ALBERT

Description: Abstract 259 Objectives: Silent cerebral infarction (SCI) occurs in approximately 27% of children with sickle cell disease (SCD) by age 6 years, and is associated with decreased neurocognitive function and a 14-fold increased risk of progression to overt stroke. While several clinical parameters, such as increased white blood cell (WBC) and platelet counts and decreased hemoglobin (Hb) or hematocrit, have been reported in the literature to be associated with SCI, to date no validated biomarkers exist to predict SCI in patients with SCD. Furthermore, recent unpublished data from the Silent Infarct Transfusion (SIT) Trial has identified systolic blood pressure and total hemoglobin as risk factors. The aim of this study was to identify candidate biomarker plasma proteins that correlate with SCI in patients with SCD. Methods: We used a proteomic discovery approach involving three sequential separation steps to compare the plasma proteomes of 15 children with SCD (7 with SCI and 8 without SCI), aged 5–15 years. Baseline steady-state plasma samples were obtained from the SIT Trial Biologic Repository and matched for age, Hb and WBC. Plasma samples were Hb depleted in the first dimension, separated using immunoaffinity depletion and reverse phase liquid chromatography fractionation, and then trypsin-digested for characterization using label-free quantification on a LTQ-Orbitrap (Thermo) mass spectrometer. The resulting MS/MS data were analyzed using PASS (Integrated Analysis, Bethesda, MD) with X! Tandem searches (www.thegpm.org; version 2008.12.01) of the International Protein Index peptide database (human, 3.19). We measured candidate proteins in a validation cohort of 116 children with SCD (n=65 SCI, 51 non-SCI) and 24 age-matched, healthy African American control subjects using enzyme-linked immunosorbent assays (thrombospondin 1 [TSP1], L-selectin, RandD Systems, Minneapolis, MN) and immunoassays (E- and P-selectin, Mesoscale Discovery, Maryland). All samples were run in duplicate according to the manufacturers' protocols. Statistical differences in biomarker plasma concentrations between groups were compared by the Mann-Whitney U test. Results: TSP1 (5 peptides) and L-selectin (3 peptides) were among 335 proteins that showed differential detection between the SCI and non-SCI groups based on the spectral counts. TSP1 is an extracellular matrix glycoprotein that is involved with platelet aggregation, inhibition of neovascularization and tumorigenesis and has been shown to promote the adherence of sickle erythrocytes to the vascular endothelium. L-selectin is an adhesion molecule that mediates leukocyte interaction with the vascular endothelium. In a validation cohort of 116 children with SCD (n=65 SCI, 51 non-SCI) and 24 age matched, healthy African American control subjects, TSP1 and L-selectin were both significantly increased in SCI vs. non-SCI groups (median 8.5 vs. 6.2 μ g/ml for TSP1, P =0.03; 1.5 vs. 1.4 μ g/ml for L-selectin, P =0.03). As expected, neither TSP nor L-selectin were elevated in the age-matched normal controls (median=4.6 μ g/ml for TSP1, P =0.10, 1.2 μ g/ml for L-selectin, P =0.10). The specificity of the L-selectin results was verified by demonstrating that E-selectin and P-selectin were not increased in the SCI group. TSP1 was correlated with baseline oxygen saturation in both the SCI and non-SCI groups (r=-0.51, and r=-0.35, P