Quebec platelet disorder (QPD) is an inherited bleeding disorder associated with increased urokinase plasminogen activator (uPA) in platelets but not in plasma, intraplatelet plasmin generation, and ␣-granule protein degradation. These abnormalities led us to investigate uPA expression by QPD CD34 ؉ progenitors, cultured megakaryocytes, and platelets, and whether uPA was stored in QPD ␣-granules. Although QPD CD34 ؉ progenitors expressed normal amounts of uPA, their differentiation into megakaryocytes abnormally increased expression of the uPA gene but not the flanking genes for vinculin or calcium/calmodulindependent protein kinase II␥ on chromosome 10. The increased uPA production by cultured QPD megakaryocytes mirrored their production of ␣-granule proteins, which was normal. uPA was localized to QPD ␣-granules and it showed extensive colocalization with ␣-granule proteins in both cultured QPD megakaryocytes and platelets, and with plasminogen in QPD platelets. In QPD megakaryocytes, cultured without or with plasma as a source of plasminogen, ␣-granule proteins were stored undegraded and this was associated with much less uPAplasminogen colocalization than in QPD platelets. Our studies indicate that the overexpression of uPA in QPD emerges with megakaryocyte differentiation, without altering the expression of flanking genes, and that uPA is costored with ␣-granule proteins prior to their proteolysis in QPD. (Blood. 2009;113:1535-1542)

Quebec platelet disorder (QPD) is an autosomal dominant disorder with high penetrance that is associated with increased risks for bleeding. The hallmark of QPD is a gain-of-function defect in fibrinolysis due to increased platelet content of urokinase plasminogen activator (uPA) without systemic fibrinolysis. We hypothesized that increased expression of uPA by differentiating QPD megakaryocytes is linked to PLAU. Genetic marker analyses indicated that QPD was significantly linked to a 2-Mb region on chromosome 10q containing PLAU with a maximum multipoint logarithm of the odds (LOD) score of ؉11 between markers D10S1432 and D10S1136. Analysis of PLAU by sequencing and Southern blotting excluded mutations within PLAU and its known regulatory elements as the cause of QPD. Analyses of uPA mRNA indicated that QPD distinctly increased transcript levels of the linked PLAU allele with megakaryocyte differentiation. These findings implicate a mutation in an uncharacterized cis element near PLAU as the cause of QPD.

Quebec platelet disorder (QPD) is a rare, autosomal-dominant, inherited bleeding disorder that is associated with unique abnormalities in fibrinolysis. Its hallmark features are delayed-onset bleeding following hemostatic challenges that responds to fibrinolytic inhibitor therapy and increased expression and storage of the fibrinolytic enzyme urokinase plasminogen activator in platelets, without increased plasma urokinase plasminogen activator or systemic fibrinolysis. The increased urokinase plasminogen activator in QPD platelets is only partially inhibited, and, as a result, there is intraplatelet generation of plasmin, and secondary degradation of many platelet alpha-granule proteins. During clot formation, the urokinase plasminogen activator released by QPD platelets leads to platelet-dependent increased fibrinolysis, and this is postulated to be a major contributor to QPD bleeding. The focus of the present review is to summarize the current state of knowledge on QPD, including the history of this disorder, its clinical and laboratory features, and recommended approaches for its diagnosis and treatment.

The Quebec Platelet Disorder (QPD) is an unusual bleeding disorder associated with increased platelet stores of urokinase-type plasminogen activator (u-PA) and proteolysis of platelet alpha-granule proteins. The increased u-PA and proteolyzed plasminogen in QPD platelets led us to investigate possible contributions of intracellular plasmin generation to QPD alpha-granule proteolysis. ELISA indicated there were normal amounts of plasminogen and plasmin-alpha(2)-antiplasmin (PAP) complexes in QPD plasmas. Like normal platelets, QPD platelets contained only a small proportion of the blood plasminogen, however, they contained an increased amount of PAP complexes compared to normal platelets (P < 0.005). The quantities of plasminogen stored in platelets were important to induce QPD-like proteolysis of normal alpha-granule proteins by two chain u-PA (tcu-PA) in vitro. Moreover, adding supplemental plasminogen to QPD, but not to control, platelet lysates, triggered further alpha-granule protein proteolysis to forms that comigrated with plasmin degraded proteins. These data suggest the generation of increased but limiting amounts of plasmin within platelets is involved in producing the unique phenotypic changes to alpha-granule proteins in QPD platelets. The QPD is the only known bleeding disorder associated with chronic, intracellular activation of the fibrinolytic cascade.

SummaryFactor V is an essential coagulation cofactor that circulates in plasma and platelet α-granules where it is stored complexed to multimerin 1 (MMRN1). To gain insights into the origin and processing of human platelet factor V, and factor V-MMRN1 complexes, we studied factor V in cultured megakaryocytes. Factor V mRNA was detected in all megakaryocyte cultures. However, like albumin, IgG and fibrinogen, factorV protein was detectable only in megakaryocytes cultured with exogenous protein. The amount of factor V associated with megakaryocytes was influenced by the exogenous factorV concentration. Similar to platelet factor V, megakaryocyte factor V was proteolyzed and complexed with megakaryocyte-synthesized MMRN1. With secretagogues, megakaryocytes released factorV, IgG, fibrinogen and MMRN1. Immunofluorescent and electron microscopy confirmed factorV uptake by endocytosis and its trafficking to megakaryocyte α-granules. These data provide direct evidence that human megakaryocytes process plasma-derived factor V into α-granules and generate factor V-MMRN1 complexes from endogenously and exogenously synthesized proteins.