Platelet c-mpl expression is dysregulated in patients with essential thrombocythaemia but this is not of diagnostic value

Essential thrombocythaemia (ET) can be difficult to discriminate from an occult case of reactive thrombocytosis (RT). Since thrombopoietin (TPO) is the primary regulator of thrombopoiesis, we have investigated whether levels of TPO and/or its receptor, c-mpl, are of value in the differential diagnosis of ET. Plasma TPO levels in patients with ET, RT and other myeloproliferative disorders (MPDs) did not differ significantly from normal controls. However, surface c-mpl expression was significantly reduced in platelets from 18 ET patients, 0-65.5% of controls (P < 0.001). Immunoblots on five of these and five additional patients were consistent with absent or reduced c-mpl protein levels. The surface c-mpl expression results were significantly different from those in eight RT patients (21. 3-95.5%, P = 0.0015), but there was considerable overlap between the two groups and a reduced level was not restricted to ET. Furthermore, c-mpl expression in ET patients was not different from eight patients with other MPDs (0-87.6%, P = 0.06), nor could it differentiate between ET patients with monoclonal and polyclonal haemopoiesis. Although a low or absent c-mpl level is suggestive of a primary rather than a secondary thrombocytosis, it is insufficiently discriminatory to be used as a diagnostic marker for ET.     

Thrombopoietin measurement in thrombocytosis: dysregulation and lack of feedback inhibition in essential thrombocythaemia

Essential thrombocythaemia (ET), a myeloproliferative disorder (MPD) manifested by excessive platelet production, lacks a specific diagnostic test to facilitate differentiation from other thrombocytoses. We studied thrombopoietin (TPO) levels in 41 patients with thrombocytosis: 25 ET patients, eight with other MPD, and eight with reactive thrombocytosis. Mean age and platelet counts for these groups were comparable. TPO levels for 96 healthy individuals provided a reference range for normal. The majority of ET patients (19/25 or 76%) had normal TPO levels. No patient with ET had a TPO level below 75 pg/ml, compared with 57% of healthy donors and 8/16 (50%) patients with other thrombocytoses ( P  < 0.05). TPO levels in ET are not appropriately down-regulated, as occurs with cytokines relevant to other MPD. In thrombocytosis, a TPO level <75 pg/ml indicates that ET is unlikely.     

The activating splice mutation in intron 3 of the thrombopoietin gene is not found in patients with non-familial essential thrombocythaemia

Essential thrombocythaemia (ET) is a condition of unknown aetiology characterized by sustained thrombocytosis in the absence of a detectable systemic cause. Although usually considered a clonal disease affecting myeloid cells, recent data indicate that a significant proportion of patients have polyclonal haemopoiesis. In some patients the thrombopoietin (TPO) levels are normal or raised. Recently a mutation has been described in the TPO gene in familial thrombocythaemia that results in elevated TPO levels. We have therefore screened 51 patients diagnosed with non-familial ET for the presence of this mutation, but it was not detected in any patient. The constitutional presence of this mutation is therefore unlikely to contribute to the pathogenesis of ET.     

Serial serum thrombopoietin levels in a pregnant woman with essential thrombocythaemia

Thrombopoietin (TPO) is a ligand for c-mpl, which regulates the differentiation and maturation of megakaryocytes. Essential thrombocythaemia (ET) is a clonal myeloproliferative disorder. It has been reported that the platelet count declines during pregnancy in ET patients. We examined serial changes in the serum TPO level during the course of pregnancy in a patient with ET. The serum TPO level showed significant negative correlation with the platelet count. Although it mimicked the normal feed back system, the TPO levels were consistently higher than the normal upper limit. Accumulation of these data will be helpful in revealing the pathogenesis of ET and the decline in the platelet count during pregnancy.     

Lack of pathogenic mutations in the 5'-untranslated region of the thrombopoietin gene in patients with non-familial essential thrombocythaemia

Thrombopoietin (TPO) is thought to be the major physiological regulator of thrombopoiesis, and, in general, circulating levels are inversely proportional to megakaryocyte and platelet mass. However, normal or elevated TPO levels are found in patients with essential thrombocythaemia (ET) and the reason for this is not fully understood. Recent studies have shown that four kindreds with hereditary thrombocythaemia (HT) have point mutations in the 5'-untranslated region (UTR) of the TPO gene which lead to increased TPO translation. In order to determine whether similar mutations are present in apparently acquired ET, in particular in those patients with polyclonal myelopoiesis, we have studied this region in 50 ET patients using neutrophil DNA. The known HT mutations were investigated using polymerase chain reaction with mismatch primers and restriction enzyme digestion; only wild-type alleles were detected. Single-stranded conformation polymorphism (SSCP) analysis of exons 1-4 identified a C-->T substitution at nucleotide 3767. However, this appears to be a common polymorphism, as it was present at the same frequency in haematologically normal controls and is unlikely to be of pathological significance. These results demonstrate that mutations in the 5' UTR of the TPO gene are not the cause of the normal or elevated TPO levels in acquired ET.     

Long-term evaluation of 164 patients with essential thrombocythaemia treated with pipobroman: occurrence of leukaemic evolution

Essential thrombocythaemia (ET) is usually considered an indolent disease, but it may progress during its natural course into acute leukaemia (AL); however, an influence of myelosuppressive agents in the blastic transformation of ET cannot be excluded. We performed a retrospective study to assess the incidence of AL in ET patients treated with pipobroman (PB) as first-line therapy. One hundred and sixty-four patients with ET were managed with PB at a dose of 1 mg/kg/d until a stable platelet count below 400 x 10(9)/l was achieved. Maintenance therapy was given at a planned dose ranging between 0.2 and 1 mg/kg/d according to platelet count, in all cases, with a median daily dose of 25 mg (range 7-75 mg/d). The median treatment time was 100 months (range 25-243 months). The patients were evaluated for the occurrence of AL and/or secondary malignancies and survival end-points. AL was observed in nine patients (5.5%) after a median treatment time of 153 months (range 79-227 months). The overall survival (OS) and the event-free survival (EFS) at 120 months were 95% and 97%, whereas at 180 months, they were 84% and 76% respectively. In conclusion, this retrospective analysis shows a low incidence of AL in a large group of patients consecutively treated with PB as first-line chemotherapy. Therefore, an investigation of the role of myelosuppressive agents in the blastic transformation of ET would be of interest.     

Hereditary thrombocythaemia in a Japanese family is caused by a novel point mutation in the thrombopoietin gene

Hereditary thrombocythaemia (HT) with clinical features very similar to essential thrombocythaemia (ET) has been found to be transmitted as an autosomal dominant trait in several families. Here we studied the pathogenesis of HT in a previously described Japanese kindred. We found markedly elevated thrombopoietin (TPO) serum levels in all affected individuals and identified a novel point mutation in the TPO gene, a G --> T transversion at position 516 of the TPO mRNA (G516T) that co-segregated with the HT phenotype in all affected family members. This mutation is located in the 5'-untranslated region (5'-UTR) of the TPO mRNA and when assayed in reticulocyte lysates, improved translational efficiency of in vitro transcribed TPO mRNA. Cell lines transfected with the mutant TPO cDNA secreted up to 8-fold more TPO protein than cells transfected with the normal cDNA. We provide a molecular model of how the mutation partially disables the physiologic repression of TPO translation and thereby causes thrombocytosis. This is the third family in which HT has been caused by the loss of translational inhibition of TPO mRNA.     

Interaction of thrombopoietin with the platelet c-mpl receptor in plasma: binding, internalization, stability and pharmacokinetics

Thrombopoietin (TPO) is the primary regulator of platelet production and acts through binding its receptor, c-mpl, found on megakaryocyte progenitor cells, megakaryocytes and platelets. Circulating levels of TPO are regulated primarily by the clearance of TPO after it binds to c-mpl receptors on circulating platelets. In this study the interaction of TPO with the platelet c-mpl receptor has been analysed under physiological conditions using radiochemical and pharmacokinetic approaches. 125I-rHuTPO was prepared using a novel method of gentle iodination that preserved its biological activity and used to demonstrate that platelets, but not endothelial cells, have a single class of binding sites (56 +/- 17 binding sites/platelet) with high affinity (Kd = 163 +/- 31 pM). Cross-linking experiments confirmed that TPO, but not erythropoietin (EPO), specifically associated with the 95 kD platelet c-mpl receptor. Upon addition of TPO to platelets, 80% of the TPO binding sites were internalized within an hour and were not recycled. TPO that was not bound by platelets was stable for up to 6 d in both platelet-poor and platelet-rich plasma. Using unlabelled recombinant human TPO (rHuTPO), standard pharmacokinetic analysis demonstrated that platelets have an average TPO clearance of 1.24 +/- 0.38 ml/h/109 platelets and that TPO clearance was reduced by low temperature but not by a number of drugs or metabolic inhibitors. The maximal amount of TPO removed by platelets in vitro was identical to that predicted by the total number of TPO binding sites. These results provide a biochemical and pharmacokinetic basis for the clinical use of TPO and for understanding possible disorders of platelet production.     

Relationships between thrombohemorrhagic complications and platelet function in patients with essential thrombocythaemia

Three subgroups have been distinguished in essential thrombocythaemia (ET) patients, on the basis of clinical and laboratory findings. ET patients with bleeding incidents had smaller platelet volume, lower concentrations of beta-thromboglobulin and platelet factor 4 in their plasma, 10%, 26%, and 26% lower compared to patients without complications, respectively. ATP secretion from platelets of bleeders, clotters, and "no-complications" ET patients was found to be 75%, 36%, and 45%, respectively, lower than in healthy people. Spontaneous platelet aggregation appeared to be normal in about 90% of ET patients with no complications and in all bleeders but only in 35% patients with clotting incidents. All bleeders had abnormal agonist-evoked aggregation assays. Among remaining ET patients 30%-60% displayed normal values of different evoked aggregation tests. Thus, clinically distinguished group of bleeding ET patients may be differentiated from other subgroups on the basis of laboratory findings.     

Features of essential thrombocythaemia in childhood: a study of five children

Essential thrombocythaemia (ET) is usually considered a disease of the middle-aged but, with the advent of automated platelet counting, ET is diagnosed with increasing frequency in young adults and, even more rarely, in children. We report five paediatric patients (four girls and one boy, mean age 89 months) diagnosed with ET in agreement with Polycythaemia Vera Study Group criteria. The patients had a persistent thrombocytosis over 900 x 10(9)/l and, at the time of diagnosis, their platelet count ranged between 1,112 and 3,178 x 10(9)/l. A 9-month-old girl had thrombosis of the inferior cava vein, two children had headaches and two others remained asymptomatic throughout the follow-up period. Megakaryocytes in the bone marrow were increased in number. The chromosomal analysis was normal, and bcr rearrangement was always negative. None of the patients had spontaneous BFU-E or altered levels of serum erythropoietin and thrombopoietin. Two patients showed alteration of platelet aggregation, and all had decreased levels of intraplatelet serotonin. In spite of the diagnosis of ET in our patients, we are not sure that the cases reported here are really myeloproliferative disorders. The features could suggest that the cases observed may be affected by an 'idiopathic thrombocytosis' without myeloproliferation. Possible variants of ET are described in young adults, and the heterogeneous nature of ET is also suggested by our paediatric patients. Only careful long-term follow-up of patients such as these will clarify the natural history of these disorders and suggest therapeutic management.     

Pipobroman is safe and effective treatment for patients with essential thrombocythaemia at high risk of thrombosis

Essential thrombocythaemia (ET) is a disease associated with an elevated risk of thrombosis. This study evaluated the efficacy and safety of pipobroman (PB) in the long-term control of ET patients who had, at diagnosis, one or more of the following currently known risk factors for thrombosis or haemorrhage (high-risk patients): age > 60 years, history of thrombosis or haemorrhage, platelets >1000 x 10(9)/l. From 1978 to 2000, with a median follow-up of 10 years, 118 previously untreated high-risk ET patients (median age 62 years, range 25-82), were treated with PB at the starting dose of 0.8-1 mg/kg/d. All patients reached a platelet count <600 x 10(9)/l and 91% achieved a platelet count <400 x 10(9)/l. During follow-up, 13 patients had thrombosis, with a 10-year cumulative risk of 14%. Acute myeloid leukaemia, myelofibrosis and solid tumours occurred in three, two and seven patients with a 10-year cumulative risk of 3%, 2% and 7% respectively. Actuarial survival at 20 years was 64% and the standardized mortality ratio was 1.1 (95% CI: 0.7-1.7), not statistically different from the general population (P = 0.54). Age was associated with a higher risk of death (P = 0.00009) and thrombosis (P = 0.003). The duration of PB treatment did not correlate with the occurrence of second malignancies. This study, with a median follow-up of 10 years, demonstrates that pipobroman is effective and well tolerated. The low cumulative 10-year risk of thrombosis, leukaemia and solid tumours indicates that pipobroman is an adequate treatment for patients with high risk ET.     

Anagrelide,a novel platelet lowering option in essential thrombocythaemia: treatment experience in 48 patients in Germany

Abstract: We report on our treatment experience in Germany with anagrelide, a novel platelet lowering agent, in 48 patients (27 females, 21 males) with essential thrombocythaemia. Their age was between 19 and 79 yr when anagrelide therapy was initiated. Sixteen patients were previously untreated, 15 pretreated with hydroxyurea and 17 had multiple pretreatments. Forty-one of the 48 patients had either microvascular, thromboembolic or bleeding complications. About 50% received low dose acetylsalicylic acid as an adjunct. Their platelet count prior to therapy ranged from 850,000/μl to 3,100,000/μl. Eighty-seven per cent of the patients treated with anagrelide were complete hematological responders, while 13% responded only partially or failed to respond. Twelve of our patients (25%) developed short-term (from a few days to a maximum of 4 wk) side effects including headache (most frequent), palpitations, tachycardia or nausea. Eight patients reported long-term (more than 1 month) adverse effects. However, in only 5 of all 48 patients (10%) were these side effects not acceptable so that treatment had to be discontinued. We have now treated patients for up to 7 yr (median maintenance dose 2.5 mg/d). Preliminary evidence suggests that anagrelide mediated platelet count reduction also prevents recurrence of thromboembolic complications. Hence, anagrelide has the potential to become the first-line platelet-lowering treatment in myeloproliferative disorders with high platelet counts.     

JAK2 mutation in essential thrombocythaemia: clinical associations and long-term prognostic relevance

Clinical correlates and long-term prognostic relevance of the JAK2(V617F) mutation was studied in 150 patients with essential thrombocythaemia (ET) from a single institution and followed for a median of 11.4 years. During this period, thrombotic complications were documented in 62 patients (41.3%) and transformation into acute myeloid leukaemia (AML), polycythaemia vera (PV), or myelofibrosis with myeloid metaplasia (MMM) occurred in 4 (2.7%), 8 (5.3%), and 15 (10%) patients, respectively. JAK2(V617F) was detected in either archived bone marrow or blood cells from 73 patients (48.7%) but none were homozygous for the mutant allele. Parameters at diagnosis that were significantly associated with the presence of JAK2(V617F) included advanced age and higher counts of both haemoglobin and leucocytes. During follow-up, patients with the mutation were more likely to transform into PV but the incidences of AML, MMM, or thrombotic events were similar between patients with and without the mutation. Multivariate analysis identified advanced age, higher haemoglobin level, and thrombosis history but not the presence of JAK2(V617F) as independent predictors of inferior survival. Therefore, although the presence of JAK2(V617F) in ET appears to promote a PV phenotype, it might not carry treatment-relevant information.     

Second malignancies in patients with essential thrombocythaemia treated with busulphan and hydroxyurea: long-term follow-up of a randomized clinical trial

We have previously demonstrated that hydroxyurea (HU) reduces the rate of vascular complications in patients with essential thrombocythaemia (ET) at high risk of thrombosis. However, the relatively short follow-up (median 27 months) did not enable the evaluation of the risk of developing secondary malignancies. In this study, we report the long-term outcome of the 114 patients included in the trial: 56 patients randomized to receive HU and 58 patients to receive no cytoreductive therapy. Before randomization, 15 patients had been treated with busulphan. During the observation period, 29 patients (50%) shifted from the control to the HU group mainly because of thrombosis. Median follow-up was 73 months (range 3-94). Analysis was by intention to treat and, when indicated, by treatment. When analysed by intention to treat, 46 out of 54 patients (85%) originally randomized in the HU group are alive, compared with 49 of 58 patients (84%) in the control group [not significant (n.s.)]. Five patients (9%) in the HU group and 26 patients (45%) in the control group had thrombosis (P < 0.0001). Seven patients (13%) in the HU group developed secondary acute leukaemia, myelodysplastic syndromes or solid tumours, compared with only one of the control group patients (1.7%) (P = 0.032). The occurrence of secondary malignancies was also analysed by treatment: none of the 20 patients who had never been treated with chemotherapy developed neoplasia vs. three of the 77 patients given HU only (3.9% n.s.) and five of the 15 patients given busulphan plus HU (33% P < 0. 0001). This study showed that: (a) HU reduced the risk of thrombosis in ET patients; (b) the sequential use of busulphan and HU significantly increased the risk of second malignancies; and (c) overall survival was not affected by HU therapy.     

Thrombopoietin-responsive essential thrombocythaemia with myelofibrosis

Platelet-derived growth factor (PDGF) and other cytokines released from megakaryocytes are thought to play a crucial role in the pathogenesis of myelofibrosis. We describe a patient with essential thrombocythaemia (ET) who developed myelofibrosis with an increased level of serum thrombopoietin (TPO). Recombinant human (rh) TPO stimulated the proliferation and spontaneous megakaryocyte colony formation of the neoplastic cells in the peripheral blood. Moreover, serum concentrations of PDGF, platelet factor 4, and β-thromboglobulin were elevated and the production of these growth factors from the megakaryocyte progenitors was augmented with the addition of rhTPO in vitro . These results indicate that TPO may contribute to the development of myelofibrosis in ET.     

Transient normal platelet counts and decreased requirement for interferon during pregnancy in essential thrombocythaemia

We report on the remarkable decrease in the platelet counts during pregnancy in two women with essential thrombocythaemia following treatment with recombinant interferon-α (r-IFN-α). Prior to pregnancy, the first patient was treated for 10 months with r-IFN-α 3×10⁶ units/d six times per week, and the platelet count ranged between 750 and 800×10⁹/l. Starting from the sixth week of gestation, the platelet count decreased to normal levels and remained so, resulting in a lower r-IFN-α requirement. Following successful delivery of a healthy newborn an abrupt rise of the platelet count to pre-gestation values was observed, necessitating increased r-IFN-α dosage as before pregnancy. The second patient when she became pregnant had been treated with r-IFN-α 3×10⁶ units/d six times per week for 10 weeks. Starting from the 24th week of gestation the platelet count decreased, and despite reduction in the dose of r-IFN-α reached normal values at the time of delivery. The exact mechanism for the platelet count normalization during pregnancy is unclear, and several possibilities are discussed.