Most factor VIII B domain missense mutations are unlikely to be causative mutations for severe hemophilia A: implications for genotyping

Summary. Background & Objective: The factor VIII (FVIII) B domain shares very little amino acid homology with other known proteins and is not directly necessary for procoagulant activity. Despite this, missense mutations within the B domain have been reported in patients with hemophilia A. Given that the B domain is dispensable for secretion and function of FVIII, we hypothesized that these mutations should not be causative of hemophilia A in these patients. Methods: Plasmid vectors containing B domain missense mutations that were reported to be associated with moderate/severe hemophilia A (T751S, D826E, V993L, H1047Y, T1353A, N1441K, L1462P, E1579D, A1591S, P1641L and S1669L) were analyzed for their effect on synthesis and secretion compared with FVIII wild‐type (WT) following transient transfection into COS‐1 and CHO cells in vitro. Further, H1047Y, N1441K and E1579D mutants were expressed in vivo in a hemophilia A mouse model by hydrodynamic tail‐vein injection. Results: FVIII activity and antigen levels for all mutants expressed into the conditioned media of COS‐1 and CHO cells were similar to FVIII WT. Also, plasma expression of these mutants was similar to FVIII WT in hemophilia A mice. An in vivo tail clip bleeding assay also demonstrated that blood loss from hemophilia A mice expressing FVIII WT, H1047Y, N1441K and E1579D was similar. Conclusions: We conclude that most missense mutations within the FVIII B domain would be unlikely to lead to severe hemophilia A and that the majority of such missense mutations represent polymorphisms or non‐pathologic mutations.     

Improved kinetics of rIX‐FP,a recombinant fusion protein linking factor IX with albumin,in cynomolgus monkeys and hemophilia B dogs

Summary. Background: Prophylaxis of hemophilia B, at present, requires multiple infusions of human factor (F)IX concentrates per week. A FIX molecule with a prolonged half‐life has the potential to greatly improve the convenience of, and adherence to, prophylaxis. Objectives: The aim of our studies was to investigate the pharmacokinetic (PK) and pharmacodynamic (PD) profile of a recombinant fusion protein linking coagulation FIX with albumin (rIX‐FP). Methods: Cynomolgus monkeys and hemophilia B dogs received single intravenous doses of rIX‐FP (50–500 IU kg^?1). rIX‐FP plasma levels were determined by an activity‐based assay (dogs only) and anti‐FIX ELISA methods. Additionally, activated partial thromboplastin time (APTT) was determined in hemophilia B dogs. Data were compared with a direct study comparator (recombinant FIX [rFIX]) or previously published data. Results: The terminal half‐life of rIX‐FP was prolonged in both species compared with FIX reference data. In hemophilia B dogs, human FIX antigen levels remained above 0.05 IU mL^?1 more than three times longer after rIX‐FP (7.3 days) compared with rFIX (2.3 days), whereas respective calculations based on activity levels confirmed the observed superior profile. Prolonged PDs of rIX‐FP were demonstrated with APTT < 60 s sustained around four times longer with rIX‐FP (5.9 days) than rFIX (1.5 days). Conclusions: These studies indicate that the recombinant albumin fusion technology successfully improves the PK profile of FIX. Clinical studies will test whether the improved kinetics result in a significant half‐life extension in patients with hemophilia B.     

Low molecular weight heparin inhibits plasma thrombin generation via direct targeting of factor IXa: contribution of the serpin‐independent mechanism

Summary. Background: Although heparin possesses multiple mechanisms of action, enhanced factor Xa inhibition by antithrombin is accepted as the predominant therapeutic mechanism. The contribution of FIXa inhibition to heparin activity in human plasma remains incompletely defined. Objectives: To determine the relevance of FIXa as a therapeutic target for heparins, particularly serpin‐independent inhibition of intrinsic tenase (FIXa–FVIIIa) activity. Patients/Methods: Thrombin generation was detected by fluorogenic substrate cleavage. The inhibitory potencies (EC₅₀s) of low molecular weight heparin (LMWH), super‐sulfated LMWH (ssLMWH), fondaparinux and unfractionated heparin (UFH) were determined by plotting concentration vs. relative velocity index (ratio ± heparin). Inhibition was compared under FIX‐dependent and FIX‐independent conditions (0.2 or 4 pm tissue factor [TF], respectively) in normal plasma, and in mock‐depleted or antithrombin/FIX‐depleted plasma supplemented with recombinant FIX. Results: UFH and fondaparinux demonstrated similar potency under FIX‐dependent and FIX‐independent conditions, whereas LMWH (2.9‐fold) and ssLMWH (5.1‐fold) demonstrated increased potency with limiting TF. UFH (62‐fold) and fondaparinux (42‐fold) demonstrated markedly increased EC₅₀ values in antithrombin‐depleted plasma, whereas LMWH (9.4‐fold) and ssLMWH (two‐fold) were less affected, with an EC₅₀ within the therapeutic range for LMWH. The molecular target for LMWH/ssLMWH was confirmed by supplementing FIX/antithrombin‐depleted plasma with 90 nm recombinant FIX possessing mutations in the heparin‐binding exosite. Mutated FIX demonstrated resistance to inhibition of thrombin generation by LMWH and ssLMWH that paralleled the effect of these mutations on intrinsic tenase inhibition. Conclusions: Therapeutic LMWH concentrations inhibit plasma thrombin generation via antithrombin‐independent interaction with the FIXa heparin‐binding exosite.     

Whole blood clot formation phenotypes in hemophilia A and rare coagulation disorders. Patterns of response to recombinant factor VIIa

Summary. Until now, no routinely used clotting assay has demonstrated the power to reflect significantly a patient's response to recombinant factor (rF)VIIa. Adopting a thrombelastographic principle, profiles of continuous whole blood (WB) coagulation were studied in minimally altered WB activated with a small amount of tissue factor (TF). Investigation of the WB clotting profile was performed before and after ex vivo addition of rFVIIa 20 nm to WB from 26 patients with hemophilia A, two patients with severe hemophilia B, and individuals with deficiencies of FV, FX, FXI, and FXIII. In five patients with hemophilia plus inhibitors, the response to ex vivo added rFVIIa and to activated complex concentrate (APCC) was studied. Patients with severe and moderate hemophilia A demonstrated remarkable variance in the hemostatic characteristics at baseline, even in groups with the same FVIII:C activity levels. The response to rFVIIa at 20 nm also varied extensively, the effect correlating with the continuous WB coagulation phenotype at baseline. This indicates that the efficacy of rFVIIa may be optimized by tailoring the dose according to the hemostatic response to varying doses tested prior to in vivo administration. In patients with inhibitors against FVIII and factor IX, rFVIIa and APCC substitution resulted in quite similar response patterns that appeared to be dose dependent. In severe FV, FX, and FXIII‐deficient WB, rFVIIa addition induced minor changes only. In FXI deficiency, rFVIIa normalized the dynamic properties of clotting, although a reduced clot firmness remained unchanged. In conclusion, the thrombelastographic analysis of WB clotting, as activated with a minute amount of TF, seems an interesting method that detects phenotypic variation amongst hemophilia patients. The method appears useful for assessment of the hemostatic capacity and it seems a promising tool for evaluation of the individual response to rFVIIa or APCC before and during in vivo administration.     

Laboratory testing in hemophilia: Impact of factor and non‐factor replacement therapy on coagulation assays

The advent of extended half‐life (EHL) recombinant clotting factors and innovative non‐factor replacement therapeutics, such as emicizumab, offers several advantages over existing products for the prophylactic treatment of people living with hemophilia (PwH). These include low annual bleeding rates with less frequent dosing, higher trough plasma concentrations, and a more convenient route of administration. However, increasing use of these therapies poses challenges to clinicians and coagulation laboratories due to the lack of standardized assays for monitoring of hemostatic parameters, and the potential for misinterpretation of test results, which may jeopardize patient safety. Definitive diagnosis of hemophilia and treatment monitoring is reliant on demonstrating factor VIII (FVIII; hemophilia A) or factor IX (FIX; hemophilia B) deficiency using a functional coagulation assay. The most frequently used assays are based on activated partial thromboplastin time, using a one‐stage or two‐stage process. While one‐stage and chromogenic assays have performed well with human‐derived FVIII and FIX and full‐length recombinant products, EHL recombinant factors are heterogeneous in structure and mode of action and therefore show wide variation in activity levels between different one‐stage assays, and between one‐stage and chromogenic assays. In the context of the recommended stepwise approach for laboratory diagnosis of hemophilia, we examine the diagnostic challenges associated with the use of EHL factors and novel non‐factor therapeutics and consider the optimal diagnostic approach in PwH who are receiving these treatments. Ultimately, accurate diagnostic solutions are a prerequisite for personalized therapy to minimize treatment burden and improve quality of life in PwH.     

Functional factor VIII made with von Willebrand factor at high levels in transgenic milk

Summary. Background: Current manufacturing methods for recombinant human factor VIII (rFVIII) within mammalian cell cultures are inefficient, hampering the production of sufficient amounts for affordable, worldwide treatment of hemophilia A. However, rFVIII has been expressed at very high levels by the transgenic mammary glands of mice, rabbits, sheep, and pigs. Unfortunately, it is secreted into milk with low specific activity, owing in part to the labile, heterodimeric structure that results from furin processing of its B domain. Objectives: To express biologically active rFVIII in the milk of transgenic mice through targeted bioengineering. Methods: Transgenic mice were made with a mammary‐specific FVIII gene (226/N6) bioengineered for efficient expression and stability, encoding a protein containing a B domain with no furin cleavage sites. 226/N6 was expressed with and without von Willebrand factor (VWF). 226/N6 was evaluated by ELISA, SDS‐PAGE, western blot, and one‐stage and two‐stage clotting assays. The hemostatic activity of immunoaffinity‐enriched 226/N6 was studied in vivo by infusion into hemophilia A knockout mice. Results and conclusions: With or without coexpression of VWF, 226/N6 was secreted into milk as a biologically active single‐chain molecule that retained high specific activity, similar to therapeutic‐grade FVIII. 226/N6 had > 450‐fold higher IU mL^?1 than previously reported in cell culture for rFVIII. 226/N6 exhibited similar binding to plasma‐derived VWF as therapeutic‐grade rFVIII, and intravenous infusion of transgenic 226/N6 corrected the bleeding phenotype of hemophilia A mice. This provides proof‐of‐principle for the study of expression of 226/N6 and perhaps other single‐chain bioengineered rFVIIIs in the milk of transgenic livestock.     

Integration-deficient Lentiviral Vectors Expressing Codon-optimized R338L Human FIX Restore Normal Hemostasis in Hemophilia B Mice

Integration-deficient lentiviral vectors (IDLVs) have been shown to transduce a wide spectrum of target cells and organs in vitro and in vivo and to maintain long-term transgene expression in nondividing cells. However, epigenetic silencing of episomal vector genomes reduces IDLV transgene expression levels and renders these safe vectors less efficient. In this article, we describe for the first time a complete correction of factor IX (FIX) deficiency in hemophilia B mice by IDLVs carrying a novel, highly potent human FIX cDNA. A 50-fold increase in human FIX cDNA potency was achieved by combining two mechanistically independent yet synergistic strategies: (i) optimization of the human FIX cDNA codon usage to increase human FIX protein production per vector genome and (ii) generation of a highly catalytic mutant human FIX protein in which the arginine residue at position 338 was substituted with leucine. The enhanced human FIX activity was not associated with liver damage or with the formation of human FIX-directed inhibitory antibodies and rendered IDLV-treated FIX-knockout mice resistant to a challenging tail-clipping assay. A novel S1 nuclease-based B1-quantitative polymerase chain reaction assay showed low levels of IDLV integration in mouse liver. Overall, this study demonstrates that IDLVs carrying an improved human FIX cDNA safely and efficiently cure hemophilia B in a mouse model.     

Preclinical and clinical data for the use of mesenchymal stem cells in articular cartilage tissue engineering

Introduction: Hemophilia B, the deficiency or complete absence of coagulation factor IX (FIX), affects an estimated 80,000 people throughout the world. Some of these individuals are managed with prophylaxis, which involves the intravenous infusion of FIX concentrate two to three times weekly to prevent bleeding. Because FIX prophylaxis remains underutilized, patients with hemophilia B are at risk for bleeding that may be severe and potentially life- or limb-threatening, and they may experience arthropathy resulting from recurrent hemarthroses.

Areas covered: This review focuses on recent advances in therapeutic protein fusion technology as they apply to FIX deficiency. The National Library of Medicine Medline database was searched for articles containing the term ‘Fc fusion proteins’.

Expert opinion: Genetically engineered recombinant FIX fused to the Fc portion of immunoglobulin significantly extends FIX half-life, thereby decreasing the frequency of prophylactic infusions. This in turn may increase the adoption of, and adherence to, prophylaxis, leading to better outcomes for hemophilia B patients.     

Enhancement of the enzymatic activity of activated coagulation factor IX by anti-factor IX antibodies

Summary.  Background:  Factor VIIIa (FVIIIa) binds to activated FIX and enhances the activation of FX by several orders of magnitude. Deficiency of FVIII causes the bleeding disorder hemophilia A and is treated by i.v. infusion of FVIII concentrates. Objectives:  To explore whether or not FVIII activity can be supplied by alternative molecules, e.g. molecules with FIXa-binding activity. Methods:  Conventional hybdridoma technology was used to discover antibodies exhibiting FVIII-like activity. Results:  We identified a series of antibodies specific for human FIX that mimicked the stimulatory effect of FVIIIa on FIXa. Upon binding to human FIXa, these antibodies enhanced the protease activity of FIXa towards its natural substrate FX about tenfold. A similar enhancement was also achieved with 5 p m FVIIIa (i.e . 16 mU mL⁻¹ or 1.6% activated FVIII). Procoagulant activity of these anti-FIXa antibodies was observed in model systems containing purified proteins as well as in plasma. Conclusion:  Our findings show that FVIII can, at least partially, be replaced by an unrelated molecule. Procoagulant antibodies might potentially aid the development of an FVIII substitute for hemophilia A treatment.     

Improvement of spatial fibrin formation by the anti‐TFPI aptamer BAX499: changing clot size by targeting extrinsic pathway initiation

Summary. Background: Tissue factor pathway inhibitor (TFPI) is a major regulator of clotting initiation and a promising target for pro‐ and anticoagulation therapy. The aptamer BAX499 (formerly ARC19499) is a high‐affinity specific TFPI antagonist designed to improve hemostasis. However, it is not clear how stimulation of coagulation onset by inactivating TFPI will affect spatial and temporal clot propagation. Objective: To examine the BAX499 effect on clotting in a spatial, reaction‐diffusion experimental system in comparison with that of recombinant activated factor VII (rVIIa). Methods: Clotting in plasma activated by immobilized tissue factor (TF) was monitored by videomicroscopy. Results: BAX499 dose‐dependently improved coagulation in normal and hemophilia A plasma activated with TF at 2 pmole m^?2 by shortening lag time and increasing clot size by up to ～2‐fold. The effect was TFPI specific as confirmed by experiments in TFPI‐depleted plasma with or without TFPI supplementation. Clotting improvement was half‐maximal at 0.7 nm of BAX499 and reached a plateau at 10 nm, remaining there at concentrations up to 1000 nm . The BAX499 effect decreased with TF surface density increase. RVIIa improved clotting in hemophilia A plasma activated with TF at 2 or 20 pmole m^?2, both by shortening lag time and increasing spatial velocity of clot propagation; its effects were strongly concentration dependent. Conclusions: BAX499 significantly improves spatial coagulation by inhibiting TFPI in a spatially localized manner that is different to that observed with rVIIa.     

Inhibition of tissue factor pathway inhibitor by the aptamer BAX499 improves clotting of hemophilic blood and plasma

Summary. Background: Tissue factor pathway inhibitor (TFPI) is the major inhibitor of tissue factor‐initiated coagulation, making it an interesting and novel therapeutic target in hemophilia treatment. The aptamer BAX499 (formerly ARC19499) is designed to improve hemostasis by specifically inhibiting TFPI. Objectives: The aim of the study was to examine the concentration‐dependent augmentation of clotting by BAX499. Methods: Whole blood clot formation was quantified by rotational thromboelastometry and thromboelastography, and thrombin generation in platelet‐poor plasma was assessed with the calibrated automated thrombogram, in samples from patients with congenital hemophilia A (N = 55) and B (N = 11), patients with acquired hemophilia A (N = 1), and healthy controls (N = 37). Results: BAX499 significantly improved clotting of samples from hemophilic patients in a concentration‐dependent manner, resulting in clotting profiles in samples from patients with severe hemophilia that were similar to those of healthy controls. Conclusion: BAX499 improved ex vivo clotting parameters in blood and plasma from patients with hemophilia A and B with different severity of disease, and also in a patient with acquired hemophilia. These results further support the contention that anti TFPI strategies may be an effective treatment for hemophilic patients.     

Disulfide bond-stabilized factor VIII has prolonged factor VIIIa activity and improved potency in whole blood clotting assays

BACKGROUND: Genetically engineered disulfide bonds in B-domain-deleted factor (F) VIII variants (C662-C1828 FVIII and C664-C1826 FVIII) improve FVIIIa stability by blocking A2 domain dissociation because the new disulfide covalently links the A2 and A3 domains in FVIIIa. AIM: The aim of this study was to assess the hypothesis that these variants have physiologically relevant properties because of prolonged thrombin generation and improved clot formation in whole blood. METHODS: Clot-formation properties in whole blood were measured in thromboelastogram assays. The thrombin generation capabilities of the wild-type (WT) FVIII and FVIII variants were determined, and half-lives of FVIIIa variants were determined in fresh whole blood serum. RESULTS: Thromboelastogram assays were performed with fresh, severe hemophilia whole blood reconstituted with variant and WT FVIII. The two disulfide bond-stabilized FVIII variants and WT FVIII had comparable clotting times at all studied concentrations. However, when compared with WT FVIII at low concentrations, the two FVIII variants required only 10% as much FVIII to achieve comparable clot-formation rates, clot-formation times and clot firmness values. The differences between WT and FVIII variants were quite pronounced at low FVIII concentrations. Measurement of the endogenous thrombin potential in FVIII-deficient plasma supplemented with these FVIII variants confirmed that the disulfide bond-stabilized variants supported high levels of thrombin generation at lower concentrations than did WT FVIII. During the course of clot generation in whole blood, the disulfide bond-stabilized FVIIIa variants had approximately 5-fold increased half-lives relative to WT FVIIIa. CONCLUSION: C662-C1828 FVIII and C664-C1826 FVIII have physiologically relevant superior clot-forming properties in a whole blood environment, most likely due to the increased half-life of these FVIIIa variants in whole blood.     

Factor XI promotes hemostasis in factor IX‐deficient mice

Essentials

• Mice lacking factor IX (FIX) or factor XI (FXI) were tested in a saphenous vein bleeding model.
• FIX‐deficient mice displayed a hemostatic defect and FXI‐deficient mice were similar to wild type mice.
• Infusion of FXI or over‐expression of FXI in FIX‐deficient mice improved hemostasis.
• FXI may affect the phenotype of FIX‐deficiency (hemophilia B).

Summary

Background

In humans, deficiency of coagulation factor XI may be associated with a bleeding disorder, but, until recently, FXI‐deficient mice did not appear to have a hemostatic abnormality. A recent study, however, indicated that FXI‐deficient mice show a moderate hemostatic defect in a saphenous vein bleeding (SVB) model.

Objectives

To study the effect of FXI on bleeding in mice with normal levels of the FXI substrate FIX and in mice lacking FIX (a murine model of hemophilia B).

Methods

Wild‐type mice and mice lacking either FIX (F9^?) or FXI (F11^?/?) were tested in the SVB model. The plasma levels of FXI in F11^?/? mice were manipulated by infusion of FXI or its active form FXIa, or by overexpressing FXI by the use of hydrodynamic tail vein injection.

Results

F9^? mice showed a significant defect in the SVB model, whereas F11^?/? mice and wild‐type mice were indistinguishable. Intravenous infusion of FXI or FXIa into, or overexpression of FXI in, F9^? mice improved hemostasis in the SVB model. Overexpression of a FXI variant lacking a FIX‐binding site also improved hemostasis in F9^? mice.

Conclusions

Although we were unable to demonstrate a hemostatic defect in F11^?/? mice in the SVB model, our results support the premise that supraphysiological levels of FXI improve hemostasis in F9^? mice through FIX‐independent pathways.

     

Functional analysis of the EGF-like domain mutations Pro55Ser and Pro55Leu, which cause mild hemophilia B

Summary.  We studied the functional role of two mutations, Pro55Ser and Pro55Leu, located in the N-terminal Epidermal Growth Factor-like domain (EGF1) of coagulation factor (F) IX. Both mutations cause mild hemophilia B with habitual FIX coagulant activities of 10–12% and FIX antigen levels of 50%. We found that activation by FVIIa/TF and FXIa was normal for FIXPro55Ser, but resulted in proteolysis of FIXPro55Leu at Arg318-Ser319 with a concomitant loss of amidolytic activity, suggesting intramolecular communication between EGF1 and the serine protease domain in FIX. This was further supported by experiments using an anti-EGF1 monoclonal antibody. Activation of FX by FIXaPro55Ser was impaired in both the presence and the absence of phospholipid or FVIIIa, indicating that Pro55 is not directly involved in binding to FVIIIa. We also studied the effect of the two Pro55 mutations on Ca²⁺ affinity and found only small changes. Thus, the Pro55Ser mutation causes hemophilia primarily through to an impaired ability to activate FX whereas at least in vitro the Pro55Leu defect interferes with the activation of FIX.     

重组AAV2/hFⅨ病毒制备及其基因治疗血友病B的实验研究

目的　制备携带人凝血因子Ⅸ (hFⅨ )基因的重组AAV2病毒 (rAAV2 /hFⅨ ) ,并对用rAAV2 /hFⅨ肌肉注射治疗血友病B模型小鼠的疗效进行评价。方法　通过“一株载体细胞 /一株辅助病毒”的双因素包装策略制备出rAAV2 /hFⅨ ,体外转导BHK 2 1、C2C12细胞后 ,检测细胞培养上清中hFⅨ的表达量 ;肌肉直接注射血友病B模型小鼠后 ,检测其血浆中hFⅨ的抗原水平和凝血活性等指标。结果　转导 2 4h后在细胞上清中即可检测到hFⅨ ,连续检测 12 0h都有表达 ,BHK 2 1、C2C12细胞 2 4h最高表达量分别达到 (5 1.0± 6 .5 )ng/ 10 5细胞和 (6 8.0± 7.2 )ng/ 10 5细胞。rAAV2 /hFⅨ经肌肉直接注射后 ,高、中、低三个剂量组均能检测到小鼠体内高效表达hFⅨ ,在给药后第 3周达到高峰 ,小鼠血浆中hFⅨ的表达量与对照组比较差异有显著性 (P <0 .0 1) ,之后缓慢下降 ,到第 10周仍可检测到低水平hFⅨ表达 ;取第 3周小鼠血浆样品检测凝血功能 ,高、中、低剂量组FⅨ活性均得到明显改善 ,小鼠的割尾实验出血时间明显缩短 ,5min失血量也相应显著减少 ,其中高剂量组hFⅨ最高表达量达到 (387.0± 12 .5 )ng/ml血浆 ,FⅨ活性达到正常水平的 (30 .0± 5 .5 ) % ;给药后第 10周 ,除在注射点外 ,其它主要脏器均未检测到AAV载体DNA。结论     

Major differences in bleeding symptoms between factor VII deficiency and hemophilia B

Summary.  Background:  The autosomally-inherited factor VII (FVII) deficiency and X-linked hemophilia B offer an attractive model to investigate whether reduced levels of FVII and FIX, acting in the initiation and amplification of coagulation respectively, influence hemostasis to a different extent in relation to age and bleeding site. Methods:  Hemophilia B patients ( n  = 296) and FVII-deficient males ( n  = 109) were compared for FVII/FIX clotting activity, F7/F9 genotypes and clinical phenotypes in a retrospective, multi-centre, cohort study. Results:  Major clinical differences between diseases were observed. Bleeding occurred earlier in hemophilia B (median age 2.0 years, IR 0.9–5.0) than in FVII deficiency (5.2 years, IR 1.9–15.5) and the bleeding-free survival in FVII deficiency was similar to that observed in 'mild' hemophilia B ( P  = 0.96). The most frequent disease-presenting symptoms in hemophilia B (hematomas and oral bleeding) differed from those in FVII deficiency (epistaxis and central nervous system bleeding). Differences were confirmed by analysis of FVII-deficient women. Conclusions:  Our data support the notion that low FVII levels sustain hemostasis better than similarly reduced FIX levels. On the other hand, minute amounts of FVII, differently to FIX, are needed to prevent fatal bleeding, as indicated by the rarity of null mutations and the associated life-threatening symptoms in FVII deficiency, which contributes towards shaping clinical differences between diseases in the lowest factor level range. Differences between diseases are only partially explained by mutational patterns and could pertain to the specific roles of FVII and FIX in coagulation phases and to vascular bed-specific components.     

Factor IX mutants with enhanced catalytic activity

Summary.  Background : Activated coagulation factor IX (FIXa) has low catalytic activity towards its physiologic substrate FX when activated FVIII (FVIIIa) is absent. One reason for this is that the FIX surface loop 99 stabilizes FIXa in a conformation that limits access of FX to the active site. Objectives : To investigate the effect of mutations in loop 99 and in the active site on FIXa activity with and without FVIIIa. Methods : Five full-length FIX mutants with amino acid exchanges in the catalytic domain of FIX were constructed and characterized by measuring their activity in FX activation in model systems and in plasma. Results and Conclusions : The mutants showed no or marginally improved catalytic properties in FX activation by the intrinsic tenase complex (FIXa–FVIIIa–Ca²⁺–phospholipid). The combination of mutations Y94F and K98T hardly affected FX activation in the presence of FVIIIa, but yielded a FIX molecule that, in FIX-depleted plasma, had ∼ 2.5-fold higher clotting activity and ∼ 3.5-fold higher activity in a thrombin generation assay than plasma-derived FIX (pdFIX). Two FIXa mutants had considerably increased activities towards FX in the absence of FVIIIa. FIXa-Y94F/K98T/Y177F/I213V/E219G (FIXa-L) and FIXa-Y94F/A95aK/K98T/Y177F/I213V/E219G (FIXa-M) activated FX with catalytic efficiencies ( k _cat/ K _m) that, as compared with activated pdFIX, were increased 17-fold and six-fold, respectively. However, in plasma, their zymogen forms performed similarly to pdFIX. This indicates that the introduced mutations not only affected the activity of FIXa but may have also influenced the lifetime of the activated mutant molecules in plasma by modifying their activation and/or inhibition rates.     

Activated factor X cleaves factor VIII at arginine 562, limiting its cofactor efficiency

Summary. Background: Factor VIII (FVIII) and its activated form (FVIIIa) are subject to proteolysis that dampens their cofactor function. Among the proteases that attack FVIII (activated factor X (FXa), activated protein C (APC) and plasmin), only APC cleaves within the FVIII A2 domain at R562 to fully abolish FVIII activity. Objectives: We investigated the possible involvement of the FXa cleavage at R562 within the A2 domain in the process of FVIII inactivation. Methods: An antibody (GMA012/R8B12) that recognizes the carboxy‐terminus extremity of the A2 domain (A2C) was used to evaluate FXa action. A molecule mutated at R562 was also generated to assess the functional role of this particular residue. Results and Conclusions: The appearance of the A2C domain as a function of time evidenced the identical cleavage within the A2 domain of FVIII and FVIIIa by FXa. This cleavage required phospholipids and occurred within minutes. In contrast, the isolated A2 domain was not cleaved by FXa. Von Willebrand factor and activated FIX inhibited the cleavage in a dose‐dependent manner. Mutation R562K increased both the FVIII specific activity and the generation of FXa due to an increase in FVIII catalytic efficiency. Moreover, A2C fragment could not be identified from FVIII‐R562K cleavage. In summary, this study defines a new mechanism for A2 domain‐mediated FVIII degradation by FXa and implicates the bisecting of the A2 domain at R562.     

Extravascular FIX and coagulation

This review summarizes the evidence that collagen IV binding is physiologically important, and that the extravascular compartment of FIX is composed of type IV collagen. As we have previously demonstrated, 7 days post-infusion, FIX_WT (BeneFIX) is able to control bleeding as well as the same dosage of Alprolix in hemophilia B mice, tested using the saphenous vein bleeding model (Alprolix is a chimeric FIX molecule joined at its C terminus to a Fc domain). Furthermore, we have shown that in hemophilia B mice, doses of BeneFIX or Alprolix (up to a dose of 150 IU/kg) have increased bleeding-control effectiveness in proportion to the dose up to a certain limit: higher doses are no more effective than the 150 IU/kg dose. These studies suggest that in hemophilia B mice, tested using the saphenous vein bleeding model, three things are true: first, extravascular FIX is at least as important for coagulation as is circulating FIX; second, measuring circulating levels of FIX may not be the best criterion for designing new “longer lasting” FIX molecules; and third, trough levels are less diagnostic for FIX therapy than they are for FVIII therapy.     

Gene therapy for the hemophilias

Summary.  Significant progress has recently been made in the development of gene therapy for the treatment of hemophilia A and B. These advances parallel the development of improved gene delivery systems. Long-term therapeutic levels of factor (F) VIII and FIX can be achieved in adult FVIII- and FIX-deficient mice and in adult hemophiliac dogs using adeno-associated viral (AAV) vectors, high-capacity adenoviral vectors (HC-Ad) and lentiviral vectors. In mouse models, some of the highest FVIII or FIX expression levels were achieved using HC-Ad vectors with no or only limited adverse effects. Encouraging preclinical data have been obtained using AAV vectors, yielding long-term FIX levels above 10% in primates and in hemophilia B dogs, which prevented spontaneous bleeding. Non-viral ex vivo gene therapy approaches have also led to long-term therapeutic levels of coagulation factors in animal models. Nevertheless, the induction of neutralizing antibodies (inhibitors) to FVIII or FIX sometimes precludes stable phenotypic correction following gene therapy. The risk of inhibitor formation varies depending on the type of vector, vector serotype, vector dose, expression levels and promoter used, route of administration, transduced cell type and the underlying mutation in the hemophilia model. Some studies suggest that continuous expression of clotting factors may induce immune tolerance, particularly when expressed by the liver. Several gene therapy phase I clinical trials have been initiated in patients suffering from severe hemophilia A or B. Some subjects report fewer bleeding episodes and occasionally have low levels of clotting factor activity detected. Further improvement of the various gene delivery systems is warranted to bring a permanent cure for hemophilia one step closer to reality.