five Int22h Homologous Copies in Association with Intron22 Inversion Type 3

Introduction

F8 intron22 inversion is the causative gene defect in up to 45% of severe hemophilia A (HA) patients mediated by recombination between three highly homologous copies located in intron 22 (int22h-1) and two other extragenic copies (int22h-2 and int22h-3) positioned more telomerically outside the gene. Intrachromosomal rearrangement between int22h-1 and int22h-3 provide the F8 type 1 inversion while the F8 type 2 inversion could be explained by the participation of the int22-h2 sequence in the homologous recombination. However, a third type, known since 1993*, was explained by the existence of a duplicated copy of int22h-3 or int22h-2 in case of intron22 inversion type 3A or 3B respectively.

Methods

Three unrelated HA patients with intron22 type 3A/3B were identified by southern blotting. To appreciate the length of the int22h extragenic duplicated, genomic hybridisation was performed using Affimetrix CytoScan High-Density array.

Results

Breakpoint analyses by CGH-array in all patients show same duplication of approximately 180kb delimited between intron 1 of CLIC2 gene and distal repeat int22h-3reflecting the presence of five genomic int22 copies. A fourth non-hemophiliac case was added since analysis by CGH technique identified duplication delineated by same boundaries.

Conclusion

This study suggests the existence of genotypes harboring five int22h copies mediated by 180 kb duplication at Xq28 locus probably not associated with HA. We propose that this duplication has happened by tandem inversed duplication. Such genotype is to be considered as polymorphism and could be associated with the two kinds of F8 intron22 inversion type 3 when intrachromosomal recombination has occurred between homologous copies.

*Antonarakis SE and the international consortium study. Factor VIII gene inversions in severe hemophilia A: results of an international consortium study. Blood 1995; 86: 2206-2212