An assessment of the pathogenic significance of the R924Q von Willebrand factor substitution.

BACKGROUND: Type 1 VWD is associated with mutational heterogeneity in the VWF gene. The R924Q substitution was the second most frequent sequence variation in the Canadian type 1 VWD study and this variant was also documented in other type 1 VWD studies. In this study, R924Q was detected in a compound heterozygote possessing both type 2N and 924Q substitutions whose VWF:FVIIIB and FVIII levels were disproportionately low for the heterozygous type 2N state. AIM: To determine the role of R924Q variation in the pathogenesis of type 1 VWD.
METHODS: The frequency of the R924Q variant in the normal and type 1 VWD populations was ascertained, along with the associated polymorphic VWF haplotype. The effect of the R924Q substitution on the biosynthesis and intracellular trafficking of VWF was explored by in vitro expression studies in COS-7 and AtT-20 cells. Immunofluorescent staining of VWF was performed in transfected AtT-20 cells and BOECs from the patient. RNA analysis was performed to investigate an RNA processing defect in the patient. RESULTS AND
CONCLUSIONS: In vitro expression studies demonstrated that the R924Q variation does not affect biosynthesis, intracellular trafficking and storage significantly. Storage of VWF in the patient's endothelial cells was abnormal. Analysis of the patient's VWF mRNA revealed a novel truncated transcript resulting from the activation of a cryptic splice site in exon 28. The presence of a common VWF haplotype in heterozygotes for 924Q with low VWF levels suggests a founder origin for this variant allele that may mark this splicing defect.