Unusual A alpha 16Arg-->Cys dysfibrinogenaemic family: absence of normal A alpha-chains in fibrinogen from two of four heterozygous siblings.

We describe studies of fibrinogen from an asymptomatic family, Fibrinogen Frankfurt XIII, with the substitution A alpha 16Arg-->Cys. The mother's fibrinogen was normal as assessed by plasma fibrinogen assays but the father and four children were affected. Two affected siblings and their father had thrombin times approximately 20 s longer than controls, fibrinogen of 35-52 mg/dl by clotting assay and normal levels by immunoassay. Measured by HPLC their fibrinogen had decreased amounts of releasable fibrinopeptide A (FpA) relative to normal controls. In addition, their fibrinogen was partly coagulable with batroxobin and fully coagulable with thrombin; consistent with its incomplete FpA release. In contrast, plasma from the other two siblings had thrombin times > 2 min longer than controls, and their fibrinogen was < 10 mg/dl by clotting assay and normal by immunoassay. Their isolated fibrinogen did not release FpA (n = 5) and formed thrombin clots only at cold temperatures indicating formation of desBB fibrin. Assessed by SDS-PAGE, only monomeric fibrinogen was seen in isolated fibrinogen, washed and unreduced plasma immunoprecipitates, non-crosslinked plasma clots or 56 degrees C precipitates. DNA sequencing of PCR products using oligonucleotide primers which flanked sequences coding for A alpha 7Asp to 35Phe, disclosed mutant (C-->T, coding for A alpha 16Cys) and wild type alleles present in both severely affected siblings. It is concluded that circulating fibrinogen contained only mutant FpA. A possible second mutation, not apparent from plasma assays on the mother, is believed to account for the absence of releasable normal FpA in circulating fibrinogen.