Surface-exposed hemophilic mutations across the factor VIII C2 domain have variable effects on stability and binding activities

Biophysical characterization of blood coagulation factor VIII binding to lipid nanodiscs that mimic activated platelet surfaces

BACKGROUND

Following proteolytic activation, activated blood coagulation factor (F)VIII (FVIIIa) binds to activated platelet membranes, forming the intrinsic tenase complex with activated FIX (FIXa). Previous studies have identified the C1 and C2 domains as the membrane binding domains of FVIII through conserved arginine residues. A membrane binding model for the FVIII C domains proposes that surface-exposed hydrophobic and positively charged residues at each C domain interact with the membrane, yet a comprehensive thermodynamic and structural description of this interaction is lacking.

OBJECTIVES

To determine residues of interaction, thermodynamics, and membrane binding preference for FVIII membrane association.

METHODS

The binding of FVIII constructs to lipid nanodiscs was characterized by nuclear magnetic resonance, isothermal titration calorimetry, bio-layer interferometry, and X-ray crystallography.

RESULTS

The thermodynamics of FVIII membrane binding indicated that the C1 domain associates through an enthalpically driven process while the C2 domain is entropically driven. Alanine mutations to surface-exposed hydrophobic residues in the C2 domain revealed differential effects on membrane binding, highlighting important determinants at the residue level. The structure of a C2 double mutant, L2251A/L2252A, demonstrated that its decreased affinity is likely due to decreasing the surface area hydrophobicity. Nuclear magnetic resonance studies with the C2 domain identified residues of interaction with soluble O-phospho-L-serine as well as lipid nanodiscs. Lastly, increasing phosphatidylethanolamine and decreasing phosphatidylserine content decreased overall FVIII affinity for membrane surfaces.

CONCLUSION

This study provides further insight into the molecular basis for how FVIII interacts with platelets to form the intrinsic tenase complex.

Genotype-Dependent Response to Desmopressin in Hemophilia A and Proposal of a Predictive Response Score

BACKGROUND

 Desmopressin (DDAVP) is used in patients with moderate/mild hemophilia A (PWMHs) to increase their factor VIII (FVIII) level and, if possible, normalize it. However, its effectiveness varies between individuals. The GIDEMHA study aims to investigate the influence of F8 gene variants.

MATERIAL AND METHODS

 The study collected the trajectory of FVIII levels from therapeutic intravenous DDAVP tests in four French hemophilia treatment centers. A pharmacological analysis was performed associated with efficacy scores according to F8 variants: absolute and relative responses, as well as new scores: absolute duration (based on duration with FVIII ≥ 0.50 IU.mL-1) and relative duration (based on half-life).

RESULTS

 From enrolled 439 PWMHs, 327 had a hot-spot F8 variant (with ≥5 PWMHs). For these, the median (min-max) basal and peak FVIII were 0.20 (0.02-0.040) and 0.74 (0.14-2.18) IU.mL-1 respectively, with FVIII recovery being 3.80 IU.ml-1 (1.15-14.75). The median FVIII half-life was 3.9 hours (0.7-15.9 hours). FVIII was normalized (≥0.50 IU.mL-1) in 224/327 PWMHs (69%) and the median time with normalized FVIII was 3.9 hours (0.0-54.1 hours). Following the response profiles to DDAVP defined by the four efficacy scores, four groups of F8 variants were isolated, and then compared using survival curves with normalized FVIII (p < 0.0001): "long-lastingly effective" [p.(Glu739Lys), p.(Ser2030Asn), p.(Arg2178His), p.(Gln2208Glu), and T-stretch deletion in intron 13]; "moderately effective" [p.(Ser112Phe), p.(Ala219Thr), p.(Thr2105Ile), p.Phe2146Ser), and p.(Asp2150Asn)]; "moderately ineffective" [p.Ala81Asp), p.(Gln324Pro), p.(Tyr492His), p.(Arg612Cys), p.(Met701Val), p.(Val2035Asn), and p.(Arg2178Cys)]; and "frequently ineffective" [c.-219C > T, p.(Cys2040Tyr), p.(Tyr2169His), p.(Pro2319Leu), and p.(Arg2326Gln)].

CONCLUSION

 In view of our data, we propose indications for DDAVP use in PWMH based on F8 variants for minor and major invasive procedures.

Membrane binding and lipid-protein interaction of the C2 domain from coagulation factor V

Anchoring of coagulation factors to anionic regions of the membrane involves the C2 domain as a key player. The rate of enzymatic reactions of the coagulation factors is increased by several orders of magnitude upon membrane binding. However, the precise mechanisms behind the rate acceleration remain unclear, primarily because of a lack of understanding of the conformational dynamics of the C2-containing factors and corresponding complexes. We elucidate the membrane-bound form of the C2 domain from human coagulation factor V (FV-C2) by characterizing its membrane binding the specific lipid-protein interactions. Employing all-atom molecular dynamics simulations and leveraging the highly mobile membrane-mimetic (HMMM) model, we observed spontaneous binding of FV-C2 to a phosphatidylserine (PS)-containing membrane within 2-25 ns across twelve independent simulations. FV-C2 interacted with the membrane through three loops (spikes 1-3), achieving a converged, stable orientation. Multiple HMMM trajectories of the spontaneous membrane binding provided extensive sampling and ample data to examine the membrane-induced effects on the conformational dynamics of C2 as well as specific lipid-protein interactions. Despite existing crystal structures representing presumed "open" and "closed" states of FV-C2, our results revealed a continuous distribution of structures between these states, with the most populated structures differing from both "open" and "closed" states observed in crystal environments. Lastly, we characterized a putative PS-specific binding site formed by K23, Q48, and S78 located in the groove enclosed by spikes 1-3 (PS-specificity pocket), suggesting a different orientation of a bound headgroup moiety compared to previous proposals based upon analysis of static crystal structures.

Stable binding to phosphatidylserine-containing membranes requires conserved arginine residues in tandem C domains of blood coagulation factor VIII

At sites of vascular damage, factor VIII (fVIII) is proteolytically activated by thrombin and binds to activated platelet surfaces with activated factor IX (fIXa) to form the intrinsic "tenase" complex. Previous structural and mutational studies of fVIII have identified the C1 and C2 domains in binding to negatively charged membrane surfaces through β-hairpin loops with solvent-exposed hydrophobic residues and a ring of positively charged basic residues. Several hemophilia A-associated mutations within the C domains are suggested to disrupt lipid binding, preventing formation of the intrinsic tenase complex. In this study, we devised a novel platform for generating recombinant C1, C2, and C1C2 domain constructs and performed mutagenesis of several charged residues proximal to the putative membrane binding region of each C domain. Binding measurements between phosphatidylserine (PS)-containing lipid membrane surfaces and fVIII C domains demonstrated an ionic strength dependence on membrane binding affinity. Mutations to basic residues adjacent to the surface-exposed hydrophobic regions of C1 and C2 differentially disrupted membrane binding, with abrogation of binding occurring for mutations to conserved arginine residues in the C1 (R2163) and C2 (R2320) domains. Lastly, we determined the X-ray crystal structure of the porcine fVIII C2 domain bound to o-phospho-L-serine, the polar headgroup of PS, which binds to a basic cleft and makes charge-charge contact with R2320. We conclude that basic clefts in the fVIII C domains bind to PS-containing membranes through conserved arginine residues via a C domain modularity, where each C domain possesses modest electrostatic-dependent affinity and tandem C domains are required for high affinity binding.

Structure of Blood Coagulation Factor VIII in Complex With an Anti-C2 Domain Non-Classical, Pathogenic Antibody Inhibitor

Factor VIII (fVIII) is a procoagulant protein that binds to activated factor IX (fIXa) on platelet surfaces to form the intrinsic tenase complex. Due to the high immunogenicity of fVIII, generation of antibody inhibitors is a common occurrence in patients during hemophilia A treatment and spontaneously occurs in acquired hemophilia A patients. Non-classical antibody inhibitors, which block fVIII activation by thrombin and formation of the tenase complex, are the most common anti-C2 domain pathogenic inhibitors in hemophilia A murine models and have been identified in patient plasmas. In this study, we report on the X-ray crystal structure of a B domain-deleted bioengineered fVIII bound to the non-classical antibody inhibitor, G99. While binding to G99 does not disrupt the overall domain architecture of fVIII, the C2 domain undergoes an ~8 Å translocation that is concomitant with breaking multiple domain-domain interactions. Analysis of normalized B-factor values revealed several solvent-exposed loops in the C1 and C2 domains which experience a decrease in thermal motion in the presence of inhibitory antibodies. These results enhance our understanding on the structural nature of binding non-classical inhibitors and provide a structural dynamics-based rationale for cooperativity between anti-C1 and anti-C2 domain inhibitors.

Analysis of protein missense alterations by combining sequence- and structure-based methods

BACKGROUND

Different types of in silico approaches can be used to predict the phenotypic consequence of missense variants. Such algorithms are often categorized as sequence based or structure based, when they necessitate 3D structural information. In addition, many other in silico tools, not dedicated to the analysis of variants, can be used to gain additional insights about the possible mechanisms at play.

METHODS

Here we applied different computational approaches to a set of 20 known missense variants present on different proteins (CYP, complement factor B, antithrombin and blood coagulation factor VIII). The tools that were used include fast computational approaches and web servers such as PolyPhen-2, PopMusic, DUET, MaestroWeb, SAAFEC, Missense3D, VarSite, FlexPred, PredyFlexy, Clustal Omega, meta-PPISP, FTMap, ClusPro, pyDock, PPM, RING, Cytoscape, and ChannelsDB.

RESULTS

We observe some conflicting results among the methods but, most of the time, the combination of several engines helped to clarify the potential impacts of the amino acid substitutions.

CONCLUSION

Combining different computational approaches including some that were not developed to investigate missense variants help to predict the possible impact of the amino acid substitutions. Yet, when the modified residues are involved in a salt-bridge, the tools tend to fail, even when the analysis is performed in 3D. Thus, interactive structural analysis with molecular graphics packages such as Chimera or PyMol or others are still needed to clarify automatic prediction.

Effect of F8 B domain gene variants on synthesis, secretion, activity and stability of factor VIII protein

The B domain of the coagulation factor (F)VIII comprises some unique characteristics. Though the B domain is important for processing, intracellular transport and secretion of FVIII protein, its role in the coagulation still remains unclear. This study aims to investigate the influence of 19 reported B domain variants on quantity and quality of expressed FVIII protein. F8 variants were transiently expressed in HEK293T cells. Media and cell lysates were collected after 72 hours. FVIII synthesis, relative secretion, activity and thermostability were analysed in comparison to FVIII wild-type. Eleven of 19 analysed B domain variants showed normal FVIII activity (FVIII:C), and antigen values (40–150 %). Eight variants exhibited a decreased FVIII:C, corresponding to a mild phenotype most likely due to impaired expression and secretion mechanism, reduced thermostability or combined mechanisms. One variant, p.His1066Tyr, showed markedly reduced FVIII antigen in cell lysate. The variants p.Asp845Glu, p.His998Gln, and p.Ala1610Ser revealed a significantly decreased relative secretion. Additionally, six B domain variants significantly reduced stability of FVIII. In conclusion, none of the analysed missense mutations was causative for a severe haemophilia A (HA) phenotype. Nevertheless, the mutations p.Asp845Glu, p.Pro947Arg, p.Glu1057Lys, p.His1066Tyr, p.Arg1126Trp, p.Arg1329His, p.Leu1481Pro, and p.Ala1610Ser resulted in decreased FVIII:C values that may explain mild HA phenotypes.

Molecular simulation studies of human coagulation factor VIII C domain-mediated membrane binding

The C-terminal C domains of activated coagulation factor VIII (FVIIIa) are essential to membrane binding of this crucial coagulation cofactor protein. To provide an overall membrane binding mechanism for FVIII, we performed simulations of membrane binding through coarsegrained molecular dynamics simulations of the C1 and C2 domain, and the combined C-domains (C1+C2). We found that the C1 and C2 domain have different membrane binding properties. The C1 domain uses hydrophobic spikes 3 and 4, of its total of four spikes, as major loops to bind the membrane, whereas all four of its hydrophobic loops of the C2 domain appear essential for membrane binding. Interestingly, in the C1+C2 system, we observed cooperative binding of the C1 and C2 domains such that all four C2 domain spikes bound first, after which all four loops of the C1 domain inserted into the membrane, while the net binding energy was higher than that of the sum of the isolated C domains. Several residues, mutations of which are known to cause haemophilia A, were identified as key residues for membrane binding. In addition to these known residues, we identified residues from the C1 and C2 domains, which are involved in the membrane binding process, that have not been reported before as a cause for haemophilia A, but which contribute to overall membrane binding and which are likely candidates for novel causative missense mutations in haemophilia A.

In silico analyses of missense mutations in coagulation factor VIII: identification of severity determinants of haemophilia A

Factor VIII (FVIII) mutations cause haemophilia A (HA), an X-linked recessive coagulation disorder. Over 1000 missense mutations in FVIII are known and they lead to variable clinical phenotypes (severe, moderate and mild). The exact molecular basis of this phenotypic heterogeneity by FVIII missense mutations is elusive to date. In this study, we aimed to identify the severity determinants that cause phenotypic heterogeneity of HA. We compiled and curated a data set of 766 missense mutations from the repertoire of missense mutations in FVIII. We analysed these mutations by computational programs (e.g. Swiss-PdbViewer) and different mutation analysis servers (e.g. SIFT, PROVEAN, CUPSAT, PolyPhen2, MutPred); and various sequence- and structure-based parameters were assessed for any significant distribution bias among different HA phenotypes. Our analyses suggest that 'mutations in evolutionary conserved residues', 'mutations in buried residues', mutation-induced 'steric clash' and 'surface electrostatic potential alteration' act as risk factors towards severe HA. We have developed a grading system for FVIII mutations combining the severity determinants, and the grading pattern correlates with HA phenotype. This study will help to correctly associate the HA phenotype with a mutation and aid early characterization of novel variants.

The 1.7 Å X-ray crystal structure of the porcine factor VIII C2 domain and binding analysis to anti-human C2 domain antibodies and phospholipid surfaces

The factor VIII C2 domain is essential for binding to activated platelet surfaces as well as the cofactor activity of factor VIII in blood coagulation. Inhibitory antibodies against the C2 domain commonly develop following factor VIII replacement therapy for hemophilia A patients, or they may spontaneously arise in cases of acquired hemophilia. Porcine factor VIII is an effective therapeutic for hemophilia patients with inhibitor due to its low cross-reactivity; however, the molecular basis for this behavior is poorly understood. In this study, the X-ray crystal structure of the porcine factor VIII C2 domain was determined, and superposition of the human and porcine C2 domains demonstrates that most surface-exposed differences cluster on the face harboring the "non-classical" antibody epitopes. Furthermore, antibody-binding results illustrate that the "classical" 3E6 antibody can bind both the human and porcine C2 domains, although the inhibitory titer to human factor VIII is 41 Bethesda Units (BU)/mg IgG versus 0.8 BU/mg IgG to porcine factor VIII, while the non-classical G99 antibody does not bind to the porcine C2 domain nor inhibit porcine factor VIII activity. Further structural analysis of differences between the electrostatic surface potentials suggest that the C2 domain binds to the negatively charged phospholipid surfaces of activated platelets primarily through the 3E6 epitope region. In contrast, the G99 face, which contains residue 2227, should be distal to the membrane surface. Phospholipid binding assays indicate that both porcine and human factor VIII C2 domains bind with comparable affinities, and the human K2227A and K2227E mutants bind to phospholipid surfaces with similar affinities as well. Lastly, the G99 IgG bound to PS-immobilized factor VIII C2 domain with an apparent dissociation constant of 15.5 nM, whereas 3E6 antibody binding to PS-bound C2 domain was not observed.

Epitope mapping of inhibitory antibodies targeting the C2 domain of coagulation factor VIII by hydrogen-deuterium exchange mass spectrometry

BACKGROUND

The development of anti-factor VIII antibodies (inhibitors) is a significant complication in the management of patients with hemophilia A, leading to significant increases in morbidity and treatment cost. Using a panel of mAbs against different epitopes on FVIII, we have recently shown that epitope specificity, inhibitor kinetics and time to maximum inhibition are more important than inhibitor titer in predicting responses to FVIII and the combination of FVIII and recombinant FVIIa. In particular, a subset of high-titer inhibitors responded to high-dose FVIII, which would not be predicted on the basis of their inhibitor titer alone. Thus, the ability to quickly map the epitope spectrum of patient plasma with a clinically feasible assay may fundamentally change how clinicians approach the treatment of high-titer inhibitor patients.

OBJECTIVES

To map the epitopes of anti-FVIII mAbs, three of which are classic inhibitors and one of which is a non-classic inhibitor, by the use of hydrogen-deuterium exchange coupled with mass spectrometry (HDX-MS).

METHODS

The binding epitopes of four mAbs targeting the FVIII C2 domain were mapped with HDX-MS.

RESULTS

The epitopes determined with HDX-MS are consistent with those obtained earlier through structural characterization and antibody competition assays. In addition, classic and non-classic inhibitor epitopes could be distinguished by the use of a limited subset of C2 domain-derived peptic fragments.

CONCLUSION

Our results demonstrate the effectiveness and robustness of the HDX-MS method for epitope mapping, and suggest a potential role of rapid mapping of FVIII inhibitor epitopes in facilitating individualized treatment of inhibitor patients.

Structure of the factor VIII C2 domain in a ternary complex with 2 inhibitor antibodies reveals classical and nonclassical epitopes

The factor VIII C2 domain is a highly immunogenic domain, whereby inhibitory antibodies develop following factor VIII replacement therapy for congenital hemophilia A patients. Inhibitory antibodies also arise spontaneously in cases of acquired hemophilia A. The structural basis for molecular recognition by 2 classes of anti-C2 inhibitory antibodies that bind to factor VIII simultaneously was investigated by x-ray crystallography. The C2 domain/3E6 FAB/G99 FAB ternary complex illustrates that each antibody recognizes epitopes on opposing faces of the factor VIII C2 domain. The 3E6 epitope forms direct contacts to the C2 domain at 2 loops consisting of Glu2181-Ala2188 and Thr2202-Arg2215, whereas the G99 epitope centers on Lys2227 and also makes direct contacts with loops Gln2222-Trp2229, Leu2261-Ser2263, His2269-Val2282, and Arg2307-Gln2311. Each binding interface is highly electrostatic, with positive charge present on both C2 epitopes and complementary negative charge on each antibody. A new model of membrane association is also presented, where the 3E6 epitope faces the negatively charged membrane surface and Arg2320 is poised at the center of the binding interface. These results illustrate the potential complexities of the polyclonal anti-factor VIII immune response and further define the "classical" and "nonclassical" types of antibody inhibitors against the factor VIII C2 domain.

Characterization and solution structure of the factor VIII C2 domain in a ternary complex with classical and non-classical inhibitor antibodies

The most significant complication for patients with severe cases of congenital or acquired hemophilia A is the development of inhibitor antibodies against coagulation factor VIII (fVIII). The C2 domain of fVIII is a significant antigenic target of anti-fVIII antibodies. Here, we have utilized small angle x-ray scattering (SAXS) and biochemical techniques to characterize interactions between two different classes of anti-C2 domain inhibitor antibodies and the isolated C2 domain. Multiple assays indicated that antibodies 3E6 and G99 bind independently to the fVIII C2 domain and can form a stable ternary complex. SAXS-derived numerical estimates of dimensional parameters for all studied complexes agree with the proportions of the constituent proteins. Ab initio modeling of the SAXS data results in a long kinked structure of the ternary complex, showing an angle centered at the C2 domain of ∼130°. Guided by biochemical data, rigid body modeling of subunits into the molecular envelope of the ternary complex suggests that antibody 3E6 recognizes a C2 domain epitope consisting of the Arg(2209)-Ser(2216) and Leu(2178)-Asp(2187) loops. In contrast, antibody G99 recognizes the C2 domain primarily through the Pro(2221)-Trp(2229) loop. These two epitopes are on opposing sides of the fVIII C2 domain, are consistent with the solvent accessibility in the context of the entire fVIII molecule, and provide further structural detail regarding the pathogenic immune response to fVIII.

Thermodynamic analysis of the interaction of factor VIII with von Willebrand factor

Factor VIII (FVIII) is a glycoprotein that plays an important role in the intrinsic pathway of coagulation. In circulation, FVIII is protected upon binding to von Willebrand factor (VWF), a chaperone molecule that regulates its half-life, distribution, and activity. Despite the biological significance of this interaction, its molecular mechanisms are not fully characterized. We determined the equilibrium and activation thermodynamics of the interaction between FVIII and VWF. The equilibrium affinity determined by surface plasmon resonance was temperature-dependent with a value of 0.8 nM at 35 °C. The FVIII-VWF interaction was characterized by very fast association (8.56 × 10(6) M(-1) s(-1)) and fast dissociation (6.89 × 10(-3) s(-1)) rates. Both the equilibrium association and association rate constants, but not the dissociation rate constant, were dependent on temperature. Binding of FVIII to VWF was characterized by favorable changes in the equilibrium and activation entropy (TΔS° = 89.4 kJ/mol, and -TΔS(++) = -8.9 kJ/mol) and unfavorable changes in the equilibrium and activation enthalpy (ΔH° = 39.1 kJ/mol, and ΔH(++) = 44.1 kJ/mol), yielding a negative change in the equilibrium Gibbs energy. Binding of FVIII to VWF in solid-phase assays demonstrated a high sensitivity to acidic pH and a sensitivity to ionic strength. Our data indicate that the interaction between FVIII and VWF is mediated mainly by electrostatic forces, and that it is not accompanied by entropic constraints, suggesting the absence of conformational adaptation but the presence of rigid "pre-optimized" binding surfaces.

Storage of factor VIII variants with impaired von Willebrand factor binding in Weibel-Palade bodies in endothelial cells

BACKGROUND

Point mutations resulting in reduced factor VIII (FVIII) binding to von Willebrand factor (VWF) are an important cause of mild/moderate hemophilia A. Treatment includes desmopressin infusion, which concomitantly increases VWF and FVIII plasma levels, apparently from storage pools containing both proteins. The source of these VWF/FVIII co-storage pools and the mechanism of granule biogenesis are not fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

We studied intracellular trafficking of FVIII variants implicated in mild/moderate hemophilia A together with VWF in HEK293 cells and primary endothelial cells. The role of VWF binding was addressed using FVIII variants displaying reduced VWF interaction. Binding studies using purified FVIII proteins revealed moderate (Arg2150His, Del2201, Pro2300Ser) to severe (Tyr1680Phe, Ser2119Tyr) VWF binding defects. Expression studies in HEK293 cells and primary endothelial cells revealed that all FVIII variants were present within VWF-containing organelles. Quantitative studies showed that the relative amount of FVIII storage was independent of various mutations. Substantial amounts of FVIII variants are co-stored in VWF-containing storage organelles, presumably by virtue of their ability to interact with VWF at low pH.

CONCLUSIONS

Our data suggest that the potential of FVIII co-storage with VWF is not affected in mild/moderate hemophilia A caused by reduced FVIII/VWF interaction in the circulation. These data support the hypothesis that Weibel-Palade bodies comprise the desmopressin-releasable FVIII storage pool in vivo.

Functional mapping of factor VIII C2 domain

The factor VIII (FVIII) is a cofactor of the coagulation cascade. The FVIII C2 domain is a critical domain that participates in the interactions with the von Willebrand factor and the phospholipidic surfaces. To assess the importance of each residue of this domain in the maintenance of the structure and the function of FVIII, a number (n=139) of mutants were generated by substituting the original residues, from Ser2173 to Gly2325, by an alanine. Mutants were built within a complete B domain-deleted FVIII and expressed in COS-1 cells. Mutant antigen levels and procoagulant activities were measured. Two in silico analyses, a sliding average procedure and an analysis of the mutation energy cost were conducted in parallel on the FVIII structure. Both results were in agreement with the functional data, and illustrated the benefit of using such strategies prior to targeting specific residues in the aim of generating active recombinant molecules. The functional assays identify the residues that are important to maintaining the structure of the C2 domain, mainly those forming β-sheet, and those that can afford substitution, establishing a detailed functional relation with the available crystallographic data. This study provided a comprehensive functional mapping of the FVIII C2 domain and discussed the implication of specific residues in respect to the maintenance in the activity and structure stability, the efficiency in secretion, the binding to phospholipids and the formation of epitope.

Membrane-binding properties of the Factor VIII C2 domain

Factor VIII functions as a cofactor for Factor IXa in a membrane-bound enzyme complex. Membrane binding accelerates the activity of the Factor VIIIa-Factor IXa complex approx. 100000-fold, and the major phospholipid-binding motif of Factor VIII is thought to be on the C2 domain. In the present study, we prepared an fVIII-C2 (Factor VIII C2 domain) construct from Escherichia coli, and confirmed its structural integrity through binding of three distinct monoclonal antibodies. Solution-phase assays, performed with flow cytometry and FRET (fluorescence resonance energy transfer), revealed that fVIII-C2 membrane affinity was approx. 40-fold lower than intact Factor VIII. In contrast with the similarly structured C2 domain of lactadherin, fVIII-C2 membrane binding was inhibited by physiological NaCl. fVIII-C2 binding was also not specific for phosphatidylserine over other negatively charged phospholipids, whereas a Factor VIII construct lacking the C2 domain retained phosphatidyl-L-serine specificity. fVIII-C2 slightly enhanced the cleavage of Factor X by Factor IXa, but did not compete with Factor VIII for membrane-binding sites or inhibit the Factor Xase complex. Our results indicate that the C2 domain in isolation does not recapitulate the characteristic membrane binding of Factor VIII, emphasizing that its role is co-operative with other domains of the intact Factor VIII molecule.

A human FVIII inhibitor modulates FVIII surface electrostatics at a VWF-binding site distant from its epitope

SUMMARY BACKGROUND

BO2C11 is a human monoclonal factor (F) VIII inhibitor. When bound to the C2 domain of FVIII, the Fab fragment of BO2C11 (Fab(BO2C11)) buries a surface of C2 that contains residues participating in a binding site for von Willebrand factor (VWF). BO2C11 has thus been proposed to neutralize FVIII by steric hindrance.

OBJECTIVES

The BO2C11 epitope on C2 overlaps with residues located at the periphery of the putative VWF binding site; hence, most of the residues that constitute the VWF binding site on C2 and a3 remain accessible for VWF interaction following BO2C11/FVIII complex formation. We thus investigated the contribution of alternative molecular mechanisms to FVIII inactivation by BO2C11.

METHODS

Continuum electrostatic calculations were applied to the crystal structure of C2, free or Fab(BO2C11)-complexed. In silico predictions were confirmed by site-directed mutagenesis and VWF-binding assays of the mutated FVIII.

RESULTS

Binding of Fab(BO2C11) to C2 induced perturbations in the electrostatic potential of C2 and in the local electrostatic parameters of 18 charged residues in C2, which are distant from the BO2C11 epitope. Nine of the predicted electrostatic hotspots clustered on the VWF-binding site of C2. Mutation of some of the predicted electrostatic hotspots has been associated with hemophilia A and reduced VWF binding in vitro.

CONCLUSIONS

Inhibitors may neutralize FVIII by alteration of protein surface electrostatics at a long distance from their epitope. Perturbation of the electrostatic environment of C2, either upon binding by anti-FVIII antibodies or consecutive to missense mutations in the F8 gene, may lead to hampered VWF binding and reduced FVIII residence time in circulation.

Identification of 31 novel mutations in the F8 gene in Spanish hemophilia A patients: structural analysis of 20 missense mutations suggests new intermolecular binding sites

Hemophilia A (HA) is an X-linked bleeding disorder caused by a wide variety of mutations in the factor 8 (F8) gene, leading to absent or deficient factor VIII (FVIII). We analyzed the F8 gene of 267 unrelated Spanish patients with HA. After excluding patients with the common intron-1 and intron-22 inversions and large deletions, we detected 137 individuals with small mutations, 31 of which had not been reported previously. Eleven of these were nonsense, frameshift, and splicing mutations, whereas 20 were missense changes. We assessed the impact of the 20 substitutions based on currently available information about FV and FVIII structure and function relationship, including previously reported results of replacements at these and topologically equivalent positions. Although most changes are likely to cause gross structural perturbations and concomitant cofactor instability, p.Ala375Ser is predicted to affect cofactor activation. Finally, 3 further mutations (p.Pro64Arg, p.Gly494Val, and p.Asp2267Gly) appear to affect cofactor interactions with its carrier protein, von Willebrand factor, with the scavenger receptor low-density lipoprotein receptor-related protein (LRP), and/or with the substrate of the FVIIIapi*FIXa (Xase) complex, factor X. Characterization of these novel mutations is important for adequate genetic counseling in HA families, but also contributes to a better understanding of FVIII structure-function relationship.

The tertiary structure and domain organization of coagulation factor VIII

Factor VIII (fVIII) is a serum protein in the coagulation cascade that nucleates the assembly of a membrane-bound protease complex on the surface of activated platelets at the site of a vascular injury. Hemophilia A is caused by a variety of mutations in the factor VIII gene and typically requires replacement therapy with purified protein. We have determined the structure of a fully active, recombinant form of factor VIII (r-fVIII), which consists of a heterodimer of peptides, respectively containing the A1-A2 and A3-C1-C2 domains. The structure permits unambiguous modeling of the relative orientations of the 5 domains of r-fVIII. Comparison of the structures of fVIII, fV, and ceruloplasmin indicates that the location of bound metal ions and of glycosylation, both of which are critical for domain stabilization and association, overlap at some positions but have diverged at others.

Study on the efficacy and safety of Xueyou Mixture in treating hemophilia

OBJECTIVE

To observe the effect of Xueyou Mixture (, XYM) on blood coagulation factors and its safety in treating hemophilia.

METHODS

To the randomly selected 65 inpatients of hemophilia, XYM was administered accompanied with intravenous dripping of liver cell growth factor 60-100 mg once a day to protect the liver, with no blood products like concentrated VIII and FIX factors or blood plasma given. The treatment lasted for 3 weeks. The short-term efficacy and adverse reactions were observed. The long-term efficacy in patients was observed in a follow-up study of 6-12 months after they were discharged from the hospital but continuously took XYM orally.

RESULTS

The short-term markedly effective rate in the patients was 95.38% (62/65). After they were treated for 3 weeks, the level of FVIII factor activity increased in 56 patients of type A from (3.32+/-2.21) % to (4.18+/-2.23) %, and in 9 of type B from (4.92+/-1.81) % to (5.64+/-1.96) %. Compared with that before treatment, the difference was significant in both of them (P<0.01). No obvious adverse reaction was found in the treatment period. The follow-up study showed that in 22 patients of type A, the FVIII factor activity ratio increased from (3.25+/-2.11) % to (6.31+/-2.16) %, (8.36+/-1.05) %, and (16.38+/-2.71) % in the 2nd, 3rd and 6th month after discharge respectively, all showing significant difference to that before treatment (P<0.01); and in 4 patients of type B, it increased from (4.15+/-2.26) % to 7.8% and 11.6% (mean value) in the 2nd and 6th month respectively.

CONCLUSION

XYM could raise the activity of factors VIII and IX in patients with hemophilia, and the degree of the rise is related with the duration of the therapy, with no obvious adverse reaction, which strikes out a new path and new train of thinking for the treatment of the disease by nonblood preparation.

The protein structure and effect of factor VIII

Factor VIII (FVIII) is a key component of the fluid phase of the blood coagulation system. The proteases efficiently cleave FVIII at three sites, two within the heavy and one within the light chain resulting in alteration of its covalent structure and conformation and yielding the active cofactor, FVIIIa. FVIIIa is a trimer composed of A1, A2 and A3-C1-C2 subunits. The role of FVIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. Variants of these factors frequently also lead to severe bleeding disorders.

The molecular analysis of haemophilia A: a guideline from the UK haemophilia centre doctors' organization haemophilia genetics laboratory network

Haemophilia A is a common inherited bleeding disorder that has a well-understood pathophysiology. Our understanding of the molecular genetics of the disease has allowed the development of comprehensive carrier and prenatal diagnosis for this single gene defect. Continuing technological developments improve our ability to provide genetic analysis in a rapid and cost effective manner. This guideline aims to provide advice on current best laboratory practice when approaching genetic diagnosis of haemophilia A.