Self-amplifying mRNAs

Self-amplifying mRNA (smRNA) is a special form of mRNA that is used in biomedical research and development, particularly for vaccines and gene therapies. This type of mRNA has the ability to replicate itself after it has been introduced into a cell.

Effect: While mRNA vaccines encode a defined protein, self-amplifying mRNAs (replicons) have been developed as a molecular chassis that encodes the gene of interest (GOI; transgenic gene) and all essential elements, thus enabling self-amplification of the replicon RNA. The rapid amplification of replicon RNA in target cells increases the expression of the protein of interest (e.g. a viral (glyco)protein) and induces a protective immune response, e.g. in vaccines, at a significantly lower initial RNA dose than conventional mRNA vaccines (Vogel AB et al. 2018; de Alwis R et al. 2021). The self-amplifying replicon genes were derived from a variety of positive-stranded RNA viruses. In this review, we focus on alpha- and flavivirus-based replicons as they are the best studied for use in both humans and animals. Since the viral structural genes have been replaced by a transgene, the replicon RNA cannot spread in the environment, which is a key difference from chimeric or recombinant viral vaccines (Suzuki R et al. 2014; Kamrud KI et al. 2010).

mRNA: The structure of in vitro transcribed mRNA closely resembles cellular mRNA and generally consists of a 5'-terminal 7-methylguanosine cap analog, a 5'- and 3'-untranslated region (UTR), the gene of interest (GOI) and a polyadenosine tail. In order to optimize the mRNA for vaccination purposes, modifications were made to increase vector stability, translational efficacy or immunogenicity. Since these nucleic acid vaccines exclusively encode the GOI, they cannot replicate in or spread to neighboring cells.

Replicon RNA: The structure of replicon RNA is similar to in vitro transcribed mRNA. However, this special form of mRNA also encodes viral replicase genes. These genes enable the rapid amplification of the mRNA and thereby increase the production of the GOI (gene of interest) compared to non-amplifying mRNAs. The self-amplifying viral genes originate from viruses such as alphaviruses and flaviviruses. Alphavirus-based replicons contain a separate open reading frame (ORF) upstream of the GOI that encodes all replicase proteins. In contrast, these replicase proteins in flavivirus-based replicons are encoded in a single ORF downstream of the GOI. Since replicon RNA does not encode all structural proteins of alphaviruses or flaviviruses, replicon RNA is propagation defective. Therefore, replicon RNA vaccines, similar to the non-amplifying mRNA vaccines, are selected as synthetic nucleic acid vaccines.

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