Beta-thalassemia
| Overview |
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- Beta-thalassemia is a subtype of thalassemia characterized by reduced synthesis of the beta-globin chain, resulting in a relative excess of alpha-globin within erythrocytes. See the thalassemia page for a general discussion of these disorders.
| Genetics |
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- Beta-globin is coded by a single gene on Chromosome 11; consequently, patients may have only one or both alleles of the beta-globin gene affected. Additionally, a wide variety of mutations can affect beta-globin synthesis and can reduce synthesis to differing extents. In some cases there is residual (although reduced) synthesis of the beta-globin gene, while in other cases synthesis is completely absent.
- Given the fact that mutations affect synthesis of beta-globin to different extents and that one or both alleles might be affected, the total production of beta-globin can range across a very wide spectrum from patient to patient.
| Pathogenesis |
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- If synthesis of beta-globin is only mildly reduced, typically seen in heterozygotes, then the only functional consequence is a reduced total production of normal hemoglobin and in consequence the generation of small pale erythrocytes observed clinically as a mild hypochromic microcytic anemia. With more severe reductions of beta-globin, typically seen in those with homozygous mutations, the resultant imbalance with alpha-globin yields accumulation of highly insoluble alpha-globin particles. These insoluble alpha-globin aggregates damage the erythrocyte membrane and render it less flexible. As a result, erythrocytes developing within the bone marrow are often destroyed prior to release, yielding ineffective erythropoiesis. Because of their membrane inflexibility, whatever erythrocytes that are successfully released into the periphery are recognized by the splenic reticuloendothelial system and undergo extravascular hemolysis.
| Clinical and Laboratory Consequences |
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- Overview
- The wide spectrum by which total synthesis of beta-globin may be reduced is reflected in a wide clinical spectrum. We discuss the two ends of the spectrum which are known as Beta-thalassemia Minor and Major. In general, beta-thalassemia minor is found in heterozygotes while Beta-thalassemia major is found in those who possess homozygous mutations that severely reduce beta-globin synthesis or render it completely absent.
- Beta-thalassemia Minor
- Patients with beta-thalassemia minor are typically asymptomatic and display a very mild hypochromic microcytic anemia that may be found incidentally. Occasionally, symptomology associated with a mild anemia may result.
- Beta-thalassemia Major
- As a result of chronic ineffective erythropoiesis and extravascular hemolysis, patients with beta-thalassemia major may display a severe hypochromic microcytic anemia and be transfusion-dependent. The compensatory hyperplasia of erythropoietic elements in the bone marrow can result in bone deformities. Extramedullary hematopoiesis and splenic hemolysis often result in hepatosplenomegaly..