Original ArticlesThe activated protein C (APC)-resistant phenotype of APC cleavage site mutants of recombinant factor V in a reconstituted plasma modelKolfschoten, M. van der Neut; Dirven, R. J.; Tans, G.; Rosing, J.; Vos, H. L.; Bertina, R. M.Author Information M. van der Neut Kolfschoten, R. J. Dirven, H. L. Vos and R. M. Bertina are with the Hemostasis and Thrombosis Research Center, Department of Hematology, LUMC, Leiden, The Netherlands; and G. Tans and J. Rosing are with the Department of Biochemistry, CARIM, UM, Maastricht, The Netherlands. (Received 13 August 2001; revised 10 December 2001; accepted 14 December 2001) Address correspondence to M. van der Neut Kolfschoten, Hemostasis and Thrombosis Research Center, Department of Hematology. Leiden University Medical Center, D2-19, PO Box 9600, 2300RC Leiden, The Netherlands. Tel: (+31) 71 5262076; fax: (+31) 71 5266755; e-mail: firstname.lastname@example.org Blood Coagulation & Fibrinolysis: April 2002 - Volume 13 - Issue 3 - p 207-215 Buy Abstract Recently, new missense mutations in the activated protein C (APC) cleavage sites of human factor V (FV) distinct from the R506Q (FV Leiden) mutation have been reported. These mutations affect the APC cleavage site at arginine (Arg) 306 in the heavy chain of activated FV. Whether these mutations result in APC resistance and are associated with a risk of thrombosis is not clear. The main objective of the present study was to identify the APC-resistant phenotype of FV molecules with different mutations in APC cleavage sites. To study this, recombinant FV mutants were reconstituted in FV-deficient plasma, after which normalized APC-sensitivity ratios (n-APC-SRs) were measured in activated partial thromboplastin time-based and Russell's Viper Venom time-based APC-resistance tests. The mutations introduced in FV were R306G, R306T, R506Q, R679A and combinations of these mutations. Based on the APC-sensitivity ratios, we conclude that the naturally occurring mutations at Arg306 (i.e. FV HongKong and FV Cambridge) result in a mildly reduced sensitivity for APC (n-APC-SR, 0.74–0.87), whereas much lower values (n-APC-SR, 0.41–0.51) are obtained for the mutation at Arg506 (FV Leiden). No effect on the n-APC-SR was observed for the recombinant FV mutant containing the single Ala679 mutation. Because reduced sensitivity for APC, not due to FV Leiden, is a risk factor for venous thrombosis, these data suggest that mutations at Arg306 might be associated with a mild risk of venous thrombosis. © 2002 Lippincott Williams & Wilkins, Inc.