Role of the residues of the 39-loop in determining the substrate and inhibitor specificity of factor IXa.

The activation of antithrombin (AT) by heparin facilitates the exosite-dependent interaction of the serpin with factors IXa (FIXa) and Xa (FXa), thereby improving the rate of reactions by 300- to 500-fold. Relative to FXa, AT inhibits FIXa with approximately 40-fold slower rate constant. Structural data suggest that differences in the residues of the 39-loop (residues 31-41) may partly be responsible for the differential reactivity of the two proteases with AT. This loop is highly acidic in FXa, containing three Glu residues at positions 36, 37, and 39. By contrast, the loop is shorter by one residue in FIXa (residue 37 is missing), and it contains a Lys and an Asp at positions 36 and 39, respectively. To determine whether differences in the residues of this loop contribute to the slower reactivity of FIXa with AT, we prepared an FIXa/FXa chimera in which the 39-loop of the protease was replaced with the corresponding loop of FXa. The chimeric mutant cleaved a FIXa-specific chromogenic substrate with normal catalytic efficiency, however, the mutant exhibited approximately 5-fold enhanced reactivity with AT specifically in the absence of the cofactor, heparin. Further studies revealed that the FIXa mutant activates factor X with approximately 4-fold decreased k(cat) and approximately 2-fold decreased K(m), although the mutant interacted normally with factor VIIIa. Based on these results we conclude that residues of the 39-loop regulate the cofactor-independent interaction of FIXa with its physiological inhibitor AT and substrate factor X.

Factor VIIa-antithrombin complexes in patients with arterial and venous thrombosis.

Antithrombin (AT), in the presence of heparin, is able to inhibit the catalytic activity of factor VIIa bound to tissue factor (TF) on cell surfaces. The clinical meaning of FVIIa-AT complexes plasma levels is unknown. It was the objective of this study to evaluate FVIIa-AT complexes in subjects with thrombosis. Factor VIIa-AT complexes plasma levels in 154 patients consecutively referred to our Department with arterial or venous thrombosis and in a group of 154 healthy subjects, were measured. Moreover, FVIIa-AT complexes were determined in: i) n = 53 subjects belonging to 10 families with inherited factor VII deficiency; ii) n = 58 subjects belonging to seven families with AT deficiency; iii) n = 49 patients undergoing oral anticoagulant therapy (OAT). Factor VIIa-AT levels were determined by a specific ELISA kit (R&D, Diagnostica Stago, Gennevilliers, France). Factor VIIa-AT complexes mean plasma levels were lower in patients with either acute arterial (136 +/- 40 pM) or venous (142 +/- 53 pM) thrombosis than subjects with previous thrombosis (arterial 164 +/- 33 pM and venous 172 +/- 61 pM, respectively) and than healthy controls (156 +/- 63 pM). Differences between acute and previous thrombosis, were statistically significant (p < 0.05). Subjects with inherited and acquired (under OAT) factor VII deficiency had statistically significant lower FVIIa-AT complexes plasma levels (80 +/- 23 pM and 55 +/- 22 pM, respectively) than controls (150 +/- 51 pM, p < 0.0001 and 156 +/- 63 pM, p < 0.00001, respectively). Factor VIIa-AT complexes are positively correlated with plasma factor VII/VIIa levels. Further investigations are needed to verify the possible role of higher FVIIa-AT complex plasma levels in predicting hypercoagulable states and thrombosis.

Heterozygous antithrombin deficiency improves in vivo haemostasis in factor VIII-deficient mice.

Decreased levels of factor VIII (FVIII) limit the amount of thrombin generated at the site of injury, but not the rate that thrombin is neutralised by antithrombin (AT). We hypothesised that FVIII-deficient mice with heterozygous AT deficiency will demonstrate increased thrombin generation and therefore less in vivo bleeding compared to FVIII-deficient mice with normal AT levels. Therefore, we performed tail bleeding experiments in wild-type (WT), heterozygous AT deficient (AT(+/-)) mice, FVIII-deficient (FVIII(-/-)) mice, and FVIII-deficient mice with heterozygous AT deficiency (FVIII(-/-)/AT(+/-)). Amount of bleeding was assessed by measuring absorbance of haemoglobin released from lysed red blood cells collected after tail transection. In addition, we measured thrombin generation, activated partial thromboplastin time (aPTT), and AT activity in plasma from the different mice groups. Tail bleeding was significantly reduced in FVIII(-/-)/AT(+/-) mice compared to FVIII(-/-) mice. On the other hand, there was no difference in tail bleeding between AT(+/-) and wild-type mice. Thrombin generation was dependent on the mice genotype, and increased in the following order: FVIII(-/-) < FVIII(-/-)/AT(+/-) < WT < AT(+/-). The aPTT was not influenced by reduced AT activity (i.e. AT(+/-) genotype), but was significantly prolonged in FVIII(-/-) and FVIII(-/-)/AT(+/-) mice. Using FVIII-deficient mice as an in vivo murine model of reduced thrombin generation, we demonstrated that moderately reduced AT levels increase thrombin generation and decrease bleeding after traumatic tail vessel injury. In agreement with congenital thrombotic conditions, our data elucidate that bleeding phenotypes can be modulated by the balance between procoagulant and anticoagulant proteins.

Inhibitory properties of the P1 Tyr variant of antithrombin.

Antithrombin (AT) and protein Z-dependent protease inhibitor (ZPI) are among two physiological serpin inhibitors in plasma that are involved in the regulation of the clotting cascade. Unlike AT, which can inhibit the proteolytic activity of all coagulation proteases, ZPI has narrower protease specificity, inhibiting only factors Xa (fXa) and XIa. Unlike an Arg at the P1 site of the AT reactive center loop (RCL), this residue is a Tyr in ZPI. To investigate the contribution of P1 Tyr in restricting the specificity of ZPI, we engineered an AT mutant in which the P1 Arg of the RCL was replaced with the P1 Tyr of ZPI (AT-R393Y). The reactivity of AT-R393Y with fXa and thrombin was decreased 155- and 970-fold, respectively. However, the serpin mutant inhibited chymotrypsin with an efficiency higher by >4 orders of magnitude. By contrast, chymotrypsin did not exhibit any reactivity with ZPI. The substitution of Asp-189 of fXa with the corresponding residue of chymotrypsin (Ser) did not improve the reactivity of the protease mutant with AT-R393Y; however, the fXa mutant reacted normally with ZPI. These results suggest that the contribution of P1 Tyr to restricting the protease specificity of ZPI is RCL context-dependent and that in addition to P1 Tyr, other structural features within and/or outside the ZPI RCL are involved in determining the protease specificity of the serpin. The results further suggest that thrombin is less tolerant than fXa in accommodating the nonoptimal P1 Tyr of the AT mutant in its active-site pocket.

Unraveling the thrombophilia paradox: from hypercoagulability to the prothrombotic state.

The thrombophilia paradox whereby thrombophilia testing identifies defects associated with an increased risk of a first venous thrombosis but not of a particularly high risk of recurrence is likely the result of limitations imposed by a limited dichotomous testing strategy compounded by test inaccuracy and imprecision. Consequently, the observed intermediate phenotype (defined by limited laboratory test results) is not fully concordant with the heritable genotype. The next generation of thrombophilia tests, which utilize either individual genomic analysis or global measurement of the composite plasma intermediate phenotype, may more accurately quantify the thrombophilic risk. In conjunction with clinical risk assessment a more quantitative measurement of hypercoagulability and definition of the prothrombotic state should facilitate transition of clinical management from a disease-focused to a more patient-focused strategy.

Detection and characterisation of large SERPINC1 deletions in type I inherited antithrombin deficiency.

Methods routinely used for investigating the molecular basis of antithrombin (AT) deficiency do not detect large SERPINC1 rearrangements. Between 2000 and 2008, 86 probands suspected of having AT-inherited type I deficiency were screened for SERPINC1 mutations in our laboratory. Mutations causally linked to the deficiency were identified by sequencing analysis in 63 probands. We present here results of multiplex ligation-dependent probe amplification (MLPA) analysis performed in 22 of the 23 remaining probands, in whom sequencing had revealed no mutation. Large deletions, present at the heterozygous state, were detected in 10 patients: whole gene deletions in 5 and partial deletions removing either exon 6 (n = 2), exons 1-2 (n = 1) or exons 5-7 (n = 2) in 5 others. Exon 6 partial deletions are a 2,769-bp deletion and a 1,892-bp deletion associated with a 10-bp insertion, both having 5' and/or 3' breakpoints located within Alu repeat elements. In addition, we identified the 5' breakpoint of a previously reported deletion of exons 1-2 within an extragenic Alu repeat. Distinct mutational mechanisms explaining these Alu sequence-related deletions are proposed. Overall, in this series, large deletions detected by MLPA explain almost half of otherwise unexplained type I AT-inherited deficiency cases.

Genetic susceptibility to VTE: a primary care approach.

Genetic thrombophilic disorders are variably common and primary care clinicians must be aware of them because of the increased risk of VTE. A physical examination will not be able to determine if a given VTE resulted from a genetic predisposition or not. In some instances, a patient's personal and family history will alert a clinician to the existence of a thrombophilic disorder, but diagnosis of the specific thrombophilia will require laboratory evaluation and referral to a specialist. The acute management of VTE is the same regardless of the presence of a genetic thrombophilia; therefore, laboratory testing or evaluation by a specialist is not cause to delay treatment of the acute thrombotic event. After the initial treatment and stabilization of the patient, ample time exists to perform a thrombophilia workup. Long-term management of thrombophilia disorders is complicated and needs to be individualized, so referral to specialists is necessary. Primary care clinicians need to keep abreast of the studies being conducted on thrombophilia because numerous families continue to be plagued by VTEs without a recognizable cause. Undoubtedly, new causes of inherited thrombophilias are yet to be unveiled.

The signature 3-O-sulfo group of the anticoagulant heparin sequence is critical for heparin binding to antithrombin but is not required for allosteric activation.

Heparin and heparan sulfate glycosaminoglycans allosterically activate the serpin, antithrombin, by binding through a specific pentasaccharide sequence containing a critical 3-O-sulfo group. To elucidate the role of the 3-O-sulfo group in the activation mechanism, we compared the effects of deleting the 3-O-sulfo group or mutating the Lys(114) binding partner of this group on antithrombin-pentasaccharide interactions by equilibrium binding and rapid kinetic analyses. Binding studies over a wide range of ionic strength and pH showed that loss of the 3-O-sulfo group caused a massive approximately 60% loss in binding energy for the antithrombin-pentasaccharide interaction due to the disruption of a cooperative network of ionic and nonionic interactions. Despite this affinity loss, the 3-O-desulfonated pentasaccharide retained the ability to induce tryptophan fluorescence changes and to enhance factor Xa reactivity in antithrombin, indicative of normal conformational activation. Rapid kinetic studies showed that loss of the 3-O-sulfo group affected both the ability of the pentasaccharide to recognize native antithrombin and its ability to preferentially bind and stabilize activated antithrombin. By contrast, mutation of Lys(114) solely affected the preferential interaction of the pentasaccharide with activated antithrombin. These findings demonstrate that the 3-O-sulfo group functions as a key determinant of heparin pentasaccharide activation of antithrombin both by contributing to the Lys(114)-independent recognition of native antithrombin and by triggering a Lys(114)-dependent induced fit interaction with activated antithrombin that locks the serpin in the activated state.

The critical role of hinge-region expulsion in the induced-fit heparin binding mechanism of antithrombin.

Antithrombin (AT) is the most important inhibitor of coagulation proteases. Its activity is stimulated by glycosaminoglycans, such as heparin, through allosteric and template mechanisms. AT utilises an induced-fit mechanism to bind with high affinity to a pentasaccharide sequence found in about one-third of heparin chains. The conformational changes behind this mechanism have been characterised by several crystal structures of AT in the absence and in the presence of pentasaccharide. Pentasaccharide binding ultimately results in a conformational change that improves affinity by about 1000-fold. Crystal structures show several differences, including the expulsion of the hinge region of the reactive centre loop from beta-sheet A, which is known to be critical for the allosteric activation of AT. Here, we present data that reveal an energetically distinct intermediate on the path to full activation where the majority of conformational changes have already occurred. A crystal structure of this intermediate shows that the hinge region is in a native-like state in spite of having the pentasaccharide bound in the normal fashion. We engineered a disulfide bond to lock the hinge in its native position to determine the energetic contributions of the initial and final conformational events. Approximately 60% of the free-energy contribution of conformational change is provided by the final step of hinge-region expulsion and subsequent closure of the main beta-sheet A. A new analysis of the individual structural changes provides a plausible mechanism for propagation of conformational change from the heparin binding site to the remote hinge region in beta-sheet A.

Functional consequences of the prothrombotic SERPINC1 rs2227589 polymorphism on antithrombin levels.

Genetic factors involved in the interindividual variability of antithrombin have not been identified. We studied two polymorphisms of the gene coding for antithrombin (SER-PINC1) in 298 Spanish Caucasian blood donors: rs3138521, a DNA length polymorphism located on the promoter region and rs2227589, a SNP located on intron 1 that has been described as a mild thrombotic risk factor. We detected a complete linkage disequilibrium between these polymorphisms (D'=0.999). The rs3138521 polymorphism has no functional consequences. However, the rs2227589 SNP significantly associated with plasma anti-FXa activity and antithrombin levels: carriers of the A allele had slightly but significantly lower anticoagulant activity and levels than GG subjects (97.0+/-7.3% vs. 94.6+/-8.4%; p=0.032; 99.5+/-5.8% vs. 94.8+/-5.6%; p=0.001; respectively). Our results identified a functional effect of the rs2227589 polymorphism not explained by its linkage with the promoter polymorphism that support the moderate thrombotic risk associated with the A allele.

Prevalence of genetic mutations in protein S, protein C and antithrombin genes in Japanese patients with deep vein thrombosis.

INTRODUCTION: Genetic deficiencies of PROS1, PROC, and SERPINC1 (antithrombin) are risk factors for deep vein thrombosis (DVT). Diagnosis of the inherited deficiencies of these three genes is sometimes difficult because of the phenotypic variability. This study was undertaken to reveal the frequency of nonsynonymous mutations of these three genes in Japanese DVT patients. PATIENTS/METHODS: One hundred seventy-three DVT patients were registered by the Sub-group of Blood Coagulation Abnormality, from the Study Group of Research on Measures for Intractable Diseases. We sequenced the entire coding regions of the three genes in all DNA samples and identified the nonsynonymous mutations. RESULTS AND CONCLUSIONS: For PROS1 we identified 15 nonsynonymous mutations in 28 DVT patients; for PROC, 10 nonsynonymous mutations in 17 patients; and for SERPINC1, 13 nonsynonymous mutations in 14 patients. Five patients had two mutations in PROS1 and PROC, and all of them had PROS1 K196E mutation. We previously identified one patient with a large PROS1 gene deletion. Thus, 55 out of 173 patients (32%) carried at least one genetic defect in the three genes. The PROS1 K196E mutation found in 15 Japanese DVT patients was the most prevalent. Mutations of PROC K193del and V339M were the second, each found in four patients. Our data suggested that the PROC K193del mutation caused the loss of the anticoagulant activity but not the amidolytic activity. Our effort is the first DNA resequencing study to identify the genetic variations in DVT patients without any consideration of their plasma activities and antigens. To minimize selection bias in a future evaluation of the contribution of genetic deficiency to DVT, we must recruit patients consecutively.

Hereditary deficiency of protein C or protein S confers increased risk of arterial thromboembolic events at a young age: results from a large family cohort study.

BACKGROUND: Whether hereditary protein S, protein C, or antithrombin deficiency is associated with arterial thromboembolism (ATE) and whether history of venous thromboembolism in these subjects predisposes them to subsequent ATE have yet to be determined. METHODS AND RESULTS: On the basis of pedigree analysis, we enrolled a total of 552 subjects (52% women; mean age, 46+/-17 years), belonging to 84 different kindreds, in this retrospective family cohort study. Detailed information on previous episodes of venous thromboembolism, ATE, anticoagulant use, and atherosclerosis risk factors was collected. Primary study outcome was objectively verified symptomatic ATE. Of 552 subjects, 308 had protein S (35%), protein C (39%), or antithrombin (26%) deficiency. Overall, annual incidences of ATE were 0.34% (95% confidence interval [CI], 0.23 to 0.49) in deficient versus 0.17% (95% CI, 0.09 to 0.28) in nondeficient subjects; the hazard ratio was 2.3 (95% CI, 1.2 to 4.5). Because the risk hazards varied over lifetime, we performed a time-dependent analysis. After adjusting for atherosclerosis risk factors and clustering within families, we found that deficient subjects had a 4.7-fold (95% CI, 1.5 to 14.2; P=0.007) higher risk for ATE before 55 years of age versus 1.1 (95% CI, 0.5 to 2.6) thereafter compared with nondeficient family members. For separate deficiencies, the risks were 4.6- (95% CI, 1.1 to 18.3), 6.9- (95% CI, 2.1 to 22.2), and 1.1- (95% CI, 0.1 to 10.9) fold higher in protein S-, protein C-, and antithrombin-deficient subjects, respectively, before 55 years of age. History of venous thromboembolism was not related to subsequent ATE (hazard ratio, 1.1; 95% CI, 0.5 to 2.2). CONCLUSIONS: Compared with nondeficient family members, subjects with protein S or protein C deficiency but not antithrombin deficiency have a higher risk for ATE before 55 years of age that is independent of prior venous thromboembolism.

The risk of symptomatic pulmonary embolism due to proximal deep venous thrombosis differs in patients with different types of inherited thrombophilia.

It is uncertain whether the presence of inherited thrombophilia influences the risk of developing symptomatic pulmonary embolism (PE) and whether different thrombophilic alterations are associated with different risks of symptomatic PE. To investigate such issue, we retrospectively studied 920 patients with proximal deep vein thrombosis (DVT) of the legs with or without symptomatic PE referred for thrombophilia screening; patients with overt cancer or antiphospholipid antibodies had been excluded. Three hundred fifty-four patients (38.5%) had deficiency of antithrombin (AT, n = 16), protein C (PC, n = 26), protein S (PS, n = 22), factor V Leiden (FVL, n = 168), prothrombin G20210A (PT-GA, n = 87), or multiple abnormalities (n = 35), and 566 had none of the studied thrombophilic abnormalities. Symptomatic PE complicated the first DVT in 242 patients (26%); the risk of PE was increased in patients with AT deficiency (relative risk [RR] 2.4, 95% confidence interval [CI] 1.6-3.6) or with PT-GA (RR 1.5, 95%CI 1.1-2.0) and decreased in those with FVL (RR 0.7, 95%CI 0.5-1.0) in comparison with those with unknown inherited defect. These data suggest that patients with proximal DVT have different risks of symptomatic PE according to the type of inherited thrombophilia.

Advances in understanding pathogenic mechanisms of thrombophilic disorders.

Venous thromboembolism is a major medical problem, annually affecting 1 in 1000 individuals. It is a typical multifactorial disease, involving both genetic and circumstantial risk factors that affect a delicate balance between procoagulant and anticoagulant forces. In the last 50 years, the molecular basis of blood coagulation and the anticoagulant systems that control it have been elucidated. This has laid the foundation for discoveries of both common and rare genetic traits that tip the natural balance in favor of coagulation, with a resulting lifelong increased risk of venous thrombosis. Multiple mutations in the genes for anticoagulant proteins such as antithrombin, protein C, and protein S have been identified and constitute important risk factors. Two single mutations in the genes for coagulation factor V (FV Leiden) and prothrombin (20210G>A), resulting from approximately 20,000-year-old mutations with subsequent founder effects, are common in the general population and constitute major genetic risk factors for thrombosis. In celebration of the 50-year anniversary of the American Society of Hematology, this invited review highlights discoveries that have contributed to our present understanding of the systems that control blood coagulation and the genetic factors that are involved in the pathogenesis of venous thrombosis.

[Rare thrombophilic states]. / Thrombophilies rares.

PURPOSE: Rare thrombophilic states are mostly associated with recurrent venous thrombosis or severe thrombosis such as neonatal purpura fulminans. We review here the various causes of rare thrombophilic states. CURRENT KNOWLEDGE AND KEY POINTS: Rare thrombophilic states associated with recurrent venous thrombosis include the following: antithrombin deficiencies, homozygous for protein C or protein S deficiency or compound heterozygous, double heterozygous for genetic thrombophilia and the rare thrombophilia due to auto-antibodies. Their frequency in patients with venous thromboembolism is below 2%. FUTURE PROSPECTS AND PROJECTS: These uncommon thrombophilic states require a treatment in a specialized department.

[Therapeutic consequences of thrombophilic testing]. / Conséquences thérapeutiques de la mise en évidence d'une thrombophilie.

OBJECTIVE: The objectives of this article are to review the data about the consequences of thrombophilia testing and to think about its indications. CURRENT KNOWLEDGE AND KEY POINTS: The indications of congenital thrombophilic testing have extended since the discovery of prevalent abnormalities, such as mutations of factor V or II genes. However, thrombophilia does not result in a significant increase in the risk of recurrence unlike the spontaneous occurrence of thrombotic events. The factor V Leiden mutation is associated with a moderate increase in recurrence rate, while the G20210A mutation of factor II is not associated with a significant increase in recurrence. Regarding the decrease in natural anticoagulants is concerned, there is no definite conclusion, although the decrease in antithrombin is suspected of being associated with an increase in recurrence. FUTURE PROSPECTS AND PROJECTS: Finally, identification of a constitutional thrombophilia most often do not influence the therapeutic decisions unless some rare abnormalities are found, such as a decrease in antithrombin, homozygous mutations in factors V or II genes or associations of thrombophilia. One must remember that antiphospholipid antibodies must be searched because their impact on recurrences is well-known. Diagnostic work-up for thrombophilia is not useful after a distal or a superficial venous thrombosis (except for antiphospholipid antibodies in case of distal venous thrombosis).