Fibrinogens Bern IV, Bern V and Milano XI: three dysfunctional variants with amino acid substitutions in the thrombin cleavage site of the A alpha-chain

Thrombin-induced cleavage of fibrinopeptide A is the initial step in the co nversion of fibrinogen to fibrin. Three dysfunctional fibrinogen variants a re described with an amino acid substitution at position 16 of the A alpha- chain: the fibrinogen variants Bern IV and Milano XI having an Arg-->His su bstitution and the variant Bern V having an Arg-->Cys substitution. Routine coagulation studies revealed prolonged plasma thrombin and reptilase clott ing times in all patients, and a discrepancy between the plasma levels of f ibrinogen determined by the clotting assay and electroimmunoassay. The defe ct was localized by high-performance liquid chromatography analysis of fibr inopeptide release and confirmed by polymerase chain reaction and sequencin g of exon 2 of the A alpha-chain. Immunoblotting analysis with a rabbit ant iserum against human serum albumin indicated that albumin was linked to the additional sulfhydryl group of fibrinogen Bern V. The assay of tissue-plas minogen-activator-induced plasmic degradation revealed that the fibrinolysi s of fibrin Bern V was delayed, whereas fibrin Bern IV was digested in the same way as normal fibrin. (C) 1999 Lippincort Williams & Wilkins.