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

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 Analysis of Afibrinogenemia and Hypofibrinogenemia: Novel Mutations in the FGB Gene in the Turkish Population

INTRODUCTION

Congenital afibrinogenemia is a rare autosomal recessive disorder characterized by bleeding that varies from mild to severe and by complete absence or extremely low levels of plasma and platelet fibrinogen. Hypofibrinogenemia is characterized by fibrinogen levels <1.5 g/L.

OBJECTIVE

In this study, we analyzed fibrinogen beta chain gene mutations in Turkish afibrinogenemia and hypofibrinogenemia patients.

METHODS

We evaluated 20 afibrinogenemia and hypofibrinogenemia patients and 80 healthy controls. We have sequenced all exons of the FGB gene using the DNA isolated from the peripheral blood samples of patients and controls.

RESULTS AND CONCLUSION

We found a nonsense mutation in exon 4 at nucleotide 630 that encoded serine amino acid, and in the same exon a missense mutation of T to C at nucleotide 647, resulting in a transition from leucine to proline (p.L198P) in a child with hypofibrinogenemia. These mutations have been shown for the first time in the same patient of Turkish descent. Furthermore, there was a novel heterozygous guanine-to-adenine nucleotide change in exon 3. This caused the change of arginine amino acid to threonine amino acid at position 136 (p.A136T) in a protein, which has not been described in the literature before.