Epigenetic alterations complement mutation of JAK2 tyrosine kinase in patients with BCR/ABL-negative myeloproliferative disorders

Molecular Genetics of Thrombotic Myeloproliferative Neoplasms: Implications in Precision Oncology

Classical BCR-ABL-negative myeloproliferative neoplasms (MPN) include polycythaemia vera, essential thrombocythaemia, and primary myelofibrosis. Unlike monogenic disorders, a more complicated series of genetic mutations are believed to be responsible for MPN with various degrees of thromboembolic and bleeding complications. Thrombosis is one of the early manifestations in patients with MPN. To date, the driver genes responsible for MPN include JAK2, CALR, MPL, TET2, ASXL1, and MTHFR. Affords have been done to elucidate these mutations and the incidence of thromboembolic events. Several lines of evidence indicate that mutations in JAK2, MPL, TET2 and ASXL1 gene and polymorphisms in several clotting factors (GPIa, GPIIa, and GPIIIa) are associated with the occurrence and prevalence of thrombosis in MPN patients. Some polymorphisms within XRCC1, FBG, F2, F5, F7, F12, MMP9, HPA5, MTHFR, SDF-1, FAS, FASL, TERT, ACE, and TLR4 genes may also play a role in MPN manifestation. This review aims to provide an insightful overview on the genetic perspective of thrombotic complications in patients with MPN.

Systematization of analytical studies of polycythemia vera, essential thrombocythemia and primary myelofibrosis, and a meta-analysis of the frequency of JAK2, CALR and MPL mutations: 2000-2018

BACKGROUND

Research into Philadelphia-negative chronic myeloproliferative neoplasms is heterogeneous. In addition, no systematization of studies of polycythemia vera (PV), essential thrombocythemia (ET) or primary myelofibrosis (PMF) have been carried out. The objective of this review is to characterize studies on BCR-ABL1-negative chronic myeloproliferative neoplasms and to compare the frequency of JAK2, MPL and CALR mutations in PV, ET and PMF.

METHOD

A systematic review of the scientific literature was conducted, as was meta-analysis with an ex-ante selection of protocol, according to phases of the PRISMA guide in three interdisciplinary databases. To guarantee reproducibility in the pursuit and retrieval of information, the reproducibility and methodological quality of the studies were evaluated by two researchers.

RESULTS

Fifty-two studies were included, the majority having been carried out in the United States, China, Brazil and Europe. The frequency of the JAK2V617F mutation ranged from 46.7 to 100% in patients with PV, from 31.3 to 72.1% in patients with ET, and from 25.0 to 85.7% in those with PMF. The frequency of the MPL mutation was 0% in PV, from 0.9 to 12.5% in ET, and from 0 to 17.1% in PMF. The CALR mutation occurred at a frequency of 0.0% in PV, whereas in ET, it ranged from 12.6 to 50%, and in PMF, it ranged from 10 to 100%. The risk of this mutation presenting in PV is 3.0 times that found for ET and 4.0 times that found for PMF.

CONCLUSION

Given the specificity and reported high frequencies of the JAK2V617F, MPL and CALR mutations in this group of neoplasms, the diagnosis of these diseases should not be made on clinical and hematological characteristics alone but should include genetic screening of patients.

The role of small adaptor proteins in the control of oncogenic signalingr driven by tyrosine kinases in human cancer

Protein phosphorylation on tyrosine (Tyr) residues has evolved as an important mechanism to coordinate cell communication in multicellular organisms. The importance of this process has been revealed by the discovery of the prominent oncogenic properties of tyrosine kinases (TK) upon deregulation of their physiological activities, often due to protein overexpression and/or somatic mutation. Recent reports suggest that TK oncogenic signaling is also under the control of small adaptor proteins. These cytosolic proteins lack intrinsic catalytic activity and signal by linking two functional members of a catalytic pathway. While most adaptors display positive regulatory functions, a small group of this family exerts negative regulatory functions by targeting several components of the TK signaling cascade. Here, we review how these less studied adaptor proteins negatively control TK activities and how their loss of function induces abnormal TK signaling, promoting tumor formation. We also discuss the therapeutic consequences of this novel regulatory mechanism in human oncology.

Molecular determinants of pathogenesis and clinical phenotype in myeloproliferative neoplasms

The myeloproliferative neoplasms are a heterogeneous group of clonal disorders characterized by the overproduction of mature cells in the peripheral blood, together with an increased risk of thrombosis and progression to acute myeloid leukemia. The majority of patients with Philadelphia-chromosome negative myeloproliferative neoplasms harbor somatic mutations in Janus kinase 2, leading to constitutive activation. Acquired mutations in calreticulin or myeloproliferative leukemia virus oncogene are found in a significant number of patients with essential thrombocythemia or myelofibrosis, and mutations in numerous epigenetic regulators and spliceosome components are also seen. Although the cellular and molecular consequences of many of these mutations remain unclear, it seems likely that they interact with germline and microenvironmental factors to influence disease pathogenesis. This review will focus on the determinants of specific myeloproliferative neoplasm phenotypes as well as on how an improved understanding of molecular mechanisms can inform our understanding of the disease entities themselves.

Myelofibrosis: an update on drug therapy in 2016

INTRODUCTION

Primary myelofibrosis (PMF) is the least common but the most aggressive of the classic Philadelphia chromosome-negative myeloproliferative neoplasms. Survival is much shorter in PMF than in polycythemia vera (PV) or essential thrombocythemia (ET). Post-PV/ET myelofibrosis (MF) is clinically indistinguishable from PMF and approached similarly. Areas covered: Current pharmacologic therapy of MF revolves around the Janus kinase 1/2 (JAK1/2) inhibitor ruxolitinib, which dramatically improves constitutional symptoms and splenomegaly in the majority of patients, and improves overall survival (OS). However, allogeneic stem cell transplantation remains the only potential cure. Other JAK inhibitors continue to be developed for MF, and momelotinib and pacritinib are in phase III clinical trials. Anemia is common in MF, and initially worsened by ruxolitinib. Momelotinib and pacritinib may prove advantageous in this regard. Current strategies for managing anemia of MF include danazol, immunomodulatory drugs and erythroid stimulating agents, either alone or in combination with ruxolitinib. Expert opinion: A number of other agents, representing diverse drug classes, are in various stages of development for MF. These include newer JAK inhibitors, other signaling inhibitors, epigenetic modifiers, anti-fibrotic agents, telomerase inhibitors, and activin receptor ligand traps (for anemia). Hopefully, these novel therapies will further extend the clinical benefits of ruxolitinib.

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.

Rapid and robust reversion to essential thrombocythemia on treatment with Decitabine in a case of hydroxyurea-induced t-MDS/AML

Rapid remission of MDS/AML may be induced with Decitabine; however, significant megakaryocyte expansion and subsequent thrombocytosis may occur. Decitabine‐mediated reversion of the MDS to benign ET via hypomethylation of JAK/STAT pathway repressors is one potential mechanism to explain this observed phenomenon.

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.

JAK kinase targeting in hematologic malignancies: a sinuous pathway from identification of genetic alterations towards clinical indications

Constitutive JAK-STAT pathway activation occurs in most myeloproliferative neoplasms as well as in a significant proportion of other hematologic malignancies, and is frequently a marker of poor prognosis. The underlying molecular alterations are heterogeneous as they include activating mutations in distinct components (cytokine receptor, JAK, STAT), overexpression (cytokine receptor, JAK) or rare JAK2 fusion proteins. In some cases, concomitant loss of negative regulators contributes to pathogenesis by further boosting the activation of the cascade. Exploiting the signaling bottleneck provided by the limited number of JAK kinases is an attractive therapeutic strategy for hematologic neoplasms driven by constitutive JAK-STAT pathway activation. However, given the conserved nature of the kinase domain among family members and the interrelated roles of JAK kinases in many physiological processes, including hematopoiesis and immunity, broad usage of JAK inhibitors in hematology is challenged by their narrow therapeutic window. Novel therapies are, therefore, needed. The development of more selective inhibitors is a questionable strategy as such inhibitors might abrogate the beneficial contribution of alleviating the cancer-related pro-inflammatory microenvironment and raise selective pressure to a threshold that allows the emergence of malignant subclones harboring drug-resistant mutations. In contrast, synergistic combinations of JAK inhibitors with drugs targeting cascades that work in concert with JAK-STAT pathway appear to be promising therapeutic alternatives to JAK inhibitors as monotherapies.

[SOCSJ gene mutations in patients with diffuse large B-cell lymphoma]

Diffuse large B-cell lymphoma (DLBCL) is a heterogeneous group of diseases, which accounts for 30% of all non-Hodgkin lymphomas. Current molecular studies have confirmed that there are several DLBCL subtypes characterized by different cellular origin, cytogenetic profile, molecular genetic disorders, and different pathogenesis. Impaired JAK-STAT signaling is a part of the pathogenesis of various cancers, including DLBCL. The review deals with the molecular genetic aspects of the occurrence of DLBCL and the function of the SOCSI gene that has been proven to be responsible for the development of several cancers. Mutations of this gene result from spontaneously impaired B-cell somatic hypermutation and they are frequently inactivating. The presence of point mutations in the functionally significant region of this gene in DLBCL could identify a group of patients with poor prognosis during standard chemotherapy.

Contribution of JAK2 mutations to T-cell lymphoblastic lymphoma development

The JAK-STAT pathway has a substantial role in lymphoid precursor cell proliferation, survival and differentiation. Nonetheless, the contribution of JAK2 to T-cell lymphoblastic lymphoma (T-LBL) development remains poorly understood. We have identified one activating TEL-JAK2 translocation and four missense mutations accumulated in 2 out of 16 T-LBL samples. Two of them are novel JAK2 mutations and the other two are reported for the first time in T-LBL. Notably, R683G and I682T might have arisen owing to RNA editing. Mutated samples showed different mutated transcripts suggesting sub-clonal heterogeneity. Functional approaches revealed that two JAK2 mutations (H574R and R683G) constitutively activate JAK-STAT signaling in γ2A cells and can drive the proliferation of BaF3-EpoR cytokine-dependent cell line. In addition, aberrant hypermethylation of SOCS3 might contribute to enhance the activation of JAK-STAT signaling. Of utmost interest is that primary T-LBL samples harboring JAK2 mutations exhibited increased expression of LMO2, suggesting a mechanistic link between JAK2 mutations and the expression of LMO2, which was confirmed for the four missense mutations in transfected γ2A cells. We therefore propose that active JAK2 contribute to T-LBL development by two different mechanisms, and that the use of pan-JAK inhibitors in combination with epigenetic drugs should be considered in future treatments.

Dysregulation of the MIRLET7/HMGA2 axis with methylation of the CDKN2A promoter in myeloproliferative neoplasms

Overexpression of high mobility group AT-hook 2 (Hmga2), which is negatively regulated by MIRLET7 micro RNAs through 3'-untranslated region (3'UTR), causes proliferative haematopoiesis mimicking myeloproliferative neoplasms (MPNs) and contributes to progression of myelofibrosis in mice. Thus, we investigated HMGA2 mRNA expression in 66 patients with MPNs including 23 polycythaemia vera (PV), 33 essential thrombocythaemia (ET) and 10 primary myelofibrosis (PMF). HMGA2 mRNA expression, especially variant 1 with 3'UTR that contains MIRLET7-specific sites, rather than variant 2 lacking 3'UTR, is frequently deregulated due to decreased MIRLET7 expression in granulocytes from over 20% of PV and ET, and in either granulocytes or CD34(+) cells from 100% of PMF. Patients with deregulated HMGA2 mRNA expression were significantly more likely to show splenomegaly, high serum lactate dehydrogenase values, and methylation of the CDKN2A promoter compared with other patients without deregulation of HMGA2. A histone deacetylase inhibitor, panobinostat, significantly increased MIRLET7 expression and reduced variant 1 of HMGA2 mRNA expression, but not variant 2, in both U937 cells and PMF-derived CD34(+) cells. Moreover, both panobinostat and small interfering RNA of HMGA2 demethylated the CDKN2A promoter in U937 cells. In conclusion, the frequently dysregulated MIRLET7/HMGA2 axis could be a therapeutic target in MPNs.

SOCS proteins in regulation of receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTKs) are a family of cell surface receptors that play critical roles in signal transduction from extracellular stimuli. Many in this family of kinases are overexpressed or mutated in human malignancies and thus became an attractive drug target for cancer treatment. The signaling mediated by RTKs must be tightly regulated by interacting proteins including protein-tyrosine phosphatases and ubiquitin ligases. The suppressors of cytokine signaling (SOCS) family proteins are well-known negative regulators of cytokine receptors signaling consisting of eight structurally similar proteins, SOCS1-7, and cytokine-inducible SH2-containing protein (CIS). A key feature of this family of proteins is the presence of an SH2 domain and a SOCS box. Recent studies suggest that SOCS proteins also play a role in RTK signaling. Activation of RTK results in transcriptional activation of SOCS-encoding genes. These proteins associate with RTKs through their SH2 domains and subsequently recruit the E3 ubiquitin machinery through the SOCS box, and thereby limit receptor stability by inducing ubiquitination. In a similar fashion, SOCS proteins negatively regulate mitogenic signaling by RTKs. It is also evident that RTKs can sometimes bypass SOCS regulation and SOCS proteins can even potentiate RTKs-mediated mitogenic signaling. Thus, apart from negative regulation of receptor signaling, SOCS proteins may also influence signaling in other ways.

The role of suppressors of cytokine signalling in human neoplasms

Suppressors of cytokine signalling 1-7 (SOCS1-7) and cytokine-inducible SH2-containing protein (CIS) are a group of intracellular proteins that are well known as JAK-STAT and several other signalling pathways negative feedback regulators. More recently several members have been identified as tumour suppressors and dysregulation of their biological roles in controlling cytokine and growth factor signalling may contribute to the development of many solid organ and haematological malignancies. This review explores their biological functions and their possible tumour suppressing role in human neoplasms.

Mechanistic insights into activation and SOCS3-mediated inhibition of myeloproliferative neoplasm-associated JAK2 mutants from biochemical and structural analyses

JAK2 (Janus kinase 2) initiates the intracellular signalling cascade downstream of cell surface receptor activation by cognate haemopoietic cytokines, including erythropoietin and thrombopoietin. The pseudokinase domain (JH2) of JAK2 negatively regulates the catalytic activity of the adjacent tyrosine kinase domain (JH1) and mutations within the pseudokinase domain underlie human myeloproliferative neoplasms, including polycythaemia vera and essential thrombocytosis. To date, the mechanism of JH2-mediated inhibition of JH1 kinase activation as well as the susceptibility of pathological mutant JAK2 to inhibition by the physiological negative regulator SOCS3 (suppressor of cytokine signalling 3) have remained unclear. In the present study, using recombinant purified JAK2JH1-JH2 proteins, we demonstrate that, when activated, wild-type and myeloproliferative neoplasm-associated mutants of JAK2 exhibit comparable enzymatic activity and inhibition by SOCS3 in in vitro kinase assays. SAXS (small-angle X-ray scattering) showed that JAK2JH1-JH2 exists in an elongated configuration in solution with no evidence for interaction between JH1 and JH2 domains in cis. Collectively, these data are consistent with a model in which JAK2's pseudokinase domain does not influence the activity of JAK2 once it has been activated. Our data indicate that, in the absence of the N-terminal FERM domain and thus cytokine receptor association, the wild-type and pathological mutants of JAK2 are enzymatically equivalent and equally susceptible to inhibition by SOCS3.

Methylation profiling of SOCS1, SOCS2, SOCS3, CISH and SHP1 in Philadelphia-negative myeloproliferative neoplasm

Janus kinase-signal transducer and activator of transcription (JAK/STAT) signalling, pivotal in Philadelphia-negative (Ph-ve) myeloproliferative neoplasm (MPN), is negatively regulated by molecules including SOCSs, CISH and SHP1. SOCS1, SOCS2 and SOCS3 methylation have been studied in MPN with discordant results. Herein, we studied the methylation status of SOCS1, SOCS2 and SOCS3, CISH and SHP1 by methylation-specific polymerase chain reaction (MSP) in cell lines and 45 diagnostic marrow samples of Ph-ve MPN. Moreover, we attempted to explain the discordance of methylation frequency by mapping the studied MSP primers to the respective genes. Methylation was detected in normal controls using SOCS2 MSP primers in the 3′translated exonic sequence, but not primers around the transcription start site in the 5′ untranslated regions (5′UTR). SOCS1, SOCS2, SOCS3 and CISH were completely unmethylated in primary MPN samples and cell lines. In contrast, methylation of SHP1 was detected in 8.9% primary marrow samples. Moreover, SHP1 was completely methylated in K562 cell line, leading to reversible SHP1 silencing. A review of methylation studies of SOCS1 and SOCS3 showed that spuriously high rates of SOCS methylation had been reported using MSP primers targeting CpG sites in the 3′translated exonic sequence, which is also methylated in normal controls. However, using MSP primers localized to the 5′UTR, methylation of SOCS1, SOCS2 and SOCS3 is infrequent across all studies. In summary, methylation of SOCS1, SOCS2, SOCS3 and CISH is infrequent in Ph-ve MPN. Appropriate MSP primers are important for accurate estimation of the methylation frequency. The role of SHP1 methylation in the pathogenesis of MPN warrants further investigation.

SOCS, inflammation, and cancer

Signal transduction pathways elicited by cytokines and hormones have been shown to regulate distinct stages of development. Suppressor of cytokine signaling (SOCS) proteins are negative feedback regulators of cytokine signaling mediated by the JAK-STAT signaling pathway. In particular, SOCS1 and SOCS3 are potent inhibitors of JAKs and can play pivotal roles in inflammation, as well as in the development and progression of cancers. Abnormal expression of SOCS1 and SOCS3 in cancer cells has been reported in human carcinoma associated with dysregulation of signals from cytokine receptors, Toll-like receptors (TLRs), and hormone receptors, resulting in malignancies. In this review, we focus on the role of SOCS1 and SOCS3 in cancer development. In addition, the potential of SOCS as a therapeutic target and diagnostic aid will be discussed.

JAK2 mutants (e.g., JAK2V617F) and their importance as drug targets in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) mutant V617F and other JAK mutants are found in patients with myeloproliferative neoplasms and leukemias. Due to their involvement in neoplasia and inflammatory disorders, Janus kinases are promising targets for kinase inhibitor therapy. Several small-molecule compounds are evaluated in clinical trials for myelofibrosis, and ruxolitinib (INCB018424, Jakafi®) was the first Janus kinase inhibitor to receive clinical approval. In this review we provide an overview of JAK2V617F signaling and its inhibition by small-molecule kinase inhibitors. In addition, myeloproliferative neoplasms are discussed regarding the role of JAK2V617F and other mutant proteins of possible relevance. We further give an overview about treatment options with special emphasis on possible combination therapies.

Epigenetic dysregulation of secreted frizzled-related proteins in myeloproliferative neoplasms complements the JAK2V617F-mutation

Background

Secreted frizzled-related proteins (SFRPs) are antagonists of the Wnt signaling pathway, which plays a central role in stem cell maintenance and differentiation of stem cells and hematopoietic progenitors. Epigenetic downregulation of SFRPs by promoter hypermethylation has been described to be involved in the pathogenesis of hematopoietic malignancies. There is an association between aberrant Wnt signaling and the established cancer stem cell concept. In contrast to BCR-ABL1-positive chronic myeloid leukemia CML, BCR-ABL1-negative myeloproliferative neoplasms (Ph^-MPN) are characterized by the frequent occurrence of an autoactivating mutation in the JAK2 tyrosine kinase (JAK2V617F) or other mutations in the JAK-STAT pathway. However, pathogenetic mechanisms of JAK2 mutated or unmutated Ph^-MPN remain not completely understood. We determined the promoter methylation status of SFRP-1, -2, -4, and -5 in 57 MPN patient samples by methylation-specific polymerase chain reaction (PCR) (MSP). JAK2V617F was assessed by allele-specific PCR.

Results

Aberrant methylation among primary MPN samples was 4% for SFRP-1, 25% for SFRP-2, 2% for SFRP-4, and 0% for SFRP-5. Hypermethylation of SFRP-2, which was the most frequently hypermethylated gene in our study, could not be correlated to any specific MPN subtype. However, we detected a significant correlation between SFRP-2 methylation and presence of a JAK2V617F mutation (P = 0.008). None of the 10 CML samples showed any SFRP-methylation.

Conclusions

Our data indicate that epigenetic dysregulation of the Wnt signaling pathway is a common event in MPN with aberrant methylation of at least one SFRP being detected in 25% of the primary patient samples and in 30% if only accounting for Ph^-MPN. A significant correlation between SFRP-2 methylation and presence of JAK2V617F in our data supports the hypothesis that epigenetic dysregulation may be a complementary mechanism to genetic aberrations. Aberrant methylation of crucial stem cell maintenance genes seems to contribute to disease pathogenesis in Ph^-MPN.

Disordered signaling in myeloproliferative neoplasms

The human myeloproliferative neoplasms (MPN) have long been associated with abnormal responses to cytokines and activation of signaling pathways, although the exact molecular mechanisms underlying these observations were unknown. This situation altered with the discovery of the JAK2 V617F, which presaged the ongoing description of further mutations predicted to activate canonical signaling pathways in MPN. This article covers the nature of these mutations and summarizes functional experiments in model systems and in human MPN cells to define the signaling pathways altered and how these drive and determine the MPN cellular phenotype. Also discussed are recently described, novel noncanonical signaling pathways to chromatin predicted to alter gene transcription more directly and to also contribute to the MPN phenotype.

How I treat polycythemia vera

Polycythemia vera (PV) is a clonal disorder characterized by unwarranted production of red blood cells. In the majority of cases, PV is driven by oncogenic mutations that constitutively activate the JAK-STAT signal transduction pathway, such as JAK2 V617F, or exon 12 mutations or LNK mutations. Diagnosis of PV is based on the WHO criteria. Diagnosis of post-PV myelofibrosis is established according to the International Working Group for Myeloproliferative Neoplasms Research and Treatment criteria. Different clinical presentations of PV are discussed. Prognostication of PV is tailored to the most frequent complication during follow-up, namely, thrombosis. Age older than 60 years and prior history of thrombosis are the 2 main risk factors for disease stratification. Correlations are emerging between leukocytosis, JAK2(V617F) mutation, BM fibrosis, and different outcomes of PV, which need to be confirmed in prospective studies. In my practice, hydroxyurea is still the "gold standard" when cytoreduction is needed, even though pegylated IFN-alfa-2a and ruxolitinib might be useful in particular settings. Results of phase 1 or 2 studies concerning these latter agents should however be confirmed by the ongoing randomized phase 3 clinical trials. In this paper, I discuss the main problems encountered in daily clinical practice with PV patients regarding diagnosis, prognostication, and therapy.

Clinical applications of epigenetic markers and epigenetic profiling in myeloid malignancies

Aberrant DNA methylation is frequent in the myeloid malignancies, particularly myelodysplastic syndrome (MDS) and acute myelogenous leukemia (AML). Promoter CpG methylation is correlated with silencing of tumor-suppressor genes (TSGs) in specific pathways that are also targets of mutation or other mechanisms of inactivation, and is thought to contribute to disease progression and poor prognosis. Epigenetic contributions to myeloid pathogenesis are more complex. Examples include TSG inactivation and oncogenic activation associated with formation of altered chromatin separate from CpG methylation. Epigenetic dysregulation occurs at multiple disease stages and at non-CpG island genomic sites, and also includes genomic hypomethylation and small RNA mechanisms of epigenetic regulation. Identification of recurrent mutations in potential epigenetic regulators, including TET2, IDH1, IDH2, DNMT3A, UTX, and ASXL1, were recently described. Accordingly, therapeutics directed towards epigenetic mechanisms including methylation inhibitors and histone deacetylase (HDAC) inhibitors have had some clinical success when applied to MDS and AML. However, identification of the underlying mechanisms associated with clinical responses and drug resistance remain enigmatic. Remarkably, in spite of significant molecular and translational progress, there are currently no epigenetic biomarkers in widespread clinical use. In this review, we explore the potential applications of epigenetic biomarker discovery, including epigenetic profiling for myeloid malignancy pathogenesis understanding, diagnostic classification, and development of effective treatment paradigms for these generally considered poor prognosis disorders.

The clinical impact of DNA methylation frequencies of JAK2 negative regulators in patients with essential thrombocythemia

Suppressors of cytokine signalling (SOCS) and protein tyrosine phosphatase (PTPN) proteins are negative regulators of Janus Kinase 2 (JAK2). They are thought to be involved in the molecular pathogenesis of essential thrombocythaemia (ET) particularly in patients with unmutated JAK2. In this study we compared DNA methylation of SOCS1, SOCS3 and PTPN6 in peripheral blood cells between 39 ET patients (24 JAK2 V617F mutated) and 22 healthy controls by methylation specific PCR (MSP) and analysed the clinical outcome of patients with respect to DNA methylation. In SOCS1, ET patients showed significantly less methylation (P<0.05) than healthy controls, and in SOCS3 and PTPN6 such a tendency was shown. However, there were no significant differences in the methylation frequencies between JAK2 wildtype and mutated ET patients. In addition, no correlation was detected between methylation of SOCS and PTPN and any clinical outcome parameters. Taken together, regarding the genomic regions investigated our data indicate a minor role of methylation of JAK2 negative regulators for the clinical course of ET.

New mutations and pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are clonal disorders characterized by excessive production of mature blood cells. In the majority of classic MPN—polycythemia vera, essential thrombocythemia, and primitive myelofibrosis—driver oncogenic mutations affecting Janus kinase 2 (JAK2) or MPL lead to constitutive activation of cytokine-regulated intracellular signaling pathways. LNK, c-CBL, or SOCSs (all negative regulators of signaling pathways), although infrequently targeted, may either drive the disease or synergize with JAK2 and MPL mutations. IZF1 deletions or TP53 mutations are mainly found at transformation phases and are present at greater frequency than in de novo acute myeloid leukemias. Loss-of-function mutations in 3 genes involved in epigenetic regulation, TET2, ASXL1, and EZH2, may be early events preceding JAK2V617F but may also occur late during disease progression. They are more frequently observed in PMF than PV and ET and are also present in other types of malignant myeloid diseases. A likely hypothesis is that they facilitate clonal selection, allowing the dominance of the JAK2V617F subclone during the chronic phase and, together with cooperating mutations, promote blast crisis. Their precise roles in hematopoiesis and in the pathogenesis of MPN, as well as their prognostic impact and potential as a therapeutic target, are currently under investigation.

Division of labor by dual feedback regulators controls JAK2/STAT5 signaling over broad ligand range

Cellular signal transduction is governed by multiple feedback mechanisms to elicit robust cellular decisions. The specific contributions of individual feedback regulators, however, remain unclear. Based on extensive time-resolved data sets in primary erythroid progenitor cells, we established a dynamic pathway model to dissect the roles of the two transcriptional negative feedback regulators of the suppressor of cytokine signaling (SOCS) family, CIS and SOCS3, in JAK2/STAT5 signaling. Facilitated by the model, we calculated the STAT5 response for experimentally unobservable Epo concentrations and provide a quantitative link between cell survival and the integrated response of STAT5 in the nucleus. Model predictions show that the two feedbacks CIS and SOCS3 are most effective at different ligand concentration ranges due to their distinct inhibitory mechanisms. This divided function of dual feedback regulation enables control of STAT5 responses for Epo concentrations that can vary 1000-fold in vivo. Our modeling approach reveals dose-dependent feedback control as key property to regulate STAT5-mediated survival decisions over a broad range of ligand concentrations.

Epigenetic abnormalities in myeloproliferative neoplasms: a target for novel therapeutic strategies

The myeloproliferative neoplasms (MPNs) are a group of clonal hematological malignancies characterized by a hypercellular bone marrow and a tendency to develop thrombotic complications and to evolve to myelofibrosis and acute leukemia. Unlike chronic myelogenous leukemia, where a single disease-initiating genetic event has been identified, a more complicated series of genetic mutations appear to be responsible for the BCR-ABL1-negative MPNs which include polycythemia vera, essential thrombocythemia, and primary myelofibrosis. Recent studies have revealed a number of epigenetic alterations that also likely contribute to disease pathogenesis and determine clinical outcome. Increasing evidence indicates that alterations in DNA methylation, histone modification, and microRNA expression patterns can collectively influence gene expression and potentially contribute to MPN pathogenesis. Examples include mutations in genes encoding proteins that modify chromatin structure (EZH2, ASXL1, IDH1/2, JAK2V617F, and IKZF1) as well as epigenetic modification of genes critical for cell proliferation and survival (suppressors of cytokine signaling, polycythemia rubra vera-1, CXC chemokine receptor 4, and histone deacetylase (HDAC)). These epigenetic lesions serve as novel targets for experimental therapeutic interventions. Clinical trials are currently underway evaluating HDAC inhibitors and DNA methyltransferase inhibitors for the treatment of patients with MPNs.

JAK2 V617F and beyond: role of genetics and aberrant signaling in the pathogenesis of myeloproliferative neoplasms

Dysregulated signaling is a hallmark of chronic myeloproliferative neoplasms (MPNs), as evidenced by the identification of the activating JAK2 V617F somatic mutation in almost all patients with polycythemia vera (PV) and 50-60% of essential thrombocythemia and primary myelofibrosis patients. These disorders are clinically distinct, raising the question of how a single mutation can result in such phenotypic diversity. Mouse models have demonstrated that the level of JAK2 V617F expression can modulate the phenotype, and clinical studies of JAK2 V617F allele burden have reported similar findings. It has also been hypothesized that one or more pre-JAK2 V617F events may modify the MPN phenotype. However, the molecular basis of JAK2 V617F-negative essential thrombocythemia and primary myelofibrosis remains largely unexplained. Mutations in the TET2 gene have been identified in both JAK2 V617F-positive and -negative MPNs and other myeloid neoplasms, but their functional and clinical significance have yet to be clarified. In addition, recent reports have identified a specific germline haplotype that increases the predisposition to MPNs. The role of inhibitory pathways (e.g., SOCS and LNK) in regulating JAK-STAT signaling in MPNs is being increasingly recognized. The implications of these findings and their clinical relevance are the focus of this article.

Endothelial progenitor cells are clonal and exhibit the JAK2V617F mutation in a subset of thrombotic patients with Ph-negative myeloproliferative neoplasms

In this study we investigated whether neoplastic transformation occurring in Philadelphia (Ph)–negative myeloproliferative neoplasms (MPNs) could involve also the endothelial cell compartment. We evaluated the level of endothelial colony-forming cells (E-CFCs) in 42 patients (15 with polycythemia vera, 12 with essential thrombocythemia, and 15 with primary myelofibrosis). All patients had 1 molecular abnormality (JAK2V617F or MPLW515K mutations, SOCS gene hypermethylation, clonal pattern of growth) detectable in their granulocytes. The growth of colonies was obtained in 22 patients and, among them, patients with primary myelofibrosis exhibited the highest level of E-CFCs. We found that E-CFCs exhibited no molecular abnormalities in12 patients, had SOCS gene hypermethylation, were polyclonal at human androgen receptor analysis in 5 patients, and resulted in JAK2V617F mutated and clonal in 5 additional patients, all experiencing thrombotic complications. On the whole, patients with altered E-CFCs required antiproliferative therapy more frequently than patients with normal E-CFCs. Moreover JAK2V617F-positive E-CFCs showed signal transducer and activator of transcription 5 and 3 phosphorylation rates higher than E-CFCs isolated from healthy persons and patients with MPN without molecular abnormalities. Finally, JAK2V617F-positive E-CFCs exhibited a high proficiency to adhere to normal mononuclear cells. This study highlights a novel mechanism underlying the thrombophilia observed in MPN.

Genetic and epigenetic complexity in myeloproliferative neoplasms

The past 7 years have witnessed remarkable progress in our understanding of the genetics of BCR-ABL-negative myeloproliferative neoplasms (MPNs) and has revealed layers of unexpected complexity. Deregulation of JAK2 signaling has emerged as a central feature, but despite having biological activities that recapitulate the cardinal features MPNs in model systems, JAK2 mutations are often secondary events. Several other mutated genes have been identified with a common theme of involvement in the epigenetic control of gene expression. Remarkably, the somatic mutations identified to date do not seem to be acquired in any preferred order, and it is possible that the disease-initiating events remain to be identified. The finding of complex clonal hierarchies in many cases suggests genetic instability that, in principle, may be inherited or acquired. A common haplotype has been identified that is strongly associated with the acquisition of JAK2 mutations, but the cause of relatively high-penetrance familial predisposition to MPNs remains elusive. This review summarizes the established facts relating to the genetics of MPNs, but highlights recent findings and areas of controversy.

Deactylase inhibition in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPN) are clonal haemopoietic progenitor cell disorders characterized by the proliferation of one or more of the haemopoietic lineages (myeloid, erythroid and/or megakaryocytic). The MPNs include eight haematological disorders: chronic myelogenous leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), systemic mastocytosis (SM), chronic eosinophilic leukemia, not otherwise specified (CEL, NOS), chronic neutrophilic leukemia (CNL), and unclassifiable MPN (MPN, U). Therapeutic interventions for MPNs include the use of tyrosine kinase inhibitors (TKIs) for BCR-ABL1(+) CML and JAK2 inhibitors for PV, ET and PMF. Histone deacetylase inhibitors (HDACi) are a novel class of drugs capable of altering the acetylation status of both histone and non-histone proteins, thereby affecting a repertoire of cellular functions in neoplastic cells including proliferation, differentiation, immune responses, angiogenesis and survival. Preliminary studies indicate that HDACi when used in combination with tyrosine kinase or JAK2 inhibitors may overcome resistance to the latter agents and enhance the pro-apoptotic effects on MPN cells. This review provides a review of pre-clinical and clinical studies that have explored the use of HDACi as potential therapeutics for MPNs.

Erythropoietin receptor response circuits

PURPOSE OF REVIEW

In 1985-1989, erythropoietin (EPO), its receptor (EPOR), and janus kinase 2 were cloned; established to be essential for definitive erythropoiesis; and initially intensely studied. Recently, new impetus, tools, and model systems have emerged to re-examine EPO/EPOR actions, and are addressed in this review. Impetus includes indications that EPO affects significantly more than standard erythroblast survival pathways, the development of novel erythropoiesis-stimulating agents, increasing evidence for EPO/EPOR cytoprotection of ischemically injured tissues, and potential EPO-mediated worsening of tumorigenesis.

RECENT FINDINGS

New findings are reviewed in four functional contexts: (pro)erythroblast survival mechanisms, new candidate EPO/EPOR effects on erythroid cell development and new EPOR responses, EPOR downmodulation and trafficking, and novel erythropoiesis-stimulating agents.

SUMMARY

As Current Opinion, this monograph seeks to summarize, and provoke, new EPO/EPOR action concepts. Specific problems addressed include: beyond (and before) BCL-XL, what key survival factors are deployed in early-stage proerythroblasts? Are distinct EPO/EPOR signals transduced in stage-selective fashions? Is erythroblast proliferation also modulated by EPO/EPOR signals? What functions are subserved by new noncanonical EPO/EPOR response factors (e.g. podocalyxin like-1, tribbles 3, reactive oxygen species, and nuclear factor kappa B)? What key regulators mediate EPOR inhibition and trafficking? And for emerging erythropoiesis-stimulating agents, to what extent do activities parallel EPOs (or differ in advantageous, potentially complicating ways, or both)?

Treatment of progression of Philadelphia-negative myeloproliferative neoplasms to myelodysplastic syndrome or acute myeloid leukemia by azacitidine: a report on 54 cases on the behalf of the Groupe Francophone des Myelodysplasies (GFM)

Transformation of Philadelphia (Ph)-negative myeloproliferative neoplasms (MPNs) to myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) is associated with poor response to chemotherapy and short survival. Fifty-four patients with Ph-negative MPN (including 21 essential thrombocythemia [ET], 21 polycythemia vera [PV], 7 primary myelofibrosis, and 5 unclassified MPN) who had progressed to AML (n = 26) or MDS (n = 28) were treated with azacitidine in a patient-named program. Overall response rate was 52% (24% complete response [CR], 11% partial response [PR], 8% marrow CR or CR with incomplete recovery of cytopenias, 9% hematologic improvement) and median response duration was 9 months. Prognostic factors were for overall response the underlying MPN (71% vs 33% responses in ET and PV, respectively; P = .016); prognostic factors for CR achievement were the underlying MPN (14% CR for PV vs 43% for ET; P = .040) and World Health Organization classification at transformation (36% vs 12% CR in MDS and AML, respectively, P = .038). Recurrence of chronic phase features of the initial MPN was observed in 39% of the responders. Median overall survival was 11 months. Azacitidine gives encouraging results in Ph-negative MPN having progressed to AML or MDS, but response duration is short, and consolidation treatments have to be evaluated.

Molecular and genetic bases of myeloproliferative disorders: questions and perspectives

The discovery of the JAK2V617F mutation followed by the discovery of JAK2 exon 12 and MPLW515 mutations has completely modified the understanding, diagnosis, and management of the classic myeloproliferative disorders (MPDs), which include polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). Nonetheless, genetic defects have not yet been identified in about 40% of ET and PMF. There is now strong evidence that these mutations are the oncogenic events that drive these disorders and are responsible for most biologic and clinical abnormalities. In addition, there are convincing data indicating that the number of JAK2V617F copies (homozygosity vs. heterozygosity) is important in explaining how a single mutation can be associated with several disorders. However, it is still uncertain whether these mutations are sufficient to explain the full development, heterogeneity, and progression of MPD, or if other genetic or epigenetic events are also necessary. In this review, we discuss different hypothetical models of MPD pathogenesis supported by recent findings. Further characterization of the molecular events operating in these disorders will be essential in fully understanding their pathogenesis and in developing new therapeutic approaches.

Two routes to leukemic transformation after a JAK2 mutation-positive myeloproliferative neoplasm

Acute myeloid leukemia (AML) may follow a JAK2-positive myeloproliferative neoplasm (MPN), although the mechanisms of disease evolution, often involving loss of mutant JAK2, remain obscure. We studied 16 patients with JAK2-mutant (7 of 16) or JAK2 wild-type (9 of 16) AML after a JAK2-mutant MPN. Primary myelofibrosis or myelofibrotic transformation preceded all 7 JAK2-mutant but only 1 of 9 JAK2 wild-type AMLs (P = .001), implying that JAK2-mutant AML is preceded by mutation(s) that give rise to a "myelofibrosis" phenotype. Loss of the JAK2 mutation by mitotic recombination, gene conversion, or deletion was excluded in all wild-type AMLs. A search for additional mutations identified alterations of RUNX1, WT1, TP53, CBL, NRAS, and TET2, without significant differences between JAK2-mutant and wild-type leukemias. In 4 patients, mutations in TP53, CBL, or TET2 were present in JAK2 wild-type leukemic blasts but absent from the JAK2-mutant MPN. By contrast in a chronic-phase patient, clones harboring mutations in JAK2 or MPL represented the progeny of a shared TET2-mutant ancestral clone. These results indicate that different pathogenetic mechanisms underlie transformation to JAK2 wild-type and JAK2-mutant AML, show that TET2 mutations may be present in a clone distinct from that harboring a JAK2 mutation, and emphasize the clonal heterogeneity of the MPNs.

Epigenetic therapy in myeloproliferative neoplasms: evidence and perspectives

The classic Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), which include polycythaemia vera, essential thrombocythaemia and primary myelofibrosis, originate from a stem cell-derived clonal myeloproliferation that manifests itself with variable haematopoietic cell lineage involvement; they are characterized by a high degree of similarities and the chance to transform each to the other and to evolve into acute leukaemia. Their molecular pathogenesis has been associated with recurrent acquired mutations in janus kinase 2 (JAK2) and myeloproliferative leukemia virus oncogene (MPL). These discoveries have simplified the diagnostic approach and provided a number of clues to understanding the phenotypic expression of MPNs; furthermore, they represented a framework for developing and/or testing in clinical trials small molecules acting as tyrosine kinase inhibitors. On the other hand, evidence of abnormal epigenetic gene regulation as a mechanism potentially contributing to the pathogenesis and the phenotypic diversity of MPNs is still scanty; however, study of epigenetics in MPNs represents an active field of research. The first clinical trials with epigenetic drugs have been completed recently, whereas others are still ongoing; results have been variable and at present do not allow any firm conclusion. Novel basic and translational information concerning epigenetic gene regulation in MPNs and the perspectives for therapy will be critically addressed in this review.

Methylation status of RASSF1A in patients with chronic myeloid leukemia

RASSF1A, a key cell cycle related gene, is expressed in all hematopoietic cells, it is implicated in ras signaling pathway and its promoter hypermethylation is observed in a wide variety of solid tumors. Till now, RASSF1A methylation status has not been investigated in patients with chronic myeloid leukemia (CML). In this study, we analyzed 41 patients carrying the BCR-ABL rearrangement, in different stages of the disease. No patient displayed RASSF1A promoter methylation, although the K562 erythroleukemia cell line, bearing the BCR-ABL rearrangement, was found methylated. Thus, our findings indicate that RASSF1A methylation does not appear to represent a critical step in the pathogenesis and/or the progression of CML.

Aberrant signal transduction pathways in myeloproliferative neoplasms

The BCR-ABL-negative myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), entered the spotlight in 2005 when the unique somatic acquired JAK2 V617F mutation was described in >95% of PV and in 50% of ET and PMF patients. For the very rare PV patients who do not harbor the JAK2 V617F mutation, exon 12 JAK2 mutants were discovered also to result in activated forms of JAK2. A minority of ET and PMF patients harbor mutations that constitutively activate the thrombopoietin receptor (TpoR). In bone marrow reconstitution models based on retroviral transduction, the phenotype induced by JAK2 V617F is less severe and different from the rapid fatal myelofibrosis induced by TpoR W515L. The reasons for these differences are unknown. Exactly by which mechanism(s) one acquired somatic mutation, JAK2 V617F, can promote three different diseases remains a mystery, although gene dosage and host genetic variation might have important functions. We review the recent progress made in deciphering signaling anomalies in PV, ET and PMF, with an emphasis on the relationship between JAK2 V617F and cytokine receptor signaling and on cross-talk with several other signaling pathways.

SOCS2: inhibitor of JAK2V617F-mediated signal transduction

Janus kinase 2 (JAK2)V617F-activating mutations (JAK2mu) occur in myeloproliferative disorders (MPDs) and myelodysplastic syndromes (MDSs). Cell lines MB-02, MUTZ-8, SET-2 and UKE-1 carry JAK2V617F and derive from patients with MPD/MDS histories. Challenging the consensus that expression of JAK2V617F is the sole precondition for cytokine independence in class I cytokine receptor-positive cells, two of four of the JAK2mu cell lines were growth factor-dependent. These cell lines resembled JAK2wt cells regarding JAK2/STAT5 activation: cytokine deprivation effected dephosphorylation, whereas erythropoetin or granulocyte colony-stimulating factor induced phosphorylation of JAK2 and STAT5. Cytokine independence correlated with low expression and cytokine dependence with high expression of the JAK/STAT pathway inhibitor suppressor of cytokine signaling 2 (SOCS2) suggesting a two-step mechanism for cytokine independence of MPD cells: (i) activation of the oncogene JAK2V617F and (ii) inactivation of the tumor suppressor gene SOCS2. Confirming that SOCS2 operates as a negative JAK2V617F regulator, SOCS2 knockdown induced constitutive STAT5 phosphorylation in JAK2mu cells. CpG island hypermethylation is reported to promote SOCS gene silencing in malignant diseases. Accordingly, in one of two cytokine-independent cell lines and in two of seven MPD patients, we found SOCS2 hypermethylation associated with reduced promoter access to transcription factors. Our results provide solid evidence that SOCS2 epigenetic downregulation might be an important second step in the genesis of cytokine-independent MPD clones.

Epigenetic alteration of SOCS family members is a possible pathogenetic mechanism in JAK2 wild type myeloproliferative diseases

In polycythemia vera (PV) and essential thrombocythemia (ET) specific JAK2 mutations constitutively activate the JAK-STAT pathway, explaining biologic findings such as endogenous erythroid colony (EECs) growth or PRV-1 RNA overexpression. Since these markers are detected also in JAK2 wild type patients, we hypothesized that, in these cases, the activation of the JAK-STAT pathway could be produced by a deregulation of the suppressor of cytokine signaling (SOCS) protein system. Eighty-one patients with PV and ET (53 adults and 28 children) were investigated for the methylation status of the SOCS-1, SOCS-2 and SOCS-3 CpG islands and for several myeloproliferative markers (including JAK2 and MPL mutations and clonality of hematopoiesis). SOCS-1 or SOCS-3 hypermethylation was identified in 23 patients and was associated with a significant decrease of SOCS-1 or SOCS-3 RNA and protein levels. The gene expression was restored by exposing cells to the demethylating agent 2-deoxyazacytidin. Interestingly, SOCS-1 or SOCS-3 hypermethylation was detected in 6 female patients, proved negative for JAK2 or MPL mutations and exhibiting monoclonal hematopoiesis. In conclusion, SOCS-1 or SOCS-3 hypermethylation can activate the JAK-STAT signaling pathway in alternative or together with JAK2 mutations. These alterations might represent a potential therapeutic target.

Aberrant DNA methylation of the transcription factor C/EBPalpha in acute myelogenous leukemia

We investigated the methylation status of the CCAAT/enhancer binding protein alpha (C/EBPalpha) promoter region near the transcription start site in acute myelogenous leukemia (AML). In hematopoietic tumor cell lines, CpG island hypermethylation of the proximal C/EBPalpha promoter region was associated with transcriptional silencing, and treatment with the demethylating agent 5-aza-2'-deoxycytidine resulted in C/EBPalpha reexpression and promoter demethylation. Aberrant methylation of the C/EBPalpha promoter region occurred in 10/80 diagnostic AML samples, and there was an inverse correlation between aberrant methylation of C/EBPalpha and the negative cell cycle regulator p15. Our results provide further evidence for epigenetic dysregulation of C/EBPalpha in AML.

Comparison of mutated ABL1 and JAK2 as oncogenes and drug targets in myeloproliferative disorders

Constitutively activated mutants of the non-receptor tyrosine kinases (TK) ABL1 (Abelson murine leukemia viral (v-abl) homolog (1) protein) and JAK2 (JAnus Kinase 2 or Just Another Kinase 2) play a central role in the pathogenesis of clinically and morphologically distinct chronic myeloproliferative disorders but are also found in some cases of de novo acute leukemia and lymphoma. Ligand-independent activation occurs as a consequence of point mutations or insertions/deletions within functionally relevant regulatory domains (JAK2) or the creation of TK fusion proteins by balanced reciprocal translocations, insertions or episomal amplification (ABL1 and JAK2). Specific abnormalities are correlated with clinical phenotype, although some are broad and encompass several World Health Organization-defined entities. TKs are excellent drug targets as exemplified by the activity of imatinib in BCR-ABL1-positive disease, particularly chronic myeloid leukemia. Resistance to imatinib is seen in a minority of cases and is often associated with the appearance of secondary point mutations within the TK domain of BCR-ABL1. These mutations are highly variable in their sensitivity to increased doses of imatinib or alternative TK inhibitors such as nilotinib or dasatinib. Selective and non-selective inhibitors of JAK2 are currently being developed, and encouraging data from pre-clinical experiments and initial phase-I studies regarding efficacy and potential toxicity of these compounds have already been reported.

Hypermethylation of CXCR4 promoter in CD34+ cells from patients with primary myelofibrosis

Constitutive mobilization of CD34(+) cells in patients with primary myelofibrosis (PMF) has been attributed to proteolytic disruption of the CXCR4/SDF-1 axis and reduced CXCR4 expression. We document here that the number of circulating CD34(+)/CXCR4(+) cells in PMF patients, as well as the cellular CXCR4 expression, was directly related to CXCR4 mRNA level and that reduced CXCR4 mRNA level was not due to SDF-1-induced downregulation. To address whether epigenetic regulation contributes to defective CXCR4 expression, we studied the methylation status of the CXCR4 promoter using methylation-specific polymerase chain reaction and methylation-specific sequencing in the JAK2V617F-positive HEL cell line and in CD34(+) cells. We found that CD34(+) cells from PMF patients, unlike those from normal subjects, presented hypermethylation of CXCR4 promoter CpG island 1. Following incubation with the demethylating agent 5-Aza-2'-deoxycytidine (5-AzaD), the percentage of PMF CD34(+) cells expressing CXCR4 increased 3-10 times, whereas CXCR4 mRNA level increased approximately 4 times. 5-AzaD-treated PMF CD34(+) cells displayed almost complete reversal of CpG1 island 1 hypermethylation and showed enhanced migration in vitro in response to SDF-1. These data point to abnormal methylation of the CXCR4 promoter as a mechanism contributing to constitutive migration of CD34(+) cells in PMF. Disclosure of potential conflicts of interest is found at the end of this article.