Circulating Activated Protein C in Thrombophilia Carriers
Authors: van Mens Thijs E , Meijers Joost C.M. , Middeldorp Saskia 
Published: October 2019

Abstract
Background: Inherited thrombophilias are genetic disorders in which mutation carriers have an elevated risk of venous thromboembolism through abnormalities in the coagulation cascade. These abnormalities all lead to increased thrombin generation. The mutations, of which factor V Leiden and prothrombin G20210A are the most common, therefore likely increase thrombin mediated protein C activation in plasma. Previous findings have however been inconsistent.
Increased activation of protein C in inherited thrombophilia would be interesting in light of various unexplained phenotypes described in thrombophilia carriers. Examples of such phenotypes include improved fertility, increased risk of miscarriage, protection from diabetic nephropathy, decreased susceptibility to and mortality from sepsis and decreased mortality in acute respiratory distress syndrome. These do not appear directly related to increased coagulation in carriers.
Activated protein C (APC) possesses a wide range of signaling functions and interactions with multiple pathways. These result in anti-apoptotic, anti-inflammatory, gene-expression, regenerative and endothelial stabilizing effects. Such properties can easily be thought to play a role in the above described phenotypes. APC has indeed been shown to possess beneficial properties in numerous animal injury models. Due to its pleiotropic nature, APC might be a promising candidate for further research into the unexplained phenotypes observed in inherited thrombophilia.
Aim: To investigate if plasma APC concentrations are higher in thrombophilia carriers as compared to non-carriers.
Methods: We performed a cross-sectional observational study comparing the APC plasma levels of factor V Leiden and prothrombin G20210A mutation hetero- and homozygotes with non-carriers. Exclusion criteria comprised use of anticoagulant medication and recent venous thrombosis or risk factors for venous thrombosis. We measured APC using a recently developed highly sensitive oligonucleotide-based capture assay, with a limit of detection of 0.022 ng/ml and the ability to quantify APC upward of 0.116 ng/ml (lower limit of quantification) (Müller et al., 2012). In addition we determined APC-protein C inhibitor complex (APC-PCI) as a secondary measure of protein C activation, and prothrombin fragment 1+2 (F1+2) concentration as a measure of thrombin generation using immunoassays. Parametric and non-parametric descriptive and inferential statistics were applied as appropriate.
Results: We included 19 thrombophilia carriers and 18 non-carriers (Table 1). APC was detectable in 47% of carriers and in 39% of non-carriers (p = 0.74). APC was above the lower limit of quantification in only 19% of all subjects, with no difference between the groups (Figure 1). The median APC-PCI concentration in carriers and non-carriers were 5 AU (IQR 3.5-10.5) vs. 5 AU (IQR 3.0-8.0) (p = 0.338); and mean F1+2 concentrations were 266 pmol/L and 194 pmol/L in carriers and non-carriers respectively (p = 0.075).
Discussion: We did not find increased circulating APC concentrations in thrombophilia carriers. Given the low number of subjects with quantifiable APC in the study, elevated APC levels in carriers versus non-carriers cannot be fully excluded. Local elevation at the site of thrombin formation still seems plausible, and our data do show a trend towards increased thrombin generation in thrombophilia carriers. However, we also show that systemic concentrations are generally below 0.116 ng/ml, which is an order of magnitude lower than concentrations previously reported as physiological levels. A prominent role for APC in non-coagulation related thrombophilia phenotypes might therefore be questioned.
References
Müller, J. et al. (2012). Journal of Thrombosis and Haemostasis : JTH, 10(3), 390-8.
Measured APC concentrations above the limit of quantification, according to thrombophilia carriership status. Measured APC concentrations above the limit of quantification, according to thrombophilia carriership status. Figure 1 Figure 1.

Disclosures
Middeldorp: Boehringer Ingelheim: Consultancy; GSK: Consultancy, Honoraria; Aspen: Consultancy, Honoraria; BMS/Pfizer: Consultancy, Honoraria; Bayer: Consultancy; Daiichi Sankyo: Consultancy, Honoraria; Sanquin: Consultancy.