<p> Inherited bleeding disorders can be caused by mutations affecting platelet, coagulation, or fibrinolytic proteins. Quebec platelet disorder (QPD) is a rare, autosomal dominant disorder associated with increased expression of the fibrinolytic enzyme urokinase plasminogen activator (uPA) in platelets. Individuals with QPD experience delayed-onset bleeding after hemostatic challenges that is attenuated with fibrinolytic inhibitor therapy. The aims of this thesis were to: 1) determine if increased platelet uPA contributes to QPD clot lysis in vitro; 2) investigate whether QPD individuals have increased urinary uPA, as some individuals experience hematuria; and 3) map the genetic locus of QPD, and look for the putative mutation. Studies of clot lysis indicated that QPD platelets induce a gain-of-function defect in fibrinolysis when platelets are incorporated into clots. This suggests that accelerated fibrinolysis may contribute to QPD bleeding. Studies of urinary uPA in QPD showed that uPA is not increased, indicating that hematuria in QPD is likely a consequence of increased platelet uPA. This finding also suggests that uPA overexpression in QPD may be megakaryocyte-specific. Linkage studies showed that QPD is strongly linked to a 2 megabase region on chromosome 10 that harbors the uPA gene, PLA U. No mutations in PLA U or its regulatory regions were identified; however, a common haplotype for a 32.5 kilobase region around PLA U, including inheritance of a rare, linked polymorphism, suggests this is the most likely locus for QPD. mRNA studies in QPD platelets showed that QPD selectively increases expression of the linked PLAU allele, without similar increases in megakaryocyte progenitors or in saliva. These findings implicate a cis-mutation near PLA U as the cause of QPD. This thesis provides novel insights on the fibrinolytic abnormality in QPD blood, and on the QPD genetic locus. which will be important for identifying the precise mutation that converts normally prohemostatic platelets to profibrinolytic cells. </p> / Thesis / Doctor of Philosophy (PhD)