The superiority of anti-FXa assay over anti-FIIa assay in detecting heparin-binding site antithrombin deficiency

OBJECTIVES

Antithrombin is a progressive inhibitor of active factor X (FXa) and thrombin (FIIa). Its effect is 500- to 1,000-fold accelerated by heparin or heparan sulfate. Heterozygous type I (quantitative) and most type II (qualitative) antithrombin deficiencies highly increase the risk of venous thromboembolism (VTE), while homozygous mutations are lethal. The functional defect affecting the heparin-binding site confers moderate risk of VTE to heterozygous and high risk of VTE to homozygous individuals.

METHODS

Antithrombin activity assays based on the inhibition of FIIa and FXa were compared for their efficiency in detecting heparin-binding site defects.

RESULTS

With a single exception, in heterozygotes for heparin-binding site defects (n = 20), anti-FIIa activities remained in the reference interval, while anti-FXa activities were uniformly decreased. In individuals who were homozygous for heparin-binding site mutations (n = 9), anti-FIIa activities were in the range of 48% to 80%; the range of anti-FXa activities was 9% to 25%. Anti-FIIa and anti-FXa activities in type I deficiencies and type II pleiotropic deficiency did not differ significantly.

CONCLUSIONS

Anti-FXa antithrombin assay is recommended as a first-line test to detect type II heparin-binding site antithrombin deficiency.

Factor XIII deficiency: complete phenotypic characterization of two cases with novel causative mutations

Coagulation factor XIII (FXIII) exists as heterotetramer (FXIII-A₂B₂) in the plasma and as dimer (FXIII-A₂) in cells. Activated FXIII mechanically stabilizes fibrin and protects it from fibrinolysis by cross-linking fibrin chains and α₂-plasmin inhibitor to fibrin. FXIII is essential to maintaining haemostasis, and its deficiency causes severe bleeding diathesis. Due to improper laboratory practices, FXIII deficiency is considered the most under-diagnosed bleeding disorder. The aim of this study was to demonstrate in two cases how FXIII deficiency is properly diagnosed and classified, and to compare results of laboratory analysis and clinical symptoms. FXIII activity from plasma and platelets was measured by a modified ammonia release assay, while FXIII-A₂B₂, FXIII-A and FXIII-B antigens were determined by ELISA. The exon-intron boundaries and the promoter region of F13A1 gene were amplified by PCR and the amplified products were analysed by direct fluorescent sequencing. FXIII-A mRNA in platelets was determined by RT-qPCR. Two children with severe bleeding symptoms were investigated. In both cases FXIII activity and FXIII-A antigen were undetectable in the plasma and platelet lysate. In the plasma no FXIII-A₂B₂ antigen was found, while FXIII-B antigen was >30% in both cases. Proband1 was a compound heterozygote possessing a known missense mutation (c.980G>A, p.Arg326Gln) and a novel splice-site mutation (c.1112+2T>C). Proband2 was homozygote for a novel single nucleotide deletion (c.212delA) leading to early stop codon. The discovered mutations explain the severity of clinical symptoms and the laboratory data. Methods precise in the low activity/antigen range are required to draw valid conclusion on phenotype-genotype relationship.

Comparison of a new P2Y12 receptor specific platelet aggregation test with other laboratory methods in stroke patients on clopidogrel monotherapy

Background

Clinical studies suggest that 10-50% of patients are resistant to clopidogrel therapy. ADP induced platelet aggregation, a widely used test to monitor clopidogrel therapy, is affected by aspirin and is not specific for the P2Y12 receptor inhibited by clopidogrel.

Objectives

To develop a P2Y12-specific platelet aggregation test and to compare it with other methods used for monitoring clopidogrel therapy.

Patients/Methods

Study population included 111 patients with the history of ischemic stroke being on clopidogrel monotherapy and 140 controls. The effect of clopidogrel was tested by a newly developed ADP(PGE1) aggregation test in which prostaglandin E1 treated platelets are used. Results of conventional ADP induced platelet aggregation, VerifyNow P2Y12 assay and ADP(PGE1) aggregation were compared to those obtained by flow cytometric analysis of vasodilator stimulated phosphoprotein (VASP) phosphorylation. Reference intervals for all assays were determined according to the guidelines of Clinical Laboratory Standards Institute.

Results

The P2Y12-specificity of ADP(PGE1) test was proven by comparing it with ADP aggregation in the presence of P2Y1 antagonist, adenosine 3’, 5’-diphosphate. The method was not influenced by aspirin treatment. Approximately 50% of patients were clopidogrel resistant by conventional ADP aggregation and VerifyNow tests. The ADP(PGE1) method and the VASP phosphorylation assay identified 25.9% and 11.7% of patients as non-responders, respectively. ADP(PGE1) aggregation showed good correlation with VASP phosphorylation and had high diagnostic efficiency.

Conclusion

The new ADP(PGE1) method is a reliable test for monitoring P2Y12 receptor inhibition by platelet aggregation. As a subset of patients are non-responders, monitoring clopidogrel therapy by adequate methods is essential.

Thrombomodulin-dependent effect of factor V Leiden mutation on the cross-linking of α2-plasmin inhibitor to fibrin and its consequences on fibrinolysis

INTRODUCTION

It has been shown that thrombomodulin (TM) considerably delays factor XIII (FXIII) activation and this effect is abrogated by Factor V Leiden (FV(Leiden)) mutation. The aim of the study was to explore the effect of TM on the cross-linking of α(2)-plasmin inhibitor (α(2)-PI) to fibrin in plasma samples of different FV genotypes and how this effect is related to the impaired fibrinolysis of FV(Leiden) carriers.

METHODS

In the plasma samples of fifteen individuals with different FV genotypes and in FV deficient plasma supplemented with wild type FV or FV(Leiden) coagulation was initiated by recombinant human tissue factor and phospholipids with or without recombinant human TM (rhTM). In the recovered clots the extent of α(2)-PI-fibrin cross-linking was evaluated by Western blotting and quantitative densitometry. The effect of rhTM on tissue plasminogen activator (tPA) induced clot lysis was measured by turbidimetric method.

RESULTS

rhTM significantly delayed the formation of α(2)-PI-fibrin α-chain heterodimers/oligomers in plasma samples containing wild type FV. This effect of rhTM was impaired in the presence of FV(Leiden). rhTM delayed tPA-induced clot lysis and this effect of rhTM was more pronounced in plasma containing FV(Leiden). When TAFIa was inhibited by potato carboxypeptidase inhibitor, rhTM accelerated clot lysis in the presence of wild type FV, which is explained by the delayed α(2)-PI-fibrin cross-linking. This effect of rhTM did not prevail in the presence of FV(Leiden).

CONCLUSION

FV(Leiden) abrogates the delaying effect of rhTM on α(2)-PI-fibrin cross-linking, which contributes to the impaired fibrinolysis observed in FV(Leiden) carriers.

Measurement of factor XIII activity in plasma

Coagulation factor XIII (FXIII) is converted by thrombin and Ca(2+) into an active transglutaminase (FXIIIa) in the final phase of coagulation cascade. Its main function is the mechanical stabilization of fibrin clot and its protection from fibrinolysis by cross-linking of fibrin chains and α(2)-plasmin inhibitor to fibrin. In non-substituted patients FXIII deficiency is a severe hemorrhagic diathesis, not infrequently with fatal consequences. The main reason for using FXIII assays is the diagnosis of FXIII deficiency. The aim of this review is to provide a comprehensive critical evaluation of the methods reported for the determination of FXIII activity in the plasma. Such methods are based on two principles: 1) measurement of labeled amines incorporated by FXIIIa into a glutamine residue of a substrate protein, 2) monitoring ammonia released from a peptide bound glutamine residue by FXIIIa using NAD(P)H dependent glutamate dehydrogenase indicator reaction. The incorporation assays are sensitive, but cumbersome and time-consuming, they are difficult to standardize and cannot be automated. The ammonia release assays are less sensitive, but quick, well standardized, and can be automated; this type of assay is recommended for the screening of FXIII deficiency. The traditional clot solubility assay should not be used for this purpose.

Factor XIII, clot structure, thrombosis

Blood coagulation factor XIII (FXIII) is a tetrameric protein consisting of two catalytic A (FXIII-A) and two carrier/inhibitory B (FXIII-B) subunits. It is a zymogen, which becomes transformed into an active transglutaminase (FXIIIa) in the final phase of coagulation cascade by thrombin and Ca(2+). FXIII is essential for hemostasis, its deficiency results in severe bleeding diathesis. FXIIIa mechanically stabilizes fibrin by cross-linking its α-, and γ-chains. It also protects newly formed fibrin from fibrinolysis, primarily by cross-linking α(2)-plasmin inhibitor to fibrin. Beside the above prothrombotic effects, it is involved in limiting thrombus growth by down-regulating platelet adhesion to fibrin. Elevated FXIII level seems to be a gender-specific risk factor of both coronary artery disease and peripheral arterial disease, it represents an increased risk only in females. The association of FXIII level with the risk of ischemic stroke and venous thromboembolism was investigated only in a few studies from which no clear conclusion could be drawn. Among the FXIII subunit polymorphisms, concerning their effect on the risk of thrombotic diseases, only FXIII-A p.Val34Leu was investigated extensively. Meta-analyses of reported data suggest that this polymorphism provides a moderate protection against coronary artery disease and venous thromboembolism, but not against ischemic stroke. Gene-gene and gene-environmental interactions might modify its effect. Further studies are required to explore the effect of other FXIII subunit polymorphism on the risk of thrombotic diseases.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Interaction between homocysteine and lipoprotein(a) increases the prevalence of coronary artery disease/myocardial infarction in women: a case-control study

INTRODUCTION

Our aim was to investigate the association of elevated homocysteine (Hcy) and lipoprotein(a) Lp(a) with the prevalence of coronary artery disease (CAD) and myocardial infarction (MI) and to investigate their interaction in both genders.

MATERIALS AND METHODS

955 (male/female: 578/377) consecutive patients admitted for coronary angiography were enrolled in the study. Lp(a), Hcy, vitamin B12, folic acid, MTHFR C677T polymorphism and traditional risk factors were determined.

RESULTS

619 patients had significant (≥50%) stenosis (CAD+) and 341 had MI (MI+). CAD-MI- cases (n=302) were considered as controls. Adjusted Hcy levels were significantly elevated only in the female CAD+MI+group that was related to decreased vitamin B12 levels. Lp(a) was elevated in the CAD+MI+group of both genders. Folic acid levels and MTHFR T677 allele frequency did not show significant difference. Moderate hyperhomocysteinemia (Hcy >15μmol/L) or elevated Lp(a) (>300mg/L) increased the risk of CAD (OR 2.27, CI 1.36-3.80 and OR 1.64, CI 1.03-2.61, respectively) and MI (OR 2.52, CI 1.36-4.67 and OR 1.89, CI 1.06-3.38, respectively) only in women. Only simultaneous but not isolated elevation of Hcy and Lp(a) conferred a significant, 3.6-fold risk of CAD in females and even higher (11-fold) risk in young females, which suggested an interactive effect.

CONCLUSIONS

Moderate hyperhomocysteinemia or elevated Lp(a) level associated with a risk of CAD and MI only in women. While isolated elevation of one of the two parameters represented a mild risk of CAD, their combined elevation highly increased the risk in females. No such effect was observed in males.

Thrombomodulin-dependent effect of factor VLeiden mutation on factor XIII activation

INTRODUCTION

Factor V Leiden mutation (FV(Leiden)) is associated with impaired down-regulation of activated FV procoagulant activity and loss of FV anticoagulant function that result in an increased risk of venous thromboembolism. As the downstream effects of FV(Leiden) on clot formation and fibrinolyis have only partially been revealed, we investigated its effect on the activation of factor XIII (FXIII) and the cross-linking of fibrin.

METHODS

In the plasma samples of fifteen healthy individuals with known FV genotypes coagulation was initiated by recombinant human tissue factor and phospholipids with or without recombinant human thrombomodulin (rhTM). FV deficient plasma supplemented with purified wild type FV or FV(Leiden) were also investigated. Clots were recovered and analyzed by SDS-PAGE and quantitative densitometric evaluation of Western blots.

RESULTS

rhTM considerably delayed the activation of FXIII in the plasma from FV wild type individuals. This effect of rhTM was significantly impaired in the plasma from FV(Leiden) carriers. The results were confirmed in experiments with FV deficient plasma supplemented by FV prepared from wild type individuals or FV(Leiden) homozygotes. Fibrin γ-chain dimerization was also considerably delayed by rhTM in plasma samples from individuals without Leiden mutation, but not in plasma samples from FV(Leiden) heterozygotes or homozygotes. The difference between heterozygotes and homozygotes was not statistically significant.

CONCLUSION

The highly diminished delaying effect of TM on FXIII activation and on the cross-linking of fibrin in FV(Leiden) carriers might represent a novel mechanism contributing to the increased thrombosis risk of these individuals.

Factor XIII and venous thromboembolism

Plasma factor XIII (FXIII) is a tetrameric zymogen consisting of two potentially active A subunits (FXIII-A) and two carrier/inhibitory B subunits (FXIII-B). In the final phase of the coagulation cascade, FXIII is converted into an active transglutaminase (FXIIIa) by thrombin and Ca (2 + ). FXIIIa strengthens fibrin clot mechanically by cross-linking fibrin chains. In addition, FXIIIa is a key regulator of fibrinolysis, protecting newly formed fibrin from the fibrinolytic machinery by binding α (2)-plasmin inhibitor to the fibrin meshwork. FXIII is essential for maintaining hemostasis; its severe deficiency causes a life-threatening bleeding diathesis. The involvement of FXIII in thrombotic diseases and its association with the risk of these disorders is less clear. The role of FXIII in atherothrombotic diseases has been recently reviewed. This article offers a general overview of the relationship between FXIII and venous thromboembolism (VTE), to collect individual publications on this topic, present conclusions, and examine limitations of published studies. Special attention is given to the association of FXIII-A polymorphism with the risk of VTE, which has provoked considerable interest over the last decade.

Factor XIII: novel structural and functional aspects

Factor (F)XIII is a protransglutaminase that, in addition to maintaining hemostasis, has multiple plasmatic and intracellular functions. Its plasmatic form (pFXIII) is a tetramer of two potentially active A (FXIII-A) and two inhibitory/carrier B (FXIII-B) subunits, whereas its cellular form (cFXIII) is a dimer of FXIII-A. FXIII-A belongs to the family of transglutaminases (TGs), which show modest similarity in the primary structure, but a high degree of conservatism in their domain and sub-domain secondary structure. FXIII-A consists of an activation peptide, a β-sandwich, a catalytic and two β-barrel domains. FXIII-B is a glycoprotein consisting of 10 repetitive sushi domains each held together by two internal disulfide bonds. The structural elements of FXIII-A involved in the interaction with FXIII-B have not been elucidated; in FXIII-B the first sushi domain seems important for complex formation. In the circulation pFXIII is bound to the fibrinogen γ'-chain through its B subunit. In the process of pFXIII activation first thrombin cleaves off the activation peptide from FXIII-A, then in the presence of Ca(2+) FXIII-B dissociates and FXIII-A becomes transformed into an active transglutaminase (FXIIIa). The activation is highly accelerated by the presence of fibrin(ogen). cFXIII does not require proteolysis for intracellular activation. The three-dimensional structure of FXIIIa has not been resolved. Based on analogies with transglutaminase-2, a three-dimensional structure of FXIIIa was developed by molecular modeling, which shows good agreement with the drastic structural changes demonstrated by biochemical studies. The structural requirements for enzyme-substrate interaction and for transglutaminase activity are also reviewed.

Antithrombin deficiency and its laboratory diagnosis

Antithrombin (AT) belongs to the serpin family and is a key regulator of the coagulation system. AT inhibits active clotting factors, particularly thrombin and factor Xa; its absence is incompatible with life. This review gives an overview of the protein and gene structure of AT, and attempts to explain how glucosaminoglycans, such as heparin and heparan sulfate accelerate the inhibitory reaction that is accompanied by drastic conformational change. Hypotheses on the regulation of blood coagulation by AT in physiological conditions are discussed. Epidemiology of inherited thrombophilia caused by AT deficiency and its molecular genetic background with genotype-phenotype correlations are summarized. The importance of the classification of AT deficiencies and the phenotypic differences of various subtypes are emphasized. The causes of acquired AT deficiency are also included in the review. Particular attention is devoted to the laboratory diagnosis of AT deficiency. The assay principles of functional first line laboratory tests and tests required for classification are discussed critically, and test results expected in various AT deficiency subtypes are summarized. The reader is provided with a clinically oriented algorithm for the correct diagnosis and classification of AT deficiency, which could be useful in the practice of routine diagnosis of thrombophilia.

Factor XIII subunits in human tears; their highly elevated levels following penetrating keratoplasty

BACKGROUND

As blood coagulation factor XIII (FXIII) is of high importance in wound healing, we determined the concentrations of FXIII A and B subunits (FXIII-A and FXIII-B) and their complex (FXIII-A(2)B(2)) in normal tears and in tears from patients undergoing penetrating keratoplasty (PKP).

METHODS

FXIII complex and subunit concentrations were measured by highly sensitive chemiluminescent ELISAs in tears from 60 healthy volunteers and from 31 patients undergoing corneal transplantation.

RESULTS

In non-stimulated tears from healthy volunteers, low but consistent amounts of FXIII-A and FXIII-B (medians: 2.13 μg/L and 7.22 μg/L, respectively) were measured, mostly in non-complexed form. Following stimulation of tear secretion FXIII levels moderately decreased, but if normalized to protein concentration they did not change. One day after PKP FXIII levels became highly elevated, then gradually decreased, but even on day 7 significantly exceeded pre-surgery values. The elevation of tear FXIII levels was significantly higher in PKP patients who later developed neovascularization of donor cornea.

CONCLUSIONS

FXIII subunits are low concentration components of normal tear. The striking elevation of FXIII subunit and FXIII-A(2)B(2) concentrations after PKP suggests the involvement of FXIII in corneal wound healing. Perioperatively measured high FXIII levels in tears seem to represent a risk of neovascularization.

Clinical and immunoserological characteristics of the transition from primary to overlap antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a distinct clinical entity characterized by arterial and venous thromboembolic events, recurrent fetal loss and the presence of antiphospholipid antibodies in the patients' sera. In primary APS, there is no detectable underlying disease, while overlap APS is associated with clinical syndromes including systemic autoimmune diseases, infections, or malignancies. We carried out a retrospective analysis of serological and clinical manifestations as well as assessed outcome-measures in 165 patients with primary APS. Thrombotic manifestations and possible signs of autoimmune diseases were determined at the time of the diagnosis, followed by the analysis of recurrent thrombotic events and effects of therapy during the follow-up period. Among the 165 patients with primary APS at onset, 105 patients (63%) remained primary APS after a mean 5.2 years of follow-up. In 14% of the patients, subsequently APS became associated with various characteristics of undifferentiated connective tissue disease. Finally 23% of patients evolved into a definitive systemic autoimmune disease during a mean 9.75 years of follow-up. Recurrent thrombotic events were registered in 24% of patients. Our results suggest that primary APS may be considered as a potential early phase of a dynamic transition towards a well-defined systemic autoimmune disease.

Factor XIII and atherothrombotic diseases

Factor XIII (FXIII) is a protransglutaminase that, after activation, cross-links fibrin chains and several plasma proteins, most importantly alpha (2) plasmin inhibitor, to fibrin. FXIII strengthens the fibrin clot by covalent bonds and protects fibrin from the prompt elimination by the fibrinolytic system. In the last two decades, FXIII has emerged as a key regulator of fibrinolysis. FXIII is also present in platelets, monocytes, and macrophages, but this cellular form does not contribute significantly to maintaining hemostasis. FXIII deficiency is a life-threatening bleeding diathesis whose clinical consequences are well studied. In contrast, the involvement of FXIII in thrombotic disorders and its association with the risk of such diseases are less clear. This review gives an account of the data accumulated mainly in the last decade on the association of FXIII with atherothrombotic diseases and presents conclusions and hypotheses drawn from these data as well as exposing the limitations of the published studies and our knowledge on this topic. The involvement of FXIII in atherogenesis, its role in coronary artery disease, atherothrombotic ischemic stroke, and peripheral artery disease are discussed, with particular reference to the association of FXIII levels and polymorphisms with the risk of these diseases.