Elucidation of Gross Genomic Rearrangements Involving Large Deletions in the Clotting Factor VIII Gene

Abstract

Hemophilia A is caused by the absence or impaired activity of clotting factor VIII (FVIII) resulting from various mutations of the FVIII gene (F8). Approximately half of the severely affected hemophiliacs carry a genomic inversion originating from a hot spot of intrachromosomal recombination between a 9.5 kb region within intron 22 (int22h1) and one of two extragenic copies on the X-chromosome, int22h2 or int22h3 [1], or from an analogous hot spot within the first intron [2]. In the remaining patients, hemophilia A has been attributed to a broad spectrum of mostly private mutations scattered over the entire gene comprising 186 kb and 26 exons [3]. In 5% of the cases of severe hemophilia A, multi-exon deletions in F8 resulting in complete absence of the FVIII protein predispose the patient to the development of FVIII-inactivating antibodies (inhibitors), which represent a major complication of FVIII replacement therapy. The type of F8 mutation is considered to be the most important risk factor for inhibitor formation. Whereas splice site and missense mutations are associated with a relatively low risk, approximately 21% of patients with the recurrent int22h-related inversion develop FVIII-neutralizing antibodies. Inhibitor prevalence is highest in hemophiliacs with large deletions in F8 reaching 88% in patients with deletions of exons encoding multiple domains [4]. However, only few DNA breakpoints of such large deletions have been characterized precisely so far, although their identification would facilitate molecular — including prenatal — diagnosis and carrier detection currently relying on sophisticated methods.