Increased Peripheral Platelet Destruction and Caspase-3-Independent Programmed Cell Death of Bone Marrow Megakaryocytes in Myelodysplastic Patients.

To investigate underlying mechanisms of thrombocytopenia in myelodysplastic syndrome (MDS), radiolabeled platelet studies were performed in 30 MDS patients with platelets <100 x 10⁹/L. Furthermore, plasma thrombopoietin and glycocalicin-index (a parameter of platelet or megakaryocyte destruction) were determined. Mean platelet life (MPL), corrected for the degree of thrombocytopenia, was reduced in 50% of the patients (4.3 ± 0.9 days (mean ± SD) vs 6.0 ± 1.3, p=0.0003). Platelet production rate (PPR, i.e. the number of platelets entering the circulation to maintain the platelet count) was reduced in 83% (68 ± 34 x 10⁹/d vs 220 ± 65, p<0.0001), although 80% of these patients had normal or increased numbers of bone marrow megakaryocytes. Thrombopoietin levels (patients 223 ± 194 vs controls 118 ± 33 pg/ml, p=0.04) were not significantly correlated with the PPR. However, the glycocalicin-index was significantly higher compared to controls (15 ± 16 vs 0.7 ± 0.2, p=0.001) and significantly and inversely correlated with the PPR (p=0.02, r=−0.5), but not with the MPL (p=1.8). These findings indicate to ineffective thrombopoiesis as a result of increased destruction of platelets and megakaryocytes in the bone marrow leading to a diminished release of platelets to the peripheral blood (i.e. PPR). To investigate whether programmed cell death (PCD) of bone marrow megakaryocytes might be responsible for this ineffective thrombopoiesis ultrastructural studies were performed in 5 patients MDS patients. In the majority of megakaryocytes (66 ± 15%) ultrastructural features of necrosis-like PCD were found, without signs of apoptotic or apoptotic-like PCD, especially in mature megakaryocytes. The data correspond with a negative immunohistochemical staining for activated caspase-3 of bone marrow megakaryocytes. These results indicate that the main cause of thrombocytopenia in MDS might be caspase-3-independent necrosis-like PCD resulting in a decreased PPR in conjunction with an increased glycocalicin-index.