Isolated deficiency of platelet procoagulant activity

Scott syndrome erythrocytes contain a membrane protein capable of mediating Ca2+-dependent transbilayer migration of membrane phospholipids.

“…The circulating blood cells of patients with Scott syndrome have previously been shown to exhibit a marked defect in their capacity to express procoagulant properties reflecting impaired mobilization of PS to the cell surface through Ca 2 ϩ -accelerated transbilayer movement of plasma membrane PLs (23)(24)(25)(26)(27)(28). Whereas the molecular defect responsible for this disorder remains unknown, the functional defect identified in Scott cells suggests either ( i ) a deficiency or defect in a specific protein that normally confers PL scramblase activity to the plasma membrane or, ( ii ) the expression in defective Scott cells of an inhibitor of the normal PL scramblase pathway.…”

“…Evidence for a platelet membrane protein of similar function has also been reported (22). Scott syndrome is a rare bleeding disorder in which normal cellular PL scramblase activity of blood platelets is defective despite normal secretory and aggregation responses, resulting in markedly reduced mobilization of PS to cell surfaces and reduced expression of membrane catalytic surface for the tenase and prothrombinase enzyme complexes (23)(24)(25)(26)(27)(28). The plasma membranes of these cells contain normal amounts of PS and other PLs, and exhibit normal aminophospholipid translocase activity (24,29).…”

Platelet receptor-mediated factor X activation by factor IXa. High-affinity factor IXa receptors induced by factor VIII are deficient on platelets in Scott syndrome.

“…Gel-filtered platelets were isolated as described previously (13). The patient, MS, who has been reported in detail by Weiss et al (15), is a 45-yr-old woman with a moderate bleeding disorder now referred to as Scott syndrome (16). She visited Philadelphia and her platelets were studied on three separate occasions in direct comparison with control platelets adjusted to the same platelet count.…”

“…The presence of factor VIII, which also binds to specific, high-affinity sites on activated platelets (14), together with factor X, increases fivefold the affinity of factor IXa binding to activated platelets (13). In order to investigate in more detail the mechanism of factor X activation by platelet-bound factor IXa, we have carned out studies of factor IXa binding and factor X, activation with normal platelets and with platelets obtained from a patient with a bleeding disorder termed Scott syndrome (15,16). These platelets, which have previously been shown to have an isolated deficiency of platelet procoagulant activity (15), lack factor Xa binding sites (17) and are deficient both in their capacity to promote prothrombin activation (17, 18) and factor X activation (18) and to expose phosphatidylserine at the outer surface of stimulated platelets ( 18).…”

A hereditary bleeding disorder of dogs caused by a lack of platelet procoagulant activity

“…[19][20][21][22] Characteristics of this syndrome include a moderate to severe bleeding tendency with normal platelet aggregation and secretion response but defective membrane transbilayer movement of PS and abnormal microvesiculation. 4,23,24 Molecular analyses of the candidate gene scramblase in a single family, and in lymphocytes derived from the original patient, have not revealed the molecular basis of Scott syndrome. 25,26 We report here the phenotypic characterization of a naturally occurring bleeding disorder of dogs caused by a lack of platelet procoagulant activity.…”