Citrate Synthase Activity Is Increased In Children With Sickle Cell Disease (SCD) On Hydroxyurea (HU) Therapy

Abstract

Background Mitochondrial dysfunction is associated with abnormal cell function, oxidative stress and cell death. A progressive impairment of mitochondrial function and/or increased oxidative damage likely contribute to the pathophysiology of SCD, however little is known about bioenergetics in SCD. While previous work demonstrated mitochondrial dysfunction in the platelets of adult patients with SCD characterized by deficient complex IV and V activity which was associated with hemolysis, no data has yet been described in children with SCD. Objective To initiate investigation into mitochondrial function in children with SCD. Methods Children with a diagnosis of SCD requiring parenteral opioids for vaso-occlusive pain episodes (VOE) were prospectively enrolled within 24 hours of presentation to an urban emergency department (ED). Blood was obtained for biomarkers of hemolytic rate, and estimated pulmonary pressures were noninvasively measured by a point-of-care Doppler echocardiography (echo). Platelet rich plasma was isolated from patient whole blood by differential centrifugation and flash frozen for mitochondrial studies. The activities of Complexes V and IV were measured since these enzymes are the sites of oxygen consumption (Cx IV) and ATP production (CxV). The activity of citrate synthase, a mitochondrial matrix protein, was also determined as an indicator of mitochondrial number. Results 11 children with SCD seen in the ED or admitted for VOE were included in this pilot study. Mean age was 17±3 years (range: 9-21 years), 64% were male, and 45.5% had HbSS, 45.5% HbSC and 9% HbSbThal. Mean Hb was 9.9±2 g/dL, reticulocyte (retic) count was 7.6±5%, and 45.5% were on HU. Mean tricuspid regurgitant jet velocity (TRV) = 2.0±0.4 m/s. Of interest patients on HU had significantly higher citrate synthase activity (Figure 1), although Complex IV and V activities were similar in both groups. Citrate synthase activity inversely correlated with TRV (r=-0.63, p=0.04). Similar to the previously reported adult data, complex IV activity inversely correlated with retic count (R2=0.27), although not reaching statistical significance, likely due to small sample size. No difference in mitochondrial studies were found when HbSS patients were compared to those with HbSC. Conclusions These data suggest mitochondrial dysfunction and potentially decreased mitochondrial number in SCD children with increased hemolytic rate (TRV). Further, the greater citrate synthase activity in platelets of patients with SCD on HU compared to those not treated with HU may be indicative of increased mitochondrial number with HU therapy. These observations have never been previously described, and may suggest a novel mechanism of action for HU warranting further study. Disclosures: No relevant conflicts of interest to declare.