Corrigendum to 'The Hemophilia Joint Health Score version 2.1 Validation in Adult Patients Study: A multicenter international study' [Research and Practice in Thrombosis and Haemostasis, 6/2, (2022) e12690]

[This corrects the article DOI: 10.1002/rth2.12690.].

Natural anticoagulant discovery, the gift that keeps on giving: finding FV-Short

The complex reactions of blood coagulation are balanced by several natural anticoagulants resulting in tuned hemostasis. During several decades, the knowledge base of the natural anticoagulants has greatly increased and we have also learned about antiinflammatory and cytoprotective activities expressed by antithrombin and activated protein C (APC). Some coagulation proteins have also been found to function as anticoagulants; e.g., thrombin when bound to thrombomodulin activates protein C. Another example is factor V (FV), which in addition to being a procofactor to FVa has emerged as an anticoagulant. The discovery of APC resistance, caused by FVLeiden, as a thrombosis risk factor resulted in the identification of FV as an APC cofactor working in synergy with protein S in the regulation of FVIIIa in the Xase complex. More recently, a natural anticoagulant FV splice isoform (FV-Short) was discovered when investigating the East Texas bleeding disorder. In FV-Short, the truncated B domain exposes a high-affinity binding site for tissue factor pathway inhibitor alpha (TFPIα), and together with protein S a high-affinity trimolecular complex is generated. The FXa-inhibitory activity of TFPIα is synergistically stimulated by FV-Short and protein S. The circulating FV-Short/protein S/TFPIα complex concentration is normally low (≈0.2 nM) but provides an anticoagulant threshold. In the East Texas bleeding, the concentration of the complex, and thus the threshold, is increased 10-fold, which results in bleeding manifestations. The anticoagulant properties of FV were discovered during investigations of individual patients and follow the great tradition of bed-to-bench and bench-to-bed research in the coagulation field.

Diagnosis and management of severe congenital protein C deficiency (SCPCD): Communication from the SSC of the ISTH

Severe congenital protein C deficiency (SCPCD) is rare and there is currently substantial variation in the management of this condition. A joint project by three Scientific and Standardization Committees of the ISTH: Plasma Coagulation Inhibitors, Pediatric/Neonatal Thrombosis and Hemostasis, and Women's Health Issues in Thrombosis and Hemostasis, was developed to review the current evidence and help guide on diagnosis and management of SCPCD. We provide a summary of the clinical presentations, differential diagnoses, appropriate investigations to confirm the diagnosis, approaches for management of the acute situation, and options for long-term management including subsequent pregnancies. We finally provide a set of recommendations to help in this regard.

A hydrophobic patch (PLVIVG; 1481-1486) in the B-domain of factor V-short is crucial for its synergistic TFPIα-cofactor activity with protein S and for the formation of the FXa-inhibitory complex comprising FV-short, TFPIα, and protein S

BACKGROUND

Factor V-short (FV756-1458) is a natural splice variant functioning in synergy with protein S as tissue factor pathway inhibitor alpha (TFPIα)-cofactor in inhibition of factor Xa (FXa). An exposed acid region (AR2; 1493-1537) in the B domain binds TFPIα. The preAR2 (1458-1492) is crucial for the synergistic TFPIα-cofactor activity between FV-short and protein S and for assembly of a trimolecular FXa-inhibitory complex among FV-short, protein S, and TFPIα.

OBJECTIVE

To identify which part of preAR2 is required for the synergistic TFPIα-cofactor activity between FV-short and protein S.

METHODS

A FXa-inhibition assay was used to test the synergistic TFPIα cofactor activity between protein S and new FV-short variants FV709-1476, FV712-1478, FV712-1481, FV712-1484, FV712-1487, and FV712-1490. A microtiter-based assay analyzed binding among FV-short variants, protein S, and TFPIα.

RESULTS

FV709-1476, FV712-1478, and FV712-1481 were fully active as synergistic TFPIα cofactors with protein S; FV712-1484 showed intermediate activity; and FV712-1487 and FV712-1490 were inactive. TFPIα interacted with all variants in the absence of protein S but FV712-1478 and FV712-1481 bound TFPIα with highest affinity. None of the FV-short variants bound directly to protein S in the absence of TFPIα. In the presence of TFPIα, efficient cooperative binding was demonstrated between protein S, TFPIα, and FV709-1476, FV712-1478, or FV712-1481. In contrast, no cooperativity among TFPIα, protein S, and FV712-1484, FV712-1487, or FV712-1490 was seen.

CONCLUSION

A short hydrophobic patch in preAR2 (PLVIVG, 1481-1486) in FV-short is crucial for the synergistic TFPIα-cofactor activity between FV-short and protein S and for the assembly of a trimolecular FXa-inhibitory complex among FV-short, protein S, and TFPIα.

Classic Thrombophilias and Thrombotic Risk Among Middle-Aged and Older Adults: A Population-Based Cohort Study

Background

Five classic thrombophilias have been recognized: factor V Leiden (rs6025), the prothrombin G20210A variant (rs1799963), and protein C, protein S, and antithrombin deficiencies. This study aimed to determine the thrombotic risk of classic thrombophilias in a cohort of middle‐aged and older adults.

Methods and Results

Factor V Leiden, prothrombin G20210A and protein‐coding variants in the PROC (protein C), PROS1 (protein S), and SERPINC1 (antithrombin) anticoagulant genes were determined in 29 387 subjects (born 1923–1950, 60% women) who participated in the Malmö Diet and Cancer study (1991–1996). The Human Gene Mutation Database was used to define 68 disease‐causing mutations. Patients were followed up from baseline until the first event of venous thromboembolism (VTE), death, or Dec 31, 2018. Carriership (n=908, 3.1%) for disease‐causing mutations in the PROC, PROS1, and SERPINC1 genes was associated with incident VTE: Hazard ratio (HR) was 1.6 (95% CI, 1.3–1.9). Variants not in Human Gene Mutation Database were not linked to VTE (HR, 1.1; 95% CI, 0.8–1.5). Heterozygosity for rs6025 and rs1799963 was associated with incident VTE: HR, 1.8 (95% CI, 1.6–2.0) and HR, 1.6 (95% CI, 1.3–2.0), respectively. The HR for carrying 1 classical thrombophilia variant was 1.7 (95% CI, 1.6–1.9). HR was 3.9 (95% CI, 3.1–5.0) for carriers of ≥2 thrombophilia variants.

Conclusions

The 5 classic thrombophilias are associated with a dose‐graded risk of VTE in middle‐aged and older adults. Disease‐causing variants in the PROC, PROS1, and SERPINC1 genes were more common than the rs1799963 variant but the conferred genetic risk was comparable with the rs6025 and rs1799963 variants.

The preAR2 region (1458-1492) in factor V-Short is crucial for the synergistic TFPIα-cofactor activity with protein S and the assembly of a trimolecular factor Xa-inhibitory complex comprising FV-Short, protein S, and TFPIα

BACKGROUND

Factor V-Short (FV756-1458) is a natural splice variant in which 702 residues are deleted from the B domain. It exposes an acid region (AR2; 1493-1537) that binds tissue factor pathway inhibitor alpha (TFPIα). Protein S also interacts with TFPIα and serves as TFPIα-cofactor in factor Xa (FXa) inhibition. FV-Short and protein S function as synergistic TFPIα-cofactors in inhibition of FXa. FV810-1492 is an artificial FV-Short variant that cannot synergize with protein S as TFPIα cofactor even though it contains AR2 and binds TFPIα.

OBJECTIVE

To elucidate the mechanisms for the synergism between FV756-1458 and protein S as TFPIα cofactors.

METHODS

Four FV-Short variants were created, FV756-1458 and FV712-1458 contained the preAR2 region (1458-1492), whereas FV810-1492 and FV713-1492 lacked this region. The synergistic TFPIα cofactor activity between FV-Short variants and protein S was analyzed by FXa-inhibition. A microtiter-based assay tested binding between FV-Short variants, protein S, and TFPIα.

RESULTS

The two preAR2-containing FV-Short variants were active as synergistic TFPIα cofactors, whereas the other two were inactive. All variants bound to TFPIα. None of the FV-Short variants bound directly to protein S. The combination of TFPIα and preAR2-containing FV-Short variants bound protein S, whereas TFPIα together with the preAR2-minus variants did not. Protein S potentiated TFPIα-binding to the preAR2-containing variants and binding between TFPIα and protein S was stimulated only by the preAR2-containing variants.

CONCLUSION

The preAR2 region is demonstrated to be crucial for the synergistic TFPIα-cofactor activity between FV-Short and protein S and for the assembly of a trimolecular FXa-inhibitory complex comprising FV-Short, protein S, and TFPIα.

Pleiotropic anticoagulant functions of protein S, consequences for the clinical laboratory. Communication from the SSC of the ISTH

Hereditary deficiencies of protein S (PS) increase the risk of thrombosis. However, assessing the plasma levels of PS is complicated by its manifold physiological interactions, while the large inter-individual variability makes it problematic to establish reliable cut-off values. PS has multiple physiological functions, with only two appearing to have significant anticoagulant properties: the activated protein C (APC) and tissue factor pathway inhibitor alpha (TFPIα) cofactor activities. Current clinical laboratory investigations for deficiency in PS function rely only on the APC-dependent activity. This communication presents an argument for reclassifying the qualitative PS deficiencies to differentiate the two major anticoagulant functions of PS. Reliable assays are necessary for accurate evaluation of PS function when making a specific diagnosis of PS deficiency based on the anticoagulant phenotype alone. This report emphasizes the pleiotropic anticoagulant functions of PS and presents evidence-based recommendations for their implementation in the clinical laboratory.

Reduced Apolipoprotein M and Adverse Outcomes Across the Spectrum of Human Heart Failure

BACKGROUND

Apo (apolipoprotein) M mediates the physical interaction between high-density lipoprotein (HDL) particles and sphingosine-1-phosphate (S1P). Apo M exerts anti-inflammatory and cardioprotective effects in animal models.

METHODS

In a subset of PHFS (Penn Heart Failure Study) participants (n=297), we measured apo M by Enzyme-Linked ImmunoSorbent Assay (ELISA). We also measured total S1P by liquid chromatography-mass spectrometry and isolated HDL particles to test the association between apo M and HDL-associated S1P. We confirmed the relationship between apo M and outcomes using modified aptamer-based apo M measurements among 2170 adults in the PHFS and 2 independent cohorts: the Washington University Heart Failure Registry (n=173) and a subset of TOPCAT (Treatment of Preserved Cardiac Function Heart Failure With an Aldosterone Antagonist Trial; n=218). Last, we examined the relationship between apo M and ≈5000 other proteins (SomaScan assay) to identify biological pathways associated with apo M in heart failure.

RESULTS

In the PHFS, apo M was inversely associated with the risk of death (standardized hazard ratio, 0.56 [95% CI, 0.51-0.61]; P<0.0001) and the composite of death/ventricular assist device implantation/heart transplantation (standardized hazard ratio, 0.62 [95% CI, 0.58-0.67]; P<0.0001). This relationship was independent of HDL cholesterol or apo AI levels. Apo M remained associated with death (hazard ratio, 0.78 [95% CI, 0.69-0.88]; P<0.0001) and the composite of death/ventricular assist device/heart transplantation (hazard ratio, 0.85 [95% CI, 0.76-0.94]; P=0.001) in models that adjusted for multiple confounders. This association was present in both heart failure with reduced and preserved ejection fraction and was replicated in the Washington University cohort and a cohort with heart failure with preserved ejection fraction only (TOPCAT). The S1P and apo M content of isolated HDL particles strongly correlated (R=0.81, P<0.0001). The top canonical pathways associated with apo M were inflammation (negative association), the coagulation system (negative association), and liver X receptor/retinoid X receptor activation (positive association). The relationship with inflammation was validated with multiple inflammatory markers measured with independent assays.

CONCLUSIONS

Reduced circulating apo M is independently associated with adverse outcomes across the spectrum of human heart failure. Further research is needed to assess whether the apo M/S1P axis is a suitable therapeutic target in heart failure.

Advances in Understanding Mechanisms of Thrombophilic Disorders

Venous thromboembolism constitutes a major medical problem afflicting millions of individuals worldwide each year. Its pathogenesis is multifactorial, involving both environmental and genetic risk factors. The most common genetic risk factor known to date is a mutation in the factor V (FV) gene (R506Q or FV Leiden), which impairs the normal regulation of FV by activated protein C (APC). APC is an important regulator of blood coagulation, cleaving and inactivating not only FV/FVa but also activated factor VIII (FVIIIa). In FVa, APC cleaves several sites, Arg506 (R506) being one of them. The R506Q mutation results in the APC resistance phenotype and a lifelong hypercoagulable state. A prothrombin gene mutation is another relatively frequent thrombosis risk factor, whereas deficiencies of the anticoagulant proteins antithrombin, protein C, or protein S are less common. As a result of the high prevalence of FV and prothrombin mutations in the general population, combinations of genetic defects are relatively common. Such individuals have highly increased risk of thrombosis.

Plasma apoM and S1P levels are inversely associated with mortality in African Americans with type 2 diabetes mellitus

apoM is a minor HDL apolipoprotein and carrier for sphingosine-1-phosphate (S1P). HDL apoM and S1P concentrations are inversely associated with atherosclerosis progression in rodents. We evaluated associations between plasma concentrations of S1P, plasma concentrations of apoM, and HDL apoM levels with prevalent subclinical atherosclerosis and mortality in the African American-Diabetes Heart Study participants (N = 545). Associations between plasma S1P, plasma apoM, and HDL apoM with subclinical atherosclerosis and mortality were assessed using multivariate parametric, nonparametric, and Cox proportional hazards models. At baseline, participants' median (25th percentile, 75th percentile) age was 55 (49, 62) years old and their coronary artery calcium (CAC) mass score was 26.5 (0.0, 346.5). Plasma S1P, plasma apoM, and HDL apoM were not associated with CAC. After 64 (57.6, 70.3) months of follow-up, 81 deaths were recorded. Higher concentrations of plasma S1P [odds ratio (OR) = 0.14, P = 0.01] and plasma apoM (OR = 0.10, P = 0.02), but not HDL apoM (P = 0.89), were associated with lower mortality after adjusting for age, sex, statin use, CAC, kidney function, and albuminuria. We conclude that plasma S1P and apoM concentrations are inversely and independently associated with mortality, but not CAC, in African Americans with type 2 diabetes after accounting for conventional risk factors.

Low plasma concentrations of apolipoprotein M are associated with disease activity and endothelial dysfunction in systemic lupus erythematosus

BACKGROUND

Apolipoprotein M (apoM) is a 25-kDa apolipoprotein present in 5% of high-density lipoprotein (HDL) particles. It is suggested to be anti-atherogenic and to play a key role in sustaining endothelial barrier integrity. SLE patients have increased cardiovascular disease risk, and we aimed to investigate if apoM levels reflect endothelial function in SLE. Since apoM plasma levels decrease during inflammatory conditions, our aim was also to determine the impact of SLE disease activity on apoM plasma levels.

METHODS

Plasma concentrations of apoM were measured by ELISA in two patient groups with systemic lupus erythematosus (SLE) and in 79 healthy control individuals. In patient group I (n = 84), evaluation time points were selected with the objective to include a wide range of clinical and laboratory variables reflecting disease activity which was measured as SLEDAI. In patient group II consisting of 140 consecutive patients, endothelial function was measured by a finger plethysmograph. A low Reactive Hyperemia Index (RHI) value indicates endothelial dysfunction.

RESULTS

SLE patients had decreased levels of apoM compared to healthy controls (p < 0.01), with apoM levels correlating inversely with SLEDAI (r = - 0.31, p < 0.01) as well as with levels of CRP (r = - 0.26, p = 0.02) and positively with levels of C3 (r = 0.29, p < 0.01). ApoM levels were particularly low in patients with active disease from the kidney and skin and in patients with leukopenia or positive anti-dsDNA antibody test (p < 0.05). ApoM levels correlated with RHI values in young SLE patients (r = 0.32, p = 0.01), consistent with the important role of apoM in regulating endothelial integrity.

CONCLUSIONS

ApoM levels may be regulated by SLE-related inflammatory processes and could be a marker of disease activity and endothelial dysfunction, in particular in young SLE patients. Further studies are needed to investigate the predictive value of apoM in the development of a cardiovascular disease.

New functional test for the TFPIα cofactor activity of Protein S working in synergy with FV-Short

Essentials Protein S and FV-Short are synergistic cofactors to Tissue Factor Pathway Inhibitor α (TFPIα). An assay for the TFPIα synergistic cofactor activity of protein S with FV-Short was developed. The assay was specific for the synergistic TFPIα-cofactor activity of free protein S. Protein S deficient individuals with known mutations were correctly distinguished from controls. SUMMARY: Background Protein S is an anticoagulant cofactor to both activated protein C and tissue factor pathway inhibitor (TFPIα). The TFPIα-cofactor activity of protein S is stimulated by a short isoform of factor V (FV-Short), the two proteins functioning in synergy. Objective Using the synergistic TFPIα-cofactor activity between protein S and FV-Short to develop a functional test for plasma protein S. Patients/Methods TFPIα-mediated inhibition of FXa in the presence of FV-Short, protein S and negatively charged phospholipid vesicles was monitored in time by synthetic substrate S2765. TFPIα, FXa and FV-Short were purified proteins, whereas diluted plasma from protein S deficient patients or controls were used as source for protein S. Results The assay was specific for free protein S demonstrating good correlation to free protein S plasma levels (r = 0.92) with a Y-axis intercept of -5%. Correlation to concentrations of total protein S (free and C4BPβ+-bound) was lower (r = 0.88) and the Y-axis intercept was +46%, which is consistent with the specificity for free protein S. The test distinguished protein S-deficient individuals from 6 families with known ProS1 mutations from family members having no mutation. Protein S levels of warfarin-treated protein S deficient cases were lower than protein S in cases treated with warfarin for other causes. Conclusions We describe a new assay measuring the TFPIα-cofactor activity of plasma protein S. The test identifies type I/III protein S deficiencies and will be a useful tool to detect type II protein S deficiency having defective TFPIα-cofactor activity.

Elevated Levels of Prothrombin Activation Fragment 1+2 in Plasma from Patients with Heterozygous Arg506 to Gin Mutation in the Factor V Gene (APC-Resistance) and/or Inherited Protein S Deficiency

Inherited resistance to activated protein C (APC-resistance), caused by a point mutation in the factor V gene leading to replacement of Arg(R)506 with a Gin (Q), and inherited protein S deficiency are associated with functional impairment of the protein C anticoagulant system, yielding lifelong hypercoagulability and increased risk of thrombosis. APC-resistance is often an additional genetic risk factor in thrombosis-prone protein S deficient families. The plasma concentration of prothrombin fragment 1+2 (F1+2), which is a marker of hyper-coagulable states, was measured in 205 members of 34 thrombosis-prone families harbouring the Arg506 to Gin mutation (APC-resistance) and/or inherited protein S deficiency. The plasma concentration of F1+2 was significantly higher both in 38 individuals carrying the FV:Q506 mutation in heterozygous state (1.7 ± 0.7 nM; mean ± SD) and in 48 protein S deficient cases (1.9 ± 0.9 nM), than in 100 unaffected relatives (1.3 ±0.5 nM). Warfarin therapy decreased the F1+2 levels, even in those four patients who had combined defects (0.5 ± 0.3 nM). Our results agree with the hypothesis that individuals with APC-resistance or protein S deficiency have an imbalance between pro- and anti-coagulant forces leading to increased thrombin generation and a hypercoagulable state.

Homozygous APC-resistance Combined with Inherited Type I Protein S Deficiency in a Young Boy with Severe Thrombotic Disease

Inherited resistance to activated protein C (APC) is a frequent cause of familial thrombosis. It is associated with a factor V gene point mutation replacing arginine506 in the APC-cleavage site with a glutamine. Thrombotic events are rare during childhood even in patients with homozygous APC-resistance. We now wish to report on a case of severe venous thrombosis, in a 10-year-old boy. He was found to have pronounced APC-resistance due to homozygous factor V gene mutation in combination with inherited type I protein S deficiency. The two traits were independently inherited in the family. The APC-resistance was partially corrected by adding factor V, whereas added protein S was without effect. This is the first reported case of homozygous APC-resistance combined with another inherited prothrombotic disorder. It illustrates how multiple genetic defects may provoke thrombosis at young age and emphasizes the need of complete evaluation of thrombotic patients in order to determine whether multiple risk factors exist.

Inhibition of Microarterial Thrombosis by Activated Protein C in a Rabbit Model

Protein C is the key component in a natural anticoagulant pathway. After its activation by the thrombin-thrombomodulin complex, it degrades the activated forms of coagulation cofactors VIII and V, which leads to downregulation of the coagulation process. Due to its specific anticoagulant activity, activated protein C (APC) is potentially interesting as an antithrombotic agent. The effect of bovine activated protein C on thrombus formation and haemostasis was investigated in a rabbit model of microarterial thrombosis. Segments of both central ear arteries were prepared and blood-flow interrupted with double vascular clamps.

Longitudinal arteriotomies (7 mm) and deep vessel wall trauma (5 mm) were performed, whereafter the arteriotomies were closed with running sutures. Five minutes prior to opening of the clamps (reperfusion), boluses of APC (0.8 mg/kg body weight) or vehicle alone were administered to two groups, each of 10 rabbits, in a blind random fashion. Vessel patency-rates were drastically improved by the administration of APC compared to vehicle. Correspondingly, thrombus weights were significantly lower in the APC group than in the control group. The activated partial thromboplastin time was prolonged to approximately twice the baseline throughout the 2 h observation interval in the APC group. Levels of circulating platelets were unaffected by the APC infusion, but the arteriotomy bleeding times were significantly longer in the APC group. In summary, activated protein C exerted powerful and long-acting antithrombotic effects in a microarterial model of thrombosis in rabbits.

Protein S and C4b-Binding Protein: Components Involved in the Regulation of the Protein C Anticoagulant System

The protein C anticoagulant system provides important control of the blood coagulation cascade. The key protein is protein C, a vitamin K-dependent zymogen which is activated to a serine protease by the thrombin-thrombomodulin complex on endothelial cells. Activated protein C functions by degrading the phospholipid-bound coagulation factors Va and VIIIa. Protein S is a cofactor in these reactions. It is a vitamin K-dependent protein with multiple domains. From the N-terminal it contains a vitamin K-dependent domain, a thrombin-sensitive region, four EGF)epidermal growth factor (EGF)-like domains and a C-terminal region homologous to the androgen binding proteins. Three different types of post-translationally modified amino acid residues are found in protein S, 11 γ-carboxy glutamic acid residues in the vitamin K-dependent domain, a β-hydroxylated aspartic acid in the first EGF-like domain and a β-hydroxylated asparagine in each of the other three EGF-like domains. The EGF-like domains contain very high affinity calcium binding sites, and calcium plays a structural and stabilising role. The importance of the anticoagulant properties of protein S is illustrated by the high incidence of thrombo-embolic events in individuals with heterozygous deficiency. Anticoagulation may not be the sole function of protein S, since both in vivo and in vitro, it forms a high affinity non-covalent complex with one of the regulatory proteins in the complement system, the C4b-binding protein (C4BP). The complexed form of protein S has no APC cofactor function. C4BP is a high molecular weight multimeric protein with a unique octopus-like structure. It is composed of seven identical α-chains and one β-chain. The α-and β-chains are linked by disulphide bridges. The cDNA cloning of the β-chain showed the α- and β-chains to be homologous and of common evolutionary origin. Both subunits are composed of multiple 60 amino acid long repeats (short complement or consensus repeats, SCR) and their genes are located in close proximity on chromosome 1, band 1q32. Available experimental data suggest the β-chain to contain the single protein S binding site on C4BP, whereas each of the α-chains contains a binding site for the complement protein, C4b. As C4BP lacking the β-chain is unable to bind protein S, the β-chain is required for protein S binding, but not for the assembly of the α-chains during biosynthesis. Protein S has a high affinity for negatively charged phospholipid membranes, and is instrumental in binding C4BP to negatively charged phospholipid. This constitutes a novel mechanism for control of the complement system on phospholipid surfaces. Recent findings have shown circulating C4BP to be involved in yet another calcium-dependent protein-protein interaction with a protein known as the serum amyloid P-component (SAP). The binding sites on C4BP for protein S and SAP are independent. SAP, which is a normal constituent in plasma and in tissue, is a so-called pentraxin being composed of 5 non-covalently bound 25 kDa subunits. It is homologous to C reactive protein (CRP) but its function is not yet known. The specific high affinity interactions between protein S, C4BP and SAP suggest the regulation of blood coagulation and that of the complement system to be closely linked.

Multicenter Evaluation of a Kit for Activated Protein C Resistance on Various Coagulation Instruments Using Plasmas from Healthy Individuals

Recently a new hemostatic disorder has been described which appears to be an important risk factor for familial thromboembolism. The disorder is characterized by a poor anticoagulant response to activated Protein C (APC) and has been shown to be due to lack of an APC cofactor activity which is a property of factor V.

A kit for determining the response of plasma samples towards addition of APC in an APTT-based assay - COATEST APC Resistance -has been evaluated on 35 coagulation instruments in a multicenter study involving 32 laboratories. A lyophilized normal plasma and identical plasma aliquots from 20 individuals, one of whom had a borderline resistance to APC, were analysed in each laboratory and the sensitivity of each plasma to APC was determined as the ratio between the clotting times obtained in the presence and absence of APC (APC ratio).

The plasma from the individual with a borderline resistance to APC activity was correctly classified as the lowest responder in each laboratory, with an APC ratio in the range 1.6-2.4. In comparison, plasmas from individuals with a pronounced response to APC activity resulted in APC ratios above 3.4 in most cases. Interestingly, although the actual APT time for a plasma from a given individual showed a more than 10 s difference due to the type of instrumentation used, the variation in the APC ratio was limited. A similar discrimination was also obtained from evaluation of the actual prolongation of the clotting time in the presence of APC.

The intra-laboratory coefficient of variation for the clotting times were on average 2.0% and 3.9% in the absence and presence of APC, respectively, indicating that the precision for the prolonged clotting times obtained in the presence of APC is sufficient to allow a safe assignment of the APC response. The APC ratio for the lyophilized normal plasma was 2.7 ± 0.2 (2 S.D.) illustrating a narrow distribution between instruments which shows the feasibility of including such plasma for assay validation. Altogether, the results indicate that all the coagulation instruments included in the study can be used for detection of individuals with resistance to APC activity through determination of the APC ratio or the prolongation time.

Synergistic Cofactor Function of Factor V and Protein S to Activated Protein C in the Inactivation of the Factor Villa – Factor IXa Complex

Human factor V has been shown not only to be a precursor to procoagulant factor Va but also to express anticoagulant properties. Thus, factor V was recently found to potentiate the effect of protein S as cofactor to activated protein C (APC) in the inactivation of the factor VIIIa-factor IXa complex. The purpose of this study was to determine whether the APC-cofactor function of factor V was also expressed in the bovine protein C system and to elucidate the molecular background for the species specificity of APC. For this purpose, the effects of protein S and factor V on APC-mediated inactivation of factor VIIIa were studied using purified APC, protein S and factor V of human and bovine.origin. The factor VIIIa investigated here was part of a Xase complex (i.e. factor IXa, factor VIIIa, phospholipid and calcium) and the APC-mediated inhibition of factor VIIIa was monitored by the ability of the Xase complex to activate factor X. Synergistic APC-cofactor function of factor V and protein S was demonstrated in the bovine system. The effect of bovine APC was potentiated by bovine protein S but not by human protein S, whereas both human or bovine protein S stimulated the function of human APC. Factor V did not express species specificity in its APC-cofactor activity even though bovine factor V was more potent than its human counterpart. Recombinant human/bovine protein S chimeras were used to demonstrate that the thrombin sensitive region and first epidermal growth factor-like module of protein S determine the species specificity of the APC-protein S interaction. In conclusion, both human and bovine factor V were found to express APC-cofactor activity which depends on the presence of protein S. The species specificity of APC was shown to be caused by the interaction between APC and protein S.

Female Gender and Resistance to Activated Protein C (FV:Q506) as Potential Risk Factors for Thrombosis after Elective Hip Arthroplasty

Resistance to activated protein C (APC) caused by the R506Q mutation in factor V is the most common inherited risk factor for venous thrombosis. To elucidate whether APC-resistance is a risk factor for venous thrombosis after elective total hip replacement, the association between APC-resistance (presence of FV:Q506 allele) and postoperative thrombosis was investigated in patients (n = 198) randomised to received short (during hospitalisation, n = 100) or prolonged prophylaxis (three weeks after hospitalisation, n = 98) with low molecular weight heparin (LMWH). Among APC-resistant individuals receiving short prophylaxis, 7/10 developed thrombosis as compared to 2/12 receiving long prophylaxis (p <0.0179). Odds ratio for association between APC-resistance and thrombosis in the short prophylaxis group was 4.2 (CI 95% 1.02-17.5) (p <0.0465). Among those receiving prolonged, prophylaxis, there was no increased incidence of thrombosis associated with APC-resistance. Two unexpected observations were made. One was that APC-resistance was much more common in women (19/109) than in men (3/89) (p <0.001). The other was that even women without APC-resistance were much more thrombosis-prone than men. Thus, 24/48 of women with normal FV genotype and short prophylaxis developed thrombosis vs 8/42 among men, p = 0.002. The increased risk of thrombosis associated with female gender and APC-resistance was neutralised by the prolonged treatment. In conclusion, among patients receiving short prophylaxis, female gender was found to be a strong risk factor for venous thrombosis. Even though APC-resistance appeared to be a risk factor for postoperative thrombosis, the uneven distribution of APC-resistance between men and women, taken together with the increased risk of thrombosis among women, precluded valid conclusions to be drawn about the association between APC-resistance and an increased risk of thrombosis. Our results suggest that prolonged prophylaxis with LMWH after hip surgery is more important for women than for men.

Evaluation of Original and Modified APC-Resistance Tests in Unselected Outpatients with Clinically Suspected Thrombosis and in Healthy Controls

APC-resistance is the most common hereditary condition associated with venous thrombosis. It is in a majority of cases due to a single point mutation in the factor V gene (FVR506Q). Currently used functional APC-resistance tests have 85-90% sensitivity and specificity for the FVR506Q mutation. A modified test which includes predilution of patient plasma in factor V depleted plasma has increased the sensitivity and specificity for the factor V mutation. However, neither the original nor the modified APC-resistance test have been evaluated in patients with acute thrombotic events. We have therefore used the original and the modified APC-resistance tests in 220 patients with clinically suspected acute deep venous thrombosis and in 278 healthy controls. The FVR506Q mutation was determined in all patients. The patients were classified as either DVT (deep venous thrombosis)-negative or DVT-positive depending on the outcome of contrast phlebography. In individuals with normal factor V genotype, the original APC-resistance test gave significantly lower APC-ratio values both in DVT-positive and DVT-negative patients than in healthy controls. The specificity of the original APC-resistance test for the FVR506Q mutation in controls and in DVT-negative and DVT-positive patients were 85%, 54% and 28%, respectively, when a cut off APC-ratio of 3.2 which insured 100% sensitivity was used. Using the modified APC-resistance test, essentially no difference in APC-ratios between patients with normal factor V genotype and healthy controls with normal factor V genotype was observed. The modified APC-resistance test had a specificity for the FVR506Q mutation of 98.8% at an APC-ratio cut off of 2.1 which ensured 100% sensitivity. The original APC-resistance test gave lower APC-ratios in women than in men and in patients with acute thrombosis as compared to controls. In conclusion, the modified APC-resistance test is highly sensitive and specific for the FVR506Q mutation. This test can be used in clinical practice as an easy to perform screening test for the FVR506Q allele. Moreover, the test performs equally well in patients with acute suspected venous thrombosis as in healthy controls.

Rabbit Plasma, unlike Its Human Counterpart, Contains no Complex between Protein S and C4b-Binding Protein

In human plasma, the anticoagulant vitamin K-dependent protein S exists in two molecular forms, as free protein and complexed to C4b- binding protein (C4BP), a complement regulatory protein. It has been suggested that rabbit plasma also contains two forms of protein S and that the interaction between protein S and C4BP m rabbits can be modulated by synthetic peptides corresponding to a sequence (residues 605-614) in the carboxy-lerminal part of protein S. In this report, we provide itsulls which challenge the conclusion that rabbit plasma contains the complexed form of protein S. The two forms of protein S in human plasma were separated by gel filtration chromatography on Sephacryl S-300 and the presence of protein S in the various fractions analyzed by Western blotting using a monoclonal antibody (HPS 21) directed against the γ-carboxyglutamic acid rich module of human protein S. This antibody, which was found to cross-react with rabbit protein S on Western blotting, was used in affinity purification of protein S from rabbit plasma as well as of recombinant rabbit protein S. HPS 21 specifically recognized protein S in rabbit plasma and did not cross-react with the other vitamin K-depeudenl plasma proteins. To elucidate whether rabbit plasma contained two forms of protein S, rabbit plasma was subjected to gelfiltration chromatography followed by Western blotting of the fractions with monoclonal antibody HPS 21. Protein S was found only in fractions eluting at a position corresponding to that of free protein S. A radiolabelled trace amount of recombinant rabbit protein S added to rabbit plasma chromatographed as free protein S and no high molecular weight form corresponding to a C4BP-protein S complex was detected. Rabbit protein S had the capacity to bind human C4BP and the addition of human C4BP to rabbit plasma changed the elution profile, of rabbit plasma protein S. After the addition of human C4BP, rabbit plasma protein S quantitatively eluted as a high molecular weight complex, suggesting that all the protein S in rabbit plasma was bound to human C4BP. The anticoagulant activity of human protein S is modulated by the complex formation with C4BP. Our results demonstrate that this function of C4BP and the protein S-C4BP complex formation has not been conserved throughout the evolution even though protein S has a preserved C4BP binding site.

Thromboembolic Disease – Critical Evaluation of Laboratory Investigation

Previous studies of patients with thromboembolic disease have revealed an association either with hereditary anticoagulant protein deficiencies or with defects in the fibrinolytic system. To obtain a more comprehensive picture and to investigate which analyses are useful in the evaluation of such patients, we have performed an extensive laboratory investigation in 439 individuals with thromboembolic disease. Anticoagulant protein deficiencies were found in 24 patients. Deficiencies of protein C (n = 10) and protein S (n = 9) were most common followed by deficiencies of antithrombin III (n = 3) and plasminogen (n = 2). Six of the nine protein S deficient patients demonstrated a selective deficiency of free protein S with normal total protein S concentrations. To diagnose protein C and S deficiencies among the 201 patients receiving oral vitamin K antagonists, the concentrations of protein C and S were compared with the mean concentration of several other vitamin K-dependent proteins. One protein C and three protein S deficiencies were identified among the treated patients. The number of protein C deficiencies found in this group was significantly lower than the number found among untreated patients. Although fewer protein S deficiencies were also identified among the treated patients, than in the untreated group, the difference was not statistically significant. The results suggest that protein C deficiencies went undetected in the treated group and that oral anticoagulant therapy should be discontinued before efforts to diagnose protein C deficiency are made. We found no cases with heparin cofactor II deficiency. Lupus anticoagulant was present in 10 patients. Evaluation of the fibrinolytic system revealed that the patient group had slightly lower mean euglobulin fibrinolytic activity (EFA) after venous occlusion than controls and a subgroup (approximately 15%) of patients with EFA below the level of the 5th percentile of controls, could be distinguished. Repeated analysis demonstrated a substantial individual day-to-day variation in both patients and controls and the combined EFA results did not clearly distinguish patients from controls. There was a significant negative correlation between EFA and plasminogen activator inhibitor (PAI) levels in both patients and controls and the patient group had significantly higher levels of PAI than the control group. In contrast, there was no difference between controls and patients in tissue plasminogen activator (tPA) release after venous occlusion and no correlation between EFA and tPA was observed. These results suggest that although a statistically significant difference between patients and controls in values of fibrinolytic parameters was found, an extensive laboratory evaluation of the fibrinolytic system in individual patients may not be warranted. The association between patients with thrombosis and deficiencies of anticoagulant proteins suggests that the investigation of individual patients should focus on these components.

Protein C, Protein S and c4b-Binding Protein in Severe lnfection and Septic Shock

We measured concentrations of the natural anticoagulant protein C; its cofactor, protein S; and the carrier protein C4bbinding protein (C4BP), in 24 patients with severe infection and 13 with septic shock. Decreased antithrombin III levels were found in 16 of 24 infection patients and all shock patients; high thrombin-antithrombin (TAT) complexes were present in 16 of 24 infection and 12 of 13 shock patients. Protein C concentrations were significantly reduced compared to healthy blood donors, to 60 ± 14% (infection) and 47 ± 20% (septic shock) (mean ± 1 SD). Total protein S levels were not reduced (119 ± 36.7 and 88 ± 20.0%, normal value 96±15%). Free protein S was also normal (27 ± 9.4 and 30 ± 8.7%, normal value 29 ± 9%). The percentage free of total protein S was normal in shock patients (35 ± 8.5%), but significantly reduced in patients without shock (23 ± 5.3%). C4BP was significantly higher than normal in the latter group (135 ± 43%), but not in the shock group (118 ± 40%), possibly due to increased consumption. Thus, no deficiency of total or free protein S was found in these patients, who had evidence of activated coagulation but no clinical DIC.

Lean ApoM-/- Mice with Hyperactive Brown Adipose Tissue

In Cell Reports, Christoffersen et al. [1] demonstrate that sphingosine 1-phosphate (S1P) bound to apolipoprotein M (apoM) regulates the activity and mass of brown adipose tissue (BAT). They found mice lacking apoM to have hyperactive BAT with high triglyceride (TG) utilization, resulting in low white adipose tissue (WAT) mass and low body weight.

High-density lipoprotein from end-stage renal disease patients exhibits superior cardioprotection and increase in sphingosine-1-phosphate

BACKGROUND

Chronic kidney disease (CKD) exacerbates the risk of death due to cardiovascular disease (CVD). Modifications to blood lipid metabolism which manifest as increases in circulating triglycerides and reductions in high-density lipoprotein (HDL) cholesterol are thought to contribute to increased risk. In CKD patients, higher HDL cholesterol levels were not associated with reduced mortality risk. Recent research has revealed numerous mechanisms by which HDL could favourably influence CVD risk. In this study, we compared plasma levels of sphingosine-1-phosphate (S1P), HDL-associated S1P (HDL-S1P) and HDL-mediated protection against oxidative stress between CKD and control patients.

METHODS

High-density lipoprotein was individually isolated from 20 CKD patients and 20 controls. Plasma S1P, apolipoprotein M (apoM) concentrations, HDL-S1P content and the capacity of HDL to protect cardiomyocytes against doxorubicin-induced oxidative stress in vitro were measured.

RESULTS

Chronic kidney disease patients showed a typical profile with significant reductions in plasma HDL cholesterol and albumin and an increase in triglycerides and pro-inflammatory cytokines (TNF-alpha and IL-6). Unexpectedly, HDL-S1P content (P = .001) and HDL cardioprotective capacity (P = .034) were increased significantly in CKD patients. Linear regression analysis of which factors could influence HDL-S1P content showed an independent, negative and positive association with plasma albumin and apoM levels, respectively.

DISCUSSION

The novel and unexpected observation in this study is that uremic HDL is more effective than control HDL for protecting cardiomyocytes against oxidative stress. It is explained by its higher S1P content which we previously demonstrated to be the determinant of HDL-mediated cardioprotective capacity. Interestingly, lower concentrations of albumin in CKD are associated with higher HDL-S1P.

Factor V-short and protein S as synergistic tissue factor pathway inhibitor (TFPIα) cofactors

BACKGROUND

FV-Short is a normal splice variant of Factor V (FV) having a short B domain, which exposes a high affinity-binding site for tissue factor pathway inhibitor α (TFPIα). FV-Short and TFPIα circulate in complex in plasma.

OBJECTIVES

The aim was to elucidate whether FV-Short affects TFPIα as inhibitor of coagulation FXa and to test whether the TFPIα-cofactor activity of protein S is influenced by FV-Short.

METHODS

Recombinant FV, wild-type FV-Short and a FV-Short thrombin-cleavage resistant variant were expressed and purified. The influence of FV and FV-Short variants and/or protein S on the FXa inhibitory activity of TFPIα was monitored both in a purified system and in a plasma-based thrombin generation assay.

RESULTS

FV-Short had intrinsically weak TFPIα-cofactor activity but with protein S present, FV-Short yielded efficient inactivation of FXa. Protein S alone did not promote full TFPIα-activity. Intact FV was inefficient at low protein S concentrations and had 10-fold lower activity compared to FV-Short at physiological protein S levels. Activation of FV-Short by thrombin resulted in the loss of the TFPIα-cofactor activity. The synergistic TFPIα-cofactor activity of FV-Short and protein S was also demonstrated in plasma using a thrombin generation assay.

CONCLUSIONS

FV-Short and protein S are highly efficient, synergistic cofactors to TFPIα in the regulation of FXa activity, whereas full length FV has lower activity. Our results suggest the formation of an efficient FXa-inhibitory complex between FV-Short, TFPIα and protein S on the surface of negatively charged phospholipids.

Early days of APC resistance and FV Leiden

Venous thrombosis is a major medical problem annually affecting millions of individuals worldwide. It is a typical multifactorial disease, the pathogenesis involving both environmental and genetic risk factors. A single point mutation in the gene of coagulation factor V (FV), which results in the replacement of Arg506 with a Gln (FV Leiden) is the most common genetic risk factor known to date. The anti - coagulant activated protein C (APC) regulates the activity of FVa by cleaving several sites in FVa, and the Arg506 is one of them. APC resistance, which is the consequence of the FV Arg506Gln mutation, results a lifelong hypercoagulable state that increases the risk of thrombosis. APC resistance was discovered in my laboratory and the first paper was published in 1993. This was the starting point for an avalanche of research in many laboratories and several thousands of articles have been published since on this topic. The medical community amazingly quickly accepted the concept of APC resistance/FV Leiden as a major risk factor for thrombosis and millions of individuals are today tested for this condition. This review is a personal historical annotation about the early days of APC resistance.

Protein S Thr103Asn Mutation Associated with Type II Deficiency Reproduced In Vitro and Functionally Characterised

Protein S functions as a cofactor to activated protein C (APC) in the degradation of FVa and FVIIIa. In protein S, the thrombin sensitive region (TSR) and the first EGF-like domain are important for expression of the APC cofactor activity. A naturally occurring Thr103Asn (T103N) mutation in the first EGF-like domain of protein S has been associated with functional (type II) protein S deficiency. To elucidate the functional consequences of the T103N mutation, recombinant protein S mutant was expressed in mammalian cells and functionally characterised. The expression level of protein S T103N from transiently transfected COS 1 cells was equal to that of wild type protein S. The mutant protein S and wild type protein S were also expressed in 293 cells after stable transfection, and the recombinant proteins purified. In APTT-and PT-based coagulation assays, the mutant protein demonstrated approximately 50% lower anticoagulant activity as compared to wild type protein S. The functional defect was further investigated in FVa-and FVIIIa-degradation assays. The functional defect of mutant protein S was attenuated at increasing concentrations of APC. The results demonstrate the region around residue 103 of protein S to be of functional importance, possibly through a direct interaction with APC.

The Second Laminin G-type Domain of Protein S Is Indispensable for Expression of Full Cofactor Activity in Activated Protein C-catalysed Inactivation of Factor Va and Factor VIIIa

Vitamin K-dependent protein S is a cofactor to the anticoagulant serine protease activated protein C (APC) in the proteolytic inactivation of the procoagulant, activated factor V (FVa) and factor VIII (FVIIIa). In the FVa degradation, protein S selectively accelerates the cleavage at Arg306, having no effect on the Arg506 cleavage. In the FVIIIa inactivation, the APC-cofactor activity of protein S is synergistically potentiated by FV, which thus has the capacity to function both as a pro- and an anticoagulant protein. The SHBG-like region of protein S, containing two laminin G-type domains, is required for the combined action of protein S and FV. To elucidate whether both G domains in protein S are needed for expression of APC-cofactor activities, chimeras of human protein S were created in which the individual G domains were replaced by the corresponding domain of the homologous Gas6, which in itself has no anticoagulant activity. In a plasmabased assay, chimera I (G1 from Gas6) was as efficient as wild-type recombinant protein S, whereas chimera II (G2 from Gas6) was less effective. The synergistic cofactor activity with FV in the inactivation of FVIIIa was lost by the replacement of the G2 domain in protein S (chimera II). However, chimera I did not exert full APC-cofactor activity in the FVIIIa degradation, indicating involvement of both G domains or the entire SHBG-like region in this reaction. Chimera I was fully active in the degradation of FVa in contrast to chimera II, which exhibited reduced cofactor activity compared to protein S. In conclusion, by using protein S-Gas6 chimeric proteins, we have identified the G2 domain of protein S to be indispensable for an efficient inactivation of both FVIIIa and FVa, whereas the G1 domain was found not to be of direct importance in the FVa-inactivation experiments.

Large deletions of the PROS1 gene in a large fraction of mutation-negative patients with protein S deficiency

Protein S deficiency is an autosomal dominant disorder that results from mutations in the PROS1 gene. Conventional mutation detection techniques fail to detect a pathogenic PROS1 mutation in approximately 50% of cases. The present study investigates whether large deletions of PROS1 are found in families where mutations in the PROS1 gene have not been found despite sequencing. For this purpose, a dense set of SNP and microsatellite markers were used in segregation analysis to identify deletions. Large deletions were identified by this technique in three out of eight investigated families (38%). The deletions encompassed at least 35 kb, 437 kb and 449 kb respectively. The deletions were confirmed by quantitative PCR. Haplotype analysis showed that the three large deletions and the five other disease haplotypes were all different. All of the eight disease haplotypes co-segregated with protein S deficiency, but each of the five non-deletion haplotypes were present also in normal individuals. In conclusion: Large deletions of PROS1 are relatively common in protein S deficiency patients and screening for large deletions in PROS1 mutation-negative individuals are therefore warranted.

Analytical Considerations for Free Protein S Assays in Protein S Deficiency

Protein S is an anticoagulant protein that circulates in plasma in complex with C4b-binding protein (C4BP) or in free form. Deficiency of protein S increases the risk of venous thrombosis. Measurement of free protein S, as compared to total levels, has been shown to be superior for prediction of protein S deficiency. We studied the effects of different handling protocols for an immuno- and a ligand (C4BP)-based assay for free protein S. When the assay was performed at 37° C, the levels of free protein S in plasma from protein S deficient patients were approximately twice those obtained at room temperature. The reason for this phenomenon was that plasmas from protein S deficient patients exhibited a time-, temperature-, and dilution-dependent increase in free protein S, which was more pronounced than corresponding dilution of the normal plasma that was used to create the standard curve. These findings demonstrate the importance of assay procedure and sample handling in assays for free protein S.

Involvement of Lys 62[217] and Lys 63[218] of Human Anticoagulant Protein C in Heparin Stimulation of Inhibition by the Protein C Inhibitor

Inhibition of activated protein C (APC) by protein C inhibitor (PCI) is stimulated by heparin, whereas inhibition by α1-antitrypsin (AAT) is heparin-independent. Three lysine residues located in a positively charged cluster in the serine protease domain of protein C (PC) were mutated to probe their involvement in the heparin stimulation of inhibition by PCI. These mutations were selected after analysis of the three-dimensional structure of APC and of molecular models for PCI and the APC-PCI complex. A double mutant, K62[217]N/K63[218]D, a single mutant, K86[241]S, and wild-type PC were expressed in embryonic human kidney 293 cells. Heparin stimulated the rate of inhibition of wt-APC by PCI approximately 400-fold, with second order rate constants (k 2 ) in the absence and presence of heparin of 0.72 × 103 M–1s–1 and 2.87 × 105 M–1s–1, respectively. In contrast, heparin only yielded a 52-fold stimulation of the rate of inhibition of the double mutant APC by PCI as the rate constants in the absence and presence of heparin were k 2 = 2.44 × 103 M–1s–1 and k 2 = 1.26 × 105 M–1s–1, respectively. The double mutant K62N/K63D eluted at approximately 10% lower NaCl concentration from a heparin Sepharose column than the K86S mutant or wt-APC. These data suggest K62 and K63 in APC to be part of a heparin binding site which is important for heparin-mediated stimulation of inhibition of APC by PCI.

Abbreviations: APC, activated protein C; PC, protein C; PCI, protein C inhibitor, AAT, α1-antitrypsin also called α1-proteinase inhibitor, AT, antithrombin; TM, thrombomodulin. The chymotrypsinogen nomenclature for APC (1) is used in the text while the PC numbering is indicated between brackets whenever appropriate. P1, P2... and P1’, P2’.. designate inhibitor residues amino- and carboxy-terminal to the scissile peptide bond, respectively, and S1, S2.. and S1’, S2’.. the corresponding subsites of the protease (2). The antitrypsin numbering (3) for PCI is used along this article while the PCI numbering is mentioned between brackets whenever appropriate.

In Vitro Characterisation of Two Naturally Occurring Mutations in the Thrombin-sensitive Region of Anticoagulant Protein S

The molecular consequences of two naturally occurring mutations in the thrombin-sensitive region of protein S were investigated using a combination of recombinant protein expression, functional analysis and molecular modelling. Both mutations (R49H and R70S) have been found in thrombosis patients diagnosed as having type I protein S deficiency. Molecular modelling analysis suggested the R49H substitution not to disrupt the structure of thrombin-sensitive region, whereas the R70S substitution could affect the 3D structure mildly. To elucidate the molecular consequences of these substitutions experimentally, site directed mutagenesis of protein S cDNA and expression in mammalian cells created the two mutants. The secretion profiles and functional anticoagulant activities of the protein S mutants were characterised. Secretion of the R49H mutant was similar to that of wild type protein S, whereas the R70S mutant showed moderately decreased expression. Neither of the mutants showed any major functional defects as cofactors to activated protein C (APC) in an APTT-based assay or in degradation of factor Va. However, both mutants demonstrated decreased activity in a factor VIIIa degradation assay, which in addition to APC and protein S also included factor V as synergistic APC cofactor. In conclusion, the R49H substitution did not produce a quantitative abnormality in vitro, raising doubts as to whether it caused the type I deficiency. In contrast, the experimental data obtained for the R70S mutant agrees well with the observed type I deficiency. Our study illustrates that in vitro experimental characterisation together with computer-based structural analysis are useful tools in the analysis of the relationship between naturally occurring mutations and clinical phenotypes.

Multicentre Evaluation of IL Test™ Free PS: A Fully Automated Assay to Quantify Free Protein S

Deficiency of the anticoagulant vitamin K-dependent protein S (PS) is associated with increased risk of venous thrombosis. In human plasma, PS circulates in two forms: as free protein (free PS) and PS bound to C4b-binding protein (C4BP), a regulator of the complement system. Assays for free PS have higher sensitivity and specificity for protein S deficiency than assays for total protein S. We have extensively evaluated the analytical performance of a novel assay for free PS, the IL Test™ Free Protein S, which takes advantage of the affinity of C4BP for free PS, and compared its performance to existing methods. IL Test™ Free Protein S is a rapid, fully automated turbidimetric assay consisting of two reagents: a C4BP coated latex and an anti-PS monoclonal antibody coated latex. The test range, precision and linearity were adequate and the assay tolerated high concentrations of interfering substances of clinical significance. The reference range agreed with previously published studies. The analysis of 903 patient samples belonging to 20 different clinical categories with the new assay yielded free PS results that agreed well with those obtained using the assays established in the participating laboratories. The study demonstrated the IL Test™ Free Protein S to be rapid, reliable and easy to perform.

Factor V Q506 (Resistance to Activated Protein C) and Prognosis after Acute Coronary Syndrome

Factor V:Q506 causing resistance to activated protein C (APC-resistance), is a risk factor for venous thrombosis. Some studies have indicated an association with arterial disease, especially in women. We investigated the prevalence of the FV:Q506 allele prospectively in 295 patients with acute coronary syndrome. Mortality and myocardial infarction rate were evaluated after 30 days and after 2 years. The FV:Q506 allele was found in 38 patients. In a Cox proportional hazards model, smokers carrying FV:Q506 had a higher risk of infarction or death within 30 days, compared to non-smokers with a normal genotype (relative risk 2.9 [95% CI 1.2-7.0]). The difference remained significant after 2 years (relative risk 2.8 [95% CI 1.2-6.5]). The effect of the FV:Q506 allele on clinical outcome in acute coronary syndrome has not previously been described. Our results demonstrate a gene-environment interaction between smoking and the FV:Q506 allele, with an increased risk of early complications after an acute ischemic event.

Activated Protein C Resistance (FV:Q506) and Pregnancy

Activated protein C (APC) resistance, due to a point mutation in the factor V gene (FV:Q506), is a major risk factor for venous thromboembolism. To determine the prevalence of APC resistance in a large series of pregnant women, and to elucidate its obstetric consequences, we performed a prospective study in Malmö, Sweden, comprising 2,480 women enrolled in early pregnancy. The presence of APC resistance (the FV:Q506 allele) was determined. The women were interviewed about their medical histories including venous thromboembolic events (VTE) in relatives. The outcome variables were the VTE rate, intrapartum blood loss, and the prevalence of selected pregnancy complications such as fetal loss, pre-eclampsia, and intrauterine growth retardation.

The overall prevalence of APC resistance was 11% (270/2480). The APC-resistant subgroup did not differ significantly from the non-APC-resistant subgroup in terms of pregnancy complications, but was characterized by an 8-fold higher risk of VTE (3/270 vs. 3/2210), a lower rate of profuse intrapartum haemorrhage (3.7% vs. 7.9%) (p = 0.02), and less intrapartum blood loss (340 ml vs. 361 ml) (p = 0.04). Despite the high prevalence of APC resistance in this series of gravidae (11%), its presence was unrelated to adverse pregnancy outcome apart from an 8-fold increased risk of VTE.

Factor V Q506 Mutation (Activated Protein C Resistance) Associated with Reduced Intrapartum Blood Loss – a Possible Evolutionary Selection Mechanism

Objectives. To ascertain whether relationship exists between the presence of APC resistance [a hypercoagulable state due to a mutation (R506Q) in the factor V gene] and the occurrence of pre-eclampsia (PE), intrauterine growth retardation (IUGR), and pregnancy bleeding complications. Design. A retrospective study. Subjects. A study group of 122 women with PE and/or IUGR during a recent pregnancy and a control group of 465 healthy pregnant women. Results. A significantly reduced risk of intrapartum bleeding complications in the APC-resistant subgroup as compared to the non-APC-resistant subgroup was suggested by reduced intrapartum blood loss, and pre- and postpartum haemoglobin measurements. The prevalence of APC resistance in the PE and IUGR subgroups did not differ significantly from that in the control group. Conclusion. The remarkably high prevalence of the potentially harmful factor V gene mutation in the general population may be the result of an evolutionary selection mechanism conferring such survival advantages as reduction in the risk of intrapartum bleeding on carriers of the FV:Q506 allele.

Cleavage Requirements of Factor V in Tissue-factor Induced Thrombin Generation

Factor V (FV) activation is the result of cleavages at Arg709, Arg1018 and Arg1545 by thrombin or FXa. The relative importance of these cleavages in tissue factor (TF) induced thrombin generation in plasma and in a purified system was elucidated with recombinant FV in which the three sites had been eliminated one by one or in combinations. The mutants were analyzed with a clotting assay using FV-deficient plasma and in a TF induced thrombin generation system using plasma or purified components. Surprisingly, in the standard FV clotting assay, all mutants gave similar clotting activities and the thrombin generation curves obtained with wild-type and thrombin-resistant FV were similar. Differences in clotting activities and thrombin generation patterns between wild-type and thrombin-resistant FV were only observed when lower TF concentrations were used. The thrombin generation curve obtained in plasma containing wt FV was characterized by a short lag phase and a subsequent phase of rapid thrombin generation (propagation phase). The Arg709 to Gln mutation yielded a slightly prolonged lag phase and the rate of thrombin generation during the propagation phase was approximately 5-fold lower than that observed with wt FV. The Arg1018 to Ile mutation only slightly affected the thrombin generation curve, whereas the Arg1545 to Gln mutation yielded a prolonged lag phase and decreased maximum thrombin activity. Thrombin-resistant FV (mutated at all three sites) yielded a prolonged lag phase and poor thrombin generation during the propagation phase. The purified system further demonstrated the importance of the three cleavage sites for rapid and sustained thrombin generation. The results demonstrate that cleavages at positions 709, 1018 and 1545 are not required for assembly of a FXa-FV complex expressing low but significant prothrombinase activity but that all three sites in different ways are important for the creation of a FVa which maximally supports the FXa-mediated activation of prothrombin.

A New Direct, Fast and Quantitative Enzyme-linked Ligandsorbent Assay for Measurement of Free Protein S Antigen

A new method to determine the concentration of free protein S in plasma is described. It is an enzyme-linked ligandsorbent assay (ELSA) which utilises the protein S binding capacity of the natural ligand C4b-binding protein (C4BP) to capture the free protein S from plasma samples. The use of C4BP as ligand in the assay is possible due to the high affinity (Kd = 0.1 nM) of the interaction between protein S and C4BP and to a slow rate of complex dissociation. A monoclonal antibody (HPS 54) was conjugated with horseradish peroxidase and used as target antibody. This antibody recognises a Ca2+ dependent epitope in the first EGF-like domain of protein S and does not interfere with C4BP binding sites of protein S. Addition of calcium in the assay helped prevent dissociation of the C4BP-protein S-HPS 54 complex. Three different experiments demonstrated the assay to be specific for free protein S. First, near-identical dose response curves were obtained with protein S in plasma and with purified protein S. Second, addition of purified C4BP to normal plasma resulted in loss of free protein S. Third, protein S depleted plasma gave zero values and around 80% of purified protein S added to protein S depleted plasma, and approximately 70% of protein S added to protein S deficient plasma samples, was recovered with the assay. The assay is fast (involves only a single incubation step of 30 min), sensitive and the range of measurement is 3% to 200% of free protein S when plasma dilution 1 : 20 represents 100%. Intra- and inter-assay coefficients of variation at two levels were 2.3-4.3% and 5.1-7.4%, respectively. In a large protein S deficient family, the assay showed 100% sensitivity and specifity for the causative mutation. Moreover, free protein S levels in anticoagulated protein S deficient patients were completely separated from those obtained in non-anticoagulated controls. The new assay for free protein S is suitable for automation and it provides a useful means for routine clinical purposes to detect protein S deficiencies.