Severe Sickle Cell Disease Is Characterized by Low ADAMTS13/VWF: Ag Ratio

Abstract 3235

Endothelial damage is one of the major mechanisms causing most of the manifestations in sickle cell disease (SCD). We hypothesized that a possible defect in the cleavage of large vWF multimers through the protease ADAMTS13 could add further to the well established chronic activation of the endothelium and consequently to the severity of the disease. For this purpose we measured the activity of the enzyme and looked for any correlations with regard to the clinical phenotype of patients with SCD. We studied 22 steady state patients (9 men/13 women) with a median age of 35(22–74) years. Of them, 3 were in homozygous state(HbSS) and 19 were compound heterozygous for thalassamia β(HbS/β⁺, HbS/β⁰). The clinical phenotype was mild in 12 patients, moderate in 3 and 7 were suffering from severe symptoms (history of frequent vaso-occlusive and acute chest crises and need for regular exchange transfusion).

We applied FRET (fluorescence resonance energy transfer) technique to measure ADAMTS13 activity (normal values>65%). Factor VIII and vWF: Ag was measured using 1-stage clotting assay and immunoassay respectively (normal values 50–150%). We considered all relevant clinical parameters (spleen size, transfusion frequency, comorbidities, and medications with special regard to iron chelation, hydroxyurea and antiplatelet agents) together with laboratory readings (markers of haemolysis, proportion of haemoglobin fractions using high-performance liquid chromatography HPLC).We used as control group 22 healthy volunteers blood donors with their written consent matched for age, sex, body mass index and ABO blood group. All samples were taken at least 30 days post last transfusion.

We found higher levels of both vWF: Ag [189(33–345)] and ADAMTS13 [164(75–310)] in the patients compared to the control group [87(44–152) και 126(55–171) (p<0.001/p<0.025)] respectively. When we analyzed the ratio ADAMTS13/vWF: Ag we found significant difference between the two groups: 1,041(0.3–4.6) vs 1.56(0.5–2.6) (p<0.020). Furthermore, patients with severe disease experienced significantly lower ratio ADAMTS/vWF 0.7(0.5–1.2) compared to both control group (p<0.001) as well as patients with milder clinical phenotype [1.30(0.3–4.6)].Of note, in patients with severe SCD despite the fact that vWF: Ag levels were found considerably higher and in significant difference with the control group [196(130–345) vs 87(44–152)p<0.001], we could not find the same pattern for ADAMTS13 [140(75–210) vs 126(55–171)].The lower ratio found in patients with SCD and in particular in those with severe disease might indicate the need for higher protease levels in comparison to healthy controls. It looks like in this group of patients ADAMTS13, although normal, is not sufficient enough to manage the reasonably high vWF levels that characterize the chronic endothelial activation. This relevant deficiency of the metaloprotease might be caused by either biggest consumption due to large and constant release of its substrate-vWF or by an acquired inhibition of its action as the free intravascular hemoglobin might exhibit an antagonistic role for the binding site of vWF. In conclusion, the low ADAMTS13/vWF: Ag ratio seems to detect patients with severe SCD and would be of interest to prospectively investigate its role both as a prognostic tool as well as a potential therapeutic target.