The thalasassemias are classified according to which chain of the globin molecule is affected: in alpha thalassemia, the production of alpha globin is deficient, while in beta thalassemia the production of beta globin is defective.
alpha thalassemias result in excess beta chain production in adults and excess gamma chains in newborns. The excess beta chains form unstable tetramers that have abnormal oxygen dissociation curves.
There are four genetic loci for alpha globin. The more of these loci that are deleted or affected by mutation, the more severe will be the manifestations of the disease.
If all four loci are affected, the fetus cannot live once outside the uterus: most such infants are dead at birth with hydrops fetalis, and those who are born alive die shortly after birth. They are edematous and have little circulating hemoglobin, and the hemoglobin that is present is all tetrameric gamma chains (hemoglobin Barts).
If three loci are affected, Hemoglobin H disease results. Two unstable hemoglobins are present in the blood, both hemoglobin Barts and hemoglobin H (tetrameric beta chains). There is a microcytic hypochromic anemia with target cells and Heinz bodies on the peripheral blood smear. The disease may first be noticed in childhoood or in early adult life, when the anemia and splenomegaly is noted.
If two of the four alpha loci are affected, alpha-thalassemia trait results. Two alpha loci permit nearly normal erythropoiesis, though there is a middle microcytic hypochomic anemia. There is a high prevalence (about 30%) of deletion of one of the two alpha loci on chromosomes of people of recent African origin, and so the inheritance of two such chormosomes is not uncommon. The disease in this form can be mistaken for iron deficiency anemia and treated inappropriately with iron.
If one of the four alpha loci is affected, there is minimal effect. Three alpha-globin loci are enough to permit normal hemoglobin production, and there is no anemia or hypochromia in these people. They have been called alpha thalassemia carriers.
In beta thalassemia, excess alpha chains are produced, but these do not form tetramers: rather, they bind to the red blood cell membranes, producing membrane damage. The severity of the damage depends on the nature of the mutation. Some mutations (beta-o) prevent any formation of beta chains; others (beta+) allow some beta chain formation to occur.
There are two beta globin genes. If both have thalassemia mutations, a severe anemia called beta thalassemia major or Cooley's anemia results. Untreated, this results in death before age twenty: treatment consists of periodic transfusion; splenectomy if splenomegaly is present, and treatment of transfusion-caused iron overload. Cure is possible by bone marrow transplantation.
If only one beta globin gene bears a mutation, beta thalassemia minor results. This is a mild anemia with microcytosis. Symptoms are weakness and fatigue.
Thalassemia in combination with other hemoglobinopathies
Thalassemia can co-exist with other hemoglobinopathies. The most common of these are:
hemoglobin E/thalassemia: common in Cambodia and Thailand, clinically similar to beta thalassemia major
hemoglobin S/thalassemia, common in African and Mediterranean populations; clinically similar to sickle cell anemia, with the additional feature of splenomegaly
hemoglobin C/thalassemia: common in Mediterranean and African populations, hemoglobin C/beta-o thalassemia causes a moderately severe hemolytic anemia with splenomegaly; hemoglobin C/beta+ thalassemia produces a milder disease.
Microcytic hypochromic anemia without increase in reticulocyte count not caused by iron deficiency with morphological abnormalities in peripheral blood smear. In severe cases bone abnormalities & splenomegaly.