Resistance of γA/γ' fibrin clots to fibrinolysis

Elevated plasma fibrinogen levels are a major risk factor for thrombosis. This report shows two mechanisms by which fibrinogen can affect the fibrinolysis rate in vitro and thus may lead to thrombosis. First, the lysis rate of fibrin decreases as the initial concentration of fibrinogen increases. Second, a minor variant form of fibrinogen decreases the rate of fibrinolysis. This variant, γA/γ' fibrinogen, has one altered γ chain and is known to bind to factor XIII zymogen. In a fibrinolysis assay containing purified thrombin, fibrinogen, tissue-type plasminogen activator, and plasminogen, clots from γA/γA and γA/γ' fibrinogen lysed at similar rates. However, when factor XIII was added, slower lysis was seen in γA/γ' fibrin clots when compared with γA/γA fibrin clots. A D-dimer agglutination assay showed that the γA/γ' clots were more highly cross-linked than the γA/γA clots. The lysis rates of γA/γ' clots were similar to γA/γA clots in the presence of N-ethylmaleimide, a specific inhibitor of factor XIIIa. The γA/γ' fibrin clots made in the presence of factor XIII showed increased proteolytic resistance to both plasmin and trypsin. Clots made from afibrinogenemic plasma reconstituted with γA/γ' fibrinogen also showed significant resistance to lysis compared with γA/γA fibrinogen. These data demonstrate γA/γ' fibrin is resistant to fibrinolysis, possibly as a result of concentrating factor XIII on the clot. The total fibrinogen concentration and the amount of γA/γ' fibrinogen increase clot stability in vitro and thus may contribute independently to the risk of thrombosis in humans.