MOLECULAR MECHANISM OF HEMOLYTIC ANEMIA IN HOMOZYGOUS HEMOGLOBIN C DISEASE

Abstract

Erythrocytes from a patient with homozygous hemoglobin C disease were subjected to gradual osmotic dehydration by incubation in hypertonic saline. Serial observations of these cells before and after 4 and 12 hr incubation were carried out by means of interference, Soret absorption, polarization microscopy, and the electron microscope employing the freeze-etching technique. Light microscopic studies showed a progressive contraction of cellular contents into central masses which, after 12 hr dehydration, formed birefringent intracellular hemoglobin crystals in 50–75% of the cells. Electron microscopic study of freeze-etched replicas of these cells at 0, 4, and 12 hr of dehydration reveals progressive aggregation, alignment, and crystallization of hemoglobin molecules. Molecular aggregation found in C-C cells prior to osmotic dehydration was not seen in normal erythrocytes. Aggregation and packing varied from cell to cell. Reticulocytes showed a loosely packed aggregate mesh-work; older cells showed variation of molecular packing, which appeared tightest in cells corresponding to microspherocytes. With further loss of intracellular water, aggregates coalesced into patterns of tighter molecular packing with small regions of alignment, and, finally, crystallization occurred. Hemoglobin molecules measuring 70 A in diameter were readily identified within the period patterns of intracellular crystals. These findings suggest that the hemoglobin C molecules within C-C erythrocytes exist in an aggregated state. As the cell ages, intracellular water is lost and intermolecular distance decreases, hemoglobin C molecules polymerize into intracellular crystals. This pathological behavior of hemoglobin C is associated with a charge alteration conferred by the substitution of beta-6-lysine for glutamic acid on the external surface in the A-helix region of the beta-chain of the molecule, possibly increasing intermolecular attraction. Molecular aggregation accounts for the increased rigidity of C-C cells which leads to accelerated membrane and water loss with resultant microspherocyte formation. The microspherocyte, with highest intracellular hemoglobin concentration, rapidly undergoes intracellular crystallization, and is sequestered and destroyed by reticuloendothelial elements.