Mild hemophilia A associated with a cryptic donor splice site mutation in intron 4 of the factor VIII gene

Identification of deep intronic variants in 15 haemophilia A patients by next generation sequencing of the whole factor VIII gene

Current screening methods for factor VIII gene (F8) mutations can reveal the causative alteration in the vast majority of haemophilia A patients. Yet, standard diagnostic methods fail in about 2 % of cases. This study aimed at analysing the entire intronic sequences of the F8 gene in 15 haemophilia A patients by next generation sequencing. All patients had a mild to moderate phenotype and no mutation in the coding sequence and splice sites of the F8 gene could be diagnosed so far. Next generation sequencing data revealed 23 deep intronic candidate variants in several F8 introns, including six recurrent variants and three variants that have been described before. One patient additionally showed a deletion of 9.2 kb in intron 1, mediated by Alu-type repeats. Several bioinformatic tools were used to score the variants in comparison to known pathogenic F8 mutations in order to predict their deleteriousness. Pedigree analyses showed a correct segregation pattern for three of the presumptive mutations. In each of the 15 patients analysed, at least one deep intronic variant in the F8 gene was identified and predicted to alter F8 mRNA splicing. Reduced F8 mRNA levels and/or stability would be well compatible with the patients’ mild to moderate haemophilia A phenotypes. The next generation sequencing approach used proved an efficient method to screen the complete F8 gene and could be applied as a one-stop sequencing method for molecular diagnostics of haemophilia A.

Note: The work was carried out at the Department of Human Genetics, University of Würzburg, Biocenter, Am Hubland, 97074 Würzburg, Germany. New

A novel splicing acceptor mutation of the factor VIII gene producing skipping of exon 25

A gross deletion in the factor VIII (FVIII) mRNA was determined by reverse transcriptase polymerase chain reaction (RT-PCR) for a patient with moderately severe hemophilia A. Sequencing of the RT-PCR product depicted a 177-bp deletion ranging from nucleotide (nt) 6724 to nt 6900 of FVIII cDNA, exactly corresponding to the whole exon 25. Further study of the genomic DNA revealed the presence of a single base pair substitution (G >A) at position -1 of intron 24. The absolute consensus AG doublet of the intron 24 splicing acceptor changed to AA. In the novel splice site mutation, exon 24 was erroneously spliced to exon 26, skipping exon 25. The FVIII antigen level was normal despite the markedly reduced functional activity. Since exon 25 corresponds to part of the C2 domain, we speculate that for this patient the aberrant C2 domain markedly reduces binding affinity of FVIII protein to the phospholipid membrane, thus severely impairing the protein function.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene, second edition

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene, second edition

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene

Mutations at the factor VIII gene locus causing Haemophilia A have now been identified in many patients from many ethnic groups. Earlier studies used biased methods which detected repetitive mutations at a few CG dinucleotides. More recently rapid gene scanning methods have uncovered an extreme diversity of mutations. Over 80 different point mutations, 6 insertions, 7 small deletions, and 60 large deletions have been characterised. Repetitive mutation has been proved for at least 16 CpG sites. All nonsense mutations cause severe disease. Most missense mutations appear to cause instability of the protein, but some are associated with production of dysfunctional factor VIII molecules, thereby localising functionally critical regions of the cofactor. Variable phenotype has been observed in association with three of the latter class of genotype. This catalogue of gene lesions in Haemophilia A will be updated annually.

Direct characterization of factor VIII in plasma: detection of a mutation altering a thrombin cleavage site (arginine-372----histidine)

An immunoadsorbent method has been developed for the direct analysis of normal and variant plasma factor VIII. Using this method, the molecular defect responsible for mild hemophilia A has been identified for a patient whose plasma factor VIII activity is 0.05 unit/ml, even though the factor VIII antigen content is 3.25 units/ml. Although the variant factor VIII has an apparently normal molecular mass and chain composition, the 92-kDa heavy chain accumulates when the variant protein is incubated with thrombin and the 44-kDa heavy chain fragment cannot be detected. In contrast, thrombin cleavage of the 80-kDa light chain to the 72-kDa fragment is normal. As these data indicate a loss of factor VIII cleavage by thrombin at arginine-372, the genetic defect was determined by polymerase-chain-reaction amplification of exon 8 of the factor VIII gene and direct sequencing of the amplified product. A single-base substitution (guanine----adenine) was identified that produces an arginine to histidine substitution at amino acid residue 372. These data identify the molecular basis of an abnormal factor VIII, "factor VIII-Kumamoto," that lacks procoagulant function because of impaired thrombin activation.

A recurrent missense mutation (Arg----Gln) and a partial deletion in factor VIII gene causing severe haemophilia A

The presence of gene lesions in coagulation factor VIII (FVIII) gene was investigated in 70 Italian patients severely affected by haemophilia A. cDNA probes specific for exons 14-26 of the FVIII gene were used. In two related patients a gene deletion removes exon 26, a gene lesion similar to that described previously in a British haemophiliac. In exon 24 a C to T transition in the reverse complement strand causes a missense mutation in the coding strand (CGA----CAA, 2209 Arg----Gln). The mutation is located in a very conserved FVIII homology region and severely reduces FVIII activity. By restriction analysis and hybridizations with oligonucleotide probes this gene alteration was found in two unrelated haemophiliacs and in their relatives.