Mutation of the translation initiation codon in FGA causes congenital afibrinogenemia

Pathogenic Mechanisms in Congenital Afibrinogenemia: A Systematic Review of Genetic Variants

INTRODUCTION

Congenital afibrinogenemia is a rare bleeding disorder characterized by the complete absence of plasma fibrinogen, primarily caused by homozygous or compound heterozygous mutations in the FGA, FGB and FGG genes.

AIM

To deepen our understanding of the pathogenic mechanisms of afibrinogenemia through the study of natural variants.

METHODS

We conducted a literature review of all publications up to 2024 that report cases of afibrinogenemia with confirmed genetic diagnoses, focusing on the impact of mutations on fibrinogen synthesis, assembly and secretion.

RESULTS

We classified the pathogenic mechanisms of afibrinogenemia into the following seven categories: (1) Chromosomal structural variations, such as large deletions, disrupt the integrity of the fibrinogen gene cluster. (2) Splice site mutations interfere with the proper splicing of precursor mRNA, resulting in abnormal transcripts that cannot encode functional fibrinogen chains. (3) Start codon mutations prevent the initiation of translation, halting the synthesis of fibrinogen polypeptides. (4) Nonsense and frameshift mutations introduce termination codons, resulting in truncated fibrinogen chains. (5) Signal peptide mutations disrupt the targeting of polypeptides to the endoplasmic reticulum, preventing further post-translational modifications. (6) Mutations affecting disulphide bonds in the coiled-coil region hinder the assembly of fibrinogen chains, preventing the formation of complete hexamers. (7) Mutations affecting the correct conformation of β and γ nodules cause intra-cellular retention of fibrinogen and prevent its secretion.

CONCLUSIONS

This review provides a comprehensive summary of mutations associated with afibrinogenemia, offering insights that contribute to the phenotypic prediction of novel mutations and providing a framework for understanding the molecular mechanisms of afibrinogenemia.

Life-threatening spontaneous splenic rupture in congenital afibrinogenemia: Two case reports and systematic literature review

BACKGROUND

Spontaneous splenic rupture is a rare life-threatening finding with a challenging diagnosis which is largely ignored in the literature. Hematological disorders such as afibrinogenemia are reported to cause bleeding disorders mostly cerebral hemorrhage. Despite being a life-threatening condition, data about spontaneous splenic rupture in patients with Afibrinogenemia remain scarce. Such fatal incidents should warrant diligent efforts to establish and adopt prevention measures in this particularly vulnerable population. We report two new cases of spontaneous splenic rupture in two siblings, diagnosed with congenital afibrinogenemia since early childhood. In addition, we report the results of a systematic review of the literature of cases that have reported the occurrence of splenic rupture in afibrinogenemia, with the aim of filling the gap in the clinical course of such an uncommon event.

METHODS

We conduced a systematic review of English and French language publications cataloged in Pubmed, and Google Scholar up to December 2022. We opted to include articles written in other languages if an English language abstract was available and contained the information needed for our report. The following MeSH (medical subject heading) search criteria were used: ''splenic rupture'' AND ''afibrinogenemia.'' The above search strategy was further combined with a supplementary strategy that included the MeSH terms ''spleen'', AND ''rupture'' AND ''Afibrinogenemia''. In order to reduce the selection bias in searching on the Google Scholar database, we searched for articles containing these MeSH terms in their titles.

RESULTS

We reported two cases of splenic rupture in two family members who were diagnosed with afibrinogenemia at the age of 1 year. The splenic rupture was misdiagnosed by clinicians in the brother's case, resulting in his death and presenting as a forensic case. However, the sister underwent a splenectomy one week before her brother's death following the onset of asthenia and acute abdominal pain. Then we conducted a literature review of relevant studies of spontaneous splenic rupture in Afibrinogenemia. A selection of 13 relevant studies was included in the final review, describing a total of 25 cases of spontaneous splenic rupture in Afibrinogenemia. One paper was an international cross-sectional study across 25 different countries involving 211 patients with fibrinogen disorders, showing that 11 patients with afibrinogenemia had developed spontaneous splenic rupture. The remaining 12 articles were case reports or case series, describing a total of 14 cases of atraumatic splenic rupture in afibrinogenemia, ranging from 0 to 28 years old, with a median age of 14.5 years, where the majority were infants (age inferior to 18) with the most cases occurred in the age group of 10-15 years old.

CONCLUSION

Congenital afibrinogenemia is a rare disease with a wide range of clinical symptoms and with challenging management. While the most common presenting symptoms are bleeding and thrombosis, spontaneous spleen rupture is rare and probably underdiagnosed which consequently may lead to death. Studies about the topic remain scarce, hence, further studies are needed to ascertain this risk and establish target preventive measures for this vulnerable population.

Heterogeneity of congenital afibrinogenemia, from epidemiology to clinical consequences and management

Fibrinogen is a complex protein playing a major role in coagulation. Congenital afibrinogenemia, characterized by the complete absence of fibrinogen, is associated with major hemostatic defects. Even though the clinical course is unpredictable and can be completely different among patients, severe bleeding is the prominent symptom. Patients are also at increased risk of thrombosis and sometimes suffer from spontaneous spleen rupture, bone cysts and defective wound healing. Due to the relative rarity of afibrinogenemia, there are no evidence-based strategies for helping physicians in care of these patients. Fibrinogen supplementation is the keystone to prevent or treat bleeding events. In addition, fibrinogen, a pleiotropic protein with numerous physiological roles in immunity, angiogenesis and tissue repair, is involved in many diseases. Indeed, depletion of fibrinogen in animal models of infections, tumors and neurological diseases has an effect on the clinical course. The consequences for patients with afibrinogenemia still need to be investigated.

Identification and characterization of novel mutations in Chinese patients with congenital fibrinogen disorders

INTRODUCTION

Congenital fibrinogen disorders are characterized by heterogeneous clinical manifestations with mutations in the fibrinogen gene cluster. We aimed to describe the molecular genetics and clinical manifestations of fibrinogen abnormalities and perform genotype-phenotype correlations.

MATERIALS AND METHODS

Genetic analysis of fibrinogen genes was performed by direct sequencing. The effect of the specific missense variants on fibrinogen structure and function was analyzed using PROVEAN and PolyPhen-2 algorithms and was predicted by protein modeling.

RESULTS

Thirteen mutations, including five novel mutations, were identified in the three fibrinogen genes. There was poor correlation between genotypes and phenotypes. All but one of the novel mutations in subjects were predicted to be deleterious. Protein modeling predicted that multiple ienteractions with surrounding residues for novel variants were likely to result in congenital fibrinogen disorders.

CONCLUSION

This study in a relatively large cohort of Chinese patients with congenital fibrinogen disorders enabled the identification of five new fibrinogen missense mutations. In silico modeling may represent a valuable tool for understanding amino acid residues from novel variants leading to congenital fibrinogen disorders, but it should be followed by functional studies. Clinical presentation of fibrinogen disorders was variable, possibly due to genetic and environmental modifiers.

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 and characterization of novel mutations implicated in congenital fibrinogen disorders

INTRODUCTION

Fibrinogen is a complex molecule comprised of two sets of Aα, Bβ, and γ chains. Fibrinogen deficiencies can lead to the development of bleeding or thromboembolic events. The objective of this study was to perform DNA sequence analysis of patients with clinical fibrinogen abnormalities, and to perform genotype-phenotype correlations.

MATERIALS AND METHODS

DNA from 31 patients was sequenced to evaluate disease-causing mutations in the three fibrinogen genes: FGA,FGB, and FGG. Clinical data were extracted from medical records or from consultation with referring hematologists. Fibrinogen antigen and functional (Clauss method) assays, as well as reptilase time (RT) and thrombin time (TT) were obtained for each patient. Molecular modeling was used to simulate the functional impact of specific missense variants on the overall protein structure.

RESULTS

Seventeen mutations, including six novel mutations, were identified in the three fibrinogen genes. There was little correlation between genotype and phenotype. Molecular modeling predicted a substantial conformational change for a novel variant, FGG p.Ala289Asp, leading to a more rigid molecule in a region critical for polymerization and alignment of the fibrin monomers. This mutation is associated with both bleeding and clotting in the two affected individuals.

CONCLUSIONS

Robust genotype-phenotype correlations are difficult to establish for fibrinogen disorders. Molecular modeling might represent a valuable tool for understanding the function of certain missense fibrinogen mutations but those should be followed by functional studies. It is likely that genetic and environmental modifiers account for the incomplete penetrance and variable expressivity that characterize fibrinogen disorders.

Human Fibrinogen: Molecular and Genetic Aspects of Congenital Disorders

Congenital fibrinogen disorders can be quantitative (afibrinogenemia, hypofibrinogenemia) or functional (dysfibrinognemia). To date, several genetic variants have been identified in individuals with fibrinogen disorders. The complexity of the fibrinogen molecules, formed by three non-identical chains and with a trinodal organization, renders the identification of molecular causes and of clinical and biochemical phenotypes very challenging. However, the acknowledgement of the type of molecular defect is crucial for a safer therapy, which is going to improve the clinical management of these patients. In this review, some aspects concerning molecular and clinical findings available on congenital fibrinogen disorders will be discussed.