Novel examples of NMD escape through alternative intronic polyadenylation

The nonsense-mediated mRNA decay (NMD) surveillance system detects and selectively eliminates transcripts with premature stop codons. A stop codon is considered premature if it is followed by an exon-exon junction more than 50 nucleotides downstream. Pruning of the 3-untranslated region containing such junctions through alternative polyadenylation may provide a mechanism of NMD escape. Here, we systematically examine a subclass of poison exons that carry a premature stop codon for the presence of polyadenylation sites in the downstream intron. Using data from the GTEx consortium, we observed that poison exons are more often followed by an active polyadenylation site compared with cassette exons. We also identified tissue-specific switches between NMD-targeted and NMD-escape isoforms in several human genes, including the vaccinia-related kinase VRK3, nuclear transcription factor NFX1, Notch pathway regulator TM2D3, and RNA helicase DDX31. Blocking the cleavage and polyadenylation sites in these genes using antisense oligonucleotides in human cells led to a switch from NMD-escape to NMD-target isoform, accompanied by a decrease in gene expression levels. This study reveals that NMD escape via alternative polyadenylation is a widespread, yet currently overlooked post-transcriptional mechanism of gene expression regulation.