What is thalassemia?
Thalassemia is a group of inherited disorders that affects the amount and type of hemoglobin a person produces. Hemoglobin (Hb or Hgb) is found in all red blood cells. A hemoglobin molecule is made up of heme, an iron-containing complex, and globin, protein chains that surround the heme complex. The types of protein chains found in the hemoglobin molecule affect its structure and function. Different types of hemoglobin are classified according to the type of protein chains they contain. Normal hemoglobin types include:
- Hemoglobin A - makes up about 95-98% of the Hb found in adults; Hb A contains two alpha (α) protein chains and two beta (ß) protein chains.
- Hb A2 - makes up about 2-3.5% of Hb found in adults; has two alpha (α) and two delta (δ) protein chains.
- Hb F - makes up to 2% of Hb found in adults; has two alpha (α) and two gamma (γ) protein chains. This is the primary hemoglobin produced by the fetus during gestation. Its production usually falls to a low level within a year after birth.
Mutations in the genes coding for the globin chains can cause disorders in hemoglobin production. There are four genes that code for the alpha globin chains and two genes that code for the beta globin chains. (For general information on genetic testing, see The Universe of Genetic Testing.)
Inherited disorders of hemoglobin production caused by mutations in these genes fall into two categories:
- Thalassemia, or decreased production of normal hemoglobin (discussed in this article)
- Production of an abnormal hemoglobin molecule (see Hemoglobin Abnormalities)
With thalassemias, mutations in one or more of the globin genes cause a reduction in the amount of the particular globin chain produced. This can upset the balance of alpha to beta chains, causing unusual forms of hemoglobin to accumulate or causing an increase of minor hemoglobin components, such as Hb A2 or Hb F.
The thalassemias are usually classified by the type of globin chain whose synthesis is decreased.