Congenital afibrinogenemia: expression and analysis of novel mutations affecting fibrinogen assembly or secretion

Abstract number: OC025

Vu^* D., de Moerloose† P., Abu-Libdeh‡ B., Bouchardy^* I., Taher§ A., Bolton-Maggs¶ P. H. B., Morris^* M. A., Neerman-Arbez^* M.

‡Department of Pediatrics and Genetics, Jerusalem, Israel; †Division of Angiology and Hemostasis, Geneva, Switzerland; §Division of Hematology-Oncology, Beirut, Lebanon; ^*Division of Medical Genetics, Geneva, Switzerland; ¶Royal Liverpool Children's Hospital, Liverpool, UK

Congenital afibrinogenemia (MIM #202400) is a rare, autosomal recessive disorder characterized by the complete absence of circulating fibrinogen. Since our identification of the first genetic defect for this disease, a deletion of 11 kb eliminating nearly the entire FGA gene, numerous causative mutations have been described. The great majority of these have been identified in the FGA gene, although intuitively, all three genes were expected to be equally implicated. Apart from two missense mutations in FGB (Duga et al. 2000), all mutations described so far are null mutations, i.e. frameshift, nonsense or splice-site mutations, predicted to cause total lack of the corresponding fibrinogen chain. During the course of a prenatal diagnosis for a Palestinian family with two affected daughters, we identified a novel nonsense mutation in the FGB gene, W467X (exon 8) which was predicted to lead to the production of a truncated protein missing only 25 amino acids from the C-terminus. The identification of this relatively mild afibrinogenemia mutation in homozygosity in the two affected patients prompted us to study the W467X mutation further. Expression of the W467X mutant FGB cDNA in combination with wild-type FGA and FGG cDNAs showed that the W467X mutant protein was stably expressed. However, fibrinogen molecules containing the mutant beta-chain were not secreted into the media, although hexamer assembly appeared unimpaired, confirming the necessity of intact C-terminal portions of the fibrinogen beta-chain for the secretion of functional fibrinogen hexamers. Two new missense mutations identified in afibrinogenemia patients: FGA R178P, in the coiled coil region of the fibrinogen alpha chain, and FGB G444S in the C-terminal portion of the beta chain, are currently under analysis in order to investigate their putative effect on fibrinogen chain assembly and secretion.