In this video lecture, Dr. Jill Johnsen discusses:
- The genetic basis and inheritance patterns of von Willebrand disease, including the molecular variants responsible for different VWD subtypes.
- The clinical applications, strengths, and limitations of VWF genetic testing for diagnosis, classification, and family counseling.
- Challenges in VWF variant interpretation, including incomplete penetrance, variable expressivity, pseudogene interference, and variants of uncertain significance.
Dr. Jill Johnsen is an adult hematologist and scientist with expertise in classical hematology. She is a Professor of Medicine at the University of Washington and Associate Director of the Bleeding Disorders Lab at Bloodworks Northwest, a laboratory that provides clinical genotyping services for VWD and hemophilia throughout the U.S. Dr. Johnsen serves as co-chair of the ClinGen Variant Curation Expert Panel for VWD, and she leads a research program focused on genetics, von Willebrand factor, factor VIII, factor IX, and bleeding in females.
(Video Lecture Summary)
Introduction
Dr. Jill Johnsen reviews the principles and clinical applications of genetic testing for von Willebrand disease (VWD). She explains how advances in sequencing technologies have improved the ability to identify disease-causing variants while also introducing new challenges in variant interpretation. Throughout the lecture, she emphasizes that genetic testing complements, rather than replaces, careful clinical and laboratory evaluation, and that accurate interpretation requires an understanding of VWD biology, inheritance patterns, and the technical complexities of sequencing the VWF gene.
The Genetic Basis of von Willebrand Disease
VWD is an inherited bleeding disorder caused by variants in the VWF gene that affect either the quantity or function of von Willebrand factor.
Dr. Johnsen reviews the major disease classifications. Type 1 VWD results from a partial quantitative deficiency of VWF, whereas type 3 reflects near-complete absence of the protein. The type 2 variants represent qualitative defects that impair specific functions of VWF, including multimer formation, platelet binding, collagen binding, or factor VIII binding.
She explains that different forms of VWD arise from different classes of genetic variation. Single nucleotide variants are the most common and can cause any VWD subtype. Insertions, deletions, and larger structural variants are more frequently associated with type 1 and type 3 disease, although they may occasionally occur in type 2 VWD.
Inheritance Patterns Are More Complex Than They Appear
Understanding inheritance is an essential component of genetic evaluation.
Dr. Johnsen explains that most type 1 and type 2 forms of VWD are inherited in an autosomal dominant pattern, whereas type 3 and most type 2N disease are inherited recessively. However, these patterns are not absolute. Some forms of type 1 and type 2 disease demonstrate alternative inheritance patterns depending on the specific genetic variant.
She also discusses two important concepts that complicate pedigree interpretation. Incomplete penetrance means that some individuals who inherit a disease-causing variant may never develop clinically apparent VWD, giving the impression that the disease skips generations. Variable expressivity refers to the wide range of bleeding severity observed among relatives carrying the same pathogenic variant, making phenotype difficult to predict from genotype alone.
Linking VWF Biology to Disease Mechanisms
To understand how genetic variants produce disease, Dr. Johnsen reviews the normal biosynthesis of VWF.
VWF is synthesized primarily by endothelial cells and, to a lesser extent, megakaryocytes. After transcription, the protein undergoes processing, multimerization, intracellular storage within Weibel-Palade bodies, secretion into the circulation, and subsequent regulation through ADAMTS13-mediated proteolysis and physiologic clearance.
Genetic variants can disrupt virtually any step in this pathway. Defects in transcription generally reduce VWF production, while abnormalities in multimerization, intracellular trafficking, secretion, proteolysis, or clearance produce distinct forms of VWD. For example, accelerated ADAMTS13 cleavage contributes to type 2A disease, whereas increased VWF clearance produces type 1C VWD.
Why Laboratory Phenotypes Can Be Difficult to Interpret
Dr. Johnsen emphasizes that VWF levels are influenced by many factors beyond genetics. Although inherited variation accounts for much of the variability in normal VWF levels, pregnancy, inflammation, stress, aging, and even time of day can substantially alter circulating VWF concentrations. Pregnancy, in particular, produces marked increases in VWF levels, although the degree of increase varies among VWD subtypes.
Additional genetic modifiers further complicate interpretation. ABO blood group is the best recognized example, but several other genes also influence circulating VWF levels. These modifiers appear to have their greatest effect among individuals with low-normal VWF levels, whereas pathogenic VWF variants become increasingly important at lower levels.
Diagnostic Yield of Genetic Testing
The likelihood of identifying a disease-causing variant depends on the VWD subtype. Dr. Johnsen notes that genetic testing has a very high diagnostic yield for type 2 disease, with pathogenic variants identified in more than 90% of patients. Detection rates remain high for severe type 1 disease but decline substantially among patients with VWF levels in the 30 to 50 IU/dL range, reflecting the greater contribution of non-genetic modifiers and the possibility that additional genetic mechanisms remain undiscovered.
She also explains that type 2 variants often cluster within functional domains responsible for the affected biologic activity, whereas type 1 and type 3 variants may occur throughout the gene.
Technical Challenges of Sequencing the VWF Gene
The VWF gene presents several unique technical challenges.
It spans more than 170 kilobases and contains 52 exons, making comprehensive sequencing considerably more complex than many other disease genes. Historically, exon 28 received the greatest attention because it contains functionally important domains involved in platelet binding and ADAMTS13 cleavage.
A major challenge is the presence of a highly homologous partial pseudogene on chromosome 22. Because portions of the pseudogene closely resemble the authentic VWF gene, sequencing methods may inadvertently analyze the wrong genomic region or incorrectly assign variants. In addition, gene conversion events between the pseudogene and VWF can introduce genuine pathogenic variation.
For these reasons, Dr. Johnsen emphasizes that VWF sequencing should be performed in laboratories with specific expertise in VWD genetics.
Interpreting Genetic Variants
Finding a variant is only the first step; determining its clinical significance is often much more difficult. Dr. Johnsen reviews the ACMG framework for classifying variants as pathogenic, likely pathogenic, benign, likely benign, or variants of uncertain significance (VUS). More than half of currently cataloged VWF variants remain classified as VUS because insufficient evidence exists to determine whether they cause disease.
She also highlights the importance of recognizing benign variants with clinical implications. One example is the common p.D1472H variant, which reduces ristocetin-dependent laboratory activity without impairing VWF function or causing bleeding. Identifying this variant can prevent misdiagnosis of VWD, particularly in individuals of African ancestry, where it is more common.
Clinical Applications of Genetic Testing
Dr. Johnsen outlines several situations in which genetic testing provides important clinical value. Genotyping helps distinguish type 1 from type 2 disease, accurately classify type 2 subtypes, differentiate VWD from disorders with similar laboratory findings, and distinguish inherited VWD from acquired von Willebrand syndrome. In patients with type 3 VWD, genetic testing may also identify individuals at increased risk of developing inhibitors based on the underlying mutation.
Beyond diagnosis, genetic testing supports reproductive counseling, cascade testing of family members, and earlier identification of affected relatives. Studies have shown that incorporating genetic information into well-characterized patient cohorts can lead to reclassification of VWD diagnoses in a substantial proportion of patients, improving disease classification and subsequent management.
Conclusion
Dr. Johnsen concludes that genetic testing has become an increasingly valuable tool in the diagnosis and classification of von Willebrand disease. While sequencing identifies pathogenic variants in most patients with well-defined disease, successful interpretation requires integration of molecular findings with clinical history, laboratory phenotype, inheritance patterns, and expert variant analysis. As sequencing technologies continue to improve and additional evidence accumulates, genetic testing is expected to play an increasingly important role in refining diagnosis, guiding management, and advancing understanding of VWD biology.