The B4galnt2 Regulatory Polymorphism, Mvwf1, Causes Low VWF Levels and Segregates in Natural Mouse Populations

We previously identified the cause of low levels of von Willebrand factor (VWF) in the RIIIS/J mouse strain to be a regulatory mutation, Mvwf1, in an N-acetylgalactosaminyltransferase, B4galnt2 (previously designated Galgt2). Mvwf1 causes a tissue-specific switch in B4galnt2 expression from intestinal epithelium to vascular endothelium, resulting in aberrant glycosylation of VWF and accelerated clearance from circulation. We have identified thirteen Mvwf1 inbred mouse strains that share this remarkable tissue-specific switch and a common 97kb haplotype block, including a 30kb region of 2–3% sequence divergence that flanks Exon 1. An RIIIS/J BAC transgene containing the entire Mvwf1 haplotype block and B4galnt2 gene confers vascular gene expression, while C57BL6/J BAC transgenes spanning the homologous region confer “wild-type” vessel(−), intestine(+) gene expression, indicating that one or more tissue-specific regulatory elements sufficient to recapitulate the Mvwf1 tissue-specific switch lie within the genomic region covered by these BACs. A wild-derived recombinant Mvwf1 allele containing the 3′ half of the Mvwf1 haplotype block confers the vessel(−), intestine(+) B4galnt2 expression pattern, placing the regulatory mutation(s) responsible for the Mvwf1 switch well upstream of the proximal promoter region. Sequence analysis of DNA from wild-caught individual mice confirmed the presence of a highly conserved wild mouse Mvwf1 founder allele that likely pre-dates the development of the inbred mouse strains. PCR of genomic DNA from wild caught mice representing M. m. musculus, M. m. domesticus, M. m. castaneus, M. m. molossinus, M. spretus, M. hortulanis, and M. macedonicus revealed that the Mvwf1 allele is common in wild M. m. domesticus populations in North America, Europe, and Africa, with an allele frequency as high as 60% in French mice. Population samples of M. m. domesticus from Cologne, Germany and the Massif Central region of France drastically differ in the frequency of this allele (0% vs. 60%). A significant reduction in microsatellite variability at B4galnt2 in the French population, as measured by the lnRH statistic, suggests a recent, local shift in the frequency of Mvwf1, likely due to a recent change in selective pressure. Analysis of independently trapped wild mice from the Massif Central region of France confirmed that Mvwf1 confers vascular endothelial B4galnt2 expression and causes significantly lower VWF levels (5.5+2.0 vs. 13.3+2.3, p<1x10⁻⁷) in this population. These data support a B4Galnt2 allele-specific survival advantage in wild mice, leading us to speculate that a similar survival advantage could account for the high prevalence of VWD in human populations.