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Defective fibrinogen polymerization associated with a novel γ279Ala→Asp mutation

A woman with menorrhagia was investigated for a suspected fibrinogen mutation when coagulation tests revealed prolonged thrombin (55 s) and reptilase (43 s) times together with a functional and an antigenic fibrinogen concentration of 0.7 and 2.8 mg/ml respectively. Heterozygosity for a γ‐chain mutation was suggested by a doublet γ band on SDS–PAGE and an increased negative charge was observed on isoelectric focusing of HPLC‐isolated γ‐chains. Electrospray ionization mass spectrometry revealed a γ‐chain mass of 48 411 Da, which was 20 Da more than the control value of 48 391 Da. Because the normal and variant γ‐chains were not resolved, this implied a 40‐Da increase in 50% of the γ‐molecules. An increased negative charge and a 44‐Da increase in mass was verified when DNA sequencing showed heterozygosity for an Ala (GCC)→Asp (GAC) substitution at codon 279 of the γ‐gene. Fibrin polymerization curves indicated a delay in the onset, and a decrease in the rate, of polymerization. Examination of crystal structures showed that the adjacent Tyr‐γ280 side chain is involved in bonding across the D–D interface, and from the proximity of the γ279Ala→Asp mutation it would appear that this perturbs the end‐to‐end DD interactions between fibrin units of the growing polymer.