The determination of spontaneous megakaryocyte colony formation is an unequivocal test for discrimination between essential thrombocythaemia and reactive thrombocytosis

Proliferative characteristics of Philadelphia-negative myeloproliferative neoplasms - clinical implications

INTRODUCTION

Philadelphia-negative myeloproliferative neoplasms (Ph^- MPN) are characterized by overproduction of one or more blood cell lines.

METHODS

We studied the proliferative characteristics of 91 patients with de novo Ph^- MPN. Colony-forming cells (CFC) and endogenous colonies (EC), from bone marrow (BM) and/or peripheral blood (PB), were analyzed by colony assay based on methylcellulose. The level of circulating CD34⁺ cells was determined by flow cytometry.

RESULTS

The total number of PB CFC in primary myelofibrosis (PMF) was increased compared to the control sample (P < 0.01) and essential thrombocythemia (ET) (P < 0.05). The highest number of BM and PB EC was observed in polycythemia vera (PV) (P < 0.01). Increased levels of CD34⁺ cells characterized early-prefibrotic (57%) and advanced-fibrotic PMF (90%) as compared to PV (34%) and ET (32%) (P < 0.01). In the whole Ph^- MPN group, the total number of PB CFC (P < 0.01), PB EC (P < 0.05), and CD34⁺ cells (P < 0.01) correlated with the degree of BM fibrosis. Higher levels of circulating CD34⁺ cells in PMF correlated with the total number of PB EC (P < 0.05) and degree of BM fibrosis (P < 0.01).

CONCLUSIONS

Exploration of the PB proliferative characteristics of Ph^- MPN on diagnosis may be helpful in revealing early-prefibrotic PMF. Monitoring the levels of circulating CD34⁺ cells may provide a sensitive indicator of fibrotic evolution in PV and PMF.

Platelet glycoprotein IIIa gene expression in normal and malignant megakaryopoiesis

The platelet glycoprotein GPIIb/IIIa functions as a receptor for fibrinogen in platelet aggregation process and is an example of an early megakaryocytic marker. One of a chronic myeloproliferative disorder, essential thrombocythemia, is caused by abnormal megakaryopoiesis. Due to the lack of reliable method for the diagnosis of that disease and the importance of GPIIIa as a marker for identifying early megakaryocytes, the expression level of GPIIIa in mononuclear and CD34(+) cells and during megakaryopoiesis was compared between normal individuals and patients with essential thrombocythemia. For this purpose, surface markers GPIIIa and CD34 were analyzed with flow cytometer, and GPIIIa expression level was measured with real-time polymerase chain reaction (PCR) method. Mononuclear and CD34(+) cells from normal individuals and patients were isolated, analyzed, and seeded into serum-free medium Stemspantrade mark Medium enriched with IL-6, IL-3, thrombopoietin, and stem cell factor. The difference between normal individuals and patients was noticed in the expression level of GPIIIa in the CD34(+) cells and in the time course of cell surface markers. CD34(+) cells from patients has 33% higher of GPIIIa antigens on the surface and 34% higher GPIIIa messenger RNA (mRNA) expression level. The negative effect of IL-3 on the maturation of megakaryocytes was not noticed; there were 56.46% of megakaryoblasts at the end of the cultivation, and after 14 days of culturing, 111.09 times increase of GPIIIa mRNA in patients was detected. This study is therefore offering the method that could serve as reliable tool for discriminating ET from other similar myeloproliferative disorders.

Recent advances in the bcr-abl negative chronic myeloproliferative diseases

The chronic myeloproliferative disorders are clonal hematopoietic stem cell disorders of unknown etiology. In one of these (chronic myeloid leukemia), there is an associated pathognomonic chromosomal abnormality known as the Philadelphia chromosome. This leads to constitutive tyrosine kinase activity which is responsible for the disease and is used as a target for effective therapy. This review concentrates on the search in the other conditions (polycythemia vera, essential thrombocythemia and idiopathic mylofibrosis) for a similar biological marker with therapeutic potential. There is no obvious chromosomal marker in these conditions and yet evidence of clonality can be obtained in females by the use of X-inactivation patterns. PRV-1mRNA over expression, raised vitamin B₁₂ levels and raised neutrophil alkaline phosphatase scores are evidence that cells in these conditions have received excessive signals for proliferation, maturation and reduced apoptosis. The ability of erythroid colonies to grow spontaneously without added external erythropoietin in some cases, provided a useful marker and a clue to this abnormal signaling. In the past year several important discoveries have been made which go a long way in elucidating the involved pathways. The recently discovered JAK2 V617F mutation which occurs in the majority of cases of polycythemia vera and in about half of the cases with the two other conditions, enables constitutive tyrosine kinase activity without the need for ligand binding to hematopoietic receptors. This mutation has become the biological marker for these conditions and has spurred the development of a specific therapy to neutralize its effects. The realization that inherited mutations in the thrombopoietin receptor (c-Mpl) can cause a phenotype of thrombocytosis such as in Mpl Baltimore (K39N) and in a Japanese family with S505A, has prompted the search for acquired mutations in this receptor in chronic myeloproliferative disease. Recently, two mutations have been found; W515L and W515K. These mutations have been evident in patients with essential thrombocythemia and idiopathic myelofibrosis but not in polycythemia vera. They presumably act by causing constitutional, activating conformational changes in the receptor. The discovery of JAK2 and Mpl mutations is leading to rapid advancements in understanding the pathophysiology and in the treatment of these diseases.

Spontaneous megakaryocytic colony formation does not discriminate between essential thrombocythemia and polycythemia vera

Laboratory detection of spontaneous growth of colony-forming unit-megacaryocytes (CFU-MK), allowing us to distinguish essential thrombocythemia (ET) from reactive thrombocytosis, is therefore useful for the diagnostic of this myeloproliferative disorder. Whether CFU-MK assays allow us to discriminate at least partly between ET and other myeloproliferative disorders such as polycythemia vera (PV) remains, however, to be established. To gain insights about this point, we have performed CFU-MK cultures from bone marrow cells of patients diagnosed with ET (n = 42) or PV (n = 50) using a standardized collagen-based serum-free method. Spontaneous growth of CFU-MK was similarly detected in both 40/42 patients with ET and 47/50 patients with PV. These data suggest clearly that the CFU-MK assay is useful to detect not only ET, but also PV, but fails to discriminate, even partly, between these two myeloproliferative disorders.

Molecular pathogenesis of Philadelphia chromosome negative myeloproliferative disorders

We summarize the current knowledge on molecular alterations in myeloproliferative disorders (MPD), in particular altered in vitro responses of progenitor cells, cytokine signaling, gene expression patterns and genetic lesions. Newly characterized markers, such as altered expression of polycythemia rubra vera-1 (PRV-1) and the thrombopoietin receptor (c-MPL) as well as deletions on chromosome 20q (del20q) and loss of heterozygosity on chromosome 9p (9pLOH) provide an opportunity to diagnose and identify subpopulations of MPD patients. Furthermore, we review familial syndromes that share phenotypic features with sporadic MPD. In some of these families, mutations in the genes for thrombopoietin (TPO), c-MPL, EPO-receptor and the von Hippel-Lindau (VHL) gene have been shown to cause the disease. However, in the majority of familial cases the molecular causes remain unknown. Some of these families display clonal hematopoiesis and other features previously only found in sporadic MPD. Elucidating the molecular defect(s) in these pedigrees will likely be relevant for understanding sporadic MPD pathogenesis.

Maladie inflammatoire cryptogénétique de l’intestin et thrombocytémie latente : nouvelle étiologie de thrombose des veines hépatiques ?

It has been suggested in previous studies, that inflammatory bowel disease can induce hepatic vein thrombosis. However, the main weakness of those studies was the lack of extensive screening of prothrombotic factors. We report an unusual association of severe inflammatory bowel disease, hepatic vein thrombosis and latent platelet proliferation disorder. Early treatment with heparin and cyclosporin administered intravenously induce a remission of intestinal disease and total disappearance of hepatic vein thrombosis. Extensive screening excluded the classical causes of hepatic vein thrombosis, inherited coagulation disorders and latent polycythemia by culture of erythroid progenitors without added erythropoietin. However, using recent bone marrow culture conditions, we observed spontaneous colony formation of megakaryocyte progenitors revealing latent thrombocythemia. In summary, progress in the diagnosis of haemostasis disorders, will probably confirm that one or more additional predisposing prothrombotic factors are needed to the development of hepatic vein thrombosis in inflammatory bowel disease. This hypothesis could explain the scarcity of this complication in inflammatory bowel disease. Moreover, this case-report suggests that administration of heparin at the onset of the thrombosis may induce a complete regression of the thrombus in hepatic veins.

The predictive value of megakaryocytic and erythroid colony formation and platelet function tests on the risk of thromboembolic and bleeding complications in essential thrombocythaemia

The predictive value of spontaneous in vitro colony formation of megakaryocytic and erythroid progenitors (154 patients), and defective platelet aggregation responses (55 patients) on the risk of thrombohaemorrhagic complications in patients with essential thrombocythaemia (ET) was evaluated retrospectively. In the in vitro cultures of haematopoietic progenitors, 114/154 patients (74%) showed either spontaneous megakaryocytic or erythroid colony formation or both. Forty-three per cent of patients with any spontaneous colony growth and only 20% of those without this phenomenon had an arterial thrombosis at diagnosis or during the follow-up (P = 0.02). In the whole patient group neither spontaneous megakaryocytic nor spontaneous erythroid colony formation alone predicted the risk of arterial thrombosis. In patients younger than 45 yr of age, the prognostic value of spontaneous megakaryocytic growth was statistically significant: 44% of the patients with spontaneous megakaryocytic colony formation, but only 14% of those without it, experienced arterial thrombosis (P = 0.04). The presence of spontaneous colony formation had no effect on the risk of bleeding complications. Forty-one of the 55 patients (75%) showed abnormalities in the platelet aggregation responses. There was no statistically significant correlation between the platelet function response and the risk of bleeding or thrombotic complications. No correlation was found between the platelet aggregation responses and the presence of spontaneous colony growth. In conclusion, spontaneous colony formation indicated an increased risk of thrombohaemorragic events but the platelet function test had no predictive value for these complications.

A novel t(2;17) in transformation of essential thrombocythemia to acute myelocytic leukemia

A transformation of essential thrombocythemia to acute myelocytic leukemia (AML), myelodysplastic syndrome, or agnogenic myelocytic metaplasia is a relatively rare event. It occurs in 1%-4.5% of all patients with either treated or untreated essential thrombocythemia. Cytogenetic changes in the transformation to AML are common. We report the case of a patient treated for essential thrombocythemia with hydroxyurea for 49 months. He developed AML with a t(2;17), which to our knowledge has not been described in the literature.

Coexistence of chronic lymphocytic leukemia and essential thrombocythemia

The association of chronic lymphocytic leukemia (CLL) with essential thrombocythemia (ET) is an extremely rare event and until now 3 patients with such coexistence have been reported in the literature. We report a 77-year-old white woman in whom these two disorders were diagnosed concomitantly on the basis of peripheral blood count and cytology, bone marrow cytology and histology, immunophenotyping, as well as exclusion criteria. The diagnosis of ET was also supported by spontaneous in-vitro erythroid colony growth and by evaluation of thrombopoietin (TPO) serum level. Interphase FISH analysis allowed to detect 13q14.3 deletion in 98% of lymphocytes nuclei. In contrast this aberration was not observed in the megakaryocytes. The results of PCR analysis of IgG gene rearrangement showed distinct bands characteristic for monoclonal lymphoid population in bone marrow, peripheral blood and inguinal lymph node. The patient was started on hydroxyurea 1 g/day and normalization of the platelet count was achieved. Possible etiopathogenic relationships between both disorders and differential diagnosis of ET and reactive thrombocytosis (RT) are discussed.

Molecular markers in myeloproliferative disorders: from classification to prognosis?

The recent description of molecular markers in patients with myeloproliferative disorders (MPDs) has raised several questions: does the presence of multiple markers coincide in individual patients or can a patient acquire some markers selectively? Do the markers distinguish molecular categories of MPDs? Do these categories coincide with the clinically defined subgroups of MPDs: PV, ET and IMF? If not, which system of categorization is more useful to the patient and his physician, the molecular one or the clinical one, and why? The present review will summarize the current knowledge of molecular markers in MPDs and discuss today's answers to the above questions. Since our knowledge of the molecular basis of MPDs is rapidly expanding, it is my hope that this review will soon be outdated.

Clinical and scientific advances in the Philadelphia-chromosome negative chronic myeloproliferative disorders

The chronic myeloproliferative disorders are clonal hematopoietic stem cell disorders and include chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), and agnogenic myeloid metaplasia (AMM). These diseases are characterized by clonal expansion of the myeloid compartment, increased marrow angiogenesis, and varying risks for blastic transformation. A clear molecular abnormality exists (t(9;22) leading to the fusion of BCR-Abl) only for CML, which led to effective targeted therapy (STI-571). Since no similar pathogenetic mechanism has been discovered for the t(9;22) negative chronic myeloproliferative disorders, their respective diagnosis is currently based on a variety of rather cumbersome diagnostic criteria. Polycythemia vera is distinguished from reactive erythrocytosis through erythropoietin independent growth of erythroid progenitors in vitro, suppressed levels of endogenous erythropoietin, possible overexpression of PRV-1 (polycythemia rubra vera-1), decreased c-Mpl expression on megakaryocytes, as well as overexpression of bcl-xL, and potentially aberrant activity of the Jak-Stat pathway. ET is defined by thrombocytosis and is distinguished from reactive states by decreased megakaryocyte c-Mpl expression, and a propensity for thrombosis. AMM has been associated with a variety of observations including increased concentrations of pro-fibrotic cytokines, increased angiogenesis, and myeloid expansion. AMM is often indistinguishable clinically and prognostically from the advanced phases of other CMPD (specifically post-polycythemic and post-thrombocythemia myeloid metaplasia), all of which are subentities of a diagnosis of myelofibrosis with myeloid metaplasia (MMM). The management of CMPD patients is quite varied given the broad range of disease severity and survival observed. The role of stem cell transplantation is limited by the age and comorbidities encountered in CMPD patients. Since no broadly applicable therapy effects the mortality of the CMPD, management currently focuses on the prevention/palliation of disease morbidity (i.e. vascular complications, pruritus, organomegaly, constitutional symptoms). Palliative strategies which currently focus on non-specific myelosuppresion, will hopefully be soon replaced by targeted therapies as insight into pathogenetic mechanisms of these diseases evolves.

Diagnostic and prognostic value of bone marrow angiogenesis and megakaryocyte c-Mpl expression in essential thrombocythemia

The lack of diagnostic certainty in some patients makes it difficult to distinguish between primary and secondary forms of thrombocytosis. To augment current diagnostic studies for thrombocytosis, we retrospectively evaluated clinical records and bone marrow trephine specimens of 183 patients with thrombocytosis-164 with essential thrombocythemia (ET), 19 with reactive thrombocytosis (RT)-for bone marrow angiogenesis, bone marrow megakaryocyte c-Mpl staining, and morphologic evidence of megakaryocyte proliferation. Angiogenesis was increased in patients with ET compared with healthy controls (P <.0001) and patients with RT (P =.006). In addition, an increase in angiogenesis was associated with certain disease features such as splenomegaly (P =.004) and reticulin fibrosis (P =.005). Decreased megakaryocyte c-Mpl staining was observed in a heterogeneous pattern in ET compared with healthy controls (P <.0001) and RT (P <.0001). Histologic stratifying criteria incorporating increased angiogenesis, decreased megakaryocyte c-Mpl expression, and marked megakaryocyte proliferation in the bone marrow was highly sensitive (97%) and specific (95%) for distinguishing ET from RT (P <.0001). However, with the current duration of follow-up available on the patients, none of the histologic features evaluated have yet demonstrated prognostic value for subsequent clinical course, vascular events, or survival.

The role of blood component removal in essential and reactive thrombocytosis

An elevated platelet count is now a common finding in both hospitalized and ambulatory patients with the advent of automated complete blood cell counters. Clinicians may be called upon to make a distinction between a reactive process and a primary hematologic disorder as the cause of a thrombocytosis and to determine whether treatment is indicated. Essential thrombocythemia and other myeloproliferative disorders may present with marked increases in the platelet counts and may be associated with thrombohemorrhagic complications. Reactive thrombocytosis can be caused by iron deficiency and a variety of inflammatory conditions, infections, malignancy, bleeding or hemolysis, splenectomy, and drugs. Acute therapy for all of these disorders has included blood component removal, specifically plateletpheresis. The role of plateletpheresis in current management of thrombocytosis is considered, based on current knowledge of pathophysiology and a review of the literature.

Thrombocytosis and thrombocythemia

Thrombocytosis is caused by three major pathophysiological mechanisms: (1) reactive or secondary thrombocytosis; (2) familial thrombocytosis; and (3) clonal thrombocytosis, including essential thrombocythemia and related myeloproliferative disorders. Recent work has begun to elucidate the abnormal megakaryocytopoiesis of essential thrombocythemia, which is associated with paradoxically elevated plasma levels of thrombopoietin. The clonal nature of all cases of essential thrombocythemia has been challenged. Thrombotic complications are the major causes of morbidity and mortality in this disease. Indications for platelet cytoreduction and antiplatelet therapy, as well as complications of treatment, are being clarified.

Endogenous megakaryocytic colony formation and thrombopoietin sensitivity of megakaryocytic progenitor cells are useful to distinguish between essential thrombocythemia and reactive thrombocytosis

Diagnosis of essential thrombocythemia (ET) is controversial and remains mainly an exclusion diagnosis. Endogenous megakaryocyte colony (EMC) formation have been largely evaluated to identify specific criteria for ET, but results are impeded by the lack of medium standardization. We evaluated megakaryocyte (MK) colony formation in a serum-free collagen-based medium, without cytokine and in the presence of various concentrations of thrombopoietin (TPO). Thirty-six bone marrows from patients diagnosed with ET (n = 11), polycythemia vera (PV; n = 12), reactive thrombocytosis (RT; n = 6) and healthy donors (n = 7) were assessed. We demonstrate that 11 out 11 of the ET patients had spontaneous megakaryocyte colony-forming unit (CFU-MK) formation, in contrast to none of the RT patients and healthy donors. MK progenitors from ET patients remained responsive to TPO, because exogenous addition of TPO significantly increased cloning efficiency. Moreover, at low doses of TPO (0.5 ng/ml and 5 ng/ml), the number of positive cultures and mean number of TPO stimulated CFU-MK were significantly higher in cultures of cells from patients with ET than in patients with RT. In summary, we have described a standardized serum-free, collagen-based assay that allows differential diagnosis of ET and RT, according to endogenous CFU-MK formation and sensitivity to TPO.

Recent progress in the pathogenesis and management of essential thrombocythemia

In the last decade, the diagnosis of essential thrombocythemia (ET) has been refined by appreciation of the occurrence of karyotypically occult but molecularly evident chronic myelogenous leukemia and morphologically subtle myelodysplastic syndrome (MDS) and cellular-phase agnogenic myeloid metaplasia (AMM). Although ET continues to be defined by the presence of nonreactive thrombocythemia that is not accounted for by another chronic myeloid disorder, recent studies of clonality and other laboratory parameters have suggested clinically relevant biologic heterogeneity among affected patients. Furthermore, randomized, prospective, and controlled retrospective data have provided additional clinical information that has resulted in the development of risk categories and risk-adjusted treatment recommendations.

Myelofibrosis: experimental models and human studies

Thrombopoietin (TPO) is the central regulator of megakaryocytopoiesis and thrombocytopoiesis. Preclinical data and human studies have so far shown that the recombinant molecule is safe to administer and associated with very little toxicity. Nevertheless, different experimental animal models have revealed that a chronic exposure to very high doses of TPO could result in myeloproliferative syndromes with a spectrum of pathological features in common with human idiopathic myelofibrosis (PMF). A number of investigators have researched whether TPO or its receptor Mpl were involved in the pathogenesis of human myeloproliferative syndromes which are also characterized by a predominant megakaryocytic involvement, in PMF and primitive essential thrombocythemia. In both diseases, megakaryocyte (MK) progenitors develop autonomously in serum-deprived cultures. This spontaneous MK development is also observed at limiting dilution demonstrating that MK escape the normal regulatory controls. Furthermore, this abnormal MK proliferation and maturation is neither due to an autocrine stimulation by TPO nor by point mutation or deletion in the coding region of the c-mpl gene. This paper will review the data that have been reported to date on the effects of an overexpression of Mpl ligand and related molecules on the induction of experimental myelofibrosis and highlight recent insights into the pathogenesis of PMF.

Endogenous erythroid and megakaryocytic colony formation in serum-free, cytokine-free collagen gels

We studied the suitability of collagen-based semisolid medium for assay of endogenous erythroid colony formation performed in myeloproliferative disorders. Bone marrow (BM) mononuclear cells (MNC) from 103 patients suspected of having polycythemia vera (PV, 76 patients) or essential thrombocythemia (ET, 27 patients) were grown in collagen-based, serum-free, cytokine-free semisolid medium. Colony analysis at day 8 or 10 showed that this collagen assay is specific, as endogenous growth of erythroid colonies was never observed in cultures of 16 healthy donors and 6 chronic myelogenous leukemia (CML) patients. Endogenous erythroid colony formation was observed in 53.3% of patients suspected of PV, with only 15.4% of positive cultures for patients with 1 minor PV criterion and 72% (p = 0.009) of positive cultures for patients with > or =2 minor or 1 major PV criterion. Similarly, endogenous growth of erythroid colonies was found in 44.4% of patients suspected of ET, with 31.6% of positive cultures for patients with 1 ET criterion versus 75% for patients with > or =2 ET criteria. In addition, we found that in collagen gels, tests of erythropoietin (EPO) hypersensitivity in the presence of 0.01 or 0.05 U/ml of EPO and tests of endogenous colony-forming units-megakaryocyte (CFU-MK) formation cannot be used to detect PV or ET, as these tests were positive for, respectively, 21.4% and 50% of healthy donors and 83% and 50% of CML patients. A retrospective analysis suggests that collagen assays are more sensitive than methylcellulose assays to assess endogenous growth of erythroid colonies. In summary, serum-free collagen-based colony assays are simple and reliable assays of endogenous growth of erythroid colonies in myeloproliferative diseases. They also appear to be more sensitive than methylcellulose-based assays.

Autonomous Megakaryocyte Growth in Essential Thrombocythemia and Idiopathic Myelofibrosis Is Not Related to a c-mpl Mutation or to an Autocrine Stimulation by Mpl-L

Essential thrombocythemia (ET) and idiopathic myelofibrosis (PMF) are two myeloproliferative diseases characterized by a marked megakaryocytic (MK) involvement. The pathogenesis of these two diseases is unknown. Recently it has been shown that overexpression of Mpl-ligand (Mpl-L) in mice induces thrombocytosis and myelofibrosis. In this study, we investigated whether Mpl-L was responsible for the pathogenesis of ET and PMF. Using in vitro cultures of blood or marrow CD34+ cells, we investigated whether MK growth was abnormal in these two diseases. Spontaneous MK growth involving only a fraction (20%) of the MK progenitors, as compared with growth in the presence of pegylated recombinant human megakaryocyte growth and development factor (PEG-rhuMGDF), was found in both diseases (21ET and 14PMF) using serum-free semisolid and liquid cultures, including cultures at one cell per well. We first searched for ac-mpl mutation/deletion by sequencing the entire coding region of the gene by polymerase chain reaction (PCR) in nine ET patients and five PMF patients, but no mutation was found. We subsequently investigated whether an autocrine stimulation by Mpl-L could explain the autonomous MK growth. Addition of different preparations of soluble Mpl receptor (sMpl) containing a Fc domain of IgG1 (sMpl-Fc) markedly inhibited MK spontaneous growth in both ET and PMF patients. This effect was specific for sMpl because a control soluble receptor (s4-1BB-Fc) had no inhibitory effect and an sMpl devoid of the Fc fragment had the same inhibitory efficacy as the sMpl-Fc. This inhibition was reversed by addition of PEG-rhuMGDF or a combination of cytokines. The sMpl-Fc markedly altered the entry into cell cycle of the CD34+ cells and increased the apoptosis that occurs in most patient CD34+ cells in the absence of exogenous cytokine, suggesting an autocrine stimulation. In contrast, a neutralizing antibody against Mpl-L did not alter the spontaneous MK growth, whereas it totally abolished the effects of 10 ng/mL PEG-rhuMGDF on patient or normal CD34+ cells. Mpl-L transcripts were detected at a very low level in the patient CD34+cells and MK and only when a highly sensitive fluorescent PCR technique was used. By quantitative reverse-transcription (RT)-PCR, the number of Mpl-L transcripts per actin transcripts was lower than detected in human Mpl-L–dependent cell lines, suggesting that this synthesis of Mpl-L was not biologically significant. In favor of this hypothesis, the Mpl-L protein was not detected in culture supernatants using either an enzyme-linked immunosorbent assay (ELISA) or a biological (Ba/F3huc-mpl) assay, except in one PMF patient. Investigation of Mpl-L signaling showed an absence of constitutive activation of STATs in spontaneously growing patient MKs. Addition of PEG-rhuMGDF to these MKs activated STATs 3 and 5. This result further suggests that spontaneous growth is neither related to a stimulation by Mpl-L nor to ac-mpl mutation. In conclusion, our results show that Mpl-L or Mpl are not directly implicated in the abnormal proliferation of MK cells from ET and PMF. The mechanisms by which the sMpl mediates a growth inhibition will require further experiments.

Autonomous Megakaryocyte Growth in Essential Thrombocythemia and Idiopathic Myelofibrosis Is Not Related to a c-mpl Mutation or to an Autocrine Stimulation by Mpl-L

Essential thrombocythemia (ET) and idiopathic myelofibrosis (PMF) are two myeloproliferative diseases characterized by a marked megakaryocytic (MK) involvement. The pathogenesis of these two diseases is unknown. Recently it has been shown that overexpression of Mpl-ligand (Mpl-L) in mice induces thrombocytosis and myelofibrosis. In this study, we investigated whether Mpl-L was responsible for the pathogenesis of ET and PMF. Using in vitro cultures of blood or marrow CD34+ cells, we investigated whether MK growth was abnormal in these two diseases. Spontaneous MK growth involving only a fraction (20%) of the MK progenitors, as compared with growth in the presence of pegylated recombinant human megakaryocyte growth and development factor (PEG-rhuMGDF), was found in both diseases (21ET and 14PMF) using serum-free semisolid and liquid cultures, including cultures at one cell per well. We first searched for ac-mpl mutation/deletion by sequencing the entire coding region of the gene by polymerase chain reaction (PCR) in nine ET patients and five PMF patients, but no mutation was found. We subsequently investigated whether an autocrine stimulation by Mpl-L could explain the autonomous MK growth. Addition of different preparations of soluble Mpl receptor (sMpl) containing a Fc domain of IgG1 (sMpl-Fc) markedly inhibited MK spontaneous growth in both ET and PMF patients. This effect was specific for sMpl because a control soluble receptor (s4-1BB-Fc) had no inhibitory effect and an sMpl devoid of the Fc fragment had the same inhibitory efficacy as the sMpl-Fc. This inhibition was reversed by addition of PEG-rhuMGDF or a combination of cytokines. The sMpl-Fc markedly altered the entry into cell cycle of the CD34+ cells and increased the apoptosis that occurs in most patient CD34+ cells in the absence of exogenous cytokine, suggesting an autocrine stimulation. In contrast, a neutralizing antibody against Mpl-L did not alter the spontaneous MK growth, whereas it totally abolished the effects of 10 ng/mL PEG-rhuMGDF on patient or normal CD34+ cells. Mpl-L transcripts were detected at a very low level in the patient CD34+cells and MK and only when a highly sensitive fluorescent PCR technique was used. By quantitative reverse-transcription (RT)-PCR, the number of Mpl-L transcripts per actin transcripts was lower than detected in human Mpl-L–dependent cell lines, suggesting that this synthesis of Mpl-L was not biologically significant. In favor of this hypothesis, the Mpl-L protein was not detected in culture supernatants using either an enzyme-linked immunosorbent assay (ELISA) or a biological (Ba/F3huc-mpl) assay, except in one PMF patient. Investigation of Mpl-L signaling showed an absence of constitutive activation of STATs in spontaneously growing patient MKs. Addition of PEG-rhuMGDF to these MKs activated STATs 3 and 5. This result further suggests that spontaneous growth is neither related to a stimulation by Mpl-L nor to ac-mpl mutation. In conclusion, our results show that Mpl-L or Mpl are not directly implicated in the abnormal proliferation of MK cells from ET and PMF. The mechanisms by which the sMpl mediates a growth inhibition will require further experiments.

Haematopoietic progenitors and signal transduction in polycythaemia vera and primary thrombocythaemia

While significant progress has been made in understanding the cellular defect and molecular basis of polycythaemia vera (PV), elucidation of the primary mutation leading to PV remains elusive. While clinically useful, the PV diagnostic criteria put forward by the Polycythemia Vera Study Group are not based on the pathophysiology of this disorder and in some instances may lead to false diagnosis or may not be sufficient to diagnose an early PV. In diagnostically unclear situations, clinical and laboratory findings must take into account the acquired nature of PV, its clonality, and the presence of endogenous erythroid colony formation in serum-containing media. It is likely that other simpler assays may be developed based on the rapidly emerging knowledge of the cellular pathology of PV. Several intriguing observations of abnormalities pertaining to the erythroid signal transduction have been recently reported; these remain to be validated in other laboratories and to be proven specific for PV. The clinical concept of primary thrombocythaemia (PT) lags behind what we know about PV. While the diagnosis of PT is still based on the exclusion of other known causes of thrombocytosis, new knowledge is emerging. Recent clonality studies of a large number of PT females show that the majority are clonal. It is our belief that thrombocythaemic subjects who are not found to be clonal are those with secondary thrombocytosis. Multiple in vitro-based assays of megakaryocytic and erythroid progenitors have been developed and conflicting data published. It is likely that standardized assays of megakaryocytic progenitors will soon become available and a reproducible PT specific defect will be found. Such a specific test would be of immense diagnostic value in this most elusive of all myeloproliferative disorders.

Megakaryocyte progenitors in paroxysmal nocturnal haemoglobinuria are sensitive to complement

We have investigated growth in vitro of bone marrow megakaryocytic progenitors (CFU-Mk) in 7 patients with paroxysmal nocturnal haemoglobinuria (PNH) to determine the sensitivity of CFU-Mk to complement. Bone marrow light density mononuclear cells were exposed to fresh or heat-inactivated AB human serum in the presence of medium or isotonic sucrose solution. We found that the proliferative activity of bone marrow CFU-Mk in PNH patients was significantly lower than in controls. In addition, the number of CFU-Mk in PNH bone marrow cells exposed to isotonic sucrose and complement was reduced to 25% of that in PNH cells exposed to isotonic sucrose without complement. In conclusion, our finding showed an increased sensitivity of CFU-Mk in PNH bone marrow cells to complement, supporting the hypothesis that the PNH defect is present at the level of CFU-Mk.

Diagnostic and differential criteria of essential thrombocythemia and reactive thrombocytosis

Among the chronic myeloproliferative disorders essential thrombocythemia (ET) is known to be a distinct clinical entity in which an excessive number of morphologically and functionally abnormal platelets are produced. The clonal nature of the disease is well established. Based on a review of the literature the present authors propose the following novel criteria for the diagnosis of ET: A1. Platelet count in excess of 600 x 10(9)/L. A2. No increase in red-cell mass (RCM) in the presence of stainable iron in the bone marrow or failure of iron trial (RCM < 36 mL/kg in males and < 32 mL/kg in females; or RCM < 25% above mean normal predicted value*). A3. No Philadelphia chromosome. A4. Megakaryocytic hyperplasia (= increased megakaryocyte number and size) in histological sections of bone marrow and/or increased megakaryocytic ploidy (two-color flow cytometry); no collagen fibrosis. B1. Splenomegaly on isotopic scan or echogram. B2. Unstimulated growth of BFU-E and/or CFU-Meg present. B3. Normal ESR/fibrinogen. The diagnosis of ET is considered to be established if A1 + A2 + A3 + A4 or A1 + A2 + A3 + two B-criteria are fulfilled. (* Br J Haematol 1995; 89:748-756.)