β° Thalassemia in Sardinia is caused by a nonsense mutation

Abstract

We report the characterization of a molecular lesion of β thalassemia in Sardinia. Beta thalassemia in this area is predominantly the β° type with low levels of β-globin mRNA. Translation assay of this messenger RNA in a cell-free system showed β-globin chain synthesis only with the addition of an amber (UAG) suppressor transfer RNA. Double-stranded complementary DNA prepared from recticulocyte mRNA from a Sardinian patient was cloned in a bacterial plasmid and a β-globin complementary DNA containing clone was isolated and sequenced. At the position corresponding to amino acid number 39, a single nucleotide mutation converted a glutamine codon (CAG) to an amber termination codon (UAG). We previously reported an amber nonsense mutation at amino acid 17 as a cause of Chinese β° thalassemia. Thus, β° thalassemia in Sardinia represents the second example of a nonsense mutation, and we predict that other β° thalassemias with mutations at various points along the β-globin chain will be found to form a discrete subgroup of β° thalassemia. These experiments further illustrate the heterogeneity of lesions that lead to defective globin chain synthesis in β thalassemia.