Mechanism of escape from nonsense-mediated mRNA decay of human β-globin transcripts with nonsense mutations in the first exon

Abstract

The degradation of nonsense-mutated b-globin mRNA by nonsense-mediated mRNA decay (NMD) limits the synthesis of C-terminally truncated dominant negative b-globin chains and thus protects the majority of heterozygotes from symptomatic b-thalassemia. b-globin mRNAs with nonsense mutations in the first exon are known to bypass NMD, although current mechanistic models predict that such mutations should activate NMD. A systematic analysis of this enigma reveals that (1) b-globin exon 1 is bisected by a sharp border that separates NMD-activating from NMD-bypassing nonsense mutations and (2) the ability to bypass NMD depends on the ability to reinitiate translation at a downstream start codon. The data presented here thus reconcile the current mechanistic understanding of NMD with the observed failure of a class of nonsense mutations to activate this important mRNA quality-control pathway. Furthermore, our data uncover a reason why the position of a nonsense mutation alone does not suffice to predict the fate of the affected mRNA and its effect on protein expression.

Repair of Single-Point Mutations by Polypurine Reverse Hoogsteen Hairpins

“…7C). In this approach, the point mutation could not activate NMD, since it is localized in the last exon of the dhfr gene and there is no downstream exon-exon junction (Maquat 2005;Neu-Yilik et al, 2011). Thus, negative control DA5 can show a very low level of DHFR protein.…”

Nonsense-mediated mRNA decay of collagen – emerging complexity in RNA surveillance mechanisms

“…The stop codons at positions 174,308,407,502,518 and 582 that did not cause NMD (Tan et al, 2008) are all upstream of at least one Kozak translation initiation sequence, providing a theoretical explanation for why these are resistant to NMD. However, reinitiation downstream of PTCs that have escaped from NMD has only been described for mutations that are preceded by a short open reading frame of 23 or less codons (Hamid et al, 2010;Neu-Yilik et al, 2011). In contrast, the open reading frames in Col10a1 transcripts that are resistant to NMD are much longer (between 174 and 582 codons) and, moreover, reinitiation appears unlikely based on our current understanding of the parameters that are important, such as a sufficiently short upstream open reading frame of less than about 30 codons and a sufficiently long intercistronic distance of greater than about 30 codons (Kochetov et al, 2008;Kozak, 1987;Morris and Geballe, 2000;Neu-Yilik et al, 2011).…”

“…However, reinitiation downstream of PTCs that have escaped from NMD has only been described for mutations that are preceded by a short open reading frame of 23 or less codons (Hamid et al, 2010;Neu-Yilik et al, 2011). In contrast, the open reading frames in Col10a1 transcripts that are resistant to NMD are much longer (between 174 and 582 codons) and, moreover, reinitiation appears unlikely based on our current understanding of the parameters that are important, such as a sufficiently short upstream open reading frame of less than about 30 codons and a sufficiently long intercistronic distance of greater than about 30 codons (Kochetov et al, 2008;Kozak, 1987;Morris and Geballe, 2000;Neu-Yilik et al, 2011). Thus, although translation reinitiation as the underlying cause of NMD resistance in the Regions of exons that are non-coding and coding are depicted as thin and thick boxes, respectively, with introns represented by lines.…”

Mutations inTJP2, encoding zona occludens 2, and liver disease

“…In addition, escape from NMD has been identified. 10 The investigation of the consequences of TJP2 protein-truncation was undertaken initially evaluating the quantitative relative expression of mRNA and showing a reduction of the gene expression, suggesting a possible activation of the NMD pathway. This finding was confirmed with subsequent studies undertaken at protein level, in which TJP2 protein, also known as zona occludens-2 (ZO-2), showed a complete absence of both isoforms by western blotting, and was not detected at the border of the canalicular membrane or cholangiocytes of liver tissues when examined by immunohistochemistry (Fig.…”