N1-Methyl-Pseudouridine: The Evolution, Impact, and Future of a Key mRNA Vaccine Modification

Nucleoside modifications to the template of mRNA vaccines were important in achieving the desired level of antigen expression and immune response for the vaccine platform to function successfully during the COVID-19 pandemic. The innovation solved a crucial issue and resulted in the awarding of the 2023 Nobel Prize in Medicine to Katalin Karikó and Drew Weissman. Despite the spotlight on nucleoside modifications, several important aspects of its behavior in living systems were only discovered recently, particularly context specific translational accuracy, and the stimulation of +1 ribosomal frameshifting at specific “slippery sequences”. The latter has been observed in living subjects (mouse and human) to produce both off-target antigens and an off-target immune response. While adverse effects cannot currently be attributed to poor translational fidelity, this may spell issues for future iterations of mRNA therapeutics if not addressed. Fortunately, in most cases, the transcript sequence can be modified to allay these concerns and produce faithful protein products, by avoiding sequence contexts resulting in either incorrect amino acid incorporation or +1 frameshifting. The modularity of the mRNA platform makes these fixes easy to incorporate, as long as this is accounted for in future mRNA therapeutics design.