Leu 697-->Val mutation in mature von Willebrand factor is responsible for type IIB von Willebrand disease
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L Hilbert1, C Gaucher, C de Romeuf
1Laboratoire de Recherche sur l'Hémostase, Centre Régional de Transfusion Sanguine de Lille, France.
Abstract:
Type IIB von Willebrand disease is characterized by the selective loss of high molecular weight von Willebrand factor (vWF) multimers from plasma and enhanced platelet agglutination of platelet-rich-plasma in the presence of low concentrations of ristocetin. We identified, in two related patients, a C-->G transversion resulting in the substitution of Valine for Leucine at position 697 of the mature subunit of vWF. We reproduced this mutation in vWF cDNA and expressed the recombinant protein in Cos-7 cells. The subunit composition and multimeric structure of mutated protein (rvWFLeu697Val) were similar to the wild-type recombinant (WTrvWF). Ristocetin-induced binding of rvWFLeu697Val to platelets was markedly increased in the presence of low doses of ristocetin and slightly increased with botrocetin as compared with that for WTrvWF, whereas collagen binding was not affected by the mutation. These data show that the Leu 697-->Val substitution is not a rare polymorphism but is responsible for the subtype IIB characteristic abnormalities identified in the two affected patients; however, it is not located in the area of vWF (amino acid 540 to amino acid 578) where most of the other type IIB mutations have already been reported.
Insights
A novel mutation in von Willebrand factor (vWF) causes Type IIB von Willebrand disease by increasing platelet aggregation. This finding identifies a new genetic cause for the bleeding disorder.
Area of Science:
- Hematology
- Molecular Biology
- Genetics
Background:
- Type IIB von Willebrand disease involves loss of high molecular weight von Willebrand factor (vWF) multimers.
- This leads to increased platelet agglutination with ristocetin.
Purpose of the Study:
- To investigate the genetic basis of Type IIB von Willebrand disease in two related patients.
- To characterize the functional impact of a novel vWF mutation.
Main Methods:
- Identified a C-->G transversion in vWF cDNA.
- Recreated the mutation in recombinant vWF (rvWFLeu697Val).
- Expressed and analyzed the mutated vWF in Cos-7 cells, assessing platelet and collagen binding.
Main Results:
- The Leu697Val substitution in vWF did not alter subunit composition or multimeric structure.
- Mutated rvWFLeu697Val showed markedly increased ristocetin-induced platelet binding.
- Botrocetin binding was slightly increased, while collagen binding remained unaffected.
Conclusions:
- The Leu697Val substitution is causative for Type IIB von Willebrand disease in the affected patients.
- This mutation, responsible for characteristic IIB abnormalities, is located outside the previously identified mutation hotspot (amino acids 540-578).