Starting with the clinical application of two vaccines in 2020, mRNA therapeutics are currently being investigated for a variety of applications. Removing immunogenic uncapped mRNA from transcribed mRNA is critical in mRNA research and clinical applications. Commonly used capping methods provide maximum capping efficiency of around 80–90% for widely used Cap-0- and Cap-1-type mRNAs. However, uncapped and capped mRNA possesses almost identical physicochemical properties, posing challenges to their physical separation. In this work, we develop hydrophobic photocaged tag-modified cap analogs, which separate capped mRNA from uncapped mRNA by reversed-phase high-performance liquid chromatography. Subsequent photo-irradiation recovers footprint-free native capped mRNA. This approach provides 100% capping efficiency even in Cap-2-type mRNA with versatility applicable to 650 nt and 4,247 nt mRNA. We find that the Cap-2-type mRNA shows up to 3- to 4-fold higher translation activity in cultured cells and animals than the Cap-1-type mRNA prepared by the standard capping method.

Site-specific chemical modification of mRNA can improve its translational efficiency and stability. For this purpose, it is desirable to develop a complete chemical synthesis method for chemically modified mRNA. The key is a chemical reaction that introduces a cap structure into the chemically synthesized RNA. In this study, we developed a fast and quantitative chemical capping reaction between 5′-phosphorylated RNA and N7-methylated GDP imidazolide in the presence of 1-methylimidazole in the organic solvent dimethyl sulfoxide. It enabled quantitative preparation of capping RNA within 3 h. We prepared chemically modified 107-nucleotide mRNAs, including N 6-methyladenosine, insertion of non-nucleotide linkers, and 2′-O-methylated nucleotides at the 5′ end and evaluated their effects on translational activity in cultured HeLa cells. The results showed that mRNAs with non-nucleotide linkers in the untranslated regions were sufficiently tolerant to translation and that mRNAs with the Cap_2 structure had higher translational activity than those with the Cap_0 structure.

Recently messenger RNA (mRNA) therapeutics is received much attention as one of the vaccination therapies to compete against the coronavirus disease 2019 (COVID-19) pandemic.mRNA therapeutics are generally produced by in vitro transcription utilizing RNA polymerase mediated elongation. However, its purity, stability, and protein synthesis ability, are difficult to be precisely controlled, which is pointed out as drawbacks that must be overcome. To overcome these issues, the introduction of chemically modified nucleic acids is focusing attention. However, it is difficult to flexible molecular design due to the requirement of RNA polymerase recognition ability of chemically modified nucleic acids under in vitro transcription reaction. In the future, the development of a new mRNA design concept based on a flexible molecular design by the progress of chemically modified mRNA therapeutics synthesis method. Under the situation, the authors are focusing on the translation mechanism of mRNA and proposing a new mRNA molecular design to accelerate the translation reaction cycle. In this paper, we introduce an update on therapeutic mRNA design.

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Removing immunogenic uncapped mRNA from in vitro transcribed mRNA is critical in mRNA research and clinical applications. Commonly used capping methods provide a maximum capping efficiency of around 80-90% for widely used Cap-0- and Cap-1-type mRNAs. However, uncapped and capped mRNA possesses almost identical physicochemical properties, posing challenges to their physical separation. Herein, we developed hydrophobic photocaged tag-modified cap analogs, which separated capped mRNA from uncapped mRNA by reversed-phase HPLC. Subsequent photo-irradiation recovers footprint-free native capped mRNA. This approach provided 100% capping efficiency even in Cap-2-type mRNA with versatility applicable to 650 nt and 4,247 nt mRNA. The Cap-2-type mRNA showed up to 3 to 4-fold higher translational activity in cultured cells and animals than mRNA prepared by the standard capping method. Notably, the purification process simultaneously removed immunogenic double-stranded mRNA, another major contaminant of in vitro transcribed mRNA, drastically reducing mRNA immunogenicity in cultured cells.