During the last 15 years, it has become apparent that inherited abnormalities of blood coagulation play an important role in determining which individuals are at a higher risk for development of venous thrombosis. The right-hand column of Table 17-1 lists monogenic defects linked to the development of venous thrombosis. All consist of mutations in clotting factors or in inhibitors of clotting. Figure 17-1 depicts where these proteins are situated in the coagulation cascade.
Proteins that participate in the control or limitation of clot formation. Familial thrombophilia can be caused by mutations that affect antithrombin, prothrombin (Pro), factor V, protein C, and protein S. The "a" signifies activation; thus Ca = activated protein C. The circle with the X indicates inhibition or inactivation. EPCR: endothelial protein C receptor.
Two of these genetic variants are mutations in genes encoding clotting proteins—factor V and prothrombin:
Factor V Leiden is a genetic variant created by a single base change that results in the replacement of arginine by glutamine at position 506. In normal factor Va, Arg506 is the first site cleaved by activated protein C (Fig. 17-2). Subsequent cleavage by this serine protease at Arg306 and Arg679 inactivates factor Va. The absence of the initial cleavage at position 506 in factor V Leiden markedly delays cleavage at the two other sites and thereby results in higher levels of factor Va, which acts as a cofactor for factor Xa.
Prothrombin G20210A is a single base substitution in the 3' untranslated region of the prothrombin gene that increases the biosynthesis of prothrombin protein. As a result, individuals heterozygous for this mutation have approximately 30% higher plasma levels of structurally normal prothrombin compared with normal individuals. This seemingly modest increase in concentration significantly increases the risk of venous thrombosis.
Proteolytic degradation of the heavy chain of factor Va by activated protein C. (APC) Normal factor Va is rapidly cleaved at Arg506 with subsequent cleavage at Arg306 and Arg679. In the factor V Leiden mutation, the absence of the Arg cleavage site at 506 greatly delays subsequent cleavage, K: rate of cleavage.
Both factor V Leiden and prothrombin G20210A mutations are surprisingly common in White populations, with prevalences of about 5% and 2%, respectively. In fact, the high frequency of these two genetic variants suggests that they confer some type of selective advantage. However, their clinical importance lies in the striking finding that one, or sometimes both, of these mutations is encountered in a substantial percentage of individuals who develop deep venous thrombosis either spontaneously or in the presence of acquired risk factors.
Inherited defects that increase risk of deep venous thrombosis also include deficiencies of protein C, protein S, and antithrombin. The mutations in these proteins differ from factor V Leiden and prothrombin G20210A mutations in three major respects:
They involve inhibitors of coagulation rather than procoagulant factors.
They are much less common.
They are located at multiple sites in the respective genes.
The activities of protein C, protein S, and antithrombin also can be decreased in genetically normal individuals by certain conditions or medications.
Because of the low frequency of these mutations, homozygous individuals are very rarely encountered. A functional antithrombin level of less than 10% of normal is incompatible with life. Homozygous individuals with severe protein C or protein S deficiency have a very severe phenotype marked by thrombosis in childhood, adolescence, or even infancy (neonatal purpura fulminans). In contrast, the more frequently encountered heterozygotes are at increased risk of developing thrombosis in adulthood, albeit at low penetrance. The responsible mutations may either suppress expression of the protein (type I) or impair the functional activity of the protein (type II). In the former, both the antigen and activity levels are low, whereas in the latter, the antigen level is normal but the activity level is low.
In contrast to factor V Leiden and prothrombin G20210A mutations, which can be identified unequivocally by DNA analysis, inherited deficiencies of antithrombin, protein S, and protein C are diagnosed by measurements of plasma levels. Interpretation of these levels is often confounded by the fact that they can be lowered by a number of acquired factors (Table 17-2). Of most concern is the impact of acute thrombosis and anticoagulant therapy. Thus, in order to make a convincing diagnosis of an inherited deficiency of antithrombin, protein S, or protein C, measurements should be made after the patient has recovered from the thrombotic episode and is no longer receiving anticoagulants.
TABLE 17-2Causes of Acquired Deficiencies in Antithrombin, Protein C, or Protein S
An appreciation of the clinical significance of deficiencies in antithrombin, protein C, and protein S requires an understanding of the role of these proteins in pathways that inhibit clot formation. Figure 17-3 depicts how protein C and protein S limit the extent of clot formation. Thrombomodulin is a transmembrane protein expressed on endothelial cells that has on its extracellular domain a binding site for thrombin. Upon docking onto thrombomodulin, thrombin's substrate specificity becomes restricted to protein C. Protein C, like factors VII, IX, X, and prothrombin, is a vitamin K–dependent zymogen. Protein C binds to a receptor termed the endothelial protein C receptor on the endothelial cell surface, where protein C is activated by the thrombin-thrombomodulin complex. The rate of protein C activation is accelerated about 20,000-fold by binding to this complex. Activated protein C cleaves and degrades factors Va and VIIIa, as depicted in Figure 17-3. Importantly, the inactivation rate is enhanced by protein S, which is also vitamin K–dependent but not a zymogen.
Activation of protein C on the endothelial cell membrane and assembly of protein Ca and protein S on platelet and other cell membranes, enabling the rapid proteolysis of factors Va and VIIIa.
Individuals with protein C deficiency are at risk for a rare but serious complication termed warfarin-induced skin necrosis. When treatment with the anticoagulant warfarin (Chapter 13) is initiated, these patients occasionally develop painful ecchymoses in fat-containing areas of the body such as the breasts or buttocks, which can be quite extensive and debilitating (Fig. 17-4). Microscopically, these lesions contain fibrin thrombi within venules, accompanied by hemorrhagic necrosis. The explanation for warfarin-induced skin necrosis lies in wide differences in the stability of vitamin K–dependent coagulation proteins in the plasma. Factor VII and protein C have the shortest half-lives (6 and 14 hours, respectively), whereas the other vitamin K–dependent proteins have half-lives of 24 to 60 hours. In normal individuals, the fall in protein C activity following the start of warfarin is generally not of clinical significance, but in heterozygotes with levels of protein C that are about half of normal, a further drop in protein C activity during the first day of therapy can trigger thrombosis in the skin.
Warfarin-induced skin necrosis.
Antithrombin is a member of a large family of serine protease inhibitors. Although initially described as the primary physiologic inhibitor of thrombin, antithrombin was subsequently shown to also neutralize factors IXa and Xa and, to a lesser extent, factors XIa and XIIa. Antithrombin is also known as heparin cofactor I. In the absence of heparin, antithrombin is a relatively slow inactivator of these activated clotting factors. In contrast, as shown in Figure 17-5, the binding of the heparin polysaccharide to antithrombin changes its conformation in a way that markedly enhances its affinity for thrombin as well as for factors IXa, Xa, XIa, and XIIa. Heparin's binding site to antithrombin has been localized to a sulfated pentasaccharide (Fig. 17-5). This small domain is sufficient to inactivate factor Xa but not thrombin, because thrombin inhibition by heparin requires longer saccharides. This insight has permitted the development of heparin-like drugs that target factor Xa only (such as the synthetic pentasaccharide fondaparinux). Endogenous heparan sulfate polysaccharides on the surface of vascular endothelium also have domains with structures very similar to this pentasaccharide and appear to activate antithrombin, thereby serving as natural anticoagulants.
Individuals with inherited antithrombin deficiency are at an increased risk of developing venous thrombosis, similar to those with protein C and protein S deficiency. Among the inherited thrombotic disorders, the clinical penetrance of venous thromboembolism appears to be highest in families with type I antithrombin deficiency.