VIII.127 - Sickle-Cell Anemia

Summary

Sickle-cell disease is an inherited disorder resulting from an abnormality in the structure of a protein in the red blood cell called hemoglobin. It represents a spectrum of disorders ranging from the full-blown form, sickle-cell anemia, to the carrier state called sickle-cell trait. Also included in this spectrum are several other variant hemoglobin disorders, which all have the sickle hemoglobin. Sickle-cell anemia is the prototype for most molecular diseases and was the first disease to have its cause isolated to a single molecular change in the human genetic structure. This single change is responsible for all of the dramatic physiological changes and clinical events that occur in this disease.

Sickle-cell trait occurs when the individual is heterozygous for the sickle-cell gene and results in red blood cell concentrations of the abnormal hemoglobin (hemoglobin S) of less than 50 percent. It generally does not result in serious illness although this generalization has recently been disputed. In addition to sickle-cell trait, several sickle-cell syndromes occur when hemoglobin S is present in a heterozygous state with other hemoglobin variants – some with similar properties. Common examples of these include hemoglobin C and hemoglobin E.

Distribution and Incidence

Sickle-cell anemia is found in as many as 4 percent of Africans and in 1 percent of black Americans (1 per 500). Upward of 40 percent of Africans carry the sickle-cell trait as compared to 9 percent of black Americans. In some Mediterranean cultures the trait is also present. It is now generally believed that the sickle-cell gene mutation occurred independently in several areas of Africa.