Increased Von Willebrand Factor and High Circulating Placental Growth Factor Correlate with Inflammation and Iron Overload in Patients with Compound Heterozygous Sickle Cell and Beta-Thalassemia

Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by pathological polymerization of hemoglobin, increased red cell rigidity, poor microvascular blood flow, with consequent tissue ischemia and infarction. Thus, hemolytic anemia, vaso-occlusion and vasculopathy are the hallmarks of its clinical presentation. Several other factors contribute to the clinical variability, which is present in SCD, including leukocyte dysfunction, platelet interactions with endothelial cells, pro-inflammatory cytokines, oxidative stress, reduced nitric oxide (NO) availability and hemostatic activation. Placental growth factor (PlGF) is a member of the vascular endothelial growth factor superfamily, which plays an important role in both inflammation and neoangiogenesis. Recently it has been reported that markers of iron overload are associated with high plasma level of PlGF and early mortality [Wang et al, Blood 2014]. Furthermore, in SCD patients, hemolysis can lead to a prothrombotic state by increasing the activity of von Willebrand factor (vWF), a multimeric plasma glycoprotein secreted by the endothelium [Chen et al, Blood 2011]. ADAMTS-13 is a member of the ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats) family that cleaves vWF. Recent observations suggest that SCD patients suffer from an acquired ADAMTS-13 deficiency primarily because Hb competitively binds and blocks the proteolysis of vWF, leading to the accumulation of ultra-large VWF multimers in circulation and on endothelium.

The aim of this study was to evaluate inflammation, endothelial dysfunction and angiogenesis in patients with compound heterozygous SCD and beta-thalassemia (HbS/βthal) and explore possible association with iron overload and other disease features.

Eighty-nine adult caucasian patients with HbS/bthal were included in the study, while 20 apparently healthy individuals served as controls. Patients with HbS/βthal divided in two groups: group A included 49 patients under hydroxycarbamide (HC+) treatment and group B included 40 patients without hydroxycarbamide (HC-) treatment. Along with hematology and blood chemistry parameters determination, measurements of circulating high-sensitivity C-reactive protein (hs-CRP), vWF, D-Dimers, ADAMTS-13, hs-Troponin-T (hs-TnT) and PlGF were measured in patients with HbS/βthal and controls using immunoenzymatic techniques.

Levels of hs-CRP, vWF and PlGF were elevated in patients with HbS/βthal compared to controls (6.9±5.3 vs. 0.4±0.4 mg/L, 170.1±78.3 vs. 85.3±22.1 IU/dL and 20.2±7.6 vs, 15.3±2.4 pg/mL, respectively, p<0.001), while ADAMTS-13 levels were decreased in patients with HbS/βthal compared to controls (965.2±244.0 vs. 1144.2±187.0 pg/mL, p<0.001). No significant differences were found for the above parameters between patients of groups A and B (p=0.211, p=0.710, p=0.589 and p=0.384, respectively. Hs-CRP correlated positively with vWF, PlGF and ADAMTS-13 (r=0.360, p<0.001; r=0.300, p=0.005 and r=0.382, p=0.002, respectively). PlGF levels in patients with HbS/βthal correlated positively with markers of hemolysis such as reticulocyte counts, LDH and with uric acid levels (r=0.674, p<0.001; r=0.425, p<0.001 and r=0.357, p<0.001, respectively). Ferritin levels correlated positively with hs-CRP, vWF, PlGF and D-Dimers (r=0.360, p<0.001; r=0.317, p=0.004; r=0.354, p<0.001; and r=0.344, p=0.002, respectively) as well as with markers of hemolysis (p<0.001). Furthermore, only 6 patients had slightly increased hs-TnT levels and almost all patients had pathologic D-Dimers levels.

Our findings demonstrate that patients with HbS/βthal have a significant degree of endothelial dysfunction as assessed by increased vWF. The increased levels of the D-Dimers in almost all patients indicate the activation of coagulation and fibrinolytic systems even in the steady state of the disease. Iron overload and inflammation along with reduced circulating ADAMTS-13 contribute, at least partially, to the increased levels of vWF. Similarly, inflammation and iron overload enhance the production of angiogenesis markers, such as PlGF suggesting a possible pathogenetic role for iron load in SCD biology. Further studies will reveal if iron chelation is able to restore some of the above phenomena and improve endothelial dysfunction and inflammation in this entity.