Insights into abnormal hemostasis in the Quebec platelet disorder from analyses of clot lysis

Abstract

Summary. Background: The Quebec platelet disorder (QPD) is inherited and characterized by delayed-onset bleeding following hemostatic challenge. Other characteristics include increased expression and storage of active urokinase-type plasminogen activator (u-PA) in platelets in the setting of normal to increased u-PA in plasma. There is also consumption of platelet plasminogen activator inhibitor-1 and increased generation of plasmin in platelets accompanied by proteolysis of stored a-granule proteins, including Factor V. Aims and Methods: Although fibrinolysis has been proposed to contribute to QPD bleeding, the effects of QPD blood and platelets on clot lysis have not been evaluated. We used thromboelastography Ò (TEG Ò ), biochemical evaluations of whole blood clot lysis, assessments of clot ultrastructure, and perfusion of blood over preformed fibrin to gain insights into the disturbed hemostasis in the QPD. Results: Thromboelastography was not sensitive to the increased u-PA in QPD blood. However, there was abnormal plasmin generation in QPD whole blood clots, generated at low shear, with biochemical evidence of increased fibrinolysis. The incorporation of QPD platelets into a forming clot led to progressive disruption of fibrin and platelet aggregates unless drugs were added to inhibit plasmin. In whole blood perfusion studies, QPD platelets showed normal adherence to fibrin, but their adhesion was followed by accelerated fibrinolysis. Conclusions: The QPD is associated with Ôgain-of-functionÕ abnormalities that increase the lysis of forming or preformed clots. These findings suggest accelerated fibrinolysis is an important contributor to QPD bleeding.

Improving blood disorder diagnosis: reflections on the challenges

“…We found that the abnormalities reflected degradation of the platelet but not plasma pool of fibrinogen, resulting in normal levels of D-dimer in plasma but significantly elevated levels of fibrinogen degradation products, recognized by polyclonal antibody assays, in serum from whole blood, but not in serum made from platelet-poor plasma [18]. The euglobulin clot lysis times and whole blood clot lysis times are normal in the disorder [15,16,18,19].…”

“…Diagnostic testing for the disorder was expanded to include assays for increased platelet uPA (in research laboratories) 2003 The unique pattern of Quebec platelet disorder a-granule proteolysis is attributed to uPA-triggered, intracellular generation of plasmin (from the plasminogen within platelets), without systemic fibrinolysis [21] 2004 Quebec platelet disorder is reported to increase risks for joint bleeds, delayed-onset bleeding after surgery and dental extraction, and for large bruises and to be associated with episodic, spontaneous hematuria [17]. Pregnancy outcomes are reported to be good, with no treatments required for uncomplicated deliveries [17] 2006 Quebec platelet disorder is reported to cause a unique gain-of-function defect in fibrinolysis, from release of platelet uPA during clot formation [19]. Thromboelastography is excluded as a possible diagnostic screening test [19] 2008 Quebec platelet disorder is reported to have minimal effect on urinary uPA [24] 2009 Quebec platelet disorder is linked to PLAU (uPA gene) with increased expression of uPA, by the linked allele, during megakaryocyte differentiation [22].…”

“…Pregnancy outcomes are reported to be good, with no treatments required for uncomplicated deliveries [17] 2006 Quebec platelet disorder is reported to cause a unique gain-of-function defect in fibrinolysis, from release of platelet uPA during clot formation [19]. Thromboelastography is excluded as a possible diagnostic screening test [19] 2008 Quebec platelet disorder is reported to have minimal effect on urinary uPA [24] 2009 Quebec platelet disorder is linked to PLAU (uPA gene) with increased expression of uPA, by the linked allele, during megakaryocyte differentiation [22]. uPA is demonstrated to be targeted to a-granules in the disorder [23].…”

Thrombopoietin levels in Quebec platelet disorder—Implications for the mechanism of thrombocytopenia

“…QPD markedly and selectively increases the production of normal PLAU transcripts by the disease chromosome in megakaryocytes but not in leukocytes through unknown mechanisms,6 and this increases megakaryocyte and platelet uPA levels by more than 100‐fold 7, 8. As QPD megakaryocytes and platelets sequester uPA in α‐granules,8 the disorder results in a unique, platelet activation‐dependent, gain‐of‐function defect in fibrinolysis, without systemic fibrinolysis 9. In QPD, platelet morphology (by light and electron microscopy), size,3 and dense granule release are normal, and secondary platelet aggregation is absent with epinephrine, with or without reduced maximal aggregation with collagen, adenosine diphosphate, and/or thromboxane analog U46619 10.…”

“…As TPO levels in QPD plasma were <31.3 pg/mL, we were unable to investigate whether QPD causes increased TPO proteolysis. The sequestration of uPA in QPD α‐granules (which prevents systemic fibrinolysis)9 might limit TPO proteolysis by uPA and plasmin in QPD. It is also possible that the increased uPA in QPD megakaryocytes is insufficient to increase TPO effects on megakaryocytes given that there are normal expansion and differentiation of QPD peripheral blood CD34+ stem cells into megakaryocytes in serum‐free cultures with added TPO 8…”