Molecular Mechanisms of Hypo‐ and Afibrinogenemia

Abstract

Point mutations responsible for hypo‐ and afibrinogenemia are yielding new insights into amino acid side chains involved in the molecular processing, assembly, secretion, and domain stability of fibrinogen. Reverse phase chromatography, isoelectric focussing, electrospray mass spectrometry, and tryptic peptide mass mapping have shown that chains with heterozygous mutations of γ284 Gly→Arg, Bβ316 Asp→Tyr and γ371 Thr→Ile are absent from plasma fibrinogen. The nonexpression of these mutations appears to result from perturbation of the five‐stranded β sheet of the D domain. We propose that this is due to retention of the variant in the endoplasmic reticulum and that in turn this leads to hypofibrinogenemia. Other mutations effect intracellular proteolysis and chain assembly. For example the mutation, Aα20 Val→Asp, makes the protein a substrate for furin, which removes the first 19 residues of the Aα chain as the mature molecule transits the trans golgi complex. Transient expression of γ153 Cys→Arg chains together with Aα and Bβ chains suggests this mutation might perturb chain assembly, and the incorporation of mutations of Bβ353 Leu→Arg or Bβ400 Gly→Asp into intracellular fibrinogen precludes its subsequent export from host cells expressing fibrinogen genes. The graded severity of the hypo‐ and afibrinogenemias associated with homozygous Aα chain truncations suggest the absolute minimal requirement for molecular assembly is the formation of the C terminal disulfide ring of the coiled coil.

Mutant Fibrinogen Cleared from the Endoplasmic Reticulum via Endoplasmic Reticulum-Associated Protein Degradation and Autophagy

“…19 Because of their metastable state and the mechanism by which the D domains of fibrinogen ␥-chains polymerize in mature fibrinogen, 7 we suggested that ␥D may be aggregation prone and form aggregates within the ER and that clearance of these aggregates may be autophagy-dependent.…”

“…5,6 The identification of the genetic defects associated with low levels of circulating fibrinogen (hypofibrinogenemia) has shown that mutations within any of the three chains can lead to misfolding of that chain, recognition of the aberrant polypeptide by ER quality control, and subsequent degradation. 7 However, studies of individuals carrying various mutant forms of fibrinogen chains have shown that a subset of those individuals with hypofibrinogenemia also exhibit hepatic ER accumulation of fibrinogen that appears to lead to liver disease. 8 -10 For example, we previously described one family harboring a novel ␥-chain mutation, Arg375Trp, termed fibrinogen Aguadilla, in which all carriers present with hypofibrinogenemia, but only a subset have liver inclusions.…”

Fibrinogen Philadelphia, a hypodysfibrinogenemia characterized by abnormal polymerization and fibrinogen hypercatabolism due to γ S378P mutation

“…Two other hypodysfibrinogenemias, fibrinogens Marburg 44 and Otago, 45 are structurally characterized by truncations of the A␣ chain that cause delay in fibrin polymerization and the low plasma fibrinogen could be due to impaired synthesis, intracellular assembly, or defects in secretion, as demonstrated with other hypofibrinogenemias. 16,46 However, because the mechanism involved in the disposal of fibrinogen is not known, we cannot differentiate between the possibility that this mutation accelerates the regular mechanism of fibrinogen catabolism or that this mutation uncovers a new catabolic pathway.…”

“…[12][13][14] Approximately 300 families with dysfibrinogenemia have been described and more than 50 molecular defects have been identified. 12,15,16 The clinical expression of dysfibrinogenemias varies; roughly 60% are clinically silent, and the other 40% are almost equally associated with bleeding and thrombosis, and in few instances the abnormal fibrinogen is associated with both manifestations. Rarely, the abnormal fibrinogens have been associated with spontaneous abortions, 12,13 delayed wound healing, 13 or amyloidosis of the kidney.…”

Thrombophilic dysfibrinogen Tokyo V with the amino acid substitution of γ Ala327Thr: formation of fragile but fibrinolysis-resistant fibrin clots and its relevance to arterial thromboembolism

“…3 Many reports have shown that a variety of genetic mutations result in molecular defects in fibrinogen molecules (dysfibrinogen) 4,5 or decreased levels of fibrinogen in blood (afibrinogenemia and hypofibrinogenemia). 6 Analyses of molecular and functional abnormalities of dysfibrinogens participate in elucidation of structure-function relationships and fibrin polymerization mechanisms. Patients with dysfibrinogen show bleeding tendencies, thromboembolisms, or no apparent symptoms, depending on the molecular abnormality.…”