OBJECTIVETo identify the genetic defect underlying congenital afibrinogenemia in a Chinese family.

METHODSPlasma 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.

RESULTSNo 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.

CONCLUSIONThe 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.