Beta thalassemias are a group of inherited blood disorders characterized by the reduced production of hemoglobin, a protein component of red blood cells that carries oxygen from the lungs to the rest of the body.
What Causes Beta Thalassemia?
The most common hemoglobin in a normal adult is hemoglobin A (HbA), made up of two α-globin chains and two β-globin chains. People with β-thalassemia have defects in the HBB gene that encodes β-globin. Genetic mutations can reduce the level of β-globin production (β+ mutations) or completely eliminate the production of β-globin (β0 mutations). Reduced β-globin production means that not enough hemoglobin can be made, resulting in the diminished transport of oxygen around the body.
The severity of β-thalassemia depends on the number and type of HBB mutations that a person has inherited. We inherit two copies of the HBB gene – one from each parent. The most serious form of β-thalassemia is β-thalassemia major, where a person has two defective HBB alleles and each allele contains a β0 mutation, preventing the production of any β-globin. These individuals require life-long blood transfusions to provide adequate hemoglobin levels. β-thalassemia intermedia occurs in people that have also inherited two defective HBB alleles, but at least one allele carries a β+ mutation so some β-globin can still be produced. β-thalassemia intermedia symptoms vary from mild to severe, depending how much β-globin is able to be produced. β-thalassemia minor (or β-thalassemia trait) refers to people who have just one defective HBB allele and one normal HBB allele.
How Common is Beta Thalassemia?
Beta thalassemia is a common inherited disorder in individuals of Mediterranean, African and South Asian descent. These are regions of the world where malaria is commonly transmitted by mosquitoes. Carriers of a beta thalassemia mutation have some protection against malaria infection, hence there is a selective advantage for retaining HBB genetic mutations in areas affected by malaria. Genetic prevalence of beta thalassemia carriers according to geographical regions is:
- Eastern Mediterranean – Carriers represent 2-18% of the population
- Europe – Carrier frequency can be as high as 19%
- Western Pacific – Approximately 13% of the people are carriers
- Sub-Saharan Africa – Up to 12% of the people are carriers
- Southeast Asia – Carriers represent 11% of the population
- North America – Carrier frequency is lower, ranging from 0-3%
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