Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic riskfactors

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation a t position 460 results in the loss of glycosylation of N458, This polymorph ism has been found to be slightly more prevalent in thrombophilic populatio ns than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele ma y have an increased risk of thrombosis. We have now characterized the expre ssion in cell cultures of recombinant protein S Heerlen and investigated th e anticoagulant functions of the purified recombinant protein in vitro. Pro tein S Heerlen was synthesized and secreted equally well as wild-type prote in S by transiently transfected COS-1 cells. The recombinant protein S Heer len interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S, Protein S He erlen displayed reduced anticoagulant activity as cofactor to activated pro tein C (APC) in plasma-based assays, as well as in a factor Villa-degradati on system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. Howev er, when recombinant activated factor V Leiden (FVa:Q506) was used as APC s ubstrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S, These in vitro results suggest a possible mechan ism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both gen etic traits. (C) 2000 by The American Society of Hematology.