一种新型纤维蛋白原β链错义突变导致的遗传性低纤维蛋白原血症

目的：对临床发现的1例遗传性低纤维蛋白原血症进行基因及表型分析，探讨其分子发病机制。方法：采集先证者及其家系成员共2代6人的外周血，采用STAGO自动化血凝仪检测凝血酶原时间（PT）、活化部分凝血活酶时间（APTT）、凝血酶时间（TT）、D-D二聚体（D-D）和纤维蛋白降解产物（FDPs）；凝血酶凝固法（Clauss法）与免疫比浊法分别测定血浆纤维蛋白原活性（Fg:C）和抗原水平（Fg:Ag）。PCR扩增纤维蛋白原FGA、FGB和FGG基因的所有外显子和侧翼序列，PCR产物纯化后直接测序进行基因分析。分别用Swiss-PDViewer、在线分析系统对突变蛋白的结构、突变位点的保守性进行预测分析。结果：先证者PT和TT均延长，Fg:C（0.82 g/L）和Fg:Ag（1.19 g/L）明显降低；基因分析发现先证者FGB基因存在c.425T＞G杂合突变，导致纤维蛋白原Bβ链上121位亮氨酸突变为精氨酸（Leu121Arg）。其父亲及儿子也存在该突变位点。蛋白模型分析显示Leu121突变为Arg121后，氨基酸极性发生改变，并且新增与Try117、Met118、Trp125之间的氢键；蛋白同源性分析显示：Leu121在脊椎动物间有较高的保守性。结论：纤维蛋白原Bβ链Leu121Arg杂合突变是引起该家系遗传性低纤维蛋白原血症的主要原因。

Phenotypic and genetic analysis of congenital hypofibrinogenemia associated with a novel heterozygous mutation in fibrinogen β Chain

Objective: To identify the genetic defect underlying congenital hypofibrinogenamia in a Chinese pedigree. Methods: Totally 6 family members were enrolled in this study. Routine coagulation tests, including activated partial thromboplastin time (APTT), thrombin time (TT), the prothrombin time (PT), D-Dimer (DD) and fibrinogen degradation products (FDPs) were determined. The activity of fibrinogen (Fg:C) was measured using Clauss method and fibrinogen antigen (Fg:Ag) was measured using immunoturbidimetry. All exons and exon-intron boundaries of the fibrinogen Aα, Bβ and γ chain gene were amplified using PCR, followed by direct sequencing. Suspected mutation was further confirmed by reverse sequencing. The mutant fibrinogen was analyzed by the Swiss-PdbViewer, ClustalX-2.1-win and other online software. Results: PT and TT were prolonged in the proband. Her functional fibrinogen (Fg:C) and antigen fibrinogen (Fg:Ag) levels were reduced, which was 0.82 g/L and 1.19 g/L, respectively. Genetic analysis revealed a heterozygous T>G change at nucleotide 425 in exon 3 of FGB gene in the proband, predicting a novel Leu121Arg mutation. This mutation was also found in her father and son. Model analysis showed that the Leu121Arg mutation added a hydrogen bonding among Try117, Met118, Trp125 and Leu121. Moreover, the mutation also changed the mutual electrostatic forces, affecting the folding and instability of the mutant fibrinogen. Conclusion: The heterozygous Leu121Arg mutation leading to the hypofibrinogenemia in this pedigree.