Sickle cell disease is a group of disorders that affects hemoglobin, the molecule in red blood cells that delivers oxygen to cells throughout the body. People with this disorder have atypical haemoglobin molecules called haemoglobin S, which can distort red blood cells into a sickle, or crescent, shape.
Haemoglobin is produced by genes that control the expression of the hemoglobin protein. Defects in these genes can produce abnormal hemoglobins and anemia, which are conditions termed "haemoglobinopathies". Abnormal haemoglobins appear in one of three basic circumstances:
Structural defects in the haemoglobin molecule. Alterations in the gene for one of the two haemoglobin subunit chains, alpha (a) or beta (b), are called mutations. Often, mutations change a single amino acid building block in the subunit. Most commonly the change is innocuous, perturbing neither the structure nor function of the haemoglobin molecule. Occasionally, alteration of a single amino acid dramatically disturbs the behavior of the haemoglobin molecule and produces a disease state. Sickle haemoglobin exemplifies this phenomenon.
Diminished production of one of the two subunits of the haemoglobin molecule. Mutations that produce this condition are termed "thalassemias." Equal numbers of haemoglobin alpha and beta chains are necessary for normal function. Haemoglobin chain imbalance damages and destroys red cells thereby producing anemia. Although there is a dearth of the affected haemoglobin subunit, with most thalassemias the few subunits synthesized are structurally normal.
Abnormal associations of otherwise normal subunits. A single subunit of the alpha chain (from the a-globin locus) and a single subunit from the b-globin locus combine to produce a normal haemoglobin dimer. With severe a-thalassemia, the b-globin subunits begin to associate into groups of four (tetramers) due to the paucity of potential a-chain partners. These tetramers of b-globin subunits are functionally inactive and do not transport oxygen. No comparable tetramers of alpha globin subunits form with severe beta-thalassemia. Alpha subunits are rapidly degraded in the absence of a partner from the beta-globin gene cluster (gamma, delta, beta globin subunits).