Fibrinogen Otago: a major a chain truncation associated with severe hypofibrinogenaemia and recurrent miscarriage H AYLEY J. R I D G WAY, 1 S TEPHEN O. B RENNAN, 1 J AMES M. FAED 2 AND P ETER M. G EORGE 1 1 Molecular Pathology Laboratory, Christchurch Hospital, Christchurch, and 2 Department of Pathology, University of Otago Medical School, Dunedin, New Zealand Received 5 February 1997; accepted for publication 5 June 1997 Summary. A woman with a preliminary diagnosis of afib- rinogenaemia was later found to have a functional fibrino- gen of 0 . 06 mg/ml and markedly prolonged thrombin and reptilase times. The stoichiometry of fibrinopeptide release was normal but there was a gross delay in the polymeriz- ation of purified fibrin. Plasma protein electrophoresis showed an absence of normal fibrinogen and a novel anodal component which was confirmed as fibrinogen by immunofixation. Western blots of non-reducing SDS-PAGE gels indicated a molecular weight of 270 kD, compared to 340 kD for normal fibrinogen and similar analysis of reducing gels showed that the expected 67 kD Aa chain was missing and replaced by a 30 kD band. This aberrant chain was not detected by the monoclonal antibody F-103, which recognizes the epitope formed by residues 259–276 of the Aa chain. Cycle sequencing of the DNA encoding the F-103 epitope revealed the homozygous insertion of cytosine at position 4133 of the gene sequence. Predictably this translates as three new amino acids ( 268 Gln-Glu-Pro) before termination at a new (TAG) stop codon. No abnormal Aa chains could be detected in plasma from the woman’s heterozygous son. The hypofibrinogenaemia observed is likely to be the result of diminished assembly and/or secretion of the truncated Aa chains rather than enhanced extracellular degradation. Keywords: fibrinogen, transfusion, hypofibrinogenaemia, aC domain, miscarriage. Fibrinogen, a major 340 kD blood glycoprotein, is a dimer of three polypeptide chains, the Aa (67 kD), Bb (56 kD) and g (47 kD) chains. The N-termini of these six chains are clustered in the central (E) domain where the two half molecules are linked by a network of disulphide bonds. The C-terminal of the Bb and g chains fold to form lateral globular (D) domains which are connected to the central domain by a coiled coil of a, b and g chains (Doolittle, 1984; Henschen & McDonagh, 1986) and the C-termini of the Aa chains fold independently to form mobile aC domains (Veklich et al, 1993) which are found associated with the central E domain in fibrinogen. Thrombin converts fibrinogen to its active form, fibrin monomer, by cleaving fibrinopeptides A and B from the N-termini of the Aa and Bb chains, a process that exposes polymerization sites which then interact with preformed complementary sites located at the C-termini of the b and g chains (Kudryk et al, 1974; Hasegawa & Sasaki, 1990; Olexa & Budzynski, 1980). This spontaneous self-assembly process culminates in the production of the three-dimensional fibrin lattice which forms the haemostatic plug (Hantgan & Herman, 1979). Recent evidence indicates that the C-terminal region of the Aa chains has an important role in polymerization and in the formation of the final clot architecture. Electron micro- scopy and polymerization studies suggest that aC domains participate in intermolecular interactions during polymer- ization (Veklich et al, 1993) and that they accelerate polymerization and promote lateral aggregation of fibrin molecules. Interestingly, this intermolecular association of the aC domains appear to occur only in desAB fibrin and not in desA fibrin (Gorkun et al, 1994). A number of hereditary dysfibrinogenaemias alter, or partially delete, the aC domain, and all of these variants display defective polymerization. These fibrinogen variants have in recent studies been used to illustrate the importance of the aC domain in polymerization kinetics and final clot British Journal of Haematology , 1997, 98, 632–639 632  1997 Blackwell Science Ltd Correspondence: Dr Hayley J. Ridgway, Molecular Pathology Lab- oratory, Biochemistry Unit, Christchurch Hospital, Christchurch, New Zealand.