Endogenous oncogenic Nras mutation initiates hematopoietic malignancies in a dose- and cell type-dependent manner

Loss of Dnmt3a and endogenous Kras(G12D/+) cooperate to regulate hematopoietic stem and progenitor cell functions in leukemogenesis

Oncogenic NRAS and KRAS mutations are prevalent in human juvenile and chronic myelomonocytic leukemia (JMML/CMML). However, additional genetic mutations cooperating with oncogenic RAS in JMML/ CMML progression and/or their transformation to acute myeloid leukemia (AML) remain largely unknown. Here we tested the potential genetic interaction of DNMT3A mutations and oncogenic RAS mutations in leukemogenesis. We found that Dnmt3a(-/-) induces multiple hematopoietic phenotypes after a prolonged latency, including T-cell expansion in the peripheral blood, stress erythropoiesis in the spleen and myeloid malignancies in the liver. Dnmt3a(-/-) significantly promoted JMML/CMML progression and shortened the survival of Kras(G12D/+) mice in a cell-autonomous manner. Similarly, downregulating Dnmt3a also promoted myeloid malignancies in Nras(G12D/+) mice. Further studies show that Dnmt3a deficiency rescues Kras(G12D/+)-mediated depletion of hematopoietic stem cells and increases self-renewal of Kras(G12D/+) myeloid progenitors (MPs). Moreover, ~33% of animals developed an AML-like disease, which is driven by Kras(G12D/+); Dnmt3a(-/-) MPs. Consistent with our result, COSMIC database mining demonstrates that the combination of oncogenic RAS and DNMT3A mutations exclusively occurred in patients with JMML, CMML or AML. Our results suggest that DNMT3A mutations and oncogenic RAS cooperate to regulate hematopoietic stem and progenitor cells and promote myeloid malignancies.

NRASG12V oncogene facilitates self-renewal in a murine model of acute myelogenous leukemia

Mutant RAS oncoproteins activate signaling molecules that drive oncogenesis in multiple human tumors including acute myelogenous leukemia (AML). However, the specific functions of these pathways in AML are unclear, thwarting the rational application of targeted therapeutics. To elucidate the downstream functions of activated NRAS in AML, we used a murine model that harbors Mll-AF9 and a tetracycline-repressible, activated NRAS (NRAS(G12V)). Using computational approaches to explore our gene-expression data sets, we found that NRAS(G12V) enforced the leukemia self-renewal gene-expression signature and was required to maintain an MLL-AF9- and Myb-dependent leukemia self-renewal gene-expression program. NRAS(G12V) was required for leukemia self-renewal independent of its effects on growth and survival. Analysis of the gene-expression patterns of leukemic subpopulations revealed that the NRAS(G12V)-mediated leukemia self-renewal signature is preferentially expressed in the leukemia stem cell-enriched subpopulation. In a multiplexed analysis of RAS-dependent signaling, Mac-1(Low) cells, which harbor leukemia stem cells, were preferentially sensitive to NRAS(G12V) withdrawal. NRAS(G12V) maintained leukemia self-renewal through mTOR and MEK pathway activation, implicating these pathways as potential targets for cancer stem cell-specific therapies. Together, these experimental results define a RAS oncogene-driven function that is critical for leukemia maintenance and represents a novel mechanism of oncogene addiction.

Ras/Raf/MEK/ERK Pathway Activation in Childhood Acute Lymphoblastic Leukemia and Its Therapeutic Targeting

Deregulation of the Ras/Raf/MEK/extracellular signal-regulated kinase pathway is a common event in childhood acute lymphoblastic leukemia and is caused by point mutation, gene deletion, and chromosomal translocation of a vast array of gene types, highlighting its importance in leukemia biology. Pathway activation can be therapeutically exploited and may guide new therapies needed for relapsed acute lymphoblastic leukemia and other high risk subgroups.

Combined MEK and JAK inhibition abrogates murine myeloproliferative neoplasm

Overactive RAS signaling is prevalent in juvenile myelomonocytic leukemia (JMML) and the myeloproliferative variant of chronic myelomonocytic leukemia (MP-CMML) in humans, and both are refractory to conventional chemotherapy. Conditional activation of a constitutively active oncogenic Nras (NrasG12D/G12D) in murine hematopoietic cells promotes an acute myeloproliferative neoplasm (MPN) that recapitulates many features of JMML and MP-CMML. We found that NrasG12D/G12D-expressing HSCs, which serve as JMML/MP-CMML-initiating cells, show strong hyperactivation of ERK1/2, promoting hyperproliferation and depletion of HSCs and expansion of downstream progenitors. Inhibition of the MEK pathway alone prolonged the presence of NrasG12D/G12D-expressing HSCs but failed to restore their proper function. Consequently, approximately 60% of NrasG12D/G12D mice treated with MEK inhibitor alone died within 20 weeks, and the remaining animals continued to display JMML/MP-CMML-like phenotypes. In contrast, combined inhibition of MEK and JAK/STAT signaling, which is commonly hyperactivated in human and mouse CMML, potently inhibited human and mouse CMML cell growth in vitro, rescued mutant NrasG12D/G12D-expressing HSC function in vivo, and promoted long-term survival without evident disease manifestation in NrasG12D/G12D animals. These results provide a strong rationale for further exploration of combined targeting of MEK/ERK and JAK/STAT in treating patients with JMML and MP-CMML.

Copy-neutral loss of heterozygosity is prevalent and a late event in the pathogenesis of FLT3/ITD AML

Patients with high FLT3 internal tandem duplication allelic ratios (FLT3/ITD-ARs) have a poor prognosis. Single-nucleotide polymorphism/comparative genomic hybridization, single-cell PCR and colony-forming assays were used to evaluate genotypic evolution of high FLT3/ITD-ARs in 85 acute myeloid leukemia (AML) patients. Microarrays were used to examine molecular pathways disrupted in leukemic blasts with high FLT3/ITD-ARs. Copy-neutral loss of heterozygosity (CN-LOH) was identified at the FLT3 locus in diagnostic samples with high FLT3/ITD-ARs (N=11), but not in samples with low FLT3/ITD-ARs (N=24), FLT3-activating loop mutations (N=11) or wild-type FLT3 (N=39). Single-cell assays showed that homozygous FLT3/ITD genotype was present in subsets of leukemic blasts at diagnosis but became the dominant clone at relapse. Less differentiated CD34⁺/CD33⁻ progenitor colonies were heterozygous for FLT3/ITD, whereas more differentiated CD34⁺/CD33⁺ progenitor colonies were homozygous for FLT3/ITD. Expression profiling revealed that samples harboring high FLT3/ITD-ARs aberrantly expressed genes within the recombination/DNA repair pathway. Thus, the development of CN-LOH at the FLT3 locus, which results in high FLT3/ITD-ARs, likely represents a late genomic event that occurs after the acquisition of the FLT3/ITD. Although the etiology underlying the development of CN-LOH remains to be clarified, the disruption in recombination/DNA repair pathway, which is present before the development of LOH, may have a role.

BRAF kinase domain mutations are present in a subset of chronic myelomonocytic leukemia with wild-type RAS

The frequency of RAS mutations in chronic myelomonocytic leukemia (CMML) suggests that activation of the MAPK pathway is important in CMML pathogenesis. Accordingly, we hypothesized that mutations in other members of the MAPK pathway might be overrepresented in RAS(wt) CMML. We performed targeted next generation sequencing analysis on 70 CMML patients with known RAS mutation status. The study group included 37 men and 33 women with a median age of 67.8 years (range, 28-86 years). Forty patients were RAS(wt) and 30 were RAS(mut) ; the latter included KRAS = 17; NRAS = 12; KRAS + NRAS = 1. Five patients (7.1% of total group; 12.5% of RAS(wt) group) with RAS(wt) had kinase domain BRAF mutations. The BRAF mutations were of missense type and involved exon 11 in one patient and exon 15 in four patients. All BRAF(mut) patients had CMML-1 with low-risk cytogenetic findings. Two (40%) of the five patients with BRAF(mut) patients transformed to acute myeloid leukemia during follow-up. In summary, we demonstrate that a subset of patients with RAS(wt) CMML harbors BRAF kinase domain mutations that are potentially capable of activating the MAPK signaling pathway.

Concurrent juvenile myelomonocytic leukemia and T-lymphoblastic lymphoma with a shared missense mutation in NRAS

Single cases of B- and T-lymphoblastic leukemia/lymphoma occurring after remission of JMML, and JMML occurring after remission of B-lymphoblastic leukemia have been reported in the literature. We present a unique case of a child with concurrent JMML and T-lymphoblastic lymphoma in which an identical missense mutation in NRAS was found in both the neoplastic JMML and T-LBL cells. JMML has been considered a stem cell disorder, and our case provides additional molecular evidence for a stem cell lesion underlying the two different disease phenotypes.

Rasgrp1 mutation increases naive T-cell CD44 expression and drives mTOR-dependent accumulation of Helios⁺ T cells and autoantibodies

Missense variants are a major source of human genetic variation. Here we analyze a new mouse missense variant, Rasgrp1^Anaef, with an ENU-mutated EF hand in the Rasgrp1 Ras guanine nucleotide exchange factor. Rasgrp1^Anaef mice exhibit anti-nuclear autoantibodies and gradually accumulate a CD44^hi Helios⁺ PD-1⁺ CD4⁺ T cell population that is dependent on B cells. Despite reduced Rasgrp1-Ras-ERK activation in vitro, thymocyte selection in Rasgrp1^Anaef is mostly normal in vivo, although CD44 is overexpressed on naïve thymocytes and T cells in a T-cell-autonomous manner. We identify CD44 expression as a sensitive reporter of tonic mTOR-S6 kinase signaling through a novel mouse strain, chino, with a reduction-of-function mutation in Mtor. Elevated tonic mTOR-S6 signaling occurs in Rasgrp1^Anaef naïve CD4⁺ T cells. CD44 expression, CD4⁺ T cell subset ratios and serum autoantibodies all returned to normal in Rasgrp1^AnaefMtor^chino double-mutant mice, demonstrating that increased mTOR activity is essential for the Rasgrp1^Anaef T cell dysregulation.

DOI:http://dx.doi.org/10.7554/eLife.01020.001

Our DNA contains more than three billion nucleotides. Each of these nucleotides can be an A, C, G or T, and groups of three neighboring nucleotides within our DNA are used to represent the 20 amino acids that are used to make proteins. This means that changing just one nucleotide can cause one amino acid to be replaced by a different amino acid in the protein encoded by that stretch of DNA: AAA and AAG code for the amino acid lysine, for example, but AAC and AAT code for asparagine. Known as missense gene variants, these changes can also increase or decrease the expression of the gene.

Every person has thousands of missense gene variants, including about 12,000 inherited from their parents. Sometimes these variants have no consequence, but they can be harmful if replacing the correct amino acid with a different amino acid prevents the protein from performing an important task. In particular, missense gene variants in genes that encode immune system proteins are likely to play a role in diseases of the immune system. For example, variants near a gene called Rasgrp1 have been linked to two autoimmune diseases – type 1 diabetes and Graves’ disease—in which the immune system mistakenly attacks the body’s own cells and tissues.

Now Daley et al. have shed new light on the mechanism by which a missense gene variant in Rasgrp1 can cause autoimmune diseases. Mice with this mutation show signs of autoimmune disease, but their T cells—white blood cells that have a central role in the immune system – develop normally despite this mutation. Instead, Daley et al. found that a specific type of T cell, called T helper cells, accumulated in large numbers in the mutant mice and stimulated cells of a third type—immune cells called B cells—to produce autoantibodies. The production of autoantibodies is a common feature of autoimmune diseases.

Daley et al. traced the origins of the T helper cells to excessive activity on a signaling pathway that involves a protein called mTOR, and went on to show that treatment with the drug rapamycin counteracted the accumulation of the T helper cells and reduced the level of autoimmune activity. In addition to exemplifying how changing just one amino acid change can have a profound effect, the work of Daley et al. is an attractive model for exploring how missense gene variants in people can contribute to autoimmune diseases.

DOI:http://dx.doi.org/10.7554/eLife.01020.002

Cancer-associated IDH2 mutants drive an acute myeloid leukemia that is susceptible to Brd4 inhibition

Somatic mutations in the isocitrate dehydrogenase (IDH) genes IDH1 and IDH2 occur frequently in acute myeloid leukemia (AML) and other cancers. These genes encode neomorphic proteins that produce the presumed oncometabolite 2-hydroxyglutarate (2-HG). Despite the prospect of treating AML and other cancers by targeting IDH mutant proteins, it remains unclear how these mutants affect tumor development and maintenance in vivo, and no cancer models exist to study the action of IDH2 mutants in vivo. We show that IDH2 mutants can cooperate with oncogenic Flt3 or Nras alleles to drive leukemia in mice by impairing the differentiation of cells of the myeloid lineage. Pharmacologic or genetic inhibition of IDH2 triggers the differentiation and death of AML cells, albeit only with prolonged IDH2 inhibition. In contrast, inhibition of the bromodomain-containing protein Brd4 triggers rapid differentiation and death of IDH2 mutant AML. Our results establish a critical role for mutant IDH2 in leukemogenesis and tumor maintenance and identify an IDH-independent strategy to target these cancers therapeutically.

Dominant role of oncogene dosage and absence of tumor suppressor activity in Nras-driven hematopoietic transformation

Biochemical properties of Ras oncoproteins and their transforming ability strongly support a dominant mechanism of action in tumorigenesis. However, genetic studies unexpectedly suggested that wild-type (WT) Ras exerts tumor suppressor activity. Expressing oncogenic Nras(G12D) in the hematopoietic compartment of mice induces an aggressive myeloproliferative neoplasm that is exacerbated in homozygous mutant animals. Here, we show that increased Nras(G12D) gene dosage, but not inactivation of WT Nras, underlies the aggressive in vivo behavior of Nras(G12D/G12D) hematopoietic cells. Modulating Nras(G12D) dosage had discrete effects on myeloid progenitor growth, signal transduction, and sensitivity to MAP-ERK kinase (MEK) inhibition. Furthermore, enforced WT N-Ras expression neither suppressed the growth of Nras-mutant cells nor inhibited myeloid transformation by exogenous Nras(G12D). Importantly, NRAS expression increased in human cancer cell lines with NRAS mutations. These data have therapeutic implications and support reconsidering the proposed tumor suppressor activity of WT Ras in other cancers.

SIGNIFICANCE

Understanding the mechanisms of Ras -induced transformation and adaptive cellular responses is fundamental. The observation that oncogenic Nras lacks tumor suppressor activity, whereas increased dosage strongly modulates cell growth and alters sensitivity to MEK inhibition, suggests new therapeutic opportunities in cancer.

Nras(G12D/+) promotes leukemogenesis by aberrantly regulating hematopoietic stem cell functions

Oncogenic NRAS mutations are frequently identified in human myeloid leukemias. In mice, expression of endogenous oncogenic Nras (Nras(G12D/+)) in hematopoietic cells leads to expansion of myeloid progenitors, increased long-term reconstitution of bone marrow cells, and a chronic myeloproliferative neoplasm (MPN). However, acute expression of Nras(G12D/+) in a pure C57BL/6 background does not induce hyperactivated granulocyte macrophage colony-stimulating factor signaling or increased proliferation in myeloid progenitors. It is thus unclear how Nras(G12D/+) signaling promotes leukemogenesis. Here, we show that hematopoietic stem cells (HSCs) expressing Nras(G12D/+) serve as MPN-initiating cells. They undergo moderate hyperproliferation with increased self-renewal. The aberrant Nras(G12D/+) HSC function is associated with hyperactivation of ERK1/2 in HSCs. Conversely, downregulation of MEK/ERK by pharmacologic and genetic approaches attenuates the cycling of Nras(G12D/+) HSCs and prevents the expansion of Nras(G12D/+) HSCs and myeloid progenitors. Our data delineate critical mechanisms of oncogenic Nras signaling in HSC function and leukemogenesis.

Notch1 gene mutations target KRAS G12D-expressing CD8+ cells and contribute to their leukemogenic transformation

Acute T-cell lymphoblastic leukemia/lymphoma (T-ALL) is an aggressive hematopoietic malignancy affecting both children and adults. Previous studies of T-ALL mouse models induced by different genetic mutations have provided highly diverse results on the issues of T-cell leukemia/lymphoma-initiating cells (T-LICs) and potential mechanisms contributing to T-LIC transformation. Here, we show that oncogenic Kras (Kras G12D) expressed from its endogenous locus is a potent inducer of T-ALL even in a less sensitized BALB/c background. Notch1 mutations, including exon 34 mutations and recently characterized type 1 and 2 deletions, are detected in 100% of Kras G12D-induced T-ALL tumors. Although these mutations are not detected at the pre-leukemia stage, incremental up-regulation of NOTCH1 surface expression is observed at the pre-leukemia and leukemia stages. As secondary genetic hits in the Kras G12D model, Notch1 mutations target CD8(+) T-cells but not hematopoietic stem cells to further promote T-ALL progression. Pre-leukemia T-cells without detectable Notch1 mutations do not induce T-ALL in secondary recipient mice compared with T-ALL tumor cells with Notch1 mutations. We found huge variations in T-LIC frequency and immunophenotypes of cells enriched for T-LICs. Unlike Pten deficiency-induced T-ALL, oncogenic Kras-initiated T-ALL is not associated with up-regulation of the Wnt/β-catenin pathway. Our results suggest that up-regulation of NOTCH1 signaling, through either overexpression of surface NOTCH1 or acquired gain-of-function mutations, is involved in both T-ALL initiation and progression. Notch1 mutations and Kras G12D contribute cooperatively to leukemogenic transformation of normal T-cells.

Activated N-Ras signaling regulates arterial-venous specification in zebrafish

BACKGROUND

The aberrant activation of Ras signaling is associated with human diseases including hematological malignancies and vascular disorders. So far the pathological roles of activated Ras signaling in hematopoiesis and vasculogenesis are largely unknown.

METHODS

A conditional Cre/loxP transgenic strategy was used to mediate the specific expression of a constitutively active form of human N-Ras in zebrafish endothelial and hematopoietic cells driven by the zebrafish lmo2 promoter. The expression of hematopoietic and endothelial marker genes was analyzed both via whole mount in situ hybridization (WISH) assay and real-time quantitative PCR (qPCR). The embryonic vascular morphogenesis was characterized both by living imaging and immunofluorescence on the sections with a confocal microscopy, and the number of endothelial cells in the embryos was quantified by flow cytometry. The functional analyses of the blood circulation were carried out by fluorescence microangiography assay and morpholino injection.

RESULTS

In the activated N-Ras transgenic embryos, the primitive hematopoiesis appeared normal, however, the definitive hematopoiesis of these embryos was completely absent. Further analysis of endothelial cell markers confirmed that transcription of arterial marker ephrinB2 was significantly decreased and expression of venous marker flt4 excessively increased, indicating the activated N-Ras signaling promotes the venous development at the expense of arteriogenesis during zebrafish embryogenesis. The activated N-Ras-expressing embryos showed atrophic axial arteries and expansive axial veins, leading to no definitive hematopoietic stem cell formation, the blood circulation failure and subsequently embryonic lethality.

CONCLUSIONS

Our studies revealed for the first time that activated N-Ras signaling during the endothelial differentiation in vertebrates can disrupt the balance of arterial-venous specification, thus providing new insights into the pathogenesis of the congenital human vascular disease and tumorigenic angiogenesis.

Direct reprogramming by oncogenic Ras and Myc

Genetically or epigenetically defined reprogramming is a hallmark of cancer cells. However, a causal association between genome reprogramming and cancer has not yet been conclusively established. In particular, little is known about the mechanisms that underlie metastasis of cancer, and even less is known about the identity of metastasizing cancer cells. In this study, we used a model of conditional expression of oncogenic KrasG12D allele in primary mouse cells to show that reprogramming and dedifferentiation is a fundamental early step in malignant transformation and cancer initiation. Our data indicate that stable expression of activated KrasG12D confers on cells a large degree of phenotypic plasticity that predisposes them to neoplastic transformation and acquisition of stem cell characteristics. We have developed a genetically tractable model system to investigate the origins and evolution of metastatic pancreatic cancer cells. We show that metastatic conversion of KrasG12D-expressing cells that exhibit different degrees of differentiation and malignancy can be reconstructed in cell culture, and that the proto-oncogene c-Myc controls the generation of self-renewing metastatic cancer cells. Collectively, our results support a model wherein non-stem cancer cells have the potential to dedifferentiate and acquire stem cell properties as a direct consequence of oncogene-induced plasticity. Moreover, the disturbance in the normally existing dynamic equilibrium between cancer stem cells and non-stem cancer cells allows the formation of cancer stem cells with high metastatic capacity at any time during cancer progression.

The IRP1-HIF-2α axis coordinates iron and oxygen sensing with erythropoiesis and iron absorption

Red blood cell production is a finely tuned process that requires coordinated oxygen- and iron-dependent regulation of cell differentiation and iron metabolism. Here, we show that translational regulation of hypoxia-inducible factor 2α (HIF-2α) synthesis by iron regulatory protein 1 (IRP1) is critical for controlling erythrocyte number. IRP1-null (Irp1(-/-)) mice display a marked transient polycythemia. HIF-2α messenger RNA (mRNA) is derepressed in kidneys of Irp1(-/-) mice but not in kidneys of Irp2(-/-) mice, leading to increased renal erythropoietin (Epo) mRNA and inappropriately elevated serum Epo levels. Expression of the iron transport genes DCytb, Dmt1, and ferroportin, as well as other HIF-2α targets, is enhanced in Irp1(-/-) duodenum. Analysis of mRNA translation state in the liver revealed IRP1-dependent dysregulation of HIF-2α mRNA translation, whereas IRP2 deficiency derepressed translation of all other known 5' iron response element (IRE)-containing mRNAs expressed in the liver. These results uncover separable physiological roles of each IRP and identify IRP1 as a therapeutic target for manipulating HIF-2α action in hematologic, oncologic, and other disorders.

Signaling profiling at the single-cell level identifies a distinct signaling signature in murine hematopoietic stem cells

Hematopoietic stem cell (HSC) function is tightly regulated by cytokine signaling. Although phospho-flow cytometry allows us to study signaling in defined populations of cells, there has been tremendous hurdle to carry out this study in rare HSCs due to unrecoverable critical HSC markers, low HSC number, and poor cell recovery rate. Here, we overcame these difficulties and developed a "HSC phospho-flow" method to analyze cytokine signaling in murine HSCs at the single-cell level and compare HSC signaling profile to that of multipotent progenitors (MPPs), a cell type immediately downstream of HSCs, and commonly used Lin(-) cKit(+) cells (LK cells, enriched for myeloid progenitors). We chose to study signaling evoked from three representative cytokines, stem cell factor (SCF) and thrombopoietin (TPO) that are essential for HSC function and granulocyte macrophage-colony-stimulating factor (GM-CSF) that is dispensable for HSCs. HSCs display a distinct TPO and GM-CSF signaling signature from MPPs and LK cells, which highly correlates with receptor surface expression. In contrast, although majority of LK cells express lower levels of cKit than HSCs and MPPs, SCF-evoked ERK1/2 activation in LK cells shows a significantly increased magnitude for a prolonged period. These results suggest that specific cellular context plays a more important role than receptor surface expression in SCF signaling. Our study of HSC signaling at the homeostasis stage paves the way to investigate signaling changes in HSCs under conditions of stress, aging, and hematopoietic diseases.

Targeting oncogenic Ras signaling in hematologic malignancies

Ras proteins are critical nodes in cellular signaling that integrate inputs from activated cell surface receptors and other stimuli to modulate cell fate through a complex network of effector pathways. Oncogenic RAS mutations are found in ∼25% of human cancers and are highly prevalent in hematopoietic malignancies. Because of their structural and biochemical properties, oncogenic Ras proteins are exceedingly difficult targets for rational drug discovery, and no mechanism-based therapies exist for cancers with RAS mutations. This article reviews the properties of normal and oncogenic Ras proteins, the prevalence and likely pathogenic role of NRAS, KRAS, and NF1 mutations in hematopoietic malignancies, relevant animal models of these cancers, and implications for drug discovery. Because hematologic malignancies are experimentally tractable, they are especially valuable platforms for addressing the fundamental question of how to reverse the adverse biochemical output of oncogenic Ras in cancer.

Constitutional NRAS mutations are rare among patients with Noonan syndrome or juvenile myelomonocytic leukemia

Recently, germline mutations of NRAS have been shown to be associated with Noonan syndrome (NS), a relatively common developmental disorder characterized by short stature, congenital heart disease, and distinctive facial features. We report on the mutational analysis of NRAS in a cohort of 125 French patients with NS and no known mutation for PTPN11, KRAS, SOS1, MEK1, MEK2, RAF1, BRAF, and SHOC2. The c.179G>A (p.G60E) mutation was identified in two patients with typical NS, confirming that NRAS germline mutations are a rare cause of this syndrome. We also screened our cohort of 95 patients with juvenile myelomonocytic leukemia (JMML). Among 17 patients with NRAS-mutated JMML, none had clinical features suggestive of NS. None of the 11 JMML patients for which germline DNA was available had a constitutional NRAS mutation.

Nras overexpression results in granulocytosis, T-cell expansion and early lethality in mice

NRAS is a proto-oncogene involved in numerous myeloid malignancies. Here, we report on a mouse line bearing a single retroviral long terminal repeat inserted into Nras. This genetic modification resulted in an increased level of wild type Nras mRNA giving the possibility of studying the function and activation of wild type NRAS. Flow cytometry was used to show a variable but significant increase of immature myeloid cells in spleen and thymus, and of T-cells in the spleen. At an age of one week, homozygous mice began to retard compared to their wild type and heterozygous littermates. Two weeks after birth, animals started to progressively lose weight and die before weaning. Heterozygous mice showed a moderate increase of T-cells and granulocytes but survived to adulthood and were fertile. In homozygous and heterozygous mice Gfi1 and Gcsf mRNA levels were upregulated, possibly explaining the increment in immature myeloid cells detected in these mice. The short latency period indicates that Nras overexpression alone is sufficient to cause dose-dependent granulocytosis and T-cell expansion.

PRAK suppresses oncogenic ras-induced hematopoietic cancer development by antagonizing the JNK pathway

The p38 mitogen-activated protein kinase (MAPK) pathway regulates multiple physiologic and pathologic processes, including cancer development. PRAK, a p38 substrate protein kinase, has previously been implicated in the suppression of skin carcinogenesis. In the current study, we show that PRAK deletion accelerates hematopoietic cancer development in a mouse model harboring an oncogenic ras allele, Eμ-N-Ras(G12D), specifically expressed in hematopoietic cells. Further investigation reveals that enhanced hematopoietic tumorigenesis by PRAK deficiency is associated with hyperactivation of the c-jun-NH(2)-kinase (JNK) pathway both in vivo and in primary hematopoietic cells isolated from spleens. In primary splenocytes, PRAK deficiency further enhanced oncogenic ras-induced cell proliferation and promoted ras-mediated colony formation on semisolid medium in a JNK-dependent manner. In addition, deletion of PRAK leads to abrogation of ras-induced accumulation of senescence markers. These findings indicate that PRAK suppresses hematopoietic cancer formation in this mouse model by antagonizing oncogenic ras-induced activation of the JNK pathway. Our results suggest that PRAK may function as a tumor suppressor in multiple types of cancers.

An NF-κB pathway-mediated positive feedback loop amplifies Ras activity to pathological levels in mice

Genetic mutations that give rise to active mutant forms of Ras are oncogenic and found in several types of tumor. However, such mutations are not clear biomarkers for disease, since they are frequently detected in healthy individuals. Instead, it has become clear that elevated levels of Ras activity are critical for Ras-induced tumorigenesis. However, the mechanisms underlying the production of pathological levels of Ras activity are unclear. Here, we show that in the presence of oncogenic Ras, inflammatory stimuli initiate a positive feedback loop involving NF-κB that further amplifies Ras activity to pathological levels. Stimulation of Ras signaling by typical inflammatory stimuli was transient and had no long-term sequelae in wild-type mice. In contrast, these stimuli generated prolonged Ras signaling and led to chronic inflammation and precancerous pancreatic lesions (PanINs) in mice expressing physiological levels of oncogenic K-Ras. These effects of inflammatory stimuli were disrupted by deletion of inhibitor of NF-κB kinase 2 (IKK2) or inhibition of Cox-2. Likewise, expression of active IKK2 or Cox-2 or treatment with LPS generated chronic inflammation and PanINs only in mice expressing oncogenic K-Ras. The data support the hypothesis that in the presence of oncogenic Ras, inflammatory stimuli trigger an NF-κB-mediated positive feedback mechanism involving Cox-2 that amplifies Ras activity to pathological levels. Because a large proportion of the adult human population possesses Ras mutations in tissues including colon, pancreas, and lung, disruption of this positive feedback loop may be an important strategy for cancer prevention.

Inhibiting the palmitoylation/depalmitoylation cycle selectively reduces the growth of hematopoietic cells expressing oncogenic Nras

The palmitoylation/depalmitoylation cycle of posttranslational processing is a potential therapeutic target for selectively inhibiting the growth of hematologic cancers with somatic NRAS mutations. To investigate this question at the single-cell level, we constructed murine stem cell virus vectors and assayed the growth of myeloid progenitors. Whereas cells expressing oncogenic N-Ras(G12D) formed cytokine-independent colonies and were hypersensitive to GM-CSF, mutations within the N-Ras hypervariable region induced N-Ras mislocalization and attenuated aberrant progenitor growth. Exposing transduced hematopoietic cells and bone marrow from Nras and Kras mutant mice to the acyl protein thioesterase inhibitor palmostatin B had similar effects on protein localization and colony growth. Importantly, palmostatin B-mediated inhibition was selective for Nras mutant cells, and we mapped this activity to the hypervariable region. These data support the clinical development of depalmitoylation inhibitors as a novel class of rational therapeutics in hematologic malignancies with NRAS mutations.

Distinct requirements of hematopoietic stem cell activity and Nras G12D signaling in different cell types during leukemogenesis

We previously showed that widespread expression of Nras G12D/G12D from its endogenous locus in mice leads to an acute myeloproliferative disease (MPD) with a complete penetrance, whereas bone marrow-specific expression of Nras G12D/G12D in recipient mice did not result in sustained MPD phenotypes but 100% penetrant acute T-cell lymphoblastic leukemia/lymphoma (TALL). Such a phenotypic switch also is seen in the case of endogenous oncogenic Kras. Two possibilities might account for this observation and they are not necessarily mutually exclusive. First, the MPD phenotypes in primary Nras G12D/G12D mice might be a transient phenomenon attributable to microenvironmental factors that do not necessarily imply the potential for long-term maintenance in a hematopoietic-cell autonomous manner. Second, it is likely that MPD phenotypes are maintained by genetically altered hematopoietic stem cells (HSCs). Nras G12D/G12D signaling might substantially alter HSC behaviors (e.g. induce their proliferative exhaustion) so that these HSCs no longer sustain MPD phenotypes to a lethal stage in recipient mice. Here, we show some preliminary results to support the second hypothesis. Our results suggest that different lineages of hematopoietic cells might have differential requirements of HSC activity and Nras G12D signaling during leukemogenesis.