Oncogenic NRAS rapidly and efficiently induces CMML- and AML-like diseases in mice

Leukemic conversion involving RAS mutations of type 1 CALR-mutated primary myelofibrosis in a patient treated for HCV cirrhosis: a case report

Somatic frameshift mutations in exon 9 of calreticulin (CALR) gene are recognized as disease drivers in primary myelofibrosis (PMF), one of the three classical Philadelphia-negative myeloproliferative neoplasms (MPNs). Type 1/type 1-like CALR mutations particularly confer a favorable prognostic and survival advantage in PMF patients. We report an unusual case of PMF incidentally diagnosed in a 68-year-old woman known with hepatitis C virus (HCV) cirrhosis who developed a progressive painful splenomegaly, without anomalies in blood cell counts. While harboring a type 1 CALR mutation, the patient underwent a leukemic transformation in less than 1 year from diagnosis, with a lethal outcome. Analysis of paired DNA samples from chronic and leukemic phases by a targeted next-generation sequencing (NGS) panel and single-nucleotide polymorphism (SNP) microarray revealed that the leukemic clone developed from the CALR-mutated clone through the acquisition of genetic events in the RAS signaling pathway: an increased variant allele frequency of the germline NRAS Y64D mutation present in the chronic phase (via an acquired uniparental disomy of chromosome 1) and gaining NRAS G12D in the blast phase. SNP microarray analysis showed five clinically significant copy number losses at regions 7q22.1, 8q11.1-q11.21, 10p12.1-p11.22, 11p14.1-p11.2, and Xp11.4, revealing a complex karyotype already in the chronic phase. We discuss how additional mutations, detected by NGS, as well as HCV infection and antiviral therapy, might have negatively impacted this type 1 CALR-mutated PMF. We suggest that larger studies are required to determine if more careful monitoring would be needed in MPN patients also carrying HCV and receiving anti-HCV treatment.

RAB27B controls palmitoylation-dependent NRAS trafficking and signaling in myeloid leukemia

RAS mutations are among the most prevalent oncogenic drivers in cancers. RAS proteins propagate signals only when associated with cellular membranes as a consequence of lipid modifications that impact their trafficking. Here, we discovered that RAB27B, a RAB family small GTPase, controlled NRAS palmitoylation and trafficking to the plasma membrane, a localization required for activation. Our proteomic studies revealed RAB27B upregulation in CBL- or JAK2-mutated myeloid malignancies, and its expression correlated with poor prognosis in acute myeloid leukemias (AMLs). RAB27B depletion inhibited the growth of CBL-deficient or NRAS-mutant cell lines. Strikingly, Rab27b deficiency in mice abrogated mutant but not WT NRAS-mediated progenitor cell growth, ERK signaling, and NRAS palmitoylation. Further, Rab27b deficiency significantly reduced myelomonocytic leukemia development in vivo. Mechanistically, RAB27B interacted with ZDHHC9, a palmitoyl acyltransferase that modifies NRAS. By regulating palmitoylation, RAB27B controlled c-RAF/MEK/ERK signaling and affected leukemia development. Importantly, RAB27B depletion in primary human AMLs inhibited oncogenic NRAS signaling and leukemic growth. We further revealed a significant correlation between RAB27B expression and sensitivity to MEK inhibitors in AMLs. Thus, our studies presented a link between RAB proteins and fundamental aspects of RAS posttranslational modification and trafficking, highlighting future therapeutic strategies for RAS-driven cancers.

Understanding the Continuum between High-Risk Myelodysplastic Syndrome and Acute Myeloid Leukemia

Myelodysplastic syndrome (MDS) is a clonal hematopoietic neoplasm characterized by bone marrow dysplasia, failure of hematopoiesis and variable risk of progression to acute myeloid leukemia (AML). Recent large-scale studies have demonstrated that distinct molecular abnormalities detected at earlier stages of MDS alter disease biology and predict progression to AML. Consistently, various studies analyzing these diseases at the single-cell level have identified specific patterns of progression strongly associated with genomic alterations. These pre-clinical results have solidified the conclusion that high-risk MDS and AML arising from MDS or AML with MDS-related changes (AML-MRC) represent a continuum of the same disease. AML-MRC is distinguished from de novo AML by the presence of certain chromosomal abnormalities, such as deletion of 5q, 7/7q, 20q and complex karyotype and somatic mutations, which are also present in MDS and carry crucial prognostic implications. Recent changes in the classification and prognostication of MDS and AML by the International Consensus Classification (ICC) and the World Health Organization (WHO) reflect these advances. Finally, a better understanding of the biology of high-risk MDS and the mechanisms of disease progression have led to the introduction of novel therapeutic approaches, such as the addition of venetoclax to hypomethylating agents and, more recently, triplet therapies and agents targeting specific mutations, including FLT3 and IDH1/2. In this review, we analyze the pre-clinical data supporting that high-risk MDS and AML-MRC share the same genetic abnormalities and represent a continuum, describe the recent changes in the classification of these neoplasms and summarize the advances in the management of patients with these neoplasms.

Mouse Models of Frequently Mutated Genes in Acute Myeloid Leukemia

Acute myeloid leukemia is a clinically and biologically heterogeneous blood cancer with variable prognosis and response to conventional therapies. Comprehensive sequencing enabled the discovery of recurrent mutations and chromosomal aberrations in AML. Mouse models are essential to study the biological function of these genes and to identify relevant drug targets. This comprehensive review describes the evidence currently available from mouse models for the leukemogenic function of mutations in seven functional gene groups: cell signaling genes, epigenetic modifier genes, nucleophosmin 1 (NPM1), transcription factors, tumor suppressors, spliceosome genes, and cohesin complex genes. Additionally, we provide a synergy map of frequently cooperating mutations in AML development and correlate prognosis of these mutations with leukemogenicity in mouse models to better understand the co-dependence of mutations in AML.

Molecular Pathogenesis of Chronic Myelomonocytic Leukemia and Potential Molecular Targets for Treatment Approaches

Numerous examples in oncology have shown that better understanding the pathophysiology of a malignancy may be followed by the development of targeted treatment concepts with higher efficacy and lower toxicity as compared to unspecific treatment. The pathophysiology of chronic myelomonocytic leukemia (CMML) is heterogenous and complex but applying different research technologies have yielded a better and more comprehensive understanding of this disease. At the moment treatment for CMML is largely restricted to the unspecific use of cytotoxic drugs and hypomethylating agents (HMA). Numerous potential molecular targets have been recently detected by preclinical research which may ultimately lead to treatment concepts that will provide meaningful benefits for certain subgroups of patients.

Prognostic impact of RAS-pathway mutations in patients with myelofibrosis

RAS-pathway mutations are recurrent events in myeloid malignancies. However, there is limited data on the significance of RAS-pathway mutations in patients with myelofibrosis (MF). We analyzed next-generation sequencing data of 16 genes, including RAS-pathway genes, from 723 patients with primary and secondary MF across three international centers and evaluated their significance. N/KRAS variants were present in 6% of patients and were typically sub-clonal (median VAF = 20%) relative to other genes variants. RAS variants were associated with advanced MF features including leukocytosis (p = 0.02), high somatic mutation burden (p < 0.01) and the presence of established "molecular high-risk" (MHR) mutations. MF patients with N/KRAS mutations had shorter 3-year overall survival (OS) (34% vs 58%, p < 0.001) and higher incidence of acute myeloid leukemia at 3 years (18% vs 11%, p = 0.03). In a multivariate Cox model, RAS mutations were associated with decreased OS (HR 1.93, p < 0.001). We created a novel score to predict OS incorporating RAS mutations, and it predicted OS across training and validation cohorts. Patients with intermediate risk/high-risk DIPSS with RAS mutations who received ruxolitinib had a nonsignificant longer 2-year OS relative to those who did not receive ruxolitinib. These data demonstrate the importance of identifying RAS mutations in MF patients.

Cooperation of Dnmt3a R878H with Nras G12D promotes leukemogenesis in knock-in mice: a pilot study

BACKGROUND

DNMT3A R882H, a frequent mutation in acute myeloid leukemia (AML), plays a critical role in malignant hematopoiesis. Recent findings suggest that DNMT3A mutant acts as a founder mutation and requires additional genetic events to induce full-blown AML. Here, we investigated the cooperation of mutant DNMT3A and NRAS in leukemogenesis by generating a double knock-in (DKI) mouse model harboring both Dnmt3a R878H and Nras G12D mutations.

METHODS

DKI mice with both Dnmt3a R878H and Nras G12D mutations were generated by crossing Dnmt3a R878H knock-in (KI) mice and Nras G12D KI mice. Routine blood test, flow cytometry analysis and morphological analysis were performed to determine disease phenotype. RNA-sequencing (RNA-seq), RT-PCR and Western blot were carried out to reveal the molecular mechanism.

RESULTS

The DKI mice developed a more aggressive AML with a significantly shortened lifespan and higher percentage of blast cells compared with KI mice expressing Dnmt3a or Nras mutation alone. RNA-seq analysis showed that Dnmt3a and Nras mutations collaboratively caused abnormal expression of a series of genes related to differentiation arrest and growth advantage. Myc transcription factor and its target genes related to proliferation and apoptosis were up-regulated, thus contributing to promote the process of leukemogenesis.

CONCLUSION

This study showed that cooperation of DNMT3A mutation and NRAS mutation could promote the onset of AML by synergistically disturbing the transcriptional profiling with Myc pathway involvement in DKI mice.

Proposed diagnostic criteria for classical chronic myelomonocytic leukemia (CMML), CMML variants and pre-CMML conditions

Chronic myelomonocytic leukemia (CMML) is a myeloid neoplasm characterized by dysplasia, abnormal production and accumulation of monocytic cells and an elevated risk of transforming into acute leukemia. Over the past two decades, our knowledge about the pathogenesis and molecular mechanisms in CMML has increased substantially. In parallel, better diagnostic criteria and therapeutic strategies have been developed. However, many questions remain regarding prognostication and optimal therapy. In addition, there is a need to define potential pre-phases of CMML and special CMML variants, and to separate these entities from each other and from conditions mimicking CMML. To address these unmet needs, an international consensus group met in a Working Conference in August 2018 and discussed open questions and issues around CMML, its variants, and pre-CMML conditions. The outcomes of this meeting are summarized herein and include diag nostic criteria and a proposed classification of pre-CMML conditions as well as refined minimal diagnostic criteria for classical CMML and special CMML variants, including oligomonocytic CMML and CMML associated with systemic mastocytosis. Moreover, we propose diagnostic standards and tools to distinguish between 'normal', pre-CMML and CMML entities. These criteria and standards should facilitate diagnostic and prognostic evaluations in daily practice and clinical studies in applied hematology.

Subtype-specific regulatory network rewiring in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease caused by a variety of alterations in transcription factors, epigenetic regulators and signaling molecules. To determine how different mutant regulators establish AML subtype-specific transcriptional networks, we performed a comprehensive global analysis of cis-regulatory element activity and interaction, transcription factor occupancy and gene expression patterns in purified leukemic blast cells. Here, we focused on specific subgroups of subjects carrying mutations in genes encoding transcription factors (RUNX1, CEBPα), signaling molecules (FTL3-ITD, RAS) and the nuclear protein NPM1). Integrated analysis of these data demonstrates that each mutant regulator establishes a specific transcriptional and signaling network unrelated to that seen in normal cells, sustaining the expression of unique sets of genes required for AML growth and maintenance.

Mechanistic and Preclinical Insights from Mouse Models of Hematologic Cancer Characterized by Hyperactive Ras

RAS genes are mutated in 5%-40% of a spectrum of myeloid and lymphoid cancers with NRAS affected 2-3 times more often than KRAS Genomic analysis indicates that RAS mutations generally occur as secondary events in leukemogenesis, but are integral to the disease phenotype. The tractable nature of the hematopoietic system has facilitated generating accurate mouse models of hematologic malignancies characterized by hyperactive Ras signaling. These strains provide robust platforms for addressing how oncogenic Ras expression perturbs proliferation, differentiation, and self-renewal programs in stem and progenitor cell populations, for testing potential therapies, and for investigating mechanisms of drug response and resistance. This review summarizes recent insights from key studies in mouse models of hematologic cancer that are broadly relevant for understanding Ras biology and for ongoing efforts to implement rational therapeutic strategies for cancers with oncogenic RAS mutations.

MEK1 is required for the development of NRAS-driven leukemia

The dual-specificity kinases MEK1 and MEK2 act downstream of RAS/RAF to induce ERK activation, which is generally considered protumorigenic. Activating MEK mutations have not been discovered in leukemia, in which pathway activation is caused by mutations in upstream components such as RAS or Flt3. The anti-leukemic potential of MEK inhibitors is being tested in clinical trials; however, downregulation of MEK1 promotes Eμ-Myc-driven lymphomagenesis and MEK1 ablation induces myeloproliferative disease in mice, raising the concern that MEK inhibitors may be inefficient or counterproductive in this context. We investigated the role of MEK1 in the proliferation of human leukemic cell lines and in retroviral models of leukemia. Our data show that MEK1 suppression via RNA interference and genomic engineering does not affect the proliferation of human leukemic cell lines in culture; similarly, MEK1 ablation does not impact the development of MYC-driven leukemia in vivo. In contrast, MEK1 ablation significantly reduces tumorigenesis driven by Nras alone or in combination with Myc. Thus, while MEK1 restricts proliferation and tumorigenesis in some cellular and genetic contexts, it cannot be considered a tumor suppressor in the context of leukemogenesis. On the contrary, its role in NRAS-driven leukemogenesis advocates the use of MEK inhibitors, particularly in combination with PI3K/AKT inhibitors, in hematopoietic malignancies involving RAS activation.

Targeting the RAS/MAPK pathway with miR-181a in acute myeloid leukemia

Deregulation of microRNAs' expression frequently occurs in acute myeloid leukemia (AML). Lower miR-181a expression is associated with worse outcomes, but the exact mechanisms by which miR-181a mediates this effect remain elusive. Aberrant activation of the RAS pathway contributes to myeloid leukemogenesis. Here, we report that miR-181a directly binds to 3′-untranslated regions (UTRs); downregulates KRAS, NRAS and MAPK1; and decreases AML growth. The delivery of miR-181a mimics to target AML cells using transferrin-targeting lipopolyplex nanoparticles (NP) increased mature miR-181a; downregulated KRAS, NRAS and MAPK1; and resulted in decreased phosphorylation of the downstream RAS effectors. NP-mediated upregulation of miR-181a led to reduced proliferation, impaired colony formation and increased sensitivity to chemotherapy. Ectopic expression of KRAS, NRAS and MAPK1 attenuated the anti-leukemic activity of miR-181a mimics, thereby validating the relevance of the deregulated miR-181a-RAS network in AML. Finally, treatment with miR-181a-NP in a murine AML model resulted in longer survival compared to mice treated with scramble-NP control. These data support that targeting the RAS-MAPK-pathway by miR-181a mimics represents a novel promising therapeutic approach for AML and possibly for other RAS-driven cancers.

Turning the tide in myelodysplastic/myeloproliferative neoplasms

Myelodysplastic syndromes/myeloproliferative neoplasms (MDS/MPN) are aggressive myeloid malignancies recognized as a distinct category owing to their unique combination of dysplastic and proliferative features. Although current classification schemes still emphasize morphology and exclusionary criteria, disease-defining somatic mutations and/or germline predisposition alleles are increasingly incorporated into diagnostic algorithms. The developing picture suggests that phenotypes are driven mostly by epigenetic mechanisms that reflect a complex interplay between genotype, physiological processes such as ageing and interactions between malignant haematopoietic cells and the stromal microenvironment of the bone marrow. Despite the rapid accumulation of genetic knowledge, therapies have remained nonspecific and largely inefficient. In this Review, we discuss the pathogenesis of MDS/MPN, focusing on the relationship between genotype and phenotype and the molecular underpinnings of epigenetic dysregulation. Starting with the limitations of current therapies, we also explore how the available mechanistic data may be harnessed to inform strategies to develop rational and more effective treatments, and which gaps in our knowledge need to be filled to translate biological understanding into clinical progress.

BCR/ABL can promote CD19+ cell growth but not render them long-term stemness

BACKGROUND

Cancer stem cells are a subpopulation of malignant cells that have the capacity of both self-renewal and reconstitution of the cancer. Eradication of cancer stem cells is crucial for curing the malignant disease. Previous studies in hematopoietic malignancies showed that leukemia stem cells (LSCs) in chronic myelogenous leukemia (CML) chronic phase are originated from a hematopoietic stem cell (HSC), while LSCs in acute myeloid leukemia (AML) can either be derived from HSCs or be transformed from myeloid progenitors. But in B-cell acute lymphoblastic leukemia (B-ALL), the origin of leukemia stem cells is not clear. In this study, we tested whether BCR/ABL could transform B-lineage committed CD19⁺ cells to LSCs.

METHODS

The B-cell lymphoblastic leukemia mouse model was generated by transplanting BCR/ABL-containing retrovirus infected bone marrow (BM) cells or CD19⁺ cells into recipient mice. In the secondary or tertiary transplantation experiment, the GFP⁺ cells (leukemic cells) were isolated from primary or secondary B-ALL mice. In addition, the frequency of leukemia stem cells was determined by limited dilution assay.

RESULTS

We found that transducing BCR/ABL in CD19⁺ cells can promote their colony formation in vitro and induce B-ALL like disease in vivo. However, only BCR/ABL transduced whole BM cells can be transplanted multiple times in recipient mice, and the frequency of long-term LSCs from the latter ranges from 1/135 to 1/629.

CONCLUSIONS

These studies suggest that BCR/ABL is unable to confer the long-term stemness to committed B-lymphoid progenitors and imply that CD19 chimeric antigen receptor (CAR) modified T cell therapy may not be effective in eradicating LSCs in BCR/ABL⁺ B-ALL.

Roles of palmitoylation and the KIKK membrane-targeting motif in leukemogenesis by oncogenic KRAS4A

Background

We have previously shown that palmitoylation is essential for NRAS leukemogenesis, suggesting that targeting RAS palmitoylation may be an effective therapy for NRAS-related cancers. For KRAS-driven cancer, although much research has been focused on the KRAS4B splice variant, which does not undergo palmitoylation, KRAS4A has recently been shown to play an essential role in the development of carcinogen-induced lung cancer in mice and to be widely expressed in human cancers. However, the role of palmitoylation in KRAS4A tumorigenesis is not clear.

Methods

The expression of KRAS4A in KRAS-mutated leukemia cell lines and acute myeloid leukemia (AML) cells were checked using western blotting and reverse transcriptions-quantitative polymerase chain reaction (RT-qPCR) analysis, respectively. The leukemogenic potentials of oncogenic KRAS4A and its palmitoylation-defective mutants were examined by a mouse bone marrow transduction and transplantation model and the in vitro transformation assays. The activation of the RAS downstream signaling pathways and the membrane localizations of the KRAS4A and its mutants were analyzed via western blot analysis and confocal microscopy, respectively.

Results

We show here that KRAS4A is expressed in human leukemia cell lines and in AML cells harboring KRAS mutations and that mutation at the palmitoylation site of oncogenic KRAS4A significantly abrogates its leukemogenic potential. However, unlike NRAS, palmitoylation-defective KRAS4A still induces leukemia in mice, albeit with a much longer latency. Using NRAS/KRAS4A chimeric constructs, we found that the KIKK motif of KRAS4A contributes to the transforming activity of KRAS4A. Mutations at both palmitoylation site and the KIKK motif abolish the ability of oncogenic KRAS4A to induce leukemia in mice.

Conclusions

Our studies suggest that therapies targeting RAS palmitoylation may also be effective in treating KRAS4A associated malignancies and that interfering the KIKK membrane-targeting motif would enhance the therapeutic effectiveness.

Oncogenic NRAS hyper-activates multiple pathways in human cord blood stem/progenitor cells and promotes myelomonocytic proliferation in vivo

Oncogenic NRAS mutations are prevalent in human myeloid leukemia, especially in chronic myelomonocytic leukemia (CMML). NrasG12D mutation at its endogenous locus in murine hematopoietic stem cells (HSCs) leads to CMML and acute T-cell lymphoblastic lymphoma/leukemia in a dose-dependent manner. Hyper-activated MAPK and STAT5 pathways by oncogenic Nras contribute to the leukemogenesis in vivo. However, it is unclear whether these conclusions remain true in a more human relevant model. Here, we evaluated the effects of NRASG12D on human hematopoiesis and leukemogenesis in vitro and in vivo by ectopically expressing NRASG12D in human cord blood stem/progenitor cells (hSPCs). NRASG12D expressing hSPCs preferentially differentiated into myelomonocytic lineage cells, demonstrated by forming more colony forming unit-macrophages than control hSPCs in cultures. Transplantation of NRASG12D expressing hSPCs initiated myeloproliferative neoplasm in immune deficiency mice. All the recipient mice died of myeloid tumor burdens in spleens and bone marrows and none developed lymphoid leukemia. Phospho-flow analysis of CD34(+) CD38(-) hSPCs confirmed that NRASG12D hyper-activated MAPK, AKT and STAT5 pathways. Our study provides the strong evidence that NRASG12D mutation mainly targets monocytic lineage cells and leads to myelomonocytic proliferation in vivo in a highly human relevant context.

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.

The differential palmitoylation states of N-Ras and H-Ras determine their distinct Golgi subcompartment localizations

Despite a high degree of structural homology and shared exchange factors, effectors and GTPase activating proteins, a large body of evidence suggests functional heterogeneity among Ras isoforms. One aspect of Ras biology that may explain this heterogeneity is the differential subcellular localizations driven by the C-terminal hypervariable regions of Ras proteins. Spatial heterogeneity has been documented at the level of organelles: palmitoylated Ras isoforms (H-Ras and N-Ras) localize on the Golgi apparatus whereas K-Ras4B does not. We tested the hypothesis that spatial heterogeneity also exists at the sub-organelle level by studying the localization of differentially palmitoylated Ras isoforms within the Golgi apparatus. Using confocal, live-cell fluorescent imaging and immunogold electron microscopy we found that, whereas the doubly palmitoylated H-Ras is distributed throughout the Golgi stacks, the singly palmitoylated N-Ras is polarized with a relative paucity of expression on the trans Golgi. Using palmitoylation mutants, we show that the different sub-Golgi distributions of the Ras proteins are a consequence of their differential degree of palmitoylation. Thus, the acylation state of Ras proteins controls not only their distribution between the Golgi apparatus and the plasma membrane, but also their distribution within the Golgi stacks.

What's different about atypical CML and chronic neutrophilic leukemia?

Atypical chronic myeloid leukemia (aCML) and chronic neutrophilic leukemia (CNL) are rare myeloid neoplasms defined largely by morphologic criteria. The discovery of CSF3R mutations in aCML and CNL have prompted a more comprehensive genetic profiling of these disorders. These studies have revealed aCML to be a genetically more heterogeneous disease than CNL, however, several groups have reported that SETBP1 and ASXL1 mutations occur at a high frequency and carry prognostic value in both diseases. We also report a novel finding-our study reveals a high frequency of U2AF1 mutations at codon Q157 associated with CSF3R mutant myeloid neoplasms. Collectively, these findings will refine the WHO diagnostic criteria of aCML and CNL and help us understand the genetic lesions and dysregulated signaling pathways contributing to disease development. Novel therapies that emerge from these genetic findings will need to be investigated in the setting of a clinical trial to determine the safety and efficacy of targeting various oncogenic drivers, such as JAK1/2 inhibition in CSF3R-T618I-positive aCML and CNL. In summary, recent advances in the genetic characterization of CNL and aCML are instrumental toward the development of new lines of therapy for these rare leukemias that lack an established standard of care and are historically associated with a poor prognosis.

Natural history of chronic myelomonocytic leukemia: gene sequencing identifies multiple clonal molecular abnormalities associated with rapid progression to acute myeloid leukemia

Key Clinical Message

Gene panel sequencing in a CMML patient without any detectable genetic abnormality by conventional genetic studies identified four concurrent somatic mutations in three genes. Gene panel mutation analysis is a rapidly emerging clinical tool to demonstrate the clonality in hematologic malignancies, and to identify the potential targets for therapy.

Translational hematology

Translational research is scientific research that helps to make findings from basic science useful for practical applications in the clinic. The successful use of a drug that interferes with the specific molecular pathophysiology of cancer remains the ultimate vision in cancer medicine. Translational research is a multistep process including the discovery of a cytogenetic/molecular aberration as well as the demonstration of its pathophysiological relevance and its druggability by in vitro experiments and in vivo animal models. Information obtained from preclinical research paves the way for clinical trials in which a drug of interest is developed until its clinical application. Modern pathophysiology-oriented anticancer drugs that have been developed by translational research are available for clinical applications since the beginning of this millennium. By using these drugs higher efficacy and lower toxicity could be achieved as compared with previous treatments. In this article, we will present some of the most prominent examples of this translational approach.

Upregulation of IGF1R by mutant RAS in leukemia and potentiation of RAS signaling inhibitors by small-molecule inhibition of IGF1R

PURPOSE

Activating mutations in the RAS oncogene occur frequently in human leukemias. Direct targeting of RAS has proven to be challenging, although targeting of downstream RAS mediators, such as MEK, is currently being tested clinically. Given the complexity of RAS signaling, it is likely that combinations of targeted agents will be more effective than single agents.

EXPERIMENTAL DESIGN

A chemical screen using RAS-dependent leukemia cells was developed to identify compounds with unanticipated activity in the presence of an MEK inhibitor and led to identification of inhibitors of IGF1R. Results were validated using cell-based proliferation, apoptosis, cell-cycle, and gene knockdown assays; immunoprecipitation and immunoblotting; and a noninvasive in vivo bioluminescence model of acute myeloid leukemia (AML).

RESULTS

Mechanistically, IGF1R protein expression/activity was substantially increased in mutant RAS-expressing cells, and suppression of RAS led to decreases in IGF1R. Synergy between MEK and IGF1R inhibitors correlated with induction of apoptosis, inhibition of cell-cycle progression, and decreased phospho-S6 and phospho-4E-BP1. In vivo, NSG mice tail veins injected with OCI-AML3-luc+ cells showed significantly lower tumor burden following 1 week of daily oral administration of 50 mg/kg NVP-AEW541 (IGF1R inhibitor) combined with 25 mg/kg AZD6244 (MEK inhibitor), as compared with mice treated with either agent alone. Drug combination effects observed in cell-based assays were generalized to additional mutant RAS-positive neoplasms.

CONCLUSIONS

The finding that downstream inhibitors of RAS signaling and IGF1R inhibitors have synergistic activity warrants further clinical investigation of IGF1R and RAS signaling inhibition as a potential treatment strategy for RAS-driven malignancies.

Differential in vivo tumorigenicity of diverse KRAS mutations in vertebrate pancreas: A comprehensive survey

Somatic activation of the KRAS proto-oncogene is evident in almost all pancreatic cancers, and appears to represent an initiating event. These mutations occur primarily at codon 12 and less frequently at codons 13 and 61. While some studies have suggested that different KRAS mutations may have variable oncogenic properties, to date there has been no comprehensive functional comparison of multiple KRAS mutations in an in vivo vertebrate tumorigenesis system. We generated a Gal4/UAS-based zebrafish model of pancreatic tumorigenesis in which the pancreatic expression of UAS-regulated oncogenes is driven by a ptf1a:Gal4-VP16 driver line. This system allowed us to rapidly compare the ability of 12 different KRAS mutations (G12A, G12C, G12D, G12F, G12R, G12S, G12V, G13C, G13D, Q61L, Q61R, and A146T) to drive pancreatic tumorigenesis in vivo. Among fish injected with one of five KRAS mutations reported in other tumor types but not in human pancreatic cancer, 2/79 (0.25%) developed pancreatic tumors, with both tumors arising in fish injected with A146T. In contrast, among fish injected with one of seven KRAS mutations known to occur in human pancreatic cancer, 22/106 (20.8%) developed pancreatic cancer. All eight tumorigenic KRAS mutations were associated with downstream MAPK/ERK pathway activation in preneoplastic pancreatic epithelium, while non-tumorigenic mutations were not. These results suggest that the spectrum of KRAS mutations observed in human pancreatic cancer reflects selection based upon variable tumorigenic capacities, including the ability to activate MAPK/ERK signaling.

Gene mutations differently impact the prognosis of the myelodysplastic and myeloproliferative classes of chronic myelomonocytic leukemia

Initially classified in the myelodysplastic syndromes (MDSs), chronic myelomonocytic leukemia (CMML) is currently considered as a MDS/myeloproliferative neoplasm. Two classes-myelodysplastic and myeloproliferative-have been distinguished upon the level of the white blood cell count (threshold 13 G/L). We analyzed mutations in 19 genes reported in CMML to determine if and how these mutations impacted the respective prognosis of the two classes. We defined four major mutated pathways (DNA methylation, ASXL1, splicing, and signaling) and determined their prognostic impact. The number of mutated pathways impacted overall survival in the myelodysplastic class but not in the myeloproliferative class. The myeloproliferative class had a worse prognosis than the myelodysplastic class and was impacted by RUNX1 mutations only. Our results argue for a reclassification of CMML based on the myelodysplastic/myeloproliferative status.

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.

The role of RAS effectors in BCR/ABL induced chronic myelogenous leukemia

BCR/ABL is the causative agent of chronic myelogenous leukemia (CML). Through structure/function analysis, several protein motifs have been determined to be important for the development of leukemogenesis. Tyrosine177 of BCR is a Grb2 binding site required for BCR/ABL-induced CML in mice. In the current study, we use a mouse bone marrow transduction/transplantation system to demonstrate that addition of oncogenic NRAS (NRASG12D) to a vector containing a BCR/ABL(Y177F) mutant "rescues" the CML phenotype rapidly and efficiently. To further narrow down the pathways downstream of RAS that are responsible for this rescue effect, we utilize well-characterized RAS effector loop mutants and determine that the RAL pathway is important for rapid induction of CML. Inhibition of this pathway by a dominant negative RAL is capable of delaying disease progression. Results from the present study support the notion of RAL inhibition as a potential therapy for BCR/ABL-induced CML.

Toward a NOTCH1/FBXW7/RAS/PTEN-based oncogenetic risk classification of adult T-cell acute lymphoblastic leukemia: a Group for Research in Adult Acute Lymphoblastic Leukemia study

PURPOSE

The Group for Research in Adult Acute Lymphoblastic Leukemia (GRAALL) recently reported a significantly better outcome in T-cell acute lymphoblastic leukemia (T-ALL) harboring NOTCH1 and/or FBXW7 (N/F) mutations compared with unmutated T-ALL. Despite this, one third of patients with N/F-mutated T-ALL experienced relapse.

PATIENTS AND METHODS

In a series of 212 adult T-ALLs included in the multicenter randomized GRAALL-2003 and -2005 trials, we searched for additional N/K-RAS mutations and PTEN defects (mutations and gene deletion).

RESULTS

N/F mutations were identified in 143 (67%) of 212 patients, and lack of N/F mutation was confirmed to be associated with a poor prognosis. K-RAS, N-RAS, and PTEN mutations/deletions were identified in three (1.6%) of 191, 17 (8.9%) of 191, and 21 (12%) of 175 patients, respectively. The favorable prognostic significance of N/F mutations was restricted to patients without RAS/PTEN abnormalities. These observations led us to propose a new T-ALL oncogenetic classifier defining low-risk patients as those with N/F mutation but no RAS/PTEN mutation (97 of 189 patients; 51%) and all other patients (49%; including 13% with N/F and RAS/PTEN mutations) as high-risk patients. In multivariable analysis, this oncogenetic classifier remained the only significant prognostic covariate (event-free survival: hazard ratio [HR], 3.2; 95% CI, 1.9 to 5.15; P < .001; and overall survival: HR, 3.2; 95% CI, 1.9 to 5.6; P < .001).

CONCLUSION

These data demonstrate that the presence of N/F mutations in the absence of RAS or PTEN abnormalities predicts good outcome in almost 50% of adult T-ALL. Conversely, the absence of N/F or presence of RAS/PTEN alterations identifies the remaining cohort of patients with poor prognosis.

TEL (ETV6)-AML1 (RUNX1) initiates self-renewing fetal pro-B cells in association with a transcriptional program shared with embryonic stem cells in mice

The initial steps involved in the pathogenesis of acute leukemia are poorly understood. The TEL-AML1 fusion gene usually arises before birth, producing a persistent and covert preleukemic clone that may convert to precursor B cell leukemia following the accumulation of secondary genetic "hits." Here, we show that TEL-AML1 can induce persistent self-renewing pro-B cells in mice. TEL-AML1+ cells nevertheless differentiate terminally in the long term, providing a "window" period that may allow secondary genetic hits to accumulate and lead to leukemia. TEL-AML1-mediated self-renewal is associated with a transcriptional program shared with embryonic stem cells (ESCs), within which Mybl2, Tgif2, Pim2, and Hmgb3 are critical and sufficient components to establish self-renewing pro-B cells. We further show that TEL-AML1 increases the number of leukemia-initiating cells that are generated in collaboration with additional genetic hits, thus providing an overall basis for the development of novel therapeutic and preventive measures targeting the TEL-AML1-associated transcriptional program.

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.

Prognostic impact of RAS mutations in patients with myelodysplastic syndrome

RAS is an oncogene frequently mutated in human cancer. RAS mutations have been reported in 10-15% of cases of acute myeloid leukemia (AML) but they appear to be less frequent among patients with myelodysplastic syndrome (MDS). The impact of RAS mutations in patients with MDS is unclear. We conducted a retrospective study in 1,067 patients with newly diagnosed MDS for whom RAS mutational analysis was available. Overall, 4% of patients carried mutant RAS alleles. Notably, FLT3 mutations, which were found in 2% of patients, were mutually exclusive with RAS mutations. Patients with RAS mutations had a higher white blood cell count as well as bone marrow blasts compared with patients carrying wild-type RAS. However, no differences were observed between both groups regarding the risk of AML transformation (9% vs. 7%) and overall survival (395 days vs. 500 days, P = 0.057). In summary, RAS mutations are infrequent in patients with MDS and do not appear to negatively impact their outcome.

Engineering mouse models with myelodysplastic syndrome human candidate genes; how relevant are they?

Myelodysplastic syndromes represent particularly challenging hematologic malignancies that arise from a large spectrum of genetic events resulting in a disease characterized by a range of different presentations and outcomes. Despite efforts to classify and identify the key genetic events, little improvement has been made in therapies that will increase patient survival. Animal models represent powerful tools to model and study human diseases and are useful pre-clinical platforms. In addition to enforced expression of candidate oncogenes, gene inactivation has allowed the consequences of the genetic effects of human myelodysplastic syndrome to be studied in mice. This review aims to examine the animal models expressing myelodysplastic syndrome-associated genes that are currently available and to highlight the most appropriate model to phenocopy myelodysplastic syndrome disease and its risk of transformation to acute myelogenous leukemia.

Heterogeneity of molecular markers in chronic myelomonocytic leukemia: a disease associated with several gene alterations

The relatively homogenous clinical features and poor prognosis of chronic myelomonocytic leukemia (CMML) are associated with a molecular heterogeneity, with various mutations impacting several convergent pathways. Due to the restricted understanding of the mechanism involved in leukemogenesis, CMML still appears as a diagnostic and therapeutic undertaking, and poor prognosis of leukemia. Contrary to chronic myelogenous leukemia, BCR-ABL1-positive, cytogenetic, and molecular abnormalities of CMML are not specific and not pathognomonic, confirming the different levels of heterogeneity of this disease. Various mutations can be associated with a common phenotype not distinct at the clinical level, further demonstrating that molecular probings are needed for choosing individual targeted therapies.

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.

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.

Selection for Evi1 activation in myelomonocytic leukemia induced by hyperactive signaling through wild-type NRas

Activation of NRas signaling is frequently found in human myeloid leukemia and can be induced by activating mutations as well as by mutations in receptors or signaling molecules upstream of NRas. To study NRas-induced leukemogenesis, we retrovirally overexpressed wild-type NRas in a murine bone marrow transplantation (BMT) model in C57BL/6J mice. Overexpression of wild-type NRas caused myelomonocytic leukemias ∼3 months after BMT in the majority of mice. A subset of mice (30%) developed malignant histiocytosis similar to mice that received mutationally activated NRas(G12D)-expressing bone marrow. Aberrant Ras signaling was demonstrated in cells expressing mutationally active or wild-type NRas, as increased activation of Erk and Akt was observed in both models. However, more NRas(G12D) were found to be in the activated, GTP-bound state in comparison with wild-type NRas. Consistent with observations reported for primary human myelomonocytic leukemia cells, Stat5 activation was also detected in murine leukemic cells. Furthermore, clonal evolution was detected in NRas wild-type-induced leukemias, including expansion of clones containing activating vector insertions in known oncogenes, such as Evi1 and Prdm16. In vitro cooperation of NRas and Evi1 improved long-term expansion of primary murine bone marrow cells. Evi1-positive cells upregulated Bcl-2 and may, therefore, provide anti-apoptotic signals that collaborate with the NRas-induced proliferative effects. As activation of Evi1 has been shown to coincide with NRAS mutations in human acute myeloid leukemia, our murine model recapitulates crucial events in human leukemogenesis.

Janus kinase deregulation in leukemia and lymphoma

Genetic alterations affecting members of the Janus kinase (JAK) family have been discovered in a wide array of cancers and are particularly prominent in hematological malignancies. In this review, we focus on the role of such lesions in both myeloid and lymphoid tumors. Oncogenic JAK molecules can activate a myriad of canonical downstream signaling pathways as well as directly interact with chromatin in noncanonical processes, the interplay of which results in a plethora of diverse biological consequences. Deciphering these complexities is shedding unexpected light on fundamental cellular mechanisms and will also be important for improved diagnosis, identification of new therapeutic targets, and the development of stratified approaches to therapy.

Aberrant expression of RasGRP1 cooperates with gain-of-function NOTCH1 mutations in T-cell leukemogenesis

Ras guanyl nucleotide-releasing proteins (RasGRPs) are activators of Ras. Previous studies have indicated the possible involvement of RasGRP1 and RasGRP4 in leukemogenesis. Here, the predominant role of RasGRP1 in T-cell leukemogenesis is clarified. Notably, increased expression of RasGRP1, but not RasGRP4, was frequently observed in human T-cell malignancies. In a mouse bone marrow transplantation model, RasGRP1 exclusively induced T-cell acute lymphoblastic leukemia/lymphoma (T-ALL) after a shorter latency when compared with RasGRP4. Accordingly, Ba/F3 cells transduced with RasGRP1 survived longer under growth factor withdrawal or phorbol ester stimulation than those transduced with RasGRP4, presumably due to the efficient activation of Ras. Intriguingly, NOTCH1 mutations resulting in a gain of function were found in 77% of the RasGRP1-mediated mouse T-ALL samples. In addition, gain-of-function NOTCH1 mutation was found in human T-cell malignancy with elevated expression of RasGRP1. Importantly, RasGRP1 and NOTCH1 signaling cooperated in the progression of T-ALL in the murine model. The leukemogenic advantage of RasGRP1 over RasGRP4 was attenuated by the disruption of a protein kinase C phosphorylation site (RasGRP1(Thr184)) not present on RasGRP4. In conclusion, cooperation between aberrant expression of RasGRP1, a strong activator of Ras, and secondary gain-of-function mutations of NOTCH1 have an important role in T-cell leukemogenesis.

Functional specificity of ras isoforms: so similar but so different

H-ras, N-ras, and K-ras are canonical ras gene family members frequently activated by point mutation in human cancers and coding for 4 different, highly related protein isoforms (H-Ras, N-Ras, K-Ras4A, and K-Ras4B). Their expression is nearly ubiquitous and broadly conserved across eukaryotic species, although there are quantitative and qualitative differences of expression depending on the tissue and/or developmental stage under consideration. Extensive functional studies have determined during the last quarter century that these Ras gene products are critical components of signaling pathways that control eukaryotic cell proliferation, survival, and differentiation. However, because of their homology and frequent coexpression in various cellular contexts, it remained unclear whether the different Ras proteins play specific or overlapping functional roles in physiological and pathological processes. Initially, their high degree of sequence homology and the observation that all Ras isoforms share common sets of downstream effectors and upstream activators suggested that they were mostly redundant functionally. In contrast, the notion of functional specificity for each of the different Ras isoforms is supported at present by an increasing body of experimental observations, including 1) the fact that different ras isoforms are preferentially mutated in specific types of tumors or developmental disorders; 2) the different transforming potential of transfected ras genes in different cell contexts; 3) the distinct sensitivities exhibited by the various Ras family members for modulation by different GAPs or GEFs; 4) the demonstration that different Ras isoforms follow distinct intracellular processing pathways and localize to different membrane microdomains or subcellular compartments; 5) the different phenotypes displayed by genetically modified animal strains for each of the 3 ras loci; and 6) the specific transcriptional networks controlled by each isoform in different cellular settings.

Role of Ras/Raf/MEK/ERK signaling in physiological hematopoiesis and leukemia development

Recent research on hematological malignancies has shown that malignant cells often co-opt physiological pathways to promote their growth and development. Bone marrow homeostasis requires a fine balance between cellular differentiation and self-renewal; cell survival and apoptosis; and cellular proliferation and senescence. The Ras/Raf/MEK/ERK pathway has been shown to be important in regulating these biological functions. Moreover, the Ras/Raf/MEK/ERK pathway has been estimated to be mutated in 30% of all cancers, thus making it the focus of many scientific studies which have lead to a deeper understanding of cancer development and help to elucidate potential weaknesses that can be targeted by pharmacological agents [1]. In this review, we specifically focus on the role of this pathway in physiological hematopoiesis and how augmentation of the pathway may lead to hematopoietic malignancies. We also discuss the challenges and success of targeting this pathway.

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

Both monoallelic and biallelic oncogenic NRAS mutations are identified in human leukemias, suggesting a dose-dependent role of oncogenic NRAS in leukemogenesis. Here, we use a hypomorphic oncogenic Nras allele and a normal oncogenic Nras allele (Nras G12D(hypo) and Nras G12D, respectively) to create a gene dose gradient ranging from 25% to 200% of endogenous Nras G12D/+. Mice expressing Nras G12D(hypo)/G12D(hypo) develop normally and are tumor-free, whereas early embryonic expression of Nras G12D/+ is lethal. Somatic expression of Nras G12D/G12D but not Nras G12D/+ leads to hyperactivation of ERK, excessive proliferation of myeloid progenitors, and consequently an acute myeloproliferative disease. Using a bone marrow transplant model, we previously showed that ∼ 95% of animals receiving Nras G12D/+ bone marrow cells develop chronic myelomonocytic leukemia (CMML), while ∼ 8% of recipients develop acute T-cell lymphoblastic leukemia/lymphoma [TALL] (TALL-het). Here we demonstrate that 100% of recipients transplanted with Nras G12D/G12D bone marrow cells develop TALL (TALL-homo). Although both TALL-het and -homo tumors acquire Notch1 mutations and are sensitive to a γ-secretase inhibitor, endogenous Nras G12D/+ signaling promotes TALL through distinct genetic mechanism(s) from Nras G12D/G12D. Our data indicate that the tumor transformation potential of endogenous oncogenic Nras is both dose- and cell type-dependent.

C-KIT mutation cooperates with full-length AML1-ETO to induce acute myeloid leukemia in mice

The full-length AML1-ETO (AE) fusion gene resulting from t(8;21)(q22;q22) in human acute myeloid leukemia (AML) is not sufficient to induce leukemia in animals, suggesting that additional mutations are required for leukemogenesis. We and others have identified activating mutations of C-KIT in nearly half of patients with t(8;21) AML. To test the hypothesis that activating C-KIT mutations cooperate with AE to cause overt AML, we generated a murine transduction and transplantation model with both mutated C-KIT and AE. To overcome the intracellular transport block of human C-KIT in murine cells, we engineered hybrid C-KIT (HyC-KIT) by fusing the extracellular and transmembrane domains of the murine c-Kit in-frame to the intracellular signaling domain of human C-KIT. We showed that tyrosine kinase domain mutants HyC-KIT N822K and D816V, as well as juxtamembrane mutants HyC-KIT 571+14 and 557-558Del, could transform murine 32D cells to cytokine-independent growth. The protein tyrosine kinase inhibitor dasatinib inhibited the proliferation of 32D cells expressing these C-KIT mutants, with potency in the low nanomolar range. In mice, HyC-KIT N822K induced a myeloproliferative disease, whereas HyC-KIT 571+14 induces both myeloproliferative disease and lymphocytic leukemia. Interestingly, coexpression of AE and HyC-KIT N822K led to fatal AML. Our data have further enriched the two-hit model that abnormalities of both transcription factor and membrane/cytosolic signaling molecule are required in AML pathogenesis. Furthermore, dasatinib prolonged lifespan of mice bearing AE and HyC-KIT N822K-coexpressing leukemic cells and exerted synergic effects while combined with cytarabine, thus providing a potential therapeutic for t(8;21) leukemia.

Chromatin mechanisms regulating gene expression in health and disease

It is now well established that the interplay of sequence-specific DNA binding proteins with chromatin components and the subsequent expression of differential genetic programs is the major determinant of developmental decisions. The last years have seen an explosion of basic research that has significantly enhanced our understanding of the basic principles of gene expression control. While many questions are still open, we are now at the stage where we can exploit this knowledge to address questions of how deregulated gene expression and aberrant chromatin programming contributes to disease processes. This chapter will give a basic introduction into the principles of epigenetics and the determinants of chromatin structure and will discuss the molecular mechanisms of aberrant gene regulation in blood cell diseases, such as inflammation and leukemia.

Hematopoiesis and leukemogenesis in mice expressing oncogenic NrasG12D from the endogenous locus

NRAS is frequently mutated in hematologic malignancies. We generated Mx1-Cre, Lox-STOP-Lox (LSL)-Nras(G12D) mice to comprehensively analyze the phenotypic, cellular, and biochemical consequences of endogenous oncogenic Nras expression in hematopoietic cells. Here we show that Mx1-Cre, LSL-Nras(G12D) mice develop an indolent myeloproliferative disorder but ultimately die of a diverse spectrum of hematologic cancers. Expressing mutant Nras in hematopoietic tissues alters the distribution of hematopoietic stem and progenitor cell populations, and Nras mutant progenitors show distinct responses to cytokine growth factors. Injecting Mx1-Cre, LSL-Nras(G12D) mice with the MOL4070LTR retrovirus causes acute myeloid leukemia that faithfully recapitulates many aspects of human NRAS-associated leukemias, including cooperation with deregulated Evi1 expression. The disease phenotype in Mx1-Cre, LSL-Nras(G12D) mice is attenuated compared with Mx1-Cre, LSL-Kras(G12D) mice, which die of aggressive myeloproliferative disorder by 4 months of age. We found that endogenous Kras(G12D) expression results in markedly elevated Ras protein expression and Ras-GTP levels in Mac1(+) cells, whereas Mx1-Cre, LSL-Nras(G12D) mice show much lower Ras protein and Ras-GTP levels. Together, these studies establish a robust and tractable system for interrogating the differential properties of oncogenic Ras proteins in primary cells, for identifying candidate cooperating genes, and for testing novel therapeutic strategies.

Clonal analysis of NRAS activating mutations in KIT-D816V systemic mastocytosis

Cooperating genetic events are likely to contribute to the phenotypic diversity of KIT-D816V systemic mastocytosis. In this study, 44 patients with KIT-D816V systemic mastocytosis were evaluated for coexisting NRAS, KRAS, HRAS or MRAS mutations. Activating NRAS mutations were identified in 2 of 8 patients with advanced disease. NRAS mutations were not found in patients with indolent systemic mastocytosis. To better understand the clonal evolution of mastocytosis, we evaluated the cell compartments impacted by the NRAS and KIT mutations. Clonal mast cells harbored both mutations. KIT-D816V was not detected in bone marrow CD34(+) progenitors, whereas the NRAS mutation was present. These findings suggest that NRAS mutations may have the potential to precede KIT-D816V in clonal development. Unlike other mature lineages, mast cell survival is dependent on KIT and the presence of these two activating mutations may have a greater impact on the expansion of this cell compartment and in resultant disease severity.

Endogenous oncogenic Nras mutation promotes aberrant GM-CSF signaling in granulocytic/monocytic precursors in a murine model of chronic myelomonocytic leukemia

Oncogenic NRAS mutations are frequently identified in myeloid diseases involving monocyte lineage. However, its role in the genesis of these diseases remains elusive. We report a mouse bone marrow transplantation model harboring an oncogenic G12D mutation in the Nras locus. Approximately 95% of recipient mice develop a myeloproliferative disease resembling the myeloproliferative variant of chronic myelomonocytic leukemia (CMML), with a prolonged latency and acquisition of multiple genetic alterations, including uniparental disomy of oncogenic Nras allele. Based on single-cell profiling of phospho-proteins, a novel population of CMML cells is identified to display aberrant granulocyte-macrophage colony stimulating factor (GM-CSF) signaling in both the extracellular signal-regulated kinase (ERK) 1/2 and signal transducer and activator of transcription 5 (Stat5) pathways. This abnormal signaling is acquired during CMML development. Further study suggests that aberrant Ras/ERK signaling leads to expansion of granulocytic/monocytic precursors, which are highly responsive to GM-CSF. Hyperactivation of Stat5 in CMML cells is mainly through expansion of these precursors rather than up-regulation of surface expression of GM-CSF receptors. Our results provide insights into the aberrant cytokine signaling in oncogenic NRAS-associated myeloid diseases.

Lyn- and PLC-beta3-dependent regulation of SHP-1 phosphorylation controls Stat5 activity and myelomonocytic leukemia-like disease

Hyperactivation of the transcription factor Stat5 leads to various leukemias. Stat5 activity is regulated by the protein phosphatase SHP-1 in a phospholipase C (PLC)-β3-dependent manner. Thus, PLC-β3-deficient mice develop myeloproliferative neoplasm, like Lyn (Src family kinase)- deficient mice. Here we show that Lyn/PLC-β3 doubly deficient lyn(-/-);PLC-β3(-/-) mice develop a Stat5-dependent, fatal myelodysplastic/myeloproliferative neoplasm, similar to human chronic myelomonocytic leukemia (CMML). In hematopoietic stem cells of lyn(-/-);PLC-β3(-/-) mice that cause the CMML-like disease, phosphorylation of SHP-1 at Tyr(536) and Tyr(564) is abrogated, resulting in reduced phosphatase activity and constitutive activation of Stat5. Furthermore, SHP-1 phosphorylation at Tyr(564) by Lyn is indispensable for maximal phosphatase activity and for suppression of the CMML-like disease in these mice. On the other hand, Tyr(536) in SHP-1 can be phosphorylated by Lyn and another kinase(s) and is necessary for efficient interaction with Stat5. Therefore, we identify a novel Lyn/PLC-β3-mediated regulatory mechanism of SHP-1 and Stat5 activities.

Acute myeloid leukemia with t(9;11)(p21-22;q23): common properties of dysregulated ras pathway signaling and genomic progression characterize de novo and therapy-related cases

We compared pathogenetic features of 32 de novo and 29 therapy-related (t) t(9;11)(p21-22;q23)/MLLT3-MLL acute myeloid leukemia (AML) cases to identify progression factors and to assess whether distinction between these manifestations is warranted. MLLT3-MLL rearrangement was commonly the sole karyotypic abnormality at diagnosis, with many secondary chromosomal changes emerging at relapse in both subgroups. Ras point mutations were common in both groups (overall, 18/50 [36%]) and associated with monocytic phenotype and aneuploid progression. Expression patterns of 675 microRNAs profiled in 7 cases were also similar, with let-7 species linked to Ras down-modulation expressed at low levels. Outcome for both groups was poor (relapsed or refractory in 49/61 [80%] cases); however, patients with t-AML were generally older and female, with worse outcome (P = .03), likely secondary to t-AML mostly arising in patients with breast cancer following topoisomerase inhibitor-containing chemotherapy. Ras activation seems to complement the MLLT3-MLL oncogene in transformation with features of de novo and t-AML with MLLT3-MLL being similar.