Summary
We have investigated a 53-yr-old asymptomatic white man with decreased functional,
but not immunologic, fibrinogen plasma levels together with prolonged thrombin and
reptilase times, detected through routine coagulation studies prior to a surgical
procedure. A new heterozygous single nucleotide deletion (C) at position Ala499 within the Aα-chain gene was identified, which predicted changes of the corresponding
aminoacids encoded by the subsequent portion of the exon V and the appearance of a
premature stop codon at position 518 (A [499]Ala frameshift stop). The new dysfunctional
fibrinogen, San Giovanni Rotondo variant, was confirmed in vivo by SDS-PAGE analysis
of HPLC-purified fibrinogen chains. Mass spectrum examination of the abnormal HPLC-purified
peak gave an estimated mass (56,088 Da) similar to that predicted by DNA analysis
of the mutated Aα-chain gene (56,088 Da) and, after tryptic digestion, the truncated
Aα-chain was shown only in the propositus, who also carried normal Aα-chain. In addition,
mass spectrum analysis of the tryptic digest of the abnormal chain confirmed the presence
of a new and unpaired cysteine at the last position that was predicted to form a disulfide
bridge with human serum albumin. Immuno-blot analysis confirmed that fibrinogen San
Giovanni Rotondo variant, but not normal fibrinogen, contained substantial amounts
of albumin. Present findings confirm that truncated Aα-chain lacking part of the terminal
domain may be incorporated into mature fibrinogen molecules and normally secreted
in the bloodstream.
Keywords
Dysfibrinogenemia - fibrinogen genes - mass spectrometry - truncated protein