The paper investigates how incorporating modified ribonucleotides like N1-methylpseudouridine (1-methylΨ) into mRNA vaccines affects their translation fidelity. They found that 1-methylΨ significantly increases +1 ribosomal frameshifting during mRNA translation, resulting in mistranslated proteins. This was shown using reporter mRNAs in vitro and in cells.
The +1 ribosomal frameshifting is likely caused by ribosome stalling during translation elongation when 1-methylΨ is present. It occurs at specific “slippery sequences” in the mRNA. In mice and humans vaccinated with a 1-methylΨ-containing mRNA vaccine (BNT162b2), T cell responses were detected against +1 frameshifted peptide products. This suggests cellular immunity can develop against mistranslated proteins.