Alternatively spliced T-cell receptor transcripts are up-regulated in response to disruption of either splicing elements or reading frame

Nonsense mutations create premature termination codons (PTCs), leading to the generation of truncated proteins, some of which have deleterious gain-of-function or dominant-negative activity. Protecting cells from such aberrant proteins is nonsense-mediated decay (NMD), an RNA surveillance pathway that degrades transcripts harboring PTCs. A second response to nonsense mutations is the up-regulation of alternatively spliced transcripts that skip the PTC. This nonsense-associated altered splicing (NAS) response has the potential to rescue protein function, but the mechanism by which it is triggered has been controversial. Some studies suggest that, like NMD, NAS is triggered as a result of nonsense mutations disrupting reading frame, whereas other studies suggest that NAS is triggered when nonsense mutations disrupt exonic splicing enhancers (ESEs). Using T-cell receptor-β (TCRβ), which naturally acquires PTCs at high frequency, we provide evidence that both mechanisms act on a single type of mRNA. Mutations that disrupt consensus ESE sites up-regulated an alternatively spliced TCRβ transcript that skipped the mutations independently of reading frame disruption and the NMD factor UPF1. In contrast, reading frame disrupting mutations that did not disrupt consensus ESE sites elicited UPF1-dependent up-regulation of the alternatively spliced TCRβ transcript. Restoration of reading frame prevented this up-regulation. Our results suggest that the response of an mRNA to a nonsense mutation depends on its context.