Do RNA vaccines obviate the need for genotoxicity studies?

In the context of preliminary concerns with vaccine vectors, it may be also be constructive to discuss whether RNA vaccines would have qualities that obviate any apprehension for cellular transformation and a need for regulatory genotoxicity studies (1). Advances in the application of these vaccines are no less an emerging reality as evidenced by the successful mRNA vaccine production for COVID-19 which has progressed to Phase 3 human studies (2,3). Enthusiasm for RNA vaccines has been accompanied by the belief that genomic integration is not possible (4,5). With the uptake into the cytoplasm rather than nucleus, and with the presumed lack of ability for cytoplasmic RNA to be integrated without reverse transcription, others have continued to echo this feature of presumed safety (6,7). Hinz et al. pose this issue as ‘… not even a theoretical risk for insertional mutagenesis …’ (8).

It was near half of a century ago that Howard Temin and his contemporaries projected on the concept of RNA-directed DNA synthesis and its integrative potential, ‘Finding an RNA-directed DNA polymerase in uninfected tissues of normal organisms may be the first direct evidence bearing on this hypothesis’ (9). Such a concept was realised early in the study of retroviruses. In contemporary times, however, the ability of eukaryotic cells to provide reverse transcriptase expression is now accepted (10–13). The diversity of this class of enzymes as well as their common roles in routine biological processes has been confirmed (14,15). Furthermore, the potential role for the same enzymes to modulate or promote oncogenesis or cellular proliferation otherwise has also been the subject of research (16,17). Shimizu et al. drive this issue further with the finding of cytoplasmic DNA in cells that is complementary to non-retroviral RNA viruses and that is perhaps explanatory of the integration of bornavirus and filovirus sequences in the genomes of several mammals (18).

The use of modified nucleosides in the development of stabilised mRNA vaccines may also add another measure of complexity (19,20). These chemical modifications yield an mRNA that is less immunogenic since unmodified mRNA can be more commonly recognised by Toll-like receptors (7). Different such modifications may also be more stable or yield greater protein expression (21). Nucleoside modification however may affect secondary RNA structures and as well may have an impact on a variety of cellular functions (22,23). It is unclear how the latter attributes may act in tandem to any potential for mRNA integration or first-hit events. Whether any such modifications in an mRNA vaccine could have deleterious impact on eukaryotic cell function also remains to be seen on a larger scale.

In the regulatory field, several research groups have considered vaccine safety and RNA vaccines in particular (6,8,24–27). mRNA vaccines are not considered to be gene therapy products (25). Whereas it is generally believed that vaccine formulations do not require genotoxicity studies, newer versions of vaccines such as DNA or RNA vaccines attract a different level of discussion. Authors in this regulatory field carefully advise consultation with national regulatory agencies for specific guidance (25). More recent specific discussion of COVID-19-targeted vaccines focuses commentary on the developmental and reproductive toxicity and biodistribution (26).

The potential for exogenous RNA, viral or other, to integrate into human DNA from vaccination is at best theoretical at this time. The potential for any such RNA to fundamentally drive altered or oncogenic processes in human cells is also theoretical. Indeed, some may argue that the infections from RNA (DNA) viruses better present viral RNA (or DNA) into the cellular locations where integration or first-hit events occur more than any potential vaccine. Until any such hypothetical concerns are tested or observed from vaccination or natural infection, regulatory safety assessments of RNA vaccines should include genotoxicity studies. The requisite to provide such data to regulatory bodies and the experience of the regulators will be reassuring, but it is yet surprising that there is a dearth of such data published widely in the scientific and medical communities.

Funding

Funding was not sought for this publication. There is no third party support including that from the pharmaceutical industry.

Conflict of interest statement: None declared.