Background and Objectives: Von Willebrand factor (VWF) is an essential component of hemostasis. It is known that multimer size and the amount of circulating VWF impact its hemostatic function. Genetic factors play a major role in regulating VWF synthesis and clearance and are reported to contribute to 66% of the variation in plasma VWF antigen level. We have analyzed 78 SNPs distributed throughout the VWF gene and haplotypes constructed from those SNPs for an association with VWF antigen level in 7,856 subjects of European descent in the Atherosclerosis Risk in Communities cohort. Methods and Results: Blood was drawn after an 8 hour fasting period and VWF antigen level was determined by a commercial ELISA kit. All subjects underwent analysis for 78 SNPs available from Affymetrix 6.0 chip in the region encompassing the VWF gene. We used the fastPHASE 1.2 program to resolve haplotypes from the unphased SNP genotype data. Using Haploview we determined SNPs in strong linkage disequilibrium (LD), their co-segregation rates and underlying haplotypes. Linear models were used to evaluate the association of actual or log VWF antigen levels with each SNP and haplotypes derived from those SNPs. Eighteen (16 are intronic) of the 78 SNPs were found to significantly associate with levels of VWF antigen (Table 1). All are clustered in a 50 kb region of the VWF gene in spite of the fact that the 78 SNPs studied are distributed throughout the VWF gene without apparent clusters. All 78 SNPs are co-segregated in 9 LD haplotype blocks, but a majority of positive SNPs (88.9%) are located in Block 5 and 6, which significantly correlate with VWF antigen level. The O blood type contributes to ~ 15% of variation in VWF antigen levels. The association for SNPs and Haplotypes grows stronger after ABO effects are removed. Among non-genomic factors, age and BMI are the most significant factors and contribute to 4.8% and 1.4% of variation in VWF antigen level. Conclusions: We have found a strongly positive association between VWF level and SNPs and haplotypes from a strikingly concentrated region of the VWF gene. This region encodes the D2, D' and D3 domains of VWF, including the propeptide and multimerization and Factor VIII binding sites. The physiological significance of these clustered SNPs on VWF synthesis or clearance remains to be further investigated. Our data suggest that this region plays an important role in regulating VWF antigen level and support the idea that combinations of SNPs in LD can provide additive affects.
