Thalassemia is a group of inherited blood disorders that can be passed from parents to their children and affect the amount and type of hemoglobin the body produces.
Hemoglobin (Hb or Hgb) is a substance present in all red blood cells (RBCs). It is important for proper red blood cell function because it carries the oxygen that RBCs deliver around the body. One portion of hemoglobin called heme is the molecule with iron at the center. Another portion is made of up four protein chains called globins. Each of the four globin chains holds a heme group containing one iron atom. Depending on their structure, the globin chains are designated as alpha, beta, gamma, or delta.
Not all hemoglobin is the same. Different types of hemoglobin are classified according to the type of globin chains they contain. The type of globin chains present is important in hemoglobin's ability to transport oxygen.
Normal hemoglobin types include:
- Hemoglobin A – this is the predominant type of Hb in adults (about 95-98%); 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; it has two alpha (α) and two delta (δ) protein chains.
- Hb F – makes up to 2% of Hb found in adults; it has two alpha (α) and two gamma (γ) protein chains. Hb F is the primary hemoglobin produced by a developing baby (fetus) during pregnancy. Its production usually falls to a low level within a year after birth.
People with thalassemia have one or more genetic mutations that they have inherited and that result in a decreased production of normal hemoglobin. When the body doesn't make enough normal hemoglobin, red blood cells do not function properly and oxygen delivery suffers. This can lead to anemia with signs and symptoms that can range from mild to severe, depending on the type of thalassemia that a person has. Examples of signs and symptoms include weakness, fatigue, and pale skin (pallor). See the Classifications section for more about the signs, symptoms, and complications of the different types of thalassemia.
For hemoglobin, there are four genes in our DNA that code for the alpha globin chains and two genes (each) for the beta, delta, and gamma globin chains. Since everyone inherits a set of chromosomes from each parent, each person inherits two alpha globulin genes and one beta globulin gene from each parent. (For general information on genetics, see The Universe of Genetic Testing.) A person may inherit mutations in either the alpha or beta globin genes.
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, resulting in unusual forms of hemoglobin or an increase in the amount of normally minor hemoglobin, such as Hb A2 or Hb F. The thalassemias are usually classified by the type of globin chain whose synthesis is decreased. For example, the most common alpha chain-related condition is called alpha thalassemia. The severity of this condition depends on the number of genes affected.
Other types of mutations in the genes coding for the globin chains can result in a globin that is structurally altered, such as hemoglobin S, which causes sickle cell. The inherited disorders that result in the production of an abnormal hemoglobin molecule are described in the article on Hemoglobin Abnormalities. Together, thalassemia and hemoglobin abnormalities are called hemoglobinopathies.