Severe hemophilia A due to a 1.3 kb factor VIII gene deletion including exon 24: homologous recombination between 41 bp within an Alu repeat sequence in introns 23 and 24.
Partial or complete factor (F)VIII gene deletions are found in about 5% of families with severe hemophilia A. Relatively few deletions have been well characterized and, of these, recombination occurred between either common repeat elements or non-homologous sequences. In evaluating a family with severe hemophilia A, an exon 24 deletion was suspected when no fragment was obtained on attempted PCR amplifications. A combination of the 5' primer of exon 23 and the 3' primer of exon 25 fragments was used with prolonged extension times to amplify a normal 2.9 kb fragment that included exons 23 through 25; the patient's amplified product was 1.6 kb indicating a 1.3 kb deletion. A mixture of normal and patient DNA showed both sized fragments as did that from an obligate carrier. Carrier detection was applied to two women at risk; one was and one was not a carrier. Sequencing the proband's 1.6 kb fragment revealed that a 1328 bp deletion occurred between homologous sequences of 287 and 285 bp in introns 23 and 24, respectively; these share 85% identity. Blast nucleotide search revealed that these represent Alu elements. Comparison with an alignment of each of the two homologous sequences further localized recombination to a 41-bp segment. However, a simple recombination event would not account for the proband's sequence. The most likely explanation is that the homologous recombination was accompanied by incomplete mismatch repair.
Nakaya
S M
SM
Puget Sound Blood Center and the University of Washington, Seattle, Washington 98104, USA.
Hsu
T-C
TC
Geraghty
S J
SJ
Manco-Johnson
M J
MJ
Thompson
A R
AR
D020087:Alu Elements; D001483:Base Sequence; D004252:DNA Mutational Analysis; D005091:Exons; D005169:Factor VIII / Q000235:genetics*; D005192:Family Health; D005260:Female; D017353:Gene Deletion; D006580:Genetic Carrier Screening; D006467:Hemophilia A / Q000209:etiology / Q000235:genetics*; D006801:Humans; D007438:Introns / Q000235:genetics; D008297:Male; D008969:Molecular Sequence Data; D010375:Pedigree; D011995:Recombination, Genetic
D005169:Factor VIII