Domain Distribution of Type 1 VWD Sequence Variants and Impact on Clinical and Laboratory Phenotype in the Zimmerman Program

Type 1 von Willebrand disease (VWD) is a common inherited bleeding disorder, characterized by a quantitative deficiency of von Willebrand factor (VWF). The molecular basis continues to merit exploration, especially since the genetic cause has not been determined in 30-40% of cases in previous studies. As part of the Zimmerman Program for the Molecular and Biology of VWD (ZPMCB-VWD) study, we sought to identify the sequence variants (SV) within the VWF gene that result in type 1 VWD and correlate these to clinical laboratory phenotype and clinical severity and determine the frequency of type 1 subjects who do not have a gene defect. We also wanted to examine the types of sequence variants that cause type 1, type 1-Severe (1S), type 1C and low VWF (LVWF), and determine the influence of SV location on VWF levels and risk of bleeding.

310 type 1 Index Cases (IC) enrolled in the ZPMCB-VWD were analyzed, including 69 type 1 (VWF:Ag<30), 9 type 1S (VWF:Ag<5), 57 type 1C (VWF:Ag<30; VWFpp/VWF:Ag>3), and 175 LVWF (VWF:Ag or VWF:RCo 30-50). Bleeding symptoms were quantified using the ISTH bleeding score (ISTH BS). Clinical VWF testing was done in a central laboratory at BloodCenter of Wisconsin and included FVIII, VWF:Ag, VWF:RCo, VWFpp, and VWF:CB. Full-length VWF sequencing was performed, including 5' and 3' regions and intron/exon boundaries. SV found in >1% of the healthy control population were excluded from this analysis. Mutation negative cases were further investigated by comparative genomic hybridization (CGH) to identify any large deletions or duplications in the VWF gene.

SV or large deletions were identified in 62% (n=192) of this cohort with quantitative VWF deficiency, whereas 38% (n=118) had no variants found. Of those with no sequence variant identified, 10% were type 1, 5% type 1C and 85% LVWF. There were 142 SV found in this cohort. 75% of the subjects had missense mutations, 8% small insertions/deletions, 9% splice site mutations, 3% nonsense mutations and 1% large deletions. The median number of SV found in individuals was 1, however 23% had more than 1 SV present (range 1 to 4).

Mean VWF:Ag (IU/dl) was examined to determine the influence of SV location on levels and median ISTH BS was assessed as a measure of bleeding severity by domain:

Type 1S subjects had SV that were enriched in D1, D2 and A3 domains. 52% of type 1C variants were found in the A1 domain followed by 19% in D'D3 and 17% in D4. Both type 1 and LVWF subjects had SV that were evenly distributed throughout the gene, but LVWF had the majority of SV found in the A2, D2 and C1-C6 domains.

Type 1S subjects showed a difference in mutation type compared to rest of the type 1 cohort with 31% intronic/splice site mutations, 19% nonsense mutations, 19% insertions/deletions and only 25% missense mutations. In contrast, missense mutations accounted for 68% of the mutations in type 1, 91% in type 1C and 80% in LVWF. The presence of predominantly truncating or loss of function variants in type 1S correlates with lower VWF levels and increased bleeding.

In conclusion, we found that 62% of our type 1 cohort did have potentially causative SVs identified, with the majority caused by missense mutations (75%). Type 1C and type 1S tend to have SV isolated in certain domains whereas SV in type 1 and LVWF tend to be spread throughout the gene. SVs in the A3 domain demonstrated the lowest VWF:Ag (12 IU/dl) and highest ISTH BS (9) while changes in the CK domain had the highest VWF:Ag (38 IU/dl) and lowest ISTH BS (1). SVs or large deletions were not found in 38% of this cohort, indicating that other molecular factors either intronic or outside of the VWF gene may be the cause of low VWF and bleeding in these subjects.