Fusion oncogenes in acute myeloid leukemia (AML) promote self-renewal from committed progenitors, thereby linking transformation and self-renewal pathways. Like most cancers, AML is a genetically and biologically heterogeneous disease, but it is unclear whether transformation results from common or overlapping genetic programs acting downstream of multiple mutations, or by the engagement of unique genetic programs acting cooperatively downstream of individual mutations. This distinction is important, because the involvement of common programs would imply the existence of common molecular targets to treat AML, no matter which fusion oncogenes are involved. Here we demonstrate that the ability to promote self-renewal is a generalized property of leukemia-associated oncogenes. Disparate oncogenes initiated overlapping transformation and self-renewal gene expression programs, the common elements of which were defined in established leukemia stem cells from an animal model as well as from a large cohort of patients with differing AML subtypes, where they strongly predicted pathobiological character. Notably, individual genes commonly activated in these programs could partially phenocopy the self-renewal function of leukemia-associated oncogenes in committed murine progenitors. Further, they could generate AML following expression in murine bone marrow. In summary, our findings reveal the operation of common programs of self-renewal and transformation downstream of leukemia-associated oncogenes, suggesting mechanistically common therapeutic approaches to AML are likely to be possible, regardless of the identity of the driver oncogene involved.

SUMMARY

We report a Jak2V617F knock-in mouse myeloproliferative neoplasm (MPN) model resembling human polycythemia vera (PV). The MPN is serially transplantable and we demonstrate that the hematopoietic stem cell (HSC) compartment has the unique capacity for disease initiation but does not have a significant selective competitive advantage over wild type HSCs. In contrast, myeloid progenitor populations are expanded and skewed towards the erythroid lineage, but cannot transplant the disease. Treatment with a JAK2 kinase inhibitor ameliorated the MPN phenotype, but did not eliminate the disease-initiating population. These findings provide insights into the consequences of JAK2 activation on HSC differentiation and function and have the potential to inform therapeutic approaches to JAK2V617F positive MPN.

SIGNIFICANCE

The JAK2V617F mutation is a promising candidate for molecularly targeted therapy in MPN. Early data from JAK2 inhibitor clinical trials have called into question the capacity of these compounds to alter the natural history of JAK2V617F mediated MPN. Determining the effect of JAK2 inhibitors on the disease-initiating population requires a model in which the JAK2V617F allele is expressed at physiological levels in hematopoietic stem and progenitor cells, as it is in humans. Our model demonstrates that JAK2V617F causes expansion of erythroid progenitors but that only the HSC compartment can initiate disease in a transplanted mouse. We further demonstrate that the HSC compartment, the definitive target for curative therapy of JAK2V617F mediated MPN, is resistant to treatment with a JAK2 inhibitor.

Most patients with non-small cell lung cancer who initially respond to gefitinib or erlotinib (tyrosine kinase inhibitors) ultimately develop resistance and disease relapse. What is the mechanism for this resistance?

RNA-binding proteins of the Musashi (Msi) family are expressed in stem cell compartments and in aggressive tumors, but they have not yet been widely explored in the blood. Here we demonstrate that Msi2 is the predominant form expressed in hematopoietic stem cells (HSCs), and its knockdown leads to reduced engraftment and depletion of HSCs in vivo. Overexpression of human MSI2 in a mouse model increases HSC cell cycle progression and cooperates with the chronic myeloid leukemia–associated BCR-ABL1 oncoprotein to induce an aggressive leukemia. MSI2 is overexpressed in human myeloid leukemia cell lines, and its depletion leads to decreased proliferation and increased apoptosis. Expression levels in human myeloid leukemia directly correlate with decreased survival in patients with the disease, thereby defining MSI2 expression as a new prognostic marker and as a new target for therapy in acute myeloid leukemia (AML).

SUMMARY

Pten deficiency depletes hematopoietic stem cells (HSCs) but expands leukemia-initiating cells and the mTOR inhibitor, rapamycin, blocks these effects. Understanding the opposite effects of mTOR activation on HSCs versus leukemia-initiating cells could improve anti-leukemia therapies. We found that the depletion of Pten-deficient HSCs was not caused by oxidative stress and could not be blocked by N-acetyl-cysteine. Instead, Pten deletion induced, and rapamycin attenuated, the expression of p16Ink4a and p53 in HSCs, and p19Arf and p53 in other hematopoietic cells. p53 suppressed leukemogenesis and promoted HSC depletion after Pten deletion. p16Ink4a also promoted HSC depletion but had a limited role suppressing leukemogenesis. p19Arf strongly suppressed leukemogenesis but did not deplete HSCs. Secondary mutations attenuated this tumor suppressor response in some leukemias that arose after Pten deletion. mTOR activation therefore depletes HSCs by a tumor suppressor response that is attenuated by secondary mutations in leukemogenic clones.

Purpose

To understand the changes in gene expression in PV progenitor cells and their relationship to JAK2V617F

Experimental Design

mRNA isolated from CD34+ cells from 9 PV patients and normal controls was profiled using Affymetrix arrays. Gene expression change mediated by JAK2V617F was determined by profiling CD34+ cells transduced with the kinase and by analysis of leukemia cell lines harboring JAK2V617F, treated with an inhibitor.

Results

A PV expression signature was enriched for genes involved in hematopoietic development, inflammatory responses and cell proliferation. By quantitative RT-PCR, 23 genes were consistently deregulated in all patient samples. Several of these genes such as WT1 and KLF4 were regulated by JAK2, while others such as NFIB and EVI1 appeared to be deregulated in PV by a JAK2 independent mechanism. Using cell line models and comparing gene expression profiles of cell lines and PV CD34+ PV specimens, we have identified panels of JAK2 dependent 14 genes and JAK2 independent 12 genes. These two 14 and 12 gene sets could separate not only PV from normal CD34+ specimens, but also other MPN such as ET and MF from their normal counterparts.

Conclusions

A subset of the aberrant gene expression in PV progenitor cells can be attributed to the action of the mutant kinase, but there remain a significant number of genes characteristic of the disease but deregulated by as yet unknown mechanisms. Genes deregulated in PV as a result of the action of JAK2V617F or independent of the kinase may represent other targets for therapy.

Fanconi anemia (FA) is a human genetic disease characterized by a DNA repair defect and progressive bone marrow failure. Central events in the FA pathway are the monoubiquitination of the Fancd2 protein and the removal of ubiquitin by the deubiquitinating enzyme, Usp1. Here, we have investigated the role of Fancd2 and Usp1 in the maintenance and function of murine hematopoietic stem cells (HSCs). Bone marrow from Fancd2−/− mice and Usp1−/− mice exhibited marked hematopoietic defects. A decreased frequency of the HSC populations including Lin-Sca-1+Kit+ cells and cells enriched for dormant HSCs expressing signaling lymphocyte activation molecule (SLAM) markers, was observed in the bone marrow of Fancd2-deficient mice. In addition, bone marrow from Fancd2−/− mice contained significantly reduced frequencies of late-developing cobblestone area-forming cell activity in vitro compared to the bone marrow from wild-type mice. Furthermore, Fancd2-deficient and Usp1-deficient bone marrow had defective long-term in vivo repopulating ability. Collectively, our data reveal novel functions of Fancd2 and Usp1 in maintaining the bone marrow HSC compartment and suggest that FA pathway disruption may account for bone marrow failure in FA patients.

Direct inhibition of transcription factor complexes remains a central challenge in the discipline of ligand discovery. In general, these proteins lack surface involutions suitable for high-affinity binding by small molecules. Here we report the design of synthetic, cell-permeable, stabilized α-helical peptides that target a critical protein–protein interface in the NOTCH transactivation complex. We demonstrate that direct, high-affinity binding of the hydrocarbon-stapled peptide SAHM1 prevents assembly of the active transcriptional complex. Inappropriate NOTCH activation is directly implicated in the pathogenesis of several disease states, including T-cell acute lymphoblastic leukaemia (T-ALL). The treatment of leukaemic cells with SAHM1 results in genome-wide suppression of NOTCH-activated genes. Direct antagonism of the NOTCH transcriptional program causes potent, NOTCH-specific anti-proliferative effects in cultured cells and in a mouse model of NOTCH1-driven T-ALL.

Progress in the management of patients with myelodysplastic syndromes (MDS) has been hampered by the inability to detect cytogenetic abnormalities in 40-60% of cases. We prospectively analyzed matched pairs of bone marrow and buccal cell (normal) DNA samples from 51 MDS patients by single nucleotide polymorphism (SNP) arrays, and identified somatically acquired clonal genomic abnormalities in 21 patients (41%). Among the 33 patients with normal bone marrow cell karyotypes, five (15%) had clonal, somatically acquired aberrations by SNP array analysis, including four with segmental uniparental disomies (UPD) and one with three separate microdeletions. Each abnormality was detected more readily in CD34+ cells then in unselected bone marrow cells. Paired analysis of bone marrow and buccal cell DNA from each patient was necessary to distinguish true clonal genomic abnormalities from inherited copy number variations and regions with apparent LOH. UPDs affecting chromosome 7q were identified in two patients who had a rapidly deteriorating clinical course despite a low-risk International Prognostic Scoring System score (IPSS). Further studies of larger numbers of patients will be needed to determine whether 7q UPD detected by SNP array analysis will identify higher-risk MDS patients at diagnosis, analogous to those with 7q cytogenetic abnormalities.

The Warburg effect describes a pro-oncogenic metabolism switch such that cancer cells take up more glucose than normal tissue and favor incomplete oxidation of glucose even in the presence of oxygen. To better understand how tyrosine kinase signaling, which is commonly increased in tumors, regulates the Warburg effect, we performed phosphoproteomic studies. We found that oncogenic forms of fibroblast growth factor receptor type 1 inhibit the pyruvate kinase M2 (PKM2) isoform by direct phosphorylation of PKM2 tyrosine residue 105 (Y105). This inhibits the formation of active, tetrameric PKM2 by disrupting binding of the PKM2 cofactor fructose-1,6-bisphosphate. Furthermore, we found that phosphorylation of PKM2 Y105 is common in human cancers. The presence of a PKM2 mutant in which phenylalanine is substituted for Y105 (Y105F) in cancer cells leads to decreased cell proliferation under hypoxic conditions, increased oxidative phosphorylation with reduced lactate production, and reduced tumor growth in xenografts in nude mice. Our findings suggest that tyrosine phosphorylation regulates PKM2 to provide a metabolic advantage to tumor cells, thereby promoting tumor growth.

The infant leukemia-associated gene Ott1 (Rbm15) has broad regulatory effects within murine hematopoiesis. However, germ line Ott1 deletion results in fetal demise prior to embryonic day 10.5, indicating additional developmental requirements for Ott1. The spen gene family, to which Ott1 belongs, has a transcriptional activation/repression domain and RNA recognition motifs and has a significant role in the development of the head and thorax in Drosophila melanogaster. Early Ott1-deficient embryos show growth retardation and incomplete closure of the notochord. Further analysis demonstrated placental defects in the spongiotrophoblast and syncytiotrophoblast layers, resulting in an arrest of vascular branching morphogenesis. The rescue of the placental defect using a conditional allele with a trophoblast-sparing cre transgene allowed embryos to form a normal placenta and survive gestation. This outcome showed that the process of vascular branching morphogenesis in Ott1-deficient animals was regulated by the trophoblast compartment rather than the fetal vasculature. Mice surviving to term manifested hyposplenia and abnormal cardiac development. Analysis of global gene expression of Ott1-deficient embryonic hearts showed an enrichment of hypoxia-related genes and a significant alteration of several candidate genes critical for cardiac development. Thus, Ott1-dependent pathways, in addition to being implicated in leukemogenesis, may also be important for the pathogenesis of placental insufficiency and cardiac malformations.

Neuroblastoma, an embryonal tumor of the peripheral sympathetic nervous system, accounts for approximately 15% of all deaths due to childhood cancer1. High-risk neuroblastomas, prevalent in the majority of patients, are rapidly progressive; even with intensive myeloablative chemotherapy, relapse is common and almost uniformly fatal2,3. Here we report the detection of previously unknown mutations in the ALK gene, which encodes a receptor tyrosine kinase, in 8% of primary neuroblastomas. Five non-synonymous sequence variations were identified in the kinase domain of ALK, of which three were somatic and two were germline. The most frequent mutation, F1174L, was also identified in three different neuroblastoma cell lines. ALK cDNAs encoding the F1174L and R1275Q variants, but not the wild-type ALK cDNA, transformed IL-3-dependent murine hematopoietic Ba/F3 cells to cytokine-independent growth. Ba/F3 cells expressing these mutations were sensitive to a small-molecule inhibitor of ALK, TAE6844. Furthermore, two human neuroblastoma cell lines harboring the F1174L mutation were sensitive to the inhibitor. Cytotoxicity was associated with increased levels of apoptosis as measured by TUNEL-labeling. shRNA-mediated knockdown of ALK expression in neuroblastoma cell lines with the F1174L mutation also resulted in apoptosis and impaired cell proliferation. Thus, activating alleles of the ALK receptor tyrosine kinase are present in primary neuroblastoma tumors and in established neuroblastoma cell lines, and confer sensitivity to ALK inhibition with small molecules, providing a molecular rationale for targeted therapy of this disease.

SUMMARY

Despite their known transforming properties, the effects of leukemogenic FLT3-ITD mutations on hematopoietic stem and multipotent progenitor cells and on hematopoietic differentiation are not well understood. We report a mouse model harboring an ITD in the murine Flt3 locus that develops myeloproliferative disease resembling CMML and further identified FLT3-ITD mutations in a subset of human CMML. These findings correlated with an increase in number, cell cycling and survival of multipotent stem and progenitor cells in an ITD dose-dependent manner in animals that exhibited alterations within their myeloid progenitor compartments and a block in normal B-cell development. This model provides insights into the consequences of constitutive signaling by an oncogenic tyrosine kinase on hematopoietic progenitor quiescence, function, and cell fate.

SIGNIFICANCE

Activating FLT3 mutations are among the most common genetic events in AML and confer a poor clinical prognosis. Essential to our understanding of how these lesions contribute to myeloid leukemia is the development of a Flt3-ITD ‘knock-in’ murine model that has allowed examination of the consequences of constitutive FLT3 signaling on primitive hematopoietic progenitors when expressed at appropriate physiologic levels. These animals informed us to the existence of FLT3-ITD-positive human CMML, which has clinical importance given the availability of FLT3 small molecule inhibitors. This model will not only serve as a powerful biological tool to identify mutations that cooperate with FLT3 in leukemogenesis, but also to assess molecular therapies that target either FLT3 or components of its signaling pathways.

Activated phosphoinositide 3-kinase (PI3K)-AKT signalling appears to be an obligate event in the development of cancer. The highly related members of the mammalian FoxO transcription factor family, FoxO1, FoxO3 and FoxO4, represent one of several effector arms of PI3K-AKT signalling, prompting genetic analysis of the role of FoxOs in the neoplastic phenotypes linked to PI3K-AKT activation. While germline or somatic deletion of up to 5 FoxO alleles produced remarkably modest neoplastic phenotypes, broad somatic deletion of all FoxOs engendered a progressive cancer-prone condition characterized by thymic lymphomas and hemangiomas, demonstrating that the mammalian FoxOs are indeed bona fide tumor suppressors. Transcriptome and promoter analyses of differentially affected endothelium identified novel direct FoxO targets and revealed that FoxO regulation of these targets in vivo is highly context-specific, even in the same cell type. Functional studies validated Sprouty2 and PBX1, among others, as FoxO-regulated mediators of endothelial cell morphogenesis and vascular homeostasis.

The homeobox transcription factor CDX2 plays an important role in embryonic development and regulates the proliferation and differentiation of intestinal epithelial cells in the adult. We have found that CDX2 is expressed in leukemic cells of 90% of patients with acute myeloid leukemia (AML) but not in hematopoietic stem and progenitor cells derived from normal individuals. Stable knockdown of CDX2 expression by RNA interference inhibited the proliferation of various human AML cell lines and strongly reduced their clonogenic potential in vitro. Primary murine hematopoietic progenitor cells transduced with Cdx2 acquired serial replating activity, were able to be continuously propagated in liquid culture, generated fully penetrant and transplantable AML in BM transplant recipients, and displayed dysregulated expression of Hox family members in vitro and in vivo. These results demonstrate that aberrant expression of the developmental regulatory gene CDX2 in the adult hematopoietic compartment is a frequent event in the pathogenesis of AML; suggest a role for CDX2 as part of a common effector pathway that promotes the proliferative capacity and self-renewal potential of myeloid progenitor cells; and support the hypothesis that CDX2 is responsible, in part, for the altered HOX gene expression that is observed in most cases of AML.

Mutations constitutively activating FLT3 kinase are detected in ∼30% of acute myelogenous leukemia (AML) patients and affect downstream pathways such as extracellular signal–regulated kinase (ERK)1/2. We found that activation of FLT3 in human AML inhibits CCAAT/enhancer binding protein α (C/EBPα) function by ERK1/2-mediated phosphorylation, which may explain the differentiation block of leukemic blasts. In MV4;11 cells, pharmacological inhibition of either FLT3 or MEK1 leads to granulocytic differentiation. Differentiation of MV4;11 cells was also observed when C/EBPα mutated at serine 21 to alanine (S21A) was stably expressed. In contrast, there was no effect when serine 21 was mutated to aspartate (S21D), which mimics phosphorylation of C/EBPα. Thus, our results suggest that therapies targeting the MEK/ERK cascade or development of protein therapies based on transduction of constitutively active C/EBPα may prove effective in treatment of FLT3 mutant leukemias resistant to the FLT3 inhibitor therapies.

Small molecule inhibitors, such as imatinib, are effective therapies for tyrosine kinase fusions BCR-ABL–TEL-PDGFβR–mediated human leukemias, but resistance may develop. The unique fusion junctions of these molecules are attractive candidates for molecularly targeted therapeutic intervention using RNA interference (RNAi), which is mediated by small interfering RNA (siRNA). We developed a retroviral system for stable expression of siRNA directed to the unique fusion junction sequence of TEL-PDGFβR in transformed hematopoietic cells. Stable expression of the siRNA resulted in approximately 90% inhibition of TEL-PDGFβR expression and its downstream effectors, including PI3K and mammalian target of rapamycin (mTOR). Expression of TEL-PDGFβR–specific siRNA (TPsiRNA) significantly attenuated the proliferation of TEL-PDGFβR–transformed Ba/F3 cells or disease latency and penetrance in mice induced by intravenous injection of these Ba/F3 cells. Although a 90% reduction in TEL-PDGFβR expression was insufficient to induce cell death, stable siRNA expression sensitized transformed cells to the PDGFβR inhibitor imatinib or to the mTOR inhibitor rapamycin. TPsiRNA also inhibited an imatinib-resistant TEL-PDGFβR mutant, and the inhibition was enhanced by siRNA in combination with PKC412, another PDGFβR inhibitor. Although siRNA delivery in vivo is a challenging problem, stable expression of siRNA, which targets oncogenic fusion genes, may potentiate the effects of conventional therapy for hematologic malignancies.

Oncogenic ras alleles are among the most common mutations found in patients with acute myeloid leukemia (AML). Previously, the role of oncogenic ras in cancer was assessed in model systems overexpressing oncogenic ras from heterologous promoters. However, there is increasing evidence that subtle differences in gene dosage and regulation of gene expression from endogenous promoters play critical roles in cancer pathogenesis. We characterized the role of oncogenic K-ras expressed from its endogenous promoter in the hematopoietic system using a conditional allele and IFN-inducible, Cre-mediated recombination. Mice developed a completely penetrant myeloproliferative syndrome characterized by leukocytosis with normal maturation of myeloid lineage cells; myeloid hyperplasia in bone marrow; and extramedullary hematopoiesis in the spleen and liver. Flow cytometry confirmed the myeloproliferative phenotype. Genotypic and Western blot analysis demonstrated Cre-mediated excision and expression, respectively, of the oncogenic K-ras allele. Bone marrow cells formed growth factor–independent colonies in methylcellulose cultures, but the myeloproliferative disease was not transplantable into secondary recipients. Thus, oncogenic K-ras induces a myeloproliferative disorder but not AML, indicating that additional mutations are required for AML development. This model system will be useful for assessing the contribution of cooperating mutations in AML and testing ras inhibitors in vivo.

TEL-JAK2 fusion proteins, which are a result of t(9;12)(p24;p13) translocations associated with human leukemia, activate Stat5 in vitro and in vivo and cause a myelo- and lymphoproliferative disease in a murine bone marrow transplant model. We report that Socs-1, a member of the SOCS family of endogenous inhibitors of JAKs and STATs, inhibits transformation of Ba/F3 cells by TEL-JAK2 but has no effect on Ba/F3 cells transformed by BCR-ABL, TEL-ABL, or TEL–platelet-derived growth factor receptor beta. TEL-JAK2, in addition to activating Stat5, associates with Shc and Grb2 and induces activation of Erk2, and expression of Socs-1 inhibits engagement of each of these signaling molecules. TEL-JAK2 kinase activity is inhibited by Socs-1, as assessed by in vitro kinase assays. In addition, Socs-1 induces proteasomal degradation of TEL-JAK2. Mutational analysis indicates that the SOCS box of Socs-1 is required for proteasomal degradation and for abrogation of growth of TEL-JAK2-transformed cells. Furthermore, murine bone marrow transplant assays demonstrate that expression of Socs-1 prolongs latency of TEL-JAK2-mediated disease in vivo. Collectively, these data indicate that Socs-1 inhibits TEL-JAK2 in vitro and in vivo through inhibition of kinase activity and induction of TEL-JAK2 protein degradation.

The t(5;12)(q33;p13) translocation associated with chronic myelomonocytic leukemia (CMML) generates a TEL/PDGFβR fusion gene. Here, we used a murine bone marrow transplant (BMT) assay to test the transforming properties of TEL/PDGFβR in vivo. TEL/PDGFβR, introduced into whole bone marrow by retroviral transduction, caused a rapidly fatal myeloproliferative disease that closely recapitulated human CMML. TEL/PDGFβR transplanted mice developed leukocytosis with Gr-1+ granulocytes, splenomegaly, evidence of extramedullary hematopoiesis, and bone marrow fibrosis, but no lymphoproliferative disease. We assayed mutant forms of the TEL/PDGFβR fusion protein — including 8 tyrosine to phenylalanine substitutions at phosphorylated PDGFβR sites to which various SH2 domain–containing signaling intermediates bind — for ability to transform hematopoietic cells. All of the phenylalanine (F-) mutants tested conferred IL-3-independence to a cultured murine hematopoietic cell line, but, in the BMT assay, different F-mutants displayed distinct transforming properties. In transplanted animals, tyrosines 579/581 proved critical for the development of myeloproliferative phenotype. F-mutants with these residues mutated showed no sign of myeloproliferation but instead developed T-cell lymphomas. In summary, TEL/PDGFβR is necessary and sufficient to induce a myeloproliferative disease in a murine BMT model, and PDGFβR residues Y579/581 are required for this phenotype.

SUMMARY

We report the unexpected finding that loss of Hh signaling through conditional deletion of Smoothened (Smo) in the adult hematopoietic compartment has no apparent effect on adult hematopoiesis, including peripheral blood count, number or cell-cycle status of stem or progenitor cells, hematopoietic colony-forming potential, long-term repopulating activity in competitive repopulation assays, or stress response to serial 5-fluorouracil treatment. Furthermore, pharmacologic inhibition of Hh signaling with a potent and selective small molecule antagonist has no substantive effect on hematopoiesis in the mouse. In addition, Hh signaling is not required for the development of MLL-AF9-mediated acute myeloid leukemia (AML). Taken together, these data demonstrate that Hh signaling is dispensable for normal hematopoietic development and hematopoietic stem cell function, indicating that targeting of Hh signaling in solid tumors is not likely to result in hematopoietic toxicity. Furthermore, the Hh pathway may not be a compelling target in certain hematopoietic malignancies.

BACKGROUND

The V617F mutation, which causes the substitution of phenylalanine for valine at position 617 of the Janus kinase (JAK) 2 gene (JAK2), is often present in patients with polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis. However, the molecular basis of these myeloproliferative disorders in patients without the V617F mutation is unclear.

METHODS

We searched for new mutations in members of the JAK and signal transducer and activator of transcription (STAT) gene families in patients with V617F-negative polycythemia vera or idiopathic erythrocytosis. The mutations were characterized biochemically and in a murine model of bone marrow transplantation.

RESULTS

We identified four somatic gain-of-function mutations affecting JAK2 exon 12 in 10 V617F-negative patients. Those with a JAK2 exon 12 mutation presented with an isolated erythrocytosis and distinctive bone marrow morphology, and several also had reduced serum erythropoietin levels. Erythroid colonies could be grown from their blood samples in the absence of exogenous erythropoietin. All such erythroid colonies were heterozygous for the mutation, whereas colonies homozygous for the mutation occur in most patients with V617F-positive polycythemia vera. BaF3 cells expressing the murine erythropoietin receptor and also carrying exon 12 mutations could proliferate without added interleukin-3. They also exhibited increased phosphorylation of JAK2 and extracellular regulated kinase 1 and 2, as compared with cells transduced by wild-type JAK2 or V617F JAK2. Three of the exon 12 mutations included a substitution of leucine for lysine at position 539 of JAK2. This mutation resulted in a myeloproliferative phenotype, including erythrocytosis, in a murine model of retroviral bone marrow transplantation.

CONCLUSIONS

JAK2 exon 12 mutations define a distinctive myeloproliferative syndrome that affects patients who currently receive a diagnosis of polycythemia vera or idiopathic erythrocytosis.

The proto-oncogene KRAS is mutated in a wide array of human cancers, most of which are aggressive and respond poorly to standard therapies. Although the identification of specific oncogenes has led to the development of clinically effective, molecularly targeted therapies in some cases, KRAS has remained refractory to this approach. A complementary strategy for targeting KRAS is to identify gene products that, when inhibited, result in cell death only in the presence of an oncogenic allele1,2. Here we have used systematic RNA interference (RNAi) to detect synthetic lethal partners of oncogenic KRAS and found that the non-canonical IκB kinase, TBK1, was selectively essential in cells that harbor mutant KRAS. Suppression of TBK1 induced apoptosis specifically in human cancer cell lines that depend on oncogenic KRAS expression. In these cells, TBK1 activated NF-κB anti-apoptotic signals involving cREL and BCL-XL that were essential for survival, providing mechanistic insights into this synthetic lethal interaction. These observations identify TBK1 and NF-κB signaling as essential in KRAS mutant tumors and establish a general approach for the rational identification of co-dependent pathways in cancer.

Acute megakaryoblastic leukemia (AMKL) is a form of acute myeloid leukemia (AML) associated with a poor prognosis. The genetics and pathophysiology of AMKL are not well understood. We generated a knockin mouse model of the one twenty-two–megakaryocytic acute leukemia (OTT-MAL) fusion oncogene that results from the t(1;22)(p13;q13) translocation specifically associated with a subtype of pediatric AMKL. We report here that OTT-MAL expression deregulated transcriptional activity of the canonical Notch signaling pathway transcription factor recombination signal binding protein for immunoglobulin κ J region (RBPJ) and caused abnormal fetal megakaryopoiesis. Furthermore, cooperation between OTT-MAL and an activating mutation of the thrombopoietin receptor myeloproliferative leukemia virus oncogene (MPL) efficiently induced a short-latency AMKL that recapitulated all the features of human AMKL, including megakaryoblast hyperproliferation and maturation block, thrombocytopenia, organomegaly, and extensive fibrosis. Our results establish that concomitant activation of RBPJ (Notch signaling) and MPL (cytokine signaling) transforms cells of the megakaryocytic lineage and suggest that specific targeting of these pathways could be of therapeutic value for human AMKL.