Polycythemia vera and other primary polycythemias

Molecular Research of Regulation of Red Blood Cells in Health, Hereditary or Acquired Diseases

During the first era of humanity, the conditions of life, including hunting, fighting, obtaining food, and diseases, were associated with frequent hemorrhages, anemia, and infections, which led to death or untreatable conditions [...].

The transcriptional and regulatory identity of erythropoietin producing cells

Erythropoietin (Epo) is the master regulator of erythropoiesis and oxygen homeostasis. Despite its physiological importance, the molecular and genomic contexts of the cells responsible for renal Epo production remain unclear, limiting more-effective therapies for anemia. Here, we performed single-cell RNA and transposase-accessible chromatin (ATAC) sequencing of an Epo reporter mouse to molecularly identify Epo-producing cells under hypoxic conditions. Our data indicate that a distinct population of kidney stroma, which we term Norn cells, is the major source of endocrine Epo production in mice. We use these datasets to identify the markers, signaling pathways and transcriptional circuits characteristic of Norn cells. Using single-cell RNA sequencing and RNA in situ hybridization in human kidney tissues, we further provide evidence that this cell population is conserved in humans. These preliminary findings open new avenues to functionally dissect EPO gene regulation in health and disease and may serve as groundwork to improve erythropoiesis-stimulating therapies.

MTAP-related increased erythroblast proliferation as a mechanism of polycythaemia vera

Polycythaemia vera (PV) is a haematological disorder caused by an overproduction of erythroid cells. To date, the molecular mechanisms involved in the disease pathogenesis are still ambiguous. This study aims to identify aberrantly expressed proteins in erythroblasts of PV patients by utilizing mass spectrometry-based proteomic analysis. Haematopoietic stem cells (HSCs) were isolated from newly-diagnosed PV patients, PV patients who have received cytoreductive therapy, and healthy subjects. In vitro erythroblast expansion confirmed that the isolated HSCs recapitulated the disease phenotype as the number of erythroblasts from newly-diagnosed PV patients was significantly higher than those from the other groups. Proteomic comparison revealed 17 proteins that were differentially expressed in the erythroblasts from the newly-diagnosed PV patients compared to those from healthy subjects, but which were restored to normal levels in the patients who had received cytoreductive therapy. One of these proteins was S-methyl-5′-thioadenosine phosphorylase (MTAP), which had reduced expression in PV patients’ erythroblasts. Furthermore, MTAP knockdown in normal erythroblasts was shown to enhance their proliferative capacity. Together, this study identifies differentially expressed proteins in erythroblasts of healthy subjects and those of PV patients, indicating that an alteration of protein expression in erythroblasts may be crucial to the pathology of PV.

Congenital erythrocytosis

Erythrocytosis, or increased red cell mass, may be labeled as primary or secondary, depending on whether the molecular defect is intrinsic to the red blood cells/their precursors or extrinsic to them, the latter being typically associated with elevated erythropoietin (EPO) levels. Inherited/congenital erythrocytosis (CE) of both primary and secondary types is increasingly recognized as the cause in many patients in whom acquired, especially neoplastic causes have been excluded. During the past two decades, the underlying molecular mechanisms of CE are increasingly getting unraveled. Gain-in-function mutations in the erythropoietin receptor gene were among the first to be characterized in a disorder termed primary familial and congenital polycythemia. Another set of mutations affect the components of the oxygen-sensing pathway. Under normoxic conditions, the hypoxia-inducible factor (HIF), upon hydroxylation by the prolyl-4-hydroxylase domain protein 2 (PHD2) enzyme, is degraded by the von Hippel-Lindau protein. In hypoxic conditions, failure of prolyl hydroxylation leads to stabilization of HIF and activation of the EPO gene. CE has been found to be caused by loss-of-function mutations in VHL and PHD2/EGLN1 as well as gain-of-function mutations in HIF-2α (EPAS1), all resulting in constitutive activation of EPO signaling. Apart from these, globin gene mutations leading to formation of high oxygen affinity hemoglobins also cause CE. Rarely, bisphosphoglycerate mutate mutations, affecting the 2,3-bisphosphoglycerate levels, can increase the oxygen affinity of hemoglobin and cause CE. This narrative review examines the current mutational spectrum of CE and the distinctive pathogenetic mechanisms that give rise to this increasingly recognized condition in various parts of the world.

Mouse Models of Myeloid Malignancies

Mouse models of human myeloid malignancies support the detailed and focused investigation of selected driver mutations and represent powerful tools in the study of these diseases. Carefully developed murine models can closely recapitulate human myeloid malignancies in vivo, enabling the interrogation of a number of aspects of these diseases including their preclinical course, interactions with the microenvironment, effects of pharmacological agents, and the role of non-cell-autonomous factors, as well as the synergy between co-occurring mutations. Importantly, advances in gene-editing technologies, particularly CRISPR-Cas9, have opened new avenues for the development and study of genetically modified mice and also enable the direct modification of mouse and human hematopoietic cells. In this review we provide a concise overview of some of the important mouse models that have advanced our understanding of myeloid leukemogenesis with an emphasis on models relevant to clonal hematopoiesis, myelodysplastic syndromes, and acute myeloid leukemia with a normal karyotype.

JAK2 and Beyond: Mutational Study of JAK2V617 in Myeloproliferative Disorders and Haematological Malignancies in Kashmiri population

BACKGROUND

Janus Tyrosine Kinase-2 (JAK2 V617F), a novel point mutation affecting the MPD'S is a somatic gain-of-function mutation. It alters a highly conserved amino acid valine in the negative regulatory JH2 domain to phenylalanine predicted to dysregulate kinase activity.

AIM

To evaluate the prevalence and clinical significance of JAK2 V617F mutation in various MPD's as well as in hematological malignancies.

SUBJECTS AND METHODS

JAK2 mutation was assessed in 90 patients with myeloproliferative disorders and 47 leukemic patients. In addition, peripheral blood samples from 90 healthy donors were also collected as control. We used a highly sensitive Allele-Specific polymerase chain reaction (AS-PCR) for the detection and confirmed the mutation further by direct sequencing.

RESULTS

Our results showed significant differences between various disorders with respect to either the proportion of positivity or that of mutant alleles. JAK2-V617F was detected in 67/90 MPD patients and 02/17 for AML,01/11 for ALL-L1,02/12 for ALL-L2 and 02/07 for CML and 90 healthy controls.

CONCLUSION

From the above findings it is evident that the JAK2 V617F mutation is widespread not only in MPD's but also in hematological malignancies, which might as well lead to the new classification of MPD'S. Our data also suggest that different genetic events may lead to JAK-STAT pathway activation in different malignancies.

Normal and pathological erythropoiesis in adults: from gene regulation to targeted treatment concepts

Pathological erythropoiesis with consequent anemia is a leading cause of symptomatic morbidity in internal medicine. The etiologies of anemia are complex and include reactive as well as neoplastic conditions. Clonal expansion of erythroid cells in the bone marrow may result in peripheral erythrocytosis and polycythemia but can also result in anemia when clonal cells are dysplastic and have a maturation arrest that leads to apoptosis and hinders migration, a constellation typically seen in the myelodysplastic syndromes. Rarely, clonal expansion of immature erythroid blasts results in a clinical picture resembling erythroid leukemia. Although several mechanisms underlying normal and abnormal erythropoiesis and the pathogenesis of related disorders have been deciphered in recent years, little is known about specific markers and targets through which prognosis and therapy could be improved in anemic or polycythemic patients. In order to discuss new markers, targets and novel therapeutic approaches in erythroid disorders and the related pathologies, a workshop was organized in Vienna in April 2017. The outcomes of this workshop are summarized in this review, which includes a discussion of new diagnostic and prognostic markers, the updated WHO classification, and an overview of new drugs used to stimulate or to interfere with erythropoiesis in various neoplastic and reactive conditions. The use and usefulness of established and novel erythropoiesis-stimulating agents for various indications, including myelodysplastic syndromes and other neoplasms, are also discussed.

Somatic Mutations in Philadelphia Chromosome-Negative Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPN) include polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). MPN are characterized by clonal proliferation of myeloid progenitors leading to erythrocytosis, thrombocytosis, or leukocytosis, and risk of hemorrhagic and thrombotic events, as well as myelofibrosis and blast transformation. The discovery of somatic mutations in MPN, namely JAK2 V617F, JAK2 exon 12, MPL, and CALR mutations, has permitted a more specific approach to diagnosis and treatment. The prevalence of JAK2 V617F mutations is higher than 95% in PV, 50%-75% in ET and 40%-75% in PMF. JAK2 exon 12 mutations are specific of PV. A 20%-30% of patients with ET and PMF present a CALR mutation. The screening of mutations strengthens the diagnosis of MPN since 97% of MPN have at least 1 somatic mutation. Interestingly, different mutations grant different phenotype and prognosis. Of particular importance, CALR mutations grant a favorable prognosis in ET and PMF, while ASXL1 mutations confer a poorer outcome. In fact, the use of CALR/ASXL1 status for the prognostication of patients has increased clinical value and is now suggested for guidance of therapy in PMF. The increasing importance of mutations in the management of MPN warrants a more frequent revision of current diagnostic criteria and prognostic models and a better understanding of the mechanisms leading to MPN subset differentiation.

A novel splicing site IRP1 somatic mutation in a patient with pheochromocytoma and JAK2V617F positive polycythemia vera: a case report

Background

The role of the hypoxia signaling pathway in the pathogenesis of pheochromocytoma/paraganglioma (PPGL)-polycythemia syndrome has been elucidated. Novel somatic mutations in hypoxia-inducible factor type 2A (HIF2A) and germline mutations in prolyl hydroxylase type 1 and type 2 (PHD1 and PHD2) have been identified to cause upregulation of the hypoxia signaling pathway and its target genes including erythropoietin (EPO) and its receptor (EPOR). However, in a minority of patients presenting with this syndrome, the genetics and molecular pathogenesis remain unexplained. The aim of the present study was to uncover novel genetic causes of PPGL-polycythemia syndrome.

Case presentation

A female presented with a history of JAK2^V617F positive PV, diagnosed in 2007, and right adrenal pheochromocytoma diagnosed and resected in 2011. Her polycythemia symptoms and hematocrit levels continued to worsen from 2007 to 2011, with an increased frequency of phlebotomies. Postoperatively, until early 2013, her hematocrit levels remained normalized. Following this, the hematocrit levels ranged between 46.4 and 48.9% [35–45%]. Tumor tissue from the patient was further tested for mutations in genes related to upregulation of the hypoxia signaling pathway including iron regulatory protein 1 (IRP1), which is a known regulator of HIF-2α mRNA translation. Functional studies were performed to investigate the consequences of these mutations, especially their effect on the HIF signaling pathway and EPO. Indel mutations (c.267-1_267delGGinsTA) were discovered at the exon 3 splicing site of IRP1. Minigene construct and splicing site analysis showed that the mutation led to a new splicing site and a frameshift mutation of IRP1, which caused a truncated protein. Fluorescence in situ hybridization analysis demonstrated heterozygous IRP1 deletions in tumor cells. Immunohistochemistry results confirmed the truncated IRP1 and overexpressed HIF-2α, EPO and EPOR in tumor cells.

Conclusions

This is the first report which provides direct molecular genetic evidence of association between a somatic IRP1 loss-of-function mutation and PHEO and secondary polycythemia. In patients diagnosed with PHEO/PGL and polycythemia with negative genetic testing for mutations in HIF2A, PHD1/2, and VHL, IRP1 should be considered as a candidate gene.

MLF1IP promotes normal erythroid proliferation and is involved in the pathogenesis of polycythemia vera

Myelodysplasia/myeloid leukemia factor 1-interacting protein (MLF1IP) appears to be an erythroid lineage-specific gene in mice; however, its role in normal erythropoiesis and erythropoietic disorders have not yet been elucidated. Here, we found that MLF1IP is abundantly expressed in human erythroid progenitor cells and that MLF1IP-deficiency reduces cell proliferation resulting from cell cycle arrest. Moreover, MLF1IP expression is exclusively elevated in CFU-E cells from polycythemia vera (PV) patients, and MLF1IP transgenic mice develop a PV-like disorder. Further analyses revealed that the erythroid progenitors and early-stage erythroblasts from these transgenic mice expand by up-regulating cyclin D2 and down-regulating p27 and p21. Thus, our data demonstrate that MLF1IP promotes erythroid proliferation and is involved in the pathogenesis of PV, suggesting that it might be a novel molecular target for erythropoietic disorders.

Comparative long-term effects of interferon α and hydroxyurea on human hematopoietic progenitor cells

Interferon α (IFNα) is used clinically to restore polyclonal hematopoiesis in patients with the myeloproliferative neoplasms polycythemia vera and essential thrombocythemia and to improve chemosensitivity in chronic myeloid leukemia patients. However, the mechanisms by which IFNα affects disease-initiating hematopoietic stem and progenitor cells (HSPCs) remain poorly understood. Although IFNα has been found to transiently impair quiescence of murine hematopoietic stem cells, its effects on human HSPCs have not been studied in vivo. Here, we compared bone marrow serially obtained from patients with myeloproliferative neoplasms before and during pegylated IFNα treatment against marrow serially obtained from patients on hydroxyurea. The percentage of HSPCs actively undergoing cell cycle was increased after pegylated IFNα treatment in a majority of patients compared with hydroxyurea-treated controls, suggesting that IFNα promotes cell division. Furthermore, transcriptional profiling revealed that cell cycle-associated genes were induced, whereas genes involved in HSPC quiescence were repressed during IFNα treatment. Compared with hydroxyurea-treated controls, pegylated IFNα-treated patients had similar numbers of HSPCs, but increased numbers of hematopoietic progenitors as determined by colony formation assay, indicating an increase in myeloid proliferation/differentiation. These effects occurred regardless of JAK2 mutational status. Together, these data provide the first in vivo evidence that pegylated IFNα promotes cell division and differentiation of human HSPCs.

Back to biology: new insights on inheritance in myeloproliferative disorders

The myeloproliferative disorders (MPDs) are a group of hematologic diseases with significant overlap in both clinical phenotype and genetic etiology. While most often caused by acquired somatic mutations in hematopoietic stem cells, the presence of familial clustering in MPD cases suggests that inheritance is an important factor in the etiology of this disease. Though far less common than sporadic disease, inherited MPDs can be clinically indistinguishable from sporadic disease. Recently, germline mutations in Janus kinase 2 (JAK2) and MPL, two genes frequently mutated in sporadic MPD, have been shown to cause inherited thrombocytosis. Study of the function of these mutant proteins has led to a new understanding of the biological mechanisms that produce myeloproliferative disease. In this review, we summarize the data regarding inherited mutations that cause or predispose to MPDs, with a focus on the biological effects of mutant proteins. We propose that defining inherited MPDs in this manner has the potential to simplify diagnosis in a group of disorders that can be difficult to differentiate clinically.

Abnormal adhesion of red blood cells in polycythemia vera: a prothrombotic effect?

Polycythemia vera (PV) is a myeloproliferative neoplasm (MPN) characterised by the V617F activating mutation in the tyrosine kinase JAK2. PV patients exhibit increased haemoglobin levels and red cell mass because of uncontrolled proliferation of the erythroid lineage. Thrombosis and transformation to acute leukaemia are the major causes of morbidity and mortality in this disease. Increased thrombotic risk in PV patients is multifactorial and complex; it is associated with high levels of haemoglobin, impaired rheology and increased viscosity resulting from erythrocytosis. An additional parameter that might contribute to this risk was recently brought to light by work from our group showing abnormal activation of adhesion proteins in PV RBCs. In this review we provide an overview of these recent findings and discuss how the pro-adhesive features of JAK2V617F-positive red blood cells might initiate and contribute to the circulatory complications described in PV.

Red cell adhesion in human diseases

PURPOSE OF REVIEW

This review discusses the unexpected role of red blood cell (RBC) adhesiveness in the pathophysiology of two red cell diseases, hereditary spherocytosis and polycythemia vera, and two 'nonerythroid' disorders, central retinal vein occlusion and Gaucher disease. These pathologies share common clinical manifestations, that is vaso-occlusion and/or thrombotic events.

RECENT FINDINGS

Recently, the direct involvement of RBC adhesion to the vascular endothelium has been demonstrated in the occurrence of vaso-occlusive events, in particular in sickle cell disease (SCD). Several erythroid adhesion molecules and their ligands have been identified that belong to different molecular classes (integrins, Ig-like molecules, lipids...) and are activated by a variety of signaling pathways. Among these, the laminin receptor, Lutheran/basal cell adhesion molecule, which is activated by phosphorylation, appears to play a central role in several pathologies.

SUMMARY

RBC adhesiveness might be involved in complications such as the vaso-occlusive crisis in SCD, thrombosis in polycythemia vera, splenic sequestration in hereditary spherocytosis, occlusions in central retinal vein occlusion and bone infarcts in Gaucher disease. Characterization of this pathological process at the cellular and molecular levels should prove useful to develop new therapeutic approaches based on the blockade of RBC abnormal interactions with vascular endothelium and/or circulating blood cells.

Germ-line PHD1 and PHD2 mutations detected in patients with pheochromocytoma/paraganglioma-polycythemia

We have investigated genetic/pathogenetic factors associated with a new clinical entity in patients presenting with pheochromocytoma/paraganglioma (PHEO/PGL) and polycythemia. Two patients without hypoxia-inducible factor 2α (HIF2A) mutations, who presented with similar clinical manifestations, were analyzed for other gene mutations, including prolyl hydroxylase (PHD) mutations. We have found for the first time a germ-line mutation in PHD1 in one patient and a novel germ-line PHD2 mutation in a second patient. Both mutants exhibited reduced protein stability with substantial quantitative protein loss and thus compromised catalytic activities. Due to the unique association of patients' polycythemia with borderline or mildly elevated erythropoietin (EPO) levels, we also performed an in vitro sensitivity assay of erythroid progenitors to EPO and for EPO receptor (EPOR) expression. The results show inappropriate hypersensitivity of erythroid progenitors to EPO in these patients, indicating increased EPOR expression/activity. In addition, the present study indicates that HIF dysregulation due to PHD mutations plays an important role in the pathogenesis of these tumors and associated polycythemia. The PHD1 mutation appears to be a new member contributing to the genetic landscape of this novel clinical entity. Our results support the existence of a specific PHD1- and PHD2-associated PHEO/PGL-polycythemia disorder.

KEY MESSAGE

• A novel germ-l i n e PHD1 mutation causing heochromocytoma/paraganglioma and polycythemia. • Increased EPOR activity and inappropriate hypersensitivity of erythroid progenitors to EPO.

Could hypoxia increase the prevalence of thrombotic complications in polycythemia vera?

Thromboses represent a major cause of morbidity and mortality in polycythemia vera but the contributing mechanisms are not fully described. To evaluate whether environmental conditions such as altitude/hypoxia could impact thrombosis history, we retrospectively analyzed thrombosis history in 71 polycythemia vera patients living at an elevation of 5000 feet or more in the Salt Lake City (SLC) area and 166 polycythemia vera patients living near sea level in the Baltimore (BLM) area. The SLC cohort was older with a longer disease duration. No significant differences in type of anticoagulation therapy or prothrombotic factors were present between the two cohorts. After adjusting for age, sex and disease duration, SLC patients experienced an estimated 3.9-fold increase in the odds of a history of thrombosis compared with BLM patients (95% confidence interval 1.8-7.6; P=0.0004). A history of a cardiovascular event was present in 58% of the SLC patients compared with 27% of the BLM patients (P<0.0001). Before diagnosis, thrombosis occurred in 18 and 4% of the SLC and BLM groups, respectively (P=0.003). No correlation between the JAK2 allele burden and thrombosis was observed in this study. This retrospective study suggests that even moderate hypoxia associated with 5000 feet elevation should be considered as an independent prothrombotic risk factor. This observation needs to be confirmed by prospective studies.

Inherited predisposition to myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are haematological disorders characterized by an overproduction of mature myeloid cells with a tendency to transform to acute myeloid leukaemia. Clonal proliferation of myeloid progenitor cells is driven by somatically acquired mutations, most notably JAK2 V617F, but there are important features relating to pathogenesis and phenotypic diversity that cannot be explained by acquired mutations alone. In this review we consider what is currently known about the role that inherited factors play in the development and biology of both sporadic and familial forms of MPN. Although most MPN cases appear to be sporadic, familial predisposition has been recognized for many years in a subset of cases and epidemiological studies have indicated the presence of common susceptibility alleles. Currently the JAK2 46/1 haplotype (also referred to as 'GGCC') is the strongest known predisposition factor for sporadic MPNs carrying a JAK2 V617F mutation, explaining a large proportion of the heritability of this disorder. Less is known about what genetic variants predispose to MPNs that lack JAK2 V617F, but there have been recent reports of interesting associations in biologically plausible candidates, and more loci are set to emerge with the application of systematic genome-wide association methodologies. Several highly penetrant predisposition variants that affect erythropoietin signalling, thrombopoietin signalling or oxygen sensing have been characterized in families with nonclonal hereditary erythrocytosis or thrombocytosis, but much less is known about familial predisposition to true clonal MPN. The heterogeneous pattern of inheritance and presumed genetic heterogeneity in these families makes analysis difficult, but whole exome or genome sequencing should provide novel insights into these elusive disorders.

JAK2V617F activates Lu/BCAM-mediated red cell adhesion in polycythemia vera through an EpoR-independent Rap1/Akt pathway

Polycythemia vera (PV) is characterized by an increased RBC mass, spontaneous erythroid colony formation, and the JAK2V617F mutation. PV is associated with a high risk of mesenteric and cerebral thrombosis. PV RBC adhesion to endothelial laminin is increased and mediated by phosphorylated erythroid Lu/BCAM. In the present work, we investigated the mechanism responsible for Lu/BCAM phosphorylation in the presence of JAK2V617F using HEL and BaF3 cell lines as well as RBCs from patients with PV. High levels of Rap1-GTP were found in HEL and BaF3 cells expressing JAK2V617F compared with BaF3 cells with wild-type JAK2. This finding was associated with increased Akt activity, Lu/BCAM phosphorylation, and cell adhesion to laminin that were inhibited by the dominant-negative Rap1S17N or by the specific Rap1 inhibitor GGTI-298. Surprisingly, knocking-down EpoR in HEL cells did not alter Akt activity or cell adhesion to laminin. Our findings reveal a novel EpoR-independent Rap1/Akt signaling pathway that is activated by JAK2V617F in circulating PV RBCs and responsible for Lu/BCAM activation. This new characteristic of JAK2V617F could play a critical role in initiating abnormal interactions among circulating and endothelial cells in patients with PV.

Myeloproliferative disorders and the hyperviscosity syndrome

Myeloproliferative disorders and the serum hyperviscosity syndrome can rapidly manifest with emergent presentations. Hyperviscosity occurs from pathologic elevations of either the cellular or acellular (protein) fractions of the circulating blood. Classic hyperviscosity syndrome presents with the triad of bleeding diathesis, visual disturbances, and focal neurologic signs. Emergency medicine providers should be aware of these conditions and be prepared to rapidly initiate supportive and early definitive management, including plasma exchange and apharesis. Early consultation with a hematologist is essential to managing these complex patients.

Advances in understanding the pathogenesis of primary familial and congenital polycythaemia

Primary familial and congenital polycythemia (PFCP) is an autosomal-dominant proliferative disorder characterized by erythrocytosis and hypersensitivity of erythroid progenitors to erythropoietin (Epo). Several lines of evidence suggest a causal role of truncated erythropoietin receptor (EpoR) in this disease. In this review, we discuss PFCP in the context of erythrocytosis and EpoR signalling. We focus on recent studies describing mechanisms underlying Epo-dependent EpoR down-regulation. One mechanism depends on internalization mediated through the p85 regulatory subunit of the Phosphoinositide 3-Kinase, and the other utilizes ubiquitin-based proteasomal degradation. Truncated PFCP EpoRs are not properly down-regulated upon stimulation, underscoring the importance of these mechanisms in the pathogenesis of PFCP.

Erythropoiesis in polycythemia vera is hyper-proliferative and has accelerated maturation

Polycythemia vera (PV) is an acquired myeloproliferative clonal disorder, characterized by augmented erythropoiesis. To better define PV pathogenesis, we performed an in vitro erythroid expansion from peripheral blood mononuclear cells of controls and PV patients and evaluated the cells for proliferation, apoptosis, erythroid differentiation, and morphology at the defined time points. PV erythroid progenitors exhibited increased proliferation at days 9-14 and accelerated maturation at days 7-14, with a larger S-phase population (40%) than controls (20%) at day 11; however, the proportion of apoptotic cells was comparable to controls. Previously, we have identified PV-specific dysregulation of several microRNAs (i.e. miR-150, 451, 222, 155, 378). We had analyzed expression profiles of selected target genes of these microRNAs based on in silico prediction and their known function pertinent to the observed PV-specific erythropoiesis differences. p27, cMYB and EPOR showed differential expression in PV erythroid progenitors at the specific stages of erythroid differentiation. In this study, we identified accelerated maturation and hyper-proliferation at early stages of PV erythropoiesis. We speculate that aberrant expression of p27, c-MYB, and EPOR may contribute to these abnormal features in PV erythropoiesis.

Clonal heterogeneity in polycythemia vera patients with JAK2 exon12 and JAK2-V617F mutations

We studied the lineage distribution of JAK2 mutations in peripheral blood of 8 polycythemia vera (PV) patients with exon 12 mutations and in 21 PV patients with JAK2-V617F. Using a quantitative allele discrimination assay, we detected exon 12 mutations in purified granulocytes, monocytes, and platelets of 8 patients studied, but lymphoid cells showed variable involvement and the mutation was absent in T cells. Endogenous erythroid colonies grew in all patients analyzed. One patient displayed erythroid colonies homozygous for the exon 12 mutation with evidence for mitotic recombination on chromosome 9p. In some patients with exon 12 mutations or JAK2-V617F, a proportion of endogenous erythroid colonies were negative for both JAK2 mutations. One patient carried 2 independent clones: one with an exon 12 mutation and a second with JAK2-V617F. The finding of clonal heterogeneity is compatible with the hypothesis that additional clonal events are involved in the pathogenesis of PV.

Study of two tyrosine kinase inhibitors on growth and signal transduction in polycythemia vera

OBJECTIVE

An activating somatic mutation of Janus kinase 2 V617F (JAK2V617F) is present in most polycythemia vera (PV) patients. We studied efficacy of two potent tyrosine kinase inhibitors (TKI), AEE788 and AMN107, in vitro on cells bearing this mutation.

MATERIALS AND METHODS

We employed reporter cells expressing wild-type JAK2 and mutant JAK2V617F, human erythroleukemic cells (HEL) carrying JAK2V617F)to study the efficacy of these TKIs by cell proliferation assay, Annexin-V/propidium iodide staining, and on relevant cell-signaling and apoptotic events. These data were compared to ex vivo expanded native human erythroid PV progenitor cells grown in liquid cultures.

RESULTS

AEE788 showed a time- and dose-dependent growth inhibitory effect that was greater in FDCP cells expressing JAK2V617F and HEL cells than in cells expressing wild-type JAK2. AEE788 caused dephosphorylation of Akt(S243) and signal transducer and activator of transcription 5(Y694) proteins, increase in Annexin-V binding and caspase-3 cleavage, suggesting induction of apoptosis. We also observed AEE788-mediated decrease in heat shock protein 70 and 90 antiapoptotic proteins. Similarly, native PV erythroid progenitors showed more sensitivity to AEE788 than normal erythroid progenitors. AEE788 also exerted dose-dependent inhibition of PV-specific erythroid colonies. Nilotinib (AMN107) however, lacked specificity and required high (>8 microM) concentrations to inhibit growth of JAK2V617F-carrying cells.

CONCLUSION

Our data suggest that AEE788 exerts its apoptotic activity via downregulation of proliferative and antiapoptotic regulatory proteins. To our knowledge, this is the first report demonstrating the effect of AEE788 on PV erythroid progenitors. Differential effects on PV and normal progenitor cells suggest AEE788 has potential in the treatment of PV and other JAK2V617F positive hematologic malignancies.

Increased adhesion to endothelial cells of erythrocytes from patients with polycythemia vera is mediated by laminin alpha5 chain and Lu/BCAM

Patients with polycythemia vera (PV) have a JAK2 (a cytosolic tyrosine kinase) mutation and an increased risk of vascular thrombosis related to red blood cell (RBC) mass and platelet activation. We investigated functional RBC abnormalities that could be involved in thrombosis. RBC adhesion to human umbilical vein endothelial cells (HUVECs) was measured by a radiometric technique and in a flow system by video microscopy, and adhesion molecule expression was determined using specific antibodies (against CD36, CD49d, ICAM-4, Lu/BCAM, CD147, and CD47) and flow cytometry in a group of 38 patients with PV and a group of 36 healthy volunteers. Adhesion of PV RBCs was 3.7-fold higher than that of normal RBCs (P < .001). Adhesion was inhibited when PV RBCs were incubated with anti-Lutheran blood group/basal cell adhesion molecule (Lu/BCAM) or when HUVECs were treated with anti-laminin alpha(5) and to a lesser extent with anti-alpha(3) integrin. Lu/BCAM was constitutively phosphorylated in PV RBCs. Transfection of K562 cells with JAK2 617V>F resulted in increased expression and phosphorylation of Lu/BCAM. Phosphorylation of Lu/BCAM increases RBC adhesion. Our results indicate that JAK2 mutation might be linked to Lu/BCAM modification and increased RBC adhesiveness, which may be a factor favoring thrombosis in PV.

In vitro expansion of erythroid progenitors from polycythemia vera patients leads to decrease in JAK2 V617F allele

OBJECTIVES

A G>T transversion in a tyrosine kinase JAK2 (V617F) was reported in over 80% of patients with polycythemia vera (PV). Current evidence suggests that JAK2(V617F) somatic mutation is involved in the pathogenesis of PV, as it confers erythropoietin-independent proliferation to erythroid progenitor cells. However, several unanswered questions regarding the essential role of JAK2(V617F) arose as 1) it is not a dominant mutation, 2) it is not PV-specific as it is found in several myeloproliferative disorders, and 3) some ( approximately 20%) PV patients lack the JAK2(V617F) mutation. We investigated the relative frequency of JAK2(V617F) in in vitro-expanded PV progenitors.

METHODS

In vitro expansion of erythroid progenitors from mononuclear cells was optimized. Frequency of JAK2(V617F) allele was measured by using allele-specific real-time polymerase chain reaction. Clonality was performed using established procedure.

RESULTS

In vitro expansion of PV erythroid progenitors and differentiated dendritic cells resulted in a decrease of the frequency of JAK2(V617F) allele compared with granulocytes or CD235(+) erythroid progenitors. Clonality analysis demonstrated that although granulocytes of these PV patients were clonal, expanded erythroid cells were polyclonal. However, in vitro-expanded PV erythroid progenitors still had approximately a twofold increased proliferative capacity in comparison with erythroid progenitors from healthy individuals. Erythropoietin favors the cells without JAK2(V617F) allele. Dendritic cells in one out of three patients remained clonal.

CONCLUSION

JAK2(V617F) mutation does not provide a proliferative/survival advantage to the PV clone during in vitro expansion. These data suggest that the JAK2(V617F) mutation plays an important role in the biology of PV, yet it may not be the PV-initiating event.

The genetic basis of myeloproliferative disorders

For many decades, myeloproliferative disorders (MPD) were largely neglected orphan diseases. The conceptual work of William Dameshek in 1951 provided the basis for understanding MPD as a continuum of related syndromes, possibly with a common pathogenetic cause. Recognition of the clonal origin of peripheral blood cells in MPD in 1976 and the ability to grow erythroid colonies in vitro in the absence of added growth factors in 1974 initiated the search for genetic alterations that might be responsible for myeloproliferation. Mutations in the genes for the erythropoietin receptor, thrombopoietin and the von Hippel-Lindau protein were found to cause familial syndromes resembling MPD, but despite their phenotypic similarities, none of these mutations were later found in patients with the sporadic form of MPD. The discovery of activating mutations in the Janus kinase 2 (JAK2) in most patients with MPD has fully transformed and energized the MPD field. Sensitive assays for detecting the JAK2-V617F mutation have become an essential part of the diagnostic work-up, and JAK2 now constitutes a prime target for developing specific inhibitors for the treatment of patients with MPD. Despite this progress, many questions remain unsolved, including how a single JAK2 mutation causes three different MPD phenotypes, what other genes might be involved in the pathogenesis, and what are the factors determining the progression to acute leukemia.

Polycythemia vera is not initiated by JAK2V617F mutation

OBJECTIVE

The somatic JAK2(V617F) mutation is seen in most polycythemia vera (PV) patients; however, it is not clear if JAK2(V617F) is the PV-initiating mutation.

METHODS

In order to examine this issue, we developed a novel real-time quantitative allele-specific PCR, in which allelic discrimination is enhanced by the synergistic effect of a mismatch in the -1 position, and a locked nucleic acid (LNA) nucleoside at the -2 position.

RESULTS

Determination of allelic frequencies was reproducible (SD = 1.5%) and sensitive--0.1% mutant allele detected in 40 ng of DNA. The JAK2(V617F) frequency in clonal granulocytes from 3 PV females was less than 50% (27.5 +/- 11) and in 7 females greater than 50% (75 +/- 10.5). We also found that wild-type JAK2 BFU-E colonies from PV patients can grow without erythropoietin. The identification of the primary genetic lesion resulting in PV is essential for the development of novel therapeutic strategies.

CONCLUSION

Our studies correlating the frequency of JAK2(V617F) mutant allele and clonality, as well as the presence of homozygous wild-type JAK2 erythropoietin-independent erythroid colonies, provide compelling evidence that the JAK2(V617F) is not the PV-initiating mutation. This supports a model wherein the JAK2(V617F) mutation arises as a secondary genetic event. Furthermore, our results indicate that an undefined molecular lesion, preceding JAK2(V617F), is responsible for clonal hematopoiesis in PV. We conclude that development of therapeutic strategies that target the JAK2(V617F) clonal cells may not be sufficient for eradication of PV.

Excessive erythrocytosis in adult mice overexpressing erythropoietin leads to hepatic, renal, neuronal, and muscular degeneration

To investigate the consequences of inborn excessive erythrocytosis, we made use of our transgenic mouse line (tg6) that constitutively overexpresses erythropoietin (Epo) in a hypoxia-independent manner, thereby reaching hematocrit levels of up to 0.89. We detected expression of human Epo in the brain and, to a lesser extent, in the lung but not in the heart, kidney, or liver of tg6 mice. Although no acute cardiovascular complications are observed, tg6 animals have a reduced lifespan. Decreased swim performance was observed in 5-mo-old tg6 mice. At about 7 mo, several tg6 animals developed spastic contractions of the hindlimbs followed by paralysis. Morphological analysis by light and electron microscopy showed degenerative processes in liver and kidney characterized by increased vascular permeability, chronic progressive inflammation, hemosiderin deposition, and general vasodilatation. Moreover, most of the animals showed severe nerve fiber degeneration of the sciatic nerve, decreased number of neuromuscular junctions, and degeneration of skeletal muscle fibers. Most probably, the developing demyelinating neuropathy resulted in muscular degeneration demonstrated in the extensor digitorum longus muscle. Taken together, chronically increased Epo levels inducing excessive erythrocytosis leads to multiple organ degeneration and reduced life expectancy. This model allows investigation of the impact of excessive erythrocytosis in individuals suffering from polycythemia vera, chronic mountain sickness, or in subjects tempted to abuse Epo by means of gene doping.

Polycythemia vera and its molecular basis: an update

The molecular basis of polycythemia vera is discussed in the context of the JAK2 V617F mutation, in our view the most important advance in understanding the pathogenesis of polycythemia vera. This chapter discusses the nature of the JAK2 V617F mutation including the studies demonstrating its role in erythropoietin independence and hypersensitivity and endogenous erythroid colony formation. The evolving evidence that JAK2 V617F is not specific for polycythemia vera pathogenesis and the development of disease phenotype is presented as well as alternative candidates for pathogenic mutations such as the protein tyrosine phosphatases and SOCS-3. Finally, the clinical correlations and implications of the JAK2 V617F mutation are discussed.

Evidence that the JAK2 G1849T (V617F) mutation occurs in a lymphomyeloid progenitor in polycythemia vera and idiopathic myelofibrosis

The JAK2 V617F mutation has recently been described as an essential oncogenic event associated with polycythemia vera (PV), idiopathic myelofibrosis (IMF), and essential thrombocythemia. This mutation has been detected in all myeloid lineages but has not yet been detected in lymphoid cells. This raises the question whether this molecular event occurs in a true lymphomyeloid progenitor cell. In this work, we studied the presence of the mutation in peripheral blood cells and sorted B, T, and natural killer (NK) cells from PV and IMF. We detected the JAK2 V617F mutation in B and NK cells in approximately half the patients with IMF and a minority of those with PV. Moreover, in a few cases patients with IMF had mutated peripheral T cells. The mutation (homozygous or heterozygous) could be subsequently detected in B/NK/myeloid progenitors from PV and IMF, with a much higher frequency in clones derived from IMF. Using the fetal thymus organ culture (FTOC) assay, the mutation was also detected in all T-cell fractions derived from IMF and PV CD34+ cells. These results demonstrate that myeloproliferative disorders take their origin in a true myeloid/lymphoid progenitor cell but that their phenotype is related to a downstream selective proliferative advantage of the myeloid lineages.

Idiopathic erythrocytosis and other non-clonal polycythemias

Idiopathic erythrocytosis (IE) is characterized by an increase of red blood cell mass without an identified cause. Its diagnosis is based on the exclusion of polycythemia vera (PV), secondary acquired polycythemias and various congenital primary and secondary polycythemias. The frequency of IE has been estimated to be 1.1 per 1000 subjects, which is higher than that observed in PV. Heterogeneous mechanisms underlying IE have been suggested, including 'early' PV and unrecognized secondary or congenital polycythemia. However, the transition of a patient initially classified as IE into PV is a rare occurrence, when more sophisticated diagnostic techniques are employed. IE is a stable disease with a low thrombotic risk and a low, if any, tendency to spontaneous progression to acute leukemia or myelofibrosis. Phlebotomy in patients with IE is controversial. Myelosuppressive drugs should be avoided since their use is associated with evolution into acute leukemia in about 10% of patients.

Polycythemia Vera: New Clinicopathologic Perspectives

Context.—Polycythemia vera (PV) is a clonal myeloproliferative disease characterized by an erythroid dominant trilineage proliferation of hematopoietic precursor cells. Classified as a chronic myeloproliferative disease, PV represents a histopathologic spectrum of 2 recognized stages, the polycythemic and postpolycythemic phase. The clinical manifestations of hemorrhage, thrombosis, and increased red cell mass are directly related to primary bone marrow dysfunction. Prognosis is strongly associated with thrombosis risk and disease progression; thus, treatment is directed toward minimizing coagulopathic complications and preventing leukemic transformation. Recently, a specific point mutation in the Janus kinase 2 (JAK2) gene was described in a majority of PV patients. The potential diagnostic and/or prognostic value of JAK2V617F is discussed.

Objective.—To review important developments from the recent and historical literature. Modern diagnostic criteria and emerging molecular findings are emphasized.

Data Sources.—A comprehensive review was performed of the relevant literature indexed in PubMed (National Library of Medicine) and referenced medical texts.

Conclusions.—Modified clinical, histologic, and laboratory criteria have clarified the diagnosis of PV. Also, continuing studies on the recently discovered JAK2V617F gene mutation may significantly improve our understanding of PV pathogenesis and facilitate its medical management.

Lessons from familial myeloproliferative disorders

By definition, myeloproliferative disorders (MPDs) are caused by an acquired somatic mutation of a hematopoietic progenitor/stem cell and have sporadic occurrence. However, well-documented families exist with first-degree relatives acquiring one or several MPDs. It is reasonable to assume that the germ-line mutation(s) or genetic background must facilitate or predispose for one or several somatic mutation(s) that lead to the MPD that is indistinguishable from the sporadic form. This is best documented in familial polycythemia vera (PV), which appears to be inherited as an autosomal dominant disorder with incomplete penetrance. However, there are also families wherein members develop any combination of MPDs, including PV, essential thrombocythemia (ET), chronic myelocytic leukemia (CML), and idiopathic myelofibrosis (IMF). A separate group of familial diseases is the familial thrombocythemias, wherein germ-line mutations in the genes for thrombopoietin or its receptor, MPL, cause polyclonal hereditary thrombocythemia, which may be clinically indistinguishable from ET. Patients with the congenital polycythemic condition "primary familial and congenital polycythemia" (PFCP) have characteristically decreased erythropoietin (Epo) levels similar to PV, hypersensitive erythroid progenitors, and low Epo levels; as such, this condition is often confused with PV. Therefore, PFCP will also be discussed here, while other congenital polycythemic states such as the Chuvash polycythemia that have elevated or inappropriately normal Epo levels will be omitted from this review in view of their distinct phenotype and unique laboratory features.

JAK2 mutation 1849G>T is rare in acute leukemias but can be found in CMML, Philadelphia chromosome-negative CML, and megakaryocytic leukemia

An activating 1849G>T mutation of JAK2 (Janus kinase 2) tyrosine kinase was recently described in chronic myeloproliferative disorders (MPDs). Its role in other hematologic neoplasms is unclear. We developed a quantitative pyrosequencing assay and analyzed 374 samples of hematologic neoplasms. The mutation was frequent in polycythemia vera (PV) (86%) and myelofibrosis (95%) but less prevalent in acute myeloid leukemia (AML) with an antecedent PV or myelofibrosis (5 [36%] of 14 patients). JAK2 mutation was also detected in 3 (19%) of 16 patients with Philadelphia-chromosome (Ph)-negative chronic myelogenous leukemia (CML), 2 (18%) of 11 patients with megakaryocytic AML, 7 (13%) of 52 patients with chronic myelomonocytic leukemia, and 1 (1%) of 68 patients with myelodysplastic syndromes. No mutation was found in Ph(+)CML (99 patients), AML M0-M6 (28 patients), or acute lymphoblastic leukemia (20 patients). We conclude that the JAK2 1849G>T mutation is common in Ph(-) MPD but not critical for transformation to the acute phase of these diseases and that it is generally rare in aggressive leukemias.

A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera

Myeloproliferative disorders are clonal haematopoietic stem cell malignancies characterized by independency or hypersensitivity of haematopoietic progenitors to numerous cytokines. The molecular basis of most myeloproliferative disorders is unknown. On the basis of the model of chronic myeloid leukaemia, it is expected that a constitutive tyrosine kinase activity could be at the origin of these diseases. Polycythaemia vera is an acquired myeloproliferative disorder, characterized by the presence of polycythaemia diversely associated with thrombocytosis, leukocytosis and splenomegaly. Polycythaemia vera progenitors are hypersensitive to erythropoietin and other cytokines. Here, we describe a clonal and recurrent mutation in the JH2 pseudo-kinase domain of the Janus kinase 2 (JAK2) gene in most (> 80%) polycythaemia vera patients. The mutation, a valine-to-phenylalanine substitution at amino acid position 617, leads to constitutive tyrosine phosphorylation activity that promotes cytokine hypersensitivity and induces erythrocytosis in a mouse model. As this mutation is also found in other myeloproliferative disorders, this unique mutation will permit a new molecular classification of these disorders and novel therapeutical approaches.