Phenotypic Variability in Carriers of Von Willebrand Factor Truncating Sequence Variants in the Zimmerman Program

Von Willebrand disease (VWD) is caused by quantitative (types 1 and 3) and qualitative (type 2) defects in von Willebrand factor (VWF). VWD type 1 and 2 are autosomal dominant with variable expression while type 3 is autosomal recessive. The majority of patients have quantitative defects in VWF. The clinical phenotype is highly variable. This variability is evident in the normal population as the VWF antigen levels range from 50-240 IU/dl. In order to investigate variability in clinical expression of VWD, we examined type 1 or type 3 VWD index cases and their family members enrolled in the Zimmerman program where a VWF truncating variant wasfound. These variants included nonsense, splice site, deletions and duplications. All subjects had factor VIII (FVIII), VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), and multimer distribution analyzed in a central laboratory. VWF gene sequencing was performed for all index cases. Targeted sequencing was performed for family members to ascertain the presence or absence of sequence variations found in the index case. Bleeding symptoms were quantified using the ISTH bleeding assessment tool and reported as bleeding scores.

The VWF:Ag level was significantly different between all three groups (P<0.0001). As expected, type 3 subjects with two truncating variants had the significantly lower VWF:Ag and higher bleeding scores. There was less variability in VWF:Ag and bleeding score compared to the subjects in the other two groups. In subjects with no truncating variants, the unaffected family members, the average VWF:Ag and bleeding score were similar to values observed in healthy controls. The subjects with one variant were the largest group and the most variable in VWD phenotype. The average VWF:Ag was 45±20, slightly below the cutoff of the normal range but the antigen level in this group ranged from 6-116. Thirty-five percent of subjects had VWF:Ag greater than 50 and in the normal range. Approximately 65% had VWF:Ag below 50; about one half were blood type O. 17% had VWF:Ag levels below 30. Although the bleeding score was only slightly higher than the subjects with no variants (P=0.038), the bleeding score ranged from 0-17. Even within a family, highly variable Ag levels were observed.

Although numbers were small, when only families with type 3 VWD were considered, mean antigens were higher and mean bleeding scores lower for subjects heterozygous for a truncating allele compared to a non-truncating allele. For a series of subjects with identical truncating alleles, those heterozygous subjects from families with a diagnosis of type 3 VWD had lower bleeding scores than heterozygous subjects from families with only type 1 VWD, despite similar antigen levels, suggesting less concern for mild bleeding symptoms in families where some members have severe bleeding.

In summary, carriers of a single VWF truncating allele have a variable phenotype. A wide range of Ag levels and bleeding scores were observed. In 35% of subjects, the Ag level is normal (>50) suggesting the lack of expression from one VWF allele can be compensated by the other VWF allele. The majority of subjects with a single VWF truncating allele had antigen levels typical for type 1 VWD. This suggests that additional environmental or genetic factors are required for symptomaticVWD.