Beta thalassemia is a blood disorder that reduces the production of hemoglobin. Hemoglobin is the iron-containing protein in red blood cells that carries oxygen to cells throughout the body.
Hemoglobin 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 "hemoglobinopathies". Abnormal hemoglobins appear in one of three basic circumstances:
Structural defects in the hemoglobin molecule. Alterations in the gene for one of the two hemoglobin 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 hemoglobin molecule. Occasionally, alteration of a single amino acid dramatically disturbs the behavior of the hemoglobin molecule and produces a disease state. Sickle hemoglobin exemplifies this phenomenon.
Diminished production of one of the two subunits of the hemoglobin molecule. Mutations that produce this condition are termed "thalassemias." Equal numbers of hemoglobin alpha and beta chains are necessary for normal function. Hemoglobin chain imbalance damages and destroys red cells thereby producing anemia. Although there is a dearth of the affected hemoglobin 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 hemoglobin 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).
In people with beta thalassemia, low levels of hemoglobin lead to a lack of oxygen in many parts of the body. People with beta thalassemia are at an increased risk of developing abnormal blood clots.