Endogenous megakaryocyte colonies from peripheral blood in precursor cell cultures of patients with myeloproliferative disorders

Pathogenesis of myelofibrosis with myeloid metaplasia

The primary disease process in myelofibrosis with myeloid metaplasia (MMM) is clonal myeloproliferation with varying degrees of phenotypic differentiation. This is characteristically accompanied by secondary intramedullary collagen fibrosis, osteosclerosis, angiogenesis, and extramedullary hematopoiesis. Modern clonality studies have confirmed the multipotent stem-cell origin of the neoplastic process in MMM. The nature of the specific oncogenic mutation(s) is currently being unraveled with the recent discovery of an association between a somatic point mutation of JAK2 tyrosine kinase (V617F) and bcr/abl-negative myeloproliferative disorders, including MMM. The pathogenetic mechanisms that underlie the secondary bone marrow stromal changes in MMM are also incompletely understood. Mouse models of this latter disease aspect have been constructed by either in vivo overexpression of thrombopoietin (TPOhigh mice) or megakaryocyte lineage restricted underexpression of the transcription factor GATA-1 (GATA-1low mice). Gene knockout experiments using such animal models have suggested the essential role of hematopoietic cell-derived transforming growth factor beta1 in inducing bone marrow fibrosis and stromal cell-derived osteoprotegerin in promoting osteosclerosis. However, experimental myelofibrosis in mice does not recapitulate clonal myeloproliferation that is fundamental to human MMM. Other cytokines that are implicated in mediating myelofibrosis and angiogenesis in MMM include basic fibroblast, platelet-derived, and vascular endothelial growth factors. It is currently assumed that such cytokines are abnormally released from clonal megakaryocytes as a result of a pathologic interaction with neutrophils (eg, emperipolesis). This latter phenomenon, through neutrophil-derived elastase, could also underlie the abnormal peripheral-blood egress of myeloid progenitors in MMM.

Polycythemia Vera: Scientific Advances and Current Practice

Polycythemia vera (PV) is a clonal disorder of unknown etiology involving a multipotent hematopoietic progenitor cell that is characterized by the accumulation of phenotypically normal red blood cells, white blood cells, and platelets in the absence of a definable cause; extramedullary hematopoiesis, marrow fibrosis, and, in a few patients, transformation to acute leukemia can also occur. First described in 1892, the cause of the disease remains unknown and no potentially curative therapy other than bone marrow transplantation is currently available. It is commonly held that PV is a rare disorder, when in fact with a minimum incidence of 2.6 per 100,000 it is more common than chronic myelogenous leukemia (CML) and is particularly prevalent in persons of Ashkenazi Jewish ancestry. However, the incidence of PV is not as high as that of erythrocytosis from other causes collectively, which poses a problem in differential diagnosis when PV presents as isolated erythrocytosis. Characteristic features of PV are erythropoietin (Epo)-independent in vitro erythroid colony formation, as well as hypersensitivity to many other hematopoietic growth factors. Recently, a remarkable association between PV and a somatic point mutation of the JAK2 tyrosine kinase (JAK2 V617F) was described. Functional assays have revealed that JAK2 V617F is capable of inducing constitutive STAT5-mediated signaling in vitro, as well as erythrocytosis in vivo in mice. These data suggest that the JAK2 V617F mutation participates in the pathogenesis of PV. In current clinical practice, two different clinical approaches have been used to diagnose PV. One approach requires establishing the presence of absolute erythrocytosis by directly determining the red cell mass (RCM). A second approach utilizes a RCM-independent diagnostic algorithm based on the serum Epo level and bone marrow histology. Screening for JAK2 V617F can now be added to both diagnostic algorithms. However, it is very clear that some patients with classical PV lack the JAK2 V617F mutation, while some patients with other chronic myeloproliferative disorders such as idiopathic myelofibrosis (IMF) and essential thrombocytosis (ET) also express the JAK2 V617F mutation. Therefore, by necessity, any discussion of PV must take into consideration these companion myeloproliferative disorders, and since erythrocytosis is the single clinical feature that sets PV apart from IMF and ET, it is clear that the presence of the JAK2 V617F mutation cannot by itself establish a diagnosis of PV. Phlebotomy remains the mainstay of therapy for PV. In addition, both aspirin and cytoreductive therapy have been employed to control thrombocytosis and in the case of the latter, leukocytosis and extramedullary hematopoiesis as well. Despite recent progress in the field, several important issues remain controversial. In this review, we will present the areas of agreement, but also point out where the authors' personal viewpoints differ.

The JAK2V617F tyrosine kinase mutation in myeloproliferative disorders: status report and immediate implications for disease classification and diagnosis

Janus kinase 2 (JAK2) is a cytoplasmic protein-tyrosine kinase that catalyzes the transfer of the gamma-phosphate group of adenosine triphosphate to the hydroxyl groups of specific tyrosine residues in signal transduction molecules. JAK2 mediates signaling downstream of cytokine receptors after ligand-induced autophosphorylation of both receptor and enzyme. The main downstream effectors of JAK2 are a family of transcription factors known as signal transducers and activators of transcription (STAT) proteins. The myeloproliferative disorders (MPD), a subgroup of myeloid malignancies, are clonal stem cell diseases characterized by an expansion of morphologically mature granulocyte, erythroid, megakaryocyte, or monocyte lineage cells. Among the traditionally classified MPD, the disease-causing mutation has been delineated, thus far, for only chronic myeloid leukemia (ie, bcr/abl). In the past 3 months, 7 different studies have Independently described a close association between an activating JAK2 mutation (JAK2V617F) and the classic bcr/abi-negative MPD (ie, polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia) as well as the less frequent occurrence of the same mutation in both atypical MPD and the myelodysplastic syndrome. The particular finding is consistent with previous observations that have implicated the JAK/STAT signal transduction pathway in the pathogenesis of bcr/abl-negative MPD, Including the phenotype of growth factor independence and/or hypersensitivity. The current article summarizes this new information and discusses its implications for both classification and diagnosis of MPD.

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.

Polycythemia vera: a comprehensive review and clinical recommendations

More than a century has elapsed since the appearance of the modern descriptions of polycythemia vera (PV). During this time, much has been learned regarding disease pathogenesis and PV-associated molecular aberrations. New information has allowed amendments to traditional diagnostic criteria. Phlebotomy remains the cornerstone treatment of PV, whereas myelosuppressive agents may augment the benefit of using phlebotomy for thrombosis prevention in high-risk patients. Excessive aspirin use is contraindicated in PV, although the use of lower-dose aspirin has been shown to be safe and effective in alleviating microvascular symptoms including erythromelalgia and headaches. Recent studies have shown the utility of selective serotonin receptor antagonists for treating PV-associated pruritus. Nevertheless, many questions remain unanswered. What is the specific genetic mutation or altered molecular pathway that is causally related to the disease? In the absence of a specific molecular marker, how is a working diagnosis of PV made? What evidence supports current practice in the management of PV? This article summarizes both old and new information on PV; proposes a modern diagnostic algorithm to formulate a working diagnosis; and provides recommendations for patient management, relying whenever possible on an evidence-based approach.

Overexpression of FKBP51 in idiopathic myelofibrosis regulates the growth factor independence of megakaryocyte progenitors

Idiopathic myelofibrosis (IMF) is a chronic myeloproliferative disorder characterized by megakaryocyte hyperplasia and bone marrow fibrosis. Biologically, an autonomous megakaryocyte growth and differentiation is noticed, which contributes to the megakaryocyte accumulation. To better understand the molecular mechanisms involved in this spontaneous growth, we searched for genes differentially expressed between normal megakaryocytes requiring cytokines to grow and IMF spontaneously proliferating megakaryocytes. Using a differential display technique, we found that the immunophilin FKBP51 was 2 to 8 times overexpressed in megakaryocytes derived from patients' CD34(+) cells in comparison to normal megakaryocytes. Overexpression was moderate and confirmed in 8 of 10 patients, both at the mRNA and protein levels. Overexpression of FKBP51 in a UT-7/Mpl cell line and in normal CD34(+) cells induced a resistance to apoptosis mediated by cytokine deprivation with no effect on proliferation. FKBP51 interacts with both calcineurin and heat shock protein (HSP)70/HSP90. However, a mutant FKBP51 deleted in the HSP70/HSP90 binding site kept the antiapoptotic effect, suggesting that the calcineurin pathway was responsible for the FKBP51 effect. Overexpression of FKBP51 in UT-7/Mpl cells induced a marked inhibition of calcineurin activity. Pharmacologic inhibition of calcineurin by cyclosporin A mimicked the effect of FKBP51. The data support the conclusion that FKBP51 inhibits apoptosis through a calcineurin-dependent pathway. In conclusion, FKBP51 is overexpressed in IMF megakaryocytes and this overexpression could be, in part, responsible for the megakaryocytic accumulation observed in this disorder by regulating their apoptotic program.

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.

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.

Involvement of the fibrogenic cytokines, TGF-beta and bFGF, in the pathogenesis of idiopathic myelofibrosis

Idiopathic Myelofibrosis (IMF), is a chronic myeloproliferative disorder characterized by the association of myeloproliferation and myelofibrosis. The pathophysiological mechanisms resulting in this disease remain still unclear. The myeloproliferation appeared to result from the clonal amplification of hematopoietic progenitors. In contrast, fibroblasts participating in myelofibrosis were shown to be polyclonal, thus suggesting that myelofibrosis was a reactive process. We studied the role of two growth factors TGF-beta and bFGF, which display potent fibrogenic properties and are major regulators of primitive hematopoiesis, in IMF pathogenesis. We demonstrated an increase of TGF-beta and bFGF expression in circulating megakaryocytic cells and platelets, together with alterations of the expression of these cytokines and their receptors in hematopoietic CD34+ progenitor cells from IMF patients. Our results suggested that TGF-beta and bFGF are involved both in myelofibrosis and myeloproliferation which characterize IMF.

Myelofibrosis with myeloid metaplasia: diagnostic definition and prognostic classification for clinical studies and treatment guidelines

PURPOSE

Myelofibrosis with myeloid metaplasia (MMM) is a chronic myeloproliferative disorder characterized by bone marrow fibrosis and extramedullary hematopoiesis. Recent studies provide definite diagnostic criteria and prognostic classifications of the disease, and allogeneic stem-cell transplantation (SCT) now offers a chance of curing the disease. In order to put diagnostic criteria and prognostic classifications of the disease into the perspective of developing guidelines for treatment strategies, all studies published in the English literature over the last 30 years were reviewed.

MATERIALS AND METHODS

Studies were identified through a MEDLINE search (1966 to present) and from the bibliographies of relevant articles.

RESULTS

The Italian Consensus Conference on diagnostic criteria is a structured enterprise aimed at formulating a definition of MMM that will be used for enrolling patients onto clinical studies. It relies on the obligatory presence of myelofibrosis and on the exclusion of the BCR-ABL rearrangement or Philadelphia chromosome, in association with combinations of traditional features. Prognostic scores allow us to identify classes of patients on the basis of hemoglobin, age, WBC count, and chromosomal abnormalities. Several nonrandomized studies have indicated that allogeneic SCT for patients under the age of 55 is effective in prolonging survival in more than 50% of cases and in possibly curing the disease. Patients with the most severe prognosis are candidates.

CONCLUSION

"Consensus" methodology offers a definition of MMM useful for conducting and reporting clinical studies. A detailed knowledge of prognostic factors can help to delineate guidelines for addressing patients with allogeneic SCT.

Molecular detection of c-mpl thrombopoietin receptor gene expression in chronic myeloproliferative disorders

BACKGROUND

Chronic myeloproliferative disorders (CMPD) originate from a pluripotent haematopoietic progenitor cell but show a marked degree of heterogeneity, especially between Philadelphia chromosome positive and negative disease entities. Abnormal megakaryopoiesis is a frequent finding in CMPD, often associated with thrombocythaemic cell counts. Recent experimental data have suggested that the c-Mpl thrombopoietin receptor, together with its ligand thrombopoietin, are not only the major physiological regulators of megakaryopoiesis and platelet production, but also play a crucial role in chronic myeloproliferation.

METHODS

A total of 18 peripheral blood mononuclear cell samples obtained from patients with CMPD (chronic myelocytic leukaemia (CML), n = 10; polycythaemia vera (PV), n = 6; and primary thrombocythaemia (PTH), n = 2) were analysed for c-mpl mRNA using the reverse transcriptase polymerase chain reaction (RTPCR). In another 20 patients (CML, n = 10; chronic megakaryocytic granulocytic myelosis (CMGM), n = 3; PV, n = 3; PTH, n = 4), we compared the number of haematopoietic progenitors expressing c-Mpl, as characterised by coexpression with the CD34 antigen, in the bone marrow using double immunofluorescence staining.

RESULTS

c-mpl mRNA was detected in all samples from patients with CML analysed, whereas only two of six PV and one of two PTH samples were positive (p < or = 0.008; chi 2 test). Expression of the c-mpl receptor gene was absent in healthy subjects used as controls. Similarly, an increase of c-Mpl expressing CD34 positive haematopoietic cells was detected in seven of 10 bone marrow aspirates obtained from patients with CML. Increased numbers of c-Mpl positive CD34 positive cells were found in only one of four patients with PTH, whereas in PV and CMGM the numbers of c-Mpl positive CD34 positive cells did not exceed normal values, despite thrombocythaemic cell counts.

CONCLUSIONS

These data confirm recent findings showing an impaired expression of the c-mpl thrombopoietin receptor gene in Philadelphia chromosome negative CMPD when compared with patients with Philadelphia chromosome positive CML. The relevance of this observation to the functional and morphological characteristics of abnormal megakaryopoiesis remains unclear. Thrombocythaemic cell counts and a mature phenotype in megakaryocytes occur frequently in Philadelphia chromosome negative CMPD but require an intact c-Mpl receptor under physiological conditions. Therefore, further studies are warranted to elucidate the mechanisms contributing to megakaryopoiesis in CMPD disease entities with decreased c-mpl gene expression.

Functional roles of thrombopoietin-c-mpl system in essential thrombocythemia

Thrombopoietin (TPO) is implicated as a primary regulator of megakaryopoiesis and thrombopoiesis through binding to the cytokine receptor c-Mpl (the product of the c-mpl-proto-oncogene). In addition to its physiologic role, the TPO-c-mpl system has been suggested to participate in the pathophysiology of essential thrombocythemia (ET) which is a clonal disorder characterized by a sustained elevation of the circulating platelet count and bone-marrow hyperplasia with excessive proliferation of megakaryocytes. Recent studies have demonstrated that serum TPO levels are slightly elevated or within normal range in ET patients, whereas serum TPO levels tend to be inversely correlated with platelet mass. Flow cytometric, Western blot, and Northern blot analyses have revealed that the expression of platelet c-Mpl is strikingly reduced in all of patients with ET, possibly due to the decreased expression of c-mpl mRNA. These results suggest that normal or slightly elevated levels of serum TPO in ET patients may be attributable to the impaired uptake and catabolism of TPO owing to the low c-Mpl expression. Furthermore, immunoblotting with anti-phosphotyrosine antibody showed that no aberrant protein-tyrosine phosphorylation was observed in platelets of ET patients before treatment with TPO, and the levels of TPO-induced protein-tyrosine phosphorylation, including c-Mpl-tyrosyl phosphorylation, roughly paralleled those of c-Mpl expression, suggesting that c-Mpl-mediated signaling pathway was not constitutively activated in platelets of ET patients. Although activating mutation in the TPO gene, which leads to overexpression of TPO mRNA, has been reported in familial thrombocythemia, these results suggest that TPO-c-Mpl system may not be directly linked to pathogenesis of sporadic ET.

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.

Markedly Reduced Expression of Platelet c-mpl Receptor in Essential Thrombocythemia

Thrombopoietin (TPO) is implicated as a primary regulator of megakaryopoiesis and thrombopoiesis through binding to the cytokine receptor c-Mpl (the product of the c-mpl proto-oncogene). In an effort to determine the pathophysiological role of TPO-c-Mpl system in essential thrombocythemia (ET), we have examined the levels of serum TPO and the expression and function of platelet c-Mpl in 17 patients with ET. In spite of extreme thrombocytosis, serum TPO levels were slightly elevated or within normal range in most, if not all, patients with ET (mean ± SD, 1.31 ± 1.64 fmol/mL), as compared with normal subjects (0.76 ± 0.21 fmol/mL). Flow cytometric and Western blot analyses revealed that the expression of platelet c-Mpl was strikingly reduced in all patients with ET. Furthermore, the expression of platelet c-mpl mRNA was found to be significantly decreased in the ET patients tested. In contrast, almost identical levels of GPIIb/IIIa protein and mRNA were expressed in platelets from ET patients and normal controls. In addition to expression level, activation state of platelet c-Mpl was investigated in ET patients. Immunoblotting with anti-phosphotyrosine antibody showed that no aberrant protein-tyrosine phosphorylation was observed in platelets of ET patients before treatment with TPO, and the levels of TPO-induced protein-tyrosine phosphorylation, including c-Mpl-tyrosyl phosphorylation, roughly paralleled those of c-Mpl expression, suggesting that c-Mpl–mediated signaling pathway was not constitutively activated in platelets of ET patients. These results suggested that the TPO-c-Mpl system may not be directly linked to pathogenesis of ET, and that gene(s) mutated in ET may be important in regulating the levels of c-mpl gene expression in addition to the growth and differentiation of multipotential hematopoietic stem cells.

Clonality assays and megakaryocyte culture techniques in essential thrombocythemia

The development of techniques permitting in vitro growth of human megakaryocytes progenitors and more recently identification of the proto oncogene c-mpl (Mpl-R) and its ligand (Mpl-L) have created new opportunities for studying pathophysiology of E.T. Plasma or serum of E.T. patients was unable to overestimulate MK colony formation by normal bone marrow cells. Significant increases in circulating CFU MK in E.T. patients have been repeatedly observed while in E.T. marrow, due to inappropriate sampling, colony number was not significantly different from normal. Spontaneous colony formation is observed in approximately 100% bone marrow and 85% blood from E.T. patients. Spontaneous colony formation persisted in plasma clot assay without added plasma or serum and in serum free agar cultures but only at a slightly lower rate than in plasma clot. Spontaneous colony formation in culture condition without plasma and serum were never observed with normal bone marrow and blood. Spontaneous MK growth was observed in a higher proportion of E.T. patients than erythroid colony formation but both phenomenon can occur in about 50% of the patients. CFU MK colony formation disappeared in serum free cultures using highly purified CD 34 cells. MK development is not completely independent of regular control. An hypersensitivity of E.T. MK progenitors to growth factors known to stimulate normal hematopoiesis (IL3.IL6, GM CSF, has been shown as well as a decreased sensitivity to negative regulators (TGF beta), has been suggested. The number of spontaneous MK colonies was not significantly decreased by added anti IL3, IL6 or anti GM CSF, antibodies in culture medium. Pre incubation of blood non adherent mononuclear cells of E.T. patients with antisense oligonucleotides to c-mpl significantly decreased the cloning efficiency of spontaneous megakaryocyte growth as compared to the introduction of scrambled oligomers. Finally m RNA expression of the Mpl-L (TPO) was not formed in MK spontaneously grown in serum free liquid cultures after 12 days. These results suggest that human c-mpl proto oncogene may be implicated in the pathway of spontaneous megakaryocytopoiesis in MPD but an absence of autocrine-stimulation by TPO of spontaneous growth in MPD. Analysis of peripheral blood cell clonality was performed in 55 E.T. patients using either the DNA methylation pattern of the androgen receptor (AR) gene or mRNA transcripts of G6PD or IDS genes. 51 out of 55 patients were informative. Non random X inactivation was found on unfractioned blood in 73% as compared with 23% in normal females (skewed Lyonisation). In 12 patients monoclonality of hematopoiesis was definitely confirmed by recording polyclonality of the mononuclear fraction or of T lymphocytes. In 4 patients monoclonal hematopoiesis was limited to platelets, 7 patients remained polyclonal in whole blood and all cellular fractions studied. MK colony formation (provided that the serum free agar culture system is clearly standardised) and clonality studies on whole blood or granulocyte, T lymphocyte and platelet fractions may be proposed as positive criteria for diagnosis of E.T.

Circulating megakaryocytic progenitor cells in patients with primary thrombocythaemia and reactive thrombocytosis: results using a serum-deprived culture assay and a positive detection technique

A serum-free culture method was used to study the growth of megakaryocytic progenitor cells (CFU-Meg) from patients with elevated platelet counts. The culture technique was combined with immunocytochemistry (APAAP, CD61) for the identification of CFU-Meg derived cells in cytopreparations of cells eluted from the culture dishes. Twenty-six patients with primary thrombocythaemia (14 untreated patients, UPT, 12 treated patients, TPT), 14 patients with reactive thrombocytosis (RT) and 9 normal individuals were studied. Unstimulated growth of CD61-positive cells was detected in 8/14 UPT, 8/12 TPT, 12/14 RT and 5/9 normal subjects (with mean CD61-positive cell counts of 75, 579, 236 and 7 per cytopreparation respectively). Cultures supplemented with interleukin 3 contained CD61-positive cells in 11/14 UPT, 7/12 TPT, 14/14 RT and 5/9 normal subjects (with mean CD61-positive cell counts of 157, 589, 250 and 7 per cytopreparation respectively). Thus, this serum-free culture technique combined with sensitive positive identification of CFU-Meg derived cells failed to discriminate between PT and RT. These results cast doubt on the usefulness of serum-free culture assays for the detection of unstimulated CFU-Meg growth in the differential diagnosis of patients with elevated platelet counts.

Spontaneous megakaryocyte colony formation in myeloproliferative disorders is not neutralizable by antibodies against IL3, IL6 and GM-CSF

Megakaryocyte progenitor growth in 42 patients with myeloproliferative disorders (MPD), including 23 essential thrombocythaemia (ET), eight polycythaemia vera (PV), six chronic myelogenous leukaemia (CML) and five primary myelofibrosis (PMF), was studied in vitro using plasma clot assay and serum-free agar culture. Spontaneous megakaryocyte colonies (CFU-MK) were found in 34/40 (80%) blood and 14/18 (77.8%) bone marrow plasma clot cultures, and also observed in 27/35 (77.1%) blood and 10/18 (55.6%) bone marrow serum-free agar cultures. In the blood of 27 patients with MPD (15 ET, four PV, four CML and four PMF) and the bone marrow of 10 patients (five ET, four CML and one PV), spontaneous colony formation was observed in both plasma clot and serum-free agar cultures. However, spontaneous CFU-MK was only found in plasma clot culture, but not in agar culture in two blood (one ET and one CML) and four bone marrow cultures (one ET, two PV, one CML). The colony numbers were greatly increased in the presence of aplastic anaemia serum (AAS) under both conditions. In 17 patients (12 ET, two CML and three PV) with spontaneous megakaryocyte colonies, anti-cytokine antibody neutralizing experiments were carried out in blood cultures. Anti-IL3, anti-IL6 and anti-GM-CSF antibody, alone or in combination, at different concentrations (1, 5 and 10 micrograms/ml), were added into plasma clot or agar cultures without exogenous stimulating growth factors. The results showed that the numbers of spontaneous megakaryocyte colonies were not significantly decreased in the presence of these monoclonal antibodies in the cultures. The data indicated that the megakaryocyte progenitor growth in MPD under in vitro conditions was heterogenous, and independent of exogenous stimulatory factors in most patients and that optimal megakaryocyte colony development in MPD still requires exogenous growth factors. Three possibilities are discussed with regard to the phenomenon that the spontaneous colony formation was not decreased with the addition of anti-IL3, anti-IL6 and anti-GM-CSF antibodies.

Circulating megakaryocyte progenitors in myeloproliferative disorders are hypersensitive to interleukin-3

Previous studies have reported that megakaryocyte progenitors in myeloproliferative disorders (MPD) formed spontaneous megakaryocyte colonies without the addition of megakaryocyte colony-stimulating factor (Meg-CSF). To determine whether this spontaneous colony formation is due to autocrine proliferation of MPD megakaryocyte progenitors or to hypersensitivity to Meg-CSF that might exist in the culture system, we investigated colony-forming unit-megakaryocytes (CFU-Meg) in the peripheral blood of 11 MPD patients, using serum-free cultures. Spontaneous megakaryocyte colonies were observed in serum-free cultures of nonadherent mononuclear cells (NAdMNC) obtained from MPD patients with thrombocytosis, whereas the NAdMNC of MPD patients without thrombocytosis, that of patients with reactive thrombocytosis and normal subjects never formed spontaneous colonies. However, the spontaneous colonies from MPD patients with thrombocytosis disappeared in cultures using highly purified CD34-positive cells as target cells. To study the hypersensitivity of megakaryocyte progenitors to Meg-CSF, dose-response experiments were performed with interleukin-3 (IL-3). CFU-Meg from MPD patients with thrombocytosis showed maximal growth at the concentrations of IL-3 lower than those for normal subjects. CFU-Meg of MPD patients without thrombocytosis and that of patients with reactive thrombocytosis showed the same colony growth response to IL-3 as that of normal subjects. This result indicates that the CFU-Meg of MPD patients with thrombocytosis are hypersensitive to IL-3. It also suggests that spontaneous colony formation by NAdMNC is not due to the autocrine growth of megakaryocyte progenitors but is due to the hypersensitivity of megakaryocyte progenitors to Meg-CSF, such as IL-3, released by accessory cells. Furthermore, it is possible that such hypersensitivity of CFU-Meg to IL-3 might be a pathogenic factor in MPD with accompanying thrombocytosis.

Reduced responsiveness of bone marrow megakaryocyte progenitors to platelet-derived transforming growth factor beta 1, produced in normal amount, in patients with essential thrombocythaemia

In this study we evaluated the amount of transforming growth factor-beta 1 (TGF-beta 1) in platelet lysates obtained from 12 patients affected by essential thrombocythaemia (ET) in comparison with five patients affected by myelofibrosis with myeloid metaplasia (MMM) and 15 healthy donors. The levels of both bioactive and latent TGF-beta 1, evaluated in a bioassay on CCL64 cells, before and after transient acidification, were similar in platelet lysates from ET patients and normal donors and significantly (P < 0.01) elevated in platelet lysates from MMM patients. Moreover, platelet lysates from ET patients and normal controls, showed a similar degree of colony suppression when tested on haematopoietic progenitor (CD34+) cells, purified from normal bone marrows, whereas platelet lysates from MMM patients showed a higher (P < 0.01) inhibitory activity on normal CFU-meg and BFU-E growth. In parallel, platelet lysates form ET patients and normal controls were tested on CD34+ cells, purified from ET bone marrows. ET bone marrow BFU-E, similarly to normal bone marrow BFU-E, were markedly inhibited by platelet lysates, whereas ET bone marrow CFU-meg were significantly (P < 0.05) less responsive to the inhibitory activity of platelet lysates than normal bone marrow CFU-meg. The main factor responsible for the inhibitory activity contained in platelet lysates was transforming growth factor-beta 1 (TGF-beta 1), as demonstrated by the ability of a polyclonal neutralizing anti-TGF-beta 1 antibody to almost completely reverse the suppressive effect of platelet lysates on CFU-meg and BFU-E growth. Our data demonstrate that the amount of intraplatelet TGF-beta 1 is similar in ET patients and normal controls, whereas it is increased in platelets from MMM patients. Moreover, megakaryocyte progenitors in ET show a reduced sensitivity to platelet-derived inhibitors and, in particular, to TGF-beta 1.

Essential thrombocythemia: impaired regulation of megakaryocyte progenitors

In this paper, the in vitro growth of bone marrow early (megakaryocyte burst-forming units, BFU-meg) and late (megakaryocyte colony-forming units, CFU-meg) progenitors was evaluated in 18 essential thrombocythemia (ET) patients and 22 normal control subjects. BFU-meg clonality was demonstrated both in normal and ET bone marrows, cultivating these primitive progenitors at limiting dilutions in plasma clot assay: 1 to 7 BFU-meg/2.5 x 10(4) mononuclear non-adherent cells were observed, with a strong correlation in ET [r = 0.955 stimulated by recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) plus recombinant human interleukin (rhIL) 3], as well as in normal controls (r = 0.969). In order to clearly elucidate the in vitro response of ET megakaryocyte (meg) progenitors to recombinant growth factors, the interference of accessory cells (i.e., monocytes, T lymphocytes, and natural killer cells) and human serum were avoided by performing experiments on CD34+ cells in a serum-free fibrin clot assay. The number of both early and late meg progenitors in ET was significantly increased in response to rhIL-3, rhIL-3 plus rhIL-6, and rhIL-3 plus rhGM-CSF, but not in response to rhGM-CSF alone. Furthermore, both meg progenitors were investigated for their response to rh transfer growth factor (TGF)-beta 1, tested at concentrations from 0.01 to 10 ng/ml. rhTGF-beta 1 was able to inhibit CFU-meg and BFU-meg in a dose-response manner normal, whereas ET CFU-meg appeared less sensitive to the lower doses investigated (p less than 0.05) and ET BFU-meg were slightly reduced in number only at the higher concentrations of rhTGF-beta 1 (p less than 0.01). Our data suggest that the increased thrombopoiesis in ET may depend on an increased sensitivity of meg progenitors to some of the physiological growth factors and to a disrupted sensitivity to at least one negative regulator of megakaryocytopoiesis. Since these abnormalities involve both meg progenitors, this can be considered a demonstration that the neoplastic event hits the most primitive hemopoietic progenitors.

Megakaryocyte precursors (pro- and megakaryoblasts) in bone marrow tissue from patients with reactive thrombocytosis, polycythemia vera and primary (essential) thrombocythemia. An immunomorphometric study

To determine the number of megakaryocyte precursors (pro- and megakaryoblasts), an immunomorphometric study was performed on paraffin-embedded trephine biopsies of the bone marrow using a monoclonal antibody against platelet glycoprotein IIIa. Eighteen control specimens from patients with no evidence of any hematological disorder and a normal platelet count were selected and assessed together with the same number of specimens from patients with reactive thrombocytosis, polycythemia vera rubra (P. vera) or primary (essential) thrombocythemia (PTH). A strikingly proportionate increase in early megakaryocytes occurred in all patients enrolled in this study, compared with the controls. Moreover, there were no significant correlations between counts for precursors or total megakaryocytes per square millimeter of bone marrow with the corresponding values for platelets. This indicates that despite an orderly increase in immature forms in the bone marrow, the number of platelets circulating in the blood is influenced by other additional factors, such as the expanded platelet pool in the enlarged spleen. The non-disproportionate expansion of megakaryocyte precursors extends previous findings on progenitor cells of this lineage in vitro, particularly in PTH. Histological evaluation of the bone marrow of patients with P. vera and PTH indicated that megakaryopoiesis proceeded to the production of appropriate mature forms with no obvious excess of very small or blastic elements.