The molecular basis of quantitative fibrinogen disorders

Hereditary fibrinogen disorders include type I deficiencies (afibrinogenemia and hypofibrinogenemia, i.e. quantitative defects), with low or unmeasurable levels of immunoreactive protein; and type II deficiencies (dysfibrinogenemia and hypodysfibrinogenemia, i.e. qualitative defects), showing normal or altered antigen levels associated with reduced coagulant activity. While dysfibrinogenemias are in most cases autosomal dominant disorders, type I deficiencies are generally inherited as autosomal recessive traits. Patients affected by congenital afibrinogenemia or severe hypofibrinogenemia may experience bleeding manifestations varying from mild to severe. This review focuses on the genetic bases of type I fibrinogen deficiencies, which are invariantly represented by mutations within the three fibrinogen genes (FGA, FGB, and FGG) coding for the three polypeptide chains Aalpha, Bbeta, and gamma. From the inspection of the mutational spectrum of these disorders, some conclusions can be drawn: (i) genetic defects are scattered throughout the three fibrinogen genes, with only few sites appearing to represent relative mutational hot spots; (ii) several different types of genetic lesions and pathogenic mechanisms have been described in affected individuals (including gross deletions, point mutations causing premature termination codons, missense mutations affecting fibrinogen assembly/secretion, and uniparental isodisomy associated with a large deletion); (iii) the possibility to express recombinant fibrinogen mutants in eukaryotic cells is rapidly shedding light into the molecular mechanisms responsible for physiologic and pathologic properties of the molecule; (iv) though mutation analysis of the fibrinogen cluster does not yield precise information for predicting genotype/phenotype correlations, it still provides a valuable tool for diagnosis confirmation, identification of potential carriers, and prenatal diagnosis.