Upstream Mechanisms Causing Type 1C Von Willebrand Disease (VWD): Contribution Of Defective Von Willebrand Factor (VWF) Multimerization, Regulated Storage, and Secretion

One mechanism causing type 1 VWD is the reduced survival of VWF in plasma (type 1C VWD), characterized by markedly decreased VWF:Ag and VWF half-life, essentially normal multimers, increased ratio of VWF propeptide (VWFpp) to VWF:Ag, robust response to DDAVP, and normal ratios of VWF:CB, FVIII, or VWF:RCo to VWF:Ag. We enrolled 502 index cases with a pre-existing diagnosis of type 1 VWD through the Zimmerman Program for the Molecular and Clinical Biology of VWD. We confirmed 262 of the index cases as type 1 VWD (VWF:Ag or VWF:RCo ≤ 40 IU/dL). Of these, 58 met the criteria for type 1C VWD with VWFpp/VWF:Ag ≥ 3 and VWF:Ag ≤ 30 IU/dL. Sequence variations were identified in the VWF D3, A1, A2, and D4 domains. Little is known regarding the mechanisms causing type 1C VWD, but it has been assumed that VWF undergoes normal intracellular processing and secretion with rapid clearance upon release into plasma. We hypothesized that defective intracellular processing may contribute to the type 1C phenotype. We studied 10 type 1C variants including C1130Y, W1144G, R1205H, N1231S, R1315C, V1411E, R1527W, N2041S, Y2160C, and S2179F. Variants were expressed alone (homozygously) or with wild-type (WT) VWF (heterozygously) in HEK293T cells and VWF secretion, multimer structure, and binding to collagen (types III and VI), GPIb-alpha, and FVIII was analyzed. To assess regulated storage, variants were expressed homozygously in HEK293 cells where WT VWF forms elongated pseudo-Weibel-Palade bodies (pWPB). Five variants (C1130Y, R1315C, V1411E, N2041S, Y2160C) had severely decreased secretion and defective multimerization when homozygously expressed. These variants did not form pWPB, but appeared to co-localize with the endoplasmic reticulum, consistent with the severely impaired secretion. One variant, W1144G, had mildly reduced secretion, formed only dimeric VWF, and unexpectedly did not form pWPB. These multimer defective variants demonstrated decreased collagen binding and GPIb-alpha binding as would be predicted. The remaining variants (R1205H, N1231S, R1527W, S2179F) were normally secreted, multimerized, stored in pWPB, and had normal binding to FVIII, collagen, and GPIb-alpha. Interestingly, FVIII binding to homozygous VWF D3 variants C1130Y and W1144G was substantially reduced. This result is not entirely unexpected as the FVIII binding region in VWF has been mapped to the D’-D3 region. Co-expression with WT VWF essentially corrected defective secretion, although some variants still had moderately reduced secretion. Multimer structure appeared normal for all heterozygous variants, although staining which discriminates between variant and WT alleles revealed that for some variants, little variant VWF was actually expressed when transfected at a 1:1 ratio with WT. In sum, when variants were homozygously expressed, we observed a constellation of processing and functional defects. Only R1205H, N1231S, R1527W, and S2179F variants demonstrated normal processing and function. Heterozygous expression (consistent with patients) corrected most of the observed defects, although reduced secretion persisted for a subset of variants. We can conclude that while reduced plasma survival of VWF is a major determinant of the type 1C phenotype, additional upstream processing defects may contribute to the severity of the overall VWD phenotype.