Van gen naar ziekte; van mutaties in het Von Willebrand-factorgen naar hemorragische diathese en trombocytopenie

The broad spectrum of both clinical presentation and severity of bleeding complications observed in Von Willebrand's disease is the result of a high number of mutations. Most are clustered in specific functional domains, leading to quantitative (types 1 and 3) or qualitative (types 2: A, B, M and N or Normandy) defects in the Von Willebrand factor (VWF) protein. Unlike all the other subtypes, type 2B is the result of a 'gain of function' phenotype characterised by an enhanced binding affinity of the VWF protein to the glycoprotein (Gp) Ib complex of platelets, which causes the platelets to disappear from the circulation, resulting in thrombocytopenia. The thrombocytopenia can be triggered by stress or desmopressin, among other causes, and therefore the latter is contraindicated. Type 2B is characterised in vitro by enhanced platelet agglutination with ristocetin. All 2B mutations are located in exon 28, in a small region encoding the Gp-Ib binding site within the A1 domain of VWF. These mutations inactivate a regulatory function of the A1 domain which facilitates Gp-Ib binding.