Genetic analyses of novel compound heterozygous hypodysfibrinogenemia, Tsukuba I: FGG c.1129+62_65 del AATA and FGG c.1299+4 del A

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.

Congenital fibrinogen disorder with a compound heterozygote possessing two novel FGB mutations, one qualitative and the other quantitative

INTRODUCTION

Congenital fibrinogen disorders result from genetic mutations in FGA, FGB, or FGG resulting in quantitative fibrinogen deficiencies (afibrinogenemia or hypofibrinogenemia) or qualitative fibrinogen deficiencies (dysfibrinogenemia). Hypodysfibrinogenemia sharing features with hypo- and dysfibrinogenemia is rare. We performed genetic and functional analyses of a 31-year-old woman with suspected hypodysfibrinogenemia.

MATERIALS AND METHODS

Functional and antigenic fibrinogen values of patient were 1.05 and 1.24 g/L, respectively. DNA sequence and western blotting analyses for plasma fibrinogen were performed. A minigene incorporating the mutational region was transfected into a Chinese hamster ovary cell line (CHO), and reverse transcription products were analyzed. Assembly and secretion were examined using the recombinant variant fibrinogen. We purified the patient's plasma fibrinogen and analyzed thrombin-catalyzed fibrin polymerization (TCFP).

RESULTS AND CONCLUSIONS

DNA sequencing revealed compound heterozygous nucleotide mutations with FGB 35 bp c.1245-17_1262 or -16_1263 del and FGB c.510T>A (resulting in Bβp.N170K substitution) on different alleles. We did not detect shortened Bβ-chain peptides in the plasma using western blotting analysis. A minigene incorporating the deletion DNA showed two aberrant mRNA products. The secretion of Bβp.N170K-fibrinogen-CHO was almost same as normal Bβ-fibrinogen-CHO. TCFP of plasma Bβp.N170K fibrinogen was slightly lower than that of normal plasma fibrinogen. Aberrant splicing products derived from the 35 bp deletion caused hypofibrinogenemia due to nonsense-mediated mRNA decay and suggested the presence of only Bβp.N170K fibrinogen in patient's plasma. Bβp.N170K caused dysfibrinogenemia due to a delay in lateral aggregation. These findings demonstrated that these mutations respectively affected the fibrinogen quality and quantity, resulting in hypodysfibrinogenemia.

A Novel Mutation in the Fibrinogen Bβ Chain (c.490G>A; End of Exon 3) Causes a Splicing Abnormality and Ultimately Leads to Congenital Hypofibrinogenemia

We found a novel heterozygous mutation in the fibrinogen Bβ chain (c.490G>A) of a 3-year-old girl with congenital hypofibrinogenemia. To clarify the complex genetic mechanism, we made a mini-gene including a FGB c.490G>A mutation region, transfected it into a Chinese Hamster Ovary (CHO) cell line, and analyzed reverse transcription (RT) products. The assembly process and secretion were examined using recombinant mutant fibrinogen. Direct sequencing demonstrated that the mutant RT product was 99 bp longer than the wild-type product, and an extra 99 bases were derived from intron 3. In recombinant expression, a mutant Bβ-chain was weakly detected in the transfected CHO cell line, and aberrant fibrinogen was secreted into culture media; however, an aberrant Bβ-chain was not detected in plasma. Since the aberrant Bβ-chain was catabolized faster in cells, the aberrant Bβ-chain in a small amount of secreted fibrinogen may catabolize in the bloodstream. FGB c.490G>A indicated the activation of a cryptic splice site causing the insertion of 99 bp in intron 3. This splicing abnormality led to the production of a Bβ-chain possessing 33 aberrant amino acids, including two Cys residues in the coiled-coil domain. Therefore, a splicing abnormality may cause impaired fibrinogen assembly and secretion.

Management of dysfibrinogenemia in pregnancy: A case report

BACKGROUND

Dysfibrinogenemia is a rare coagulation disorder caused by mutations in the fibrinogen gene that results in abnormal fibrinogen function. Dysfibrinogenemia has a wide spectrum of clinical manifestations including asymptomatic(55%), hemorrhage (25%), and thrombosis (20%).

METHODS

We reported a 30-year-old woman with 35 weeks gestation. She was misdiagnosed with hypofibrinogenemia in a local hospital, and then she was treated with fibrinogen concentrate. However, she was diagnosed as dysfibrinogenemia in our hospital base on her low function fibrinogen level (0.55 g/L) and her normal immunologic fibrinogen level (3.80 g/L). This patient had neither bleeding symptom nor thromboembolic event. Her obstetrical history included one normal pregnancy in 2008 with uneventful full-term delivery.

RESULTS

Multidisciplinary experts suggested that there should be no specific intervention in this case because of the patient had no previous episodes of abnormal bleeding or thrombotic. She had an uneventful delivery with no abnormal bleeding symptom or thromboembolic.

CONCLUSION

Dysfibrinogenemia patients without personal or family history of bleeding and thromboembolic events, do not need specific therapeutic intervention.