mRNA Medicine: what’s next after the COVID-19 vaccine?

By Antonia Du Bois, MScReviewed by Danielle Ellis, B.Sc.

What is mRNA, and how can it be used therapeutically?
Nucleic acid therapies vs. protein-based therapies
Protein replacement therapy
Immunotherapies
Gene editing 
Challenges facing clinical approval of mRNA therapies
References 
Further reading 

The coronavirus 2019 (COVID-19) pandemic has brought cutting-edge scientific approaches like messenger ribonucleic acid (mRNA) therapeutic technologies into the public eye. In addition to the breakthrough vaccines protecting against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), mRNA technologies possess a massive potential for gene therapy, anticancer therapy, and regenerative medicine.

What is mRNA, and how can it be used therapeutically?

Where deoxyribonucleic acid (DNA) acts primarily as a method of storing information and instructions for cellular function, RNA has a host of other responsibilities, including protein coding, post-translational gene silencing, and gene expression regulation. mRNA is critical to the former, encoding proteins necessary for cellular function.

The use of mRNA and DNA in therapeutic endeavors such as gene therapy, vaccines, and immunotherapy reached pre-clinical stages beginning in the 1990s. While DNA was initially explored as an option, mRNA has drawn more recent attention.

While both nucleic acid-based approaches, DNA-based therapies have the drawback of requiring rapid entry into the cytoplasm and nucleus of the cell to be effective. This necessity for nuclear entry represents a potential drawback. mRNA-based therapies improve upon this by not requiring nuclear entry. Additionally, there is no risk of internal mutagenesis of the genome.

mRNA-based therapies also allow for greater control of gene expression levels, and structural customizability can offer tailored and optimized pharmacokinetics. Unfortunately, mRNA therapeutics can be prone to instability in physiological conditions, transient beneficial effects, and significant immunogenicity. Current research is examining potential methods of mitigating these drawbacks, such as drug delivery systems to counter instability.

Nucleic acid therapies vs. protein-based therapies

Protein-based therapies have also been explored but are surpassed by nucleic acid technologies in several respects.

Firstly, nucleic acid therapies are far cheaper to produce than protein-based therapies. Secondly, nucleic acids avoid replicating specific post-translational modifications, which can be very difficult to guarantee in protein production. Finally, certain proteins, such as membrane proteins, are unable to be synthesized in vitro, which limits what may be available to researchers.

Protein replacement therapy

In pre-clinical development, the use of mRNA in protein replacement therapy seeks to address monogenic diseases through increasing expression and replacing or restoring the function of key proteins. Often targeting organs such as the liver, this is often approached through the expression of foreign proteins that modulate cellular pathways or by supplementing non-functional or under-expressed native proteins.

As an example, Baba et al. used mRNA-based experimental therapeutics to provide native and mature forms of neural cellular proteins to treat neurological disorders. Using mRNA-loaded nanocarriers, mouse models of nasal administration showed significant promise in the form of recovered olfactory function.

Immunotherapies

The general goal of mRNA-based immunotherapy is to elicit immune responses of a specific nature against a target cell. The SARS-CoV-2 vaccines by Pfizer and Moderna represent a well-known example of mRNA-based immunotherapies in the form of a vaccination. Showing impressive effectiveness in mitigating the COVID-19 pandemic, these vaccines represent the promise of mRNA-based immunotherapy.

As a vaccine approach, mRNA provides many benefits such as low production costs, the possibility of large-scale production, and self-adjuvant effects. Beyond infectious disease, this approach has applications in cancer treatment as well.

Lei et al. (2020) explored these applications using mRNA-simulated lymphocytes to express cytokines which subsequently induced cytotoxicity in cancerous models. These technologies have also demonstrated promise in other areas of cancer research, such as colon cancer.

These approaches improve on previous antibody-based therapeutics, which carried problems of reliance on antibody production, low stability, and aggregation. 

Gene editing

An alternative approach to mRNA-based therapeutics is using those nucleic acids in treating disease by encoding genome-editing nucleases. These can include the encoding of zinc finger nucleases (ZFNs), transcription activator effector nucleases (TALENs), and CRISPR-Cas9. The endonucleases in question behave as tools to replace genes and/or modify their expression by introducing deletions and protective mutations.

With such targeted modifications of the genome, these approaches are expected to offer permanent cures to genetic diseases once perfected. The use of Cas-9 with mRNA delivery systems, as opposed to DNA has the added benefit of not requiring nuclear entry. Additionally, mRNA delivery shows reduced off-target effects when compared to plasmid DNA delivery.

Current studies go beyond CRISPR-Cas-9 to other systems into related approaches, such as Cas-13, to optimize effects in mammalian cells. Cas-13 has shown promise in SARS-CoV-2 detection and the study of facioscapulohumeral muscular dystrophy. Rashnonejad et al. developed Cas-13b gRNAs capable of improving cell viability and decreasing cell death in vivo.

Challenges facing clinical approval of mRNA therapies

While the aforementioned mRNA technologies and therapeutic approaches are highly encouraging, there are many hurdles still to be overcome and questions to be researched to gain clinical approval for a market-ready therapy.

Among these, the inherent instability of mRNA must be addressed, with current efforts focusing on the optimization of delivery systems and mRNA structures. Additionally, inherent immunogenicity requires that research examines methods of regulating the mRNA’s activation of the immune system.

Despite the long road to travel for many experimental mRNA-based therapeutics, the use of mRNA technology in currently available SARS-CoV-2 vaccines is an example of the possibilities of this new frontier in molecular therapeutics.

References

• Baptista, B., et al. (2021). mRNA, a Revolution in Biomedicine. Pharmaceutics, 13(12) pp. 2090, doi: 10.3390/pharmaceutics13122090
• Chabanovska, O., et al. (2021). mRNA – A game changer in regenerative medicine, cell-based therapy, and reprogramming strategies. Advanced Drug Delivery Reviews, 179, pp. 114002, doi: 10.1016/j.addr.2021.114002
• Hogan, M.J., and Pardi, N., (2022). mRNA Vaccines in the COVID-19 Pandemic and Beyond. Annual Review of Medicine, 73(1) pp. 17-39, doi: 10.1146/annurev-med-042420-112725
• Qin, S., et al. (2022). mRNA-based therapeutics: powerful and versatile tools to combat diseases. Signal Transduction and Targeted Therapy, 7(1) pp. 166, doi: 10.1038/s41392-022-01007-w

Further Reading

• All RNA Content
• What is RNA?
• RNA Structure
• Types of RNA: mRNA, rRNA and tRNA
• RNA Synthesis