Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal myeloid disorder characterized by an acquired defect in the X-linked phosphatidylinositol glycan class A (PIG-A) gene, leading to loss of cell surface glycosylphosphatidylinositol-anchored proteins (for review see Rosse, 1997). Loss of the complement-regulating glycosylphosphatidylinositol-linked surface proteins, decay-accelerating factor and membrane attack complex inhibitory factor, causes the red cells to be exquisitely sensitive to complement-mediated hemolysis. Some patients have thrombocytope-nia, and an increased risk for unusual, life-threatening venous thrombotic events, such as hepatic vein thrombosis, occurs in some patients. Thus, the clinical profile of HIT potentially can be mimicked. The thrombocytopenia could be related either to decreased platelet production or to complement-mediated formation of procoagulant platelet-derived microparticles (Wiedmer et al., 1993).
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