Nonsense-mediated RNA decay (NMD) is an RNA-based quality control mechanism that eliminates transcripts bearing premature translation termination codons (PTC). Approximately, one-third of all inherited disorders and some forms of cancer are caused by nonsense or frame shift mutations that introduce PTCs, and NMD can modulate the clinical phenotype of these diseases. 5-azacytidine is an analogue of the naturally occurring pyrimidine nucleoside cytidine, which is approved for the treatment of myelodysplastic syndrome and myeloid leukemia. Here, we reveal that 5-azacytidine inhibits NMD in a dose-dependent fashion specifically upregulating the expression of both PTC-containing mutant and cellular NMD targets. Moreover, this activity of 5-azacytidine depends on the induction of MYC expression, thus providing a link between the effect of this drug and one of the key cellular pathways that are known to affect NMD activity. Furthermore, the effective concentration of 5-azacytidine in cells corresponds to drug levels used in patients, qualifying 5-azacytidine as a candidate drug that could potentially be repurposed for the treatment of Mendelian and acquired genetic diseases that are caused by PTC mutations.

Due to their capability to transport chemicals or proteins into target cells, cell‐penetrating peptides (CPPs) are being developed as therapy delivery tools. However, and despite their interesting properties, arginine‐rich CPPs often show toxicity for reasons that remain poorly understood. Using a (PR)n dipeptide repeat that has been linked to amyotrophic lateral sclerosis (ALS) as a model of an arginine‐rich CPP, we here show that the presence of (PR)n leads to a generalized displacement of RNA‐ and DNA‐binding proteins from chromatin and mRNA. Accordingly, any reaction involving nucleic acids, such as RNA transcription, translation, splicing and degradation, or DNA replication and repair, is impaired by the presence of the CPPs. Interestingly, the effects of (PR)n are fully mimicked by protamine, a small arginine‐rich protein that displaces histones from chromatin during spermatogenesis. We propose that widespread coating of nucleic acids and consequent displacement of RNA‐ and DNA‐binding factors from chromatin and mRNA accounts for the toxicity of arginine‐rich CPPs, including those that have been recently associated with the onset of ALS.