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

Coagulation factor and protease pathways in thrombosis and cardiovascular disease

The biochemical characterisation of the proteolytic pathways that constitute blood coagulation was one of the most relevant achievements in biomedical research during the second half of the 20th century. Understanding these pathways was of crucial importance for improving global health through application in haemostasis and thrombosis pathologies. Immediately after the cloning of the genes corresponding to these proteins, mutations were discovered in them that were associated with imbalances in haemostasis. Later, the importance of coagulation pathways in other pathological processes was demonstrated, such as in atherosclerosis and inflammation, both essential processes involved in vascular disease. In the present review we evaluate the concepts that have allowed us to reach the integrated vision on coagulation that we have today. The thrombo-inflammation model encompassing these aspects includes a pivotal role for the proteases of the coagulation pathway as well as the regulatory proteins thereof. These concepts illustrate the importance of the coagulation cascade in cardiovascular pathology, not only in thrombotic processes, but also in atherosclerotic processes and in the response to ischaemia-reperfusion injury, making it a central mechanism in cardiovascular disease.

Protein S Heerlen mutation heterozygosity is associated with venous thrombosis risk

Hereditary Protein S (PS) deficiency is a rare coagulation disorder associated with an increased risk of venous thrombosis (VT). The PS Heerlen (PSH) mutation is a rare S501P mutation that was initially considered to be a neutral polymorphism. However, it has been later shown that PSH has a reduced half-life in vivo which may explain the association of PSH heterozygosity with mildly reduced levels of plasma free PS (FPS). Whether the risk of VT is increased in PSH carriers remains unknown. We analyzed the association of PSH (rs121918472 A/G) with VT in 4,173 VT patients and 5,970 healthy individuals from four independent case-control studies. Quantitative determination of FPS levels was performed in a subsample of 1257 VT patients. In the investigated populations, the AG genotype was associated with an increased VT risk of 6.57 [4.06–10.64] (p = 1.73 10⁻¹⁴). In VT patients in whom PS deficiency was excluded, plasma FPS levels were significantly lower in individuals with PSH when compared to those without [72 + 13 vs 91 + 21 UI/dL; p = 1.86 10⁻⁶, mean + SD for PSH carriers (n = 21) or controls (n = 1236) respectively]. We provide strong evidence that the rare PSH variant is associated with VT in unselected individuals.

Influence of single nucleotide polymorphisms on thrombin generation in factor V Leiden heterozygotes

Carriership of the factor V (FV) Leiden mutation increases the risk of venous thromboembolism (VTE) ~4-fold, but the individual risk of each FV Leiden carrier depends on several co-inherited risk and protective factors. Under the hypothesis that thrombin generation might serve as an intermediate phenotype to identify genetic modulators of VTE risk, we enrolled 188 FV Leiden heterozygotes (11 with VTE) and determined the following parameters: thrombin generation in the absence and presence of activated protein C (APC); plasma levels of prothrombin, factor X, antithrombin, protein S and tissue factor pathway inhibitor; and the genotypes of 24 SNPs located in the genes encoding these coagulation factors and inhibitors. Multiple regression analysis was subsequently applied to identify the (genetic) determinants of thrombin generation. The endogenous thrombin potential (ETP) showed a striking inter-individual variability among different FV Leiden carriers and, especially when measured in the presence of APC, correlated with VTE risk. Several SNPs in the F2 (rs1799963, rs3136516), F10 (rs693335), SERPINC1 (rs2227589), PROS1 (Heerlen polymorphism) and TFPI (rs5940) genes significantly affected the ETP-APC and/or the ETP+APC in FV Leiden carriers. Most of these SNPs have shown an association with VTE risk in conventional epidemiological studies, suggesting that the genetic dissection of thrombin generation leads to the detection of clinically relevant SNPs. In conclusion, we have identified several SNPs that modulate thrombin generation in FV Leiden heterozygotes. These SNPs may help explain the large variability in VTE risk observed among different FV Leiden carriers.

Protein S: A multifunctional anticoagulant vitamin K-dependent protein at the crossroads of coagulation, inflammation, angiogenesis, and cancer

Since its discovery in 1970, protein S (PS) has emerged as a key vitamin K-dependent natural anticoagulant protein at the crossroads of multiple biological processes, including coagulation, apoptosis, atherosclerosis, angiogenesis/vasculogenesis, and cancer progression. Following the binding to a unique family of protein tyrosine kinase receptors referred to as Tyro-3, Axl and Mer (TAM) receptors, PS can lead to regulation of coagulation, phagocytosis of apoptotic cells, cell survival, activation of innate immunity, vessel integrity and angiogenesis, and local invasion and metastasis. Because of these dynamics and multiple functions of PS, which are largely lost following invalidation of the mouse PROS1 gene, this molecule is currently intensively studied in biomedical research. The purpose of this review is to provide a brief chronicle of the discovery and current understanding of the mechanisms of PS signaling, and how PS and their signaling partners regulate various cellular functions, with a particular focus on TAM receptors.

Elucidating the role of carbohydrate determinants in regulating hemostasis: insights and opportunities

Recent improvement in modern analytical technologies has stimulated an explosive growth in the study of glycobiology. In turn, this has lead to a richer understanding of the crucial role of N- and O-linked carbohydrates in dictating the properties of the proteins to which they are attached and, in particular, their centrality in the control of protein synthesis, longevity, and activity. Given their importance, it is unsurprising that both gross and subtle defects in glycosylation often contribute to human disease pathology. In this review, we discuss the accumulating evidence for the significance of glycosylation in mediating the functions of the plasma glycoproteins involved in hemostasis and thrombosis. In particular, the role of naturally occurring coagulation protein glycoforms and inherited defects in carbohydrate attachment in modulating coagulation is considered. Finally, we describe the therapeutic opportunities presented by new insights into the role of attached carbohydrates in shaping coagulation protein function and the promise of carbohydrate modification in the delivery of novel therapeutic biologics with enhanced functional properties for the treatment of hemostatic disorders.

Protein S inherited qualitative deficiency: novel mutations and phenotypic influence

BACKGROUND

Only a few mutations associated with qualitative protein S deficiency have already been described. Sensitivity and specificity for type II PROS1 mutations of commercially available reagents for measuring Protein S (PS) activity are not well established. Whether these mutations are significant risk factors for thrombosis remains an unresolved question.

METHODS

In order to address the first point, we present and discuss the results of PROS1 analysis performed in the 30 probands with type II PS-inherited deficiency suspicion and 35 relatives, studied in our laboratory between 2000 and 2008. In order to investigate the influence of type II mutations on the coagulability level, thrombin generation tests were performed on plasma from 102 PROS1 type II, type I/III or PS Herleen mutation heterozygous carriers and controls.

RESULTS

Mutations (12 novel, six already described) which probably explain the qualitative phenotype, were found in 27 (90%) out of the 30 probands studied. In relatives, 78% of heterozygotes presented with a type II phenotype. An APC resistance phenotype was documented in type II and type I/III defects heterozygous carriers; however, the effect of type II was milder than the effect of type I/III PS mutations.

CONCLUSIONS

A PS functional assay (Staclot PS, Stago) was efficient in screening for PROS1 type II defects, particularly in probands. A significant positive influence of type II mutations on ex vivo thrombin generation was demonstrated. However, whether these mutations increase the risk of venous thromboembolism requires further investigation.

Molecular diagnostics in hemostatic disorders

The use of molecular diagnostic techniques in clinical and research hemostasis laboratories is increasing as genetic factors that affect the procoagulant and anticoagulant systems are identified. Many of these molecular alterations are associated with thrombotic tendencies, whereas others tip the hemostatic balance in favor of bleeding. In either scenario, molecular testing may serve as a primary diagnostic modality or may provide information that complements clot-based "functional" assays. The clinical application of DNA-based testing continues to expand since the discoveries of the factor V Leiden and prothrombin G20210A gene mutations. Indications for genetic testing continue to evolve as the underlying causes of hemostatic disorders are better understood. Further development of molecular assays depends on their proved utility in the clinical management and treatment of these complex multifactorial disorders.

Protein S deficiency: a clinical perspective

Protein S (PS) is an extensively studied protein with an important function in the down-regulation of thrombin generation. Because of the presence of a pseudogene and two different forms of PS in plasma, a bound and a free form, it is one of the most difficult thrombophilias to study. A deficiency of PS predisposes subjects to (recurrent) venous thromboembolism (VTE) and foetal loss. However, the conundrum of diagnosing PS deficiency has led to conflicting reports of PS as a risk factor for VTE. In this review, we aim to present a clinical perspective of PS deficiency.

Denaturing high-performance liquid chromatography mutation analysis in patients with reduced Protein S levels

BACKGROUND

Patients with congenital Protein S deficiency have increased risk of venous thromboembolism. However, Protein S levels show large intra-individual variation and the biochemical assays have low accuracy and a high interlaboratory variability. Genetic analysis might aid in a more precise diagnosis and risk estimation. The aim was to design a high-throughput genetic analysis based on denaturing high-performance liquid chromatography to identify sequence variations in the gene coding for Protein S.

PATIENTS

In total, 55 patients referred to the Section of Thrombosis and Haemostasis, Odense University Hospital, in the period 1998-2004 were included in the study.

RESULTS

Mutations were found in ten of the 55 patients: Six different variants were identified, of which four were not previously reported: One were a nonsense mutation substituting a glutamine with a stopcodon (c.790C>T) and the rest were missense mutations (c.932T>G; c.1367A>G; c.1378T>C). Furthermore, four patients carried the same mutation (c.1045G>A), while two carried the Heerlen mutation (c.1378T>C).

CONCLUSIONS

The reported method will be useful for rapidly detecting sequence variations in the gene coding for Protein S, giving a precise diagnosis and subsequently a better risk estimation.

The γ-carboxyglutamic acid domain of anticoagulant protein S is involved in activated protein C cofactor activity, independently of phospholipid binding

We expressed 2 chimeras between human protein S (PS) and human prothrombin (FII) in which the prothrombin γ-carboxyglutamic acid (Gla) domain replaced the PS Gla domain in native PS (GlaFII-PS) or in PS deleted of the thrombin-sensitive region (TSR) (GlaFII-ΔTSR-PS). Neither PS/FII chimera had activated protein C (APC) cofactor activity in plasma clotting assays or purified systems, but both bound efficiently to phospholipids. This pointed to a direct involvement of the PS Gla domain in APC cofactor activity through molecular interaction with APC. Using computational methods, we identified 2 opposite faces of solvent-exposed residues on the PS Gla domain (designated faces 1 and 2) as potentially involved in this interaction. Their importance was supported by functional characterization of a PS mutant in which the face 1 and face 2 PS residues were reintroduced into GlaFII-PS, leading to significant APC cofactor activity, likely through restored interaction with APC. Furthermore, by characterizing PS mutants in which PS face 1 and PS face 2 were individually replaced by the corresponding prothrombin faces, we found that face 1 was necessary for efficient phospholipid binding but that face 2 residues were not strictly required for phospholipid binding and were involved in the interaction with APC.

Molecular diversity and thrombotic risk in protein S deficiency: The PROSIT study

The Protein S Italian Team (PROSIT) enrolled 79 protein S (PS) deficient families and found 38 PROS1 variations (19 novel) in 53 probands. Of these, 23 variants were selected for expression in'vitro, to evaluate their role as possible causative variants. Transient expression showed high secretion levels (>75%) for three variants, which were considered neutral. Seven missense and five nonsense variants showed low (<or=11%) expression levels and were classified as severe defects. Intermediate expression was observed for eight variants, which were evaluated by factor Va inactivation assay in order to be globally classified as severe or intermediate. Based on the cumulative data, the hazard ratio associated with causative variants was 4.9 (95% CI: 1.4-17.7) for deep vein thrombosis and/or pulmonary embolism, 5.1 (95% CI: 1.1-23.9) for superficial thrombophlebitis, and 4.8 (95% CI: 1.8-13.0) for any venous thrombosis. The hazard ratio for deep vein thrombosis and/or pulmonary embolism in carriers of severe defects only was 7.4 (95% CI: 1.6-24.1). PROSIT showed that dysfunctional variants causing PS deficiency are more common than expected and confirmed that PS deficiency is associated with increased thrombotic risk, although risk assessment is complicated by molecular heterogeneity.

Direct anticoagulant activity of protein S-C4b binding protein complex in Heerlen heterozygotes and normals

BACKGROUND

Plasma protein S normally circulates free (40%) or complexed with C4b-binding protein (PS-C4BP); only free protein S is a cofactor for activated protein C during factor (F) Va inactivation. Protein S-Heerlen lacks a carbohydrate group, leading to low plasma free protein S levels, but normal levels of PS-C4BP.

OBJECTIVES

Because protein S-Heerlen is not associated with thrombosis, we investigated whether PS-C4BP is directly anticoagulant in plasma and whether PS-Heerlen-C4BP has enhanced direct anticoagulant activity.

METHODS

An assay for protein S direct activity was applied to Heerlen-heterozygous plasmas. Free and complexed protein S were repeatedly isolated from normal and Heerlen-heterozygous plasmas and tested for direct anticoagulant activity in prothrombinase assays and in plasma.

RESULTS

Heerlen-heterozygous plasmas were deficient in free and total protein S antigen but had normal to high protein S direct anticoagulant activity. Purified Heerlen-heterozygous PS-C4BP was 7-fold more potent than normal PS-C4BP in inhibiting full prothrombinase activity, and 22-fold more potent in inhibiting prothrombin activation in the absence of FVa; it also specifically prolonged plasma clotting times 14-fold more than normal PS-C4BP. Heerlen-heterozygous PS-C4BP did not compete for limiting phospholipids any better than normal PS-C4BP. However, ligand blots and surface plasmon resonance studies showed that Heerlen-heterozygous PS-C4BP bound more avidly to FXa than did normal PS-C4BP (apparent Kd = 4.3 nm vs. 82 nm).

CONCLUSIONS

Plasma-derived PS-C4BP has direct anticoagulant activity in plasma and in purified systems. Enhanced direct activity of PS-Heerlen-C4BP may compensate for low free protein S levels and low cofactor activity in individuals with protein S-Heerlen.

The prevalence of, and molecular defects underlying, inherited protein S deficiency in the general population

The molecular basis of protein S (PS) deficiency was investigated in seven of eight donors identified with persistently low plasma PS levels from a survey of PS levels in 3788 Scottish blood donors. PROS1 gene analysis identified at least one defect in six donors. Five were heterozygous for the Heerlen polymorphism predicting a Ser460Pro substitution. Haplotype analysis revealed the possibility that this allele was inherited with the same haplotype in four of the five donors, suggesting a founder effect for the Heerlen allele in this population. One Heerlen allele carrier was also heterozygous for a 3 bp deletion 68-72 bp upstream of exon 2. Platelet PROS1 transcript analysis showed no reduction in mRNA expression from the affected allele in this donor. A T to G transversion 3 bp upstream of exon 12 was identified in one donor, which is predicted to reduce the efficiency of PS mRNA splicing. However, PROS1 transcript analysis showed no evidence of exon skipping or cryptic splicing. No PROS1 gene defect was detected in the remaining donor. This genetic information enabled us to refine our estimate of the prevalence of heritable PS deficiency in the Scottish population to between 0.16% and 0.21%, predominantly resulting from the presence of the Heerlen allele.

Coagulation, inflammation, and apoptosis: different roles for protein S and the protein S-C4b binding protein complex

Protein S (PS) has an established role as an important cofactor to activated protein C (APC) in the degradation of coagulation cofactors Va and VIIIa. This anticoagulant role is evident from the consequences of its deficiency, when there is an increased risk of venous thromboembolism. In human plasma, PS circulates approximately 40% as free PS (FPS) and 60% in complex with C4b-binding protein (C4BP). Formation of this complex results in loss of PS cofactor function, and C4BP can then modulate the anticoagulant activity of APC. It had long been predicted that the complex could act as a bridge between coagulation and inflammation due to the involvement of C4BP in regulating complement activation. This prediction was recently supported by the demonstration of binding of the PS-C4BP complex to apoptotic cells. This review aims to summarize recent findings on the structure and functions of PS, the basis and importance of its deficiency, its interaction with C4BP, and the possible physiologic and pathologic importance of the PS-C4BP interaction.

Treatment of autoimmune hemolytic anemias

Treatment of autoimmune hemolytic anemias varies depending on whether the patient has autoimmune hemolytic anemia of warm antibody type, cold agglutinin syndrome, paroxysmal cold hemoglobinuria, or autoimmune hemolytic anemia secondary to an underlying disorder. Initial therapy for warm antibody autoimmune hemolytic anemia should be corticosteroids, such as prednisone at conventional doses of 1 to 1.5 mg/kg/d orally. Criteria must be established to determine whether the therapeutic response is adequate, because long-term therapy may lead to significant detrimental side effects. Splenectomy has the advantage over therapeutic options in that it has the potential for complete and long-term remission. The major adverse effect is the syndrome of overwhelming postsplenectomy infection. Other therapeutic options, which are less likely to have long-term benefit, are immunosuppressive drugs, danazol, intravenous immunoglobulin, and plasma exchange. Therapy of cold agglutinin syndrome often is unsatisfactory. All patients should avoid exposure to cold, and if additional therapy is necessary, the therapies used for warm antibody autoimmune hemolytic anemia may be tried with less likelihood of response. Paroxysmal cold hemoglobinuria requires aggressive supportive therapy, generally supplemented by corticosteroids. Hemolysis usually terminates spontaneously. Patients with secondary autoimmune hemolytic anemia may be treated similarly to those with idiopathic autoimmune hemolytic anemia, and additional therapy for the underlying disorder also may result in remission of the hemolysis.

What causes the antiphospholipid syndrome?

The antiphospholipid syndrome (APS) is characterized by unpredictable, sporadic, thrombotic events. The cause of the thrombosis is probably multifactorial and may involve disparate effects of the autoantibodies associated with the syndrome, which are known to interfere with various protein regulators of hemostasis. An integrated theory of pathogenesis that accounts for the diversity of autoantibodies and their effects suggests that cellular inflammation or apoptosis within the vasculature may lead to oxidation or turnover in phospholipid membranes. Thus, normally cryptic, functionally important epitopes of phospholipid-binding proteins are subjected to increased exposure to immune surveillance.