Scott syndrome: an inherited defect of the procoagulant activity of platelets

Anionic phospholipids, chiefly phosphatidylserine, are essential for the assembly of the characteristic enzyme complexes of the blood coagulation cascade at the surface of stimulated platelets and derived microparticles. In the resting cell, these phospholipids are sequestered in the inner leaflet of the plasma membrane. Scott syndrome is an extremely rare bleeding disorder that confirms the essential role of these anionic procoagulant phospholipids. In Scott patients, phosphatidylserine externalization and microparticle shedding are dramatically impaired. This functional deficiency is clearly evidenced by the measurement of residual prothrombin in serum. The recent detection of a familial Scott syndrome testifies to the genetic origin of the defect. Symptomatic Scott patients present provoked hemorrhages and are probably homozygous for the disorder whereas asymptomatic children are probably heterozygous. The Scott phenotype can be detected in platelets, red cells and lymphocytes by functional prothrombinase assay and flow cytometry. Intermediate degrees of phosphatidylserine exposure and vesiculation are observed in cells from the asymptomatic heterozygous offspring when compared to those from their homozygous defective parent and healthy subjects. The functional and molecular characterization of mutated element(s) in Scott syndrome should be of valuable help for the understanding of phospholipid transmembrane migration, also termed flip-flop, its possible links with membrane vesiculation, and the eventual implications in thrombotic or apoptotic processes.