Fibrinogen is an abundant protein synthesized in the liver, present in human blood plasma at concentrations ranging from 1.5-4 g/L in healthy individuals with a normal half-life of 3-5 days. With fibrin, produced by thrombin-mediated cleavage, fibrinogen plays important roles in many physiological processes. Indeed, the formation of a stable blood clot, containing polymerized and cross-linked fibrin, is crucial to prevent blood loss and drive wound healing upon vascular injury. Balance between clotting, notably the conversion of fibrinogen to fibrin, and fibrinolysis, the proteolytic degradation of the fibrin mesh, is essential. Disruption of this equilibrium can cause disease in distinct manners. While some pathological conditions are the consequence of altered levels of fibrinogen, others are related to structural properties of the molecule. The source of fibrinogen expression and the localization of fibrin(ogen) protein also has clinical implications. Low level fibrinogen expression has been detected in extra-hepatic tissues including carcinomas, potentially contributing to disease. Fibrin(ogen) deposits at aberrant sites including the central nervous system (CNS) or kidney can also be pathological. In this review, disorders in which fibrinogen and fibrin are implicated will be discussed, highlighting mechanisms which may contribute to disease.

Hereditary fibrinogen abnormalities comprise two classes of plasma fibrinogen defects: Type I, afibrinogenemia or hypofibrinogenemia, which has absent or low plasma fibrinogen antigen levels (quantitative fibrinogen deficiencies), and Type II, dysfibrinogenemia or hypodysfibrinogenemia, which shows normal or reduced antigen levels associated with disproportionately low functional activity (qualitative fibrinogen deficiencies). In afibrinogenemia and hypofibrinogenemia, most mutations of the FGA, FGB, or FGG fibrinogen encoding genes are null mutations. In some cases, missense or late truncating nonsense mutations allow synthesis of the corresponding fibrinogen chain but intracellular fibrinogen assembly and/or secretion are impaired. Afibrinogenemia is associated with mild-to-severe bleeding, whereas hypofibrinogenemia is most often asymptomatic. Thromboembolism may occur either spontaneously or in association with fibrinogen substitution therapy. Women with afibrinogenemia suffer from recurrent pregnancy loss but this can also occur in women with hypofibrinogenemia. Dysfibrinogenemia, caused mainly by missense mutations, is commonly associated with bleeding, thrombophilia, or both; however, most individuals are asymptomatic. Hypodysfibrinogenemia is a subcategory of this disorder. Even in specialized laboratories, the precise diagnosis of some fibrinogen disorders may be difficult. Determination of the molecular defects is important because it gives the possibility to confirm the diagnosis, to elaborate a diagnostic strategy, to distinguish in some cases that the patient is at risk of thrombosis rather than bleeding, and to enable prenatal diagnosis. However, genotype–phenotype correlations are not easy to establish. Replacement therapy is effective in treating bleeding episodes, but because the pharmacokinetics of fibrinogen after replacement therapy is highly variable among patients, it is important to adjust the treatment individually.

Key Points: Major bleeding, thrombosis, and postpartum hemorrhage are frequent in propositi and relatives with congenital dysfibrinogenemia. Hotspot mutations were not predictive of either phenotype or outcome.