A novel fibrinogen gamma-chain mutation, p.Cys165Arg, causes disruption of the γ165Cys–Bβ227Cys disulfide bond and ultimately leads to hypofibrinogenemia

Abstract

BackgroundCongenital hypofibrinogenemia is a type of hereditary disease characterized by impaired fibrinogen synthesis and/or secretion induced by mutations in the fibrinogen gene. ObjectivesWe investigated the phenotypes, genotypes, and pathogenesis of congenital hypofibrinogenemia in an affected family. Patients/methodsThe proband had a risk of bleeding; therefore, conventional coagulation screening was performed for the proband and her family members. Mutation sites in all exons and flanking sequences of FGA, FGB, and FGG were identified, with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) performed to indicate the expression of abnormal chains. The effect of the mutation sites on fibrinogen structure and function was predicted by molecular modeling, and purified plasma fibrinogen from the proband was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and scanning electron microscopy. Thromboelastography was applied to assess the risk of bleeding and clotting in the proband. ResultsFibrinogen levels in the proband were 1.21 g/L, 1.31 g/L, and 1.38 g/L according to Clauss assay, the prothrombin time method, and enzyme-linked immunosorbent assay, respectively. A novel heterozygous mutation (γCys165Arg), a heterozygous mutation (AαIle6Val), and two genetic polymorphisms (AαThr331Ala and BβArg478Lys) in fibrinogen were found in the proband, and MALDI-TOF MS indicated absence of the mutated chain in patient plasma. Additionally, the heterozygous mutation (γCys165Arg) displayed substitution of a nonpolar γ165Cys (low mass) with a positively charged Arg (high mass) along with a small fiber diameter and loose network structure. ConclusionsFibrinogen γCys165Arg mutations cause damage to the interchain disulfide bonds of fibrinogen and hinder fibrinogen secretion, possibly explaining the pathological mechanism associated with congenital hypofibrinogenemia.