Recurrent inversion with concomitant deletion and insertion events in the coagulation factor VIII gene suggests a new mechanism for X-chromosomal rearrangements causing hemophilia A.

Recurrent int22h-related inversions in the coagulation factor VIII gene (F8) are the most common cause of severe hemophilia A. Such inversions have repeatedly been hypothesized to be associated with concomitant deletions that are responsible for an increased risk of immune responses against therapeutic exogenous factor VIII. However, exact DNA breakpoints have not yet been reported. In a patient with persistent factor VIII-inactivating antibodies, molecular analysis of F8 including Southern Blot, long-range PCR and primer walking techniques revealed a combination of an int22h2-related inversion, deletion of exons 16-22 and insertion of a duplicated part of the X-chromosomal MPP1 gene. This novel genomic rearrangement was also detectable in the patient's mother, but absent in both maternal grandparents. The genetic defect most likely originated from a complex X-chromosomal recombination event during spermatogenesis due to the formation of a DNA loop stabilized by Alu and LINE repeat elements. Elucidation of such combined mutations may allow early identification of patients at high risk of developing factor VIII-neutralizing antibodies and will help to understand the mechanisms behind gross chromosomal rearrangements causing hemophilia A and other diseases.