Type IIB von Willebrand Disease (vWD) is characterized by the selective loss of large von Willebrand Factor (vWF) multimers from plasma, presumably due to their increased reactivity with platelets and subsequent clearance from the circulation. Using the
Type IIA von Willebrand disease (vWD) is an autosomal dominant bleeding disorder characterized by a qualitative defect in von Willebrand factor (vWF). A number of missense mutations responsible for type IIA vWD have recently been identified. This report examines the type IIA vWD mutations Leu777-->Pro and Ile865-->Thr by expression of recombinant vWF containing mutant and wild-type (WT) sequences. Recombinant vWF containing the L777P mutation (vWFL777P) showed markedly impaired secretion compared with that for wild-type vWF (vWFWT) after DNA transfection into mammalian cells. Multimer analysis of secreted vWFL777P showed predominantly low molecular weight forms. In contrast, recombinant vWF containing the I865T mutation (vWFI865T) was processed in a pattern similar to vWFWT, with secretion of the full spectrum of vWF multimers. Thus, L777P and I865T are subclassified as type IIA group I and group II mutations, respectively. Analysis of platelet vWF from a patient heterozygous for the L777P mutation shows reduced large vWF multimers in a pattern similar to plasma, consistent with the intracellular transport defect predicted for a group I mutation. An increase in the proportion of high molecular weight multimers observed in type IIA vWD patient plasma, after renal transplantation from a normal donor, suggests that the kidney endothelium may be a major source of plasma vWF.
An animal model for human type I von Willebrand disease (vWD) has been previously described in the inbred mouse strain RIIIS/J. Murine vWD is characterized by a prolonged bleeding time, normal von Willebrand factor (vWF) multimer distribution, autosomal dominant inheritance, and proportionately decreased plasma vWF antigen, ristocetin cofactor, and factor VIII (FVIII) activities. To study the molecular genetics of murine vWD, a portion of the vWF gene surrounding exon 28 was cloned, sequenced, and used to develop two informative DNA sequence polymorphisms for rapid genotyping by DNA polymerase chain reaction. RIIIS/J mice were crossed with PWK/Ph mice, an inbred line of Mus musculus musculus, and the F1 progeny backcrossed to the parental PWK/Ph strain. vWF antigen levels in F1 mice were not significantly different from the parental RIIIS/J strain but were markedly decreased compared with the parental PWK/Ph mice. Genetic linkage analysis of 104 backcross progeny showed no correlation between vWF antigen level and vWF genotype. These data indicate that murine vWD is caused by a defect at a novel genetic locus, distinct from the murine vWF gene. The distribution of vWF antigen levels among backcross progeny suggests the presence of one major dominant vWD gene in the RIIIS/J mouse with possible modifying contributions from one or more additional minor loci. These observations may provide new insights into the molecular basis and variable expressivity of human vWD.
An animal model for human type I von Willebrand disease (vWD) has been previously described in the inbred mouse strain RIIIS/J. Murine vWD is characterized by a prolonged bleeding time, normal von Willebrand factor (vWF) multimer distribution, autosomal dominant inheritance, and proportionately decreased plasma vWF antigen, ristocetin cofactor, and factor VIII (FVIII) activities. To study the molecular genetics of murine vWD, a portion of the vWF gene surrounding exon 28 was cloned, sequenced, and used to develop two informative DNA sequence polymorphisms for rapid genotyping by DNA polymerase chain reaction. RIIIS/J mice were crossed with PWK/Ph mice, an inbred line of Mus musculus musculus, and the F1 progeny backcrossed to the parental PWK/Ph strain. vWF antigen levels in F1 mice were not significantly different from the parental RIIIS/J strain but were markedly decreased compared with the parental PWK/Ph mice. Genetic linkage analysis of 104 backcross progeny showed no correlation between vWF antigen level and vWF genotype. These data indicate that murine vWD is caused by a defect at a novel genetic locus, distinct from the murine vWF gene. The distribution of vWF antigen levels among backcross progeny suggests the presence of one major dominant vWD gene in the RIIIS/J mouse with possible modifying contributions from one or more additional minor loci. These observations may provide new insights into the molecular basis and variable expressivity of human vWD.
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