Abstract
While von Willebrand disease (VWD) is the most common inherited bleeding disorder, most patients have quantitative defects in von Willebrand factor (VWF). The qualitative variants, collectively termed type 2 VWD, are less common, but also in general more severe than type 1 VWD. However, despite a common laboratory phenotype of decreased VWF:RCo/VWF:Ag ratio for types 2A, 2B, and 2M VWD, the clinical phenotype is highly variable. We examined index cases and affected family members enrolled in the Zimmerman Program with a phenotypic diagnosis of type 2 VWD. All subjects had factor VIII (FVIII), VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), and multimer distribution analyzed in a central laboratory. For calculation of mean VWF:RCo values, a level of 5 was assigned to subjects with VWF:RCo below the laboratory lower limit of detection of 10 IU/dL. A platelet binding assay was also performed using a gain of function GPIb containing 2 mutations that enable spontaneous binding to VWF in the absence of ristocetin (VWF:GPIbM). Full length 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 (BS). Mean FVIII, VWF:Ag, VWF:RCo, and BS are listed in the table below for each type 2 variant.
For type 2A VWD, 113 subjects have been enrolled to date. All had an abnormal multimer distribution with loss of high molecular weight multimers. 6 type 2A subjects had a VWF:RCo/VWF:Ag ratio of ≤0.7. The lowest VWF:RCo levels were seen in the type 2A cohort with 60% <10. 98% of type 2A subjects had an identified sequence variation on full length sequencing. 25% had the p.R1597W sequence variation and an additional 4 subjects had p.R1597Q. The mean bleeding score for the subjects with sequence variations at 1597 was 10.6. 11% of subjects had p.R1374H, which correlated with a higher mean bleeding score of 12.4. Mean bleeding score for the remainder of the type 2A subjects was lower, at 6.6, suggesting that differences in VWF genetics may account for differences in phenotype, despite the common type 2A laboratory presentation of reduced VWF:RCo and loss of high molecular weight multimers.
44 type 2B subjects have been enrolled to date, all with abnormal multimer distribution and either documented abnormal VWF-platelet binding or a presence of a known type 2B sequence variation. Sequence variations were found in 100% of subjects. The most common sequence variations were p.V1316M (20%), p.R1306W (18%), p.R1341Q (11%), and p.H1268Y (9%). Mean VWF:RCo/VWF:Ag ratios ranged from 0.32-1.12, suggesting that a normal VWF:RCo/VWF:Ag ratio cannot completely exclude the possibility of type 2B VWD. Most (94%) had increased VWF:GPIbM. Subjects with p.V1316M and p.R1306W/Q sequence variations had lower VWF:RCo compared to subjects with p.R1341Q/W but mean bleeding scores did not differ.
59 type 2M subjects have been enrolled to date. Mean VWF:RCo/VWF:Ag ratio was 0.46 (range 0.14-0.7). Sequence variations were found in 93% of subjects. R1374C was found in 13 members from one family. While mean VWF levels were similar to the entire 2M group, a wide range in VWF:Ag and VWF:RCo/VWF:Ag ratio was observed, accompanied by a corresponding range in BS from 0-8. This suggests that other modifiers of phenotype may be present aside from the VWF sequence variation.
11 type 2N subjects have been enrolled to date, all with low VWF binding to FVIII. Sequence variations were found in 100% of this cohort. R854Q was present in 89% of subjects. Bleeding scores were highest for homozygous 2N sequence variations.
Overall, the mean BS for type 1 VWD subjects was 6.3, the mean BS for type 2 VWD subjects was 7.5, and the mean BS for type 3 VWD subjects was 16.8. Types 2A and 2N had higher bleeding scores on average than type 2B, and type 2M subjects had on average the lowest bleeding scores. Although heterogeneity was seen across all the type 2 variants, both laboratory testing and genetic testing are useful in categorizing and phenotyping type 2 VWD.
Table. FVIII (mean) VWF:Ag (mean) VWF:RCo (mean) BS (mean) Type 2A 47 34 12 8.7 Type 2B 45 36 23 7.1 Type 2M 62 54 21 5.4 Type 2N 30 69 76 8.3
Disclosures
Montgomery: Immucor: Patents & Royalties.
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