Molecular cloning of a cDNA encoding human antihaemophilic factor

Factor VIII Structure and Function

Factor VIII, a non-covalent heterodimer comprised of a heavy chain (A1-A2-B domains) and light chain (A3-C1-C2 domains), circulates as an inactive procofactor in complex with von Willebrand factor. Metal ions are critical to the integrity of factor VIII, with Cu and Ca ions stabilizing the heterodimer and generating the active conformation, respectively. Activation of factor VIII catalyzed by thrombin appears dependent upon interactions with both anion-binding exosites I and II, and converts the heterodimer to the active cofactor, factor VIIIa. This protein, comprised of A1, A2, and A3-C1-C2 subunits, is labile due to weak affinity of the A2 subunit. Association of factor VIIIa with factor IXa to form the intrinsic factor Xase complex is membrane-dependent and involves multiple inter-protein contacts that remain poorly characterized. This complex catalyzes the conversion of factor X to factor Xa, a reaction that is essential for the propagation phase of coagulation. The role of factor VIIIa in this complex is to increase the catalytic efficiency for factor Xa generation by several orders of magnitude. Mechanisms for the down-regulation of factor Xase focus upon inactivation of the cofactor and include dissociation of the A2 subunit as well as activated protein C-catalyzed proteolysis.

Highly Conserved Antigenic Structure of the Factor VIII C2 Domain in Some Mammals

To elucidate differences in the antigenic structure of factor VIII (FVIII) among mammals, we evaluated cross-reactivities of well-defined antihuman FVIII antibodies with canine and other mammalian FVIII proteins. Monoclonal antibodies against human FVIII recognizing the A1 domain in the heavy chain and the A3 domain in the light chain showed 2.7% and 0% cross-reactivities, respectively, with canine FVIII. The cross-reactivities of 2 alloantibodies and a monoclonal antibody that recognized the A2 domain in the heavy chain were 10%, 38%, and 0%, respectively. On the other hand, 2 kinds of alloantibodies and a monoclonal antibody recognizing the C2 domain in the light chain showed 160%, 390%, and 130% cross-reactivity, respectively, with canine FVIII. The anti-C2 monoclonal antibody (NMC-VIII/5) showed a type 2 inactivating property when tested with canine and human plasma. Moreover, cross-reactivities of NMC-VIII/5 with simian and feline FVIII were 54.5% and 82.8%, respectively, while the cross-reactivities of the anti-A2 monoclonal antibody (JR8) with simian and feline FVIII were 1.3% and 0%, respectively. These findings suggest that the antigenic structure of the C2 epitope has remained relatively conserved throughout mammalian evolution in contrast to the A2 epitope and that a canine model of hemophilia A is useful for FVIII inhibitor experiments.

Lipid binding region (2303-2332) is involved in aggregation of recombinant human FVIII (rFVIII)

Factor VIII (FVIII) is a multi-domain protein that is important in the clotting cascade. Its deficiency causes Hemophilia A, a bleeding disorder. The unfolding of protein domains can lead to physical instability such as aggregation, and hinder their use in replacement therapy. It has been shown that the aggregation of rFVIIII is initiated by small fluctuations in the protein's tertiary structure (Grillo et al., 2001, Biochemistry 40:586-595). We have investigated the domain(s) involved in the initiation of aggregation using circular dichroism (CD), size exclusion chromatography (SEC), fluorescence anisotropy, domain specific antibody binding, and clotting activity studies. The studies indicated that aggregation may be initiated as a result of conformational change in the C2 domain encompassing the lipid-binding region (2303-2332). The presence of O-phospho-L-Serine (OPLS), which binds to the lipid-binding region of FVIII, prevented aggregation of the protein.

The promise and challenges of bioengineered recombinant clotting factors

The past 10 years of clinical experience have demonstrated the safety and efficacy of recombinant clotting factors. With the adoption of prophylactic strategies, there has been considerable progress in avoiding the complications of hemophilia. Now, insights from our understanding of clotting factor structure and function, mechanisms of hemophilia and inhibitors, gene therapy advances and a worldwide demand for clotting factor concentrates leave us on the brink of embracing targeted bioengineering strategies to further improve hemophilia therapeutics. The ability to bioengineer recombinant clotting factors with improved function holds promise to overcome some of the limitations in current treatment, the high costs of therapy and increase availability to a broader world hemophilia population. Most research has been directed at overcoming the inherent limitations of rFVIII expression and the inhibitor response. This includes techniques to improve rFVIII biosynthesis and secretion, functional activity, half-life and antigenicity/immunogenicity. Some of these proteins have already reached commercialization and have been utilized in gene therapy strategies, while others are being evaluated in pre-clinical studies. These novel proteins partnered with advances in gene transfer vector design and delivery may ultimately achieve persistent expression of FVIII leading to an effective long-term treatment strategy for hemophilia A. In addition, these novel FVIII proteins could be partnered with new advances in alternative recombinant protein production in transgenic animals yielding an affordable, more abundant supply of rFVIII. Novel rFIX proteins are being considered for gene therapy strategies whereas novel rVIIa proteins are being evaluated to improve the potency and extend their plasma half-life. This review will summarize the status of current recombinant clotting factors and the development and challenges of recombinant clotting factors bioengineered for improved function.

Seminal Factor VIII and von Willebrand Factor: a possible role of the conventional clotting system in human semen?

Factor (F) VIII circulates in blood complexed with von Willebrand Factor (vWF). Deficiency or defect accounts for haemophilia A and vWF disease. In blood, FVIII functions as a co-factor for FIXa in the activation of FX. Human semen coagulates and liquefies in a process that resembles and has some links with the conventional haemostatic process. A study elsewhere has detected traces, but not measurable levels, of FVIII coagulant activity (FVIII:C). In the present study we have assessed FVIII antigen (FVIII:Ag), FVIII:C and vWF antigen (vWF:Ag) levels in 159 semen specimens obtained from sub-fertile (n = 21), normally fertile (n = 38), fertile donors (n = 32), and vasectomized men (n = 57). Seminal FVIII:Ag levels were also measured in a group defined by several parameters derived from the World Health Organization (WHO) fertility criteria, termed "pooled normal semen parameters" (PNSP). Factor VIII:Ag levels were compared with conventional fertility parameters. In addition, both FVIII:C and vWF:Ag were assessed in a separate group of normal individuals (n = 11). Factor VIII:Ag, FVIII:C and vWF were present and quantifiable in human semen. Factor VIII:Ag levels were significantly lower in vasectomy subjects compared with donors (p = 0.01) or PNSP group (p = 0.01). Several trends taken together suggest an associations between FVIII:Ag and semen quality. Parallel investigations demonstrate FV, FVII, FVIIa, FIX, FIXa, FXa, FXI, FXII, tissue factor (TF) and tissue factor pathway inhibitor (TFPI) in semen. The present report therefore provides further evidence for the presence of a functioning clotting system in human semen.

Lower inhibitor development in hemophilia A mice following administration of recombinant factor VIII-O-phospho-L-serine complex

Factor VIII is a multidomain protein composed of A1, A2, B, A3, C1, and C2 domains. Deficiency or dysfunction of factor VIII causes hemophilia A, a bleeding disorder. Administration of exogenous recombinant factor VIII as a replacement leads to development of inhibitory antibodies against factor VIII in 15-30% of hemophilia A patients. Hence, less immunogenic preparations of factor VIII are highly desirable. Inhibitory antibodies against factor VIII are mainly directed against immunodominant epitopes in C2, A3, and A2 domains. Further, several universal epitopes for CD4+ T-cells have been identified within the C2 domain. The C2 domain is also known to interact specifically with phosphatidylserine-rich lipid vesicles. Here, we have investigated the hypothesis that complexation of O-phospho-l-serine, the head group of phosphatidylserine, with the C2 domain can reduce the overall immunogenicity of factor VIII. The biophysical (circular dichroism and fluorescence) and biochemical studies (ELISA and size exclusion chromatography) showed that O-phospho-l-serine binds to the phospholipid-binding region in the C2 domain, and this interaction causes subtle changes in the tertiary structure of the protein. O-Phospho-l-serine also prevented aggregation of the protein under thermal stress. The immunogenicity of the factor VIII-O-phospho-l-serine complex was evaluated in hemophilia A mice. The total and inhibitory antibody titers were lower for factor VIII-O-phospho-l-serine complex compared with factor VIII alone. Moreover, factor VIII administered as a complex with O-phospho-l-serine retained in vivo activity in hemophilia A mice. Our results suggest that factor VIII-O-phospho-l-serine complex may be beneficial to increase the physical stability and reduce immunogenicity of recombinant factor VIII preparations.

Mutating factor VIII: lessons from structure to function

Factor VIII, a metal ion-dependent heterodimer, circulates in complex with von Willebrand factor. At sites of vessel wall damage, this procofactor is activated to factor VIIIa by limited proteolysis and assembles onto an anionic phospholipid surface in complex with factor IXa to form the intrinsic factor Xase; an enzyme complex that efficiently converts factor X to factor Xa during the propagation phase of coagulation. Factor Xase activity is down-regulated by mechanisms that include self-dampening by dissociation of a critical factor VIIIa subunit and proteolytic inactivation by the activated protein C pathway. Recent studies identify putative metal ion coordination sites as well as ligands involved in the catabolism of the activated and procofactor forms of the protein. Our knowledge of these multiple intra- and inter-molecular interactions has been facilitated by the application of naturally occurring and site-directed mutations to study factor VIII structure and function. In this review, we document important and novel contributions following this line of investigation.

Recent advances in the management of the child who has hemophilia

This article discusses recent advances in the management of the child who has hemophilia.

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

Factor VIII (fVIII) is a plasma glycoprotein that functions as an essential cofactor in blood coagulation. Its carboxyl-terminal "C2" domain is responsible for binding to both activated platelet surfaces and von Willebrand factor. We characterized the effect of 20 hemophilia-associated missense mutations across this domain (that all occur in patients in vivo) on its stability and its binding activities. At least six of these mutations were severely destabilizing, and another four caused moderate destabilization and corresponding reductions in both binding functions. One mutant (A2201P) displayed a significant reduction in its membrane binding activity but normal von Willebrand factor binding, while two others (P2300S and R2304H) caused the opposite effect. Several mutations (including L2210P, V2223M, M2238V, and R2304C) displayed near wild-type stabilities and binding activities and may instead affect mRNA splicing or alternative properties or functions of the protein. This study demonstrated that von Willebrand factor and membrane binding activities can be uncoupled and uniquely disrupted by different mutations and that either effect can lead to similar reductions in clotting activity. It also illustrated how a heterogeneous genetic disorder causes diverse molecular phenotypes that result in similar disease states.

Disruption of Protein-Membrane Binding and Identification of Small-Molecule Inhibitors of Coagulation Factor VIII

Factor VIII is a critical member of the blood coagulation cascade. It binds to the membrane surfaces of activated platelets at the site of vascular injury via a highly specific interaction between factor VIII's carboxy-terminal C2 domain and their phosphatidylserine-rich lipid bilayer. We have identified small-molecule inhibitors of factor VIII's membrane binding activity that have IC50 values as low as 2.5 microM. This interaction is approximately 10(3)-fold tighter than that of free o-phospho-L-serine. These compounds also inhibit factor VIII-dependent activation of factor X, indicating that disruption of membrane lipid binding leads to inhibition of the intrinsic coagulation pathway. The tightest binding inhibitor is specific and does not prevent membrane binding by the closely related coagulation factor V. These results indicate that this and related compounds may be used as leads to develop novel antithrombotic agents.

Molecular basis of haemophilia A

Technologies in molecular biology have greatly advanced the knowledge regarding the origin of haemophilia A and the physiology of the factor VIII (FVIII) protein. A variety of different mutations in the FVIII gene have been identified and their effects on the FVIII protein described. It has been shown that the frequency of haemophilia A is due to a high mutation rate predominantly in male germ cells. A significant proportion is originating de novo in early embryogenesis from somatic mutations, a finding that has implications for genetic counselling. The life-cycle of the FVIII protein and its structure-function relationships are continuously clarified. Most recently it has been shown that FVIII clearance from the circulation is mediated by the low-density lipoprotein receptor-related protein (LRP) and cell-surface heparan sulphate proteoglycans (HSPGs). These findings raise hope for novel recombinant FVIII molecules with prolonged half-life that may improve therapies for haemophlia A.

Activation of factor VIII and mechanisms of cofactor action

The factor VIII procofactor circulates as a metal ion-dependent heterodimer of a heavy chain and light chain. Activation of factor VIII results from limited proteolysis catalyzed by thrombin or factor Xa, which binds the factor VIII substrate over extended interactive surfaces. The proteases efficiently cleave factor VIII 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, factor VIIIa. The role of factor VIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. This effect is manifested in a dramatic increase in the catalytic rate constant, k(cat), by mechanisms that remain poorly understood.

Localization of a pH-dependent, A2 subunit-interactive surface within the factor VIIIa A1 subunit

Factor VIIIa can be reconstituted from A2 subunit and A1/A3-C1-C2 dimer in a reaction that is facilitated by slightly acidic pH. We recently demonstrated that a truncated A1 (A1(37-336)) possessed markedly reduced affinity for A2 compared with intact A1, but retained 30% of native factor VIIIa activity in the presence of A3-C1-C2. We now identify A1-interactive regions for A2 using A1 fragments derived from a limited tryptic digest. Unfractionated trypsin-cleaved A1 inhibited reconstituted factor VIIIa activity. Two fragments, designated A1(37-121) and A1(221-336), markedly inhibited factor VIIIa reconstitution with either native A1 (K(i)=340 and 194 nM, respectively) or with A1(37-336) (K(i)=69 and 116 nM, respectively) at pH 6.0. A third fragment designated A1(122-206) did not possess inhibitory activity. At pH 7.2, the A1(221-336) partially inhibited reconstitution, whereas the A1(37-121) possessed little if any inhibitory activity. Both fragments inhibited factor VIIIa reconstitution as judged by fluorescence energy transfer using acrylodan-labeled A2 and fluorescein-labeled A1 forms at pH 6.0. Furthermore, covalent cross-linking between A2 and A1(37-121) but not A1(221-336) was observed following reaction with a zero-length cross-linker. These findings demonstrate the presence of an extended, pH-dependent A2-interactive surface within regions 37-121 and 221-336 of A1. This interactive surface appears conformationally labile in the truncated A1 as judged by its apparent stabilization following association with A3-C1-C2.

Relationships between factor VIII:Ag and factor VIII in recombinant and plasma-derived factor VIII concentrates

A variety of plasma-derived (pd) and recombinant (r) factor VIII (FVIII) concentrates are used to prevent and treat bleeding in severe hemophilia A patients. A significant side effect of FVIII replacement is the development of FVIII neutralizing antibodies (inhibitors) in up to 30% of patients receiving FVIII concentrates. The FVIII protein content (FVIII:Ag) per unit of FVIII:C in FVIII concentrates, and how effectively the FVIII:Ag in FVIII concentrates binds to von Willebrand factor (VWF) may provide information relevant for the survival of FVIII:C in vivo and for estimating the risk for inhibitor development. The FVIII:Ag content of nine r-FVIII and nine pd-FVIII concentrates were quantified in this study using two enzyme-linked immunosorbent assay (ELISA) platforms. The two ELISA platforms were based on the use of a monoclonal anti-(FVIII light chain)-IgG and polyclonal anti-FVIII antibodies as capture antibodies and both ELISAs were equally able to detect > or =0.005 IU of FVIII:Ag. Measured in international units, the r-FVIII concentrates contained significantly higher FVIII:Ag per unit of FVIII:C than the pd-FVIII concentrates. The VWF-binding profiles of the r-FVIII and pd-FVIII concentrates were also determined by gel filtration chromatography. Unlike the plasma-derived products, the r-FVIII concentrates invariably contained a fraction of FVIII:Ag molecules (approximately 20%) which was unable to associate with VWF. Given that VWF regulates both factor VIII proteolysis and survival of FVIII:Ag in vivo, the fraction of FVIII:Ag unable to bind to VWF may have a reduced survival and be more susceptible to proteolytic degradation in vivo. The extent to which the fractions of FVIII:Ag in concentrates able and unable to bind to VWF contribute to inhibitor development in severe FVIII-deficient patients is unknown.

Home management of haemophilia

The demonstrated benefits of home care for haemophilia include improved quality of life, less pain and disability, fewer hospitalizations, and less time lost from work or school. Although reduced mortality has not been demonstrated, the substantial increase in longevity since the early 1980s correlates with the introduction of home treatment and prophylaxis programmes. These programmes must be designed and monitored by haemophilia treatment centres (HTC), which are staffed with professionals with broad and complementary expertise in the disease and its complications. In return, patients and their families must be willing to accept the reciprocal responsibilities that come from administering blood products or their recombinant equivalents at home. Patients with inhibitors to factors VIII or IX pose special challenges, but these complications do not obviate participation in home care programmes. Home care was an essential prerequisite to the introduction of effective prophylactic factor replacement therapy. Prophylaxis offers significant improvements in quality of life, but requires a substantial commitment. The use of implantable venous access devices can eliminate some of the difficulty and discomfort of peripheral venous access in small children, but brings additional risks. The future holds the promise of factor concentrates for home use that have longer half-lives, or can be administered by alternate routes. Knowledge of patient genotypes may allow treatments tailored to avoid complications such as inhibitor development. Gene therapy trials, which are currently ongoing, will ultimately lead to gene-based treatments as a complement to traditional protein-based therapy.

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.

Identification of Porcine Coagulation Factor VIII Domains Responsible for High Level Expression via Enhanced Secretion

Blood coagulation factor VIII has a domain structure designated A1-A2-B-ap-A3-C1-C2. Human factor VIII is present at low concentration in normal plasma and, comparably, is produced at low levels in vitro and in vivo using transgenic expression techniques. Heterologous expression of B domain-deleted porcine factor VIII in mammalian cell culture is significantly greater than B domain-deleted human or murine factor VIII. Novel hybrid human/porcine factor VIII molecules were constructed to identify porcine factor VIII domains that confer high level expression. Hybrid human/porcine factor VIII constructs containing the porcine factor VIII A1 and ap-A3 domains expressed at levels comparable with recombinant porcine factor VIII. A hybrid construct containing only the porcine A1 domain expressed at intermediate levels between human and porcine factor VIII, whereas a hybrid construct containing the porcine ap-A3 domain expressed at levels comparable with human factor VIII. Additionally, hybrid murine/porcine factor VIII constructs containing the porcine factor VIII A1 and ap-A3 domain sequences expressed at levels significantly higher than recombinant murine factor VIII. Therefore, the porcine A1 and ap-A3 domains are necessary and sufficient for the high level expression associated with porcine factor VIII. Metabolic radiolabeling experiments demonstrated that high level expression was attributable to enhanced secretory efficiency.

Bioengineering of coagulation factor VIII for improved secretion

Factor VIII (FVIII) functions as a cofactor within the intrinsic pathway of blood coagulation. Quantitative or qualitative deficiencies of FVIII result in the inherited bleeding disorder hemophilia A. Expression of FVIII (domain structure A1-A2-B-A3-C1-C2) in heterologous mammalian systems is 2 to 3 orders of magnitude less efficient compared with other proteins of similar size compromising recombinant FVIII production and gene therapy strategies. FVIII expression is limited by unstable mRNA, interaction with endoplasmic reticulum (ER) chaperones, and a requirement for facilitated ER to Golgi transport through interaction with the mannose-binding lectin LMAN1. Bioengineering strategies can overcome each of these limitations. B-domain-deleted (BDD)-FVIII yields higher mRNA levels, and targeted point mutations within the A1 domain reduce interaction with the ER chaperone immunoglobulin-binding protein. In order to increase ER to Golgi transport we engineered several asparagine-linked oligosaccharides within a short B-domain spacer within BDD-FVIII. A bioengineered FVIII incorporating all of these elements was secreted 15- to 25-fold more efficiently than full-length FVIII both in vitro and in vivo. FVIII bioengineered for improved secretion will significantly increase potential for success in gene therapy strategies for hemophilia A as well as improve recombinant FVIII production in cell culture manufacturing or transgenic animals.

Residues 110-126 in the A1 domain of factor VIII contain a Ca2+ binding site required for cofactor activity

Generation of factor VIII cofactor activity requires divalent metal ions such as Ca2+ or Mn2+. Evaluation of cofactor reconstitution from isolated factor VIIIa subunits revealed the presence of a functional Ca2+ binding site within the A1 subunit. Isothermal titration calorimetry demonstrated at least two Ca2+ binding sites of similar affinity (K(d) = 0.74 microm) within the A1 subunit. Mutagenesis of an acidic residue-rich region in the A1 domain (residues 110-126) homologous to a putative Ca2+ binding site in factor V (Zeibdawi, A. R., and Pryzdial, E. L. (2001) J. Biol. Chem. 276, 19929-19936) and expression of B-domainless factor VIII molecules yielded reagents to probe Ca2+ and Mn2+ binding in a functional assay. Basal activity observed for wild type factor VIII in a metal ion-free buffer was enhanced approximately 2-fold with saturating Ca2+ or Mn2+ and yielded functional K(d) values of 1.2 and 1.40 microm, respectively. Ca2+ binding affinity was greatly reduced (or lost) in several mutants including E110A, E110D, D116A, E122A, D125A, and D126A. Alternatively, E113A, D115A, and E124A showed wild type-like activity with little or no reduction in Ca2+ affinity. However, Mn2+ affinity was minimally altered except for mutant D125A (and D116A). These results are consistent with region 110-126 serving a critical role for Ca2+ coordination with selected residues capable of contributing to a partially overlapping site for Mn2+, and that occupancy of either site is required for maximal cofactor activity.

Factor VIII ectopically expressed in platelets: efficacy in hemophilia A treatment

Activated platelets release their granule content in a concentrated fashion at sites of injury. We examined whether ectopically expressed factor VIII in developing megakaryocytes would be stored in alpha-granules and whether its release from circulating platelets would effectively ameliorate bleeding in a factor VIIInull mice model. Using the proximal glycoprotein 1b alpha promoter to drive expression of a human factor VIII cDNA construct, transgenic lines were established. One line had detectable human factor VIII that colocalizes with von Willebrand factor in platelets. These animals had platelet factor VIII levels equivalent to 3% to 9% plasma levels, although there was no concurrent plasma human factor VIII detectable. When crossed onto a factor VIIInull background, whole blood clotting time was partially corrected, equivalent to a 3% correction level. In a cuticular bleeding time study, these animals also had only a partial correction, but in an FeCl3 carotid artery, thrombosis assay correction was equivalent to a 50% to 100% level. These studies show that factor VIII can be expressed and stored in platelet alpha-granules. Our studies also suggest that platelet-released factor VIII is at least as potent as an equivalent plasma level and perhaps even more potent in an arterial thrombosis model.

Topology of factor VIII bound to phosphatidylserine-containing model membranes

Factor VIII (FVIII), a plasma glycoprotein, is an essential cofactor in the blood coagulation cascade. It is a multidomain protein, known to bind to phosphatidylserine (PS)-containing membranes. Based on X-ray and electron crystallography data, binding of FVIII to PS-containing membranes has been proposed to occur only via the C2 domain. Based on these models, the molecular topology of membrane-bound FVIII can be envisioned as one in which only a small fraction of the protein interacts with the membrane, whereas the majority of the molecule is exposed to an aqueous milieu. We have investigated the topology of the membrane-bound FVIII using biophysical and biochemical techniques. Circular dichroism (CD) and fluorescence studies indicate no significant changes in the secondary and tertiary structure of FVIII associated with the membranes. Acrylamide quenching studies show that the protein is predominantly present on the surface of the membrane, exposed to the aqueous milieu. The light scattering and electron microscopy studies indicate the absence of vesicle aggregation and fusion. Binding studies with antibodies directed against specific epitopes in the A1, A2 and C2 domains suggest that FVIII binds to the membrane primarily via C2 domain including the specific phospholipid binding epitope (2303-2332) and may involve subtle conformational changes in this epitope region.

Factor VIII determination in patient's plasma and concentrates: a novel test equally suited for both matrices

A novel assay for the determination of factor VIII (FVIII) is described. The assay uses a fluorescence-based detection system comparable with the common chromogenic test. At the same time, the assay is analogous to the clotting test as it does not involve pre-activation of FVIII. The assay was adjusted to perform equally well with patient's plasma and FVIII concentrates as samples. The combined employment of two different FVIII-deficient plasmas turned out to be of crucial importance in order to render the matrices as similar as possible to patient's plasma and, simultaneously, to obtain maximum sensitivity. Samples with a FVIII content down to 0.01 IU/ml are readily measured as are samples with a FVIII content of 1 IU/ml or somewhere in between. Upon dilution of samples, concentrates and plasma exhibited the same dose-response characteristics.

The future of recombinant coagulation factors

Hemophilias A and B are X chromosome-linked bleeding disorders, which are mainly treated by repeated infusions of factor (F)VIII or FIX, respectively. In the present review, we specify the limitations in expression of recombinant (r)FVIII and summarize the bioengineering strategies that are currently being explored for constructing novel rFVIII molecules characterized by high efficiency expression and improved functional properties. We present the strategy to prolong FVIII lifetime by disrupting FVIII interaction with its clearance receptors and demonstrate how construction of human-porcine FVIII hybrid molecules can reduce their reactivity towards inhibitory antibodies. While the progress in improving rFIX is impeded by low recovery rates, the authors are optimistic that the efforts of basic science may ultimately lead to higher efficiency of replacement therapy of both hemophilias A and B.

Polymorphisms in coagulation factor genes and their impact on arterial and venous thrombosis

Arterial and venous thromboses, with their clinical manifestations such as stroke, myocardial infarction (MI), or pulmonary embolism, are the major causes of death in developed countries. Several studies in twins and siblings have shown that genetic factors contribute significantly to the development of these diseases. Since the advent of molecular genetics in medicine, it has been a focus of interest to elucidate the role of mutations in various candidate genes and their impact on hemostatic disorders such as arterial and venous thromboses. In this article, we review the current knowledge of the contribution of polymorphisms in coagulation factors to the development of thrombotic diseases. We show that in arterial thrombosis, results are controversial. Only for factor XIII 34Leu a protective effect on the development of myocardial infarction has been demonstrated in several studies. No other single polymorphism in a coagulation factor could be confirmed as a relevant risk factor, although there is evidence for a role of factor V Arg506Gln, factor VII Arg353Gln, and vWF Thr789Ala polymorphisms in patient subgroups. Further studies will be necessary to confirm the value of testing for genetic polymorphisms in arterial thrombosis. A large body of data is available on the role of factor V Arg506Gln and the prothrombin G20210A mutation in venous thrombosis. Some papers already recommend diagnosis and treatment strategies. We will discuss these recent publications on venous thrombosis in our review.

Altered interactions between the A1 and A2 subunits of factor VIIIa following cleavage of A1 subunit by factor Xa

Factor VIIIa consists of subunits designated A1, A2, and A3-C1-C2. The limited cofactor activity observed with the isolated A2 subunit is markedly enhanced by the A1 subunit. A truncated A1 (A1(336)) was previously shown to possess similar affinity for A2 and retain approximately 60% of its A2 stimulatory activity. We now identify a second site in A1 at Lys(36) that is cleaved by factor Xa. A1 truncated at both cleavage sites (A1(37-336)) showed little if any affinity for A2 (K(d)>2 microm), whereas factor VIIIa reconstituted with A2 plus A1(37-336)/A3-C1-C2 dimer demonstrated significant cofactor activity ( approximately 30% that of factor VIIIa reconstituted with native A1) in a factor Xa generation assay. These affinity values were consistent with values obtained by fluorescence energy transfer using acrylodan-labeled A2 and fluorescein-labeled A1. In contrast, factor VIIIa reconstituted with A1(37-336) showed little activity in a one-stage clotting assay. This resulted in part from a 5-fold increase in K(m) for factor X when A1 was cleaved at Arg(336). These findings suggest that both A1 termini are necessary for functional interaction of A1 with A2. Furthermore, the C terminus of A1 contributes to the K(m) for factor X binding to factor Xase, and this parameter is critical for activity assessed in plasma-based assays.

Mn2+ binding to factor VIII subunits and its effect on cofactor activity

Metal ions, such as Ca2+ and Mn2+, are necessary for the generation of cofactor activity following reconstitution of factor VIII from its isolated light chain (LC) and heavy chain (HC). Titration of EDTA-treated factor VIII with Mn2+ showed saturable binding with high affinity (K(d) = 5.7 +/- 2.1 microM) as detected using a factor Xa generation assay. No significant competition between Ca2+ and Mn2+ for factor VIII binding (K(i) = 4.6 mM) was observed as measured by equilibrium dialysis using 20 microM Ca2+ and 8 microM factor VIII in the presence of 0-1 mM Mn2+. The intersubunit affinity measured by fluorescence energy transfer of an acrylodan-labeled LC (fluorescence donor) and fluorescein-labeled HC (fluorescence acceptor) in the presence of 20 mM Mn2+ (K(d) = 53.0 +/- 17.1 nM) was not significantly different from the affinity value previously obtained in the absence of metal ion (K(d) = 53.8 +/- 14.2 nM). The sensitization of phosphorescence of Tb3+ bound to factor VIII subunits was utilized to detect Mn2+ binding to the subunits. Mn2+ inhibited the phosphorescence of Tb3+ bound to HC and LC, as well as the HC-derived A1 and A2 subunits with a relatively wide range of estimated inhibition constant values (K(i) values = 169-1147 microM), whereas Ca2+ showed no effect on Tb3+ phosphorescence. These results suggest that factor VIII cofactor activity can be generated by Mn2+ binding to site(s) on factor VIII that are different from the high-affinity Ca2+ binding site. However, like Ca2+, Mn2+ did not alter the affinity for HC and LC association. Thus, Mn2+appears to generate factor VIII cofactor activity by a similar mechanism as observed for Ca2+following its association at nonidentical sites on the protein.

Treatment strategies in children with hemophilia

Hemophilia is an inherited bleeding disorder caused by quantitative or qualitative defects in the synthesis of factor VIII (FVIII) or factor IX (FIX). Clinically, it is divided into severe, moderate and mild disease depending on the levels of FVIII or FIX in the blood. The bleeding tendency is most pronounced and can start at a very young age in severe hemophilia, which is characterized by repeated hemorrhage into the joints and muscles. Without treatment, these episodes lead to severe arthropathy, and there is also a high risk of lethal cerebral hemorrhage. The treatment of bleeding symptoms requires the correction of the coagulation defect. Factor concentrates have been available for 30 years, initially with the development of cryoprecipitate, subsequently with increasingly purified plasma-derived forms, and ultimately with recombinant clotting factor concentrates. The advantage of this highly effective therapy has been subdued by the outbreak of HIV and Hepatitis C infections in patients with hemophilia treated with factor concentrates which did not have adequate viral inactivation steps in the purification process. Plasma-derived and recombinant factor concentrates are today considered to have a good safety profile, but are only available for a small group of hemophilia patients worldwide. A multidisciplinary team approach is important for early diagnosis, communication with the patient and parents, and to tailor the best treatment possible with the amount of clotting factor concentrates available. The main goal of hemophilia treatment is to prevent bleeding symptoms and allow normal integration in social life. In patients with severe hemophilia, this can best be achieved by early home treatment and primary prophylaxis. Future developments in gene therapy may transform severe hemophilia to a mild form, with no need for regular injections of clotting factor concentrates.

The safety and efficacy of B-domain deleted recombinant factor VIII concentrate in patients with severe haemophilia A

BACKGROUND

B-domain-deleted recombinant factor VIII (BDDrFVIII) was developed when the B-domain was found to be redundant for maintaining haemostasis. This allows formulation of the final product without albumin added as a stabilizer.

METHODS

Three multicentre clinical studies and one pharmacokinetic study were conducted in 218 patients to evaluate the safety and haemostatic efficacy of BDDrFVIII.

RESULTS

Previously treated patients (n = 113; median duration, 1711 days; median exposure days, 385; total 98,096,287 IU infused) rated 97-99% of all infusions as good or excellent efficacy. FVIII inhibitor was noted in one patient in the previously treated patient cohort after 113 exposure days. Among 101 previously untreated patients, responses to BDDrFVIII were rated as excellent or good in 92-95% of infusions (median duration, 1413 days; median exposure days, 148; total 12,636,458 IU infused). Thirty-two previously untreated patients developed inhibitors after a median duration of 12 exposure days (range, 3-49). Sixteen of 32 (50%) patients had low levels (< or = 5 Bethesda units) and 16 had high levels of inhibitors. Inhibitors disappeared in six of 14 (43%) of the high-level and six of eight (75%) of the low-level patients who underwent immune tolerance induction therapy. A total of 42 patients underwent surgery and the overall efficacy of BDDrFVIII was rated as excellent or good for 99.6% of infusions.

CONCLUSIONS

The results of these clinical studies indicate that BBDrFVIII is safe and effective and has haemostatic activity similar to that of full-length FVIII concentrates.

The assembly of the factor X-activating complex on activated human platelets

Platelet membranes provide procoagulant surfaces for the assembly and expression of the factor X-activating complex and promote the proteolytic activation and assembly of the prothrombinase complex resulting in normal hemostasis. Recent studies from our laboratory and others indicate that platelets possess specific, high-affinity, saturable, receptors for factors XI, XIa, IX, IXa, X, VIII, VIIIa, V, Va and Xa, prothrombin, and thrombin. Studies described in this review support the hypothesis that the factor X-activating complex on the platelet surface consists of three receptors (for the enzyme, factor IXa; the substrate, factor X; and the cofactor, factor VIIIa), the colocalization of which results in a 24 million-fold acceleration of the rate of factor X activation. Whether the procoagulant surface of platelets is defined exclusively by procoagulant phospholipids, or whether specific protein receptors exist for the coagulant factors and proteases, is currently unresolved. The interaction between coagulation proteins and platelets is critical to the maintenance of normal hemostasis and is pathogenetically important in human disease.

History of pediatric hematology oncology

Pediatric Hematology Oncology as a specialty was possible because of the evolution of the science of Hematology, which developed microscopy for describing blood cell morphology and methods for quantitation of these elements. Before pediatric blood diseases could be defined, it was necessary to establish the normal blood values of infancy and childhood. The unique features of the blood of the newborn were the focus of many of the early studies. After normal values were established, specific blood disease and hematologic syndromes of children began to be described in Europe and the United States. Pediatric Hematology Oncology is a broad and complex area that encompasses perturbations of the several-formed elements of the blood and their precursors in the bone marrow, as well as the coagulation-fibrinolytic systems in the plasma, the reticuloendothelial system, and malignancies of the blood and solid tissues and organs. The interactions of the blood and nutrition have long been important areas of study. Advances in Pediatric Oncology have been particularly spectacular in the last 50 years. Using multi-modal therapy including combination chemotherapy, more than 80% of children with cancer can now be cured. During the last 50 years, Pediatric Hematology Oncology has increasingly used tools of the "new biology": immunology, biochemistry, enzymology, genetics and molecular genetics, and others. During the last century, many diseases have been recognized and defined by biochemical and genetic mechanisms, and in some instances they have been prevented or cured.

Gene therapy for bleeding disorders

The goal of gene therapy for bleeding disorders is to provide stable insertion and expression of a particular gene whose absence is responsible for a particular disease. The bleeding disorders for which the most basic and clinical research has been completed are the hemophilias factor VIII deficiency and factor IX deficiency. These X-linked diseases are caused by single-gene mutations; replacement of the defective gene has not only resulted in clinical and biochemical improvement in animal models but also provided promising results in phase I clinical trials. An ideal gene transfer approach to the treatment of hemophilia would require a minimally invasive procedure for gene delivery, have minimal associated morbidity, and result in long-term transgene expression, ideally yielding factor levels in the therapeutic range. Multiple approaches to gene transfer in the hemophilias are currently under investigation.

Structural requirements for the activation of human factor VIII by thrombin

The coagulation factors V (FV) and VIII (FVIII) are important at sites of vascular injury for the amplification of the clotting cascade. Natural variants of these factors frequently lead to severe bleeding disorders. To understand the mechanisms of activation of FVIII by thrombin, we used a bank of mutant thrombins to define residues important for its activation. From the initial screening of 53 mutant thrombins for the activation of human recombinant FVIII, we mapped thrombin mutants with 50% or less activity to anion-binding exosite-I (Lys21Ala, His66Ala, Lys65Ala, Arg68Ala, Arg70Ala, and Tyr71Ala) and anion-binding exosite-II (Arg98Ala), the Na(+)-binding site (Glu229Ala, Arg233Ala, Asp234Ala, and Asp193Ala/Lys196Ala), and the 50-insertion loop (Trp50Ala), which were similar to our results for the activation of FV. The role of these residues for cleavage at Arg372 and Arg1689 was investigated using plasma FVIII. Anion-binding exosite-I appears to be important for cleavage at both sites, whereas the anion-binding exosite-II residue Arg98Ala is important for cleavage at Arg372 alone. The Glu229Ala mutant, which contributes to the Na(+)-binding site, and the 50-insertion loop mutant W50A have severely impaired cleavage at Arg372 and Arg1689. This suggests that the integrity of the active site and the Na(+)-bound form of thrombin are important for its procoagulant activity against FVIII. Detailed mutagenic analysis of thrombin can assist in understanding the pathogenesis of bleeding disorders and may lead to the rational design of selective thrombin inhibitors.

High level expression of recombinant porcine coagulation factor VIII

Recombinant human factor VIII expression levels, in vitro and in vivo, are significantly lower than levels obtained for other recombinant coagulation proteins. Here we describe the generation, high level expression and characterization of a recombinant B-domain-deleted porcine factor VIII molecule. Recombinant B-domain-deleted porcine factor VIII expression levels are 10- to 14-fold greater than recombinant B-domain-deleted human factor VIII levels by transient and stable expression in multiple cell lines. Peak expression of 140 units x 10(6) cells(-1) x 24 h(-1) was observed from a baby hamster kidney-derived cell line stably expressing recombinant porcine factor VIII. Factor VIII expression was performed in serum-free culture medium and in the absence of exogenous von Willebrand factor, thus greatly simplifying protein purification. Real time reverse transcription-PCR analysis demonstrated that the differences in protein production were not caused by differences in steady-state factor VIII mRNA levels. The identification of sequence(s) in porcine factor VIII responsible for high level expression may lead to a better understanding of the mechanisms that limit factor VIII expression.

Generation and characterization of human hematopoietic cell lines expressing factor VIII

Considering the plasticity of hematopoietic stem cells (HSC), they would be ideal targets for gene therapy of hemophilia A by virtue of their progeny providing immediate access to the bloodstream. However, several attempts to show expression of recombinant factor VIII (rFVIII) by primary hematopoietic cells and cell lines have failed; this failure was attributed to the inability of HSC to secrete rFVIII. Here we describe the generation of stable, FVIII-secreting hematopoietic cell lines representing different blood-cell types using a bicistronic lentiviral vector encoding for a B-domain-deleted FVIII (FVIII Delta B) and enhanced green fluorescence protein (EGFP). Transduced cell lines with erythroid and/or megakaryocytic background, (K562-F8 and TF-1-F8) secrete high levels of FVIII in the order of 76.4 and 41.6 ng FVIII:C/ml, whereas moderate and low levels are observed in B lymphoblastoid Raji-F8 cells and the T leukemia line Jurkat-F8 which secrete 6.73 and 1.83 ng FVIII:C/ml, respectively. The capacity to secrete rFVIII appeared to depend on factors related to the cell lineage rather than on the transduction efficacy. Stimulation of transduced cells with the protein kinase C (PKC)-activator phorbol myristate acetate (PMA) resulted in a marked augmentation of rFVIII secretion and enhanced green fluorescent protein (EGFP). Incubation with 0.1 and 1 ng/ml PMA resulted in up to 2.7-fold (K562-F8, Raji-F8) and 1.8-fold (293T-F8) increased rFVIII secretion. The established cell lines should be helpful in further elucidating mechanisms that are able to improve FVIII secretion in hematopoietic cells on a post-translational level and suggest reanalysis of hematopoietic cells as target for gene therapy of hemophilia.

Ca(2+) binding to both the heavy and light chains of factor VIII is required for cofactor activity

Previously, we demonstrated that Ca(2+) was necessary for the generation of cofactor activity following reconstitution of factor VIII from its isolated light chain (LC) and heavy chain (HC) but that Ca(2+) did not affect HC-LC binding affinity (Wakabayashi et al. (2001) Biochemistry 40, 10293-10300). Titration of EDTA-treated factor VIII with Ca(2+) followed by factor Xa generation assay showed a two-site binding pattern, with indicated high-affinity (K(d) = 8.9 +/- 1.8 microM) and low-affinity (K(d) = 4.0 +/- 0.6 mM) sites. Analysis by equilibrium dialysis using (45)Ca and <400 microM free Ca(2+) verified a high-affinity binding (K(d) = 18.9 +/- 3.7 microM). Preincubation of either HC or LC with 6 mM Ca(2+) followed by reassociation with the untreated complementary chain in the presence of 0.12 mM Ca(2+) failed to generate significant cofactor activity (<0.5 nM min(-1) (nM LC)(-1)). However, pretreatment of both HC and LC with 6 mM Ca(2+) followed by reassociation (at 0.12 mM Ca(2+)) generated high activity (7.5 +/- 0.4 nM min(-1) (nM LC)(-1)). Progress curves for activity regain following factor VIII-Ca(2+) association kinetics fitted well to a series reaction scheme rather than one of simple association (p < 0.0001), suggesting a multistep process which may include a Ca(2+)-dependent conformational change. These results suggest that factor VIII contains two Ca(2+) binding sites with different affinities and that active factor VIII can be reconstituted from HC and LC only when both chains are preactivated by Ca(2+).

A novel mechanism of factor VIII protection by von Willebrand factor from activated protein C-catalyzed inactivation

The protective effect of von Willebrand factor (VWF) toward activated protein C (APC)-catalyzed inactivation of factor VIII (FVIII) has been attributed mainly to inhibition of FVIII binding to phospholipid. In the present study, we demonstrated that VWF-mediated FVIII protection from APC also results from direct inhibition of FVIII binding to APC. Inhibition of FVIII binding to anhydro-APC by VWF would be consistent with partial or complete overlap of the FVIII binding sites for APC and VWF. We examined, therefore, the inhibitory effects of 6 synthetic peptides spanning residues 1996 to 2028 around the previously localized APC binding region (FVIII residues 2009-2018). Peptide 2009 to 2018 inhibited FVIII binding to anhydro-APC by 83% (50% inhibition, 55 microM). Similarly, peptide 2013 to 2022 inhibited FVIII binding to VWF by 84% (50% inhibition, 25 microM). It was also found that peptides 2009 to 2018 and 2013 to 2022 optimally bound to anhydro-APC and VWF, respectively. A rabbit antipeptide IgG, raised against peptide 2009 to 2022, blocked the binding of both anhydro-APC and VWF to FVIII. This immunoglobulin G inhibited proteolytic cleavage of FVIII by APC. Our results indicate that the essential regions for the binding of APC and VWF to FVIII overlap and that the protective effect of VWF on APC-catalyzed FVIII inactivation includes competitive inhibition of APC binding to FVIII by VWF.

Temporal and spatial expression of biologically active human factor VIII in the milk of transgenic mice driven by mammary-specific bovine alpha-lactalbumin regulation sequences

Hemophilia A is one of the major inherited bleeding disorders caused by a deficiency or abnormality in coagulation factor VIII (FVIII). Hemophiliacs have been treated with whole plasma or purified FVIII concentrates. The risk of transmitting blood-borne viruses and the cost of highly purified FVIII are the major factors that restrict prophylaxis in hemophilia therapy. One of the challenges created by the biotechnology revolution is the development of methods for the economical production of highly purified proteins in large scales. Recent developments indicate that manipulating milk composition using transgenesis has focused mainly on the mammary gland as a bioreactor to produce pharmaceuticals. In the present study, a hybrid gene containing bovine alpha-lactalbumin and human FVIII cDNA was constructed for microinjection into the pronuclei of newly fertilized mouse eggs. The alphaLA-hFVIII hybrid gene was confirmed to be successfully integrated and stably germ-line transmitted in 12 (seven females/five males) lines. Western-blot analysis of milk samples obtained from eight of the transgenic founders and F1 offspring indicated that the recombinant hFVIII was secreted into the milk of the transgenic mice. The concentrations of rFVIII ranged from 7.0 to 50.2 microg/ml, over 35-200-fold higher than that in normal human plasma. Up to 13.4 U/ml of rFVIII was detected in an assay in which rFVIII restored normal clotting activity to FVIII-deficient human plasma.

Biochemical and genetic properties of Paenibacillus glycosyl hydrolase having chitosanase activity and discoidin domain

Cells of "Paenibacillus fukuinensis" D2 produced chitosanase into surrounding medium, in the presence of colloidal chitosan or glucosamine. The gene of this enzyme was cloned, sequenced, and subjected to site-directed mutation and deletion analyses. The nucleotide sequence indicated that the chitosanase was composed of 797 amino acids and its molecular weight was 85,610. Unlike conventional family 46 chitosanases, the enzyme has family 8 glycosyl hydrolase catalytic domain, at the amino-terminal side, and discoidin domain at the carboxyl-terminal region. Expression of the cloned gene in Escherichia coli revealed beta-1,4-glucanase function, besides chitosanase activity. Analyses by zymography and immunoblotting suggested that the active enzyme was, after removal of signal peptide, produced from inactive 81-kDa form by proteolysis at the carboxyl-terminal region. Replacements of Glu(115) and Asp(176), highly conserved residues in the family 8 glycosylase region, with Gln and Asn caused simultaneous loss of chitosanase and glucanase activities, suggesting that these residues formed part of the catalytic site. Truncation experiments demonstrated indispensability of an amino-terminal region spanning 425 residues adjacent to the signal peptide.

Cofactor activities of factor VIIIa and A2 subunit following cleavage of A1 subunit at Arg336

Factor VIIIa consists of three subunits designated A1, A2, and A3-C1-C2. The isolated A2 subunit possesses limited cofactor activity in stimulating factor IXa-catalyzed activation of factor X. This activity is markedly enhanced by the A1 subunit (inter-subunit K(d) = 1.8 microm). The C-terminal region of A1 subunit (residues 337-372) is thought to represent an A2-interactive site. This region appears critical to factor VIIIa, because proteolysis at Arg(336) by activated protein C or factor IXa is inactivating. A truncated A1 (A1(336)) showed similar affinity for A2 subunit (K(d) = 0.9 microm) and stimulated its cofactor activity to approximately 50% that observed for native A1. However, A1(336) was unable to reconstitute factor VIIIa activity in the presence of A2 and A3-C1-C2 subunits. Fluorescence anisotropy of fluorescein (Fl)-FFR-factor IXa was differentially altered by factor VIIIa trimers containing either A1 or A1(336). Fluorescence energy transfer demonstrated that, although Fl-A1(336)/A3-C1-C2 bound acrylodan-A2 with similar affinity as the native dimer, an increased inter-fluorophore separation was observed. These results indicate that the C-terminal region of A1 appears necessary to properly orient A2 subunit relative to factor IXa in the cofactor rather than directly stimulate A2 and elucidate the mechanism for cofactor inactivation following cleavage at this site.

Expression of factor VIII in recombinant and transgenic systems

Deficiency in a coagulation factor VIII (FVIII) causes a genetic disorder hemophilia A, which is treated by repeated infusions of expensive FVIII products. Recombinant FVIII (rFVIII), the culmination of years of extensive international research, is an important alternative to plasma-derived FVIII (pdFVIII) and is considered to have a higher margin of safety. Advances in biotechnology allowed production of rFVIII at industrial scale, which significantly improved treatment of hemophilia A patients. We review the contemporary methods used for FVIII expression in mammalian cell culture systems and discuss the factors responsible for insufficient recoveries of rFVIII, such as inefficient accumulation of FVIII mRNA in the cell, complexity of the mechanisms of FVIII secretion, and instability of secreted FVIII. The approaches to improve the yield of rFVIII in cell culture systems include genetic engineering of B-domain-deleted FVIII, introduction of introns into FVIII cDNA constructs for more efficient processing and accumulation of FVIII mRNA, and introduction of mutations into chaperone-binding sites of FVIII to improve its secretion. Design of FVIII with prolonged half-life in vivo is considered as another promising direction in improving rFVIII protein and efficiency of hemophilia A therapy. As an alternative to expression of rFVIII in cell culture systems, we discuss production of rFVIII in transgenic animals, where high levels of rFVIII have been successfully secreted into milk. We also pay attention to the major limitations of this approach, such as safety issues associated with potential transmission of animal pathogens. Finally, we present a brief characterization of commercial recombinant FVIII products currently available on the market for hemophilia A treatment.