Structure, Biology, and Genetics of Factor VIII. Factor VIII Function, страница 11

Duplications

Duplications of parts of human genes are very rare causes of mutations. Two such lesions are described in the factor VIII gene. In one there was a duplication of 23 kb of IVS22 inserted between exons 23–25. MLID88324432  186 This rearrangement, found in two female siblings, was apparently unstable and led to deletion of exons 23–25 in the male offspring of one of the females. In the second case there was an in-frame duplication of exon 13 in a patient with mild hemophilia A. MLID90243245  187

Point Mutations

Small Deletion/Insertions

Small deletions or insertions in the coding region of factor VIII gene that result in frameshifts have been reported. A compilation of point mutations indicates that there are 21 small deletions (of 1, 2, 4, 11, and 23 nucleotides) among 252 independent mutations recorded (8.3%). MLID93194188  188 The number of small insertions (1 and 10 nucleotides) in the mutation data base is seven (2.8% of the total point mutations). About one-half of the small deletions (10 of 21) or insertions (4 of 7) were found in exon 14. All the mutations that result in translation frameshifts cause severe hemophilia A.

Nonsense Mutations

Sixty-three independent nonsense mutations in 21 different codons are included in the point mutation data base, comprising 25% of the total number of point mutations. This percentage is perhaps biased, since many investigators have used restriction digestion analysis with TaqI that recognizes CG to TG mutations, in particular CGA to TGA (Arg to Stop) substitutions. MLID87065092  189 In two samples of 53 severe hemophilia A patients in which all point mutations have been characterized, the number of mutant nonsense codons was 7 (i.e., 13.2% [7 of 53] of the total severe mutations, or 28% [7 of 25] of the point mutations). MLID93258342  173,190 (There are 4 large deletions and 24 inversions in the sample of 53 severe hemophilia A patients).

CpG Dinucleotide Hypermutability

The study of point mutations in factor VIII uncovered two general lessons concerning human mutations. The first was the discovery of a mutation hot spot at CpG dinucleotides in which there is a common substitution CG to TG if the mutation occurs in the sense strand or CG to CA if the mutation occurs in the antisense strand. MLID87065092  189 The CG dinucleotide is the only mutation hot spot known today, and the mutations occur because cytosine 5 to guanine, is a site of methylation of mammalian DNA. MLID88113667  191 Methylation at the 5 carbon of cytosine is due to the enzyme methyltransferase, and it usually occurs in tissues in which the gene of interest is not expressed. Subsequently 5-methylcytosine is spontaneously deaminated to thymine. There are 120 independent mutations that conform to the CG to TG rule in the point mutation data base (47.6%). This high proportion of CpG mutations is probably due to the deliberate screening of these sites with restriction analysis or oligonucleotide hybridization. In 24 sites recurrent mutation at CpG dinucleotides appears to have occurred. An unbiased estimate of the frequency of CG to TG mutation may be obtained from studies in which all point mutations have been characterized in a given sample of patients. MLID91334474 MLID93258342  173,174,190,192 In these selected studies, a total of 84 point mutations have been characterized, and 32 fell under the CG-to-TG rule (38%). It has been estimated that in the factor VIII gene CG to TG or CA mutations are 10–20 times more frequent than mutations of CG to any other dinucleotide. MLID88191889  193 The mutation hot spot has subsequently been observed in a wide variety of other human genes related to disease phenotypes.