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Alpha thalassemia
Last reviewed: 23.04.2024
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Alpha-thalassemia is a group of diseases that are common in Southeast Asia, China, Africa, and the Mediterranean. Two almost identical copies of the alpha globin gene are located on chromosome 16. In 80–85% of cases of alpha thalassemia, one or more of these four genes are lost. The rest of the patients, these genes are preserved, but not functioning.
The clinical manifestations of alpha-thalassemia correlate with the degree of impaired synthesis of the alpha-globin chain, but they are usually less pronounced than with β-thalassemia. This is due, firstly, to the fact that the presence of four alpha-globin genes contributes to the formation of an adequate number of alpha chains until three or four genes are lost. A significant imbalance of hemoglobin chains occurs only if three of the four genes are affected. Secondly, β-chain aggregates (β1 tetramers are formed when alpha chains are deficient) are more soluble than alpha 4 tetramers, and therefore even in patients with significantly impaired alpha globin synthesis with alpha thalassemia hemolysis is much weaker, and erythropoiesis more effective than β-thalassemia.
Alpha-Thalassemia (alpha-tal) is a hemolytic anemia caused by a deficiency in the synthesis of a-globin as a result of the loss or damage of one or more alpha-globin genes. The decrease in alpha-chain synthesis leads to the accumulation of free γ- and β-chains and the formation of tetramers-γ 4 (Hb Bart's) and unstable β 4 (Hb H) from them, followed by acceleration of erythrocyte destruction. Possessing a very high affinity for oxygen, these tetramers cannot perform the function of oxygen transfer. Thus, the clinical picture of severe alpha-tal is characterized by a combination of hypochromic anemia, hemolysis, and defective oxygen transport due to varying amounts of physiologically ineffective hemoglobin in erythrocytes. As a result, the degree of tissue hypoxia is much higher than expected with an appropriate degree of anemia.
There are 4 groups of clinical syndromes alpha-tal:
- silent carriage;
- alpha thalassemia with minimal changes;
- hemoglobinopathy H;
- alpha-thalassemic fetal dropsy.
The severity of the phenotypic manifestation of alpha-tal is directly proportional to the decrease in alpha-globin synthesis.
Silent carrier state (alpha-tal-2 heterozygotes)
Phenotypically dumb alpha-tal carriers differ little from healthy children. MCV is usually in the range of 78-80 fl, while the MCH can correspond to the lower limit of the norm. All other hematologic parameters are normal. Some dumb carriers may even have normal MCV values in the 80-85 fl range. In the blood of some of them, in the neonatal period, small amounts of Hb Bart's (<2%) disappear during the first months of life.
Small alpha-thalassemia-2 (asymptomatic carriage) - due to the loss of two alpha-globin genes on different chromosomes (trans-form). It is found in the inhabitants of Asia, Africa, and the Mediterranean. Hematological parameters do not differ from the norm; no clinical manifestations. In the neonatal period, an increased amount of Bart Hb is determined - 0.8-5%. In adults with a-thalassemia-2, the pathological HbH fractions of hemoglobin H Bart are not detected, the HbA 2 and HbF levels are normal.
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