Dominant von Willebrand disease type 2M and 2U are variable expressions of one distinct disease entity caused by loss-of-function mutations in the A1 domain of the von Willebrand factor gene.

A complete set of laboratory investigations, including bleeding time, PFA-100 closure time, factor VIII coagulant activity (FVIII:C), von Willebrand factor (VWF) ristocetin cofactor activity (RCo), collagen binding (CB) and antigen concentration (Ag), ristocetin-induced platelet aggregation (RIPA) and multimeric analysis of VWF in low and medium SDS-agarose resolution gels, is warranted to diagnose and classify all variants of von Willebrand disease (VWD). VWD type 2M and 2U are typically characterized by decreased RIPA and a poor response of VWF:RCo to desmopressin (DDAVP), but normal VWF:CB and good responses of VWF:CB, VWF:Ag and FVIII:C to DDAVP. VWF multimeric analysis in patients with VWD 2M and 2U show relative decreases in large VWF multimers with less resolved triplet structure of each of the multimeric bands in low-, medium- or high-resolution gels. VWD type 2M or 2U are caused by a loss-of-function mutation in the A1 domain. The laboratory manifestations and molecular defects in the A1 domain causing VWD type 2M and 2U are clearly distinct from all variants of type 1 VWD and also from all other variants [VWD type 2A, 2B, 2E (IIE) and 2C (IIC)].