Definition of subtypes of essential thrombocythaemia and relation to polycythaemia vera based on JAK2 V617F mutation status: a prospective study

JAK2V617F mutation in platelets from essential thrombocythemia patients: correlation with clinical features and analysis of STAT5 phosphorylation status

OBJECTIVE

JAK2V617F mutation rate in granulocytes from essential thrombocythemia (ET) patients ranges from 12% to 57%. Our aim was to evaluate the frequency of this mutation in the megakaryocyte/platelet lineage, and to analyze its clinical associations in ET. In addition, we determined whether this mutation leads to constitutive phosphorylation of STAT5 in platelets.

MATERIALS AND METHODS

Consecutive patients with ET were included and clinical features were retrospectively reviewed. Mutation detection was performed by allele specific RT-PCR (AS-RT-PCR) and Restriction fragment length polymorphism (RFLP) analysis of platelet RNA. Constitutive phosphorylation of STAT5 in platelets was studied by Western blot.

RESULTS

Fifty patients were included, 24 (48%) were JAK2V617F-positive by both AS-RT-PCR and RFLP. Patients with the mutation were older, had significantly higher hemoglobin levels, and lower platelet counts. Besides, higher frequency of thrombotic events was found in JAK2V617F-positive patients younger than 60, 53% vs. 4%, P = 0.0008. In addition, constitutive STAT5 phosphorylation was not detected in platelets from 12 patients.

CONCLUSIONS

The frequency of the JAK2V617F mutation in platelets was similar to that reported in granulocytes in the literature, suggesting this mutation does not occur as an isolated event in the megakaryocyte lineage. If confirmed in a larger study, the observed higher frequency of thrombosis in patients younger than 60 might be a useful predictive marker for thrombosis in this subset of patients. Even though this mutation has been predicted to constitutively activate the JAK2 kinase, spontaneous phosphorylation of STAT5 does not seem to be a frequent finding in platelets from ET patients.

Practical management of classical myeloproliferative disorder patients: a clinician's guide

The classical myeloproliferative disorders (MPDs) are comprised of the clonal, BCR-ABL-negative, chronic myeloid disorders of essential thrombocythemia, polycythemia vera, and myelofibrosis with myeloid metaplasia. Management of these disorders remains a significant challenge due to the varied range of prognosis and phenotypic manifestations. Curative therapy, achieved in some patients through allogeneic stem cell transplantation, is elusive or inappropriate in most. Additionally, no available medical therapy has been shown to clearly improve survival or delay disease progression. Current management involves an emphasis on prevention of thrombohemorrhagic complications (through aspirin treatment, phlebotomy and cytoreduction in high-risk patients) in early-stage patients and symptomatic care in those with advanced disease. Leukemic transformation from MPDs remains a rapidly fatal complication, unresponsive to current therapies. Recent elucidation of the role of the activating tyrosine kinase mutation JAK2 (V617F) is anticipated to usher in an era of greater understanding and targeted therapy for the MPDs.

Current diagnostic criteria for the chronic myeloproliferative disorders (MPD) essential thrombocythemia (ET), polycythemia vera (PV) and chronic idiopathic myelofibrosis (CIMF)

The clinical criteria for the diagnosis of essential thrombocythemia (ET) according to the polycythemia vera study group (PVSG) do not distinguish between ET and thrombocythemia associated with early stage PV and prefibrotic chronic idiopathic myelofibrosis (CIMF). The clinical criteria of the PVSG for the diagnosis of polycythemia vera (PV) only detects advanced stage of PV with increased red cell mass. The bone marrow criteria of the World Health Organization (WHO) are defined by pathologists to explicitly define the pathological criteria for the diagnostic differentiation of ET, PV, and prefibrotic and fibrotic CIMF. As the clinical PVSG and the pathological WHO criteria show significant shortcomings, an updated set of European Clinical and Pathological (ECP) criteria combined with currently available biological and molecular markers are proposed to much better distinct true ET from early PV mimicking ET, to distinguish ET from thrombocythemia associated with prefibrotic CIMF, and to define the various clinical and pathological stages of PV and CIMF that has important therapeutic and prognostic implications. Comparing the finding of clustered giant abnormal megakaryocytes in a representative bone marrow as a diagnostic clue to MPD, the sensitivity for the diagnosis of MPD associated with splanchnic vein thrombosis was 63% for increased red cell mass, 52% for low serum EPO level, 72% for EEC, and 74% for splenomegaly indicating the superiority of bone marrow histopathology to detect masked early and overt MPD in this setting. The majority of PV and about half of the ET patients have spontaneous EEC, low serum EPO levels and PRV-1 over-expression and are JAK2 V617F positive. The positive predictive value for the diagnosis of PV of spontaneous growth of endogenous erythroid colonies (EEC) of peripheral blood (PB) and bone marrow (BM) cells is about 80-85% when either PB or BM EEC assays, and up to 94% when BM and PB EEC assays were performed. The diagnostic impact of low serum EPO levels (ELISA assay) in a large study of 186 patients below the normal range (<3.3 IU/l) had a sensitivity specificity and positive predictive value of 87%, 97% and 97.8%, respectively, for the diagnosis of PV. There is a significant overlap of serum EPO levels in PV versus control and controls versus SE. The specificity of a JAK2 V617F PCR test for the diagnosis of MPD is high (near 100%), but only half of ET and MF (50%) and the majority of PV (up to 97%) are JAK2 V617F positive. The use of biological markers including JAK2 V617 PCR test, serum EPO, PRV-1, EEC, leukocyte alkaline phosphatase score and peripheral blood parameters combined with bone marrow histopathology has a high sensitivity and specificity (almost 100%) to diagnose the early and overt stages of ET, PV and CIMF in JAK2 V617F positive and negative MPDs.

Telomere length dynamics in normal hematopoiesis and in disease states characterized by increased stem cell turnover

Telomeres both reflect and limit the replicative lifespan of normal somatic cells. Immature sub-populations of human CD34+38- hematopoietic stem cell (HSC) can be identified in vitro based on their growth kinetics and telomere length. Fluorescence in situ hybridization and flow cytometry (flow-FISH) has been used to characterize telomere length dynamics as a surrogate marker for HSC turnover in vivo. Investigations in normal steady-state hematopoiesis provided the basis for follow-up studies in model scenarios characterized by increased HSC turnover. Disorders with underlying malignant transformation of HSC (e.g., chronic myeloid leukemia (CML)) can be discriminated from disease states with increased HSC turnover rates secondary to depletion of the stem cell compartment, for example, as in defined bone marrow failure syndromes. In some of these model scenarios, the degree of telomere shortening can be correlated with disease duration, disease stage and severity as well as with response to disease-modifying treatment strategies. Whether increased telomere shortening represents a causal link between HSC turnover, replicative senescence and/or the induction of genetic instability in acquired HSC disorders remains to be shown. However, data from congenital disorders, like dyskeratosis congenita (DKC), suggest that disturbed telomere maintenance may play a role for replicative exhaustion of the HSC pool in vivo.

The role of Janus kinases in haemopoiesis and haematological malignancy

The production of blood cells is regulated by a number of protein growth factors and cytokines that influence cell survival, proliferation and differentiation. Many of these molecules bind to cell surface receptors, which belong to a family of closely related cytokine receptors that lack intrinsic catalytic activity but are intimately associated with tyrosine kinases of the Janus kinase (JAK) family. Ligand binding induces the activation of JAKs, which sit at the apex of a signalling cascade in which a key role is played by members of the signal transducers and activators of transcription (STAT) group. Congenital deficiencies in JAK-STAT signalling are associated with immunodeficiency states and acquired activating mutations and translocations are involved in the pathophysiology of haematological malignancy. The latter findings have raised hopes that drugs that target aberrant JAK-STAT signalling may be useful for the treatment of human disease.

Progenitors homozygous for the V617F mutation occur in most patients with polycythemia vera, but not essential thrombocythemia

An acquired V617F JAK2 mutation occurs in patients with polycythemia vera (PV) or essential thrombocythemia (ET). In a proportion of V617F-positive patients, mitotic recombination produces mutation-homozygous cells that come to predominate with time. However, the prevalence of homozygosity is unclear, as previous reports studied mixed populations of wild-type, V617F-heterozygous, and V617F-homozygous mutant cells. We therefore analyzed 1766 individual hematopoietic colonies from 34 patients with PV or ET in whom granulocyte sequencing demonstrated that the mutant peak did not predominate. V617F-positive erythroid burst-forming units (BFU-Es) were more frequent in patients with PV compared with patients with ET (P = .022) and, strikingly, V617F-homozygous BFU-Es were detected in all 17 patients with PV, but in none of the patients with ET (P < .001). Moreover, mutation-homozygous cells were present in 2 patients with ET after polycythemic transformation. These results demonstrate that V617F-homozygous erythroid progenitors are present in most patients with PV but occur rarely in those with ET.

The JAK2-V617F mutation is frequently present at diagnosis in patients with essential thrombocythemia and polycythemia vera

We determined the allelic frequency of the JAK2-V617F mutation in DNA and assessed the expression levels of the mutant and wild-type JAK2 mRNA in granulocytes from 60 patients with essential thrombocythemia (ET) and 62 patients with polycythemia vera (PV) at the time of diagnosis. Using allele-specific quantitative polymerase chain reaction (qPCR), we detected JAK2-V617F in 75% of ET and 97% of PV at diagnosis. The total JAK2 mRNA levels were elevated in ET, PV, and secondary and idiopathic erythrocytosis, suggesting that hyperactive hematopoiesis alters JAK2 expression. The expression levels of JAK2-V617F mRNA were variable but strongly correlated with the allelic ratio of JAK2-V617F determined in DNA. Thus, differences in JAK2-V617F expression, markedly lower in ET than in PV, reflected different percentages of granulocytes carrying the mutation. Moreover, allelic ratios higher than 50% JAK2-V617F, indicating the presence of granulocytes homozygous for JAK2-V617F, were found in 70% of PV at diagnosis but never in ET.

A quantitative assay for JAK2V617F mutation in myeloproliferative disorders by ARMS-PCR and capillary electrophoresis

A point mutation in the Janus tyrosine kinase 2 (JAK2) gene has been described in patients with chronic myeloproliferative disorders (MPD), but the clinical significance of JAK2(V617F), which may be harbored in either the heterozygote or homozyote status, is still largely undefined. There are indirect suggestions that clinical phenotype and also some biological characteristics are dependent on the mutated allele levels. We have designed and validated in 179 MPD patients an amplification-refractory mutation sequencing PCR assay that allows the relative quantitation of mutated and normal JAK2 mRNAs using dye-labelled mutation-specific primers and capillary electrophoresis. Direct sequencing confirmed the specificity of the assay, which has a detection limit congruent with1% and allowed to identify 9% more JAK2-mutated patients as compared to conventional allele-specific PCR. The mutated mRNA ratio ranged from 5 to 51% in the JAK2(V617F) heterozygote and from 45 to 100% in the homozygote patients. Expression levels of both PRV-1 and NF-E2 gene, previously found to be overexpressed in MPD patients, were significantly correlated to the amount of mutated JAK2 mRNA. We propose that this method might complement current technologies based on genomic DNA analysis, and lead prospectively to a better clinically oriented assessment of the impact of JAK2(V617F) mutation in MPD.

Thrombotic complications in essential thrombocythemia (ET): clinical facts and biochemical riddles

Hemostatic complications which can occur in the arterial or venous vasculature or in the microcirculation are the major causes of morbidity and mortality in patients with ET. In order to prevent these complications, often platelet reductive drugs are used. These agents are by themselves potentially toxic, i.e. may cause leukemia or cardiac side effects. In order to avoid these adverse effects, a better understanding of the mechanism of thrombus formation which is causative in ET is mandatory. Unfortunately, until now, no biomarkers have been identified which allow the estimation of the risk of thrombotic complications. Platelet number is not a good predictor per se since thrombotic complications can occur in some patients at low platelet numbers whereas others do not encounter a thrombosis even at very high platelet levels. On the other hand, lowering of the platelet count usually results in symptomatic improvement. In ET, morphological alterations of the megakaryocyte in the bone marrow and the circulating platelets are observed: megakaryocyte nuclei show a staghorn appearance, circulating platelets are characterized by anisocytosis and giant size. Functional studies indicate that these anatomically altered platelets function abnormally. When platelets are analyzed with a platelet function analyzer (PFA-100, which uses cartridges that measure how well a patient's platelets adhere and aggregate to form a platelet plug in the first phase of thrombus formation), in many patients with ET, closure time using collagen/ADP and collagen/epinephrine cartridges is prolonged. This seems paradoxical since these patients do not show an increased bleeding time. These results indicate that either receptors and/or consecutive signaling events are abnormal in ET platelets. Proteomic analysis of platelets of ET patients has revealed individual differences but not yet led to the identification of disease-specific proteins. Moreover, the search for alternative risk factors (factor V Leiden, prothrombin gene polymorphism, etc.) has not provided evidence for the contribution of these factors to the generation of the thrombotic risk in ET patients. In summary, despite intensive research over several decades, relatively little is known about the pathogenesis and risk factors for thrombosis in ET. I expect that this conference will contribute to the development of new strategies to identify patients at risk for hemostatic complications.

Anagrelide: a review of its use in the management of essential thrombocythaemia

Anagrelide (Agrylin, Xagrid) is an oral imidazoquinazoline agent which is indicated in Europe for the reduction of elevated platelet counts in at-risk patients with essential thrombocythaemia who are intolerant of or refractory to their current therapy, and in the US for the reduction of elevated platelet counts and the amelioration of thrombohaemorrhagic events in patients with thrombocythaemia associated with myeloproliferative disorders. Anagrelide is well established as an effective platelet-lowering agent in most patients with essential thrombocythaemia, including both treatment-naive patients and those refractory to other cytoreductive therapy. Results of the only randomised trial to date (the Primary Thrombocythaemia 1 [PT1] study) indicated that the composite primary endpoint (arterial or venous thrombosis, serious haemorrhage or death from vascular causes) occurred more often in recipients of anagrelide plus aspirin than in those receiving hydroxycarbamide (hydroxyurea) plus aspirin. This trial also indicated that the incidence of the secondary endpoints transient ischaemic attack and gastrointestinal bleeding favoured hydroxycarbamide plus aspirin, while the incidence of venous thrombosis favoured anagrelide plus aspirin. There were no differences between the groups in the incidence of secondary endpoints myocardial infarction, stroke, unstable angina, pulmonary embolism, hepatic-vein thrombosis, other serious haemorrhage or related deaths. The design of the PT1 study has been queried with respect to the heterogeneous nature of the study population (possible inclusion of patients with early myelofibrotic disease) and the concomitant use of aspirin (interaction with anagrelide causing increased bleeding events). Further data are therefore required before the role of anagrelide in essential thrombocythaemia can be finalized. In the meantime, when considering treatment options for patients with this disorder, anagrelide's positive effects on platelet function, lack of mutagenicity and lack of association with leukaemia or angiogenesis must be balanced against its comparative expense and positive inotropic effects. Thus, the role of anagrelide in the management of high-risk patients with essential thrombocythaemia will ultimately depend on individual patient assessment and future clarification of the potential leukaemogenicity of hydroxycarbamide.

Mutation screening for JAK2V617F: when to order the test and how to interpret the results

With the application of adequately sensitive tests, it is now becoming evident that more than 90% of patients with conventionally-defined polycythemia vera (PV) carry the somatic JAK2V617F mutation in their granulocytes. However, the specific mutation is also found in other classic and atypical myeloproliferative disorders (MPD), albeit at a lesser frequency. In contrast, JAK2V617F has not been reported in patients with either reactive myeloproliferation or lymphoid disorders. Therefore, mutation screening for JAK2V617F can be considered as a myeloid-specific clonality assay and it is diagnostically most useful in the evaluation of "polycythemia".

Idiopathic myelofibrosis: pathogenesis to treatment

Idiopathic myelofibrosis (IMF) is the least common of the chronic myeloproliferative disorders and carries the worst prognosis with a median survival of 4 years. It is a clonal haematopoietic stem-cell disorder and, although the pathogenesis remains unclear, approximately 50% of cases are known to possess an activating JAK2 V617F mutation. In contrast, the characteristic stromal proliferation is a reactive, or secondary, event that results from the aberrant release of a variety of growth factors from megakaryocytes and monocytes. Treatment for most cases is supportive, although androgens, recombinant erythropoietin, steroids and thalidomide are effective modalities for the amelioration of anaemia. Myelosuppression, splenectomy and irradiation are valuable therapeutic modalities for specific clinical situations. Prognostic scores are available to aid the identification of cases for whom bone marrow transplantation should be considered. Recently, the use of reduced intensity conditioning has resulted in prolonged survival and lower transplant-related mortality. This review summarises the recent advances in the disease's pathogenesis and discusses the role of the various therapeutic options.

JAK2 (V617F) as an acquired somatic mutation and a secondary genetic event associated with disease progression in familial myeloproliferative disorders

BACKGROUND

A somatic gain-of-function mutation of the Janus kinase 2 (JAK2) gene has been identified in chronic myeloproliferative disorders, which appear to have a sporadic occurrence in most individuals. The authors studied the biologic significance of the JAK2 (V617F) mutation in familial myeloproliferative disorders.

METHODS

Twenty pedigrees with familial chronic myeloproliferative disorders were identified through an investigation of family history in 264 patients with sporadic myeloproliferative disorders. A quantitative real-time polymerase chain reaction (qRT-PCR)-based allelic discrimination assay was employed for the detection of the V617F mutation in circulating granulocytes and T lymphocytes. An analysis of X-chromosome inactivation pattern was performed in female patients.

RESULTS

Fourteen families had homogeneous phenotypes, and 6 families had mixed phenotypes. By using a qRT-PCR-based allelic discrimination assay, the JAK2 (V617F) mutation was detected in circulating granulocytes from 20 of 31 patients, but the mutation was not detected in T lymphocytes. Granulocyte mutant alleles ranged from 2.1% to 91.5% and, on average, increased with time. Discordant distribution of the JAK2 (V617F) mutation was observed in siblings with polycythemia vera. The proportion of granulocytes that carried the JAK2 (V617F) mutation was lower than the proportion of clonal granulocytes, as determined in an analysis of X-chromosome inactivation patterns in female patients.

CONCLUSIONS

The current findings indicated that the JAK2 (V617F) mutation represents an acquired somatic mutation in patients with familial chronic myeloproliferative disorders and probably occurs as a secondary genetic event in the background of preexisting clonal hematopoiesis. Thus, a genetic predisposition to acquisition of JAK2 (V617F) is inherited in families with myeloproliferative disorders.

Classification, Diagnosis and Management of Myeloproliferative Disorders in the JAK2V617F Era

JAK2V617F, a somatic gain-of-function mutation involving the JAK2 tyrosine kinase gene, occurs in nearly all patients with polycythemia vera (PV) but also in a variable proportion of patients with other myeloid disorders; mutational frequency is estimated at approximately 50% in both essential thrombocythemia (ET) and myelofibrosis (MF), up to 20% in certain subcategories of atypical myeloproliferative disorder (atypical MPD), less than 3% in de novo myelodysplastic syndrome (MDS) or acute myeloid leukemia, and 0% in chronic myeloid leukemia (CML). Accordingly, there is now molecular justification for grouping PV, ET, and MF together in a distinct MPD category (i.e., classic, BCR-ABL− MPD) that is separate from chronic myeloid leukemia (CML), MDS, and atypical MPD. To date, JAK2V617F has not been described in patients with reactive myeloproliferation, lymphoid disorders, or solid tumor. Therefore, the presence of JAK2V617F strongly suggests an underlying MPD and it is therefore reasonable to consider JAK2V617F-based laboratory tests for the evaluation of polycythemia, primary thrombocytosis, unexplained leukocytosis, bone marrow fibrosis, or abdominal vein thrombosis. Current information on disease-specific prognostic relevance of JAK2V617F is inconclusive and confounded by inter-study differences in the performance of mutation screening assays. Regardless, the discovery of JAK2V617F has reinforced the pathogenetic contribution of JAK-STAT signaling in MPD and identifies JAK2 as a valid drug target.

Myeloproliferative disease in pregnancy and other management issues

The goal of this review is to assess the current treatment and outcomes of special clinical situations in patients with myeloproliferative disease (MPD) such as pregnancy, major thrombotic and bleeding complications and surgical interventions. However, only a limited literature to support optimal management strategies is available. Many of the proposed strategies are the results of common sense or derive from the extrapolation of data from other studies not specifically designed to solve these problems. Therefore, practical recommendations to guide clinical decisions in these settings still remain largely empirical.