Beta-thalassemia

Abstract

Beta-thalassemia is caused by the reduced (beta) or absent (beta) synthesis of the beta globin chains of the hemoglobin tetramer. Three clinical and hematological conditions of increasing severity are recognized, i.e., the beta-thalassemia carrier state, thalassemia intermedia, and thalassemia major. The beta-thalassemia carrier state, which results from heterozygosity for beta-thalassemia, is clinically asymptomatic and is defined by specific hematological features. Thalassemia major is a severe transfusion-dependent anemia. Thalassemia intermedia comprehend a clinically and genotypically very heterogeneous group of thalassemia-like disorders, ranging in severity from the asymptomatic carrier state to the severe transfusion-dependent type. The clinical severity of beta-thalassemia is related to the extent of imbalance between the alpha and nonalpha globin chains. The beta globin (HBB) gene maps in the short arm of chromosome 11, in a region containing also the delta globin gene, the embryonic epsilon gene, the fetal A-gamma and G-gamma genes, and a pseudogene (psiB1). Beta-thalassemias are heterogeneous at the molecular level. More than 200 disease-causing mutations have been so far identified. The majority of mutations are single nucleotide substitutions, deletions, or insertions of oligonucleotides leading to frameshift. Rarely, beta-thalassemia results from gross gene deletion. In addition to the variation of the phenotype resulting from allelic heterogeneity at the beta globin locus, the phenotype of beta-thalassemia could also be modified by the action of genetic factors mapping outside the globin gene cluster and not influencing the fetal hemoglobin. Among these factors, the ones best delineated so far are those affecting bilirubin, iron, and bone metabolisms. Because of the high carrier rate for HBB mutations in certain populations and the availability of genetic counseling and prenatal diagnosis, population screening is ongoing in several at-risk populations in the Mediterranean. Population screening associated with genetic counseling was extremely useful by allowing couples at risk to make informed decision on their reproductive choices. Clinical management of thalassemia major consists in regular long-life red blood cell transfusions and iron chelation therapy to remove iron introduced in excess with transfusions. At present, the only definitive cure is bone marrow transplantation. Therapies under investigation are the induction of fetal hemoglobin with pharmacologic compounds and stem cell gene therapy.