Clinical and biochemical characterization of the prothrombin Belgrade mutation in a large Serbian pedigree: new insights into the antithrombin resistance mechanism

Effect of prothrombin Belgrade mutation, causing antithrombin resistance, on fibrin clot properties

INTRODUCTION

Prothrombin Belgrade mutation is the result of the c.1787G>A substitution in the prothrombin gene. It is located in the antithrombin and sodium binding site and leads to impaired inactivation of thrombin by antithrombin, resulting in antithrombin resistance and thrombotic disorders. However, it negatively affects sodium binding and may have hypocoagulant effects. Considering that prothrombin Belgrade mutation mechanism is still not fully elucidated and that sodium binding is important for thrombin affinity towards fibrinogen, our aim was to determine whether this mutation affects fibrin clot formation and lysis.

METHODS

Using HEK293T cell line, recombinant wild type and mutated prothrombin were generated by transient transfection. Samples that correspond to plasma of a non-carrier, heterozygous and homozygous carriers were reconstituted using prothrombin deficient plasma and recombinant proteins. Reconstituted samples were used in OHP assay (Overall Hemostasis Potential) to determine kinetic profiles of coagulation and fibrinolysis. Clot turbidity assay was performed to observe kinetics of clot formation and lysis more closely. Fibrin clots formed in reconstituted plasma samples were analyzed by confocal microscopy to determine density of fibrin network. Fibrin clots were additionally observed using electron microscopy to determine thickness of individual fibrin fibers.

RESULTS

No significant difference found in OHP, OCP, OFP, and fibrin network density between wild type, heterozygous, and homozygous carrier reconstituted plasma samples. There were significant differences between samples for slope and slope time parameters in kinetic profiles and fibrin fiber thickness.

CONCLUSIONS

Results indicate that prothrombin Belgrade mutation has no significant impact on fibrinolysis, however it may affect kinetics of clot formation and its architecture.

Three Cases of Unprovoked Venous Thromboembolism with Prothrombin p.Arg596Gln Variant and a Literature Review of Antithrombin Resistance

Antithrombin resistance (ATR) is a newly identified strong genetic predisposition to venous thromboembolism (VTE) caused by genetic variations in prothrombin with substitutions of Arg at position 596 with either Leu, Gln, or Trp. In the present report, we identified a missense variant p.Arg596Gln in 3 patients from 2 families with unprovoked VTE who each experienced their first VTE event at 19, 67, and 19 years old. The three patients did not show any positive markers for thrombophilia on routine testing, suggesting that patients with unprovoked VTE who have negative findings on thrombophilia tests may carry a prothrombin variant with ATR.

A phenotype driven integrative framework uncovers molecular mechanisms of a rare hereditary thrombophilia

Antithrombin resistance is a rare subtype of hereditary thrombophilia caused by prothrombin gene variants, leading to thrombotic disorders. Recently, the Prothrombin Belgrade variant has been reported as a specific variant that leads to antithrombin resistance in two Serbian families with thrombosis. However, due to clinical data scarcity and the inapplicability of traditional genome-wide association studies (GWAS), a broader perspective on molecular and phenotypic mechanisms associated with the Prothrombin Belgrade variant is yet to be uncovered. Here, we propose an integrative framework to address the lack of genomic samples and support the genomic signal from the full genome sequences of five heterozygous subjects by integrating it with subjects' phenotypes and the genes' molecular interactions. Our goal is to identify candidate thrombophilia-related genes for which our subjects possess germline variants by focusing on the resulting gene clusters of our integrative framework. We applied a Non-negative Matrix Tri-Factorization-based method to simultaneously integrate different data sources, taking into account the observed phenotypes. In other words, our data-integration framework reveals gene clusters involved with this rare disease by fusing different datasets. Our results are in concordance with the current literature about antithrombin resistance. We also found candidate disease-related genes that need to be further investigated. CD320, RTEL1, UCP2, APOA5 and PROZ participate in healthy-specific or disease-specific subnetworks involving thrombophilia-annotated genes and are related to general thrombophilia mechanisms according to the literature. Moreover, the ADRA2A and TBXA2R subnetworks analysis suggested that their variants may have a protective effect due to their connection with decreased platelet activation. The results show that our method can give insights into antithrombin resistance even if a small amount of genetic data is available. Our framework is also customizable, meaning that it applies to any other rare disease.

Snake Venoms in Diagnostic Hemostasis and Thrombosis

Snake venoms have evolved primarily to immobilize and kill prey, and consequently, they contain some of the most potent natural toxins. Part of that armory is a range of hemotoxic components that affect every area of hemostasis, which we have harnessed to great effect in the study and diagnosis of hemostatic disorders. The most widely used are those that affect coagulation, such as thrombin-like enzymes unaffected by heparin and direct thrombin inhibitors, which can help confirm or dispute their presence in plasma. The liquid gold of coagulation activators is Russell's viper venom, since it contains activators of factor X and factor V. It is used in a range of clotting-based assays, such as assessment of factor X and factor V deficiencies, protein C and protein S deficiencies, activated protein C resistance, and probably the most important test for lupus anticoagulants, the dilute Russell's viper venom time. Activators of prothrombin, such as oscutarin C from Coastal Taipan venom and ecarin from saw-scaled viper venom, are employed in prothrombin activity assays and lupus anticoagulant detection, and ecarin has a valuable role in quantitative assays of direct thrombin inhibitors. Snake venoms affecting primary hemostasis include botrocetin from the jararaca, which can be used to assay von Willebrand factor activity, and convulxin from the cascavel, which can be used to detect deficiency of the platelet collagen receptor, glycoprotein VI. This article takes the reader to every area of the diagnostic hemostasis laboratory to appreciate the myriad applications of snake venoms available in diagnostic practice.

Venous thromboembolism is caused by prothrombin p.Arg541Trp mutation in Japanese individuals

Venous thromboembolism (VTE) is a multifactorial disease. Because low-frequency variants and rare mutations have been found to predispose carriers toward VTE, there is a need for variant discovery in clinical settings. Therefore, we used a whole-exome approach for a young VTE patient with a positive family history. We identified in the proband and his affected mother a rare, functional missense variant of prothrombin, p.Arg541Trp, which contributes to the clinical picture of VTE.

The genetics of venous thromboembolism: a systematic review of thrombophilia families

Genetic risk factors are important for the occurrence and prognosis of venous thromboembolism (VTE). The studies of thrombophilia families are important for dissecting the genetic background of the thrombotic disease. We conducted the systematic review of all published family-based studies on VTE genetics across all racial groups through PubMed and Embase prior to 13th April 2020. This systematic review of 287 families (including 225 Caucasian families, 52 East Asian families, and families of other ethnicities) revealed a total of 21 different genes; the five most reported mutated genes were F5 (88/287, 30.7%), SERPINC1 (67/287, 23.3%), PROC (65/287, 22.6%), F2 (40/287, 13.9%) and PROS1 (48/287, 16.7%). For Caucasian families, F5 mutations were most frequently reported at 37.8% (85/225), while PROS1 mutations were most frequently reported, at 40.4% (21/52), for East Asian families (Chinese, Japanese and Korean). Factor V Leiden was reported more frequently in Caucasians than in East Asians. Missense mutations were reported frequently in the SERPINC1, PROC and PROS1 genes. In conclusion, our study found the most likely mutated genes associated with VTE among different ethnic groups and provided indications for VTE genetic testing and research in the future.

Rare bleeding disorders and advances in gene therapy

: Rare bleeding disorders usually begin in childhood and manifest as varying degrees of bleeding, which can be life-threatening in severe cases. With the development of gene editing technology, it is expected that hereditary coagulation factor disorders will someday be fundamentally cured by gene therapy. On account of their rarity, comprehension of these diseases is essential for the application of new treatment strategies. We have compiled the features of some newly discovered mutations of prothrombin, factor VII, and factor X in recent years. In addition, this review introduces the advances and obstacles in gene therapy.

Vitamin K-Dependent Coagulation Factors That May be Responsible for Both Bleeding and Thrombosis (FII, FVII, and FIX)

Vitamin K-dependent clotting factors are commonly divided into prohemorrhagic (FII, FVII, FIX, and FX) and antithrombotic (protein C and protein S). Furthermore, another protein (protein Z) does not seem strictly correlated with blood clotting. As a consequence of this assumption, vitamin K-dependent defects were considered as hemorrhagic or thrombotic disorders. Recent clinical observations, and especially, recent advances in molecular biology investigations, have demonstrated that this was incorrect. In 2009, it was demonstrated that the mutation Arg338Leu in exon 8 of FIX was associated with the appearance of a thrombophilic state and venous thrombosis. The defect was characterized by a 10-fold increased activity in FIX activity, while FIX antigen was only slightly increased (FIX Padua). On the other hand, it was noted on clinical grounds that the thrombosis, mainly venous, was present in about 2% to 3% of patients with FVII deficiency. It was subsequently demonstrated that 2 mutations in FVII, namely, Arg304Gln and Ala294Val, were particularly affected. Both these mutations are type 2 defects, namely, they show low activity but normal or near-normal FVII antigen. More recently, in 2011-2012, it was noted that prothrombin defects due to mutations of Arg596 to Leu, Gln, or Trp in exon 15 cause the appearance of a dysprothrombinemia that shows no bleeding tendency but instead a prothrombotic state with venous thrombosis. On the contrary, no abnormality of protein C or protein S has been shown to be associated with bleeding rather than with thrombosis. These studies have considerably widened the spectrum and significance of blood coagulation studies.

Familial pulmonary thromboembolism with a prothrombin mutation and antithrombin resistance

We describe the case of a 45-year-old man with congenital thrombophilia induced by antithrombin resistance. He had recurrent venous thrombosis without traditional risk factors or abnormal coagulation function and had a family history of venous thrombosis which included his mother, brother, and nephew. We suspected the association of hereditary antithrombin resistance, which has been reported in some cases of familial venous thromboses due to prothrombin mutations. Although prothrombin abnormality typically shows a bleeding tendency, variations of arginine at position 596 in the gene encoding prothrombin have been reported to conversely cause thrombosis. Therefore, we tested and detected antithrombin resistance in the patient's plasma. We also performed genetic analysis for his second filial generation, and found a missense mutation (c.1787G>A), resulting in a substitution of arginine for glutamine at position 596 (p.Arg596Gln) in the gene encoding prothrombin (called prothrombin Belgrade). The Gln596 substitution caused the susceptibility to thrombosis. This variation is the same as one previously reported in Serbia and India, and it is the third report in Japan. <Learning objective: Venous thrombosis is caused by multiple factors. However, approximately 20% of the patients with venous thrombosis of unknown cause have no risk factors. The present case had no known risk factors, rather he has a family history of thrombosis. Therefore, we suspected an association with inherited thrombophilia. Although several gene mutations have been found in some families with inherited thrombophilia, many mutations remain to be detected.>.

Optimisation of antithrombin resistance assay as a practical clinical laboratory test: Development of prothrombin activator using factors Xa/Va and automation of assay

INTRODUCTION

Antithrombin resistance (ATR) is a novel thrombotic risk in abnormal prothrombins. A manual ATR assay using Oxyuranus scutellatus (Ox) venom as a prothrombin activator was established for detecting antithrombin-resistant prothrombin. However, this assay was limited because of Ox snake venom availability and its throughput capacity. Here, we have improved the ATR assay using bovine factors Xa and Va (FXa/Va) as prothrombin activators and have optimised assay conditions for an automated instrument (ACL TOP 500).

METHODS

Diluted plasma was incubated with a prothrombin activator mix (phospholipids, CaCl₂ , and bovine FXa/Va), followed by inactivation with antithrombin for 10, 20 and 30 minutes. We added a chromogenic substrate S-2238, and assessed changes in absorbance/min at 405 nm. We also adapted assay conditions for ACL TOP 500.

RESULTS

Optimum conditions for FXa/Va treatment were 6.25% phospholipids, 5 mM CaCL₂ , 0.01 μg/mL FXa and 0.1 μg/mL FVa. ATR assay kinetics with the FXa/Va activator was comparable with that with the Ox activator in heterozygous reconstituted plasma with the recombinant wild-type or antithrombin-resistant prothrombin. Using ACL TOP 500, optimum conditions for the FXa/Va treatment were 10.0% phospholipids, 5 mM CaCl₂ , 0.02 μg/mL FXa and 0.2 μg/mL FVa. The automated ATR assay with the FXa/Va activator demonstrated good detectability for antithrombin-resistant prothrombin in plasma from a heterozygous carrier with prothrombin Yukuhashi or Belgrade.

CONCLUSION

We optimised the ATR assay with the FXa/Va activator and adapted the assay for ACL TOP 500; the assay showed the ability to clearly detect antithrombin-resistant prothrombin in manual and automated procedures.

Screening and functional exploration of prothrombin Arg596 related mutations in Chinese venous thromboembolism patients

AIMS

Dysfunctional prothrombin residue Arg596 associated mutation has been found to precipitate venous thromboembolism (VTE). In the current study we investigated the prevalence of Arg596 associated mutations in Chinese patients with VTE and explored the functional impact of Arg596Gln mutation on coagulation function in affected patients.

METHODS

Prothrombin clotting activity was measured in 267 unrelated patients with unprovoked VTE. Patients with moderately decreased activities underwent further analysis of the F2 gene. Prothrombin amidolytic activity and antigen levels were detected in mutation carriers. Specific family members were investigated about their VTE histories and clinical phenotypes. The thrombin generation test (TGT) was used to evaluate thrombin function and antithrombin resistance assay was applied to assess the extent of impaired antithrombin inhibition of mutation carriers.

RESULTS

Two heterozygous mutation carriers of prothrombin Arg596Gln were identified, both of whom had moderately decreased clotting activities but normal amidolytic activities and antigen levels. Among the families of the two probands, nine out of 13 mutation carriers experienced episodes of VTE. TGTs showed that patients had elevated endogenous thrombin potential and prolonged start tail time. Thrombin generation could be inhibited in the presence of thrombomodulin. The thrombin Arg596Gln variant in patients' plasma presented strong resistance to antithrombin inhibition.

CONCLUSION

Prothrombin Arg596Gln mutation is a risk factor for Chinese patients with VTE due to its moderately decreased clotting activity but strong resistance to antithrombin inhibition. Prothrombin clotting activity screening and its encoding gene sequencing should be considered in patients with VTE when other established risk factors are absent.

In vitro exploration of latent prothrombin mutants conveying antithrombin resistance

INTRODUCTION

Antithrombin resistance (ATR) prothrombinemia is an inherited thrombophilic disorder caused by missense mutations in prothrombin gene (F2) at Arg596 of the sodium-binding region. Previously, prothrombin mutants Yukuhashi (Arg596Leu), Belgrade (Arg596Gln), and Padua 2 (Arg596Trp) were reported as ATR-prothrombins possessing a risk of familial venous thrombosis. To identify additional F2 mutations causing the ATR-phenotype, we investigated the coagulant properties of recombinant prothrombins mutated at amino acid residues within the sodium-binding region by single nucleotide substitutions (Thr540, Arg541, Glu592, and Lys599).

MATERIALS AND METHODS

We constructed expression vectors of prothrombin mutants, established stably transfected HEK293 cells, and isolated the recombinant prothrombin proteins. We evaluated procoagulant activity and ATR-phenotypes of those mutants in reconstituted plasma by mixing with prothrombin deficient plasma.

RESULTS

The secreted quantity of all prothrombin mutants was the same as that of the wild-type prothrombin. Procoagulant activity of each mutant varied from 1.7% to 79.5% in a one-stage clotting assay and from 2.0% to 104.5% in a two-stage chromogenic assay. Most prothrombin mutants tested presented with a severe ATR-phenotype. To estimate the thrombosis risk of these mutations, we determined the residual clotting activity (RCA) after 30min inactivation with antithrombin. RCA scores, normalized to the wild-type, revealed that prothrombin mutants Lys599Arg (5.35) and Glu592Gln (4.71) had high scores, which were comparable with prothrombins Yukuhashi (4.36) and Belgrade (5.19).

CONCLUSIONS

Mutation of prothrombin at the sodium-binding site caused ATR-phenotypes. Of those tested, Lys599Arg and Glu592Gln may possess a thrombosis risk as large as the known pathogenic prothrombins Yukuhashi and Belgrade.