Metabolic Profiling in Sickle Cell Disease

Abstract 4837

Sickle cell disease (SCD) is a genetic blood disorder of hemoglobin with patients (pts) suffering from multiple complications. Patients are exposed to the clinical consequences of a hemolytic anemia associated with episodes of vascular vaso-occlusion. Lifelong episodes of painful crises lead to end-organ damage with shortened lifespan. The mutated hemoglobin S in the red cell polymerizes during hypoxia (venous circulation) leading to tissue inflammation and ischemia. The FDA approved drug hydroxyurea used to treat SCD increases fetal hemoglobin production thereby inhibiting hemoglobin S polymerization. Hydroxyurea therapy has been shown to improve morbidity and mortality in SCD patients. The metabolic demands of SCD patients are increased due to chronic hemolysis. The dietary supplement folic acid is recommended to meet some of the excess metabolic requirements. However, metabolic pathways associated with SCD are poorly understood or even explored. The components of the purine nucleotide synthetic pathway were analyzed in the serum of adult SCD patients in steady state. Purine nucleotides are made available for cells via two routes, either by de novo synthesis or by reusing of catabolized purine bases, mainly hypoxanthine. Salvaging the purine ring is more efficient in terms of ATP equivalents than de novo purine synthesis. Xanthine oxidase catalyzes the final reactions of the purine catabolism by consecutive oxidation of hypoxanthine to xanthine to uric acid. Levels of hypoxanthine, xanthine, uric acid as well as xanthine oxidase enzymatic activity (μU/mL) were evaluated in (1) SCD patients (n=5-9), (2) in SCD patients treated with hydroxyurea (n=5-9), and (3) normal adult controls (n=12). Hypoxanthine (μM), xanthine (μM) and uric acid (μM) levels all showed statistically significant (p<0.05) elevations in SCD patients when compared to controls; 14 ± 1 vs. 5 ± 1, 66 ± 8 vs. 22 ± 5, 1142 ± 60 vs. 735 ± 33, respectively. Xanthine oxidase enzymatic activity (μU/mL) also showed statistically significant (p<0.05) increases in SCD pts of 4.3 ± 1.1 vs. 0.9 ± 0.3 when compared to controls. However, SCD pts treated with hydroxyurea showed a statistically significant (p<0.05) drop of hypoxanthine, xanthine, and uric acid levels and also xanthine oxidase activity when compared to SCD pts; 8 ± 1 vs. 14 ± 1, 38 ± 5 vs. 66 ± 8, 780 ± 88 vs. 1142 ± 60, respectively. Furthermore, the hypoxanthine, xanthine, and uric acid levels and xanthine oxidase activity of SCD pts who were taking hydroxyurea were not statistically different from controls. Since levels of hypoxanthine, xanthine, and uric acid were all elevated in SCD pts this suggests that the nucleotide synthetic pathway may not have switched to the more efficient salvage pathway, as would be expected if the uric acid level were low or normal. Furthermore, an unexpected added benefit of hydroxyurea therapy may include down regulation of the overactive purine nucleotide synthetic pathway in SCD pts. The significant increase of purine related compounds such as hypoxanthine and xanthine without a decrease in uric acid suggest that metabolically active cells are not shunting purine bases to the more efficient salvage pathway for synthesis of nucleic acids and higher energy needs. The high proliferate erythropoietic turnover rate of chronic hemolysis requires large quantities of purine bases for transmission of ATP and synthesis of nucleic acids. The metabolic changes in SCD suggest that there is an ineffectual shunting of purine bases to the more efficient salvage pathway. Hydroxyurea therapy may allow for shunting to the more energy efficient salvage pathway resulting in less total body metabolic expenditure. Alternatively, hydroxyurea as a ribonucleotide reductase inhibitor decreases hematopoietic cell turnover which may lead to less purine metabolic requirements. Future metabolic studies will explore this and identify other important pathways for potential therapeutic intervention in SCD.