mRNA as gene therapeutic: how to control protein expression

For many years, it was generally accepted that mRNA is too unstable to be efficiently used for gene therapy purposes. In the last decade, however, several research groups faced this challenge and not only proved the feasibility of mRNA-mediated transfection with surprising results regarding transfection efficiency and duration of protein expression, but also were able to demonstrate major advantages over the use of pDNA. These advantages will be the first issue discussed in this review, which first of all addresses the notions that mRNA does not need to cross the nuclear barrier to exert its biological activity and in addition lacks CpG motifs, which reduces its immunogenicity. Secondly, it provides insight in the (in)stability of the mRNA molecule, in how mRNA can be modified to increase its half-life and in the necessities of exogenously produced mRNA to be successfully used in transfection protocols. Furthermore, this review gives an in-depth overview of the different techniques and vehicles for intracellular mRNA delivery exploited by us and other groups, comprising electroporation, gene gun injection, lipo- and polyplexes. Finally, it covers recent literature describing specific applications for mRNA based gene delivery, showing that until now most attention has been paid to vaccination strategies. This review offers a comprehensive overview of current knowledge of the major theoretical as well as practical aspects of mRNA-mediated transfection, showing both its possibilities and its pitfalls and should therefore be useful for a diverse scientific audience.