Both genetic and environmental influences contribute to the wide variation in plasma von Willebrand factor (VWF) levels observed in humans. Inbred mouse strains also have highly variable plasma VWF levels, providing a convenient model in which to study genetic modifiers of VWF. Previously, we identified a major modifier of VWF levels in the mouse (Mvwf1) as a regulatory mutation in murine Galgt2. We now report the identification of an additional murine VWF modifier

Summary. Background: The genetic factors responsible for the wide variation in plasma von Willebrand factor (VWF) levels observed among individuals are largely unknown, although these genes are also likely to contribute to variability in the severity of von Willebrand disease (VWD) and other bleeding and thrombotic disorders. We have previously mapped two genes contributing to the regulation of plasma VWF levels in mice (Mvwf1 on chromosome 11 and Mvwf2 on chromosome 6). Objective: To identify additional quantitative trait loci (QTL) contributing to the genetic regulation of murine plasma VWF levels. Methods: To map genetic loci contributing to the > 7-fold difference in plasma VWF levels between two mouse strains (A/J and CASA/RkJ), high-density individual genotyping and R/qtl analyses were applied to a previously generated set of 200 F2 mice obtained from an intercross of these two inbred lines. Results: Genomic loci for two additional candidate VWF modifier genes were identified: Mvwf3 on chromosome 4 and Mvwf4 on chromosome 13. These loci demonstrate primarily epistatic effects when co-inherited with two CASA/ RkJ Vwf alleles, although Mvwf4 may also exert a small, independent, additive effect. Conclusions: Mvwf3 and Mvwf4, combined with the effect of Mvwf2, explain 45% of the genetic variation in plasma VWF level among the A/J and CASA/RkJ strains. Mvwf3 and Mvwf4 exhibit homology of synteny to three human chromosomal segments (on chromosomes 1, 5 and 6) previously reported by the Genetic Analysis of Idiopathic Thrombophilia (GAIT) study, suggesting that orthologs of Mvwf3 and Mvwf4 may also encode important VWF modifier genes in humans.

Type 1 von Willebrand disease (VWD) is the most common inherited human bleeding disorder. However, diagnosis is complicated by incomplete penetrance and variable expressivity, as well as wide variation in von Willebrand factor (VWF) levels among the normal population. Previous work has exploited the highly variable plasma VWF levels among inbred strains of mice to identify 2 major regulators, Mvwf1 and Mvwf2 (modifier of VWF).Mvwf1 is a glycosyltransferase and Mvwf2 is a natural variant in Vwf that alters biosynthesis. We report the identification of an additional alteration at the Vwf locus (Mvwf5), as well as 2 loci unlinked to Vwf (Mvwf6-7) using a backcross approach with the inbred mouse strains WSB/EiJ and C57BL/6J. Through positional cloning, we show that Mvwf5 is a cis-regulatory variant that alters Vwf mRNA expression. A similar mechanism could potentially explain a significant percentage of human VWD cases, especially those with no detectable mutation in the VWF coding sequence. Mvwf6 displays conservation of synteny with potential VWF modifier loci identified in human pedigrees, suggesting that its ortholog may modify VWF in human populations. Introductionvon Willebrand factor (VWF) is a central component of hemostasis, serving as the adhesive link between platelets and the injured blood vessel wall, as well as the carrier for factor VIII. Deficiencies in VWF result in von Willebrand disease (VWD), the most common inherited bleeding disorder in humans. Elevated VWF levels may also be an important risk factor for thrombosis, both through a direct role in platelet adhesion, 1 as well as indirectly by causing elevated levels of factor VIII. [2][3][4] Diagnosis of VWD is elusive in many cases because of its variable expressivity and incomplete penetrance 5 and the nonspecific nature of bleeding symptoms. 6 VWF plasma protein levels also display a broad distribution in the normal human population. Thus, it is often difficult to determine whether a person has VWD and is at risk for pathologic hemorrhage or simply has VWF levels in the low range of normal.Levels of plasma VWF have been shown to be largely determined by genetic factors, with estimates of heritability in humans ranging from 25% to 32% by pedigree analysis, 7,8 to 66% to 75% in twin studies. 9,10 ABO blood group is responsible for one-third of the genetic variability in VWF plasma levels. 11 However, the loci responsible for the remaining two-thirds of this genetic component are unknown. Recent evidence from European and Canadian cooperative studies on type 1 VWD have found that disease diagnosis does not segregate with VWF genotype in approximately 50% of families, supporting the existence of additional genetic factors. [12][13][14][15][16] Laboratory mice display wide variation in VWF levels with 65% heritability in a cross between the strains A/J and CASA/RkJ, 17 strikingly similar to the estimates for humans derived from twin studies. 9,10 This variability among inbred mouse strains has been used to identify genetic loci modifying VWF level...

Summary. Functional deficiency or absence of the human von Willebrand factor (VWF)-cleaving protease (VWF-cp), recently termed ADAMTS-13, has been shown to cause acquired and congenital thrombotic thrombocytopenic purpura (TTP), respectively. As a first step towards developing a small animal model of TTP, we have cloned the complete (non-truncated) murine Adamts-13 gene from BALB/c mice liver poly A + mRNA. Murine ADAMTS-13 is a 1426-amino-acid protein with a high homology and similar structural organization to the human ortholog. Transient expression of the murine Adamts-13 cDNA in HEK 293 cells yielded a protein with a molecular weight of approximately 180 kDa which degraded recombinant murine VWF (rVWF) in a dose-dependent manner. The cleavage products of murine rVWF had the expected size of 140 and 170 kDa. Murine ADAMTS-13 was inhibited by EDTA and the plasma from a TTP patient.