[Molecular biology of haemostasis: fibrinogen, factor XIII]

Genetic defects of fibrinogen are caused by a broad spectrum of mutations in one of the three structural genes FGA, FGB and FGG. They result in complete or partial lack of plasma fibrinogen (a- or hypofibrinogenaemia) or in structural abnormalities affecting protein function (dysfibrinogenaemia). In contrast to afibrinogenaemia mainly caused by nonsense, frameshift, and splice site mutations resulting in substantially truncated polypeptide chains (mainly Aalpha), in hypo- and dysfibrinogenaemias missense mutations lead to the exchange of single amino acids as dominating underlying defect. In the cases with quantitative disorders, bleeding with various degrees of severity is generally observed. Dysfibrinogenaemia is associated with both bleeding or thrombosis or even a combination of haemorrhagic and thromboembolic symptoms. About one half of the dysfibrinogenaemic cases is clinically asymptomatic. The plasmatic factor XIII (FXIII) is a heterotetramer composed of two A and two B subunits encoded by two different genes. FXIII deficiency is associated with bleeding, wound dehiscence and recurrent spontaneous abortions. The most frequent form is caused by defects in the A subunit with a broad spectrum of underlying mutations. Defects of the B subunit are very rare and were molecularly elucidated in only a few cases.