Nonsense mutations in the human beta globin gene affect mRNA metabolism
A common mutation causing thalassemia in Mediterranean populations is an amber (UAG) nonsense mutation at the 39th codon of the human β globin gene, the β-39 mutation. Studies of mRNA metabolism in reticulocytes from patients with β-39 thalassemia and studies using heterologous transfection systems have suggested the possibility that this mutation affects not only protein synthesis but also alters mRNA metabolism. This phenomenon has been investigated further by two approaches. A careful series of RNA expression studies were performed comparing expression of β-39 to β-normal (β-nl). These experiments led to the conclusion that the defect in expression of the β-39 mRNA resides in the nucleus. A number of nonsense and missense mutations of the β globin gene were constructed by oligonucleotide-directed site-specific mutagenesis. Their expression was studied in a heterologous transfection system. These studies strongly suggest that the presence of a nonsense mutation (but not a missense mutation) in β-globin mRNA decreases the accumulation of its mRNA. Decreased β-globin mRNA accumulation can be caused by the presence of a nonsense mutation, independent of its type or location.