Clinical phenotype and pathophysiological mechanisms underlying qualitative Low VWF

• Mild isolated functional VWF defects in the 30-50 IU/dl range are common and are associated with significant bleeding in some patients.

• Qualitative low VWF is a distinct clinic-pathological entity compared to type 2 VWD.

Previous reports have highlighted that some low VWF patients with significant bleeding were diagnosed based upon an isolated but persistent reduction in plasma VWF activity levels in the 30-50 IU/dL range. These patients had plasma VWF:Ag levels > 50 IU/dL and thus had 'qualitative' rather than 'quantitative' low VWF. Although the clinical importance of functional VWF defects in type 2 VWD is well recognized, the translational implications of mild functional defects in patients with qualitative low VWF (low VWF-QL) have not been defined. To address this clinically important question, we combined low VWF datasets from the low VWF in Ireland cohort and the low VWF in Erasmus MC studies. Overall, we observed that low VWF-QL was common and accounted for approximately 50% of our combined low VWF cohort. Importantly, our findings demonstrate that many of these patients with mild isolated functional VWF defects in the 30-50 IU/dl range had significant bleeding phenotypes, even though their plasma VWF:Ag levels were within the normal range. In addition, we further show that low VWF-QL is a distinct clinic-pathological entity compared to type 2 VWD. Finally, our studies highlight that low VWF-QL is predominantly due to abnormalities in VWF biosynthesis within endothelial cells that are occurring largely independent of identifiable pathological VWF sequence variants. Cumulatively, these novel observations have important clinical implications for the diagnosis and management of patients with mild functional VWF defects.