A novel specific immunoassay for plasma two-chain factor VIIa: investigation of FVIIa levels in normal individuals and in patients with acute coronary syndromes

We report the development of an enzyme-linked immunosorbent assay (ELISA) that is specific for factor VIIa (FVIIa). This assay uses a neoantigen specific capture antibody directed to the amino acid peptide sequence N terminal to the FVII cleavage activation site. The antibody exhibits approximately 3,000-fold greater reactivity to FVIIa than FVII on a molar basis. Experiments using plasma with added (exogenous) human FVIIa gave quantitative recovery in the ELISA over a range of 0.20 to 3.2 ng/mL of FVIIa. The intra- and inter-assay coefficient of variation (CVs) of the ELISA are 4.5% and 9.8%, respectively. The ELISA shows excellent correlation (r = .99) with a functional assay (using recombinant soluble tissue factor) in detecting FVIIa added to plasma over the range 0.05 to 18.0 ng/mL. However, a major discrepancy exists between the two assays when normal endogenous plasma concentrations of FVIIa are measured. Using normal plasma (n = 14) the functional assay reported 3.10 +/- 0.30 ng/mL (mean +/- SE) whereas only 0.025 +/- 0.010 ng/mL was detected in the same samples by the immunoassay. Patients (n = 43) presenting with acute coronary syndromes (myocardial infarction and unstable angina) exhibited elevations (P < .05) in immunologically detected FVIIa, 0.093 +/- 0.013 ng/mL (mean +/- SE) compared to patient controls (n = 20) contemporaneously admitted with noncardiac chest pain, 0.048 +/- 0.007 ng/mL (mean +/- SE). These elevations in the acute coronary syndromes were accompanied by increased (P < .05) and correlating prothrombin fragment F1 + 2 levels (Spearman correlation coefficient rs = .4, P < .01), demonstrating that thrombin generation is certainly associated with, and may even be caused by, extrinsic pathway activation.

Gene mutations in 21 unrelated cases of phenotypic heterozygous protein C deficiency and thrombosis. Protein C Study Group

Mutations have been identified in the protein C gene in 21 patients with venous thromboembolism and phenotypic heterozygous protein C deficiency. In 20 probands, single mutations were the only abnormalities identified by sequencing all coding regions, intron exon boundaries and the promoter region back to -1540. In one proband 2 mutations were identified and in another family 2 mutations were identified (but not both in the proband). Of the 23 mutations, 18 resulted in predicted amino acid substitutions, 3 were mutations resulting in stop codons, one was a mutation within a consensus splice sequence and another a 9 base pair insertion within exon 5 (this region within exon 5 is proposed as a deletion/insertion hot spot). A novel polymorphism was also, uniquely, identified in the propeptide region of the molecule (Pro-21Pro; CCT to CCC) in a kindred from Hong Kong. Cosegregation of the protein C gene mutation with protein C deficiency could be determined in 13 families. In a further family, phenotypic protein C deficiency and the genetic mutation cosegregated in only 4/5 members. The first thrombotic incident occurred in the probands between the ages of 11 and 59 years and 12 individuals suffered recurrent thrombosis. Thrombosis occurred in at least one other family member in 9/21 families, but in 2 of these it was inconsistently associated with protein C deficiency. An independent genetic risk factor, factor V Arg506Gln (FV Leiden) was identified in 2 probands (and 3 family members) and in 4 protein C deficient members of a third family but not in the proband. The results suggest that in the majority of probands with thrombosis and phenotypic protein C deficiency, a single protein C gene mutation is associated with thrombosis. However, it is also possible that additional unknown genetic risk factors contribute to the thrombotic risk. An added, acquired, risk factor leads to thrombosis at an early age (< 25 years).

Identification of 19 protein S gene mutations in patients with phenotypic protein S deficiency and thrombosis. Protein S Study Group

Protein S is a protein C-dependent and independent inhibitor of the coagulation cascade. Deficiency of protein S is an established risk factor for venous thromboembolism. We have used a strategy of specific amplification of the coding regions and intron/exon boundaries of the active protein S gene (PROS1) and direct single-strand solid phase sequencing, to seek mutations in 35 individuals with phenotypic protein S deficiency. Nineteen point mutations (16 novel) in 19 probands (or relatives of probands) with venous thromboembolism are reported here. Fifteen of the 19 mutations were expected to be causal and included 10 missense mutations (Lys9Glu, Glu26Ala, Gly54Glu, Cys145Tyr, Cys200Ser, Ser283Pro, Gly340Asp, Cys408Ser, Ser460Pro, and Cys625Arg). Three of the 15 mutations resulted in premature stop codons (delete T 635 producing a stop codon at position 126, Lys368stop and Tyr595stop) and two were at intron/exon boundaries (+1 G to A in intron d and +3 A to C in intron j). Of the remaining four mutations, three were within intronic sequence and one was a silent mutation within the coding region and did not alter amino acid composition. In two of the 10 missense mutations, reduced plasma protein S activity compared with antigen level suggested the presence of variant (type II) protein S.

Directed search for thrombomodulin gene mutations

Thrombomodulin is a principle thrombin receptor located on the vascular endothelium. Thrombomodulin alters the specificity of thrombin, redirecting its procoagulant function to an anticoagulant function by making it a more efficient activator of protein C. While mutation of the genes of other components of this anticoagulant mechanism, protein C, protein S and factor V, is known to predispose towards venous thromboembolism, there are only a few reports of the investigation of thrombomodulin gene mutation. We present the design and evaluation of a strategy to investigate thrombomodulin gene mutation in arterial and venous thrombosis.

Prenatal diagnosis in combined antithrombin and factor V gene mutation

We report prenatal diagnosis in a family with combined antithrombin deficiency (type II heparin binding site) and factor V 506 Arg to Gln mutation. Both clinically unaffected parents are heterozygous for the antithrombin mutation, which results in a 99 Leu to Phe substitution, and the father is also heterozygous for the factor V gene defect. There is one daughter, homozygous for the antithrombin and heterozygous for the factor V mutations, who suffered a right-sided hemiparesis at the age of 4 months due to occlusion of the left middle cerebral artery and a large left sided infarct followed by further thromboembolic events. The family requested prenatal diagnosis and chorionic villi was sampled at 12 weeks gestation. The fetus was shown to be heterozygous for the antithrombin and factor V gene mutations, the same genotype as the unaffected father. No further intervention was considered necessary. To our knowledge this is the first report of prenatal diagnosis in antithrombin deficiency.

High purity factor IX and prothrombin complex concentrate (PCC): pharmacokinetics and evidence that factor IXa is the thrombogenic trigger in PCC

Recent studies using assays for surrogate markers of thrombogenicity in man have demonstrated that activation of the coagulation system occurs following infusion of clinical doses of prothrombin complex concentrates (PCC) but not after the same doses of high-purity factor IX concentrates (HP-FIX) in patients with haemophilia B. Here we have investigated the mechanism of such thrombogenesis by applying assays that detect early-through to late-events in coagulation system activation in a pharmacokinetic cross-over study of 50 IU/kg PCC and a new HP-FIX product in haemophilia B patients. Satisfactory recoveries and half-lives were observed for both concentrates. HP-FIX caused no increases in thrombin-antithrombin III complex (TAT), prothrombin activation peptide fragment F1+2 (F1+2), factor X activation peptide (FXAP) or factor VIIa (FVIIa). In contrast the same dose of factor IX in the form of PCC was followed by significant increases over pre-infusion levels of TAT, F1+2 and FXAP, but not FVIIa. Elevations of FIXAP occurred after both HP-FIX and PCC but did not reach normal levels and were attributed to normalisation of the FIX concentration in those patients whose levels of FIXAP were initially low. We conclude that the thrombogenic trigger associated with PCC infusion occurs at the level of factor X activation. In the absence of any increase in FVIIa, we would attribute this to the likely presence of FIXa in the PCC.

Molecular genetics of antithrombin deficiency

Antithrombin is the major proteinase inhibitor of thrombin and other blood coagulation proteinases. Antithrombin has two functional domains, a heparin binding site and a reactive centre (that complexes and inactivates the proteinase). Its deficiency results in an increased risk of venous thromboembolism. Appreciable progress has been made in recent years in understanding the structure and function of this protein, the genetic cause of inherited deficiency and its clinical consequence. The structure of antithrombin is now considered in terms of the models derived from X-ray crystallography, which have provided explanations for the function of its heparin interaction site and of its reactive loop. The structural organization of the antithrombin gene has been defined and numerous mutations have been identified that are responsible for antithrombin deficiency: these may reduce the level of the protein (Type I deficiency), alter the function of the protein (Type II deficiency, altering heparin binding or reactive sites), or even have multiple or 'pleiotropic effects' (Type II deficiency, altering both functional domains and the level of protein).

Oral contraceptives enhance the risk of clinical manifestation of venous thrombosis at a young age in females homozygous for factor V Leiden

In 29 patients (17 females) homozygous Arg 506 Gln mutation (FV Leiden) was identified. 25 had been investigated because of venous thromboembolism (VTE); four asymptomatic patients were found during family studies. The first VTE had occurred significantly earlier in females (median age [m] 26 years, range 17-49) than in males (m=38 years, range 21-82) (P=0.01). 12 females (80%) had taken oral contraceptives (OC, estrogen content 0.02-0.1 mg) for 6-150 months prior to thrombosis. Further triggering conditions in females were hormone replacement (n=1) and pregnancy (n=2). In 8/10 males the first VTE had occurred spontaneously--in two after surgery. The sites of VTE were deep vein thrombosis, pulmonary embolism, caval vein thrombosis and superficial thrombophlebitis. From our data we conclude that OC medication is the most important precipitating factor for VTE in females with homozygous FV Leiden.

Factor V Leiden (FV R506Q) in families with inherited antithrombin deficiency

We investigated the presence of the gene mutation of factor V, FV R506Q or factor V Leiden, responsible for activated protein C resistance, in DNA samples of 127 probands and 188 relatives from 128 families with antithrombin deficiency. The factor V mutation was identified in 18 families. Nine families were available to assess the mode of inheritance and the clinical relevance of combined defects. The factor V and antithrombin genes both map to chromosome 1. Segregation of the defects on opposite chromosomes was observed in three families. Co-segregation with both defects on the same chromosome was demonstrated in four families. In one family a de novo mutation of the antithrombin gene and in another a crossing-over event were the most likely explanations for the observed inheritance patterns. In six families with type I or II antithrombin deficiency (reactive site or pleiotropic effect), 11 of the 12 individuals with both antithrombin deficiency and the factor V mutation developed thrombosis. The median age of their first thrombotic episode was 16 years (range 0-19); this is low compared with a median age of onset of 26 years (range 20-49) in 15 of 30 carriers with only a defect in the antithrombin gene. One of five subjects with only factor V mutation experienced thrombosis at 40 years of age. In three families with type II heparin binding site deficiencies, two of six subjects with combined defects experienced thrombosis; one was homozygous for the heparin binding defect. Our results show that, when thrombosis occurs at a young age in antithrombin deficiency, the factor V mutation is a likely additional risk factor. Co-segregation of mutations in the antithrombin and factor V genes provides a molecular explanation for severe thrombosis in several generations. The findings support that combinations of genetic risk factors underly differences in thrombotic risk in families with thrombophilia.

Factor V Leiden gene mutation and thrombin generation in relation to the development of acute stroke

To determine the prevalence of the factor V Leiden gene mutation in relation to the phenotypes of cerebral infarction and cerebral hemorrhage, we studied 386 randomly selected cases of acute stroke and 247 control subjects. Factor V genotype was determined by amplification of a 267-bp sequence of exon/intron 10 of the factor V gene. Levels of prothrombin fragment F(1 + 2), a marker of thrombin generation, were determined in both acute and convalescent stroke and related to factor V genotype. Prothrombin fragment F(1 + 2) was assessed by using an enzyme-linked immunosorbent assay. Sixteen stroke cases (4.1%) were identified as having the mutation compared with 14 (5.6%) control subjects. Prothrombin fragment F(1 + 2) levels were estimated in 191 cases and found to be elevated both acutely and after 3 months, but they were not related to factor V genotype. Prothrombin fragment F(1 + 2) is elevated in acute stroke and requires further evaluation in relation to cerebrovascular disease. These results suggest that the factor V Leiden gene mutation is not a risk factor for arterial thrombosis causing stroke.

An ELISA for factor X activation peptide: application to the investigation of thrombogenesis in cardiopulmonary bypass

An ELISA for measurement of factor X activation peptide (FXAP) in plasma has been developed. The capture antibody was generated by immunization with a carrier-coupled synthetic peptide based on the amino acid sequence of the C terminal region of native human FXAP: the tag antibody was a commercial polyclonal antibody to factor X. Because of limited specificity of the capture antibody to FXAP compared with factor X, a plasma processing step precipitated plasma factor X and also permitted a concentration step, enabling detection of FXAP below the lower limit of the normal range in plasma. The overall intra- and inter-assay coefficients of variation were approximately 5% and approximately 11%, respectively. 18 normal laboratory control subjects had FXAP levels of 2.12 +/- 0.82 ng/ml (mean +/- SEM). Eight patients undergoing surgery and cardiopulmonary bypass progressively generated FXAP throughout the surgery with mean FXAP rising to 11.73 +/- 4.66 ng/ml, and this resulted in increased generation of thrombin detected by measurement of plasma levels of F1 + 2. Levels of FXAP rose significantly ahead of those of factor IX activation peptide (FIXAP), supporting a suggestion that contact system activation can not be the primary stimulus to coagulation in bypass. The ELISA to FXAP will be useful in the study of mechanisms of thrombogenesis in clinical situations where the coagulation system is activated.

Antithrombins Southport (Leu 99 to Val) and Vienna (Gln 118 to Pro): two novel antithrombin variants with abnormal heparin binding

We report the characterization of three variant antithrombins with reduced heparin binding as the primary abnormality. Two of these variants, antithrombin Southport (Leu 99 to Val, 2759 C to G) and antithrombin Vienna (Gln 118 to Pro, 5349 A to C) were novel, whereas the third, Pro 41 to Leu, has been previously described as antithrombin Basel. All three variants exhibited reduced binding for heparin on crossed immunoelectrophoresis and in a quantitative monoclonal antibody-based assay. The mutations were characterized by direct sequence analysis of enzymatically amplified genomic DNA and all affected individuals were heterozygous for the mutations. These three mutations do not occur at the sites of the basic amino acids directly involved in heparin binding nor do they result in a change in charge of the affected residue. It seems probable that they reduce heparin affinity either by perturbing the initial contact site involved in the heparin-binding domain (Arg 47, Arg 129 and possibly Arg 24), or by preventing the subsequent heparin-induced conformational change.

Hereditary antithrombin deficiency: heterogeneity of the molecular basis and mortality in Dutch families

We studied the molecular basis and genetic heterogeneity of hereditary antithrombin (III) deficiency in nine Dutch families. Polymerase chain reaction (PCR) amplification and direct sequencing of all antithrombin gene exons and flanking intronic regions identified mutations in eight families. Given the opportunity to correlate the molecular basis with survival, we addressed the relevance of molecular defects to mortality in inherited antithrombin deficiency. The defects included single nucleotide deletions (7671 del G, 7768-69 del G) and insertions (5501 ins A, 2463 G-->TC) that lead to frameshifts, a single base substitution [5381 C-->T (129Arg-->stop)] leading to a premature termination codon, and single base substitutions resulting in amino acid substitutions [2652 A-->C (63Tyr-->Ser), 13380 T-->C (421Ile-->Thr), and 13407 G-->T (430Cys-->Phe)]. All affected individuals were heterozygous for the defects. Previously we found in Dutch families that antithrombin deficiency did not lead to higher mortality compared with the general population. In accordance with these findings, we observed no excess mortality in the nine families [Observed:Expected, 52:52.6; standardised mortality ratio (SMR) 1.0, 95% confidence interval (CI), 0.7-1.3]. Our findings confirmed a considerable genetic heterogeneity underlying antithrombin deficiency. We therefore concluded that the lack of excess mortality in these families is not caused by a Dutch mild defect. We suggest that the longevity is not affected by molecular defects in the antithrombin gene and hypothesize that differences in mortality or natural history between families most likely result from other (genetic) risk factors.

Role of N- and C-terminal amino acids in antithrombin binding to pentasaccharide

We have used a monoclonal antibody-based binding procedure to determine the dissociation constants of the interactions between the essential antithrombin-binding pentasaccharide and a series of 13 distinct N- and C-terminal antithrombin substitution mutation variants with defective binding interaction with heparin. The reduction in binding affinity of the pentasaccharide with the N-terminal variants (with substitution mutations Pro-41-->Leu, Arg-47-->Cys and His, Leu-99-->Val and Phe, Gln-118-->Pro, Arg-129-->Gln) compared to normal antithrombin, Kd 200 nM, ranged from 15-984-fold and was generally less than 150-fold. Reduced binding affinity is assumed to arise mostly by perturbation, direct or indirect, of the initial contact of pentasaccharide with basic residues of antithrombin. Surprisingly, the binding interaction of the pentasaccharide with the C-terminal variants (with substitution mutations in or near strand 1C/4B, Phe-402-->Leu, Cys, and Ser, Ala-404-->Thr, Pro-407-->Thr, Pro-429-->Leu) was more uniformly and yet substantially (135-482-fold) decreased, despite the spatial separation between the site of mutation and the proposed primary contact site of the pentasaccharide. These results demonstrate that strand 1C/4B region integrity is required for optimum interaction with the pentasaccharide, suggesting its involvement in transmission of the induced conformation change required for high affinity binding.

Role of factor XII in thrombin generation and fibrinolysis during cardiopulmonary bypass

During cardiopulmonary bypass, thrombin is generated, which is thought to be initiated by activation of factor XII on the surface of the bypass equipment. We present a patient with severe factor XII deficiency who underwent cardiac surgery. As much thrombin was formed during cardiopulmonary bypass (measured by the prothrombin activation fragment F1 + 2 and thrombin-antithrombin complexes) as in normal patients, showing that factor XII was not necessary for thrombin generation. Factor X, but not factor IX, was activated (as measured by their activation peptides), and this activation correlated with F1 + 2 and thrombin-antithrombin complexes, suggesting that the tissue-factor/factor-VIIa pathway is the trigger for thrombin formation.

Antithrombin III: summary of first database update

Antithrombin III is the most important inhibitor of coagulation proteinases such as thrombin and factor Xa. Inherited deficiency of antithrombin III is a well recognised risk factor for the early development of venous thromboembolism. The gene for antithrombin III is located at chromosome 1q 23-25 and its structural organisation has been described. A database of mutations of the antithrombin III gene has been compiled and a recent update lists 184 entries. These entries are listed according to subtype of deficiency and to nucleotide sequence number. There are 68 reports of type I 'classical' and 116 reports of type II 'variant' deficiencies. This summary considers the entries in terms of the number of unique molecular events, the nature of the genetic defects and the role of CpG dinucleotides in deficiency. Sample listings of type I and II deficiency entries are provided.