Analysis of megakaryocytopoiesis in children with congenital thrombocytopenia

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In this study, we analysed 15 children with congenital thrombocytopenia in order to unravel the underlying mechanisms. Plasma thrombopoietin (Tpo) and glycocalicin (GC, a marker for platelet turnover) levels were measured and megakaryocytopoiesis was studied with an in vitro liquid-culture system. Based on clinical and laboratory data included patients were divided into three groups. The first group of seven patients was diagnosed with amegakaryocytic thrombocytopenia. In bone marrow from these patients hardly any megakaryocyte was found and also in in vitro cultures no megakaryocyte formation was observed. Plasma Tpo levels were severely increased and GC levels were decreased, both indicating a platelet production defect. Sequence analysis of the gene encoding the Tpo-receptor, c-mpl, revealed mutations in five of the six patients analysed. Some mutations lead to the formation of a stopcodon and thus to a direct loss of Mpl function. We are currently investigating whether the other mutations, leading to single amino-acid replacements, which do not directly predict loss of Mpl function, are causing disruption of Mpl function or expression. The second group of three patients had normal numbers of atypical megakaryocytes in the bone marrow, showed megakaryocyte formation in vitro, normal Tpo plasma levels and severely decreased GC levels. A defect in megakaryocyte maturation and subsequent proplatelet formation of megakaryocytes is likely the cause of the thrombocytopenia in this group. The third group of five patients was diagnosed with Wiskott Aldrich Syndrome (WAS). They had normal Tpo-, normal GC-levels and megakaryocyte formation both in vivo and in vitro. Thrombocytopenia in this group is due to an increased platelet turnover, that is WAS related. Concluding, with three parameters, Tpo and GC plasma levels and in vitro megakaryocyte culture, a good distinction between different causes of congenital thrombocytopenia in children can be achieved.