Characterization of the I4399M variant of apolipoprotein(a): implications for altered prothrombotic properties of lipoprotein(a)

SummaryBackground Lipoprotein(a) (Lp(a)) is a causal risk factor for a variety of cardiovascular diseases. Apolipoprotein(a) (apo(a)), the distinguishing component of Lp(a), is homologous to plasminogen, suggesting that Lp(a) can interfere with the normal fibrinolytic functions of plasminogen. This has implications for the persistence of fibrin clots in the vasculature and hence to atherothrombotic diseases. A single nucleotide polymorphism (rs3798220) in the gene encoding apo(a) has been reported that results in an Ile to Met substitution in the protease-like domain (I4399M variant). In population studies, the I4399M variant has been correlated with elevated plasma Lp(a) levels and higher CHD risk, and carriers of the SNP had increased cardiovascular benefit from aspirin therapy. In vitro studies suggested an antifibrinolytic role for Lp(a) containing this variant. Objectives We performed a series of experiments to assess the effect of the Ile-Met substitution on fibrin clot formation and lysis as well as the architecture of the clots. Results We found that the Met variant decreased coagulation time and increased fibrin clot lysis time compared to wild-type apo(a). Further, we observed that the presence of the Met variant significantly increased fibrin fiber width in plasma clots formed ex vivo, while having no effect on fiber density. Mass spectrometry analysis of a recombinant apo(a) species containing the Met variant revealed sulfoxide modification of the methionine residue. Conclusions Our data suggest that the I4399M variant differs structurally from wild-type apo(a), which may underlie key differences related to its effects on fibrin clot architecture and fibrinolysis.