Summary
A bioRxiv preprint reports that nonsense-mediated decay suppresses many RNA isoforms created when transposable-element sequences become part of genes. The findings suggest this cellular quality-control system helps regulate a pool of potential evolutionary novelty.
A bioRxiv preprint reports that nonsense-mediated decay, a cellular process that breaks down certain RNA molecules, suppresses many gene transcripts containing sequences from transposable elements. The authors’ analyses suggest this RNA quality-control system filters some forms of genetic novelty before they can persist as active transcripts or proteins.
How RNA surveillance filters new gene transcripts
Nonsense-mediated decay (NMD) helps cells identify and degrade RNA carrying a premature stop signal, which can lead to an incomplete protein. Transposable elements are DNA sequences that can move or be copied within genomes. When a transposable-element sequence is incorporated into a gene’s RNA as an exon—a segment retained in the mature transcript—the process is called exonization. It can create an alternative version of a gene’s transcript.
To examine NMD’s role, Tobias Plowman and George Kassiotis inhibited the process in cell lines and used long-read RNA sequencing to assemble an expanded transcriptome of about 217,000 transcripts. Roughly 80% were unannotated. Transcripts regulated by NMD were enriched for transposable-element exonization, particularly involving long terminal repeat (LTR) and long interspersed nuclear element (LINE) sequences. These were largely alternative forms of already annotated genes.
The authors also compared the transposable elements across evolutionary data. Elements whose transcripts were silenced by NMD tended to be evolutionarily younger and more diverged, with younger and less conserved splice sites. They were also more likely to encode protein isoforms with functions distinct from those associated with elements tuned by NMD or found in stably expressed transcripts.
A reservoir of potential evolutionary novelty
The results point to NMD as more than a system for clearing faulty RNA: it may also regulate which transposable-element-derived changes are expressed. In this interpretation, NMD can suppress some newly formed transcript variants while leaving a pool of potential innovations whose effects depend on whether they are expressed and maintained.
The researchers report that primate-specific Alu elements created alternative forms of the DDX49, NEIL3 and SETDB1 genes. These isoforms produced stable proteins with distinct effects on biological activity. This provides examples of transposable-element-derived RNA variants making functional proteins, while the broader evolutionary significance of such variants remains an interpretation of the comparative and experimental findings.
The preprint also reports elevated NMD activity in lung cancer and an association between NMD activity and improved survival in lung squamous cell carcinoma. That survival result is an association, separate from the cell-line experiments. The work is posted on bioRxiv as a preprint.