The Quebec platelet disorder (QPD) is an autosomal dominant platelet disorder associated with delayed bleeding and ␣-granule protein degradation. The degradation of ␣-granule, but not plasma, fibrinogen in patients with the QPD led to the investigation of their platelets for a protease defect. Unlike normal platelets, QPD platelets contained large amounts of fibrinolytic serine proteases that had properties of plasminogen activators. Western blot analysis, zymography, and immunodepletion experiments indicated this was because QPD platelets contained large amounts of urokinase-type plasminogen activator (u-PA) within a secretory compartment. u-PA antigen was not increased in all QPD plasmas, whereas it was increased more than 100-fold in QPD platelets (P < .00009), which contained increased u-PA messenger RNA. Although QPD platelets contained 2-fold more plasminogen activator inhibitor 1 (PAI-1) (P < .0008) and 100-fold greater u-PA-PAI-1 complexes (P < .0002) than normal platelets, they contained excess u-PA activity, predominantly in the form of two chain (tcu-PA), which required additional PAI-1 for full inhibition. There was associated proteolysis of plasminogen in QPD platelets, to forms that comigrated with plasmin. When similar amounts of tcu-PA were incubated with normal platelet secretory proteins, many ␣-granule proteins were proteolyzed to forms that resembled degraded QPD platelet proteins. These data implicate u-PA in the pathogenesis of ␣-granule protein degradation in the QPD. Although patients with the QPD have normal to increased u-PA levels in their plasma, without evidence of systemic fibrinogenolysis, their increased platelet u-PA could contribute to bleeding by accelerating fibrinolysis within the hemostatic plug. QPD is the only inherited bleeding disorder in humans known to be associated with increased u-PA. IntroductionCongenital platelet disorders are usually associated with defective primary hemostasis. [1][2][3] The Quebec platelet disorder (QPD) is an autosomal dominant platelet disorder that has unusual clinical features: it is associated with moderate to severe delayed bleeding, that typically begins 12 to 24 hours after surgery or trauma, and its hemorrhagic manifestations can be controlled with fibrinolytic inhibitors but not with platelet transfusions. 1,[4][5][6] This disorder was initially designated as factor V Quebec because of the abnormalities found in platelet factor V of these patients. 7 Two families from Quebec have been identified with this condition, which is now known to be associated with other platelet abnormalities that include reduced to low-normal platelet counts, proteolytic degradation of soluble and membrane proteins stored in platelet ␣-granules, an apparent quantitative deficiency of the ␣-granule protein multimerin, and defective aggregation with epinephrine. 1,[4][5][6]8 Although patients with the QPD have elevated levels of fibrinogen degradation products (FDPs) in their serum (because of platelet fibrinogen degradation), their plasma contains normal amo...
Multimerin is a massive soluble, multimeric protein found in platelets and endothelial cells. Recent studies identified multimerin as a specific coagulation factor V binding protein, complexed with platelet, but not plasma, factor V. These findings led us to investigate individuals with inherited factor V deficiencies for possible multimerin abnormalities. Platelet proteins were evaluated using immunoassays, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, immunoprecipitation, and direct binding studies. Patients with factor V Quebec, a disorder with abnormal platelet factor V, had a quantitative deficiency in multimerin (n = 11 tested; mean, 12.5%; range, 5% to 27% of the normal pool; normal range, 45% to 214%) with a normal multimer pattern. Quantitative and qualitative abnormalities were detected in their platelet factor V. An unrelated patient who was deficient in platelet and plasma factor V had normal platelet multimerin. The levels of platelet beta- thromboglobulin, von Willebrand factor, thrombospondin, and fibrinogen antigen were normal in the factor V Quebec patients. However, proteins with abnormal mobility were detected in their platelet lysate and releasate, and their platelet thrombospondin, von Willebrand factor, and fibrinogen showed evidence of proteolytic degradation. Platelet counts of the factor V Quebec patients ranged from mildly thrombocytopenic to low normal (mean, 159 x 10(9)/L; range, 104 to 198 x 10(9)/L). In addition, their platelets failed to aggregate in response to 6 to 10 micromol/L epinephrine despite normal numbers of platelet alpha 2-adrenergic receptors. These data indicate that patients with factor V Quebec have an inherited bleeding disorder distinct from other platelet disorders and associated with multiple abnormalities, including multimerin deficiency, abnormal platelet factor V, thrombospondin, von Willebrand factor, and fibrinogen, and an epinephrine aggregation defect.
We recently described a Quebec family with an autosomal dominant bleeding disorder characterized by mildly reduced-low normal platelet counts, an epinephrine aggregation defect, multimerin deficiency, and proteolytic degradation of several, soluble α-granular proteins. Similar clinical features led us to investigate a second family with an unexplained, autosomal dominant bleeding disorder. The affected individuals had reduced to normal platelet counts, absent platelet aggregation with epinephrine, and multimerin deficiency. Their platelet α-granular proteins factor V, thrombospondin, von Willebrand factor, fibrinogen, fibronectin, osteonectin, and P-selectin were proteolyzed and comigrated with the degradation products found in patients from the other family. However, their platelet albumin, IgG, external membrane glycoproteins, CD63 (a lysosomal and dense granular protein), calpain, and plasma von Willebrand factor were normal, indicating restriction in the proteins proteolyzed. Electron microscopy studies indicated preserved α-granular ultrastructure, despite degradation of soluble and membrane α-granular proteins. Immunoelectron microscopy studies of the patients' platelets indicated that fibrinogen, von Willebrand factor, P-selectin, multimerin, and factor V were within α-granules, with normal to reduced labeling for these proteins. Pathologic proteolysis of α-granular contents, rather than a defect in targeting proteins to α-granules, may be the cause of the protein degradation in the Quebec platelet disorder.
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