Abnormal factor VIII Hiroshima: defect in crucial proteolytic cleavage by thrombin at Arg1689 detected by a novel ELISA

Delivery of full-length factor VIII using a piggyBac transposon vector to correct a mouse model of hemophilia A

Viral vectors have been used for hemophilia A gene therapy. However, due to its large size, full-length Factor VIII (FVIII) cDNA has not been successfully delivered using conventional viral vectors. Moreover, viral vectors may pose safety risks, e.g., adverse immunological reactions or virus-mediated cytotoxicity. Here, we took advantages of the non-viral vector gene delivery system based on piggyBac DNA transposon to transfer the full-length FVIII cDNA, for the purpose of treating hemophilia A. We tested the efficiency of this new vector system in human 293T cells and iPS cells, and confirmed the expression of the full-length FVIII in culture media using activity-sensitive coagulation assays. Hydrodynamic injection of the piggyBac vectors into hemophilia A mice temporally treated with an immunosuppressant resulted in stable production of circulating FVIII for over 300 days without development of anti-FVIII antibodies. Furthermore, tail-clip assay revealed significant improvement of blood coagulation time in the treated mice.piggyBac transposon vectors can facilitate the long-term expression of therapeutic transgenes in vitro and in vivo. This novel gene transfer strategy should provide safe and efficient delivery of FVIII.

Coagulation potential of immobilised factor VIII in flow-dependent fibrin generation on platelet surfaces

Coagulation factor VIII (FVIII) plays an essential role in haemostasis. To date, physiologic activity of FVIII circulating in the bloodstream (S-FVIII) is evaluated by classic coagulation assays. However, the functional relevance of FVIII (-von Willebrand factor complex) immobilised on thrombogenic surfaces (I-FVIII) remains unclear. We used an in vitro perfusion chamber system to evaluate the function of I-FVIII in the process of mural thrombus formation under whole blood flow conditions. In perfusion of either control or synthetic haemophilic blood, the intra-thrombus fibrin generation on platelet surfaces significantly increased as a function of I-FVIII, independent of S-FVIII, under high shear rate conditions. This I-FVIII effect was unvarying regardless of anti-FVIII inhibitor levels in synthetic haemophilic blood. Thus, our results illustrate coagulation potentials of immobilised clotting factors, distinct from those in the bloodstream, under physiologic flow conditions and may give a clue for novel therapeutic approaches for haemophilic patients with anti-FVIII inhibitors.

Combined homology modelling and evolutionary significance evaluation of missense mutations in blood clotting factor VIII to highlight aspects of structure and function

Most small lesions in the factor VIII (FVIII) gene that cause haemophilia A (HA) are single nucleotide substitutions resulting in amino acid replacing (missense) mutations and leading to various phenotypes, ranging from mild to severe. We took a combined approach of homology modelling and quantitative evaluation of evolutionary significance of amino acid replacing alterations using the Grantham Matrix Score (GMS) to assess their structural effects and significance of pathological expression. Comparative homology models of all amino acid substitutions summarized in the FVIII mutations database plus these identified and reported lately by us or by our collaborators were evaluated. Altogether 640 amino acid replacing mutations were scored for potential distant or local conformation changes, influence on the molecular stability and predicted contact residues, using available FVIII domain models. The average propensity to substitute amino acid residues by mutation was found comparable to the overall probability of de novo mutations. Missense changes reported with various HA phenotypes were all confirmed significant using GMS. The fraction of these, comprising residues apparently involved in intermolecular interactions, exceeds the average proportion of such residues for FVIII. Predicted contact residues changed through mutation were visualized on the surface of FVIII domains and their possible functional implications were verified from the literature and are discussed considering available structural information. Our predictive modelling adds on the current view of domain interface molecular contacts. This structural insight could aid in part to the design of engineered FVIII constructs for therapy, to possibly enhance their stability and prolong circulating lifetime.

Factor V C2 domain contains a major thrombin-binding site responsible for thrombin-catalyzed factor V activation

Factor (F)V is converted into its active form, FVa, by limited proteolysis. Thrombin-catalyzed activation of FV is essential for its full cofactor activation. Previously, we reported that thrombin was bound to the C2 domain in the light chain of FVIII. As FV has a similar domain structure to FVIII, we focused on the FV C2 domain as a possible binding region for thrombin. Kinetic parameters, measured by surface plasmon resonance, revealed that the K(d) values of anhydro-thrombin for FV, FVa, and the FV C2 domain were 66, 240, and 670 nmol L(-1), respectively. FV activation was increased by approximately 9-fold by the addition of thrombin. In the presence of the FV C2 domain, this increase of the FV activation was inhibited. However, FV activation was not inhibited by the addition of the FVIII C2 domain. FV was cleaved into a 105-kDa heavy chain and a 71/74-kDa light chain by thrombin-catalyzed proteolysis at Arg709, Arg1018 and Arg1545. In the presence of the FV C2 domain, the cleavage was inhibited at all sites. Proteolysis was not affected by the addition of the FVIII C2 domain. These results indicated that the FV C2 domain contains a major binding site for thrombin and that this domain is necessary for the proteolysis at all cleavage sites. Furthermore, the present results also suggested that thrombin has an independent binding site for FV different from that for FVIII.

Successful steroid pulse treatment in childhood acquired haemophilia with nephrotic syndrome

We encountered a 2-year-old boy with acquired haemophilia, which rarely occurs in children, who was complicated with nephrotic syndrome. In mid-August 2001, he was diagnosed to have nephrotic syndrome based on the presence of massive proteinuria and hypoalbuminaemia. Activated partial thromboplastin time (APTT) was normal at 42.4 s at that time. After starting prednisone administration of 2 mg kg(-1) day(-1), the proteinuria disappeared immediately. However, in early October the same year, subcutaneous ecchymosis and intramuscular bleeding occurred for no apparent reason, and from the examination results his APTT was 106.4 s, factor VIII (FVIII) activity was <1%, and the anti-FVIII inhibitor titre was 6.9 BU ml(-1). As a result, he was diagnosed to have acquired haemophilia. The anti-nuclear antibody and anti-phospholipid antibody were negative. With recombinant activated FVII, haemostasis was obtained, and after administering three courses of steroid pulse therapy (methyl prednisolone: 20 mg kg(-1) day(-1) x 3 days), the anti-FVIII inhibitory activity disappeared. An analysis of the immunological and coagulation properties of his FVIII autoantibodies showed the anti-FVIII inhibitory activity to be mediated by IgG(1) antibody. In other words, his FVIII inhibitor was a Th1 dominant and it provided a good response to treatment. These findings correlate with those of previous reports. The patient thereafter frequently demonstrated a recurrence of nephrotic syndrome. As a result, he is presently being managed with cyclosporine. However, no recurrence of the anti-FVIII titre has been observed.

Inversions of the Factor VIII Gene in Japanese Patients with Severe Hemophilia A

Hemophilia A is genetically very heterogeneous because disease-causing mutations involving deletions, point mutations, insertions, and inversions are scattered throughout the factor VIII gene. Of these mutations, inversions, which are intrachromosomal recombinations between int22h-1 (intron 22 homologous region 1) and 1 of 2 other extragenic copies located 500 kilobases upstream, are the more frequently found defects, especially in patients with severe hemophilia A. Reportedly, approximately half of all severe hemophilia A patients have inversions in intron 22. A group of unrelated patients from the middle of Japan with severe hemophilia A were screened by Southern blot analysis for gene inversions. Forty-two of 100 severely affected patients presented factor VIII gene rearrangements. Of these patients, 36 exhibited the distal type of inversion, and 6 exhibited the proximal type. No other variant type of recombination was observed. In this study, neither the prevalence of inhibitor development against factor VIII nor the frequency of sporadic cases in the group presenting gene inversions was significantly different from that in the group without chromosomal inversions. Southern blot analysis successfully detected a carrier in a hemophilia family for which no patient was available. Genetic counseling of patients with severe hemophilia A and their families will be considerably improved, because the inversions occur in 42% of the Japanese patients with severe hemophilia.

Molecular defects in coagulation Factor VIII and their impact on Factor VIII function

Molecular defects in Factor VIII (FVIII), such as haemophilia A-related mutations or denaturative conformational changes, may affect the stability of FVIII as well as its interactions with physiological activators, von Willebrand Factor, phospholipid, or conformationally sensitive antibodies. We summarize the contemporary assays which allow identification of impaired functional interactions of FVIII that cause a reduction or loss of its cofactor activity and/or increased immunogenicity. These assays can potentially be used for detection of molecular defects in FVIII and elucidation of the function impaired by these defects.

Immunological characterization of factor VIII autoantibodies in patients with acquired hemophilia A in the presence or absence of underlying disease

The development of a factor VIII autoantibody results in a severe hemorrhagic diathesis known as acquired hemophilia A. Underlying pathologies, such as autoimmune disease or chronic inflammatory disease, are observed in about half of the patients. We have investigated a total of 16 cases with acquired hemophilia A and divided the patients into two groups according to the presence or absence of other clinical conditions. Group A comprised nine cases with no detectable associated pathology. Group B consisted of seven cases with other clinical diagnoses. Significant levels of factor VIII activity (FVIII:C) and factor VIII antigen (FVIII:Ag) were detected in Group A and the pattern of FVIII:C inactivation was characteristic of Type 2 inhibitors. In contrast, no FVIII:C was detected in Group B and, in five of seven cases, the inhibitory pattern was Type 1. IgG(4) antibody subclass specificity was dominant in both groups. IgG1 antibody reactivity was higher in Group B than in Group A. Our results suggested a close relationship between the presence of underlying disease and immunological and coagulation characteristics in acquired hemophilia A.

Circulating factor VIII immune complexes in patients with type 2 acquired hemophilia A and protection from activated protein C-mediated proteolysis

Factor VIII (FVIII) inhibitor antibodies are classified into 2 groups according to the kinetic pattern of FVIII inactivation. Type 2 antibodies are more commonly observed in patients with acquired hemophilia A and do not completely inhibit FVIII activity; in most cases, substantial levels of circulating FVIII are detected. Three type 2 autoantibodies from patients who had normal levels of FVIII antigen despite having low levels of FVIII activity were studied. The antibodies reacted exclusively with the light chain of FVIII but not with the C2 domain, and their epitopes were therefore ascribed to the regions in the A3-C1 domains. Heavy and light chains of FVIII were detected in plasma-derived immune complexes extracted by using protein G Sepharose. Direct binding assays using anhydro-activated protein C (anhydro-APC), a catalytically inactive derivative of activated protein C (APC) in which the active-site serine is converted to dehydroalanine, were used to examine the relation between immune complexes and APC. The intact FVIII, 80-kd light chain, and 72-kd light chain bound in a dose-dependent manner to anhydro-APC, with K(d) values of 580, 540, and 310 nM, respectively, whereas no appreciable binding was detected for the heavy chain. The 3 autoantibodies blocked FVIII binding to anhydro-APC by approximately 80% and consequently inhibited APC-induced FVIII proteolytic inactivation. These antibodies also bound to a synthetic peptide, His2009-Val2018, which contains the APC binding site. The findings suggest that binding of type 2 autoantibodies, recognizing residues His2009 to Val2018, protects FVIII from APC-mediated proteolysis and might contribute to the presence of FVIII immune complexes in the circulation.

Factor VIII C2 domain contains the thrombin-binding site responsible for thrombin-catalyzed cleavage at Arg1689

Thrombin-catalyzed factor VIII activation is an essential positive feedback mechanism regulating intrinsic blood coagulation. A factor VIII human antibody, A-FF, with C2 epitope, exclusively inhibited factor VIII activation and cleavage at Arg(1689) by thrombin. The results suggested that A-FF prevented the interaction of thrombin with factor VIII and that the C2 domain was involved in the interaction with thrombin. We performed direct binding assays using anhydro-thrombin, a catalytically inactive derivative of thrombin in which the active-site serine is converted to dehydroalanine. Intact factor VIII, 80-kDa light chain, 72-kDa light chain, and heavy chain fragments bound dose-dependently to anhydro-thrombin, and the K(d) values were 48, 150, 106, and 180 nm, respectively. The C2 and A2 domains also dose-dependently bound to anhydro-thrombin, and the K(d) values were 440 and 488 nm, respectively. The A1 domain did not bind to anhydro-thrombin. A-FF completely inhibited C2 domain binding to anhydro-thrombin (IC(50), 18 nm), whereas it did not inhibit A2 domain binding. Furthermore, C2-specific affinity purified F(ab)'(2) of A-FF, and the recombinant C2 domain inhibited thrombin cleavage at Arg(1689). Our results indicate that the C2 domain contains the thrombin-binding site responsible for the cleavage at Arg(1689).

Role of factor VIII C2 domain in factor VIII binding to factor Xa

Factor VIII (FVIII) is activated by proteolytic cleavages with thrombin and factor Xa (FXa) in the intrinsic blood coagulation pathway. The anti-C2 monoclonal antibody ESH8, which recognizes residues 2248-2285 and does not inhibit FVIII binding to von Willebrand factor or phospholipid, inhibited FVIII activation by FXa in a clotting assay. Furthermore, analysis by SDS-polyacrylamide gel electrophoresis showed that ESH8 inhibited FXa cleavage in the presence or absence of phospholipid. The light chain (LCh) fragments (both 80 and 72 kDa) and the recombinant C2 domain dose-dependently bound to immobilized anhydro-FXa, a catalytically inactive derivative of FXa in which dehydroalanine replaces the active-site serine. The affinity (K(d)) values for the 80- and 72-kDa LCh fragments and the C2 domain were 55, 51, and 560 nM, respectively. The heavy chain of FVIII did not bind to anhydro-FXa. Similarly, competitive assays using overlapping synthetic peptides corresponding to ESH8 epitopes (residues 2248-2285) demonstrated that a peptide designated EP-2 (residues 2253-2270; TSMYVKEFLISSSQDGHQ) inhibited the binding of the C2 domain or the 72-kDa LCh to anhydro-FXa by more than 95 and 84%, respectively. Our results provide the first evidence for a direct role of the C2 domain in the association between FVIII and FXa.

Role of activation of the coagulation factor VIII in interaction with vWf, phospholipid, and functioning within the factor Xase complex

Blood coagulation factor VIII (fVIII) in its nonactivated form circulates in plasma in a complex with von Willebrand factor (vWf). Upon activation by thrombin- or factor Xa-mediated site-specific proteolysis, activated fVIII (fVIIIa) serves as a cofactor for factor IXa. This protein complex assembled on a phospholipid surface (factor Xase) activates factor X. This complex plays the key role in the intrinsic pathway of blood coagulation. We reviewed the molecular events triggered by fVIII activation, which are required for the assembly and functioning of the Xase complex, including fVIIIa dissociation from vWf and a significant increase of fVIII affinity for binding to the phospholipid surface. Both events are mediated by activation-related cleavage within fVIII light chain (LCh), releasing the 40 amino-acid N-terminal LCh peptide, which is followed by a conformational change within the C2 domain. The conformational change within LCh is also required for the optimal fVIII cofactor functioning within the factor Xase complex, exerted via fVIIIa interactions with phospholipid, factor IXa, and factor X. Since factor IXa not only stabilizes but also proteolytically inactivates fVIIIa within the factor Xase complex, the stability of the membrane-bound fVIIIa in the presence and absence of factor IXa is discussed. In conclusion, we outline some new possible directions of the research. One of them arises from the recently demonstrated ability of plasma lipoproteins to provide a phospholipid surface for the assembly of the factor Xase complex in vitro. This finding raises a possibility that lipoproteins participate in factor Xase functioning in vivo and suggests a direct link between elevated levels of lipoproteins associated with atherosclerosis and increased thrombogenicity associated with this disease.

The role of platelet von Willebrand factor in the binding of factor VIII to activated platelets

Factor VIII binds to activated platelets and contributes to the tenase complex assembled on the platelet membrane surface. We have examined the role of platelet von Willebrand factor in the binding of factor VIII to platelets using a platelet captured enzyme-linked immunosorbent assay. Purified factor VIII bound to activated normal platelets in a dose dependent manner. Factor VIII also bound to platelets obtained from a patient with Type 2N von Willebrand disease, although in this case the binding was reduced to approximately 50% of that seen with control platelets. Furthermore, factor VIII bound to Type 3 von Willebrand disease platelets in the absence of detectable von Willebrand factor. In this instance the binding reaction appeared to be approximately 30% of that seen with the same number of normal platelets. An anti-A3 domain monoclonal antibody, NMC-VIII/10, which recognizes the amino-terminal acidic region of the factor VIII light chain, and an anti-C2 domain monoclonal antibody, NMC-VIII/5, which also moderates the binding of factor VIII to phosphatidylserine, inhibited the association between factor VIII and platelets. Inhibition was more remarkable with NMC-VIII/5 than with NMC-VIII/ 10 but not complete. The findings suggest that the binding of factor VIII to activated platelets is not based on a single ligand-receptor relationship, although a predominant role exists for the platelet von Willebrand factor. Furthermore, both the amino-terminal acidic region of the A3 domain and the C2 domain participate in the binding of factor VIII to activated platelets.

A mechanism for inhibition of factor VIII binding to phospholipid by von Willebrand factor

von Willebrand factor (vWf) acts as a carrier for blood coagulation factor VIII (fVIII) in the circulation. The amino-terminal 272 residues of mature vWf contain a high affinity fVIII binding site. Upon thrombin activation, fVIII is released from vWf, thereby allowing its binding to phospholipid which is required for its procoagulant activity. Although phospholipid and vWf compete for fVIII binding, it was previously suggested that their binding sites are not closely juxtaposed within the fVIII protein because only amino-terminal vWf proteolytic fragments larger than SPIII-T4 (1-272) were able to block the binding of fVIII to phospholipid. We have demonstrated, however, that SPIII-T4 is able to inhibit fVIII binding to phosphatidylserine (PS) in a dose-dependent fashion, but only at concentrations higher than those used in previous experiments. Our demonstration that the Kd values for vWf and SPIII-T4 for fVIII are 0.52 nM and 48 nM, respectively, explain this discrepancy. Inhibition (> 95%) of SPIII-T4 binding to fVIII by a purified recombinant fVIII C2 domain polypeptide demonstrated that SPIII-T4 binds directly to C2, as we had previously shown for vWf. The similarity of the C2 binding sites for vWf and SPIII-T4 was further confirmed by the identical inhibitory effects of synthetic peptides and monoclonal antibodies (mAbs) on vWf-fVIII or SPIII-T4 fVIII binding. In both cases, binding was inhibited by synthetic peptide 2303-2332, containing a PS binding site, and by mAb NMC-VIII/5 Fab' (epitope within C2 residues 2170-2327). We propose that vWf, via residues 1-272, and PS compete for fVIII binding because they recognize overlapping sites within fVIII C2 domain residues 2303-2332.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene, second edition

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene, second edition

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.