Calibrating Sickle Cell Disease

Sickle cell disease is fundamentally a kinetic disorder, in which cells containing the mutated hemoglobin (HbS) will cause occlusion if they sickle in the microvasculature, but have minimal (or no) consequences if they sickle in the venous return. Physiologically, sickling always occurs when some ligands are present; nonetheless, the kinetics in the presence of ligands are virtually unstudied. Sickling arises from nucleation-controlled polymer formation, triggered when the HbS loses ligands (e.g. oxygen). Thus, understanding how nucleation responds to the presence of oxygen is the key to understanding how sickling proceeds in a physiological context. We have measured the rate of nucleus formation in HbS partially liganded with NO or CO, which we find have equivalent effects in reducing the nucleation rates. We find that hemoglobin must be in the T (tense) quaternary structure for nucleation, but the presence of ligands inhibits nucleus formation even when the correct quaternary structure is present. From these results we can predict the fraction of cells that will sickle at any given partial ligand saturations. It appears that symptoms are ameliorated once fewer than 30% of the cells sickle in less than 1 sec. This conclusion is supported by analysis of exchange transfusions, and suggests how to interpret the mixed results of hydroxyurea therapy. The ability to make such predictions may prove especially useful in designing future therapies, particularly those where the oxygen affinity is perturbed.