Influence of the Variable Number of Tandem Repeat and Kozak Sequence Polymorphisms in Platelet GPIbα and the C807t α₂ Integrin Polymorphism in Type 1 von Willebrand Disease.

Initial adhesion of platelets to the injured vessel wall involves binding of the glycoprotein (GP)Ib-IX-V complex and integrin α₂β₁ to von Willebrand factor (VWF) and collagen, respectively, in the subendothelium. Polymorphisms have been described in the GPIbα and α₂ genes that change the expression of these proteins or alter their configuration such that the efficiency of platelet adhesion is changed. The variable number of tandem repeat (VNTR) size polymorphism of GPIbα may extend the VWF binding domain at variable lengths above the platelet membrane, with variants A, B, C and D having 4, 3, 2 and 1 repeats, respectively; a polymorphism in the Kozak translation consensus sequence for GPIbα appears to predict receptor density. 3 haplotypes of α₂ involving C807T have been correlated with α₂β₁ expression levels. We investigated these polymorphisms in the GPIbα and α₂ integrin genes to evaluate the hypothesis that they influence the efficiency of platelet adhesion in the context of the inherited bleeding disorder, von Willebrand disease (VWD). The study population comprised 47 patients with type 1 VWD and 128 controls. Polymorphism genotyping was performed using restriction enzyme digest analysis of genomic DNA. Frequencies of the GPIbα VNTR alleles were: B=0.05, C=0.81, D=0.14 in the type 1 VWD group and B=0.11, C=0.80, D=0.08 in the controls. These frequencies were not significantly different between the 2 study groups. Genotype frequencies at the VNTR locus were: B/B=0.0, B/C=0.09, B/D=0.02, C/C=0.66, C/D=0.21, D/D=0.02 in the type 1 VWD group and B/B=0.0, B/C=0.20, B/D=0.02, C/C=0.63, C/D=0.13, D/D=0.01 in the controls (P=0.31). C and T Kozak allele frequencies were 0.12 and 0.88, respectively, in the type 1 VWD group and 0.08 and 0.91, respectively, in the controls. These allele frequencies were not significantly different between the 2 groups. Genotype frequencies were: C/C=0.02, C/T=0.20, T/T=0.78 in the type 1 VWD group and C/C=0.01, C/T=0.16, T/T=0.84 in the controls (P=0.61). Frequencies of the α₂ alleles 1, 2 and 3 were: 1=0.38, 2=0.47, 3=0.15 in the type 1 VWD group, and 1=0.39, 2=0.50, 3=0.11 in the controls; genotype frequencies were 1/1=0.07, 1/2=0.48, 1/3=0.15, 2/2=0.17, 2/3=0.11, 3/3=0.02 in the type 1 VWD group and 1/1=0.15, 1/2=0.38, 1/3=0.11, 2/2=0.26, 2/3=0.10, 3/3=0.01 in the controls. Neither the allele frequencies nor the genotype distributions at the α₂ integrin locus were significantly different between the VWD and control groups. In addition, allele frequencies and genotype distributions at all 3 polymorphic sites were similar to those for previously published controls. Thus, we did not observe any associations between the diagnosis of type 1 VWD and the GPIbα VNTR or Kozak polymorphisms or the α₂ integrin polymorphism that determines α₂β₁ receptor density. Although our observation with the α₂ integrin polymorphism is in contrast with that of a recent report that the 807C haplotype is associated with increased bleeding severity scores in type 1 VWD (Kunicki et al., Blood, prepublished online June 29, 2004), our observation with the GPIbα Kozak polymorphism is in accord with this study. Finally, our finding of the 2nd-largest B VNTR allele in the type 1 VWD group at a frequency of ½ that in controls suggests that the lower levels of this larger form of the GPIbα receptor may contribute to an increased bleeding risk in the VWD population.