Fibrinogen St. Gallen I (γ 292 Gly → Val): Evidence for Structural Alterations Causing Defective Polymerization and Fibrinogenolysis

Abstract

Fibrinogen St. Gallen I was detected in an asymptomatic Swiss woman. Routine coagulation tests revealed a prolonged thrombin and reptilase time. Functionally measured fibrinogen levels were considerably lower than those determined immunologically. Polymerization of fibrin monomers derived from purified fibrinogen was delayed in the presence of either calcium or EDTA. Normal fibrinopeptide A and B release by thrombin was established. An abnormal degradation of fibrinogen St. Gallen I by plasmin was observed. Fragment D1 of normal fibrinogen was fully protected against further proteolysis in the presence of 10 mM calcium, whereas fibrinogen St. Gallen I was partially further degraded to fragments D2 and D3. In the presence of 10 mM EDTA, the conversion of variant fragment D1 to D2 was accelerated whereas the degradation of fragment D2 to D3 was delayed in comparison to degradation of fragments D1 and D2 of normal fibrinogen. Three high-affinity calcium binding sites were found in both normal and variant fibrinogen. Mutation screening with SSCP analysis suggested a mutation in exon VIII of the gamma-chain gene. Cycle sequencing of this gene portion revealed a single base substitution from G to T of the base 7527, leading to replacement of gamma 292 glycine by valine. The same mutation has already been described for the fibrinogen variant Baltimore I. Molecular modeling was performed of a part of the gamma-chain containing the mutation site, based on recently published X-ray crystal structures of human fibrinogen fragment D and of a 30 kD C-terminal part of the gamma-chain. Significant structural alterations due to the substitution of glycine by valine at gamma 292 were observed, e.g. spreading of the protein backbone, probably leading to a modified accessibility of the plasmic cleavage sites in the gamma-chain at 356 Lys and 302 Lys. A shift of gamma 297 Asp that is involved in interactions of fragment D with the Gly-Pro-Arg-Pro-peptide was noted by molecular modeling. The latter observation is compatible with delayed polymerization of fibrin monomers.