一种新的FGA基因无义突变导致遗传性无纤维蛋白原血症

目的　对 1例遗传性无纤维蛋白原血症患者及其家系成员进行基因分析,探讨遗传性无纤维蛋白原血症发病的分子机制。方法　凝血酶法与免疫比浊法测定血浆纤维蛋白原 (Fg)含量,提取先证者及其家系成员外周血基因组DNA,PCR法扩增其Fg的FGA、FGB和FGG基因所有外显子和侧翼序列,DNA序列分析Fg的基因异常。将先证者突变序列、家系成员和 50名正常人相应序列的PCR产物用限制性内切酶RsaⅠ消化,以进一步确定基因突变位点并排除基因多态性。结果　用Clauss法检测不到先证者及其父亲的血浆Fg,用免疫比浊法测定时,Fg含量均<0. 02g/L。两人FGA基因外显子 4第 3108位核苷酸发生C→T纯合性改变,使Fg的Aα链第 150位密码子 (CAG,编码Gln)突变为终止密码TAG。先证者及其父亲的FGA基因外显子 4和侧翼序列的PCR产物,不能被RsaⅠ酶切,其母亲及部分家系成员的PCR产物被部分酶切,而正常人和该家系中的 5个成员的PCR产物可被完全酶切。结论　FGA基因(外显子 4)Q150X无义突变导致该家系先证者及其父亲遗传性无Fg血症,家系中部分成员为携带者,此突变是一种国际上尚未报道的新的突变类型。

Congenital afibrinogenemia associated with a novel nonsense mutation in the FGA gene

OBJECTIVE: To identify the genetic defect underlying congenital afibrinogenamia in a Chinese family. METHODS: Plasma fibrinogen (Fg) was assessed by both clauss method and immunonephelometry. Genomic DNA was isolated from peripheral blood of the proband and 13 members of her family. All the exons and exon-intron boundaries of the three fibrinogen genes (FGA, FGB, FGG) were amplified by PCR followed by direct sequencing. Restriction endonuclease analysis was performed for the PCR products of the family members and 50 healthy donors to exclude gene polymorphism. RESULTS: No Fg was detected in the plasma of the proband and her father by clauss method, while low levels (< 0.02 g/L) were detected by immunonephelometry. A homozygous C to T mutation was found in the two cases at nucleotide 3108 in exon 4 of FGA gene, resulting in a null mutation which encoded severely truncated alpha-chains owing to its premature termination at the Gln 150 codon. The C-->T mutation eliminated a unique recognition site for restriction enzyme RsaI. The PCR amplified fragments of the proband and her father could not be digested by RsaI, showing that they are homozygous. Her mother and some family members are heterozygous at this site since the fragment could partly be digested, while the same fragment of controls could be completely digested as expected. CONCLUSION: The Gln (CAG)-->150stop (TAG) nonsense mutation in FGA gene is a novel genetic defect of congenital afibrinogenemia which, to our knowledge, has not been described before.