Summary
Using an ELISA-based method to detect type 2N von Willebrand disease (VWD), we found
two individuals with absent FVIII binding. Direct sequencing of the FVIII binding
region of the von Willebrand factor (VWF) gene showed that one individual had an R854Q
substitution whilst the other had a T791M substitution. The very low FVIII binding
and the VWF:Ag levels in both individuals suggested a second defect on the other VWF
allele. Conformation sensitive gel electrophoresis of polymerase chain reaction amplified
DNA was used to detect an additional change in the VWF gene of each patient. Direct
sequencing confirmed a previously unreported G to A transition in the donor splice
site in intron 25 of both individuals which should result in a null allele. This was
confirmed by mRNA analysis. These two individuals therefore have compound heterozygous
VWD in which the only expressed allele has a type 2N mutation. In our population,
such compound heterozygosity appears to be a significant cause of type 2N VWD.