retroviruses can cause tumors when they integrate near a protooncogene or tumor suppressor gene of the host. We infected >2,500 mice with the SL3-3 murine leukemia virus; in 22 resulting tumors, we found provirus integrations nearby or within the gene that contains the mir-17-92 microRNA (miRNA) cistron. Using quantitative real-time PCR, we showed that expression of miRNA was increased in these tumors, indicating that retroviral infection can induce expression of oncogenic miRNAs. Our results demonstrate that retroviral mutagenesis can be a potent tool for miRNA discovery.

Neoplasia driven by mutant c-KIT is mediated by intracellular, not plasma membrane, receptor signaling

Activating mutations in c-KIT are associated with gastrointestinal stromal tumors, mastocytosis, and acute myeloid leukemia. In attempting to establish a murine model of human KIT(D816V) (hKIT(D816V))-mediated leukemia, we uncovered an unexpected relationship between cellular transformation and intracellular trafficking. We found that transport of hKIT(D816V) protein was blocked at the endoplasmic reticulum in a species-specific fashion. We exploited these species-specific trafficking differences and a set of localization domain-tagged KIT mutants to explore the relationship between subcellular localization of mutant KIT and cellular transformation. The protein products of fully transforming KIT mutants localized to the Golgi apparatus and to a lesser extent the plasma membrane. Domain-tagged KIT(D816V) targeted to the Golgi apparatus remained constitutively active and transforming. Chemical inhibition of intracellular transport demonstrated that Golgi localization is sufficient, but plasma membrane localization is dispensable, for downstream signaling mediated by KIT mutation. When expressed in murine bone marrow, endoplasmic reticulum-localized hKIT(D816V) failed to induce disease in mice, while expression of either Golgi-localized HyKIT(D816V) or cytosol-localized, ectodomain-deleted KIT(D816V) uniformly caused fatal myeloproliferative diseases. Taken together, these data demonstrate that intracellular, non-plasma membrane receptor signaling is sufficient to drive neoplasia caused by mutant c-KIT and provide the first animal model of myelomonocytic neoplasia initiated by human KIT(D816V).

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.

Sustained leukaemic phenotype after inactivation of BCR-ABLp190 in mice

Pharmacological inactivation of cancer genes or products is being used as a strategy for therapy in oncology. To investigate the potential role of BCR-ABLp190 cessation in leukaemia development, we generated mice carrying a tetracycline-repressible BCR-ABLp190 transgene. These mice were morphologically normal at birth, and developed leukaemias. Disease was characterized by the presence of B-cell blasts co-expressing myeloid markers, reminiscent of the human counterpart. BCR-ABLp190 activation can initiate leukaemia in both young and adult mice. Transitory expression of BCR-ABLp190 is enough to develop leukaemia. Suppression of the BCR-ABLp190 transgene in leukaemic CombitTA-p190 mice did not rescue the malignant phenotype, indicating that BCR-ABLp190 is not required to maintain the disease in mice. Similar results were obtained by inactivation of BCR-ABLp190 with STI571 (Gleevec; Novartis, East Hanover, NJ, USA) in leukaemic CombitTA-p190 mice. However, gradual suppression of BCR-ABLp190 in leukaemic CombitTA-p190 mice identified a minimum level of BCR-ABLp190 expression necessary to revert the specific block in B-cell differentiation in the leukaemic cells. Overall, the findings indicate that BCR-ABLp190 appears to cause epigenetic and/or genetic changes in tumour-maintaining cells that render them insensitive to BCR-ABLp190 inactivation.

Evaluation of ex vivo expanded human NK cells on antileukemia activity in SCID-beige mice

The possibility of using natural killer (NK) cells in treatment of human hematological malignancies has increased in recent years. One factor contributing to this is the introduction of new methods for ex vivo generation of enriched populations of clinical grade NK cells. The objective of the present study was to evaluate the safety and efficacy of human ex vivo expanded clinical grade NK cells against K562 leukemia cells in severe combined immunodeficiency disease (SCID)-beige mice. Irradiated SCID-beige mice were injected intravenously (i.v.) with K562 leukemia cells. Following leukemia cell injection, mice were injected with ex vivo expanded human NK cells. NK cells were followed in vivo and mice monitored for survival from leukemia. Administration of these ex vivo expanded clinical grade NK cells was safe and prevented leukemia development. In conclusion, these results imply possibilities for the use of this NK cell preparation in treatment trials of human hematological malignancies and possibly other forms of cancer.

[Overexpression of tissue inhibitor of metalloprotease-2 promotes proliferation and infiltration of human monocytic leukemia cells]

OBJECTIVE

To investigate the functional role of human tissue inhibitor of metalloprotease (TIMP)-2 gene on the proliferation and infiltrating capability of human monocytic leukemic cells.

METHODS

Human monocytic leukemic cells of the line SHI-1 were cultured and the TIMP-2 expression on the cell membrane was detected by flow cytometry (FCM). The SHI-1 cells were transfected with human TIMP-2 gene (SHI-1/TIMP-2 cells) or blank vector (SHI-1/MSCV cells). The TIMP-2 expressed on the surface of the cell membranes of SHI-1/TIMP-2 and SHI-1/MSCV cells was detected by FCM. The SHI-1/TIMP-2 and SHI-1/MSCV cells were inoculated into the 96-well plate, 24, 48, 72, and 96 hours later MMT method and ELISA were used, and the cell growth curve was drawn to detect the proliferation ability of the cells. Matrigel was put into the upper layer of Transwell chamber. Human bone marrow matrix cells (BMMC) of leukemia patient and SHI-1/TIMP-2 cells or SHI-1/MSCV cells were put into the upper layers as experimental groups, and SHI-1/TIMP-2 cells or SHI-1/MSCV cells only, without human BMMC, were put into the upper layers as control groups. 72 hours later blood cell counting plate was used to measure the number of cells migrating through Matrigel. SHI-1/TIMP-2 cells or SHI-1/MSCV cells were injected intravenously into pre-treated BALB/c nu/nu mice. Thirty days later 8 mice from each group were killed to observe the tumorigenesis in the organs, especially the central nervous system leukemia (CNL). The survival of the other mice was observed.

RESULTS

The expression level of TIMP-2 on the cell membrane of the SHI-1/TIMP-2 cells was 99.3% +/- 0.1%, significantly higher than that of the SHI-1/MSCV cells (85.9% +/- 2.6%, P < 0.05). The A values 24, 48, 72, and 96 hours later of the SHI-1/TIMP-2 cells were 0.34 +/- 0.05, 0.6 +/- 0.05, 0.97 +/- 0.12, and 1.28 +/- 0.06 respectively, all significantly higher than those of the SHI-1/MSCV cells (0.28 +/- 0.03, 0.36 +/- 0.03, 0.54 +/- 0.09, and 0.99 +/- 0.03 respectively, all P < 0.05). The SHI-1/TIMP-2 cells and SHI-1/MSCV cells only could not migrate through Matrigel basically. 24 - 48 hours after co-cultivation Shi-1 cells began to appear in the lower layer of Transwell chambers, 72 hours later the trans-Matrigel SHI-1/TIMP-2 cells accounted for 24.7% +/- 6.9% of the inoculated SHI-1/TIMP-2 cells, a proportion significantly higher than that in the case of the SHI-1/MSCV cells (12% +/- 1.4%, P < 0.05). 24 days after the inoculation the mean body weight of the mice inoculated with SHI-1/TIMP-2 cells was 21.5 g +/- 0.4 g, significantly higher than that of the mice inoculated with SHI-1/MSCV cells (17.4 g +/- 0.6 g, P < 0.01). The mice inoculated with SHI-1/TIMP-2 cells showed much more tumors in different organs and much more severe infiltration in the CNS in comparison with the mice inoculated with SHI-1/MSCV cells The mean survival time of the mice inoculated with the SHI-1/TIMP-2 cells was 33.7 days, significantly shorter than that of the mice inoculated with SHI-1/MSCV cells (40 days).

CONCLUSION

TIMP-2 expressed on the cell membrane is critical to promote the proliferation and infiltration of SHI-1 cells.

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

AID for innate immunity to retroviral transformation

AID is a cytidine deaminase essential for class switch recombination and somatic hypermutation during the humoral immune response. In this issue of Immunity, Gourzi et al. (2006) show that AID also plays a critical role in innate immunity to virally induced acute pro-B cell leukemia.

A role for activation-induced cytidine deaminase in the host response against a transforming retrovirus

Activation-induced cytidine deaminase (AID) is specifically expressed in the germinal centers of lymphoid organs, where it initiates targeted hypermutation of variable regions of immunoglobulin genes in response to stimulation by antigen. Ectopic expression of AID, however, mediates generalized hypermutation in eukaryotes and prokaryotes. Here, we present evidence that AID is induced outside the germinal center in response to infection by the Abelson murine leukemia virus. The genotoxic activity of virally induced AID resulted in checkpoint kinase-1 (chk1) phosphorylation and ultimately restricted the proliferation of the infected cell. At the same time, it induced NKG2D ligand upregulation, which alerts the immune system to the presence of virally transformed cells. Hence, in addition to its known function in immunoglobulin diversification, AID is active in innate defense against a transforming retrovirus.

Activating alleles of JAK3 in acute megakaryoblastic leukemia

Tyrosine kinases are aberrantly activated in numerous malignancies, including acute myeloid leukemia (AML). To identify tyrosine kinases activated in AML, we developed a screening strategy that rapidly identifies tyrosine-phosphorylated proteins using mass spectrometry. This allowed the identification of an activating mutation (A572V) in the JAK3 pseudokinase domain in the acute megakaryoblastic leukemia (AMKL) cell line CMK. Subsequent analysis identified two additional JAK3 alleles, V722I and P132T, in AMKL patients. JAK3(A572V), JAK3(V722I), and JAK3(P132T) each transform Ba/F3 cells to factor-independent growth, and JAK3(A572V) confers features of megakaryoblastic leukemia in a murine model. These findings illustrate the biological importance of gain-of-function JAK3 mutations in leukemogenesis and demonstrate the utility of proteomic approaches to identifying clinically relevant mutations.

Cell-autonomous and -nonautonomous contributions of STAT1 in murine models of tumorigenesis

In this issue of Cancer Cell, Kovacic and colleagues have reexamined the role of STAT1 in murine models of leukemogenesis. Their studies shed new light on the complex interplay between cell-autonomous contributions and host responsiveness to cancer and elucidate a previously unknown role of STAT1 in tumor progression.

STAT1 acts as a tumor promoter for leukemia development

The tumor suppressor STAT1 is considered a key regulator of the surveillance of developing tumors. Here, we describe an unexpected tumor-promoting role for STAT1 in leukemia. STAT1(-/-) mice are partially protected from leukemia development, and STAT1(-/-) tumor cells induce leukemia in RAG2(-/-) and immunocompetent mice with increased latency. The low MHC class I protein levels of STAT1(-/-) tumor cells enable efficient NK cell lysis and account for the enhanced tumor clearance. Strikingly, STAT1(-/-) tumor cells acquire increased MHC class I expression upon leukemia progression. These findings define STAT1 as a tumor promoter in leukemia development. Furthermore, we describe the upregulation of MHC class I expression as a general mechanism that allows for the escape of hematopoietic malignancies from immune surveillance.