Functional and structural consequences of fibrinogen γ-chain variants associated with thrombotic phenotype in congenital fibrinogen disorders.

Fibrinogen is a key determinant of clot formation and stability in the final phase of coagulation. Genetic variants in the fibrinogen γ-chain gene (FGG) are a frequent cause of congenital fibrinogen disorders (CFDs) and are associated with marked heterogeneity of clinical presentation, including thrombosis. Increasing evidence indicates that genetic findings alone are insufficient to predict thrombotic risk. We performed a comprehensive molecular, functional, and ultrastructural characterization of 20 patients from Czechia and Slovakia carrying FGG variants. Genetic analysis was combined with fibrin polymerization and fibrinolysis assays, fibrinopeptide release measurements, and scanning electron microscopy of fibrin clots. Five previously unreported pathogenic fibrinogen variants γp.T60A, γp.Y237H, γp.Y306C, γp.G310E, and γp.H333Y were identified. Although 60% of patients were clinically asymptomatic, 30% developed thrombotic manifestations in the absence of established thrombotic risk factors. Functional studies demonstrated delayed fibrin polymerization, reduced clot optical density, and prolonged fibrinolysis despite residual fibrin formation. Ultrastructural analysis revealed markedly altered fibrin clot architecture, characterized by abnormal fiber diameters and increased fiber density compared with controls, consistent with a dense, poorly lysable fibrin network. These findings indicate that FGG variants may promote thrombosis through qualitative alterations of fibrin structure and impaired fibrinolysis rather than fibrinogen deficiency alone. Integration of genetic, functional, and structural analyses is therefore essential for accurate assessment of thrombotic risk in patients with CFDs.