Background: Severe von Willebrand disease (VWD) type 3 is caused by large deletions, insertions, small truncating mutations, splice site mutations and missense mutations of the VWF gene, respectively. Large deletions have been regarded as being a rare cause of VWD type 3. Complete gene deletions have only been identified in Italian and German patients to date. However, their extent and breakpoints have not been determined yet.
Objectives: To identify the breakpoints of complete VWF deletions in patients with VWD type 3.
Patients/methods: Five index patients with large deletions from two unrelated German and three Italian families were investigated by polymerase chain reaction (PCR) and primer walking. Haplotypes were composed of eight deletion flanking markers.
Results: After initial characterization of a homozygous 253,246 bp deletion (Delta253 k) in a German patient, with the centromeric breakpoint located between CD9 and VWF and the telomeric breakpoint in intron 3 of TMEM16B, respectively, we identified the same Delta253 k in an additional two homozygous and two compound-heterozygous patients, and in their heterozygous parents. All patients share the same deletion-associated marker haplotype. The genomic structure of the breakpoint regions favors DNA double-strand breaks followed by non-homologous end-joining repair as an underlying molecular mechanism rather than a homologous recombination event.
Conclusions: Our results suggest a single genetic origin of Delta253 k. Homozygosity for Delta253 k in non-consanguineous families from two different countries may indicate a higher incidence of large deletions in VWD type 3 than previously thought. The availability of a Delta253k-specific assay allows simple and rapid detection of even heterozygous patients and carriers.