Hypofibrinogenemia due to novel 316 Asp -> Tyr substitution in the fibrinogen B beta chain

We investigated the molecular basis of hypofibrinogenemia in a woman with a plasma fibrinogen of 1.0 mg/mL. After sequencing the coding region and int ronic boundaries of all three fibrinogen genes a single heterozygous GAC -- > TAC mutation was identified at codon 316 of the B beta gene. This Asp --> Tyr substitution segregated with the hypofibrinogenemia in the only other affected family member. Examination by SDS-PAGE, isoelectric focussing, rev erse phase chromatography and electrospray ionisation (ESI) mass spectromet ery, failed to detect expression of the new B beta chain in purified plasma fibrinogen. The absence of the variant chain was confirmed by ESI tryptic mapping; while the [M + 1 H] and [M + 2 H] ions of the affected peptide (MG PTELLIEMEDWK) were clearly visible at 1,692 and 847 mit, then were no new s ignals (1741 or 871 mit) that would at indicate expression of the variant i n plasma. Asp 316 and its gamma chain homologue (Asp 252) are conserved in all known species and this is the first report of a mutation at either of t hese. The residue appears to be critical in maintaining the structure of th e five stranded sheet that forms the dominant structural feature of the D d omains.