Clinical histories and molecular characterization of two afibrinogenemic patients: insights into clinical management

Mutations Accounting for Congenital Fibrinogen Disorders: An Update

Fibrinogen is a complex protein that plays a key role in the blood clotting process. It is a hexamer composed of two copies of three distinct chains: Aα, Bβ, and γ encoded by three genes, FGA, FGB, and FGG, clustered on the long arm of chromosome 4. Congenital fibrinogen disorders (CFDs) are divided into qualitative deficiencies (dysfibrinogenemia, hypodysfibrinogenemia) in which the mutant fibrinogen molecule is present in the circulation and quantitative deficiencies (afibrinogenemia, hypofibrinogenemia) with no mutant molecule present in the bloodstream. Phenotypic manifestations are variable, patients may be asymptomatic, or suffer from bleeding or thrombosis. Causative mutations can occur in any of the three fibrinogen genes and can affect one or both alleles. Given the large number of studies reporting on novel causative mutations for CFDs since the review on the same topic published in 2016, we performed an extensive search of the literature and list here 120 additional mutations described in both quantitative and qualitative disorders. The visualization of causative single nucleotide variations placed on the coding sequences of FGA, FGB, and FGG reveals important structure function insight for several domains of the fibrinogen molecule.

Genetic Variants in the FGB and FGG Genes Mapping in the Beta and Gamma Nodules of the Fibrinogen Molecule in Congenital Quantitative Fibrinogen Disorders Associated with a Thrombotic Phenotype

Fibrinogen is a hexameric plasmatic glycoprotein composed of pairs of three chains (Aα, Bβ, and γ), which play an essential role in hemostasis. Conversion of fibrinogen to insoluble polymer fibrin gives structural stability, strength, and adhesive surfaces for growing blood clots. Equally important, the exposure of its non-substrate thrombin-binding sites after fibrin clot formation promotes antithrombotic properties. Fibrinogen and fibrin have a major role in multiple biological processes in addition to hemostasis and thrombosis, i.e., fibrinolysis (during which the fibrin clot is broken down), matrix physiology (by interacting with factor XIII, plasminogen, vitronectin, and fibronectin), wound healing, inflammation, infection, cell interaction, angiogenesis, tumour growth, and metastasis. Congenital fibrinogen deficiencies are rare bleeding disorders, characterized by extensive genetic heterogeneity in all the three genes: FGA, FGB, and FGG (enconding the Aα, Bβ, and γ chain, respectively). Depending on the type and site of mutations, congenital defects of fibrinogen can result in variable clinical manifestations, which range from asymptomatic conditions to the life-threatening bleeds or even thromboembolic events. In this manuscript, we will briefly review the main pathogenic mechanisms and risk factors leading to thrombosis, and we will specifically focus on molecular mechanisms associated with mutations in the C-terminal end of the beta and gamma chains, which are often responsible for cases of congenital afibrinogenemia and hypofibrinogenemia associated with thrombotic manifestations.

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

BACKGROUND

Congenital hypofibrinogenemia is a type of hereditary disease characterized by impaired fibrinogen synthesis and/or secretion induced by mutations in the fibrinogen gene.

OBJECTIVES

We investigated the phenotypes, genotypes, and pathogenesis of congenital hypofibrinogenemia in an affected family.

PATIENTS/METHODS

The 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.

RESULTS

Fibrinogen 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.

CONCLUSIONS

Fibrinogen γCys165Arg mutations cause damage to the interchain disulfide bonds of fibrinogen and hinder fibrinogen secretion, possibly explaining the pathological mechanism associated with congenital hypofibrinogenemia.

Identification of Two Novel Fibrinogen Bβ Chain Mutations in Two Slovak Families with Quantitative Fibrinogen Disorders

Congenital fibrinogen disorders are caused by mutations in one of the three fibrinogen genes that affect the synthesis, assembly, intracellular processing, stability or secretion of fibrinogen. Functional studies of mutant Bβ-chains revealed the importance of individual residues as well as three-dimensional structures for fibrinogen assembly and secretion. This study describes two novel homozygous fibrinogen Bβ chain mutations in two Slovak families with afibrinogenemia and hypofibrinogenemia. Peripheral blood samples were collected from all subjects with the aim of identifying the causative mutation. Coagulation-related tests and rotational thromboelastometry were performed. All exons and exon–intron boundaries of the fibrinogen genes (FGA, FGB and FGG) were amplified by PCR followed by direct sequencing. Sequence analysis of the three fibrinogen genes allowed us to identify two novel homozygous mutations in the FGB gene. A novel Bβ chain truncation (BβGln180Stop) was detected in a 28-year-old afibrinogenemic man with bleeding episodes including repeated haemorrhaging into muscles, joints, and soft tissues, and mucocutaneous bleeding and a novel Bβ missense mutation (BβTyr368His) was found in a 62-year-old hypofibrinogenemic man with recurrent deep and superficial venous thromboses of the lower extremities. The novel missense mutation was confirmed by molecular modelling. Both studying the molecular anomalies and the modelling of fibrinogenic mutants help us to understand the extremely complex machinery of fibrinogen biosynthesis and finally better assess its correlation with the patient’s clinical course.

Thromboembolism in patients with congenital afibrinogenaemia

Frequent arterial and venous thromboembolism in patients with congenital afibrinogenaemia (CA) is neither understood nor is a safe and effective treatment established. It was our objective to report on the clinical observations and laboratory data contributing to the understanding of the frequency, physiopathology, prognosis and treatment of CA. We observed the long-term clinical course and laboratory data in a cohort of four patients with CA and thromboembolic complications, and conducted a systematic review retrieving all available data. Four patients with CA developed recurrent and extensive arterial and venous thromboembolism (TE) from an age of 25–51 years. In two patients, a treatment strategy targeting at maintaining constantly measurable fibrinogen (Fbg) levels (≥0.5 g/l) either by regular Fbg replacement or by orthotopic liver transplantation resulted in long-term remissions. Radiological imaging documented resolved arterial thrombi after 6–12 months. In contrast, recurrent thromboembolic events were observed in two other patients with infrequent Fbg replacement. A systematic review of the literature revealed 48 reports of TE in patients with CA (median age at first event 31 years), and a favourable outcome in most patients with frequent application of Fbg, aimed at constantly measurable trough levels. Present data suggests that patients with CA are at high risk of arterial and venous thromboembolic events, probably caused by thrombin excess owing to lack of thrombin scavenging by Fbg/fibrin. Regular low-dose Fbg replacement might be a safe and effective treatment option in patients with CA and thromboembolic complications.

Note: Preliminary data of this report were presented as oral presentation at the XXV Congress of the International Society on Thrombosis and Haemostasis, June 20th to 25th, Toronto, Canada.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Congenital afibrinogenemia: from etiopathogenesis to challenging clinical management

INTRODUCTION

Congenital afibrinogenemia belongs to the group of autosomal recessive bleeding disorders and represents the absolute deficiency of fibrinogen detected by an antigenic test. This can lead to severe clinical manifestations of the disorder. Therefore, it is very important to take afibrinogenemia into account in the process of the differential diagnostics of the patients.

AREAS COVERED

The authors provide a summary of currently available literature about afibrinogenemia. They collected the information from the scientific journals dedicated to thrombosis and hemostasis and searched world-wide databases. Expert commentary: The most frequent clinical manifestation of this disorder is mucosal bleeding, but musculoskeletal bleeding pattern, gynecologic and obstetric issues, spontaneous bleeding, episodes provoked by minor injury or any other intervention, and even paradoxical thromboembolic events have been published. Afibrinogenemia is the consequence of mutations of the homozygous or compound heterozygous type in gene FGA, FGB or FGG encoding fibrinogen. Pregnant women with a family history, or with a history of consanguinity ought to be properly counselled. However, primary prophylaxis of bleeding events is not suggested. The article deals with actual information about afibrinogenemia contributing to its early diagnosis and effective treatment, which in many cases requires multidisciplinary approach.

Treating a Patient of Dysfibrinogenemia with Acute Thromboembolism by Rivaroxaban and Cilostazol

Congenital dysfibrinogenemia is a rare autosomal recessive bleeding disorder, which is characterized by the absence of functional fibrinogen. Patients may have bleeding and paradoxical arterial and venous thrombotic problems from early childhood. The optimal antithrombotic therapy in these patients hasn't been determined yet. In this report we present a dysfibrogenemic patient, who has suffered recurrent arterial thrombosis under aspirin treatment. Intravenous fibrinogen concentrates (fc) along with reduced doses of rivaroxaban (10 mg daily), cilostazol (50 mg bid) and aspirin (100 mg daily) were given as antithrombotic treatment. The pain and the cyanosis clinically recovered within 6 weeks. This is, to our knowledge, the first time in which a new oral anticoagulant, rivaroxaban and cilostazol combination was used in a dysfibrinogenemic patient with thrombotic episodes. We determined the type, the dosage and the duration of antithrombotic treatment according to the clinical progress of the symptoms. Rivaroxaban, cilostazol and fibrinogen concentrate replacement; combination may represent a useful alternative for the antithrombotic treatment in dysfibrinogenemic patients.

FGB mutations leading to congenital quantitative fibrinogen deficiencies: an update and report of four novel mutations

INTRODUCTION

Causative mutations leading to congenital quantitative fibrinogen are frequently clustered in FGA encoding the fibrinogen Aα-chain. Mutations of FGB encoding the Bβ-chain are less common and of interest since the Bβ-chain is considered the rate-limiting factor in the hepatic production of the fibrinogen hexamer.

METHOD

Four novel FGB mutations were identified in two afibrinogenemic (one new-born and one 30 years old male) and hypofibrinogenemic (a 49 years old female) patient, with heterogeneous thrombotic and bleeding phenotype. The human fibrinogen beta chain precursor protein sequence (P02675) was obtained from the UniProt database. The resulting models were analysed in SwissPdbViewer 4.1 and POV-Ray 3.7.

RESULTS

The FGB c.895T>C p.Y299H (numbering from the initiator Met) and the FGB c.1415G>T p.G472V were predicted to be deleterious by SIFT analysis. The first replaces an uncharged aromatic amino acid side chain by a positively charged side chain modifying the balance in the distribution of hydrophobic and hydrophilic of the 10 Å neighbourhood residues. The second replaces one non-charged aliphatic side chain by another without any changes for the 10 Å surrounding region. The FGB c.352C>T p.Q118X leads to a severe premature termination codon and the FGB intron 4: IVS4-1G>C (c719-1G>C) leads to skipping of exon 5 or usage of a cryptic acceptor site located upstream or downstream of the normal site.

CONCLUSIONS

The continuous characterization of novel molecular defects responsible for fibrinogen deficiency combined with modelling of mutant proteins will continue to provide a better comprehension of the complexity of fibrinogen synthesis and physiology.