The highly prevalent deletions in F8 intron 13 found in French mild hemophilia A patients result from both founder effect and recurrent de novo events

BACKGROUND: Recently, our group has reported a 13-bp deletion in a poly(T)-track in the F8 intron 13 as the causative variant in approximately 6% of all cases of mild haemophilia A (HA) in France. The systematic screening of mild HA patients for this deletion identified individuals carrying deletions from 9 to 14-bp in the same region. AIMS: To demonstrate that these highly prevalent deletions could result from a recurrent molecular mechanism and to determine the clinical significance of deletions other than 13-bp in size. METHODS: Haplotype analysis using five polymorphic markers was performed in 71 unrelated French mild hemophilia A patients. Minigene analysis was performed to study the splicing impact of deletions from 1 to 14-bp. RESULTS: A peculiar haplotype (H1) was identified in 22.5% of patients carrying the 13-bp deletion. Haplotypes differing from H1 only for the two most distal markers were found in more than the half of patients. These results confirmed the founder effect origin for the 13-bp deletion. However, the 9 patients carrying other sizes of deletion had a different haplotype suggesting that these deletions arose independently. Supporting the recurrent mechanism hypothesis, similar deletions were also found in 3/19 genetically unresolved mild Canadian patients. In vitro splicing analysis confirmed that deletions larger than 9-bp had a deleterious impact on splicing of F8 transcript. CONCLUSION: We demonstrated that the poly(T)-track in F8 intron 13 is a deletion hotspot. We recommend that deletions in this region should be specifically investigated in all genetically unresolved mild HA patients.