Variability of Thrombosis among Homozygous Siblings with Resistance to Activated Protein C Due to an Arg-to-Gln Mutation in the Gene for Factor V

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Abstract
The most frequent laboratory abnormality in patients with idiopathic deep-vein thrombosis is resistance to activated protein C1. Depending on the selection criteria, in vitro resistance to activated protein C can be identified in 20 to 50 percent of patients2-6. Protein C, a key element in the regulation of coagulation, circulates in plasma as an inactive precursor. On contact with thrombin bound to the thrombomodulin receptors on vascular endothelial cells, protein C rapidly becomes activated. Activated protein C enzymatically lyses two cofactors of the coagulation cascade, factor VIIIa and factor Va. It is thus a natural anticoagulant that controls the conversion of factor X to factor Xa by inactivating factor VIIIa, and that of prothrombin to thrombin by inactivating factor Va.