Beta-thromboglobulin content in megakaryocytes of patients with myeloproliferative diseases

Bleeding time and platelet function in essential thrombocythemia and other myeloproliferative syndromes

Bleeding time (BT) and platelet function tests have been widely used in patients with essential thrombocythemia (ET), with the aim to support diagnosis and to identify laboratory predictors of haemorrhagic and thrombotic complications. BT is significantly prolonged in 7-19% of ET patients and several functional abnormalities have been observed in platelet structure, biochemistry and survival. However, the attempt to relate these in vivo and in vitro platelet dysfunctions with diagnosis or clinical sequelae has been generally disappointing. Therefore, BT and platelet function tests are currently not recommended in the initial evaluation or during the follow-up of patients with ET, unless in the setting of a clinical or biological study. A noteworthy exception is represented by a subset of patients characterized by very high platelet count (> 1500 x 10(9)/L) and bleeding symptoms, who can have an acquired von Willebrand disease. In these cases, prolonged BT and abnormal multimeric pattern of von Willebrand factor are useful for diagnosing and monitoring this acquired hemorrhagic disease. BT and platelet function tests should be included in the baseline evaluation of ET patients enrolled in prospective clinical trials aiming assess their predictive role on clinical end-points.

Platelet derived growth factor messenger RNA is increased in bone marrow megakaryocytes in patients with myeloproliferative disorders

Platelet derived growth factor (PDGF) has been suggested to play an important role in the pathogenesis of myelofibrosis, which often occurs in patients with myeloproliferative disorders (MPD). We examined the expression level of PDGF mRNA in bone marrow megakaryocytes from 13 MPD patients by in situ hybridization, using cDNA probes for both human PDGF A chain and B chain (c-sis). The mRNA level for both chains in the patients was significantly higher than that in control patients, and was markedly higher for one patient with essential thrombocythemia and one with polycythemia vera. Transcripts for A chain and B chain were expressed with a positive correlation in the MPD patients. Using the marrow fibroblast proliferation assay, we found PDGF activity in purified megakaryocytes from one of the MPD patients with high mRNA level to be similar to that from one control patient. In addition, PDGF was previously shown to be decreased in circulating platelets from MPD patients. These results may suggest that, in some patients, PDGF is synthesized in megakaryocytes at a high rate, but some fraction is released into the bone marrow environment, if the level of PDGF mRNA is assumed to be linearly related to the protein synthesized. This might be one possible mechanism causing marrow fibrosis in MPD patients.

Intraplatelet levels of vWF:Ag and fibrinogen in myeloproliferative disorders

Several platelet function abnormalities have been described in the myeloproliferative syndromes. We have measured the intraplatelet vWF:Ag and fibrinogen (FI) in the platelet lysates by Laurell technique in 11 patients with polycythemia vera (PV), 10 with essential thrombocythemia (ET), 14 with chronic myelocytic leukaemia (CML) and 3 with myelofibrosis (MF) and these results were correlated with platelet function abnormalities. Decreased intraplatelet levels of vWF:Ag and FI were found in all the patients with ET and MF, in 8 out of 11 PV and 3 out of 14 CML. A statistical significant correlation was observed between the intraplatelet levels of vWF:Ag and FI in the control group and in CML and PV, but no correlation was found in ET and MF. No correlation was observed between the plasmatic and the intraplatelet levels of vWF:Ag and FI in any group. Evidences of platelet activation (spontaneous platelet aggregation or circulating platelet aggregates) were observed in 40% of the cases with ET and PV, and all these cases had low intraplatelet levels of both antigens. None of the cases with MF had evidences of platelet activation and 2 out of 14 patients with CML had platelet activation. The deficiency of the dense bodies was less frequent than the depletion of the alpha granules (5 out of 11 PV, 4 out of 10 ET, 6 out of 14 CML and 2 out of 3 MF). The low intraplatelet contents of vWF: Ag and FI, more frequently observed in ET and PV, may be the result of platelet activation and in vivo release, but megakaryocyte dysfunction is more likely in myelofibrosis.