Aberrant Hepatic Processing Causes Removal of Activation

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 a chain indicated heterozygosity for a mutation of 20 Val -+ Asp. The molar ratio of fibrinopeptide A to B released by thrombin was substantially reduced at 0.64 suggesting either impaired cleavage or that the majority of the variant a-chains lacked the A peptide. The latter novel proposal arises from the observation that the mutation changes the normal 16R G P R V ' sequence to R G P R D creating a potential furin cleavage site at Arg 19. Synthetic peptides incorporating both sequences were tested as substrates for both thrombin and furin. There was no substantial difference in the thrombin catalyzed cleavage. However, the variant peptide, but not the normal, was rapidly cleaved at Arg 19 by furin. Predictably intracellular cleavage of the Aa-chain at Arg 19 would remove fibrinopeptide A together with the G P R polymerisation site. This was confirmed by sequence analysis of fibrinogen Aa 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 verified by nonreducing SDSPAGE of the NH2-terminal disulfide knot which indicated the presence of aberrant homoand heterodimers. (J. Clin Invest. 1995. 96:2854-2858.)