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.

Chiaki Taira,Kazuyuki Matsuda,Takeshi Uehara,Nobuo Okumura,Mitsutoshi Sugano,Shinpei Arai

doi:10.3390/ijms18112470

Chiaki Taira, Kazuyuki Matsuda + Show 4 more

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https://doi.org/10.3390/ijms18112470

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Abstract

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.

Highlights

Fibrinogen is a 340-kDa plasma glycoprotein consisting of a hexameric molecule of three polypeptide chains: Aα, Bβ, and γ-chains, which are synthesized and assembled into a six-chain molecule in hepatocytes, and secreted into the blood of healthy adults at a plasma concentration of 1.8–3.5 g/L [1,2]
We identified a novel heterozygous point mutation, c.490G>A, in FGB in a 3-year-old girl with congenital hypofibrinogenemia (Kyoto IX)
We analyzed mRNA sequences derived from a mini-gene of Kyoto IX, and the assembly process and secretion of mutant fibrinogen using recombinant fibrinogen with c.490G>A in FGB

Summary

Introduction

Fibrinogen is a 340-kDa plasma glycoprotein consisting of a hexameric molecule of three polypeptide chains: Aα-, Bβ-, and γ-chains, which are synthesized and assembled into a six-chain molecule in hepatocytes, and secreted into the blood of healthy adults at a plasma concentration of 1.8–3.5 g/L [1,2]. Congenital fibrinogen disorders have been classified into four different manifestations: afibrinogenemia, hypofibrinogenemia, dysfibrinogenemia, and hypodysfibrinogenemia. Afibrinogenemia (the absence of fibrinogen) or hypofibrinogenemia (low plasma levels of fibrinogen) has been defined as a reduction in the quantity of fibrinogen in plasma. Dysfibrinogenemia has been defined as a qualitative abnormality a low functional activity and a normal antigenic fibrinogen. Hypodysfibrinogenemia is rarer and has the characteristics of both dysfibrinogenemia and hypofibrinogenemia [4,5]. Patients with hypofibrinogenemia or dysfibrinogenemia have a higher risk of bleeding during surgery or pregnancy than the general population [6]

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