Sickle cell disease is a group of disorders that affects haemoglobin, the molecule in red blood cells that delivers oxygen to cells throughout the body. People with this disorder have atypical hemoglobin molecules called hemoglobin S, which can distort red blood cells into a sickle or crescent shape.
The production of hemoglobin A, which is the principle type of hemoglobin in humans, is governed by 3 genes: HBA1, HBA2, and HBB. Each hemoglobin A molecule consists of two alpha and two beta chains, and mutations in either of the HBA or the HBB genes may result in abnormal hemoglobin molecules with reduced or diminished function. Sickle cell disease arises from a single point mutation in the 6th codon of the beta-globin gene (HBB), which results in a valine instead of a glutamic acid in the hemoglobin beta-chain.
Abnormal hemoglobin ultimately leads to anemia as well as other symptoms, depending on the exact mutations present. Diseases caused by defective hemoglobin fall into a larger category of diseases known as the "haemoglobinopathies" which also include the thalassemias, a related group of diseases that are characterized by reduced or deficient rather than abnormal haemoglobin.
