von Willebrand factor-T441N, a novel missense variant in the von Willebrand factor D2 domain, exhibits impaired multimerization and deformed Weibel-Palade bodies.

BACKGROUND

von Willebrand factor (VWF) propeptide (VWFpp) located in the D1-D2 domain of pro-VWF, is essential for multimerization and intracellular storage of VWF. Pathogenic variants of VWFpp cause various subtypes of von Willebrand disease, such as type 1, type 2A, type 2N, and type 3. In heterozygotes of these variants, VWFpp derived from the normal allele can rescue the abnormalities observed in the homozygote. This rescue effect may highlight the phenotypic difference between heterozygotes and homozygotes.

OBJECTIVES

We investigated the molecular pathogenesis of a novel missense variant of VWFpp, VWF c.1322 C>A (p.T441N), identified in a heterozygous 20-year-old woman.

METHODS

The VWF gene was analyzed using whole-exome sequencing and Sanger sequencing. Recombinant VWF (rVWF) and VWF propeptide (rVWFpp) were overexpressed in HEK293 cells. rVWF production was measured using an in-house enzyme-linked immunosorbent assay and multimer analysis. The morphology of pseudo-Weibel-Palade bodies (WPBs) and WPBs in endothelial colony-forming cells were observed by immunocytochemistry.

RESULTS

In the patient's plasma, the VWF antigen level was in the normal range, but VWF ristocetin cofactor activity was slightly decreased, by 0.38 IU/mL, and high-molecular-weight and intermediate-molecular-weight multimers were decreased. Pseudo-WPBs were also shortened and rounded, corresponding with patient-derived endothelial colony-forming cells. These abnormalities were partially rescued by coexpression of wild-type rVWFpp on separate vectors.

CONCLUSION

VWF-T441N results in impaired multimerization and deformed WPBs. In heterozygous VWF-T441N, these abnormalities are partially rescued by VWFpp derived from the normal allele. The observed rescue effect provides further insights for understanding the phenotypic heterogeneity of VWFpp variants.