FIGURE 10-34 Discovery of nonsense-mediated decay (NMD). (a) Patients with β⁰-thalassemia express very low levels of β-globin mRNA. A common cause of this syndrome is a single-base-pair deletion in exon 1 or exon 2 of the β-globin gene. Ribosomes translating the mutant mRNA read out of frame following the deletion and encounter a stop codon in the wrong reading frame before they translate across the last exon-exon junction in the mRNA. Consequently, they leave an exon-junction complex (EJC) in place on the mRNA. Cytoplasmic proteins associate with the EJC and induce degradation of the mRNA. (b) Bone marrow was obtained from a patient with a wild-type β-globin gene and from a patient with β⁰-thalassemia. RNA was isolated from the bone marrow cells shortly after collection and again 30 minutes after incubation in media with actinomycin D, a drug that inhibits transcription. The amount of β-globin RNA was measured using the S1-nuclease protection method (arrow). The patient with β⁰-thalassemia had much less β-globin mRNA than the patient with a wild-type β-globin gene (−Act D). The mutant β-globin mRNA decayed rapidly when transcription was inhibited (+Act D), whereas the wild-type β-globin mRNA remained stable.

[Part (b) republished with permission of Elsevier, from Maquat, L.E., et al., “Unstable β-globin mRNA in mRNA-deficient β⁰ thalassemia,” Cell, 1981, 27(3 Pt2):543–53; permission conveyed through Copyright Clearance Center, Inc.]