Lee A. O'brien

Genetic linkage and association analysis in type 1 von Willebrand disease: results from the Canadian type 1 VWD study.

Summary.  Background: von Willebrand disease (VWD) is the most common bleeding disorder known in humans, with type 1 VWD representing the majority of cases. Unlike the other variant forms of VWD, type 1 disease represents a complex genetic trait, influenced by both genetic and environmental factors. Aim: To evaluate the contribution of the von Willebrand factor (VWF) and ABO blood group loci to the type 1 VWD phenotype, and to assess the potential for locus heterogeneity in this condition, we have performed genetic linkage and association studies on a large, unselected type 1 VWD population. Method: We initially collected samples from 194 Canadian type 1 VWD families for analysis. After the exclusion of families found to have either type 2 or type 3 VWD, and pedigrees with samples from single generations, linkage and association analysis was performed on 155 type 1 VWD families. Results and conclusion: The linkage study has shown a low heterogeneity LOD score of 2.13 with the proportion of families linked to the VWF gene estimated to be 0.41. Linkage was not detected to the ABO locus in this type 1 VWD population. In the family‐based association test, significant association was found between the type 1 VWD phenotype, the quantitative traits, VWF:Ag, VWF:RCo, and FVIII:C and the ABO ‘O’ and ‘A’ alleles and the VWF codon 1584 variant. There was also weak association with the −1185 promoter polymorphism and VWF:Ag, VWF:RCo, and FVIII:C plasma levels. These studies provide further evidence to support the role for genetic loci other than VWF and ABO in the pathogenesis of type 1 VWD.

A novel type 2A (Group II) von Willebrand disease mutation (L1503Q) associated with loss of the highest molecular weight von Willebrand factor multimers

Summary.  Type 2A von Willebrand disease (VWD) is characterized by decreased platelet‐dependent function of von Willebrand factor (VWF) associated with an absence of high‐molecular‐weight multimers. In this study, sequence analysis of the VWF gene from a Type 2A VWD patient showed a novel, heterozygous T→A transversion at nucleotide 4510, resulting in the non‐conservative substitution of L1503Q in the mature VWF subunit. This substitution, which was not found in 55 unrelated normal individuals, was reproduced by in vitro site directed mutagenesis of a full‐length VWF cDNA and was subsequently expressed in COS‐7 cells. The corresponding recombinant mutant VWF protein was partially retained in COS‐7 cells yet the full spectrum of multimers was observed, suggesting that the absence of the highest molecular weight multimers results from increased proteolysis. The recombinant mutant VWF protein was digested with the ADAMTS13 protease from VWF‐depleted plasma and the aberrant VWF multimer pattern was observed. These results suggest that the L1503Q substitution induces a conformational change in the VWF protein, which increases the proteins susceptibility to proteolysis. A three‐dimensional model of the A2 domain demonstrates that the L1503Q mutation and the physiological proteolytic cleavage site for ADAMTS13 (Y1605‐M1606) are localized close together in two adjacent parallel β‐sheets. The mutation L1503Q does not significantly disrupt the conformation of the protein; thus the subtle loss of multimers in this patient may be due to altered interactions with the ADAMTS13 protease.

Theoretical structural explanation for Group I and Group II, type 2A von Willebrand disease mutations

L . A . O ’BR I E N , J . J . SUTHER LAND,* D . F . WEAVER and D . L I LL ICRAP Department of Pathology and Molecular Medicine, Queen’s University, Kingston, Ontario, K7L 3 N6, Canada; *Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3 N6, Canada; and Departments of Medicine and Chemistry, and School of Biomedical Engineering, Dalhousie University, Halifax, Nova Scotia, B3H 4J3, Canada