Antithrombotic potential of a single-domain antibody enhancing the activated protein C-cofactor activity of protein S

BACKGROUND

Protein S (PS) is a natural anticoagulant acting as a cofactor for activated protein C (APC) in the proteolytic inactivation of activated factors V (FVa) and VIII (FVIIIa), but also for tissue factor pathway inhibitor α (TFPIα) in the inhibition of activated factor X (FXa).

OBJECTIVE

For therapeutic purposes, we aimed at generating single-domain antibodies (sdAbs) that could specifically modulate the APC-cofactor activity of PS in vivo.

METHODS

A llama-derived immune library of sdAbs was generated and screened on recombinant human PS by phage display. PS binders were tested in a global activated partial thromboplastin time (APTT)-based APC-cofactor activity assay.

RESULTS

A PS-specific sdAb (PS003) was found to enhance the APC-cofactor activity of PS in our APTT-based assay, and this enhancing effect was greater for a bivalent form of PS003 (PS003biv). Further characterization of PS003biv demonstrated that PS003biv also enhanced the APC-cofactor activity of PS in a tissue factor (TF)-induced thrombin generation assay and stimulated APC in the inactivation of FVa, but not FVIIIa, in plasma-based assays. Furthermore, PS003biv was directed against the sex hormone-binding globulin (SHBG)-like domain but did not inhibit the binding of PS to C4b-binding protein (C4BP) and did not interfere with the TFPIα-cofactor activity of PS. In mice, PS003biv exerted an antithrombotic effect in a FeCl3 -induced thrombosis model, while not affecting physiological hemostasis in a tail-clip bleeding model.

DISCUSSION

Altogether, these results showed that pharmacological enhancement of the APC-cofactor activity of PS through an original anti-PS sdAb might constitute a promising and safe antithrombotic strategy.

Induced forms of α2-macroglobulin neutralize heparin and direct oral anticoagulant effects

Alpha₂-macroglobulin (α₂M) is a physiological macromolecule that facilitates the clearance of many proteinases, cytokines and growth factors in human. Here, we explored the effect of induced forms of α₂M on anticoagulant drugs. Gla-domainless factor Xa (GDFXa) and methylamine (MA)-induced α₂M were prepared and characterized by electrophoresis, immunonephelometry, chromogenic, clot waveform and rotational thromboelastometry assays. Samples from healthy volunteers and anticoagulated patients were included. In vivo neutralization of anticoagulants was evaluated in C57Bl/6JRj mouse bleeding-model. Anticoagulant binding sites on induced α₂M were depicted by computer-aided energy minimization modeling. GDFXa-induced α₂M neutralized dabigatran and heparins in plasma and whole blood. In mice, a single IV dose of GDFXa-induced α₂M following anticoagulant administration significantly reduced blood loss and bleeding time. Being far easier to prepare, we investigated the efficacy of MA-induced α₂M. It neutralized rivaroxaban, apixaban, dabigatran and heparins in spiked samples in a concentration-dependent manner and in samples from treated patients. Molecular docking analysis evidenced the ability of MA-induced α₂M to bind non-covalently these compounds via some deeply buried binding sites. Induced forms of α₂M have the potential to neutralize direct oral anticoagulants and heparins, and might be developed as a universal antidote in case of major bleeding or urgent surgery.

Role of oculocerebrorenal syndrome of Lowe (OCRL) protein in megakaryocyte maturation, platelet production and functions: a study in patients with Lowe syndrome

Lowe syndrome (LS) is an oculocerebrorenal syndrome of Lowe (OCRL1) genetic disorder resulting in a defect of the OCRL protein, a phosphatidylinositol-4,5-bisphosphate 5-phosphatase containing various domains including a Rho GTPase-activating protein (RhoGAP) homology domain catalytically inactive. We previously reported surgery-associated bleeding in patients with LS, suggestive of platelet dysfunction, accompanied with a mild thrombocytopenia in several patients. To decipher the role of OCRL in platelet functions and in megakaryocyte (MK) maturation, we conducted a case-control study on 15 patients with LS (NCT01314560). While all had a drastically reduced expression of OCRL, this deficiency did not affect platelet aggregability, but resulted in delayed thrombus formation on collagen under flow conditions, defective platelet spreading on fibrinogen and impaired clot retraction. We evidenced alterations of the myosin light chain phosphorylation (P-MLC), with defective Rac1 activity and, inversely, elevated active RhoA. Altered cytoskeleton dynamics was also observed in cultured patient MKs showing deficient proplatelet extension with increased P-MLC that was confirmed using control MKs transfected with OCRL-specific small interfering(si)RNA (siOCRL). Patients with LS also had an increased proportion of circulating barbell-shaped proplatelets. Our present study establishes that a deficiency of the OCRL protein results in a defective actomyosin cytoskeleton reorganisation in both MKs and platelets, altering both thrombopoiesis and some platelet responses to activation necessary to ensure haemostasis.

Genotype-Phenotype Relationships in a Large French Cohort of Subjects with Inherited Protein C Deficiency

Inherited protein C (PC) deficiency caused by mutations in the PROC gene is a well-known risk factor for venous thromboembolism. Few studies have investigated the relationship between PROC genotype and plasma or clinical phenotypes. We addressed this issue in a large retrospective cohort of 1,115 heterozygous carriers of 226 PROC pathogenic or likely pathogenic mutations. Mutations were classified in three categories according to their observed or presumed association with type I, type IIa, or type IIb PC deficiency. The study population comprised 876 carriers of type I category mutations, 55 carriers of type IIa category mutations, and 184 carriers of type IIb category mutations. PC anticoagulant activity significantly influenced risk of first venous thrombosis (p trend < 10^-4). No influence of mutation category on risk of whole or unprovoked thrombotic events was observed. Both PC anticoagulant activity and genotype significantly influenced risk of venous thrombosis. Effect of detrimental mutations on plasma phenotype was ambiguous in several carriers, whatever the mutation category. Altogether, our findings confirm that diagnosing PC inherited deficiency based on plasma measurement may be difficult but show that diagnosis can be improved by PROC genotyping.

Five Novel Mutations of the Protein S Active Gene (PROS 1) in 8 Norman Families

To further elucidate the molecular basis for hereditary thrombophilia, we screened the protein S active gene in 11 families with type I deficiency, using a strategy based on denaturating gradient gel electrophoresis (DGGE) of all the coding sequences. Fragments with an abnormal DGGE pattern were sequenced, and 5 novel mutations were identified in 8 families. The mutations were a 7-nucleotide deletion in exon II, a 4-nucleotide deletion in exon III, a T insertion in exon VII, a C to T transition transforming Leu 259 into Pro and a T to C transition transforming Cys 625 into Arg in 4 families. These mutations were the only sequence variations found in the propositus’ gene exons and co-segregated with the plasma phenotype. A total of 28 members of these 8 families were heterozygous for one of the 5 mutations. Twenty-four (58,5%) of the 41 deficient subjects over 18 years of age had clinical thrombophilia, whereas the 13 subjects under 18 were asymptomatic. Of the 28 subjects, 6 (21,5%) were also found to bear the factor V Arg 506 Gin mutation.

FXa-α2-Macroglobulin Complex Neutralizes Direct Oral Anticoagulants Targeting FXa In Vitro and In Vivo

Increasing number of patients are treated with direct oral anticoagulants (DOAC). An antidote for dabigatran inhibiting thrombin (idarucizumab) is available but no antidote is yet approved for the factor Xa (FXa) inhibitors (xabans). We hypothesized that a complex between Gla-domainless FXa and α₂-macroglobulin (GDFXa-α₂M) may neutralize the xabans without interfering with normal blood coagulation.Purified α₂M was incubated with GDFXa to form GDFXa-α₂M. Affinity of apixaban and rivaroxaban for GDFXa-α₂M was only slightly decreased compared to FXa. Efficacy and harmlessness of GDFXa-α₂M were tested in vitro and in vivo. Stoichiometric excess of GDFXa-α₂M neutralized rivaroxaban and apixaban as attested by clot waveform assay and rotational thromboelastometry, whereas GDFXa-α₂M alone had no effect on these assays. Efficacy and pro-thrombotic potential of GDFXa-α₂M were also assessed in vivo. Half-life of GDFXa-α₂M in C57BL6 mice was 4.9 ± 1.1 minutes, but a 0.5 mg/mouse dose resulted in uptake saturation such that 50% persistence was still observed after 170 minutes. Single administration of GDFXa-α₂M significantly decreased the rivaroxaban-induced bleeding time (p < 0.001) and blood loss (p < 0.01). GDFXa-α₂M did not increase D-dimer or thrombin-antithrombin complex formation, suggesting a lack of pro-thrombotic potential.GDFXa-α₂M is therefore an attractive candidate for xaban neutralization neither pro- nor anticoagulant in vitro as well as in vivo.

First Case of Sporadic Protein S Deficiency due to a Novel Candidate Mutation, Ala 484 → Pro, in the Protein S Active Gene (PROSl)

In a series of 16 propositi with symptomatic protein S deficiency and a protein S gene mutation, we identified a sporadic case of a novel mutation that probably affects gene expression. The mutation, a G to C transversion leading to the substitution of Ala 484 by Pro, was not found in the protein S gene of the patient’s parents. Transmission of the paternal and maternal protein S alleles was apparently normal, on the basis of the frequent polymorphism in exon XV. We also checked the transmission of chromosomal material by analysing protein C gene polymorphisms, Β-globin gene frameworks and four variable number of tandem repeats (VNTRs). By combining the results of these analyses, we were able to rule out nonpaternity and to confirm the de novo nature of the mutation.

Role of FCγRIIA Gene Polymorphism in Human Platelet Activation by Monoclonal Antibodies

The aim of this study was to determine if there is a correlation between the activity of a MoAb as an agonist and its ability to bind to the Fc platelet receptor, FcγRIIa. A polymorphism at amino acid 131 [arginine (Arg) or histidine (His)] of FcγRIIa was first shown to be determinant for MoAb-IgG1 binding on monocytes. To clarify the role of this polymorphism in platelet activation by MoAb-IgG1 we (i) established the FCγRIIA polymorphism at the gene level by adapting the denaturating gradient gel electrophoresis method, (ii) analyzed the binding affinity of the MoAbs to FrγRIIa on platelets from homozygous Arg, homozygous His, and heterozygous Arg/His donors, and (iii) characterized the different reactivities of platelets according to the FCγRIIA polymorphism. Among 167 Caucasian donors we found 46% heterozygous Arg/His, 36% homozygous His and 18% homozygous Arg. ALB6, an anti CD9, P256 an anti GPIIb-IIIa, and AP3 an anti-GPIIIa were chosen according to their ability (ALB6, P256) or not (AP3) to activate platelets. These 3 MoAbs-IgG1 bind to FcγRIIa with a stronger affinity for the Arg-form of FcγRIIa, a result which was confirmed with the use of diverse MoAbs directed against various antigens. The different abilities of MoAbs to bind to the two FcγRIIa forms were well correlated to the different platelet responses induced by ALB6 and P256. However, low concentrations of ALB6, which allow full activation of platelets from homozygous Arg donors, as did P256, did not induce any activation of platelets from homozygous His donors, whereas P256 is able to induce a low aggregation. The results further define the respective roles of the antigen and the Fc receptor, depending on the MoAb, and the role of the FcγRIIa polymorphism in platelet activation induced by MoAbs. In addition, the results obtained with MoAbs unable to induce platelet activation provide evidence that the binding of a MoAb on FcγRIIa does not predict its ability to activate platelets.

Influence of Six Mutations of the Protein C Gene on the Gla Domain Conformation and Calcium Affinity

The protein C Gla domain was studied in six families presenting a type II hereditary deficiency characterized by low activity in a coagulation assay and normal activity in an amidolytic assay. Five of these mutations, previously described by our group, affected Arg-5, Arg-1, Arg 229 and Ser 252. We report here the first natural Glu 7 to Asp mutation in a sixth family.

We evaluated the binding of the mutated protein C to Hn, a monoclonal antibody (mAb) known to recognize the sequence Phe 4 to Arg 9 of the Gla domain; the presence of calcium ions suppresses the recognition of this epitope by Hn. Mutation of Arg 229 to Gin and Ser 252 to Asn did not modify the inhibition of protein C binding, whereas the Arg-1 to His mutation resulted in a loss of inhibition in the presence of CaCl2. This suggests that the protein C of this patient shows impaired carboxylation.

The protein C from patients bearing the mutations Arg-5 to Trp, Arg-1 to Cys and Glu 7 to Asp bound poorly to Hn mAb, even in the absence of calcium ions. The calcium affinity of the Gla domain was studied by pseudo-affinity chromatography, in which protein C was successively eluted from a Mono Q column by CaCl210 mM and NaCl 0.6 M. Protein C from the patient bearing the Arg-5 to Asp mutation had a normal elution profile, suggesting that a modification of the propeptide cleavage site impairs the conformation of the Gla domain but not carboxylation. Conversely, the patient bearing the mutation Arg-1 to Cys had an abnormal elution profile, showing that calcium binding affinity was impaired probably due to a defective carboxylation (the behaviour of the Glu 7 to Asp mutant lacking one Gla residue validated our experimental approach).

These results confirm that mutations in the serine protease domain do not modify the conformation of the N-terminal part of the Gla domain. Arg-1 to His, Arg-1 to Cys and Glu 7 to Asp mutations strongly modified affinity for calcium ions, whereas the Arg-5 to Trp mutation probably modifies the conformation of the Gla domain.

Compound Heterozygosity in a Family with Protein C Deficiency Illustrating the Complexity of the Underlying Molecular Mechanism

The association of two missense mutations, a Leu 223 to Phe and an lie 403 to Met, is described in a family presenting with various protein C deficiency phenotypes. In this family, two subjects were compound heterozygotes with protein C levels of about 25%, the other members being heterozygous for only one of the mutations. The Leu 223 to Phe mutation was also found in 9 members of 3 other families and, in all cases but one, resulted in protein C levels below 60% associated with a high incidence of thrombotic complications. The other mutation, an lie 403 to Met, was identified in those of the family’ members who presented with borderline protein C concentrations. In such a family, the genomic DNA analysis represents the only way to differentiate between the genetic status of each family member. The results highlight the importance of the genotype determination and the poor discriminative power of the plasma assays currently used.

A Plasma Clot Lysis Assay Based on the Release of Fibrin Degradation Products: Application to the Diagnosis of Hypofibrinolytic States

Using a monoclonal antibody-based assay, we measured the fibrin degradation product release in the supernatant of plasma clots obtained before and after venous occlusion (VO) in 30 patients with definite or suspected vascular thrombosis (19 definite and 2 suspected deep vein thrombosis, 6 recurrent superficial thrombophlebitis, 3 arterial occlusions of lower limbs). tPA and PAI-1 concentrations were determined using ELISA assays; the post-occlusion values were corrected for haemoconcentration. The increase in tPA during VO was correlated with haemoconcentration (r = 0.74), but 3 patients had ineffective VO (<2% increase in proteins). The fibrinolytic response to VO was evaluated using the shortening of the time necessary for the release of 200 μg of fibrin degradation products per mg of fibrinogen (Δ T 200). Two among the 27 patients with effective VO were bad responders with a Δ T 200 <3 h (whereas all the others had Δ T 200 >10 h). These patients had respectively a deficient tPA release (Δ tPA = 1 ng/ml) and an elevated PAI-1 level at rest (33 ng/ml). Several other patients were bad responders in terms of tPA release or of shortening of the euglobulin clot lysis time but they had a normal Δ T 200. This plasma clot test reflects the ability of free tPA to bind to fibrin (the amount of which depends on the level of tPA and PAI-1), and may be useful in the diagnosis of a hypofibrinolytic state.

A Study of Fibrinogen and Fibrinolysis in 10 Adults with Nephrotic Syndrome

In 10 patients with nephrotic syndrome (NS), the coagulation inhibitors, the fibrinolytic system and several functions of the fibrinogen-fibrin molecule were studied.

Among the coagulation inhibitors, only antithrombin III (AT III) was found decreased and correlated with serum-albumin levels. Venous occlusion test provoked a normal tissue plasminogen activator (tPA) release in all patients. The plasminogen activator inhibitor (PAI) had an increased activity in 5 out of the 10 patients.

Thrombin and reptilase times were found abnormal in most patients. The thrombin time (TT) prolongation correlated with serum albumin levels and was corrected by adding purified albumin.

The fibrinogen was purified from each of the 10 patients’ plasma. Only 2 of them showed abnormal polymerization in purified system, suggesting dysfibrinogenaemia. Other functions (thrombin binding, tPA stimulating activity, lysis by purified plasmin) were found normal except in one of the 2 patients with dysfibrinogenaemia whose fibrinogen lysis by plasmin was delayed.

It is concluded that an abnormal fibrinogen molecule is not the most frequent explanation for thrombin time prolongation in NS.

Characterization of Cleaved Plasma Protein S with a Monoclonal Antibody-based Assay

A monoclonal antibody (mAb 5A5G2) recognized cleaved plasma protein S (PS) but not uncleaved PS. Interestingly, mAb 5A5G2 did not recognize thrombin-cleaved recombinant PS. Microsequencing of cleaved plasma PS showed a Q-S-T-N amino-terminal sequence, inferring cleavage after the Arg 60 residue. The mAb epitope was located within the sequence encompassing residues 61 to 73, i. e. the carboxyterminal part of the thrombin-sensitive region (TSR). We used this mAb to develop an ELISA assay to quantify in vivo cleaved PS. In plasma from 10 normal subjects, about 10% of PS was cleaved (7.1% to 15.4%), with a more than 2-fold increase in the corresponding sera. We found increased levels of cleaved PS in 8 patients with disseminated intravascular coagulation (DIC) and decreased levels in 22 patients on long-term oral anticoagulant therapy, whereas cleaved PS levels were similar in 8 hemophiliacs and the 10 normal subjects. Cleaved PS levels did not correlate with prothrombin fragment 1+2 levels released after cleavage by FXa in any of the groups, suggesting that circulating FXa is not the main factor involved in the production of cleaved PS in vivo.

The Factor V Gene A4070G Mutation and the Risk of Venous Thrombosis

The A4070G polymorphism in exon 13 of the factor V (FV) gene, which replaces His by Arg at position 1299 of the B domain, was recently shown to influence circulating FV levels and to contribute to the activated protein C (APC) resistance phenotype. We examined the impact of this polymorphism in a population of unselected patients with venous thromboembolic disease (VTE). The prevalence of the G4070 (R2) allele was determined in 205 patients and 394 healthy subjects of similar age and sex distribution. Thirty-seven patients (18%) were heterozygous for the R2 allele and 1 (0.5%) was homozygous. Forty-four controls (11.2%) were heterozygous for the R2 allele and 1 (0.2%) was homozygous. Thus, the allelic frequency was significantly higher in the patients with VTE than in the healthy controls, with respective values of 9.5% and 5.8%. The odds ratio was 1.8 (95% CI: 1.1-2.8, p = 0.02), pointing to an increased risk of VTE in carriers of the R2 allele. After excluding subjects with putative or confirmed gene defects (mainly the FV R506Q mutation), the R2 allele was still a risk factor for VTE in the remaining patients, with an odds ratio of 2.0 (95% CI: 1.2-3.5, p = 0.01), demonstrating that this polymorphism is itself a risk factor. This study also confirms that the R2 allele influences APC resistance (APCR) in the absence of the FV R506Q mutation.

Thirty-three Novel Mutations in the Protein C Gene

We analyzed the protein C gene (PROC) with the denaturing gradient gel electrophoresis (DGGE) scanning strategy in a series of 129 patients with suspected protein C (PC) deficiency (93 with low plasma PC levels and 36 with borderline level). At least one sequence variation was found in 104 of the 129 patients. Thirty-nine sequence variations (found in 72 patients) were already reported detrimental mutations. Thirty-three were novel sequence variations, of which 19 (found in 25 patients) were probably detrimental. Five novel mutations (A1T, R9H, S11R, S12R and K193Q) were associated with qualitative plasma PC deficiency, suggesting or confirming the functional importance of amino acids at these positions. This strategy confirmed the diagnosis of inherited PC deficiency in 79/93 (84.9%) patients with low plasma PC levels and 14/36 (38.8%) patients with borderline values. In order to explain abnormal PC levels observed in patients who did not carry detrimental mutations, screening for the -1654C/T and -1641A/G PROC promoter polymorphisms known to influence plasma PC concentrations was performed. The frequency of the CG allele associated with lower PC concentrations was slightly but not significantly lower in 82 heterozygotes for detrimental PROC gene mutations than in 36 patients with no identified detrimental mutations.

Identification of functional polymorphisms of the thromboxane A2 receptor gene in healthy volunteers

Thromboxane A2 receptor (TP) is an important actor in vascular physiology and plays a crucial role in the platelet activation process. Genetic polymorphisms of the gene coding for the TP have been described, but their impacts on platelet function tests are unknown. The aim of this study was to investigate the relationship between genetic polymorphisms of the coding sequence of the TP gene and platelet function tests. We investigated 100 healthy volunteers twice, one week apart by performing platelet aggregation and secretion tests. We sequenced the coding region of the TP gene and confronted the genetic variants with the phenotypic results. We identified five single nucleotide polymorphisms (SNP); one of them, T1712C, replaces Leu by Pro at position 133 of the isoform β of the TP. Homozygosity for the minor allele of the C795T, C924T or the G1686A SNP was associated with a decreased expression of CD62P when platelets were stimulated with the TP agonist U46619. As C795T and C924T have been linked to clinical disorders in which TxA2 plays a key role, the possible role of the G1686A and T1712C SNP should also be examined in selected diseases.

Low-Molecular-Weight Fucoidan Induces Endothelial Cell Migration via the PI3K/AKT Pathway and Modulates the Transcription of Genes Involved in Angiogenesis

Low-molecular-weight fucoidan (LMWF) is a sulfated polysaccharide extracted from brown seaweed that presents antithrombotic and pro-angiogenic properties. However, its mechanism of action is not well-characterized. Here, we studied the effects of LMWF on cell signaling and whole genome expression in human umbilical vein endothelial cells and endothelial colony forming cells. We observed that LMWF and vascular endothelial growth factor had synergistic effects on cell signaling, and more interestingly that LMWF by itself, in the absence of other growth factors, was able to trigger the activation of the PI3K/AKT pathway, which plays a crucial role in angiogenesis and vasculogenesis. We also observed that the effects of LMWF on cell migration were PI3K/AKT-dependent and that LMWF modulated the expression of genes involved at different levels of the neovessel formation process, such as cell migration and cytoskeleton organization, cell mobilization and homing. This provides a better understanding of LMWF’s mechanism of action and confirms that it could be an interesting therapeutic approach for vascular repair.

Three-Dimensional Environment Sustains Hematopoietic Stem Cell Differentiation into Platelet-Producing Megakaryocytes

Hematopoietic stem cells (HSC) differentiate into megakaryocytes (MK), whose function is to release platelets. Attempts to improve in vitro platelet production have been hampered by the low amplification of MK. Providing HSC with an optimal three-dimensional (3D) architecture may favor MK differentiation by mimicking some crucial functions of the bone marrow structure. To this aim, porous hydrogel scaffolds were used to study MK differentiation from HSC as well as platelet production. Flow cytometry, qPCR and perfusion studies showed that 3D was suitable for longer kinetics of CD34+ cell proliferation and for delayed megakaryocytic differentiation far beyond the limited shelf-life observed in liquid culture but also increased production of functional platelets. We provide evidence that these 3D effects were related to 1) persistence of MK progenitors and precursors and 2) prolongation of expression of EKLF and c-myb transcription factors involved in early MK differentiation. In addition, presence of abundant mature MK with increased ploidy and impressive cytoskeleton elongations was in line with expression of NF-E2 transcription factor involved in late MK differentiation. Platelets produced in flow conditions were functional as shown by integrin αIIbβ3 activation following addition of exogenous agonists. This study demonstrates that spatial organization and biological cues synergize to improve MK differentiation and platelet production. Thus, 3D environment constitutes a powerful tool for unraveling the physiological mechanisms of megakaryopoiesis and thrombopoiesis in the bone marrow environment, potentially leading to an improved amplification of MK and platelet production.

[Clinical and biological features of patients with essential thrombocythaemia according to their mutational status JAK2 or CALR: Single-center study of 40 patients and review of the literature]

BACKGROUND

Somatic mutations in the calreticulin gene (CALR) were recently described in essential thrombocythemia (ET) and primary myelofibrosis with non-mutated JAK2 or MPL. The aim of this single-center study was to compare the clinical and biological features of ET patients according to their mutational status.

METHODS

We included 40 patients with ET followed in hematology consultation. The JAK2 V617F mutation was assessed by quantitative PCR. For the detection of CALR mutations, we performed a PCR amplification of CALR exon 9 followed by direct sequencing.

RESULTS

Among 40 study patients, 23 (57.5%) harbored V617F JAK2, 12 of the 17 patients without JAK2 mutation harbored CALR, no patient expressed MPL mutation and 5 were negative for all three mutations. Five types of mutations were identified with predominance of 52bp deletion and 5bp insertion (7/12 and 2/12 respectively). The incidence of thrombotic events at diagnosis was significantly higher in JAK2 mutated patients (P<0.05). Biologically, patients with CALR mutation had significantly higher platelet count (P<0.01) and significantly lower hemoglobin level (P<0.05) than those with V617F JAK2 mutation.

CONCLUSION

JAK2 and CALR mutation screening in ET has a diagnostic value. Each mutation displays a distinct phenotype with uncertain impact on long-term outcome.

Assessment of the interplay between blood and skin vascular abnormalities in adult purpura fulminans

RATIONALE

Purpura fulminans in adults is a rare but devastating disease. Its pathophysiology is not well known.

OBJECTIVES

To understand the pathophysiology of skin lesions in purpura fulminans, the interplay between circulating blood and vascular alterations was assessed.

METHODS

Prospective multicenter study in four intensive care units. Patients with severe sepsis without skin lesions were recruited as control subjects.

MEASUREMENTS AND MAIN RESULTS

Twenty patients with severe sepsis and purpura fulminans were recruited for blood sampling, and skin biopsy was performed in deceased patients. High severity of disease and mortality rates (80%) was observed. Skin biopsies in purpura fulminans lesions revealed thrombosis and extensive vascular damage: vascular congestion and dilation, endothelial necrosis, alteration of markers of endothelial integrity (CD31) and of the protein C pathway receptors (endothelial protein C receptor, thrombomodulin). Elevated plasminogen activating inhibitor-1 mRNA was also observed. Comparison with control patients showed that these lesions were specific to purpura fulminans. By contrast, no difference was observed for blood hemostasis parameters, including soluble thrombomodulin, activated protein C, and disseminated intravascular coagulation markers. Bacterial presence at the vascular wall was observed specifically in areas of vascular damage in eight of nine patients tested (including patients with Streptococcus pneumoniae, Neisseria meningitidis, Escherichia coli, and Pseudomonas aeruginosa infection).

CONCLUSIONS

Thrombi and extensive vascular damage with multifaceted prothrombotic local imbalance are characteristics of purpura fulminans. A "vascular wall infection" hypothesis, responsible for endothelial damage and subsequent skin lesions, can be put forward.

Platelet dysfunction associated with the novel Trp29Cys thromboxane A₂ receptor variant

BACKGROUND

Genetic variations that affect the structure of the thromboxane A2 receptor (TP receptor) provide insights into the function of this key platelet and vascular receptor, but are very rare in unselected populations.

OBJECTIVES

To determine the functional consequences of the TP receptor Trp29Cys (W29C) substitution.

PATIENTS/METHODS

We performed a detailed phenotypic analysis of an index case (P1) with reduced platelet aggregation and secretion responses to TP receptor pathway activators, and a heterozygous TP receptor W29C substitution. An analysis of the variant W29C TP receptor expressed in heterologous cells was performed.

RESULTS

Total TP receptor expression in platelets from P1 was similar to that of controls, but there was reduced maximum binding and reduced affinity of binding to the TP receptor antagonist [(3) H]SQ29548. HEK293 cells transfected with W29C TP receptor cDNA showed similar total TP receptor expression to wild-type (WT) controls. However, the TP receptor agonist U46619 was less potent at inducing rises in cytosolic free Ca(2+) in HEK293 cells expressing the W29C TP receptor than in WT controls, indicating reduced receptor function. Immunofluorescence microscopy and cell surface ELISA showed intracellular retention and reduced cell surface expression of the W29C TP receptor in HEK293 cells. Consistent with the platelet phenotype, both maximum binding and the affinity of binding of [(3) H]SQ29548 to the W29C TP receptor were reduced compared to WT controls.

CONCLUSION

These findings extend the phenotypic description of the very rare disorder TP receptor deficiency, and show that the W29C substitution reduces TP receptor function by reducing surface receptor expression and by disrupting ligand binding.

Membrane binding and anticoagulant properties of protein S natural variants

INTRODUCTION

Protein S (PS) is a vitamin K-dependent plasma glycoprotein with a key role in the control of coagulation pathway on phospholipid membranes. We compared anticoagulant and membrane binding properties of PS altered by natural mutations (N217S, DelI203D204) affecting the epidermal growth factor like-domain 4 (EGF4) and causing PS deficiency.

MATERIALS AND METHODS

Binding of recombinant, immunopurified PS (rPS) to several conformation-specific antibodies, to C4BP and to phospholipid liposomes was investigated by ELISA. PS binding to cells was analysed by flow cytometry. PS inhibitory activities were studied in plasma and purified systems.

RESULTS AND CONCLUSIONS

Conformational changes produced by mutations were revealed by mapping with calcium-dependent antibodies. The immunopurified recombinant mutants (rPS) showed at 200-800 nM concentration reduced inhibition of coagulation (rPS217S, 10.2-17.3%; rPSDelI203D204, 5.8-8.9% of rPSwt) in FXa 1-stage clotting assay with APC. In thrombin generation assays the inhibition of ETP was reduced to 51.6% (rPS217S) and 24.1% (rPSDelI203D204) of rPSwt. A slightly shortened lag time (minutes) was also observed (rPS217S, 2.58; rPSDelI203D204, 2.33; rPSwt, 3.17; PS deficient plasma, 2.17). In flow cytometry analysis both mutants efficiently bound apoptotic cells in adhesion or in suspension. The affinity for phosphatidylserine-rich vesicles (apparent Kd: rPSwt 27.7+/-1.6 nM, rPS217S 146.0+/-16.1 nM and rPSDelI203D204 234.1+/-28.1 nM) was substantially increased by membrane oxidation (10.9+/-0.6, 38.2+/-3.5 and 81.4+/-6.0 nM), which resulted in a virtually normal binding capacity of mutants at physiological PS concentration. These properties help to define the molecular bases of PS deficiency, and provide further elements for PS-mediated bridging of coagulation and inflammation.

A comparative study of the protein C pathway in septic and nonseptic patients with organ failure

RATIONALE

Severe sepsis is associated with an exacerbated procoagulant state with protein C (PC) system impairment. In contrast, the inflammatory and coagulation status of nonseptic patients with organ failure (OF) is less documented.

OBJECTIVES

To compare coagulation activation, focusing on the PC system, and inflammatory status in septic and nonseptic patients with OF.

METHODS

Thirty patients with severe sepsis and 30 nonseptic patients were recruited at the onset of OF and compared with 30 matched healthy subjects. We performed an extensive analysis of the PC pathway, including plasma protein measurements and quantification of leukocyte expression of PC system receptors. In addition, we analyzed the inflammatory status, based on inflammation-related gene leukocyte expression.

MEASUREMENTS AND MAIN RESULTS

We observed coagulation activation, reflected by a similar increase in tissue factor mRNA expression, in the two patient groups when compared with the healthy subjects. Soluble thrombomodulin levels were higher in septic patients than in healthy control subjects, whereas PC, protein S, and soluble endothelial cell PC receptor levels were lower. Similar results were obtained in nonseptic patients with OF. Monocyte thrombomodulin overexpression, together with increased circulating levels of activated PC, suggests that the capacity for PC activation is at least partly preserved in both settings. No difference in the inflammatory profile was found between septic and nonseptic patients.

CONCLUSIONS

The pathogenesis of OF in critical care patients is characterized by an overwhelming systemic inflammatory response and by exacerbated coagulation activation, independently of whether or not infection is the triggering event. Clinical trial registered with www.clinicaltrials.gov (NCT 00361725).

Mutations des protéines de la coagulation et thromboses

The coagulation system is governed by a subtle balance between clotting activators and inhibitors. Many genes can contribute to the overall phenotype, and polymorphisms may act to up regulate or down regulate the generation of thrombin, the coagulation-key enzyme. An increase in coagulation factor (gain function) or/and a decrease in coagulation inhibitors (loss of function) may favor venous thromboembolism (VTE). It has been observed since a long time that VTE may be a familial disease, but it was only in 1965 that Egeberg published the first case of inherited antithrombin (AT) deficiency. This was followed by similar reports of protein C (PC) and protein S (PS) deficiencies. Hereditary thrombophilia was thus initially considered as a rare monogenic disorder with incomplete penetrance. AT, PC and PS deficiencies are due to multiple and mostly private mutations of the corresponding genes. Most patients are heterozygous and experience VTE at adult age. Homozygosity associated with severe thrombosis at birth has been observed in newborns with undetectable PC or PS concentrations. The discovery of factor (F) V Leiden and F2 g.20210 G>A, two gain of function mutations, challenged the view of thrombophilia as a rare monogenic disorder. FV Leiden and F2 g.20210 G>A are due to a founder effect and affect populations of European descent with frequencies at 5% and 3% respectively. These two mutations are moderate of risk factor for thrombosis and paved the way for gene-gene and gene-environment interactions. Patients carrying more than one genetic risk factor are at higher risk to develop VTE. The exposition to acquired risk factors such as estrogen based oral contraception may also have a synergistic effect favoring thrombosis in patients with FV Leiden or other genetic risk factors.

Molecular bases of type II protein S deficiency: the I203-D204 deletion in the EGF4 domain alters GLA domain function

OBJECTIVE

To characterize the first type II protein S (PS) deficiency affecting the epidermal growth factor (EGF)4 domain, a calcium-binding module with a poorly defined functional role.

PATIENTS

The proband suffered from recurrent deep vein thrombosis and showed reduced PS anticoagulant activity (31%), and total, free PS antigen and C4bBP levels in the normal range.

RESULTS

Reverse transcription-polymerase chain reaction analysis showed the presence of the IVSg-2A/T splicing mutation that, by activating a cryptic splice site, causes the deletion of codons Ile203 and Asp204. Free PS, immunopurified from proband's plasma, showed an altered electrophoretic pattern in native condition or in the presence of Ca2+. The recombinant PS (rPS) mutant showed reduced anticoagulant (<10%) and activated protein C-independent activities (24-38%) when compared with wild-type rPS (rPSwt). Binding of the rPS variant to phospholipid vesicles (Kd 235.7 +/- 30.8 nM, rPSwt; Kd 15.2 +/- 0.9 nM) as well as to Ca2+-dependent conformation-specific monoclonal antibodies for GLA domain was significantly reduced.

CONCLUSIONS

These data aid in the characterization of the functional role of the EGF4 domain in the anticoagulant activities of PS and in defining the thrombophilic nature of type II PS deficiency.

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.

[Validation of a diagnostic kit for factor V Leiden mutation]

The Stagen factor V Leiden mutation is a new kit allowing to perform all steps necessary to identify the Leiden mutation of Factor V, from DNA extraction from patient's blood sample to electrophoresis of amplification products. The method is based on an allele specific amplification, which allows patient's genotype to be established in a single step. Analytical properties of the kit have been tested first, and revealed the robustness of the kit. In a second step, a prospective study of a cohort of 300 thrombophilic patients demonstrated the specificity and the sensitivity of the method. In additions, several controls and methodological ingenious processes allow to minimize the risk of human or method errors, making the reliability of the kit. Finally, the Stagen factor V Leiden mutation is easy and rapid to use in hospital or private laboratories, and is well-suited for small series of patients of the latter.

A haplotype of the EPCR gene is associated with increased plasma levels of sEPCR and is a candidate risk factor for thrombosis

The endothelial cell protein C (PC) receptor (EPCR) facilitates PC activation by the thrombin-thrombomodulin complex. A soluble form of this receptor (sEPCR) found in plasma inhibits both activated PC (aPC) activity and PC activation by competing for PC with membrane-associated EPCR. Elevated sEPCR levels are found in approximately 20% of healthy subjects, but the mechanisms underlying this interindividual variability are unknown. We measured sEPCR levels in 100 healthy male volunteers, and observed 2 phenotypic groups of subjects. The temporal stability of sEPCR levels suggested genetic control. Extensive analysis of the EPCR gene in these subjects revealed 13 polymorphisms in complete linkage disequilibrium; these defined 3 haplotypes, 1 of which (A3) was strongly associated with high sEPCR levels. The high constitutive sEPCR levels observed in A3 haplotype carriers might reduce the efficiency of the PC system and predispose these subjects to venous thrombosis. By studying 338 patients with venous thrombosis and 338 age- and sex-matched healthy subjects, we found that the A3 haplotype was overrepresented in the patients. In multivariate analysis, subjects carrying the A3 haplotype had an increased risk of thrombosis (odds ratio [OR] = 1.8; P =.004). Thus, the A3 haplotype, which is associated with elevated plasma sEPCR levels, is a candidate risk factor for venous thrombosis.

Adenosine diphosphate-induced platelet aggregation is associated with P2Y12 gene sequence variations in healthy subjects

BACKGROUND

The adenosine diphosphate (ADP) receptor P2Y12 plays a pivotal role in platelet aggregation, as demonstrated by the benefit conferred by its blockade in patients with cardiovascular disease. Some studies have shown interindividual differences in ADP-induced platelet aggregation responses ex vivo, but the mechanisms underlying this variability are unknown.

METHODS AND RESULTS

We examined ADP-induced platelet aggregation responses in 98 healthy volunteers, and we identified 2 phenotypic groups of subjects with high and low responsiveness to 2 micromol/L ADP. This prompted us to screen the recently identified Gi-coupled ADP receptor gene P2Y12 for sequence variations. Among the 5 frequent polymorphisms thus identified, 4 were in total linkage disequilibrium, determining haplotypes H1 and H2, with respective allelic frequencies of 0.86 and 0.14. The number of H2 alleles was associated with the maximal aggregation response to ADP in the overall study population (P=0.007). Downregulation of the platelet cAMP concentration by ADP was more marked in 10 selected H2 carriers than in 10 noncarriers.

CONCLUSIONS

In healthy subjects, ADP-induced platelet aggregation is associated with a haplotype of the P2Y12 receptor gene. Given the crucial role of the P2Y12 receptor in platelet functions, carriers of the H2 haplotype may have an increased risk of atherothrombosis and/or a lesser clinical response to drugs inhibiting platelet function.

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.

Functional properties of the sex-hormone-binding globulin (SHBG)-like domain of the anticoagulant protein S

Protein S (PS) possesses a sex-hormone-binding globulin (SHBG)-like domain in place of the serine-protease domain found in other vitamin K-dependent plasma proteins. This SHBG-like domain is able to bind a complement fraction, C4b-binding protein (C4b-BP). To establish whether the PS SHBG-like domain can fold normally in the absence of other domains, and to obtain information on the specific functions of this region, we expressed the PS SHBG-like domain alone or together with its adjacent domain EGF4. The folding of the two recombinant modules was studied by analyzing their binding to C4b-BP. The apparent dissociation constants of this interaction indicated that both recombinant modules adopted the conformation of native PS, indicating that the PS SHBG-like region is an independent folding unit. We also obtained the first direct evidence that the SHBG-like domain alone is sufficient to support the interaction with C4b-BP. In addition, both recombinant modules were able to bind Ca2+ directly, as shown by the migration shift in agarose gel electrophoresis in the presence of Ca2+, together with the results of equilibrium dialysis and the functional effect of Ca2+ on the C4b-BP/PS interaction, confirming the presence of one Ca2+ binding site within the SHBG-like domain. Neither recombinant module exhibited activated protein C (aPC) cofactor activity in a clotting assay, suggesting that the PS SHBG-like region must be part of the intact molecule for it to contribute to aPC cofactor activity, possibly by constraining the different domains in a conformation that permits optimal interaction with aPC.

Implication of protein S thrombin-sensitive region with membrane binding via conformational changes in the gamma-carboxyglutamic acid-rich domain

In the vitamin K-dependent protein family, only protein S (PS) contains a thrombin-sensitive region (TSR), located between the domain containing the gamma-carboxyglutamic acid and the first epidermal growth factor-like domain. To better define the role of TSR in the PS molecule, we expressed a recombinant human PS (rHPS) and its analogue lacking TSR (rTSR-less), and prepared factor Xa- and thrombin-cleaved rHPS. A peptide reproducing TSR (TSR-peptide) was also synthesized in an attempt to obtain direct evidence of the domain involvement in PS anticoagulant activity. In a coagulation assay, both rTSR-less and factor Xa-cleaved PS were devoid of activated protein C cofactor activity. The TSR-peptide did not inhibit rHPS activity, showing that TSR must be embedded in the native protein to promote interaction with activated protein C. The binding of rHPS to activated platelets and to phospholipid vesicles was not modified after factor Xa- or thrombin-mediated TSR cleavage, whereas the binding of rTSR-less was markedly reduced. This suggested a role for TSR in conferring to PS a strong affinity for phospholipid membranes. TSR-peptide did not directly bind to activated platelets or compete with rHPS for phospholipid binding. The results of the present study show that TSR may not interact directly with membranes, but probably constrains the gamma-carboxyglutamic acid-rich domain in a conformation allowing optimal interaction with phospholipids.

Three-dimensional model of the SHBG-like region of anticoagulant protein S: new structure-function insights

Protein S (PS) is a vitamin K-dependent glycoprotein that consists of several modules including a C-terminal sex hormone-binding globulin (SHBG)-like domain that has been subdivided into two laminin LG-type domains. The SHBG-like region of PS is known to bind to a complement regulator molecule, C4b-binding protein (C4BP), coagulation factor Va (FVa) and receptor tyrosine kinases. Inherited PS deficiency has been associated with thromboembolic disease. Yet, study of the mechanisms by which the SHBG-like region of PS serves its essential functions has so far been hampered because of the lack of structural information. Recently, the three-dimensional (3D) structure of LG domains from plasma SHBG, laminin and neurexin have been reported and were found related to the pentraxin family. We used these X-ray structures to build homology models of the SHBG-like region of human PS. We then analyzed previously reported experimental/clinical data in the light of the predicted structures. A potential calcium-binding site is found in the first LG domain of PS and D292 could play a role in this process. This region is close to the interface between the two LG domains and is also surrounded by segments that have been suggested by synthetic peptide studies to be important for C4BP or FVa binding. The 39 point mutations linked to PS deficiencies or reported as neutral variants were rationalized in the 3D structure. Proteins 2001;43:203-216.