ABERRANT HEPATIC PROCESSING CAUSES REMOVAL OF ACTIVATION PEPTIDE AND PRIMARY POLYMERIZATION SITE FROM FIBRINOGEN CANTERBURY (A-ALPHA-20-VAL-]ASP)

A novel mechanism of molecular disease was uncovered in a patient with prolonged thrombin time and a mild bleeding tendency, DNA sequencing of the fibrinogen A alpha chain indicated heterozygosity for a mutatio n of 20 Val --> Asp. The molar ratio of fibrinopeptide A to B released by thrombin was substantially reduced at 0.64 suggesting either impai red cleavage or that the majority of the variant cu-chains lacked the A peptide, The latter novel proposal arises from the observation that the mutation changes the normal (16)R G P R V-20 sequence to R G P R D creating a potential furin cleavage site at Arg 19. Synthetic peptide s incorporating both sequences were tested as substrates for both thro mbin and furin. There was no substantial difference in the thrombin ca talyzed cleavage. However, the variant peptide, but not the normal, wa s rapidly cleaved at Arg 19 by furin, Predictably intracellular cleava ge of the A alpha-chain at Arg 19 would remove fibrinopeptide A togeth er with the G P R polymerisation site. This was confirmed by sequence analysis of fibrinogen A alpha chains after isolation by SDS-PAGE. The expected normal sequence was detected together with a new sequence(D V E R H Q S A-) commencing at residue 20, Truncation was further verif ied by nonreducing SDSPAGE of the NH2-terminal disulfide knot which in dicated the presence of aberrant homo- and heterodimers.