Type 1 VWD is caused by a partial quantitative deficiency of VWF and represents approximately 75% of VWD cases. It is the most clinically heterogeneous type, with patients having a mild to moderate bleeding phenotype.11 Bleeding in type 1 VWD results from a decrease in the concentration of VWF. The VWF function is normal without a significant abnormality in the platelet, collagen, or FVIII binding sites or a significant decrease in HMW multimers. Functional assays of VWF, such as VWF ristocetin cofactor (VWF:RCo) or VWF activity (VWF:Act) (see section on Laboratory Testing for further details), are proportionally decreased relative to the VWF antigen level (VWF:Ag), and the ratio of functional activity as compared with the VWF level is normal (ie, VWF:RCo/VWF:Ag ratio is > 0.6). As noted, VWF mutations are identified in only 65% of type 1 VWD cases and have incomplete penetrance and variable expressivity.10 Approximately 70% of mutations identified are missense mutations. Missense mutations may affect VWF levels by affecting any part of the biosynthetic pathway, including trafficking, storage, secretion, and/or clearance of VWF.
Increased VWF clearance is a well-described mechanism for type 1 VWD, known as type 1C. These patients will typically have very low VWF levels, an increased VWF propeptide to antigen ratio (VWFpp/VWF:Ag), and a marked but short-lived response to DDAVP, limiting DDAVP’s clinical applicability.12 On the other hand, the half-life of VWF/FVIII concentrates is normal in these individuals. Type 1C VWD is caused by missense mutations which occur mainly in the D3 domain and reduce the half-life of VWF up to 15-fold. R1205H, known as the “Vicenza” variant, is the most common and severe as well as the best characterized of these mutations.13
Accounting for approximately 25% of VWD cases, type 2 VWD is characterized by a qualitative deficiency of VWF activity and is further subcategorized based on the mechanism of VWF dysfunction. Type 2A, 2B, and 2M affect VWF–platelet interactions by way of loss of HMW multimers, a gain of function of the GPIbα binding site, or a loss of function of the same site, respectively. On the other hand, type 2N is caused by defective VWF binding to FVIII. Type 2 VWD is often suspected when investigations demonstrate a function-antigen discordance: the VWF:RCo or VWF:Act is decreased disproportionately to the decrease in VWF:Ag, and the VWF:RCo/VWF:Ag ratio is less than 0.6.
Type 2A VWD is the most common type 2 variant. It is characterized by disproportionately low functional activity compared to antigen level (ie, VWF:RCo/VWF:ag ratio is < 0.6) and a loss of HMW and sometimes intermediate molecular weight (IMW) multimers. Ristocetin-induced platelet agglutination (RIPA) will be decreased with standard doses of ristocetin and absent with low doses. Type 2A VWD is usually inherited as an autosomal dominant trait. This subtype encompasses missense mutations that impair dimerization or multimerization of VWF subunits (CK, D1, and D2 domains); disrupt intersubunit disulphide bonds (D3 and D2 domains); enhance susceptibility to ADAMTS13-mediated proteolysis (A2 and A1 domains); or result in intracellular retention of the HMW multimers (D3, A1, and A2 domains).10 The result is VWF that lacks HMW multimers, thereby possessing fewer GPIbα binding sites, and that is less effective in binding platelets.
Type 2B VWD is the result of gain-of-function mutations within the GPIbα binding site of VWF. Generally, the platelet-binding site of VWF within the A1 domain is only exposed once VWF is immobilized on injured collagen and subjected to shear forces, resulting in a conformational change.7 In type 2B VWD, the gain-of-function mutation results in spontaneous binding of VWF to platelets without the need for a VWF-collagen interaction and unfolding of VWF by shear forces. The VWF–platelet interaction selectively depletes the HMW multimers by the unfolding of the A2 domain and increasing ADAMTS13 proteolysis. The increased binding of mutant VWF to platelets also triggers the formation of platelet aggregates, which are removed from circulation resulting in thrombocytopenia. Increases in endogenous VWF seen with acute stressors or pregnancy can worsen thrombocytopenia and increase the risk of bleeding.14 Certain mutations, such as V1316M, alter megakaryocytopoiesis and are characterized by giant platelets with abnormal ultrastructure and further exacerbate the thrombocytopenia.15 The laboratory profile reveals a VWF:RCo/VWF:Ag ratio of < 0.6 and absence of HMW multimers. In contrast to type 2A, platelets will agglutinate with low-dose ristocetin. Missense mutations are highly penetrant dominant and occur in or close to the A1 domain.16
Type 2M VWD is characterized by loss-of-function mutations within the GPIbα binding site of VWF. Phenotypic characteristics include a reduced ratio of VWF:RCo/VWF:Ag of < 0.6 but a normal multimer pattern.17 Missense mutations are reported in the A1 domain affecting the GPIbα-binding site. In very rare instances, mutations in the A3 domain that impair the VWF/collagen interaction have been described.18 These collagen-binding mutations are not included in the last iteration of the ISTH classification in 2006,1 but fit best in the type 2M category. In these cases, VWF:RCo or VWF:Act, which reflect activity at the GPIbα-binding site, may be normal and the diagnosis requires VWF/collagen binding assays (VWF:CB).
Type 2N VWD results from mutations of the FVIII binding site or conformational changes that impair the VWF–FVIII interaction. Most (~80%) missense mutations are located in domains D’ and D3.19 These mutations are autosomal recessive, and affected individuals are either homozygous or compound heterozygous for type 2N/2N or type 1/2N mutations, or compound heterozygous for a missense mutation and a mutation resulting in a null allele (type 2N/3 mutations). The laboratory phenotype is a disproportionate reduction in the FVIII level relative to the VWF level, which may be low or normal. Most cases of type 2N VWD have a normal multimeric profile, but rare cases will demonstrate loss of HMW multimers. Definitive diagnosis requires evidence of reduced FVIII binding to VWF (VWF:FVIIIB) or the identification of causative mutations in the FVIII binding region of the VWF gene.20