Congenital afibrinogenemia is a rare autosomal recessive coagulation disorder characterised by hemorrhagic manifestations of variable entity and by severe plasma
fibrinogen deficiency. Among the 31
afibrinogenemia-causing mutations so far reported, only 2 are missense mutations and both are located in the
fibrinogen Bbeta-chain gene. Direct sequencing of the
fibrinogen gene cluster in two afibrinogenemic Iranian siblings revealed a novel homozygous T>G transversion in exon 8 (
nucleotide position 8025) of the
fibrinogen Bbeta-chain gene. The resulting W437G missense mutation involves a highly conserved
amino acid residue, located in the C-terminal globular D domain. The role of the W437G amino acid substitution on
fibrinogen synthesis, folding, and secretion was assessed by in vitro expression experiments in COS-1 cells, followed by qualitative and quantitative analyses of intracellular and secreted mutant
fibrinogen. Results of both pulse-chase experiments and
enzyme-linked
immunosorbent assays demonstrated intracellular retention of the mutant W437G
fibrinogen and marked reduction of its secretion. These data, besides elucidating the pathogenetic role of the W437G mutation in
afibrinogenemia, underline the importance of the Bbeta-chain D domain in
fibrinogen folding and secretion.