Von Willebrand Disease Genetic Subtyping, Type 2 and Platelet Type

Molecular test to confirm a phenotypic diagnosis of VWD type 2A

Molecular test to distinguish VWD type 2B from PT-VWD

Molecular test to confirm a phenotypic diagnosis of VWD type 2M

Molecular test to distinguish VWD type 2N from hemophilia A

Molecular test to distinguish VWD type 2B from PT-VWD

Related Subclassification Tests
  • Recommended panel to subclassify VWD when high suspicion for VWD exists
  • Contains VWF multimers, factor VIII activity, VWF antigen, and VWF activity (ristocetin cofactor)
  • Multimeric testing is performed when ristocetin cofactor, VWF antigen, or factor VIII activity is low or if the ratio of ristocetin cofactor to VWF antigen is <0.7

Order to assist with diagnosis and subclassification of inherited or acquired von Willebrand disease in conjunction with factor VIII activity, VWF antigen, and VWF activity

  • Not recommended except in suspected cases of acquired VWD or high suspicion of VWD
  • Contains VW factor multimeric analysis, factor VIII activity, VWF antigen, VWF activity (ristocetin cofactor)

Von Willebrand disease (VWD) is the most common inherited bleeding disorder and is classified into three major types, type 1, type 2, and type 3.  After diagnosis, subtyping may be indicated. Analysis of von Willebrand factor (VWF) multimers using a qualitative assay may help determine the type, but additional molecular genetic testing may be required to distinguish among certain types and subtypes. These genetic tests may be used to evaluate family members of individuals with known variants or confirm a phenotypic diagnosis of VWD types 2A, 2B, 2M, 2N, or platelet type, helping to distinguish type 2N from mild hemophilia A, and type 2B from platelet type VWD [PT-VWD]. An accurate phenotypic diagnosis helps guide therapeutic decision-making.

Disease Overview

Incidence

  • VWD type 2: 1-5/10,000 
  • Platelet-type VWD: <1/1,000,000 

Symptoms

Patients with VWD may demonstrate the following :

  • Mucocutaneous bleeding after brushing or flossing teeth
  • Unexplained bruising
  • Prolonged repeated nosebleeds
  • Menorrhagia
  • Prolonged bleeding following childbirth, trauma, or surgery

For clinical characteristics of subtypes, see table.

Genetics

Genes

  • Type 2: VWF
  • Platelet type: GP1BA

Inheritance

  • Autosomal dominant: most type 2A cases, types 2B and 2M, and PT-VWD  
  • Autosomal recessive: types 2N, 20% of type 2A cases

Penetrance

Autosomal dominant types 2A, 2B, and 2M

  • Incomplete penetrance when VWF:Ag and VWF:RCo levels are 25-50 IU/dL 
  • Full penetrance is expected when VWF:Ag and VWF:RCo levels are <25 IU/dL 

Structure/Function

VWF is a large multimeric glycoprotein that plays a critical role in hemostasis. VWF binds factor VIII to protect it from premature degradation, which causes platelet recruitment via the GP1BA receptor and facilitates clot formation.

Variants

GP1BA variants

  • c.746 G>T (p.Gly249Val)
  • c.746 G>A (p.Gly249Ser)
  • c. 763A>G (p.Met255Val)
  • c. 1306del27 (p.436_444 del 9)

Test Interpretation

Sensitivity/Specificity

  • Clinical sensitivity
    • 80% for VWD types 2B and 2M
    • 99% for 2A
    • Unknown for other VWD subtypes
  • Analytical specificity and sensitivity: 99% for type 2

Results

Test Result Variant(s) Detected Interpretive Data

Type 2A (VWF) Sequencing

Positive

1 pathogenic type 2A VWF gene variant detected

Individual may be affected if the variant is dominant

If the variant is recessive, individual is at least a carrier of VWD

Negative

No pathogenic VWF gene variant detected

n/a

Uncertain

1 variant of uncertain significance was detected

Significance unknown

Type 2B (VWF) Sequencing

Positive

1 pathogenic variant detected

Individual is at risk to be affected with type 2B VWD

Negative

Negative

Individual may still be affected with VWD if an undetected pathogenic variant is present

Uncertain

1 variant of uncertain significance was detected

Significance unknown

Type 2M (VWF) Sequencing

Positive

1 pathogenic variant detected

Individual is at risk to be affected with type 2M VWD

Negative

Negative

Individual may still be affected with VWD if an undetected pathogenic variant is present

Uncertain

1 variant of uncertain significance was detected

Significance unknown

Type 2N (VWF) Sequencing

Positive

2 pathogenic variants detected

Individual is predicted to be affected with VWD

1 pathogenic VWF gene variant detected

Individual is at least a carrier and may be affected if an undetected VWF variant is present

Negative

No pathogenic variants detected

Individual appears to be neither a carrier of or affected with type 2N VWD

A negative result for type 2N sequencing would be expected in patients with hemophilia A

GP1BA Variant Detection

Positive

1 pathogenic variant detected

Individual predicted to be affected with PT-VWD

Negative

Negative for 4 variants tested in the GP1BA gene

Risk for PT-VWD is reduced, but not eliminated

Individual may have a rare GP1BA variant

Limitations

  • A negative result does not eliminate the possibility of VWD, as undetected pathogenic variant(s) may be present in one of the unsequenced exons, a noncoding region, or the promoter
  • VWF sequencing may identify sequence variants with uncertain clinical significance
  • VWF variants, other than those in exons tested, will not be detected
  • Large VWF deletions/duplications will not be detected
  • No GP1BA variants, other than the four targeted, are detected by analysis for PT-VWD
  • Rare diagnostic errors may occur due to primer-site variants
Clinical Characteristics of Subtypes
Type Defect Clinical Presentation Treatment

Type 1: ~30% of cases

Partial deficiency of VWF

Mild mucocutaneous bleeding

Desmopressin or VWF/FVIII clotting factor concentrates; usually only needed for surgery or major trauma

Type 2: ~60% of cases

Subtype frequency in the Caucasian population: 2A>2M>2N>2B

Structurally or functionally abnormal VWF

Highly variable

  • 2A
    • Mild to moderate mucocutaneous bleeding
    • May have thrombocytopenia
  • 2B
    • Mild to moderate mucocutaneous bleeding
    • Thrombocytopenia may be present
    • Enhanced ability of VWF to bind platelet receptor GP1BA, causes removal of the platelet/VWF complex
  • 2M
    • Mild to moderate mucocutaneous bleeding
    • Bleeding episodes may be severe, especially in cases of very low or absent VWF:RCo
  • 2N: symptoms are similar to hemophilia A, but with predominant mucocutaneous bleeding
  • 2A: VWF/FVIII clotting factor concentrates; responsiveness to desmopressin variable; treatment for severe bleeding episodes may require clotting factor concentrates
  • 2B: clotting factor concentrates needed to treat severe bleeding;  desmopressin therapy may worsen the thrombocytopenia
  • 2M: clotting factor concentrates needed; response to desmopressin is usually very poor; may require clotting factor concentrates
  • 2N: desmopressin for minor bleeding; severe bleeding requires concentrate with both VWF and factor VIII

Type 3: <10% of cases

Complete absence of VWF

Severe mucocutaneous and musculoskeletal bleeding

Requires repeated infusions of clotting factor concentrates

PT-VWD or pseudo VWD

Abnormal high-affinity interaction between platelet glycoprotein Ib/V/IX complex and VWF

Caused by GPIBA variants

Often indistinguishable from VWD type 2B

 

VWF:RCo: VWF to ristocetin cofactor, a measure of the ability of VWF to agglutinate platelets in response to initiation by the antibiotic ristocetin.

Source: GeneReviews, 2017 

    References