The F8 gene encodes coagulation factor VIII, also known as "coagulation factor VIII, procoagulant component". Upon injury, coagulation factor VIII is proteolytically activated by a number of coagulation enzymes, such as thrombin. When factor VIII is activated, it dissociates from von Willebrand factor, with which it is usually strongly associated. Activated factor VIII interacts with another coagulation factor called factor IX, and this triggers a cascade of reactions that lead to coagulation, which has a pivotal role in hemostasis.
The F8 gene has 26 exons spanning 186 kb. Gene expression studies uncovered that F8 is expressed in the liver, spleen, lymph nodes, and a variety of other tissues. The length of the transcript detected by northern blotting is 9 kb.
Factor VIII precursor protein is 2,351 amino acids long and has a molecular mass of 267 kD, while the mature protein has 2,332 amino acids. Factor VIII is structurally related to factor V and ceruloplasmin and consists of clearly demarcated domains.
At least 270 mutations and polymorphisms affecting the F8 gene have been uncovered by researchers. These variants include deletions affecting various exons (e.g., 3, 5, 14, and 26) in addition to many inversions, nonsense and missense mutations. Many studies revealed that harboring certain molecular defects; such as these that cause premature termination, may be a predisposing factor for developing antibodies that inhibit factor VIII. On the contrary, F8 gene inversions do not seem to be a major predisposing factor for producing such antibodies.
In 1990, Nafa et al. investigated the allele frequencies of intragenic (intron 17 and intron 25) and extragenic (DXS15 and DXS52) F8 gene RFLPs in the Algerian population. Altogether, 287 X chromosomes (97 males and 95 females) were studied. A new allele (14 kb), called 1 DZ, was found in 3.1% of the chromosomes. Fifty-one families with hemophilia A were studied with the same probes (374 subjects). Of the females, 94% were informative for at least one intra- or extragenic RFLP. Two recombinations were found between DXS52 and F8, of which one occurred between the DXS15, DXS52 block and F8, indicating that the two anonymous loci are on the same side of the F8 gene. Only two obvious gene deletions were observed in 73 unrelated hemophiliacs: one was found in a patient with severe hemophilia A (patient H20) and encompassed exons 14-22 of the F8 gene (about 4.3 kb of cDNA and 36 kb of genomic DNA); the other removed the last exon (exon 26, representing 2 kb of cDNA). In a separate study, Nafa et al. (1992) found a novel mutation in a patient with 7% factor VIII activity and moderate hemophilia A. The mutation was caused by a CGA>CTA transversion at codon 1941 in exon 18 of the A3 domain, resulting in p.Arg1941Leu.
A study by Owaidah et al. (2009) evaluated the genotype-phenotype correlation in hemophilia A patients in Saudi Arabia. The study included 22 men affected with hemophilia A from 18 unrelated families. All, but two brothers, suffered severe hemophilia A with FVIII activity less than 1 IU/dl. The age range was 4-37 years and all patients experienced frequent bleeding episodes. Screening for F8 intron 22 inversions showed that 10 out of 22 cases were positive for this mutation. The remaining 12 patients were then screened for other mutations in the gene F8. The latter study unveiled six different mutations (5 missense and one deletion/insertion), of which one was a novel missense (p.R593P) and one was a novel deletion/insertion (a frameshift in codons 1226-1229). The two patients who suffered a mild phenotype were shown to have the missense mutation c.923C>T, while all other patients suffered a severe phenotype.
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