Factor VIII C domains contain key binding sites for von Willebrand factor (vWF) and phospholipid membranes. Hemophilic patients were screened for factor VIII C-domain mutations to provide a well-characterized series. Mutated residues were localized to the high-resolution C2 structure and to a homology model of C1. Of 30 families found with mutations in the C domains, there were 14 missense changes, and 9 of these were novel. Of the missense mutations, 10 were associated with reduced vWF binding and 8 were at residues with surface-exposed side chains. Six of the 10 mutants had nearly equivalent factor VIII clotting activity and antigen level, suggesting that reduced vWF binding could cause hemophilia by reducing factor VIII stability in circulation. When the present series was combined with previously described mutations from an online international database, 11 C1 and C2 mutations in patients with mild or moderately severe hemophilia A were associated with antibody-inhibitor development in at least one affected individual. Of these substitutions, 6 occurred at surface-exposed residues. As further details of the C1 structure and its interface with C2 become available, and as binding studies are performed on the plasma of more patients with hemophilic C-domain mutations, prediction of surface binding sites should improve, allowing confirmation by site-specific mutagenesis of surface-exposed residues.

A 42-year-old patient with mild hemophilia A developed spontaneous muscle hematomas 1 month after intense therapy with factor VIII concentrates. Factor VIII clotting activity was less than 1% and his factor VIII inhibitor was 10 Bethesda units (BU)/mL. The titer peaked at 128 BU despite daily infusions of factor VIII; 1 year later, the titer was 13 BU with no spontaneous bleeding for 4 months. The plasma inhibitor was 95% neutralized by factor VIII A2 domain but less than 15% neutralized by light-chain or C2 domain. His inhibitor did not cross-react with porcine factor VIII and was at least 10-fold less reactive to a series of hybrid factor VIII proteins in which human residues 484-508 are replaced by the homologous porcine sequence (Healey et al, J Biol Chem 270:14505, 1995). The inhibitor patient's DNA encoding his A2 domain and flanking sequences showed a C-T transition predicting Arg593 to Cys. Thirteen patients from 5 unrelated families with Cys593 have not developed inhibitors. Factor VIII clotting activity from one of them was inhibited similarly to diluted normal plasma by inhibitor patient plasma. In an homologous structure, ceruloplasmin (Zaitseva et al, J Biol Inorgan Chem 1:15,1996), the residue equivalent to Arg593, is in a loop distinct from residues 484-508. On solution phase immunoprecipitation with labeled factor VIII fragments, A2, light chain, and C2 domains bound. In contrast to typical immune responses to factor VIII in patients with severe hemophilia A, this patient's inhibitor was almost entirely reactive with common epitopes within the A2 domain whereas by more sensitive immunoprecipitation testing antibodies to light chain epitopes were also present. Accordingly, immune responsiveness to exogenous factor VIII (antigen burden) appears to be more critical than his endogenous, hemophilic factor VIII to his developing high-titer anti–factor VIII antibodies and loss of tolerance to both native and hemophilic factor VIII proteins.

Summary.A variety of mutations are found in haemophilia A families. Those with circulating, dysfunctional protein can provide insights into structural determinants of factor VIII function. A molecular model based upon the crystal structure of the homologous A domains in caeruloplasmin enables predictions of molecular consequences of mutations. To identify haemophilic mutations in coding regions for three A domains of factor VIII and predict amino acid substitutions important for coagulant cofactor function, amplified DNA fragments from 188 unrelated haemophilia A families were screened for heteroduplex formation. Exons 1-19 were examined. 65 families were positive for 58 distinct mutations (39 novel) on DNA sequencing. 12 were nonmissense mutations. 38 missense mutations were found in patients that circulate or potentially circulate dysfunctional factor VIII protein and are in an A domain molecular model. Of these 38, 12 have identical residues among all known species of factors V, VIII and caeruloplasmin. These 38 mutations have been localized onto a factor VIII A domain molecular model. Of these, 19 are in coiled, 15 in b-pleated sheet, and two each in turns and a-helical structures. 15 substituted residues are on the surface, nine are partially on the surface and 14 are buried within the model structure. Mutant side-chain substitutions were inserted to predict changes in surface groups or, for buried residues, potential surface areas whose structure is probably disrupted by the substitution.

Recombinant retrovirus vectors are widely used for gene transfer studies. The recent development of a pseudotyped Moloney murine leukemia virus vector that contains the G envelope protein from the vesicular stomatitis virus allows for efficient concentration of vector and offers hope for potential use of these vectors for gene expression in vivo. A standard amphotropic vector expressing a serum marker protein, human alpha 1-antitrypsin, was infused into regenerating mouse liver and was 10-fold more efficient at achieving stable gene expression than was an equivalent pseudotyped vector. Discrepant results were obtained with cultured hepatocytes infected with an Escherichia coli ␤-galactosidase-producing pseudotype and amphotropic vector. High rates of ␤-galactosidase-positive cells were detected with the vesicular stomatitis virus G glycoprotein vector under culture conditions known to be relatively nonpermissive for retrovirus-mediated gene transfer. Subsequent studies demonstrated that ␤-galactosidase protein was concentrated and copurified during pseudotype vector preparation, resulting in high rates of protein transfer rather than stable gene transfer, a process referred to as pseudotransduction. The cotransfer of protein with concentrated pseudotyped retroviruses indicates that caution must be used when interpreting gene transduction efficiencies in gene therapy experiments.

Summary. Factor VIII's exon 14 codes for its B domain that includes nearly one-third of its amino acid sequence that is not necessary for function. Frameshift mutations appear to occur more frequently within exon 14 than in other exons. To assess exon 14 frameshift mutations and their clinical correlates, a series of unrelated, severe or moderately severe haemophilia A patients were screened for heteroduplex formation in amplified exon 14 fragments. In 25 families, a frameshift mutation was identified by sequencing. Occurrence of haemophilia was isolated in 18 of these families. Moderate severity was noted in at least six out of 13 families with an A insertion or deletion at one of two sequences where the frameshift resulted in a sequence of 8±10 As. Inhibitors occurred in five of the other 12 families including one with an A insertion within a sequence of six As. Recurrent insertions into an A 8 (codons 1439±1441) or an A 9 (codons 1191±1194) sequence or of an A deletion from the A 9 sequence are common, recurrent causes of haemophilia A that may have a moderately severe phenotype.

SummaryHeteroduplex screening identified 74 small mutations in the factor VIII genes of 72 families with hemophilia A. In addition, patients from 3 families with high titer inhibitors had partial gene deletions and 5 unrelated families that were negative for heteroduplex formation had a mutation on direct sequencing. The latter had mild hemophilia A with an inhibitor, and sequencing their exon 23 fragments found a transition predicting a recurrent Arg2150 to His. Of 69 distinct mutations (including the 3 partial gene deletions), 47 are novel. Of small mutations, 51 were missense (one possibly a normal variant and two that could also alter splicing) at 39 sites, 13 were small deletions or insertions (3 inframe and one a normal variant in an intron), 13 were nonsense at 12 sites and 2 altered intron splice junctions. In 24 families, at least one affected member had evidence for an alloimmune response to factor VIII; of these, 11 were associated with missense mutations. In 14 families, de novo origin was demonstrated.

Factor VIII C domains contain key binding sites for von Willebrand factor (vWF) and phospholipid membranes. Hemophilic patients were screened for factor VIII C-domain mutations to provide a well-characterized series. Mutated residues were localized to the high-resolution C2 structure and to a homology model of C1. Of 30 families found with mutations in the C domains, there were 14 missense changes, and 9 of these were novel. Of the missense mutations, 10 were associated with reduced vWF binding and 8 were at residues with surface-exposed side chains. Six of the 10 mutants had nearly equivalent factor VIII clotting activity and antigen level, suggesting that reduced vWF binding could cause hemophilia by reducing factor VIII stability in circulation. When the present series was combined with previously described mutations from an online international database, 11 C1 and C2 mutations in patients with mild or moderately severe hemophilia A were associated with antibody-inhibitor development in at least one affected individual. Of these substitutions, 6 occurred at surface-exposed residues. As further details of the C1 structure and its interface with C2 become available, and as binding studies are performed on the plasma of more patients with hemophilic C-domain mutations, prediction of surface binding sites should improve, allowing confirmation by site-specific mutagenesis of surface-exposed residues.