New mutations and pathogenesis of myeloproliferative neoplasms

Integrative analysis of copy number and gene expression data suggests novel pathogenetic mechanisms in primary myelofibrosis

Primary myelofibrosis (PMF) is a Myeloproliferative Neoplasm (MPN) characterized by megakaryocyte hyperplasia, progressive bone marrow fibrosis, extramedullary hematopoiesis and transformation to Acute Myeloid Leukemia (AML). A number of phenotypic driver (JAK2, CALR, MPL) and additional subclonal mutations have been described in PMF, pointing to a complex genomic landscape. To discover novel genomic lesions that can contribute to disease phenotype and/or development, gene expression and copy number signals were integrated and several genomic abnormalities leading to a concordant alteration in gene expression levels were identified. In particular, copy number gain in the polyamine oxidase (PAOX) gene locus was accompanied by a coordinated transcriptional up-regulation in PMF patients. PAOX inhibition resulted in rapid cell death of PMF progenitor cells, while sparing normal cells, suggesting that PAOX inhibition could represent a therapeutic strategy to selectively target PMF cells without affecting normal hematopoietic cells' survival. Moreover, copy number loss in the chromatin modifier HMGXB4 gene correlates with a concomitant transcriptional down-regulation in PMF patients. Interestingly, silencing of HMGXB4 induces megakaryocyte differentiation, while inhibiting erythroid development, in human hematopoietic stem/progenitor cells. These results highlight a previously un-reported, yet potentially interesting role of HMGXB4 in the hematopoietic system and suggest that genomic and transcriptional imbalances of HMGXB4 could contribute to the aberrant expansion of the megakaryocytic lineage that characterizes PMF patients.

Overcoming treatment challenges in myelofibrosis and polycythemia vera: the role of ruxolitinib

Myelofibrosis (MF) and polycythemia vera (PV) are BCR-ABL1-negative myeloproliferative neoplasms associated with somatic hematopoietic stem cell mutations leading to over activation of JAK-STAT signaling. MF and PV are pathogenically related and share specific clinical features such as splenomegaly and constitutional symptoms. The MF phenotype is dominated by the effects of progressive bone marrow fibrosis resulting in shortened survival. In contrast, elevated thrombosis risk due to erythrocytosis is the primary clinical concern in PV. Ruxolitinib, an oral JAK1/JAK2 inhibitor, is approved in the USA for the treatment of patients with intermediate- or high-risk MF and patients with PV who have had an inadequate response to or are intolerant of hydroxyurea. For MF, results of two phase III studies demonstrated that ruxolitinib therapy reduced spleen volume and MF-related symptom burden, improved quality-of-life measures, and was associated with prolonged overall survival. Treatment benefits were generally sustained with continued therapy. Dose-dependent cytopenias were common but generally manageable with transfusions (for anemia), dose reduction, or treatment interruption. Optimal dosing management is critical to maintain long-term treatment benefit, because cessation of therapy resulted in rapid return of symptoms to baseline levels. Results of the phase III PV trial showed that ruxolitinib was significantly more effective than standard therapy in controlling hematocrit levels and improving splenomegaly and PV-related symptoms. Only 1 of 110 patients in the ruxolitinib arm compared with 6 of 112 patients in the control arm experienced a thromboembolic event through week 32. Grade ≥3 cytopenias were uncommon.

An International MDS/MPN Working Group's perspective and recommendations on molecular pathogenesis, diagnosis and clinical characterization of myelodysplastic/myeloproliferative neoplasms

In the 2008 WHO classification, chronic myeloid malignancies that share both myelodysplastic and myeloproliferative features define the myelodysplastic/myeloproliferative group, which includes chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, atypical chronic myeloid leukemia, refractory anemia with ring sideroblasts and thrombocytosis, and myelodysplastic/myeloproliferative unclassified. With the notable exception of refractory anemia with ring sideroblasts and thrombocytosis, there is much overlap among the various subtypes at the molecular and clinical levels, and a better definition of these entities, an understanding of their biology and an identification of subtype-specific molecular or cellular markers are needed. To address some of these challenges, a panel comprised of laboratory and clinical experts in myelodysplastic/myeloproliferative was established, and four independent academic MDS/MPN workshops were held on: 9(th) March 2013, in Miami, Florida, USA; 6(th) December 2013, in New Orleans, Louisiana, USA; 13(th) June 2014 in Milan, Italy; and 5(th) December 2014 in San Francisco, USA. During these meetings, the current understanding of these malignancies and matters of biology, diagnosis and management were discussed. This perspective and the recommendations on molecular pathogenesis, diagnosis and clinical characterization for adult onset myelodysplastic/myeloproliferative is the result of a collaborative project endorsed and supported by the MDS Foundation.

miR-203 and miR-221 regulate SOCS1 and SOCS3 in essential thrombocythemia

The biological basis of essential thrombocythemia (ET) patients lacking known mutations is still unknown. MicroRNAs (miRNA) regulate hematopoietic differentiation and are deregulated in several hematopoietic malignancies. However, miRNA expression in ET patients has been poorly explored. We performed miRNA profiling in platelets from 19 ET patients and 10 healthy controls. Hierarchical cluster analysis showed two well-separated clusters between patients and controls, indicating that ET platelets had a characteristic 70-miRNA signature (P<0.0001), 68 of which were downregulated. According to the mutational status, three differentially expressed miRNAs, miR-15a (P=0.045), miR-150 (P=0.001) and miR-519a (P=0.036), were identified. A 40-miRNA signature was identified characterizing JAK2V617F-positive ET patients. Eight genes, whose interaction with the miRNAs could activate the JAK/STAT pathway were identified. An inverse correlation was observed between miRNAs expression and their target genes for SOCS1 and miR-221, SOCS3 and miR-221, SOCS3 and miR-203, and PTPN11 and miR-23a. All three miRNAs were upregulated in JAK2V617F-negative ET patients. SOCS1 and SOCS3 were validated as targets of miR-221 and miR-203, respectively. In summary, our study shows that platelets from JAK2V617F-negative ET patients harbor a specific miRNA signature that can participate in the modulation of the JAK/STAT pathway through regulation of key genes as SOCS1 and SOCS3.

Where to Turn for Second-Line Cytoreduction After Hydroxyurea in Polycythemia Vera?

The goals of therapy in patients with polycythemia vera (PV) are to improve disease-related symptoms, prevent the incidence or recurrence of thrombosis, and possibly delay or prevent the transformation into myelofibrosis or acute myeloid leukemia (AML). Cytoreductive therapies have been used in older patients and those with a history of thrombosis to achieve these goals. Hydroxyurea (HU) remains the first-line cytoreductive choice; however, up to one in four patients treated with HU over time will develop resistance or intolerance to HU. More importantly, patients who fail HU have a 5.6-fold increase in mortality and a 6.8-fold increase risk of transformation to myelofibrosis or AML; therefore, alternative therapies are needed for these patients. Interferon-α has been used in PV and has shown significant activity in achieving hematologic responses and decreasing JAK2 V617F mutation allele burden. JAK inhibition has also been investigated and recently garnered regulatory approval for this indication. In this review, we will discuss the current treatment options that are available for patients after HU and the novel therapies that are currently under investigation.

IMPLICATIONS FOR PRACTICE

The outcomes of PV patients who fail or who are intolerant of hydroxyurea are poor. Although pegylated interferon can be considered in younger patients, currently, ruxolitinib is the only U.S. Food and Drug Administration-approved agent in this setting, representing a viable option, leading to hematocrit control and a reduction in spleen size and constitutional symptoms. Although a small number of patients will achieve a molecular response with continuous treatment, the implications of such response on the clinical outcomes are still unknown. Patients whose disease is not adequately controlled with ruxolitinib, or who lose their response, can be treated with low-dose busulfan or pipobroman; however, they should be encouraged to participate in trials with novel therapies.

Thrombopoietin receptor activation by myeloproliferative neoplasm associated calreticulin mutants

Calreticulin mutants responsible for myeloproliferative neoplasms specifically activate the thrombopoietin receptor and in turn JAK2. Activation of the thrombopoietin receptor requires the glycan binding site and a novel C-terminal tail of the mutant calreticulin.

Mutant Calreticulin Requires Both Its Mutant C-terminus and the Thrombopoietin Receptor for Oncogenic Transformation

Somatic mutations in calreticulin (CALR) are present in approximately 40% of patients with myeloproliferative neoplasms (MPN), but the mechanism by which mutant CALR is oncogenic remains unclear. Here, we demonstrate that expression of mutant CALR alone is sufficient to engender MPN in mice and recapitulates the disease phenotype of patients with CALR-mutant MPN. We further show that the thrombopoietin receptor MPL is required for mutant CALR-driven transformation through JAK-STAT pathway activation, thus rendering mutant CALR-transformed hematopoietic cells sensitive to JAK2 inhibition. Finally, we demonstrate that the oncogenicity of mutant CALR is dependent on the positive electrostatic charge of the C-terminus of the mutant protein, which is necessary for physical interaction between mutant CALR and MPL. Together, our findings elucidate a novel paradigm of cancer pathogenesis and reveal how CALR mutations induce MPN.

SIGNIFICANCE

The mechanism by which CALR mutations induce MPN remains unknown. In this report, we show that the positive charge of the CALR mutant C-terminus is necessary to transform hematopoietic cells by enabling binding between mutant CALR and the thrombopoietin receptor MPL.

New Strategies in Myeloproliferative Neoplasms: The Evolving Genetic and Therapeutic Landscape

The classical BCR-ABL1-negative myeloproliferative neoplasms (MPN) include essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis (MF). Although these clonal disorders share certain clinical and genetic features, MF in particular is distinct for its complex mutational landscape, severe disease phenotype, and poor prognosis. The genetic complexity inherent to MF has made this disease extremely challenging to treat. Pharmacologic JAK inhibition has proven to be a transformative therapy in MPNs, alleviating symptom burden and improving survival, but has been hampered by off-target toxicities and, as monotherapy, has shown limited effects on mutant allele burden. In this review, we discuss the genetic heterogeneity contributing to the pathogenesis of MPNs, focusing on novel driver and epigenetic mutations and how they relate to combination therapeutic strategies. We discuss results from ongoing studies of new JAK inhibitors and report on new drugs and drug combinations that have demonstrated success in early preclinical and clinical trials, including type II JAK inhibitors, antifibrotic agents, and telomerase inhibitors.

Historical views, conventional approaches, and evolving management strategies for myeloproliferative neoplasms

The classical Philadelphia chromosome-negative myeloproliferative neoplasms (MPN), which include essential thrombocythemia, polycythemia vera, and myelofibrosis (MF), are in a new era of molecular diagnosis, ushered in by the identification of the JAK2(V617F) and cMPL mutations in 2005 and 2006, respectively, and the CALR mutations in 2013. Coupled with increased knowledge of disease pathogenesis and refined diagnostic criteria and prognostic scoring systems, a more nuanced appreciation has emerged of the burden of MPN in the United States, including the prevalence, symptom burden, and impact on quality of life. Biological advances in MPN have translated into the rapid development of novel therapeutics, culminating in the approval of the first treatment for MF, the JAK1/JAK2 inhibitor ruxolitinib. However, certain practical aspects of care, such as those regarding diagnosis, prevention of vascular events, choice of cytoreductive agent, and planning for therapies, present challenges for hematologists/oncologists, and are discussed in this article.

N-Glycan-based ER Molecular Chaperone and Protein Quality Control System: The Calnexin Binding Cycle

Helenius and colleagues proposed over 20-years ago a paradigm-shifting model for how chaperone binding in the endoplasmic reticulum was mediated and controlled for a new type of molecular chaperone- the carbohydrate-binding chaperones, calnexin and calreticulin. While the originally established basics for this lectin chaperone binding cycle holds true today, there has been a number of important advances that have expanded our understanding of its mechanisms of action, role in protein homeostasis, and its connection to disease states that are highlighted in this review.

Evolving Therapeutic Strategies for the Classic Philadelphia-Negative Myeloproliferative Neoplasms

Despite the emergence of JAK inhibitors, there is a need for disease-modifying treatments for Philadelphia-negative myeloproliferative neoplasms (MPNs). JAK inhibitors ameliorate symptoms and address splenomegaly, but because of the heterogeneous contributors to the disease process, JAK inhibitor monotherapy incompletely addresses the burden of disease. The ever-growing understanding of MPN pathogenesis has provided the rationale for testing novel and targeted therapeutic agents, as monotherapies or in combination, in preclinical and clinical settings. A number of intriguing options have emerged, and it is hoped that further progress will lead to significant changes in the natural history of MPNs.

Molecular Pathogenesis and Clinical Significance of Driver Mutations in Primary Myelofibrosis: A Review

Primary myelofibrosis (PMF) is a rare chronic BCR-ABL1-negative myeloproliferative neoplasm characterized by progressive bone marrow fibrosis, inefficient hematopoiesis, and shortened survival. The clinical manifestations of PMF include splenomegaly, consequent to extramedullary hematopoiesis, pancytopenias, and an array of potentially debilitating constitutional symptoms. The diagnosis is based on bone marrow morphology and clinical criteria. Mutations in the JAK2 (V617F), MPL (W515), and CALR (exon 9 indel) genes are found in approximately 90% of patients whereas the remaining 10% are so-called triple negatives. Activation of the JAK/STAT pathway results in overproduction of abnormal megakaryocytes leading to bone marrow fibrosis. These mutations might be accompanied by other mutations, such as ASXL1. The commonly used prognostication scoring for PMF is based on the International Prognostic Scoring System. The subsequently developed Dynamic International Prognostic Scoring System-plus employs clinical as well as cytogenetic variables. In PMF, CALR mutation is associated with superior survival and ASXL1 with inferior outcome. Patients with triple-negative PMF have a higher incidence of leukemic transformation and lower overall survival compared with CALR- or JAK2-mutant patients. The impact of genetic lesions on survival is independent of current prognostic scoring systems. These observations indicate that driver and passenger mutations define distinct disease entities within PMF. Accounting for them is not only relevant to clinical decision-making, but should also be considered in designing clinical trials.

Clinical and molecular response to interferon-α therapy in essential thrombocythemia patients with CALR mutations

Pegylated IFNα induces hematologic and molecular remission in CALR-mutated ET patients. The analysis of additional mutations highlights the presence of subclones with variable evolutions during IFNα therapy.

The Bone Marrow-Mediated Protection of Myeloproliferative Neoplastic Cells to Vorinostat and Ruxolitinib Relies on the Activation of JNK and PI3K Signalling Pathways

The classical BCR-ABL-negative Myeloproliferative Neoplasms (MPN) are a group of heterogeneous haematological diseases characterized by constitutive JAK-STAT pathway activation. Targeted therapy with Ruxolitinib, a JAK1/2-specific inhibitor, achieves symptomatic improvement but does not eliminate the neoplastic clone. Similar effects are seen with histone deacetylase inhibitors (HDACi), albeit with poorer tolerance. Here, we show that bone marrow (BM) stromal cells (HS-5) protected MPN-derived cell lines (SET-2; HEL and UKE-1) and MPN patient-derived BM cells from the cytotoxic effects of Ruxolitinib and the HDACi Vorinostat. This protective effect was mediated, at least in part, by the secretion of soluble factors from the BM stroma. In addition, it correlated with the activation of signalling pathways important for cellular homeostasis, such as JAK-STAT, PI3K, JNK, MEK-ERK and NF-κB. Importantly, the pharmacological inhibition of JNK and PI3K pathways completely abrogated the BM protective effect on MPN cell lines and MPN patient samples. Our findings shed light on mechanisms of tumour survival and may indicate novel therapeutic approaches for the treatment of MPN.

Effect of lenalidomide treatment on clonal architecture of myelodysplastic syndromes without 5q deletion

Non-del(5q) transfusion-dependent low/intermediate-1 myelodysplastic syndrome (MDS) patients achieve an erythroid response with lenalidomide in 25% of cases. Addition of an erythropoiesis-stimulating agent could improve response rate. The impact of recurrent somatic mutations identified in the diseased clone in response to lenalidomide and the drug's effects on clonal evolution remain unknown. We investigated recurrent mutations by next-generation sequencing in 94 non-del(5q) MDS patients randomized in the GFM-Len-Epo-08 clinical trial to lenalidomide or lenalidomide plus epoetin β. Clonal evolution was analyzed after 4 cycles of treatment in 42 cases and reanalyzed at later time points in 18 cases. The fate of clonal architecture of single CD34(+)CD38(-) hematopoietic stem cells was also determined in 5 cases. Mutation frequency was >10%: SF3B1 (74.5%), TET2 (45.7%), DNMT3A (20.2%), and ASXL1 (19.1%). Analysis of variant allele frequencies indicated a decrease of major mutations in 15 of 20 responders compared with 10 of 22 nonresponders after 4 cycles. The decrease in the variant allele frequency of major mutations was more significant in responders than in nonresponders (P < .001). Genotyping of single CD34(+)CD38(-) cell-derived colonies showed that the decrease in the size of dominant subclones could be associated with the rise of founding clones or of hematopoietic stem cells devoid of recurrent mutations. These effects remained transient, and disease escape was associated with the re-emergence of the dominant subclones. In conclusion, we show that, although the drug initially modulates the distribution of subclones, loss of treatment efficacy coincides with the re-expansion of the dominant subclone. This trial was registered at www.clinicaltrials.gov as #NCT01718379.

Myeloproliferative neoplasms: Current molecular biology and genetics

Myeloproliferative neoplasms (MPNs) are clonal disorders characterized by increased production of mature blood cells. Philadelphia chromosome-negative MPNs (Ph-MPNs) consist of polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). A number of stem cell derived mutations have been identified in the past 10 years. These findings showed that JAK2V617F, as a diagnostic marker involving JAK2 exon 14 with a high frequency, is the best molecular characterization of Ph-MPNs. Somatic mutations in an endoplasmic reticulum chaperone, named calreticulin (CALR), is the second most common mutation in patients with ET and PMF after JAK2 V617F mutation. Discovery of CALR mutations led to the increased molecular diagnostic of ET and PMF up to 90%. It has been shown that JAK2V617F is not the unique event in disease pathogenesis. Some other genes' location such as TET oncogene family member 2 (TET2), additional sex combs-like 1 (ASXL1), casitas B-lineage lymphoma proto-oncogene (CBL), isocitrate dehydrogenase 1/2 (IDH1/IDH2), IKAROS family zinc finger 1 (IKZF1), DNA methyltransferase 3A (DNMT3A), suppressor of cytokine signaling (SOCS), enhancer of zeste homolog 2 (EZH2), tumor protein p53 (TP53), runt-related transcription factor 1 (RUNX1) and high mobility group AT-hook 2 (HMGA2) have also identified to be involved in MPNs phenotypes. Here, current molecular biology and genetic mechanisms involved in MNPs with a focus on the aforementioned factors is presented.

Interferon-α induces marked alterations in circulating regulatory T cells, NK cell subsets, and dendritic cells in patients with JAK2V617F-positive essential thrombocythemia and polycythemia vera

Long-term therapy with IFN-α2 is associated with sustained major molecular remissions in JAK2-positive ET and PV. The efficacy of IFN-α2 may be partly mediated by modulation of immune cells, which was investigated in twenty patients with ET (n = 6) and PV (n = 14). The frequency of CD4(+) CD25(+) Foxp3(+) T cells was significantly increased during IFN-α2 treatment in all patients (P < 0.0001). A significant expansion of the CD56(bright) NK cells (P = 0.0002) and a concomitant decrease in the frequency of CD56(dim) NK cells (P < 0.0001) were also detected. Myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) were studied in nine patients, and decreased frequencies of both cell types were observed during the course of treatment. On both mDCs and pDCs, HLA-ABC expression was upregulated (P = 0.003), but decreasing expression levels of HLA-DR was detected on mDCs. The expression of CD40 (P = 0.002), CD83 (P = 0.03), and CD86 (P = 0.01) increased, but was confined to pDCs. Furthermore, PD-L1 expression was reduced on mDC (P = 0.003) and increased on pDCs (P = 0.02). No significant correlations were found between the changes in immune cells and hematological or molecular responses achieved in our cohort of patients. So forth, it remains to be revealed whether the profound changes in circulating immune cells contribute to the beneficial effects of long-term IFN-α2 treatment in some patients.

Blast transformation and fibrotic progression in polycythemia vera and essential thrombocythemia: a literature review of incidence and risk factors

Polycythemia vera (PV) and essential thrombocythemia (ET) constitute two of the three BCR-ABL1-negative myeloproliferative neoplasms and are characterized by relatively long median survivals (approximately 14 and 20 years, respectively). Potentially fatal disease complications in PV and ET include disease transformation into myelofibrosis (MF) or acute myeloid leukemia (AML). The range of reported frequencies for post-PV MF were 4.9–6% at 10 years and 6–14% at 15 years and for post-ET MF were 0.8–4.9% at 10 years and 4–11% at 15 years. The corresponding figures for post-PV AML were 2.3–14.4% at 10 years and 5.5–18.7% at 15 years and for post-ET AML were 0.7–3% at 10 years and 2.1–5.3% at 15 years. Risk factors cited for post-PV MF include advanced age, leukocytosis, reticulin fibrosis, splenomegaly and JAK2V617F allele burden and for post-ET MF include advanced age, leukocytosis, anemia, reticulin fibrosis, absence of JAK2V617F, use of anagrelide and presence of ASXL1 mutation. Risk factors for post-PV AML include advanced age, leukocytosis, reticulin fibrosis, splenomegaly, abnormal karyotype, TP53 or RUNX1 mutations as well as use of pipobroman, radiophosphorus (P³²) and busulfan and for post-ET AML include advanced age, leukocytosis, anemia, extreme thrombocytosis, thrombosis, reticulin fibrosis, TP53 or RUNX1 mutations. It is important to note that some of the aforementioned incidence figures and risk factor determinations are probably inaccurate and at times conflicting because of the retrospective nature of studies and the inadvertent labeling, in some studies, of patients with prefibrotic primary MF or ‘masked' PV, as ET. Ultimately, transformation of MPN leads to poor outcomes and management remains challenging. Further understanding of the molecular events leading to disease transformation is being investigated.

The prevalence of CALR mutations in a cohort of patients with myeloproliferative neoplasms

INTRODUCTION

To investigate the prevalence of calreticulin (CALR) mutations in JAK2- and MPL-non-mutated patients with suspected myeloproliferative neoplasm (MPN) from a large MPN clinic and confirm a diagnosis of MPN.

METHODS

JAK2/MPL-non-mutated patients from the Belfast City Hospital (BCH) with either of the MPNs - ET or MF - and diagnosed between 1988 and 2014 were selected for CALR screen. All cases were validated according to the WHO 2008 classification for MPNs. Statistical analysis was performed with Minitab 16 Statistical Software package. Exon 9 of CALR was amplified by PCR using genomic DNA, and mutations were detected by fragment analysis.

RESULTS

Of the 62 JAK2/MPL-non-mutated MPN patients screened, 57 had ET and 5 had MF; 34 patients (53.1%) carried CALR mutations. Three of 5 MF patients were CALR positive. Thirty-one ET patients (54.3%) harboured CALR mutation, whereas 26 (45.7%) were classified as 'triple negatives'.

CONCLUSION

Detection of CALR mutations in a cohort of JAK2/MPL-non-mutated patients with suspected MPN confirmed the diagnosis of MPN in around 53% of cases. This is lower than initially reported, but similar to subsequent studies. However, a sizable cohort of patients remains lacking a specific molecular marker.

Fishing the targets of myeloid malignancies in the era of next generation sequencing

Recent advent in next generation sequencing (NGS) and bioinformatics has generated an unprecedented amount of genetic information in myeloidmalignancies. This information may shed lights to the pathogenesis, diagnosis and prognostication of these diseases and provide potential targets for therapeutic intervention. However, the rapid emergence of genetic information will quickly outpace their functional validation by conventional laboratory platforms. Foundational knowledge about zebrafish hematopoiesis accumulated over the past two decades and novel genomeediting technologies and research strategies in thismodel organismhavemade it a unique and timely research tool for the study of human blood diseases. Recent studies modeling human myeloid malignancies in zebrafish have also highlighted the technical feasibility and clinical relevance of thesemodels. Careful validation of experimental protocols and standardization among laboratorieswill further enhance the application of zebrafish in the scientific communities and provide important insights to the personalized treatment ofmyeloid malignancies.

Chelation efficacy and erythroid response during deferasirox treatment in patients with myeloproliferative neoplasms in fibrotic phase

At present, very few data are available on deferasirox (DFX) in the treatment of patients with Philadelphia-negative myeloproliferative neoplasms in fibrotic phase (FP-MPN) and transfusion dependence. To address this issue, a retrospective analysis of 28 patients (22 male and 6 female) with FP-MPN and iron overload secondary to transfusion dependence was performed, based on patients enrolled in the database of our regional cooperative group who received treatment with DFX. DFX was started after a median interval from diagnosis of 12.8 months (IR 7.1-43.1) with median ferritin values of 1415 ng/mL (IR 1168-1768). Extra-hematological toxicity was reported in 16 of 28 patients (57.1%), but only two patients discontinued treatment due to toxicity. Among 26 patients evaluable for response (≥6 months of treatment), after a median treatment period of 15.4 months (IR 8.1-22.3), 11 patients (42.3%) achieved a stable and consistent reduction in ferritin levels <1000 ng/mL. As for hematological improvement, 6 of 26 patients (23%) showed a persistent (>3 months) rise of Hb levels >1.5 g/dL, with disappearance of transfusion dependence in four cases. Treatment with DFX is feasible and effective in FP-MPN with iron overload. Moreover, in this setting, an erythroid response can occur in a significant proportion of patients.

Smoking as a contributing factor for development of polycythemia vera and related neoplasms

Smoking may be associated with accelerated erythropoiesis, leukocytosis and thrombocytosis, which are also hallmarks in patients with polycythemia vera, essential thrombocythemia and early stages of myelofibrosis (MPNs). The JAK-STAT and NF-κB signaling pathways are activated in both smokers and in patients with MPNs. Additionally, both share elevated levels of several proinflammatory cytokines, in vivo activation of leukocytes and platelets, endothelial dysfunction and increased systemic oxidative stress. Based upon experimental, epidemiological and clinical data it is herein argued and discussed, if smoking may be involved in MPN pathogenesis, considering most recent studies and reviews which are supportive of the concept that chronic inflammation with NF-κB activation and oxidative stress may have a major role - both as triggers but also as the driving force for clonal expansion in MPNs.

Clinical potential of pacritinib in the treatment of myelofibrosis

Myelofibrosis (MF) is a myeloid disorder caused by a clonal hematopoietic stem-cell proliferation associated with activation of the Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathways. Patients with MF often develop severe splenomegaly, marked symptom burden and significant cytopenias, with a consequent marked negative impact on quality of life and survival. The management of MF patients has dramatically improved with the development of a group of drugs that inhibit JAK signaling. The first of these agents to be approved was ruxolitinib, a JAK1/JAK2 inhibitor, which has been shown to improve both spleen size and symptoms in patients with MF. However, myelotoxicity, particularly of the platelet lineage, significantly limits the patient population who can benefit from this agent. Thus, there is an unmet need for novel agents with limited myelotoxicity to treat MF. Pacritinib, a JAK2 and FMS-like tyrosine kinase 3 (FLT3) inhibitor, has shown promising results in early phase trials with limited myelotoxicity and clinical responses that are comparable with those seen with ruxolitinib, even in patients with severe thrombocytopenia. Currently there are two large phase III clinical trials of pacritinib in MF, including patients with thrombocytopenia, and those previously treated with ruxolitinib. If the encouraging results observed in early phase clinical trials are confirmed, pacritinib will represent a new and exciting treatment option for patients with MF and particularly patients with significant cytopenias.

Setting Appropriate Goals for the Next Generation of Clinical Trials in Myelofibrosis

New targeted therapies administered in phase II and phase III studies have produced substantial improvements in outcomes of myelofibrosis (MF). However, strong documentation that the new agents modify the natural history of the disease is lacking, and a number of therapeutic indications of new drugs remain unaddressed. Overall survival (OS) improvement is the major goal of next-generation clinical trials in MF. This may be attained if an adequate population of patients and an unambiguous design of the trial will be selected. Another goal is preventing disease progression in early MF: this requires a controlled clinical trial with an accessible endpoint and a clinically relevant definition of disease progression. Improvement in the documentation of responsiveness of patient-reported outcomes (PROs) will allow to use them as a critical endpoint of new trials. Finally, exploiting the clinical utility of biomarkers should become a major goal of future clinical experimentation in MF.

Megakaryocytic hyperplasia in myeloproliferative neoplasms is driven by disordered proliferative, apoptotic and epigenetic mechanisms

AIMS

Myeloproliferative neoplasms (MPN) are a heterogeneous group of clonal proliferative bone marrow diseases characterised by extensive megakaryocytic hyperplasia and morphological atypia. Despite knowledge of genomic defects, the pathobiological processes driving these megakaryocytic abnormalities in MPN remain poorly explained. We have explored the proliferative, apoptotic and epigenetic profiles of megakaryocytes in human MPN.

METHODS

Immunohistochemical staining was performed on bone marrow trephine biopsies of 81 MPN (with and without JAK2(V617F) and CALR mutations) and 15 normal controls to assess the megakaryocytic expression of biomarkers associated with proliferation (Ki67), apoptosis (Bcl-XL, BNIP-3) and epigenetic regulation (EZH2, SUZ12).

RESULTS

Myeloproliferative megakaryocytes showed significantly greater expression of proliferative Ki67 and anti-apoptotic Bcl-XL, reduced pro-apoptotic BNIP-3 and increased SUZ12 compared with controls. In essential thrombocythaemia, large-giant megakaryocytes with hyperlobated nuclei showed a trend towards a proliferative signature. In contrast, myelofibrotic megakaryocytes with condensed nuclear chromatin, and cases with CALR mutations, had significant reductions in pro-apoptotic BNIP-3.

CONCLUSIONS

Uncontrolled megakaryocytic expansion in MPN results from a combination of increased proliferation, attenuated apoptosis and defective epigenetic regulation with CALR mutations favouring apoptotic failure. The higher platelet counts reported to be seen in MPN with CALR mutations may be due to greater dysregulation of megakaryocyte apoptosis.

Allogeneic stem cell transplant vs.Janus kinase inhibition in the treatment of primary myelofibrosis or myelofibrosis after essential thrombocythemia or polycythemia vera

Primary myelofibrosis is one of the Philadelphia chromosome-negative myeloproliferative neoplasms and is the member of that group with the worst survival and the most significant limitations in quality of life. Hepatosplenomegaly due to extramedullary hematopoiesis, constitutional symptoms, and cytopenias are the main manifestations. The natural history is highly variable, and up to 30% of patients can experience acceleration to acute myelogenous leukemia. Conventional therapy is only palliative and not always effective. However, huge advances have been achieved in the past 2 decades toward a better understanding of the pathogenesis of this disease, as well as improved management. Powerful risk stratification systems are now available and can reliably separate the patients into different prognostic categories to aid clinical management. Allogeneic stem cell transplant can offer cure but is still not universally applicable owing to the treatment-related mortality and toxicity. Nevertheless, outcomes of transplant are improving, owing to the introduction of reduced-intensity conditioning regimens and the optimization of remission monitoring techniques and relapse prevention strategies. The discovery of the V617F mutation of JAK2 (Janus kinase 2) and some other molecular aberrations has shed more light on the molecular pathogenesis of the disease and has led to the introduction of novel therapies such as JAK2 inhibitors. In fact, JAK inhibitors have shown promising symptomatic efficacy, and the JAK inhibitor ruxolitinib has also shown a potential survival benefit. Future effort should be made to combine allogeneic stem cell transplant with JAK inhibition.

Long noncoding RNA in hematopoiesis and immunity

Dynamic gene expression during cellular differentiation is tightly coordinated by transcriptional and post-transcriptional mechanisms. An emerging theme is the central role of long noncoding RNAs (lncRNAs) in the regulation of this specificity. Recent advances demonstrate that lncRNAs are expressed in a lineage-specific manner and control the development of several cell types in the hematopoietic system. Moreover, specific lncRNAs are induced to modulate innate and adaptive immune responses. lncRNAs can function via RNA-DNA, RNA-RNA, and RNA-protein target interactions. As a result, they affect several stages of gene regulation, including chromatin modification, mRNA biogenesis, and protein signaling. We discuss recent advances, future prospects, and challenges in understanding the roles of lncRNAs in immunity and immune-mediated diseases.

Toward point-of-care testing for JAK2 V617F mutation on a microchip

Molecular genetics now plays a crucial role in diagnosis, the identification of prognostic markers, and monitoring of hematological malignancies. Demonstration of acquired changes such as the JAK2 V617F mutation within myeloproliferative neoplasms (MPN) has quickly moved from a research setting to the diagnostic laboratory. Microfluidics-based assays can reduce the assay time and sample/reagent consumption and enhance the reaction efficiency; however, no current assay has integrated isothermal amplification for point-of-care MPN JAK2 V617F mutation testing with a microchip. In this report, an integrated microchip that performs the whole human blood genomic DNA extraction, loop-mediated isothermal nucleic acid amplification (LAMP) and visual detection for point-of-care genetic mutation testing is demonstrated. This method was validated on DNA from cell lines as well as on whole blood from patients with MPN. The results were compared with those obtained by unlabeled probe melting curve analysis. This chip enjoys a high accuracy, operability, and cost/time efficiency within 1h. All these benefits provide the chip with a potency toward a point-of-care genetic analysis. All samples identified as positive by unlabeled probe melting curve analysis (n=27) proved positive when tested by microchip assay. None of the 30 negative controls gave false positive results. In addition, a patient with polycythemia vera diagnosed as being JAK2 V617F-negative by unlabeled probe melting curve analysis was found to be positive by the microchip. This microchip would possibly be very attractive in developing a point-of-care platform for quick preliminary diagnosis of MPN or other severe illness in resource-limited settings.

Pathogenesis of myeloproliferative neoplasms

Major progress has been recently made in understanding the molecular pathogenesis of myeloproliferative neoplasms (MPN). Mutations in one of four genes-JAK2, MPL, CALR, and CSF3R-can be found in the vast majority of patients with MPN and represent driver mutations that can induce the MPN phenotype. Hyperactive JAK/STAT signaling appears to be the common denominator of MPN, even in patients with CALR mutations and the so-called "triple-negative" MPN, where the driver gene mutation is still unknown. Mutations in epigenetic regulators, transcription factors, and signaling components modify the course of the disease and can contribute to disease initiation and/or progression. The central role of JAK2 in MPN allowed development of small molecular inhibitors that are in clinical use and are active in almost all patients with MPN. Advances in understanding the mechanism of JAK2 activation open new perspectives of developing the next generation of inhibitors that will be selective for the mutated forms of JAK2.

Inhibition of Thrombopoietin/Mpl Signaling in Adult Hematopoiesis Identifies New Candidates for Hematopoietic Stem Cell Maintenance

Thrombopoietin (Thpo) signals via its receptor Mpl and regulates megakaryopoiesis, hematopoietic stem cell (HSC) maintenance and post-transplant expansion. Mpl expression is tightly controlled and deregulation of Thpo/Mpl-signaling is linked to hematological disorders. Here, we constructed an intracellular-truncated, signaling-deficient Mpl protein which is presented on the cell surface (dnMpl). The transplantation of bone marrow cells retrovirally transduced to express dnMpl into wildtype mice induced thrombocytopenia, and a progressive loss of HSC. The aplastic BM allowed the engraftment of a second BM transplant without further conditioning. Functional analysis of the truncated Mpl in vitro and in vivo demonstrated no internalization after Thpo binding and the inhibition of Thpo/Mpl-signaling in wildtype cells due to dominant-negative (dn) effects by receptor competition with wildtype Mpl for Thpo binding. Intracellular inhibition of Mpl could be excluded as the major mechanism by the use of a constitutive-dimerized dnMpl. To further elucidate the molecular changes induced by Thpo/Mpl-inhibition on the HSC-enriched cell population in the BM, we performed gene expression analysis of Lin-Sca1+cKit+ (LSK) cells isolated from mice transplanted with dnMpl transduced BM cells. The gene expression profile supported the exhaustion of HSC due to increased cell cycle progression and identified new and known downstream effectors of Thpo/Mpl-signaling in HSC (namely TIE2, ESAM1 and EPCR detected on the HSC-enriched LSK cell population). We further compared gene expression profiles in LSK cells of dnMpl mice with human CD34+ cells of aplastic anemia patients and identified similar deregulations of important stemness genes in both cell populations. In summary, we established a novel way of Thpo/Mpl inhibition in the adult mouse and performed in depth analysis of the phenotype including gene expression profiling.

A pooled analysis of overall survival in COMFORT-I and COMFORT-II, 2 randomized phase III trials of ruxolitinib for the treatment of myelofibrosis

Ruxolitinib, a potent Janus kinase 1/2 inhibitor, resulted in rapid and durable improvements in splenomegaly and disease-related symptoms in the 2 phase III COMFORT studies. In addition, ruxolitinib was associated with prolonged survival compared with placebo (COMFORT-I) and best available therapy (COMFORT-II). We present a pooled analysis of overall survival in the COMFORT studies using an intent-to-treat analysis and an analysis correcting for crossover in the control arms. Overall, 301 patients received ruxolitinib (COMFORT-I, n=155; COMFORT-II, n=146) and 227 patients received placebo (n=154) or best available therapy (n=73). After a median three years of follow up, intent-to-treat analysis showed that patients who received ruxolitinib had prolonged survival compared with patients who received placebo or best available therapy [hazard ratio=0.65; 95% confidence interval (95%CI): 0.46-0.90; P=0.01]; the crossover-corrected hazard ratio was 0.29 (95%CI: 0.13-0.63). Both patients with intermediate-2- or high-risk disease showed prolonged survival, and patients with high-risk disease in the ruxolitinib group had survival similar to that of patients with intermediate-2-risk disease in the control group. The Kaplan-Meier estimate of overall survival at week 144 was 78% in the ruxolitinib arm, 61% in the intent-to-treat control arm, and 31% in the crossover-adjusted control arm. While larger spleen size at baseline was prognostic for shortened survival, reductions in spleen size with ruxolitinib treatment correlated with longer survival. These findings are consistent with previous reports and support that ruxolitinib offers a survival benefit for patients with myelofibrosis compared with conventional therapies. (clinicaltrials.gov identifiers: COMFORT-I, NCT00952289; COMFORT-II, NCT00934544).

Myeloproliferative neoplasms and the JAK/STAT signaling pathway: an overview

Myeloproliferative neoplasms are caused by a clonal proliferation of a hematopoietic progenitor. First described in 1951 as ‘Myeloproliferative Diseases’ and reevaluated by the World Health Organization classification system in 2011, myeloproliferative neoplasms include polycythemia vera, essential thrombocythemia and primary myelofibrosis in a subgroup called breakpoint cluster region-Abelson fusion oncogene-negative neoplasms. According to World Health Organization regarding diagnosis criteria for myeloproliferative neoplasms, the presence of the JAK2 V617F mutation is considered the most important criterion in the diagnosis of breakpoint cluster region-Abelson fusion oncogene-negative neoplasms and is thus used as a clonal marker. The V617F mutation in the Janus kinase 2 (JAK2) gene produces an altered protein that constitutively activates the Janus kinase/signal transducers and activators of transcription pathway and other pathways downstream as a result of signal transducers and activators of transcription which are subsequently phosphorylated. This affects the expression of genes involved in the regulation of apoptosis and regulatory proteins and modifies the proliferation rate of hematopoietic stem cells.

Novel therapies for myelofibrosis

Myelofibrosis (MF), including primary, post-essential thrombocythemia and post-polycythemia vera MF, associates with a reduced quality of life and shortened life expectancy. Dysregulation of the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway is prominent, even in the absence of the JAK2(V617F) mutation. Therefore, all symptomatic MF patients may potentially derive benefit from JAK inhibitors. Despite the efficacy of JAK inhibitors in controlling signs and symptoms of MF, they do not eradicate the disease. Therefore, JAK inhibitors are currently being tested in combination with other novel therapies, a strategy which may be more effective in reducing disease burden, either by overcoming JAK inhibitor resistance or targeting additional mechanisms of pathogenesis. Additional targets include modulators of epigenetic regulation, pathways that work downstream from JAK/STAT (i.e. mammalian target of rapamycin/AKT/phosphoinositide 3-kinase) heat shock protein 90, hedgehog signaling, pro-fibrotic factors, abnormal megakaryocytes and telomerase. In this review, we discuss novel MF therapeutic strategies.

Deletion of Stat3 enhances myeloid cell expansion and increases the severity of myeloproliferative neoplasms in Jak2V617F knock-in mice

The JAK2V617F mutation commonly found in myeloproliferative neoplasms (MPNs) induces constitutive phosphorylation/activation of the signal transducer and activator of transcription 3 (Stat3). However, the contribution of Stat3 in MPN evoked by JAK2V617F remains unknown. To determine the role of Stat3 in JAK2V617F-induced MPN, we generated Stat3-deficient Jak2V617F-expressing mice. Whereas expression of Jak2V617F resulted in a PV-like disease characterized by increased red blood cells (RBC), hematocrit, neutrophils and platelets in the peripheral blood of Jak2V617F knock-in mice, deletion of Stat3 slightly reduced RBC, and hematocrit parameters and modestly increased platelet numbers in Jak2V617F knock-in mice. Moreover, deletion of Stat3 significantly increased the neutrophil counts/percentages and markedly reduced the survival of mice expressing Jak2V617F. These phenotypic manifestations were reproduced upon bone marrow transplantation into wild-type animals. Flow cytometric analysis showed increased hematopoietic stem cell and granulocyte-macrophage progenitor populations in the bone marrow and spleens of Stat3-deficient Jak2V617F mice. Stat3 deficiency also caused a marked expansion of Gr-1⁺/Mac-1⁺ myeloid cells in Jak2V617F knock-in mice. Histopathologic analysis revealed marked increase in granulocytes in the bone marrow, spleens and livers of Stat3-deficient Jak2V617F-expressing mice. Together, these results suggest that deletion of Stat3 increases the severity of MPN induced by Jak2V617F.

Dissecting Genomic Aberrations in Myeloproliferative Neoplasms by Multiplex-PCR and Next Generation Sequencing

In order to assess the feasibility of amplicon-based parallel next generation sequencing (NGS) for the diagnosis of myeloproliferative neoplasms (MPN), we investigated multiplex-PCR of 212 amplicons covering genomic mutational hotspots in 48 cancer-related genes. Samples from 64 patients with MPN and five controls as well as seven (myeloid) cell lines were analyzed. Healthy donor and reactive erythrocytosis samples showed several frequent single-nucleotide polymorphisms (SNPs) but no known pathogenic mutation. Sequencing of the cell lines confirmed the presence of the known mutations. In the patient samples, JAK2 V617F was present in all PV, 4 of 10 ET, and 16 of 19 MF patients. The JAK2 V617F allele burden was different in the three groups (ET, 33+/-22%; PV 48+/-28% and MF 68+/- 29%). Further analysis detected both previously described and undescribed mutations (i.e., G12V NRAS, IDH1 R132H, E255G ABL, and V125G IDH1 mutations). One patient with lymphoid BC/Ph+ ALL who harbored a T315I ABL mutation and was treated with ponatinib was found to have developed a newly acquired V216M TP53 mutation (12% of transcripts) when becoming resistant to ponatinib. Ponatinib led to a decrease of ABL T315I positive transcripts from 47% before ponatinib treatment to 16% at the time of ponatinib resistance in this patient, suggesting that both TP53 and ABL mutations were present in the same clone and that the newly acquired TP53 mutation might have caused ponatinib resistance in this patient. In conclusion, amplicon-sequencing-based NGS allows simultaneous analysis of multiple MPN associated genes for diagnosis and during treatment and measurement of the mutant allele burden.

Genomic diversity in myeloproliferative neoplasms: focus on myelofibrosis

The classical myeloproliferative neoplasms (MPNs) are a group of clonal diseases comprising essential thrombocythaemia (ET), polycythaemia vera (PV) and primary myelofibrosis (PMF). PMF is the rarest disease sub type and has been challenging to address due to the lack of a specific genetic marker, inadequate risk identification models and a highly variable clinical course. Continuous efforts have over time, seen the inclusion of cytogenetic information in prognostic scoring models that have resulted in improved risk stratification models providing further rationale for therapeutic management. Technological advances using single nucleotide polymorphism arrays increased the detection of known and novel MPN related changes and variant detection by massively parallel sequencing provided a large scale screening tool for the multitude of somatic gene mutations that have more recently been described in MPN. Some of these mutations show an association with specific cytogenetic changes or phenotypes. While PMF occurs mainly in adults, it has also been described in paediatric cases and shows distinct histopathological, genetic and clinical features in comparison. This review provides an overview of the genomics landscape of PMF and current developments in MPN therapy.

TET2 loss, a rescue of JAK2V617F HSCs

In this issue of Blood, Chen et al and Kameda et al demonstrate that Tet2 loss has 2 effects in Jak2V617F mice: it increases both the severity of the myeloproliferative disorders and the self-renewal properties of the Jak2V617F hematopoietic stem cells (HSCs).

Going for broke: targeting the human cancer pseudokinome

Protein phosphorylation lies at the heart of cell signalling, and somatic mutation(s) in kinases drives and sustains a multitude of human diseases, including cancer. The human protein kinase superfamily (the kinome) encodes approximately 50 'pseudokinases', which were initially predicted to be incapable of dynamic cell signalling when compared with canonical enzymatically active kinases. This assumption was supported by bioinformatics, which showed that amino acid changes at one or more key loci, making up the nucleotide-binding site or phosphotransferase machinery, were conserved in multiple vertebrate and non-vertebrate pseudokinase homologues. Protein kinases are highly attractive targets for drug discovery, as evidenced by the approval of almost 30 kinase inhibitors in oncology, and the successful development of the dual JAK1/2 (Janus kinase 1/2) inhibitor ruxolitinib for inflammatory indications. However, for such a large (>550) protein family, a remarkable number have still not been analysed at the molecular level, and only a surprisingly small percentage of kinases have been successfully targeted clinically. This is despite evidence that many are potential candidates for the development of new therapeutics. Indeed, several recent reports confirm that disease-associated pseudokinases can bind to nucleotide co-factors at concentrations achievable in the cell. Together, these findings suggest that drug targeting using either ATP-site or unbiased ligand-discovery approaches should now be attempted using the validation technology currently employed to evaluate their classic protein kinase counterparts. In the present review, we discuss members of the human pseudokinome repertoire, and catalogue somatic amino acid pseudokinase mutations that are emerging as the depth and clinical coverage of the human cancer pseudokinome expand.

C/EBPα in normal and malignant myelopoiesis

CCAAT/enhancer binding protein α (C/EBPα) dimerizes via its leucine zipper (LZ) domain to bind DNA via its basic region and activate transcription via N-terminal trans-activation domains. The activity of C/EBPα is modulated by several serine/threonine kinases and via sumoylation, its gene is activated by RUNX1 and additional transcription factors, its mRNA stability is modified by miRNAs, and its mRNA is subject to translation control that affects AUG selection. In addition to inducing differentiation, C/EBPα inhibits cell cycle progression and apoptosis. Within hematopoiesis, C/EBPα levels increase as long-term stem cells progress to granulocyte-monocyte progenitors (GMP). Absence of C/EBPα prevents GMP formation, and higher levels are required for granulopoiesis compared to monopoiesis. C/EBPα interacts with AP-1 proteins to bind hybrid DNA elements during monopoiesis, and induction of Gfi-1, C/EBPε, KLF5, and miR-223 by C/EBPα enables granulopoiesis. The CEBPA ORF is mutated in approximately 10 % of acute myeloid leukemias (AML), leading to expression of N-terminally truncated C/EBPαp30 and C-terminal, in-frame C/EBPαLZ variants, which inhibit C/EBPα activities but also play additional roles during myeloid transformation. RUNX1 mutation, CEBPA promoter methylation, Trib1 or Trib2-mediated C/EBPαp42 degradation, and signaling pathways leading to C/EBPα serine 21 phosphorylation reduce C/EBPα expression or activity in additional AML cases.

CD133 marks a stem cell population that drives human primary myelofibrosis

Primary myelofibrosis is a myeloproliferative neoplasm characterized by bone marrow fibrosis, megakaryocyte atypia, extramedullary hematopoiesis, and transformation to acute myeloid leukemia. To date the stem cell that undergoes the spatial and temporal chain of events during the development of this disease has not been identified. Here we describe a CD133(+) stem cell population that drives the pathogenesis of primary myelofibrosis. Patient-derived circulating CD133(+) but not CD34(+)CD133(-) cells, with a variable burden for JAK2 (V617F) mutation, had multipotent cloning capacity in vitro. CD133(+) cells engrafted for up to 10 months in immunocompromised mice and differentiated into JAK2-V617F(+) myeloid but not lymphoid progenitors. We observed the persistence of human, atypical JAK2-V617F(+) megakaryocytes, the initiation of a prefibrotic state, bone marrow/splenic fibrosis and transition to acute myeloid leukemia. Leukemic cells arose from a subset of CD133(+) cells harboring EZH2 (D265H) but lacking a secondary JAK2 (V617F) mutation, consistent with the hypothesis that deregulation of EZH2 activity drives clonal growth and increases the risk of acute myeloid leukemia. This is the first characterization of a patient-derived stem cell population that drives disease resembling both chronic and acute phases of primary myelofibrosis in mice. These results reveal the importance of the CD133 antigen in deciphering the neoplastic clone in primary myelofibrosis and indicate a new therapeutic target for myeloproliferative neoplasms.