Molecular etiology of factor VIII deficiency in hemophilia A

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

Recombinant expression of coagulation factor VIII in hepatic and non-hepatic cell lines stably transduced with third generation lentiviral vectors comprising the minimal factor VIII promoter

BACKGROUND

Lentiviral vectors have the capacity to transduce stably non-dividing, differentiated and undifferentiated cells of various tissues, including liver. To obtain high-level expression of transgenes, vectors often rely on viral promoters. However, recent data suggest that the supraphysiologic expression from ubiquitous viral promoters may not be beneficial and harbor the risk of oncogene activation. Therefore this study explored the lentiviral-mediated expression of human coagulation factor VIII (FVIII) driven by the physiologic FVIII gene promoter (FVIII-p), the liver-specific human alpha-1-antitrypsin gene promoter (hAAT-p), the ubiquitous but non-viral EF1alpha promoter (EF1alpha-p) and the viral CMV promoter.

METHODS

Hepatic and non-hepatic cell lines were stably transduced with lentiviral vectors encoding FVIIIdelB and EGFP. To compare the different promoters, lentiviral vectors were cloned to drive FVIII expression from FVIII-p, EF1alpha-p, hAAT-p and CMV-p.

RESULTS

As expected, the strong viral CMV-p and the ubiquitous EF1alpha-p resulted in the highest FVIII expression in all cell lines tested (CMV-p 1.85 IU/mL/10(6) cells for 293T, 3.15 for HepG2, 5.03 for SK-Hep, 0.91 for Hepa1-6; EF1-alpha promoter 0.30 IU/mL/10(6) cells for 293T, 0.04 for HepG2, 2.75 for SK-Hep, 0.46 for Hepa1-6). While the hAAT-p resulted in low FVIII levels (0.10 IU/mL/10(6)cells in HepG2 and 0.04 in Hepa1-6), the FVIII promoter gave reasonable expression levels in hepatic cells (0.47 IU/mL/10(6)cells in Hepa1-6 and 0.44 in SK-Hep).

DISCUSSION

These results indicate the potential usefulness of the FVIII-p for hemophilia A gene therapy.

Nucleotide changes around the splicing acceptor of intron 24 in the factor VIII gene and its impact on splicing

Nucleotide 6724 of the factor VIII gene harbors a polymorphism of low frequency. A report from Taiwan claimed that 97.9% of the 83 alleles examined were of the A nucleotide at this position, which is quite different to the data from Western populations. Furthermore, this nucleotide is the start of exon 25, located in juxtaposition to the splicing acceptor of intron 24. We wonder if the nucleotide change at this location might have any effect on the splicing process of pre-mRNA. Using genomic DNA with direct sequencing of the polymerase chain reaction-amplified intron 24/exon 25 junction site, we found that 59 of the 60 patient samples were of the GTG sequence at nucleotides 6724-6726. The polymorphism is similar between populations in Taiwan and Western countries. The sequence of intron 24 around the splicing acceptor was always TCCAACTCTATTGCCCTCAG (-20 to -1), except for one hemophiliac patient who had a mutation in which the absolute consensus AG doublet of the intron 24 splicing acceptor changed to the AA dinucleotide. Owing to the mutation, exon 24 was erroneously spliced to exon 26, and exon 25 was skipped. This finding further testifies to the importance of the invariant AG dinucleotide in the example of the factor VIII gene.

Exon skipping caused by an intronic insertion of a young Alu Yb9 element leads to severe hemophilia A

Short interspersed elements, such as Alu elements, have propagated to more than one million copies in the human genome. They affect the genome in several ways, caused by retrotransposition, recombination between elements, gene conversion, and alterations in gene expression. These events, including novel insertions into active genes, have been associated with a number of human disorders. Hemophilia A is an X-linked severe bleeding disorder and is caused by mutations in the Factor VIII gene. The spectrum of mutations includes point mutations, rearrangements, insertions, and deletions. Recently, an Alu retrotransposition event in a coding exon has been reported in a family with a severe form of hemophilia A. This was the first report of an Alu insertion in the Factor VIII gene. Here, we report a second Alu insertion event that lies in an intron of the same gene that causes exon skipping and the complete disruption of gene expression.

High throughput mutation screening of the factor VIII gene (F8C) in hemophilia A: 37 novel mutations and genotype-phenotype correlation

Hemophilia A (HEMA) is an X-linked bleeding disorder caused by mutations in the factor VIII gene (F8C). Molecular genetic testing for the factor VIII gene is challenging due to its large size. Here we present results of high throughput mutation scanning based on Southern blot analysis and direct sequencing of all PCR amplified coding exons and the exon-intron boundaries of the factor VIII gene. The results of mutation analysis on 89 hemophiliac males showed presence of a disease-causing mutation in 80 individuals (90%, 95% CI of 82%-95%). Seven out of nine mutation-negative individuals were severe cases of hemophilia A with < 1% factor VIII protein in the blood. The correlation of phenotype with genotype as observed in this study was not absolute. This finding is supported by similar observations in the international database for hemophilia A mutations (HAMSTeRS). This issue raises the importance of genotypes at other loci that can act as modifiers for the phenotype. Thirty-four novel mutations and three novel substitutions for previously reported amino acid residues were identified in this series of 80 mutations. The mutations cover the full spectrum including rearrangements, deletions, frameshift, and point mutations. The novel missense mutations require careful evaluation. Prediction of a mutation as the disease-causing allele was made from the nature of the substitution and the degree of conservation of the mutated amino acid among species that have diverged in evolution. In some cases segregation analysis of the mutation with disease condition was performed when other family members were available.

Factor VIII inhibitors in haemophiliacs: a single-centre experience over 34 years, 1964-97

A retrospective study of the natural history of factor VIII inhibitors in haemophilia A patients experienced in a single comprehensive haemophilia centre over three decades is reported. 431 haemophilia A patients of all severities have been followed-up for a total of 5626 patient-years. The frequency of inhibitors was 10% in the severe haemophilia A patients and 37% occurred in children <10 years. The majority of the patients received several products before developing the inhibitors. 59% of patients had <50 exposure days and 48% were high responders (>5 BU). An 8-year (1987-95) inhibitor-free period was seen during which all previously untreated patients were treated with an intermediate-purity factor VIII concentrate. A moderate haemophiliac with a missense mutation that has not been described in association with inhibitor is reported. Six HIV-positive patients preserved their antibody response to factor VIII even at the advanced stage of their disease.

Deletion of the fibrinogen [correction of fibrogen] alpha-chain gene (FGA) causes congenital afibrogenemia

Congenital afibrinogenemia is a rare autosomal recessive disorder characterized by the complete absence of detectable fibrinogen. Uncontrolled bleeding after birth from the umbilical cord is common, and spontaneous intracerebral bleeding and splenic rupture can occur throughout life. Patients respond well to fibrinogen replacement therapy, either prophylactically or on demand. Because the half-life of infused fibrinogen is essentially normal, the genetic defect is assumed to be at the level of synthesis, but no responsible locus has been identified. Preliminary studies using Southern blotting suggested that no gross structural changes of the fibrinogen genes were present in patients. We report the identification of causative mutations in a nonconsanguineous Swiss family with congenital afibrinogenemia. The four affected male individuals (two brothers and their two first cousins) have homozygous deletions of approximately 11 kb of the fibrinogen alpha-chain gene (FGA). Haplotype data suggest that these deletions occurred separately, on three distinct ancestral chromosomes, implying that the FGA region of the fibrinogen locus is susceptible to deletion by a common mechanism. Furthermore, our results demonstrate that humans, like mice, may be born without the capacity to synthesize functional fibrinogen.

The Missense Mutation Arg593 → Cys Is Related to Antibody Formation in a Patient With Mild Hemophilia A

The development of inhibitory antibodies to factor VIII in patients affected by a mild form of hemophilia A (factor VIII < 0.05 IU/mL) is considered a rare event. In this study, we evaluated the relationship between genotype and anti-factor VIII antibody formation in a patient with mild hemophilia A. Mutation analysis showed that a missense mutation in the factor VIII gene leading to replacement of Arg593 by Cys in the A2 domain of factor VIII was associated with hemophilia A in this patient. The anti-factor VIII antibodies present in the patient's plasma were characterized using metabolically labeled factor VIII fragments expressed in insect cells. The anti-factor VIII antibodies, composed of subclasses IgG2 and IgG4, reacted with both the fragment corresponding to the factor VIII heavy chain and the A2 domain. The Arg593 → Cys substitution was introduced into the cDNA encoding the A2 domain of factor VIII and the resulting construct was expressed in insect cells. Strikingly, the metabolically labeled A2 domain carrying the Arg593 → Cys mutation was not recognized by the anti-factor VIII antibodies present in the plasma of the patient. These data indicate that the anti-factor VIII antibodies are exclusively directed against exogenous factor VIII. This strongly suggests that the Arg593 → Cys substitution results in recognition of wild-type factor VIII as nonself and is thereby related to the formation of anti-factor VIII antibodies after factor VIII replacement therapy in this particular patient.