Scott syndrome

Scott syndrome is a rare congenital bleeding disorder caused by a defect in one step of blood clotting inside the platelet. When a blood vessel is injured, platelets are normally activated and a membrane lipid called phosphatidylserine is moved from the inner to the outer face of the platelet membrane. There it gives a binding site for the protein complexes that convert prothrombin to thrombin, the tenase and prothrombinase complexes. In Scott syndrome the machinery that moves phosphatidylserine across the membrane is defective, so thrombin forms poorly. The same defect has been shown in the red blood cells and Epstein-Barr virus transformed lymphocytes of affected people, which suggests that the underlying mutation arises in a stem cell and touches several blood cell lineages. The cause of the transport defect was long unknown, and a candidate protein, scramblase, appears normal in the platelets of patients. The first patient reported with the syndrome has since been found to carry a mutation at a splice-acceptor site of the gene for anoctamin 6, also called TMEM16F. The only treatment for bleeding episodes named by the source is transfusion of normal platelets.