A Novel Nonsense Mutation in FGB (c.1421G>A; p.Trp474Ter) in the Beta Chain of Fibrinogen Causing Hypofibrinogenemia with Bleeding Phenotype.

Tomas Simurda,Miroslava Dobrotova,Zuzana Kolkova,Alessandro Casini,Marian Grendar,Rui Vilar,Monika Brunclikova,Jan Stasko,Eliska Ceznerova,Marguerite Neerman-Arbez,Jana Zolkova,Dusan Loderer,Peter Kubisz,Ingrid Skornova

doi:10.3390/biomedicines8120605

Tomas Simurda, Miroslava Dobrotova + Show 12 more

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https://doi.org/10.3390/biomedicines8120605

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Abstract

Congenital hypofibrinogenemia is a rare bleeding disorder characterized by a proportional decrease of functional and antigenic fibrinogen levels. Hypofibrinogenemia can be considered the phenotypic expression of heterozygous loss of function mutations occurring within one of the three fibrinogen genes (FGA, FGB, and FGG). Clinical manifestations are highly variable; most patients are usually asymptomatic, but may appear with mild to severe bleeding or thrombotic complications. We have sequenced all exons of the FGA, FGB, and FGG genes using the DNA isolated from the peripheral blood in two unrelated probands with mild hypofibrinogenemia. Coagulation screening, global hemostasis, and functional analysis tests were performed. Molecular modeling was used to predict the defect of synthesis and structural changes of the identified mutation. DNA sequencing revealed a novel heterozygous variant c.1421G>A in exon 8 of the FGB gene encoding a Bβ chain (p.Trp474Ter) in both patients. Clinical data from patients showed bleeding episodes. Protein modelling confirmed changes in the secondary structure of the molecule, with the loss of three β sheet arrangements. As expected by the low fibrinogen levels, turbidity analyses showed a reduced fibrin polymerisation and imaging difference in thickness fibrin fibers. We have to emphasize that our patients have a quantitative fibrinogen disorder; therefore, the reduced function is due to the reduced concentration of fibrinogen, since the Bβ chains carrying the mutation predicted to be retained inside the cell. The study of fibrinogen molecules using protein modelling may help us to understand causality and effect of novel genetic mutations.

Highlights

Fibrinogen is a complex plasma glycoprotein with a molecular weight of 340 kDa
Patient 1 is a six-year-old boy diagnosed with mild hypofibrinogenemia upon investigation of significant delayed bleeding after 6 h of cleft lip surgery
ROTEM analysis in proband 1 showed a slight prolongation of the clotting time (CT) with EXTEM (78 mm; normal range: 38–72 mm) and limit value for maximum lysis (ML) with FIBTEM (14%; N < 15%); other all parameters were in the normal range, with EXTEM, INTEM, and FIBTEM.ROTEM analyses in proband 2 showing extended CFT in EXTEM

Summary

Introduction

Fibrinogen is a complex plasma glycoprotein with a molecular weight of 340 kDa. The structure of the molecule forms a hexamer that is composed of two sets of three polypeptide chains—Aα, Bβ, and γ—joined by a disulfide bridge [1]. The mature molecule is encoded by three genes, FGA, FGB, and FGG, located contiguously on chromosome band 4q23. The main role of fibrinogen in hemostasis is to strengthen the platelet plug after conversion into an insoluble fibrin polymer by thrombin cleavage of fibrinopeptides A and B. The fibrin polymer traps red blood cells and platelets, leading to a stable fibrin plug that stops bleeding from the site of injury [2]. Congenital hypofibrinogenemia is characterized by abnormally low levels of functional and antigen fibrinogen, generally due to heterozygous mutations in one of the three fibrinogen genes [4]. Mutations accounting to hypofibrinogenemia are frequently localized in the last exon 8 of FGB, encoding the C- terminus of the βB chain [6]

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