Expression and characterization of human factor IX and factor IX-factor X chimeras in mouse C127 cells

An integrated multitool analysis contributes elements to interpreting unclassified factor IX missense variants associated with hemophilia B

BACKGROUND

Dissection of genotype-phenotype relationships in hemophilia B (HB) is particularly relevant for challenging (mild HB) or for HB-associated but unclassified factor (F)IX missense variants.

OBJECTIVE

To contribute elements to interpret unclassified HB-associated FIX missense variants by a multiple-level approach upon identification of a reported, but uncharacterized, FIX missense variant associated with mild HB.

METHODS

Molecular modeling of wild-type and V92A FIX variants, expression studies in HEK293 cells with evaluation of protein (ELISA, western blotting) and activity (activated partial thromboplastin time-based/chromogenic assays) levels after recombinant expression, and multiple prediction tools.

RESULTS

The F9(NM_000133.4):c.275T>C (p.V92A) variant was found in a mild HB patient (antigen, 45.4 U/dL; coagulant activity, 23.6 IU/dL; specific activity, 0.52). Newly generated molecular models showed alterations in Gla/EGF1-EGF2 domain conformation impacting Ca++ affinity and protein-protein interactions with activated factor XI (FXIa). Multitool analysis indicated a moderate impact on protein structure/function of the valine-to-alanine substitution, in accordance with patient and modeling data. Expression studies on the V92A variant showed a specific activity (0.49 ± 0.07; wild-type, 1.0 ± 0.1) recapitulating that of the natural variant, and pointed toward a moderate activation impairment as the main determinant underlying the p.V92A defect. The validated multitool approach, integrated with evidence-based data, was challenged on a panel (n = 9) of unclassified FIX missense variants, which resulted in inferred protein (secretion/function) outputs and HB severity.

CONCLUSION

The rational integration of multitool and multiparameter analyses contributed elements to interpret genotype/phenotype relationships of unclassified FIX missense variants, with implications for diagnosis, management, and treatment of HB patients, and potentially translatable into other human disorders.

Characterisation of factor IX with a glycine-to-valine missense mutation at residue 190 in a patient with severe haemophilia B

A patient with severe haemophilia B with a glycine-to-valine missense mutation at residue 190 (c25, chymotrypsin numbering) in factor IX (FIX; FIX-G190V or FIX-FuChou) had <1% of normal FIX clotting activity and 36% of normal FIX antigen levels (cross-reacting material-reduced, CRMr). Residue 190 in the C-terminal protease domain of human FIX is highly conserved in mammalian species and the serine protease family, suggesting that it has an indispensable role in protein function. To explore the pathological mechanism by which this mutation contributes to dysfunction of the FIX molecule, we functionally characterised FIX-G190V in vitro and in vivo. Liver-specific FIX-G190V gene expression following hydrodynamic plasmid delivery into haemophilia B mice revealed a 5.7-fold reduction in specific clotting activity compared with FIX-WT (wild type) and a two-fold decrease in plasma FIX-G190V concentration. Pulse-chase analysis demonstrated that FIX-G190V was secreted at a significantly slower rate than was FIX-WT. Purified FIX-G190V and FIX-WT displayed normal calcium-dependent conformational changes as shown by intrinsic fluorescence quenching. The in vivo half-lives of FIX-G190V and FIX-WT were indistinguishable. FIX-G190V was, however, more readily degraded than FIX-WT, especially after being activated by the active form of FXI. The vulnerable sites were mapped to the peptide bonds at Arg116-Leu117, Lys265-Tyr266, Arg327-Val328, and Arg338-Ser339, which are in the exposed loops of the FIX molecule. Also, failure of FXIa-activated FIX-G190V to bind p-aminobenzamidine indicated an abnormal conformation of the active-site pocket. Thus, the mutation at residue 190 of FIX may result in protein misfolding that affects secretion, clotting function, and hydrolysis.

Structural insights into the interaction of blood coagulation co-factor VIIIa with factor IXa: a computational protein-protein docking and molecular dynamics refinement study

Coagulation factor X (FX) zymogen activation by factor IXa (FIXa) enzyme plays a critical role in the middle-phase of coagulation cascade. The activation process is catalytically inert and requires FIXa binding and complex formation with co-factor VIIIa (FVIIIa). In order to understand the structural details of the FVIIIa:FIXa complex, we employed knowledge-driven protein-protein docking and aqueous-phase MD refinement methods to develop a stable structural complex between FVIIIa and FIXa. The model shows that all four domains of FIXa wrap across FVIIIa that spans the co-factor binding surface of A2, A3 and C1 domains. The region surrounding the 558-helix of the A2-domain of FVIIIa is predicted to be the key interaction site with the helical segments of Lys293-Lys301 and Asp332-Arg338 residues of the serine-protease domain of FIXa. The hydrophobic helical stack between the GLA and EGF1 domains of FIXa is predicted to be primary interacting region with the A3-C2 domain interface of FVIIIa.

Contribution of the NH2-terminal EGF-domain of factor IXa to the specificity of intrinsic tenase

Factor IXa (FIXa) is a vitamin K-dependent coagulation serine protease which binds to factor VIIIa (FVIIIa) on negatively charged phospholipid vesicles (PCPS) to catalyse the activation of factor X (FX) to factor Xa (FXa) in the intrinsic pathway. Fluorescence resonance energy transfer (FRET) studies have indicated that the Gla-domain-dependent interaction of FIXa and FX with PCPS in the presence of FVIIIa positions the active-site of the protease at an appropriate height above the membrane surface to optimise the catalytic reaction. In this study, we investigated the contribution of the NH2-terminal EGF-domain (EGF1) of FIXa to the recognition specificity of intrinsic tenase by constructing an EGF1 deletion mutant of FIXa (FIXa-desEGF1) and characterising the properties of the mutant in kinetic, direct binding and FRET assays. The results of direct binding and kinetic studies demonstrated that the binding affinity of the mutant for interaction with FVIIIa on PCPS has been impaired greater than 10-fold and the catalytic efficiency of the mutant protease-FVIIIa-PCPS complex in the activation of FX has been decreased ~100-fold. By contrast, the mutant protease exhibited a normal activity toward FX in the absence of the protein cofactor. FRET measurements revealed that the distance of the active-site of the mutant FIXa relative to PCPS vesicles has been decreased 10 Å from 75 ± 2 Å for FIXa to 65 ± 2 Å for FIXa-desEGF1 independent of FVIIIa. These results suggest that the NH2-terminal EGF-domain of FIXa provides a binding-site for FVIIIa and plays an essential spacer function in the intrinsic tenase complex.

Activation mechanisms of coagulation factor IX

Blood haemostasis is accomplished by a complex network of coagulatory and fibrinolytic processes. These processes have to be delicately balanced, as clinically manifested by bleeding disorders, such as haemophilia A and B. These disorders are caused by defects in coagulation factor VIII and factor IX, respectively. Following a dual strategy, we emphasise on the one hand principles conserved in most coagulation enzymes, thus mirroring much of the underlying complexity in haemostasis; on the other hand, we identify enzymatic properties of the factor IXa-factor VIIIa system (Xase) that distinguish this proteolytic machine from other components of the coagulation system. While the exact mechanisms of its activity modulation remain baffling until today, superactive factor IX mutants significantly improve our current understanding and serve as a specific and testable model of Xase action.

Functional Analysis of the Factor IX Epidermal Growth Factor-Like Domain Mutation Ile66Thr Associated with Mild Hemophilia B

The present study focused on the functional role of the mutation Ile66Thr located in the N-terminal epidermal growth factor-like domain of coagulation factor IX (FIX). This mutation causes mild hemophilia B with approximately 25% FIX coagulant activity and FIX antigen levels of around 90% of normal. In the 3-dimensional structure of porcine FIXa and in the subsequent 3-dimensional model of human FIXa that we have previously developed, residue 66 is exposed to the solvent and can be replaced by many amino acids, including Thr, without affecting the major folding/stability of the molecule. This is consistent with the basically normal antigen levels observed. We found that the FIX Ile66Thr mutant was activated to a normal extent by FVIIa/TF and FXIa. However, the ability of FIX Ile66Thr to activate FX was impaired in both the presence and absence of FVIIIa, indicating that Ile66 is not directly involved in the binding of FIX to FVIIIa.

Molecular models of the procoagulant factor VIIIa-factor IXa complex

BACKGROUND

Formation of the intrinsic tenase complex is an essential event in the procoagulant reactions that lead to clot formation. The tenase complex is formed when the activated serine protease, Factor IXa (FIXa), and its cofactor Factor VIIIa (FVIIIa) assemble on a phospholipid surface to proteolytically convert the zymogen Factor X (FX) into its active form FXa. The physiological relevance of the tenase complex is evident in hemophilia A or B patients who present with bleeding disorders.

OBJECTIVES

The purpose of this study was to establish three-dimensional (3D) models of the FVIIIa-FIXa complex.

METHODS

First, we built two new theoretical models of FVIIIa via homology modeling, inter-domain docking and loop simulation algorithms as well as a model for FIXa. This was followed by pseudo-Brownian protein-protein docking in internal coordinates with the ICM (Internal Coordinates Mechanics) program between the two FVIIIa and the FIXa structures.

RESULTS

Ten representative models of this complex are presented based on agreements with known experimental data and according to structural criteria.

CONCLUSIONS

These novel 3D models will help guide future site directed mutagenesis aimed at improving the functionality of FVIIIa and/or FIXa and will contribute to a better understanding of the role of this macromolecular complex in the blood coagulation cascade.

Crystal structure of Mg2+- and Ca2+-bound Gla domain of factor IX complexed with binding protein

Factor IX is an indispensable protein required in the blood coagulation cascade. It binds to the surface of phospholipid membrane by means of a gamma-carboxyglutamic acid (Gla) domain situated at the N terminus. Recently, we showed that physiological concentrations of Mg2+ ions affect the native conformation of the Gla domain and in doing so augment the biological activity of factor IXa and binding affinity with its binding protein even in the presence of Ca2+ ions. Here we report on the crystal structures of the Mg2+/Ca2+-bound and Ca2+-bound (Mg2+-free) factor IX Gla domain (IXGD1-46) in complex with its binding protein (IX-bp) at 1.55 and 1.80 A resolutions, respectively. Three Mg2+ and five Ca2+ ions were bound in the Mg2+/Ca2+-bound IXGD1-46, and the Mg2+ ions were replaced by Ca2+ ions in Mg2+-free IXGD1-46. Comparison of Mg2+/Ca2+-bound with Ca2+-bound structures of the complexes showed that Mg2+ ion, which formed a bridge between IXGD1-46 and IX-bp, forced IXGD1-46 to rotate 4 degrees relative to IX-bp and hence might be the cause of a more tight interaction between the molecules than in the case of the Mg2+-free structure. The results clearly suggest that Mg2+ ions are required to maintain native conformation and in vivo function of factor IX Gla domain during blood coagulation.

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

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

Discontinuous residues of factor IX constitute a surface for binding the anti-factor IX monoclonal antibody A-5

Anti-human factor IX monoclonal antibody, A-5 (Mab A-5), has been widely used in structure-function studies for factor IX. Mab A-5 recognizes the catalytic domain of human factor IX (FIX). Regions important for Mab A-5 binding have previously been localized to the amino terminus of the heavy chain of factor IX, encompassing amino acid residues 181-310 [Blood (74) 971]. We have selected 20 positions in this region for alanine-scanning mutagenesis. We found that Mab A-5 failed to react with the recombinant factor IX mutants with substitutions at positions 228 and 252. Mab A-5 also reacted to a lesser extent to FIXD276A (factor IX with alanine substitution for aspartic acid at residue 276) and FIXK201A/D203A (double alanine substitutions at residues 201 and 203). The apparent dissociation rate constants (K(D)) in binding Mab A-5 were 6.0 x 10(-9), 1.4 x 10(-8) and 2.0 x 10(-8) M, for wild-type FIX, FIXK201A/D203A and FIXD276A, respectively. The increased K(D) values of the two FIX mutants are mainly owing to the increased dissociation rates. These affected residues constitute a surface opposite from the factor VIIIa binding surface. We conclude that the epitope for Mab A-5 is on a surface composed of residues 228, 252, 276, and 201 or 203. This surface, which may not be important for factor VIII binding, contains a strong antigenic region on factor IX.

The N-terminal epidermal growth factor-like domain of coagulation factor IX. Probing its functions in the activation of factor IX and factor X with a monoclonal antibody

The absence or reduced activity of coagulation factor IX (FIX) causes the severe bleeding disorder hemophilia B. FIX contains an N-terminal Gla domain followed by two epidermal growth factor-like (EGF) domains and a serine protease domain. In this study, the epitope of monoclonal antibody AW, which is directed against the C-terminal part of the first EGF domain in human FIX, was defined, and the antibody was used to study interactions between the EGF domain of FIX and other coagulation proteins. Antibody AW completely blocks activation of FIX by activated factor XI, but activation by activated factor FVII-tissue factor is inhibited only slightly. The antibody also causes a marginal reduction in the apparent k(cat) for factor X both in the presence and absence of activated factor VIII. Based on these results, we produced a preliminary model of the structure of the activated factor IX-activated factor VIII-AW complex on the surface of phospholipid. The model suggests that in the Xase complex, EGF1 of activated factor IX is not involved in direct binding to activated factor VIII. Studies of the interaction of antibody AW with a mutated FIX molecule (R94D) also suggest that the Glu(78)-Arg(94) salt bridge is not important for maintaining the structure of FIX.

Identification of functionally important residues of the epidermal growth factor-2 domain of factor IX by alanine-scanning mutagenesis. Residues Asn(89)-Gly(93) are critical for binding factor VIIIa

This paper describes the consequences of alanine-scanning mutagenesis on 28 positions of the second epidermal growth factor (EGF-2) domain of factor IX. We identified four positions of Gln(97), Phe(98), Tyr(115), and Leu(117) that are critical for secretion of factor IX. Of the remaining mutations, 4 mutants (V86A, E113A, K122A, and S123A) are as active as wild-type factor IX (IXwt); 16 (D85A, K100A, N101A, D104A, N105A, R116A, E119A, T87A, I90A, K91A, R94A, E96A, S102A, K106A, T112A, and N120A) retain reduced but detectable activity, and 4 (N89A, N92A, G93A, and V107A) are nearly inert in the clotting assay. Both factor XIa and the factor VIIa-tissue factor complex effectively catalyzed the activation of these mutants except N89A. The mutant V107A failed to form the factor tenase complex with factor VIIIa because of a 35-fold increase in K(d). The mutants N89A and N92A did not compete with factor IXwt for factor VIIIa binding, and G93A exhibited a 6-fold increase in K(i) values in the competitive binding assay. It appears that mutations at these positions have significantly affected the interaction between factor IX and factor VIIIa, although other mutations had little effect on the binding of factor IX to factor VIIIa. Mutations in two regions, Thr(87)-Gly(93) and Asn(101)-Val(107), significantly increased the K(m) value of factor IXa (2-10-fold) in cleavage of factor X in the absence of factor VIIIa. In the presence of factor VIIIa, the catalytic efficiency of each mutant toward factor X paralleled its clotting activity. Briefly, we propose two relatively distinctive functions of factor IX for two adjacent regions in the EGF-2 domain; the first loop region (residues 89-94) is involved with the binding of its cofactor, factor VIIIa, and the third loop with connected beta-sheets (residues 102-108) is involved in the proper binding to the substrate, factor X.

The factor IXa second epidermal growth factor (EGF2) domain mediates platelet binding and assembly of the factor X activating complex

Previously we have determined that residues 88-109 (but not Arg(94)) in the second epidermal growth factor (EGF2)-like domain of factor IXa (FIXa) are important for assembly of the factor X (FX) activating complex on phospholipid vesicles (Wilkinson, F. H., London, F. S., and Walsh, P. N. (2002) J. Biol. Chem. 277, 5725-5733). Here we report that these residues are important for platelet binding affinity, stoichiometry, and assembly of the FX activating complex. We prepared several chimeric FIXa proteins using homologous sequences from factor VII (FVII): FIXa(FVIIEGF2) (FIX Delta 88-124,inverted Delta FVII91-127), FIXa(loop1) (FIX Delta 88-99,inverted Delta FVII91-102), FIXa(loop2) (FIX Delta 95-109,inverted Delta FVII98-112), and FIXa(loop3) (FIX Delta 111-124,inverted Delta FVII114-127) and tested their ability to bind to thrombin-activated platelets. Binding affinities (K(d) values in 10(-9) m) for the proteins were as follows in the presence and absence of FVIIIa, respectively: FIXa(N) (0.55 +/- 0.06, 2.9 +/- 0.45), FIXa(WT) (0.80 +/- 0.08, 3.5 +/- 0.5), FIXa(loop1) (19 +/- 4.0, 27 +/- 5.0), FIXa(loop2) (35 +/- 9.0, 65 +/- 12.0), and FIXa(loop3) (1.1 +/- 0.09, 5.0 +/- 0.90). These K(d) values are in good agreement with K((d)(app)) values (in 10(-9) m) determined from the activation of FX (in the presence and absence of FVIIIa, respectively): FIXa(N) (0.46 +/- 0.05, 1.40 +/- 0.14), FIXa(WT) (0.72 +/- 0.08, 3.8 +/- 0.08), FIXa(loop1) (3.2 +/- 0.72, 14.0 +/- 1.60), FIXa(loop2) (18.4 +/- 1.60, 26.3 +/- 3.40), and FIXa(loop3) (0.7 +/- 0.05, 3.0 +/- 0.15). Moreover, the stoichiometry of binding (sites/platelet) showed an agreement with V(max) of FX activation and was reduced in those proteins that also showed a decreased platelet binding affinity. A peptide corresponding to the FIX EGF2 domain (Leu(84)-Val(128)) was an effective inhibitor of FIXa binding to platelets in both the presence (K(i) = 0.7 x 10(-6) m) and the absence (K(i) = 1.5 x 10(-6) m) of FVIIIa and FX. We conclude that residues 88-109 of the FIXa EGF2 domain mediate binding to platelets and assembly of the FX activating complex.ut not Ar

Residues 88-109 of factor IXa are important for assembly of the factor X activating complex

Activated platelets and phospholipid vesicles promote assembly of the intrinsic factor X (FX) activating complex by presenting high-affinity binding sites for blood coagulation FIXa, FVIIIa, and FX. Previous reports suggest that the second epidermal growth factor (EGF)-like domain of FIXa mediates assembly of the FX activating complex (Ahmad, S. S., Rawala, R., Cheung, W. F., Stafford, D. W., and Walsh, P. N. (1995) Biochem. J. 310, 427-431; Wong, M. Y., Gurr, J. A., and Walsh, P. N. (1999) Biochemistry 38, 8948-8960). To identify important residues, we prepared several chimeric FIXa proteins using homologous sequences from FVII: FIXa(FVIIEGF2) (FIX Delta 88-124,inverted Delta FVII91-127), FIXa(loop1) (FIX Delta 88-99,inverted Delta FVII91-102), FIXa(loop2) (FIX Delta 95-109,inverted Delta FVII98-112), FIXa(loop3) (FIX Delta 111-124,inverted Delta FVII114-127), and point mutants (FIXaR94D and FIXa(loop1)G94R). In the presence and absence of FVIIIa, a 2- to 10-fold reduced V(max) of FX activation (nm FXa min(-1)) was observed for FIXa(FVIIEGF2), FIXa(loop1), FIXa(loop2), and FIXa(loop1)G94R, whereas FIXa(loop3) and FIXaR94D were normal. For all of the FIXa proteins, K(m)((app)) values were normal as were EC(50) values for interactions with FVIIIa. However, K(d)((app)) (in nm) for the FX activating complex assembled on phospholipid vesicles was increased for FIXa(FVIIEGF2) (43.3 +/- 2.70), FIXa(loop1)(10.9 +/- 2.8), FIXa(loop2) (70.5 +/- 1.60), and FIXa(loop1)G94R (17.1 +/- 2.90) relative to FIXa(N) (3.9 +/- 0.11), FIXa(WT) (4.6 +/- 0.17), FIXa(loop3) (4.5 +/- 0.20), and FIXaR94D (2.2 +/- 0.09) suggesting that reduced V(max) is a result of impaired complex assembly. These data indicate that residues 88-109 (but not Arg(94)) are important for normal assembly of the FX activating complex on phospholipid vesicles.

The N-terminal epidermal growth factor-like domain in factor IX and factor X represents an important recognition motif for binding to tissue factor

Factors VII, IX, and X play key roles in blood coagulation. Each protein contains an N-terminal gamma-carboxyglutamic acid domain, followed by EGF1 and EGF2 domains, and the C-terminal serine protease domain. Protein C has similar domain structure and functions as an anticoagulant. During physiologic clotting, the factor VIIa-tissue factor (FVIIa*TF) complex activates both factor IX (FIX) and factor X (FX). FVIIa represents the enzyme, and TF represents the membrane-bound cofactor for this reaction. The substrates FIX and FX may utilize multiple domains in binding to the FVIIa*TF complex. To investigate the role of the EGF1 domain in this context, we expressed wild type FIX (FIX(WT)), FIX(Q50P), FIX(PCEGF1) (EGF1 domain replaced with that of protein C), FIX(DeltaEGF1) (EGF1 domain deleted), FX(WT), and FX(PCEGF1). Complexes of FVIIa with TF as well as with soluble TF (sTF) lacking the transmembrane region were prepared, and activations of WT and mutant proteins were monitored by SDS-PAGE and by enzyme assays. FVIIa*TF or FVIIa*sTF activated each mutant significantly more slowly than the FIX(WT) or FX(WT). Importantly, in ligand blot assays, FIX(WT) and FX(WT) bound to sTF, whereas mutants did not; however, all mutants and WT proteins bound to FVIIa. Further experiments revealed that the affinity of the mutants for sTF was reduced 3-10-fold and that the synthetic EGF1 domain (of FIX) inhibited FIX binding to sTF with K(i) of approximately 60 microm. Notably, each FIXa or FXa mutant activated FVII and bound to antithrombin, normally indicating correct folding of each protein. In additional experiments, FIXa with or without FVIIIa activated FX(WT) and FX(PCEGF1) normally, which is interpreted to mean that the EGF1 domain of FX does not play a significant role in its interaction with FVIIIa. Cumulatively, our data reveal that substrates FIX and FX in addition to interacting with FVIIa (enzyme) interact with TF (cofactor) using, in part, the EGF1 domain.

An anti-EGF monoclonal antibody that detects intramolecular communication in factor IX

Coagulation factor IX contains a gamma-carboxyglutamic acid (Gla) module, two epidermal growth factor-like (EGF) modules, and a serine protease region. We have characterized a mouse monoclonal antibody that binds the N-terminal EGF-like module of human factor IX with high affinity. Studies of recombinant factor IX mutants indicated that the epitope is located in the C-terminal end of the EGF-like module, which is consistent with the binding being non-Ca(2+)-dependent. The antibody bound factor IXa (K(D) = 7.6 x 10(-10) M) with about 10-fold higher affinity than factor IX (K(D) = 6.2 x 10(-9) M). Binding of the antibody to factor IXa did not affect the amidolytic activity of the protein, nor was binding affected by active site inhibition of factor IXa. These results are consistent with long-range interactions between the serine protease region and the N-terminal EGF-like module in factor IX.

Epitope mapping of factor VIII inhibitor antibodies of Chinese origin

Epitopes recognized by factor VIII (FVIII) inhibitors of Chinese origin were analysed by immunoblotting with full-length recombinant FVIII (rFVIII), thrombin-activated FVIII (FVIIIa) and 16 FVIII fusion proteins synthesized by bacteria. Twenty-eight patients, 12 with haemophilia A and 16 with autoimmune diseases, were recruited. Antibodies from 22 patients showed reactivity with rFVIII, 20 with FVIIIa, and one reacted only with FVIII fusion proteins. Of these 22 cases, most were reactive with A2-a2 and A3-C1-C2 of FVIII(a). Of the nine cases that depicted binding to the fusion proteins, three were reactive with the A domains, three with only the B domain, and the other three with both the A and B (or C) domains. An epitope for a neutralizing antibody of a haemophilia A patient, designated TWN-112, was localized to residues 323-390, specified by FVIII fusion proteins. The same epitope also appeared on an FVIII-expression phage library screening. Immunoabsorption of antibodies from TWN-112 with the epitope reduced the neutralizing activity of the inhibitor by 33%. The incidence of a1 of FVIII is higher, and that of a3 is lower, than previously reported. Two novel epitopes, reported for the first time in this paper, were localized on the 8B2 (amino acid residues 1022-1204) and 8A2(V) (residues 673-740) fusion proteins. These two epitopes were able to reduce inhibitory antibody activity by 24% and 25% respectively. Changes of FVIII fragment specificity were also observed in one of six patients for whom multiple samples, collected at different times, were available. Our initial finding showed that the FVIII inhibitors in these Chinese patients shared epitopes with those of patients from very different genetic backgrounds, suggesting a common mechanism for the development of FVIII inhibitors.

An Arg/Ser Substitution in the Second Epidermal Growth Factor–Like Module of Factor IX Introduces an O-Linked Carbohydrate and Markedly Impairs Activation by Factor XIa and Factor VIIa/Tissue Factor and Catalytic Efficiency of Factor IXa

Factor IXR94S is a naturally occurring hemophilia B defect, which results from an Arg 94 to Ser mutation in the second epidermal growth factor (EGF)-like module of factor IX. Recombinant factor IXR94S was activated by factor XIa/calcium with an ≈50-fold reduced rate and by factor VIIa/tissue factor/phospholipid/calcium with an ≈20-fold reduced rate compared with wild-type factor IX. The apparent molecular mass of the light chain of factor IXaR94S was ≈6 kD higher than that of plasma or wild-type factor IX, which was not corrected by N-glycosidase F digestion. This result indicated the presence of additional O-linked carbohydrate in the mutant light chain, probably at new Ser 94. The initial rate of activation of factor X by factor IXaR94S in the presence of polylysine was 7% ± 1% of the initial rate of activation of factor X by plasma factor IXa, and the kc/Km for activation of factor X by factor IXaR94S/factor VIIIa/phospholipid/calcium was 4% ± 1% of the kc/Km for activation of factor X by plasma factor IXa/factor VIIIa/phospholipid/calcium. The reduced efficiency of activation of factor X by factor IXaR94S in the tenase enzyme complex was due to a 58-fold ± 12-fold decrease in kcat with little effect on Km. In conclusion, the R94S mutation had introduced an O-linked carbohydrate, which markedly impaired both activation by factor XIa and turnover of factor X in the tenase enzyme complex.

Protease and EGF1 domains of factor IXa play distinct roles in binding to factor VIIIa. Importance of helix 330 (helix 162 in chymotrypsin) of protease domain of factor IXa in its interaction with factor VIIIa

Previous studies revealed that cleavage at Arg-318-Ser-319 in the protease domain autolysis loop of factor IXa results in its diminished binding to factor VIIIa. Now, we have investigated the importance of adjacent surface-exposed helix 330-338 (162-170 in chymotrypsin numbering) of IXa in its interaction with VIIIa. IXWT, eight point mutants mostly based on hemophilia B patients, and a replacement mutant (IXhelixVII in which helix 330-338 is replaced by that of factor VII) were expressed, purified, and characterized. Each mutant was activated normally by VIIa-tissue factor-Ca2+ or XIa-Ca2+. However, in both the presence and absence of phospholipid, interaction of each activated mutant with VIIIa was impaired. The role of IXa EGF1 domain in binding to VIIIa was also examined. Two mutants (IXQ50P and IXPCEGF1, in which EGF1 domain is replaced by that of protein C) were used. Strikingly, interactions of the activated EGF1 mutants with VIIIa were impaired only in the presence of phospholipid. We conclude that helix 330 in IXa provides a critical binding site for VIIIa and that the EGF1 domain in this context primarily serves to correctly position the protease domain above the phospholipid surface for optimal interaction with VIIIa.

Blood coagulation factor IX residues Glu78 and Arg94 provide a link between both epidermal growth factor-like domains that is crucial in the interaction with factor VIII light chain

Recently, we established that mutations at calcium-binding sites within the first epidermal growth factor (EGF)-like domain of activated factor IX affect its interaction with factor VIIIa (Lenting, P. J., Christophe, O. D., ter Maat, H., Rees, D. J. G., and Mertens, K. (1996) J. Biol. Chem. 271, 25332-25337). In the present study, we have investigated the functional role of residue Glu78, which is not involved in calcium binding. Glu78 is also located in the first EGF-like domain and, when mutated to Lys, is associated with severe hemophilia B. Because Glu78 is conserved in related vitamin K-dependent proteins, it is difficult to understand how a mutation at this position is associated with factor IX-specific function. In this study, we addressed the hypothesis that Glu78 exerts its biological activity by interacting with another residue. One candidate was found to be the second EGF-like domain residue, Arg94, which is also associated with severe hemophilia B when mutated. We constructed a series of mutants that included mutations at position 78 alone (Glu78 to Lys/Glu78 to Asp) or at both positions 78 and 94 (Glu78 to Lys and Arg94 to Asp). The functional parameters of immunopurified and activated mutants were compared with normal activated factor IX. Mutants were indistinguishable from normal factor IXa in cleaving the synthetic substrate CH3SO2-Leu-Gly-Arg-p-nitroanilide or activating factor X in the absence of factor VIIIa. In contrast, in the presence of factor VIIIa, factor IXa Glu78 to Asp and factor IXa Glu78 to Lys/Arg94 to Asp were stimulated to the same extent as normal factor IXa, whereas factor IXa Glu78 to Lys was markedly less stimulated (140-fold versus 2,000-fold). This suggests that residues 78 and 94 should carry an opposite charge for a normal interaction of factor IXa to factor VIIIa. This hypothesis was confirmed in inhibition studies employing synthetic peptides comprising the factor IXa-binding motifs of factor VIII heavy (Ser558-Gln565) or light chain (Glu1811-Lys1818) and in direct binding studies. We propose that residues 78 and 94 link both EGF-like domains and thereby maintain the integrity of the factor VIII light chain binding site.

Interaction of factor IXa with factor VIIIa. Effects of protease domain Ca2+ binding site, proteolysis in the autolysis loop, phospholipid, and factor X

We previously identified a high affinity Ca2+ binding site in the protease domain of factor IXa involving Glu235 (Glu70 in chymotrypsinogen numbering; hereafter, the numbers in brackets refer to the chymotrypsin equivalents) and Glu245[80] as putative ligands. To delineate the function of this Ca2+ binding site, we expressed IXwild type (IXWT), IXE235K, and IXE245V in 293 kidney cells and compared their properties with those of factor IX isolated from normal plasma (IXNP); each protein had the same Mr and gamma-carboxyglutamic acid content. Activation of each factor IX protein by factor VIIa.Ca2+.tissue factor was normal as analyzed by sodium dodecyl sulfate-gel electrophoresis. The coagulant activity of IXaWT was approximately 93%, of IXaE235K was approximately 27%, and of IXaE245V was approximately 4% compared with that of IXaNP. In contrast, activation by factor XIa.Ca2+ led to proteolysis at Arg318-Ser319[150-151] in the protease domain autolysis loop of IXaE245V with a concomitant loss of coagulant activity; this proteolysis was moderate in IXaE235K and minimal in IXaWT or IXaNP. Interaction of each activated mutant with an active site probe, p-aminobenzamidine, was also examined; the Kd of interaction in the absence and presence (in parentheses) of Ca2+ was: IXaNP or IXaWT 230 microM (78 microM), IXaE235K 150 microM (145 microM), IXaE245V 225 microM (240 microM), and autolysis loop cleaved IXaE245V 330 microM (350 microM). Next, we evaluated the apparent Kd (Kd,app) of interaction of each activated mutant with factor VIIIa. We first investigated the EC50 of interaction of IXaNP as well as of IXaWT with factor VIIIa in the presence and absence of phospholipid (PL) and varying concentrations of factor X. At each factor X concentration and constant factor VIIIa, EC50 was the free IXaNP or IXaWT concentration that yielded a half-maximal rate of factor Xa generation. EC50 values for IXaNP and IXaWT were similar and are as follows: PL-minus/X-minus (extrapolated), 2.8 microM; PL-minus/X-saturating, 0.25 microM; PLplus/X-minus, 1.6 nM; and PL-plus/X-saturating, 0.09 nM. Further, Kd,app of binding of active site-blocked factor IXa to factor VIIIa was calculated from its ability to inhibit IXaWT in the Tenase assay. Kd,app values in the absence and presence (in parentheses) of PL were: IXaNP or IXaWT, 0. 19 microM (0.07 nM); IXaE235K, 0.68 microM (0.26 nM); IXaE245V, 2.5 microM (1.35 nM); and autolysis loop-cleaved IXaE245V, 15.6 microM (14.3 nM). We conclude that (a) PL increases the apparent affinity of factor IXa for factor VIIIa approximately 2,000-fold, and the substrate, factor X, increases this affinity approximately 10-15-fold; (b) the protease domain Ca2+ binding site increases this affinity approximately 15-fold, and lysine at position 235 only partly substitutes for Ca2+; (c) Ca2+ binding to the protease domain increases the S1 reactivity approximately 3-fold and prevents proteolysis in the autolysis loop; and (d) proteolysis in the autolysis loop leads to a loss of catalytic efficiency with retention of S1 binding site and a further approximately 8-fold reduction in affinity of factor IXa for factor VIIIa.

Ca2+ binding to the first epidermal growth factor-like domain of human blood coagulation factor IX promotes enzyme activity and factor VIII light chain binding

Ca2+ binding to the first epidermal growth factor (EGF)-like domain of factor IX is known to be required for biological activity, but the mechanism by which Ca2+ contributes to factor IX function has remained unclear. We have studied recombinant factor IX mutants which lack Ca2+ binding to the first EGF-like domain, due to a replacement of Asp64 by Glu, Lys, or Val. The purified mutants (factors IX D64E, D64K, and D64V), were compared to plasma-derived and recombinant wild-type factor IX with regard to a number of metal-ion dependent functional parameters. In the presence of Mg2+, the activated mutants were indistinguishable from normal factor IXa in hydrolyzing the synthetic substrate CH3-SO2-Leu-Gly-Arg-p-nitroanilide. Replacing Mg2+ by Ca2+ further stimulated the activity of normal factor IXa but not of mutant factor IXa. In factor VIII-independent factor X activation, factor IXa D64K and D64E displayed reduced catalytic activity compared to normal factor IXa (apparent kcat/Km approximately 1, 2, and 4 x 10(3) M-1 s-1, respectively). In the presence of factor VIIIa, factor X activation rates by normal and mutant factor IXa were stimulated by factor VIIIa to a different extent ( approximately700- and 200-fold, respectively), indicating that Asp64 replacements affect the interaction with factor VIIIa. This possibility was addressed in inhibition studies employing synthetic peptides comprising the factor IXa-binding motifs of factor VIII heavy or light chains. Whereas the heavy chain peptide (Ser558-Gln565) inhibited factor VIII-dependent factor X activation by normal and mutant factor IXa with similar efficiency, the light chain peptide (Lys1804-Lys1818) inhibited normal factor IXa 2-3-fold more efficiently than did mutant factor IXa. This indicates that the reduced response to factor VIIIa may be due to impaired binding of mutant factor IXa to the factor VIII light chain. This was further explored in direct binding studies. In the presence of Mg2+, normal and mutant factor IXa were similar in binding to the factor VIII light chain. However, in the presence of Ca2+, factor IXa mutants were less efficient than normal factor IXa, which was illustrated by a 4-5-fold lower affinity than normal factor IXa for factor VIII light chain. Collectively, our data demonstrate that a number of factor IXa functions, including enzymatic activity and assembly into the factor IXa-factor VIIIa complex, are dependent on Ca2+ binding to the first EGF-like domain of factor IX.

The molecular basis of haemophilia A and B

Most families with haemophilia A or B carry gene defects of independent origin. Haemophilia B is mostly due to small changes in the factor IX gene affecting either its transcription, mRNA maturation, mRNA translation or the fine structure of factor IX. Only 2-3% of patients show gross deletions or rearrangements. The great variety of missense mutations reported to cause haemophilia B indicates that this multidomain protein is highly constrained. Less is known about the factor VIII gene as fully efficient mutation detection procedures only became available in 1991. This, however, led to the discovery that almost half the severe cases of haemophilia A or a fifth of all cases are due to frequently occurring inversions caused by homologous intra-chromosome (-chromatid) recombination between repeated sequences 9.5 kb long. Of the three repeats one is in intron 22 of the factor VIII gene and two are 400-500 kb more telomeric. They are 99.8% similar to each other. The spectrum of the other haemophilia A mutations is similar to that of haemophilia B. Since 1983 mounting evidence has shown that in both haemophilias the nature of the mutation is important in predisposing to the inhibitor complication.

Localization of factor IXa and factor VIIIa interactive sites

The contribution of the catalytic and noncatalytic domains of factor IXa to the interaction with its cofactor, factor VIIIa, was evaluated. Two proteolytic fragments of factor IXa, lacking some or all of the serine protease domain, failed to mimic the ability of factor IXa to enhance the reconstitution of factor VIIIa from isolated A1/A3-C1-C2 dimer and A2 subunit. Both fragments, however, inhibited this factor IXa-dependent activity. Selective thermal denaturation of the factor IXa serine protease domain eliminated its effect on factor VIIIa reconstitution. Modification of factor IXa with dansyl-Glu-Gly-Arg chloromethyl ketone (DEGR-IXa) stabilized this domain, and heat-treated DEGR-IXa retained its ability to enhance factor VIIIa reconstitution. These results indicate the importance of the serine protease domain as well as structures residing in the factor IXa light chain (gamma-carboxyglutamic acid and/or epidermal growth factor domains) for cofactor stabilizing activity. In the presence of phospholipid, the A1/A3-C1-C2 dimer produced a saturable increase in the fluorescence anisotropy of fluorescein-Phe-Phe-Arg chloromethyl ketone-modified factor IXa (Fl-FFR-IXa). This effect was inhibited by a factor IXa fragment comprised of the gamma-carboxyglutamic acid and epidermal growth factor domains. The difference in Fl-FFR-IXa anisotropy in the presence of A1/A3-C1-C2 dimer (delta r = 0.043) compared with factor VIIIa (delta r = 0.069) represented the contribution of the A2 subunit, A peptide corresponding to factor VIII A2 domain residues 558-565 decreased the factor VIIIa dependent-anisotropy of Fl-FFR-IXa to a value similar to that observed with the A1/A3-C1-C2 dimer. These results support a model of multiple interactive sites in the association of the enzyme-cofactor complex and localize sites for the A1/A3-C1-C2 dimer and the A2 subunit to the factor IXa light chain and serine protease domain, respectively.

The roles of factor VII's structural domains in tissue factor binding

Factor VIIa binds to tissue factor in one of the initial steps of blood clotting. In order to determine the role of the various domains of the factor VII molecule in this interaction, we made several chimeric factor VII proteins using recombinant DNA techniques. The molecules have factor IX domains substituted into factor VII and vice versa. The domains exchanged were the 4-carboxyglutamic acid plus aromatic stack domain (gla), the first epidermal growth factor-like domain (Egf-1), the second epidermal growth factor-like domain (Egf-2), and the catalytic domain. Using tissue factor-coated microtiter wells, competition binding studies with 125I-labeled factor VIIa indicated factor VIIa's Kd is 4.2 nM. Employing the same microtiter plate assay, koff and kon were determined and yielded a Kd of 1.5 nM. The results of competitive binding experiments and activation assays using chimeric proteins indicated the interaction between factor VIIa and tissue factor involves direct contact between tissue factor and factor VIIa's Egf-1 domain and catalytic domain. On the other hand, the gla and Egf-2 domains, while necessary for optimal binding, may merely impart structure to the rest of the molecule. However, either one or both of the latter domains might contribute a relatively small amount of energy to direct binding.

Enzymatic removal of sialic acid from human factor IX and factor X has no effect on their coagulant activity

Factor IX and factor X have sialic acid in O-linked and N-linked oligosaccharides on their activation peptides, and a terminal sialic acid is found on a recently described O-linked tetrasaccharide at Ser-61 in the light chain of human factor IXa. In studies presented here, the potential role of sialic acid residues in mediating activity of human coagulation factors IX and X was tested after enzymatic removal of sialic acid residues. In contrast to previous reports, treatment of factor IX or factor IXa with recombinant sialidase did not decrease the rate of factor IX activation or proteolytic properties of human factor IXa. The activation rates of factor IX and desialated factor IX were indistinguishable when treated with factor XIa, with factor VIIa/tissue factor complex, and with the factor X activating enzyme from Russell's viper venom. Desialated human factor IXa showed full activity in the non-activated partial thromboplastin time assay and retained full "tenase" activity in a coupled amidolytic assay. Similar experiments with human factor X showed no detectable loss of clotting activity in the prothrombin time assay after desialation. Additionally, desialated human factor X was cleaved by the factor X activating enzyme from Russell's viper venom and intrinsic tenase at the same rate as untreated factor X when analyzed by SDS-polyacrylamide gel electrophoresis. These studies have shown that factor IX and factor X clotting activity are not dependent on sialic acid content. Further studies are needed to determine whether desialated factor IX binds to endothelial cells, and whether factors IX and X are more rapidly cleared from circulation or have altered susceptibility to proteolysis after enzymatic removal of sialic acid.

Expression and functional characterization of chimeras between human and bovine vitamin-K-dependent protein-S-defining modules important for the species specificity of the activated protein C cofactor activity

Vitamin-K-dependent protein S is an anticoagulant plasma protein functioning as a cofactor to activated protein C (APC) in the degradation of factors Va and VIIIa. The APC-cofactor function of protein S is species specific, as human protein S potentiates the anticoagulant activity of human but not that of bovine APC, whereas bovine protein S is a cofactor to APC from both species. To elucidate which modules in protein S determine the species specificity, in vitro mutagenesis was used to construct six recombinant chimeric molecules between human and bovine protein S. Wild-type human and bovine protein S and the chimeras were expressed in 293 cells and the recombinant proteins purified by monoclonal antibody affinity chromatography. The recombinant proteins were found to be post-translationally modified, they bound C4b-binding protein and were functionally active as cofactors to APC. Chimeras having both the thrombin-sensitive region (TSR) and the first epidermal-growth-factor-(EGF)-like module of bovine origin expressed APC-cofactor activity similar to that of bovine protein S. Those chimeras, in which TSR or EGF1 derived from different species, manifested APC-cofactor activity similar to that of human protein S, i.e. they did not express cofactor activity to bovine APC. These data indicate that sequence differences in the TSR and EGF1 of human and bovine protein S cause the species specificity of the APC-cofactor activity. The data support the concept that these two modules of protein S interact with APC on the surface of negatively charged phospholipids.

High-affinity, specific factor IXa binding to platelets is mediated in part by residues 3-11

To identify the amino acids in the Gla domain that mediate factor IXa binding to human platelets, we have used chimeric molecules and point mutations in the Gla domain of recombinant factor IX, based on molecular modeling using the coordinates of the Gla domain of bovine prothrombin, which reveals two surface structures whose sequences differ among factor IX, factor X, and factor VII. Binding to thrombin-activated platelets of factor IXa in the presence of factor VIIIa (2 units/mL) and factor X (1.5 microM) revealed a stoichiometry of approximately 550 sites per platelet with a Kd of approximately 0.65 nM compared with a Kd of approximately 2.5 nM in the absence of factor VIIIa and factor X. In contrast, mutations of factor IX to factor X residues at positions 4 and 5 or at positions 9, 10, and 11 results in decreases in the number of sites and affinity of factor IXa binding in the presence or absence of factor VIIIa and factor X. A chimera consisting of the Gla domain of factor VII with factor IX residues at positions 33, 34, 35, 39, and 40 displayed abnormal factor IXa binding and a decreased Vmax and a normal Km for factor X activation, and the replacement of amino acid residues 3-10 with those of factor IX restored normal binding and factor X activation kinetics to this chimeric protein.(ABSTRACT TRUNCATED AT 250 WORDS)

The binding of natural variants of human factor IX to endothelial cells

The Gla-domain of human factor IX contains a specific element required for the binding of factor IX to an endothelial cell surface protein. We have investigated the dependence of this interaction on the structural integrity of the adjacent hydrophobic stack and epidermal growth factor-like domains. The ability of purified natural variants of human factor IX to compete with wild-type factor IX binding to the endothelial cell surface was used to obtain apparent Ki values of the variants. Our data suggest that the functional integrity of the Gla domain, enabling factor IX to specifically interact with an endothelial cell surface protein, depends on the structural and functional integrity of both the hydrophobic stack domain and the first epidermal growth factor-like domain.

Construction, expression, and properties of a recombinant chimeric human protein C with replacement of its growth factor-like domains by those of human coagulation factor IX

The cDNA encoding a chimeric human protein C (PC), in which its epidermal growth factor-(EGF) like regions have been replaced with equivalent structures from human factor IX (fIX), was constructed and the gene product was expressed in human 293 cells. A molecular subpopulation of the recombinant chimeric protein (r-[PC/delta EGF-1,2/delta fIXEGF-1,2]) was purified that contained the full complement (9 residues/mol) of gamma-carboxyglutamic acid (Gla). After conversion by thrombin to its activated form (r-[APC/delta EGF-1,2/delta fIXEGF-1,2]), this latter enzyme was found to possess approximately 10% of the activity of wild-type recombinant APC (wtr-APC) in an APTT assay. In assay systems employing purified components, the activity of the mutant enzyme toward prothrombinase cofactor Va (fVa) and tenase cofactor VIII (fVIII) was approximately 30% and < 10%, respectively, of that of wtr-APC. The chimeric protein displayed full reactivity with a Ca(2+)-dependent monoclonal antibody to the Gla domain of PC, yielding a C50 for Ca2+ that was very similar to that obtained with wtr-PC (ca. 3.7 mM). Titrations of the dependency on Ca2+ of the intrinsic fluorescence of r-[PC/delta EGF-1,2/delta fIXEGF-1,2] allowed calculation of a C50 value of 0.34 mM, again very similar to that of wtr-PC. As with wtr-PC, Ca2+ inhibited the thrombin-catalyzed activation of r-[PC/delta EGF-1,2/delta fIXEGF-1,2] with aKi of 148 microM, as compared to a Ki of 125 microM for wtr-PC. At a saturating level of Ca2+, activation of r-[PC/delta EGF-1,2/delta fIXEGF-1,2/] by the thrombin/thrombomodulin (thrombin/TM) complex occurred at approximately 70% of the rate of that of wtr-PC.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of calcium in the organization of fibrillin microfibrils

The microfibrillar glycoprotein fibrillin has a multidomain structure which contains forty-three epidermal growth factor-like motifs with calcium-binding consensus sequences. We have utilized intact microfibrils isolated from human dermal fibroblast cultures to investigate the putative influence of bound calcium on microfibrillar organization and integrity. Incubation with EDTA or EGTA rapidly resulted in gross disruption of microfibril morphology. The treatment induced disorganization of the interbead domains although the regular beaded arrangement was always apparent. These changes were readily reversible on replacing calcium, indicating that the treatment had not compromised microfibrillar integrity. The data localize calcium binding EGF-like repeats to the interbead domains and indicate that lateral packing of fibrillin monomers is calcium-dependent. This arrangement suggests how mutations in epidermal growth factor-like domains of fibrillin might cause the disruption in microfibril organization and interactions which underlies the clinical symptoms of some Marfan syndrome patients.

Identification and structural analysis of the tetrasaccharide NeuAc alpha(2-->6)Gal beta(1-->4)GlcNAc beta(1-->3)Fuc alpha 1-->O-linked to serine 61 of human factor IX

O-Linked fucose has been found attached to Thr/Ser residues within the sequence Cys-X-X-Gly-Gly-Thr/Ser-Cys in the N-terminal EGF domains of several coagulation/fibrinolytic proteins. Carbohydrate composition and mass spectrometric analyses of tryptic and thermolytic peptides containing the corresponding site (Ser-61) in the first EGF domain of human factor IX indicated the presence of a tetrasaccharide containing one residue each of sialic acid, galactose, N-acetylglucosamine, and fucose. The Ser-61 tetrasaccharide was not susceptible to alpha-fucosidase digestion. Fragments generated during mass spectrometric analysis indicated that fucose was the attachment sugar residue. The involvement of fucose in the carbohydrate-peptide linkage was confirmed by two-dimensional 1H NMR spectroscopic analysis of the glycopeptide containing factor IX residues 57-65. The complete structure of the tetrasaccharide was obtained by methylation analysis and two-dimensional 1H TOCSY and ROESY experiments as NeuAc alpha(2-->6)Gal beta(1-->4)GlcNAc beta(1-->3)Fuc alpha 1-->O-Ser61.

Towards gene therapy for haemophilia B using primary human keratinocytes

Haemophilia B might be permanently cured by gene therapy--the introduction of a correct copy of the factor IX gene into the somatic cells of a patient. Here, we have introduced a recombinant human factor IX cDNA into primary human keratinocytes by means of a defective retroviral vector. In tissue culture, transduced keratinocytes were found to secrete biologically active factor IX and after transplantation of these cells into nude mice, human factor IX was detected in the bloodstream in small quantities for one week. This is the first demonstration of a therapeutic protein reaching the bloodstream from transduced primary keratinocytes. This may have implications for the treatment of haemophilia B and other disorders.