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

Laminin G1 residues of protein S mediate its TFPI cofactor function and are competitively regulated by C4BP

Protein S is a cofactor in the tissue factor pathway inhibitor (TFPI) anticoagulant pathway. It enhances TFPIα-mediated inhibition of factor (F)Xa activity and generation. The enhancement is dependent on a TFPIα-protein S interaction involving TFPIα Kunitz 3 and protein S laminin G-type (LG)-1. C4b binding protein (C4BP), which binds to protein S LG1, almost completely abolishes its TFPI cofactor function. However, neither the amino acids involved in TFPIα enhancement nor the mechanisms underlying the reduced TFPI cofactor function of C4BP-bound protein S are known. To screen for functionally important regions within protein S LG1, we generated 7 variants with inserted N-linked glycosylation attachment sites. Protein S D253T and Q427N/K429T displayed severely reduced TFPI cofactor function while showing normal activated protein C (APC) cofactor function and C4BP binding. Based on these results, we designed 4 protein S variants in which 4 to 6 surface-exposed charged residues were substituted for alanine. One variant, protein S K255A/E257A/D287A/R410A/K423A/E424A, exhibited either abolished or severely reduced TFPI cofactor function in plasma and FXa inhibition assays, both in the presence or absence of FV-short, but retained normal APC cofactor function and high-affinity C4BP binding. The C4BP β-chain was expressed to determine the mechanisms behind the reduced TFPI cofactor function of C4BP-bound protein S. Like C4BP-bound protein S, C4BP β-chain-bound protein S had severely reduced TFPI cofactor function. These results show that protein S Lys255, Glu257, Asp287, Arg410, Lys423, and Glu424 are critical for protein S-mediated enhancement of TFPIα and that binding of the C4BP β-chain blocks this function.

In vitro diagnostics for the medical dermatologist. Part II: Hypercoagulability tests

The skin often provides initial clues of hypercoagulability with features such as livedo reticularis, livedo racemosa, retiform purpura, necrosis, and ulcerations. Because these cutaneous manifestations are nonspecific, laboratory testing is often needed to evaluate for underlying causes of hypercoagulability. Importantly, these disorders are reported to be the most common mimicker, resulting in an erroneous diagnosis of pyoderma gangrenosum. Understanding inherent properties of, and indications for, available tests is necessary for appropriate ordering and interpretation of results. Additionally, ordering of these tests in an indiscriminate manner may lead to inaccurate results, complicating the interpretation and approach to management. This second article in this continuing medical education series summarizes information on methodology, test characteristics, and limitations of several in vitro laboratory tests used for the work up of hypercoagulability and vasculopathic disease as it pertains to dermatologic disease.

Risk prediction of recurrent venous thromboembolism: a multiple genetic risk model

A single genetic biomarker is unable to accurately predict the risk for venous thromboembolism (VTE) recurrence. We aimed to: (a) develop a multiple single nucleotide polymorphisms (SNPs) model to predict the risk of VTE recurrence and (b) validate a previously described genetic risk score (GRS) and compare its performance with the model developed in this study. Twenty-two SNPs, including established and putative SNPs associated with VTE risk, were genotyped in the Malmö thrombophilia study cohort (MATS; n = 1465, follow-up ~ 10 years) by using TaqMan PCR. Out of 22-SNPs, 12 had an association with the risk of VTE recurrence and were included for calculating GRSs. The risk of VTE recurrence was calculated by stratifying patients according to number of risk alleles. In 12-SNP GRS, patients with ≥ 7 risk alleles were associated with higher risk of VTE recurrence compared to patients having ≤ 6 risk alleles. In a simplified model (8-SNP GRS), the discriminative power of 8-SNP GRS was similar to that of 12-SNP GRS based on post-test probabilities (PP). Furthermore, 8-SNP GRS further improved the risk prediction of VTE recurrence in unprovoked VTE and male patients (PP% = 15.4 vs 8.3, 17.1 vs 7.2 and 19.0 vs 7.1 for high risk groups vs low risk groups in whole population, males and unprovoked VTE patients respectively). In addition, we also validated previously described 5-SNP GRS in our cohort and found that the 8-SNP GRS performed better than the 5-SNP GRS in terms of higher PP. Our results show that a multiple SNP GRS consisting of 8-SNPs may be an effective model for prediction of VTE recurrence, particularly in unprovoked VTE and male patients.

Fat mass and obesity-associated gene rs9939609 polymorphism is a potential biomarker of recurrent venous thromboembolism in male but not in female patients

Multiple genetic variations have been identified in FTO (fat mass and obesity-associated) gene. Among them, FTO rs9939609 polymorphism is shown to be associated with the risk of primary venous thromboembolism (VTE). However, its role in recurrent VTE is not known. The aim of our study was to investigate the association between FTO rs9939609 polymorphism and the risk of VTE recurrence in a prospective follow-up study in both male and female patients. FTO rs9939609 polymorphism (T/A) was analyzed in the Malmö thrombophilia study (MATS, followed for ~10 years) by using TaqMan PCR. MATS patients (n = 1050) were followed from the discontinuation of anticoagulant treatment until diagnosis of VTE recurrence or the end of follow-up. A total of 126 patients (12%) had VTE recurrence during follow-up. Cox regression analyses showed that sex modified the potential effect of FTO rs9939609 polymorphism on VTE recurrence. Male patients with the AA genotype for the FTO rs9939609 polymorphism had significantly higher risk of VTE recurrence as compared to the TT or AT genotypes (univariate hazard ratio [HR] = 2.05, 95% confidence interval [CI] = 1.2-3.5, P = 0.009 and adjusted HR = 2.03, 95% CI 1.2-3.6, P = 0.013). There was no association between FTO rs9939609 polymorphism and VTE recurrence in female patients. In conclusion, our results show that FTO rs9939609 polymorphism in recurrent VTE may differ according to gender and FTO polymorphism may predict VTE recurrence in male patients.

Association between TLR9 rs5743836 polymorphism and risk of recurrent venous thromboembolism

Recent gene knockout studies on mice have shown the role of toll-like receptor 9 (TLR9) in resolution of venous thromboembolism (VTE) through sterile inflammation. However, the role of a putative functional TLR9 polymorphism (rs5743836) in risk assessment of VTE recurrence remains unknown. The aim of our study was to investigate the TLR9 rs5743836 polymorphism in VTE patients and its association with the risk of VTE recurrence. We analyzed TLR9 rs5743836 polymorphism in Malmö thrombophilia study patients; a prospective follow-up study of 1465 VTE patients by Taqman PCR. From a total of 1465 VTE patients, those who had VTE before inclusion and those who died or had VTE recurrence during anticoagulant treatment were excluded (n = 415). Cox regression analyses were performed on the remaining 1050 VTE patients, including 126 (12.5%) patients that had recurrent VTE during follow-up period. TLR9 polymorphism was significantly associated with higher risk of VTE recurrence in female patients (HR 3.46, 95% CI 1.06–11.33) independent of acquired risk factors for VTE, family history, risk of thrombophilia and deep vein thrombosis (DVT) location. Similarly, in unprovoked VTE patients, TLR9 polymorphism was significantly associated with higher risk of VTE recurrence in female patients (HR 5.94, 95% CI 1.25–28.13) after adjusting for family history, risk of thrombophilia and DVT location. No association between TLR9 polymorphism and risk of VTE recurrence was found in male patients. Our results suggest that TLR9 rs5743836 polymorphism is an independent risk factor for VTE recurrence in female patients but not in males.

Thrombomodulin gene c.1418C>T polymorphism and risk of recurrent venous thromboembolism

Thrombomodulin gene (THBD) is a critical cofactor in protein C anticoagulant system. THBD c.1418C>T polymorphism is reported to be associated with higher risk of primary venous thromboembolism (VTE) but its role in VTE recurrence is unknown. The aim of this study was to investigate the role of THBD polymorphism in VTE recurrence. THBD c.1418C>T polymorphism was genotyped by using Taqman polymerase chain reaction in a prospective population based study of 1465 consecutive objectively verified VTE patients. Uni- and multivariate Cox regression were performed for the risk assessment of VTE recurrence. Patients who had VTE before inclusion or had recurrence or died during anticoagulant treatment were excluded. Among the remaining (N = 1046) patients, 126 (12.05 %) had VTE recurrence during the follow up period (from 1998 to 2008). THBD polymorphism was not significantly associated with risk of VTE recurrence in the univariate [Hazard ratio (HR) 1.11, 95 % confidence interval (CI) 0.78-1.59, p = 0.55] as well as the multivariate analysis adjusted for age, sex and thrombophilia (HR 1.11, 95 % CI 0.78-1.59, p = 0.54). Similarly, in unprovoked first VTE (n = 614), no association was observed between THBD polymorphism and risk of VTE recurrence (HR 1.22 and 95 % CI 0.78-1.89, p = 0.38). In this prospective study, our results do not suggest a predictive role for THBD c.1418C>T polymorphism in VTE recurrence.

Identification of Genetic Aberrations in Thrombomodulin Gene in Patients With Recurrent Venous Thromboembolism

Thrombomodulin (THBD) serves as a cofactor for thrombin-mediated activation of anticoagulant protein C pathway. Genetic aberrations in THBD have been studied in arterial and venous thrombosis. However, genetic changes in THBD and their role in the risk assessment of recurrent venous thromboembolism (VTE) are not well understood. The aim of the present study was to identify the genetic aberrations in THBD and their association with the risk of VTE recurrence in a prospective population-based study. We sequenced the entire THBD gene, first in selected patients with VTE (n = 95) by Sanger sequencing and later validated those polymorphisms with minor allele frequency (MAF) ≥5% in the whole study population (n = 1465 with the follow-up period of 1998-2008) by Taqman polymerase chain reaction. In total, we identified 8 polymorphisms in THBD, and 3 polymorphisms with MAF ≥5% were further validated. No significant association between THBD polymorphisms and risk of VTE recurrence on univariate or multivariate Cox regression analysis was found (hazard ratio [HR] = 0.89, 95% confidence interval [CI] = 0.62-1.28, HR = 1.27, 95% CI = 0.88-1.85, and HR = 1.15, 95% CI = 0.80-1.66 for THBD rs1962, rs1042580, and rs3176123 polymorphisms, respectively), adjusted for family history, acquired risk factors for VTE, location of deep vein thrombosis, and risk of thrombophilia. Subanalysis of patients with unprovoked first VTE also showed no significant association of identified THBD polymorphisms with the risk of VTE recurrence. Our results show that aberrations in the THBD gene may not be useful for the assessment of VTE recurrence; however, further studies with large sample size are needed to confirm these findings.

Identification of polymorphisms in Apolipoprotein M gene and their relationship with risk of recurrent venous thromboembolism

Apolipoprotein M (ApoM) plasma levels have been reported to be associated with risk of venous thromboembolism (VTE) recurrence. However, the role of genetic alterations in the ApoM gene in VTE recurrence remains unknown. The aim of this study was to identify genetic aberrations in ApoM gene in VTE recurrence and their role in prediction of VTE recurrence in a prospective follow-up study of 1465 VTE patients. During follow-up, 156 (10.6 %) patients had VTE recurrence. First screening of whole ApoM gene was performed by Sanger's sequencing in selected age and sex matched non-recurrent and recurrent patients (n=95). In total six polymorphisms were identified and two polymorphisms (rs805297 and rs9404941) with minor allele frequency (MAF) ≥5 % were further genotyped in the whole cohort by Taqman PCR. ApoM rs805297 polymorphism was significantly associated with higher risk of VTE recurrence in males but not in females on both univariate (p= 0.038, hazard ratio = 1.72, confidence interval = 1.03-2.88) and on multivariate analysis adjusted with mild and severe thrombophilia, family history, location and acquired risk factors for VTE. However, ApoM rs9404941 polymorphism showed no significant association with risk of VTE recurrence in all patients as well as in different gender groups. Moreover, ApoM rs805297 and rs9404941 polymorphisms were not associated with the ApoM plasma levels. In conclusion, for the first time we have sequenced whole ApoM gene in VTE and identified six polymorphisms. ApoM rs805297 was significantly associated with higher risk of VTE recurrence in male but not in female patients.

Pro- and anticoagulant properties of factor V in pathogenesis of thrombosis and bleeding disorders

Factor V (FV) serves an important role in the regulation of blood coagulation, having both pro- and anticoagulant properties. The circulating high molecular weight single-chain FV molecule undergoes a series of proteolytic cleavages during both activation of coagulation and during anticoagulant regulation of coagulation by activated protein C (APC). It is noteworthy that mutations in the factor V gene (F5) either cause thrombosis or bleeding. New insights into the importance and complexity of FV functions have been generated from elucidation of the pathogenic mechanisms of two familial mutations in the F5 gene. The first mutation was identified as a result of the discovery of APC resistance as the most common risk factor for venous thrombosis. The mutation (FV Leiden) predicts the Arg(506) Gln replacement, which impairs the normal regulation of FVa by APC, as the Arg506 site is an important APC cleavage site. In addition, elucidation of APC resistance resulted in the discovery of the anticoagulant APC cofactor activity of FV. The second FV mutation (FV(A2440G) ), identified in a family with an autosomal dominant bleeding disorder, has led to the discovery of an alternative splicing generating a previously unidentified FV isoform (FV-Short), which inhibits coagulation via an unexpected and intriguing mechanism involving the coagulation inhibitor TFPI-α. These are naturally occurring mutations in the F5 gene that have generated new knowledge on the role of FV in regulation of coagulation and the importance of genetic risk factors for thrombosis and bleeding.

Plasminogen activator inhibitor-1 4G/5G polymorphism, factor V Leiden, prothrombin mutations and the risk of VTE recurrence

Plasminogen-activator inhibitor (PAI)-1 is an important inhibitor of the plasminogen/plasmin system. PAI-1 levels are influenced by the 4G/5G polymorphism in the PAI-1 promoter. We investigated the relationship between the PAI-1 polymorphism and VTE recurrence, and its possible modification by factor V Leiden (FVL) and prothrombin (PTM) mutations. Patients (n=1,069) from the Malmö Thrombophilia Study were followed from discontinuation of anticoagulant treatment until diagnosis of VTE recurrence or the end of the study (maximum follow-up 9.8 years). One hundred twenty-seven patients (11.9 %) had VTE recurrence. PAI-1 was genotyped by TaqMan PCR. Cox regression analysis adjusted for age, sex and acquired risk factors of VTE showed no evidence of an association between PAI-1 genotype and risk of VTE recurrence in the study population as a whole. However, by including an interaction term in the analysis we showed that FVL but not PTM modified the effect of PAI-1 genotype: patients with the 4G allele plus FVL had a higher risk of VTE recurrence [hazard ratio (HR) =2.3, 95 % confidence interval (CI) =1.5-3.3] compared to patients with the 4G allele but no FVL (reference group) or FVL irrespective of PAI-1 genotype (HR=1.8, 95 % CI=1.3-2.5). Compared to reference group, 5G allele irrespective of FVL was associated with lower risk of VTE recurrence only when compared with 4G allele together with FVL. In conclusion, FVL has a modifying effect on PAI-1 polymorphism in relation to risk of VTE recurrence. The role of PAI-1 polymorphism as a risk factor of recurrent VTE may be FVL dependent.

Amino acid residues in the laminin G domains of protein S involved in tissue factor pathway inhibitor interaction

Protein S functions as a cofactor for tissue factor pathway inhibitor (TFPI) and activated protein C (APC). The sex hormone binding globulin (SHBG)-like region of protein S, consisting of two laminin G-like domains (LG1 and LG2), contains the binding site for C4b-binding protein (C4BP) and TFPI. Furthermore, the LG-domains are essential for the TFPI-cofactor function and for expression of full APC-cofactor function. The aim of the current study was to localise functionally important interaction sites in the protein S LG-domains using amino acid substitutions. Four protein S variants were created in which clusters of surface-exposed amino acid residues within the LG-domains were substituted. All variants bound normally to C4BP and were fully functional as cofactors for APC in plasma and in pure component assays. Two variants, SHBG2 (E612A, I614A, F265A, V393A, H453A), involving residues from both LG-domains, and SHBG3 (K317A, I330A, V336A, D365A) where residues in LG1 were substituted, showed 50-60 % reduction in enhancement of TFPI in FXa inhibition assays. For SHBG3 the decreased TFPI cofactor function was confirmed in plasma based thrombin generation assays. Both SHBG variants bound to TFPI with decreased affinity in surface plasmon resonance experiments. The TFPI Kunitz 3 domain is known to contain the interaction site for protein S. Using in silico analysis and protein docking exercises, preliminary models of the protein S SHBG/TFPI Kunitz domain 3 complex were created. Based on a combination of experimental and in silico data we propose a binding site for TFPI on protein S, involving both LG-domains.

Genome-wide linkage scan in affected sibling pairs identifies novel susceptibility region for venous thromboembolism: Genetics In Familial Thrombosis study

BACKGROUND

Venous thromboembolism (VTE) is a multicausal disorder involving environmental and genetic risk factors. In many thrombophilic families the clustering of thrombotic events cannot be explained by known genetic risk factors, indicating that some remain to be discovered.

OBJECTIVES

We aimed to identify novel thrombosis susceptibility alleles in a large panel of small thrombophilic families: the Genetics In Familial Thrombosis (GIFT) study.

PATIENTS/METHODS

In the GIFT study, 201 families were recruited consisting of 438 siblings with an objectively confirmed VTE at a young age. Multipoint linkage analysis (402 SSR markers) and fine mapping were performed, followed by genotyping of tagging SNPs in positional candidate genes.

RESULTS

Established genetic risk factors such as factor V Leiden, ABO blood group non-O, prothrombin 20210A, fibrinogen gamma 10034T and deficiencies of antithrombin, protein C and protein S were more frequent in GIFT patients than in unselected VTE patients. Linkage supported the presence of novel thrombosis susceptibility loci on 7p21.3-22.2 (LOD score = 3.23) and Xq24-27.3 (LOD score = 1.95). Simulation analysis showed that the chr7 signal was genome-wide statistically significant (P = 0.022). Tagging SNPs (n = 157) in eight positional candidate genes (LOD drop 1.5 regions) were genotyped in GIFT patients and 332 healthy controls. Five chr7 SNPs associated with VTE. SNP THSD7A rs2074597 was responsible for part of the chr7 signal.

CONCLUSIONS

The GIFT panel is rich in established genetic risk factors for VTE, but genetic factors remain unidentified in many families. Genome-wide linkage failed to identify the previously established genetic risk factors for VTE, but identified a novel VTE susceptibility locus on chr7.

The effect of different hormonal contraceptives on plasma levels of free protein S and free TFPI

Use of combined oral contraceptives is associated with a three- to six-fold increased risk of venous thrombosis. Hormonal contraceptives induce acquired resistance to activated protein C (APC), which predicts the risk of venous thrombosis. The biological basis of the acquired APC resistance is unknown. Free protein S (PS) and free tissue factor pathway inhibitor (TFPI) are the two main determinants of APC. Our objective was to assess the effect of both hormonal and non-hormonal contraceptives with different routes of administration on free TFPI and free PS levels. We conducted an observational study in 243 users of different contraceptives and measured APC sensitivity ratios (nAPCsr), free TFPI and free PS levels. Users of contraceptives with the highest risk of venous thrombosis as reported in recent literature, had the lowest free TFPI and free PS levels, and vice versa, women who used contraceptives with the lowest risk of venous thrombosis had the highest free TFPI and free PS levels. An association was observed between levels of free TFPI and nAPCsr, and between free PS and nAPCsr. The effect of oral contraceptives on TFPI and PS is a possible explanation for the increased risk of venous thrombosis associated with oral contraceptives.

[Congenital protein S deficiencies; diagnostic difficulties]

Protein S is a physiologic inhibitor of coagulation acting as a cofactor of activated protein C (APC) that inhibits factor Va and VIII. Approximately 60% of PS is bound to C4bBP, a protein of the complement system and only the free PS has a cofactor PCa role. Congenital PS deficiencies are diagnosed by immunologic dosage of free and total PS and functional assay evaluating APC cofactor activity. However, it has been demonstrated a direct anticoagulant activity of free PS, non-dependant of APC on the cascade coagulation and even PS bound to C4bBP seems to have anticoagulant properties. So, it appears that functional assays available estimate only a part of PS anticoagulant activities and, in addition, many interferences are reported with these tests (lupus anticoagulant, factor V Leiden, factor VIII excess...). Immunologic dosages are more reliable in spite of rare qualitative PS deficiencies that could be non-diagnosed. PS deficiencies are often difficult to diagnose because of an overlapping between normal and pathological values. Familial studies are necessary to prove the hereditary origin because there are several causes of acquired and sometimes persistent PS deficiencies (liver insufficiency, vitamin K absence, hormonal therapy in women, PS auto immune deficiency). About 200 different mutations were retrieved and, therefore, molecular studies are not of current practice. It is recommended currently to measure in first intention the free PS, if possible in association with PCa cofactor activity.

Different effects of oral contraceptives containing different progestogens on protein S and tissue factor pathway inhibitor

BACKGROUND

Oral contraceptives (OC) containing different types of progestogens induce different sensitivities to activated protein C (APC) measured with the thrombin generation-based APC-resistance test. These differences in APC resistance may be the biological explanation for the differences in thrombotic risk of the various pills. The mechanistic basis of APC resistance observed in OC users is unknown. Our objective was to study the effect of OC on the two main determinants of the APC-resistance test, free protein S and free tissue factor pathway inhibitor (TFPI).

PATIENTS/METHODS

We measured free protein S and free TFPI in 156 users of various types of OC.

RESULTS

Users of desogestrel-containing OC, known to double the risk of thrombosis compared with levonorgestrel-containing OC, had lower free protein S (24 vs. 33 U dL(-1)) and TFPI free antigen (2.9 vs. 3.6 ng mL(-1)) levels than users of OC containing levonorgestrel. Women using cyproterone acetate-containing OC, known to confer a high thrombotic risk, had the lowest free protein S (19 U dL(-1)) and free TPFI antigen (2.5 ng mL(-1)) levels. Users of OC containing drospirenone had lower free protein S (23 U dL(-1)) and TFPI antigen levels (3.2 ng mL(-1)) than users of levonorgestrel-containing OC. Low free protein S and low free TFPI antigen levels were associated with an increased resistance to APC, an established risk factor for thrombosis.

CONCLUSIONS

This study observed that the differences in APC resistance induced by OC containing different progestogens can at least in part be explained by different effects of OC on free protein S and TFPI.

Prediction of maternal complications and adverse infant outcome at admission for temporizing management of early-onset severe hypertensive disorders of pregnancy

OBJECTIVE

We explored the association between clinical parameters at admission and the subsequent development of major maternal complications or adverse infant outcome in women with hypertensive complications of pregnancy remote from term.

STUDY DESIGN

We drew data from a randomized trial of temporizing management in 216 patients with hemolysis, elevated liver enzymes, and low platelets syndrome; severe preeclampsia; eclampsia; or hypertension-related fetal growth restriction and gestational ages between 24 and 34 completed weeks. End points were adverse infant outcome (perinatal death, severe morbidity) and major maternal complications (major morbidity; recurrent and newly acquired hemolysis, elevated liver enzymes, and low platelets; eclampsia) after admission. End point prevalences were comparable between the treatment and control groups. The association with age, parity, ethnicity, body mass index, gestational age, estimated fetal weight, blood pressure, antihypertensive medication, pulse rate, hemoglobin concentration, admitting center, diagnosis at inclusion, chronic hypertension, and thrombophilia was explored by logistic regression analysis.

RESULTS

Adverse infant outcome was predominantly influenced by gestational age (odds ratio 0.4 per week increment). Major maternal complications were correlated to multiparity (odds ratio 0.4) and estimated fetal weight (odds ratio 0.9 per 100-g increment).

CONCLUSION

Prediction at admission of the clinical course of the disease and the development of additional maternal complications was not feasible.

Effect of oral and transdermal estrogen replacement therapy on hemostatic variables associated with venous thrombosis: a randomized, placebo-controlled study in postmenopausal women

OBJECTIVE

The purpose of this study was to investigate whether the effect of transdermal estrogen therapy in postmenopausal women differs from that of oral therapy with regard to resistance to activated protein C (APC), an important risk factor for venous thrombosis, and levels of related proteins, such as protein S, protein C, and prothrombin.

METHODS AND RESULTS

In a randomized, double-blind, placebo-controlled study, 152 healthy hysterectomized postmenopausal women received daily either placebo (n=49), transdermal 17beta-estradiol (E2) 50 microg (tE2 group, n=33), oral E2 1 mg (oE2 group, n=37), or oral E2 1 mg combined with gestodene 25 microg (oE2+G group, n=33) for 13 28-day treatment cycles, followed by 4 cycles of placebo for each group. Plasma samples were collected at baseline and in cycles 4, 13, and 17. In cycle 13, significant increases versus baseline and placebo were found in normalized APC sensitivity ratios (nAPCsr) in all treated groups (tE2, +26.9%; oE2, +102.7%; oE2+G, +69.9%). Increases in nAPCsr were significantly higher in the oral treatment groups than in the tE2 group. In addition, compared with baseline and placebo, after 13 cycles, decreases were observed in total protein S (tE2, -4.1%; oE2, -7.9%; oE2+G, -5.8%), free protein S (tE2, -7.1%; oE2, -8.4%; oE2+G, -5.2%), and protein C in the oE2+G group (-6.4%), but these changes did not explain the increase in nAPCsr. Changes in prothrombin were small and also did not affect the nAPCsr.

CONCLUSIONS

Increases were observed in resistance to APC, which were more pronounced in the oral treatment groups than in the transdermal group. The increase in resistance to APC was not explained by changes in protein S, protein C, or prothrombin and may contribute to the increased incidence of venous thrombosis in users of hormone replacement therapy.

Cleaved protein S (PS), total PS, free PS, and activated protein C cofactor activity as risk factors for venous thromboembolism

BACKGROUND

Although hereditary protein S (PS) deficiency is clearly associated with venous thromboembolism (VTE), the importance of low PS concentrations as a risk factor for VTE in other patients is still a matter of debate. To clarify this issue, we designed a case-control study to evaluate the role of different molecular forms of plasma PS.

METHODS

We quantified plasma cleaved, total, and free PS and activated protein C (APC) cofactor activity in 87 VTE patients and 174 controls matched for age, sex, and hormonal treatment. Free PS was measured by ELISA or by enzyme-linked ligand sorbent assay (ELSA). Cleaved and total PS were measured by ELISA.

RESULTS

In controls, the mean (SD) concentration of circulating cleaved PS was 39 (14) nmol/L, corresponding to 10% (3.5%) of total PS. Concentrations of cleaved PS and total PS were not significantly different in patients with VTE compared with controls. However, in our population, low free PS measured by ELISA or ELSA, as well as APC cofactor activity values were significantly associated with VTE with odds ratios (95% confidence intervals) of 2.9 (1.3-6.3), 2.5 (1.1-5.6), and 2.9 (1.3-6.4), respectively, in multivariate analyses.

CONCLUSION

Phenotypic low PS detected by APC cofactor activity assay or by an assay specific for free PS should be considered a risk factor for VTE.

A review of the technical, diagnostic, and epidemiologic considerations for protein S assays

OBJECTIVES

To review the state of the art relating to protein S deficiency as a risk factor for thrombosis and to make recommendations regarding the use of protein S measurements in the assessment of thrombotic risk in individual patients and families.

DATA SOURCES, EXTRACTION, AND SYNTHESIS

Selection criteria were developed for the inclusion of publications from 1985 to 2001 based on the relevant literature concerned with the systematic review of diagnostic tests. Minimal selection criteria were agreed on and the articles stratified into level 1 if they met these criteria and level 2 if they did not meet these criteria. The included articles were reviewed by the authors and abstracted onto predetermined data collection forms. These forms were then scored and recommendations based on level 1 studies. As described elsewhere, results of discussions at the College of American Pathologists Conference XXXVI on Diagnostic Issues in Thrombophilia were used to revise the manuscript into its final form.

CONCLUSIONS

Consensus was reached on 16 recommendations for the use of protein S assays in the assessment of thrombotic risk in individuals and families. Two themes run through the conclusions. First, protein S assays are the most technically problematic of the assays reviewed at this conference. Second, only 2 papers evaluating the diagnostic use of protein S assays met our level 1 inclusion criteria. These 2 problems point out the need for better standardized assays and rigorous studies of the diagnostic utility of these assays.

Dynamic equilibrium between protein S and C4b binding protein is important for accurate determination of free protein S antigen

Protein S in circulation is in a dynamic equilibrium with C4b binding protein (C4bBP), thus affecting the measurement of free protein S antigen. We addressed the issue of overestimation of the free protein S concentration with current immunoassays due to the dynamic equilibrium and propose a new method for its accurate determination. Our assay system was tested at different reaction temperatures using purified free protein S, protein S-C4bBP complexes, plasma samples, and a commercially available free protein S assay kit. At a reaction temperature of 37 degrees C, the free protein S fraction increased from 0.5 ng/ml (at 4 degrees C) to 7.8 ng/ml, and from 4.5 ng/ml (at 4 degrees C) to 56 ng/ml when the concentration of the assayed protein S-C4bBP complexes was 20 ng/ml and 200 ng/ml, respectively. In plasma samples, free protein S levels were approximately 0.8 microg/ml and 6 pg/ml higher at 25 degrees C and 37 degrees C, respectively compared to measurements at 4 degrees C. Measurements of free protein S in plasma using a commercially available assay kit were approximately 0.6 microg/ml higher at 25 degrees C than measurements performed at 4 degrees C. Dynamic equilibrium between protein S and C4bBP affects the measurement of free protein S antigen. Measurement of free protein S antigen should be performed under conditions where protein S is not dissociated from protein S-C4bBP complexes, as exemplified by assay at low temperature (4 degrees C).

Structural requirements of anticoagulant protein S for its binding to the complement regulator C4b-binding protein

The vitamin K-dependent anticoagulant protein S binds with high affinity to C4b-binding protein (C4BP), a regulator of complement. Despite the physiological importance of the complex, we have only a patchy view of the C4BP-binding site in protein S. Based on phage display experiments, protein S residues 447-460 were suggested to form part of the binding site. Several experimental approaches were now used to further elucidate the structural requirements for protein S binding to C4BP. Peptides comprising residues 447-460, 451-460, or 453-460 of protein S were found to inhibit the protein S-C4BP interaction, whereas deletion of residues 459-460 from the peptide caused complete loss of inhibition. In recombinant protein S, each of residues 447-460 was mutated to Ala, and the protein S variants were tested for binding to C4BP. The Y456A mutation reduced binding to C4BP approximately 10-fold, and a peptide corresponding to residues 447-460 of this mutant was less inhibitory than the parent peptide. A further decrease in binding was observed using a recombinant variant in which a site for N-linked glycosylation was moved from position 458 to 456 (Y456N/N458T). A monoclonal antibody (HPSf) selective for free protein S reacted poorly with the Y456A variant but reacted efficiently with the other variants. A second antibody, HPS 34, which partially inhibited the protein S-C4BP interaction, reacted poorly with several of the Ala mutants, suggesting that its epitope was located in the 451-460 region. Phage display analysis of the HPS 34 antibody further identified this region as its epitope. Taken together, our results suggest that residues 453-460 of protein S form part of a more complex binding site for C4BP. A recently developed three-dimensional model of the sex hormone-binding globulin-like region of protein S was used to analyze available experimental data.

Laboratory Investigation of Thrombophilia

Until recently, laboratory diagnosis of thrombophilia was based on investigation of the plasmatic anticoagulant pathways to detect antithrombin, protein C, and protein S deficiencies and on the search for dysfibrinogenemia and anti-phospholipid antibodies/lupus anticoagulants. More recently, laboratory investigations have been expanded to include activated protein C (APC) resistance, attributable or not to the presence of the factor V Leiden mutation; hyperprothrombinemia attributable to the presence of the prothrombin gene mutation G20210A; and hyperhomocysteinemia attributable to impairment of the relevant metabolic pathway because of enzymatic and/or vitamin deficiencies. All of the above are established congenital or acquired conditions associated with an increased risk of venous and, more rarely, arterial thrombosis.

Testing is recommended for patients who have a history of venous thrombosis and should be extended to their first-degree family members. Because most of the tests are not reliable during anticoagulation, it is preferable to postpone laboratory testing until after discontinuation of treatment.

Whenever possible, testing should be performed by means of functional assays. DNA analysis is required for the prothrombin gene mutation G20210A. Laboratory diagnosis for anti-phospholipid antibodies/lupus anticoagulant should be performed by a combination of tests, including phospholipid-dependent clotting assays and solid-phase anti-cardiolipin antibodies. Hyperhomocysteinemia can be diagnosed by HPLC methods or by fluorescence polarization immunoassays.

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

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa–degradation 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. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, 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 mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits.

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

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa–degradation 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. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, 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 mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits.

Both G-type domains of protein S are required for the high-affinity interaction with C4b-binding protein

Anticoagulant protein S interacts with the complement regulatory protein C4b-binding protein (C4BP) via its sex-hormone-binding globulin (SHB6)-like region, which contains two globular (G) domains. Similar G domains are found in Gas6, a protein homologous to protein S, which is not known to bind C4BP or to have any anticoagulant activity. To determine the relative importance of the two G domains in protein S for C4BP protein binding, three recombinant protein S chimeras were produced having either of the two globular domains, or the whole SHB6-like globulin region, replaced by corresponding parts from Gas6. The chimeras were tested for binding to immobilized C4BP using surface-plasmon-resonance technology and microtiter plate-based assays. In both systems, chimeras containing either only globular domains G1 or G2 from protein S were found to bind C4BP. Binding was stimulated by Ca2+ in a manner similar to that found for wild-type protein S. The affinities for C4BP of both chimeras containing individual G domains from protein S, were lower than that of wild-type protein S. Chimera II, containing the G1 domain from protein S, consistently bound C4BP more efficiently than chimera I, which had the protein S-derived G2 domain. The chimera containing the whole SHB6-like globulin region from Gas6 interacted considerably more weakly with C4BP. Our results demonstrate that both G domains of protein S are involved in the interaction between protein S and C4BP and that full affinity binding is dependent on contributions from both domains.