The thrombocytopenia of Wiskott Aldrich syndrome is not related to a defect in proplatelet formation

The Wiskott-Aldrich syndrome (WAS) is an X-linked hereditary disease charac terized by thrombocytopenia with small platelet size, eczema, and increased susceptibility to infections. The gene responsible for WAS was recently cl oned. Although the precise function of WAS protein (WASP) is unknown, it ap pears to play a critical role in the regulation of cytoskeletal organizatio n. The platelet defect, resulting in thombocytopenia and small platelet siz e, is a consistent finding in patients with mutations in the WASP gene. How ever, its exact mechanism is unknown. Regarding WASP function in cytoskelet al organization, we investigated whether these platelet abnormalities could be due to a defect in proplatelet formation or in megakaryocyte (MK) migra tion. CD34(+) cells were isolated from blood and/or marrow of 14 WAS patien ts and five patients with hereditary X-linked thrombocytopenia (XLT) and cu ltured in serum-free liquid medium containing recombinant human Mpl-L (PEG- rHuMGDF) and stem-cell factor (SCF) to study in vitro megakaryocytopoiesis. In all cases, under an inverted microscope, normal MK differentiation and proplatelet formation were observed. At the ultrastructural level, there wa s also no abnormality in MK maturation, and normal filamentous MK were pres ent. Moreover, the in vitro produced platelets had a normal size, while per ipheral blood platelets of the same patients exhibited an abnormally small site. However, despite this normal platelet production, we observed that F- actin distribution was abnormal in MKs from WAS patients. Indeed, F-actin w as regularly and linearly distributed under the cytoplasmic membrane in nor mal MKs, but it was found concentrated in the center of the WAS MKs. After adhesion, normal MKs extended very long filopodia in which WASP could be de tected. In contrast, MKs from WAS patients showed shorter and less numerous filopodia. However, despite this abnormal filopodia formation, MKs from WA S patients normally migrated in response to stroma-derived factor-1 alpha ( SDF-1 alpha), and actin normally polymerized after SDF-1 alpha or thrombin stimulation. These results suggest that the platelet defect in WAS patients is not due to abnormal platelet production, but instead to cytoskeletal ch anges occuring in platelets during circulation. (C) 1999 by The American So ciety of Hematology.