Functional Characterization of the VWF p.Cys2163Tyr Variant Reveals Impaired Secretion and Intracellular Processing.

Von Willebrand disease (VWD) is the most common inherited bleeding disorder, yet the contribution of specific VWF domains to its pathogenesis remains incompletely understood. In particular, the role of the D4 domain in VWF secretion, intracellular maturation, and multimer formation has not been fully elucidated. Here, we investigated the functional impact of a heterozygous p.Cys2163Tyr variant located in the D4 domain, identified in a patient with a severe bleeding phenotype, using clinical evaluation, genetic analysis, family studies, and in vitro expression assays. Laboratory testing revealed markedly reduced VWF:Ag (6.8 IU/dL), VWF:GPIbR (0.1 IU/dL), and FVIII:C levels, indicating a severe VWD phenotype in the proband. Plasma VWF multimer analysis showed a markedly reduced overall VWF signal with an almost complete absence of high-molecular-weight multimers, supporting classification of the phenotype as severe type 2A VWD. The heterozygous c.6488G>A (p.Cys2163Tyr) variant was also present in asymptomatic family members, indicating incomplete segregation with the severe phenotype and suggesting that this variant alone is insufficient to explain the proband's disease severity. Notably, the proband's mother exhibited mildly reduced VWF levels in the absence of this variant, suggesting the possible contribution of an additional unidentified defect or modifier affecting the maternal allele. In vitro expression demonstrated preserved intracellular VWF antigen, markedly reduced secretion of mutant VWF, and loss of high-molecular-weight VWF multimers. Together, these findings indicate that VWF p.Cys2163Tyr is a functionally deleterious variant that markedly impairs VWF secretion and high-molecular-weight multimer formation in vitro. However, the incomplete segregation observed in the family suggests that this heterozygous variant alone may not fully account for the proband's severe type 2A VWD phenotype.