Molecular pathogenesis and therapeutic advances in haemophilia, an update of the current evidence

Table 1 Genomic and structural characteristics of the F8 and F9 genes of their variant databases

Feature	F8 gene	F9 gene
Chromosomal location	Xq28	Xq27
Gene size	Approximately 186 kb[11]	Approximately 34 kb[11]
Number of exons	26	8
Protein encoded	Coagulation factor VIII	Coagulation factor IX
Protein structure/domains	A1-a1-A2-a2-B-a3-A3-C1-C2; encoded partly by large exon 14[11]	Signal peptide, propeptide, Gla domain, EGF1, EGF2, activation peptide, serine protease domain[11]
Key genomic features	Intron 22 region containing intronic gene copies (F8A and F8B), predisposing to Inv22 (40%-50% of severe HA)[11]; intron 1 low-copy repeats predisposing to Inv1 (2%-5% of severe HA); CpG hotspots accounting for approximately 40% of small variants despite comprising approximately 2% of sequence[11]	Compact gene with modular exon-domain correspondence; promoter contains androgen-responsive elements responsible for haemophilia B Leyden phenotype[11]
Predominant mutation classes	Inv22 (40%-50% of severe); Inv1 (2%-5% of severe); nonsense, frameshift, splice-site defects dominate in severe HA; missense variants common in mild/moderate HA	Missense (approximately 68%) dominate; nonsense/frameshift variants associated with severe phenotype; splice-site variants relatively frequent; large deletions rare (1%-3%) but high inhibitor risk
Mutation hotspots	Inv22 and Inv1 mediated by homologous recombination	Noncomparable to F8 inversions; mutations distributed across gene
Typical diagnostic methods	Long-range PCR for inversion detection; Sanger/NGS sequencing for point mutations, indels; MLPA for deletions/duplications	Direct sequencing (gene small enough for full coverage); MLPA when deletions suspected

HA: Haemophilia A; Inv22: Intron 22 inversion; Inv1: Intron 1 inversion; NGS: Next gene sequence; MLPA: Multiplex ligation-dependent probe amplification.