Beta Thalassemia
Beta thalassemia results from absence or decreased of production beta globin chains. Each erythrocyte precursor cell has a beta globin gene on each chromosome 11 that determines beta globin production. Thus, there are a total of two beta globin genes.
The types of beta thalassemia result from mutation of these genes. Many mutations exist, but most of them result in an aberrant mRNA directing beta globin chain production. There have been two major variations of beta thalassemia arise in human history. Hemoglobin electrophoresis is helpful to characterize beta thalassemias.
One variation results from a gene termed beta+ thalassemia that directs reduced production of beta globin chains.
The other variation arises from the presence of a gene termed beta0 thalassemia that results in absent beta globin chain production.
Beta Thalassemia Minor
In this condition, a single aberrant beta globin gene is present on one chromosome 11. The result is mild anemia--or no anemia at all. RBC's have reduced MCV and MCH, resulting in microcytosis and hypochromia. Hemoglobin electrophoresis will demonstrate double the normal amount of hemoglobin A2, about 3 to 6%, and up to 5% hemoglobin F.
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Beta Thalassemia Major
Inheritance of two abnormal beta globin genes results in beta thalassemia major. If both genes code for beta-0, then hemoglobin electrophoresis will reveal absence of hemoglobin A, about 5 to 10% hemoglobin A2, and over 90% hemoglobin F. If one or both genes are beta+, then there can be 1 to 2% hemoglobin A.
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The RBC's will show decreased MCV and MCH, resulting in microcytosis and hypochromia. Anemia is severe, because alpha globin chains cannot form a stable and soluble tetramer. This results in formation of insoluble alpha globin chain aggregates that affect even RBC precursors, leading to ineffective erythropoiesis, as well as hemolysis. As a consequence, there is expansion of erythropoiesis in marrow spaces, leading to skeletal deformities, and extramedullary hematopoiesis in liver and spleen. Hemolysis leads to indirect hyperbilirubinemia. Increased erythropoiesis leads to increased dietary iron absorbtion, leading to hemosiderosis (but hemochromatosis is rare).
Beta Thalassemia Intermedia
In some less severe forms of beta globin gene mutations, more hemoglobin A is present and insoluble alpha globin chain aggregates are absent, and this is termed "thalassemia intermedia". Complications are not as severe as for beta thalassemia major.
The following image illustrates pathologic findings with beta thalassemia:
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