[Molecular and cellular pathophysiology of sickle cell anemia]

The beta S mutation responsible for sickle cell disease (SCD) was identified in 1949. This mutation always consists in a T for A substitution at codon 6 of the beta-globin chain. Deoxygenated HbS becomes polymerized, producing erythrocyte shape changes and vasoocclusion. Still unexplained is the wide variability in clinical expression of SCD, whose spectrum ranges from severely incapacitating forms to virtually silent forms. Recent research has focused on genetic or environmental factors that may interfere with one or more steps of the basic pathophysiologic mechanisms. These factors can be roughly classified in three groups: i) factors that modify HbS levels within erythrocytes, thereby impacting the propensity of HbS for polymerization; ii) other normal or abnormal hemoglobins present in the erythrocyte that may modify side contacts of the hemoglobin molecule within the polymer, thus either stopping or enhancing polymerization; iii) molecules other than hemoglobin expressed at the surface of the SCD reticulocytes that may adhere to the vascular endothelium, thus slowing flow in the microcirculation and delaying HbS reoxygenation. Whatever the starting point, the same vicious circle is set in motion. These mechanisms have generally been studied separately, in vitro, under artificial conditions. An important goal is to understand how they interact in vivo in a given patient.