By Kai Yee Eng
RNA has emerged as a new class of drug in the pharmaceutical industry in the 21st century. With the current pandemic, RNA as a vaccine has received more attention than any time in history. First discovered in the 1960s, the role of messenger RNA to translate codons into proteins highlights its importance in our cells. (Kim, 2020) The discovery of RNA interference in 1998, which shows that double-stranded RNA can induce suppression of genes in a homology dependent manner, further implies that RNA molecules play an extensive and tightly regulated role in cellular activity. (The 2006 Nobel Prize in Physiology or Medicine – Advanced information, 2006) Therefore, it is no surprise that there is great potential in designing RNA for use in clinical practice.
The main classes of RNA in cancer therapy include mRNA vaccines, RNA interference and antisense oligonucleotides. The application of mRNA vaccines in cancer can be separated into two pathways: immunotherapy and personalized medicine. The immune system can fight and eradicate cancer cells as they are tagged as “foreign” particles. (How Immunotherapy Is Used to Treat Cancer, 2019) However, some cancer cells are not differentiated enough, so the immune system fails to recognize these cells. Thus, the cancer cells can bypass the checkpoint and divide uncontrollably to develop into a tumour. Additionally, in some cases, the immune response is too small to outcompete the cancer cells. The first function of mRNA vaccine is thus aimed at triggering an immune response to target these cancerous cells. To initiate an immune response, an antigen is needed. The naked mRNA in the vaccine serves as a template to produce the antigen, and another component, mRNA-protamine complexes, is used to produce proteins to amplify the immune response. (Fiedler et al., 2016; Sahin & Türeci, n.d.) mRNA vaccines can be tailored to target a specific tumour, making it the best-suited vaccine for the individual. This is done via next-generation sequencing, which identifies the mutations in patients. In this case, the mRNA can be designed to better express the antigens of the mutated cells, the neoantigen. One example of a cancer vaccine recently developed is the KRAS cancer vaccine by Moderna and Merck. KRAS is an oncogene in which a mutation is commonly observed in epithelial cells, leading to carcinomas. The vaccine is therefore designed to present the mutated KRAS protein as the targeted antigen, inducing an antitumour response via T cell activation. (Anon, n.d., Anon, n.d.) This technique provides an additional protection to other cells as only the cancerous cells exhibit the mutation. (Fiedler et al., 2016)
In addition to mRNA, the RNA interference pathway triggered by non-coding RNA is a highly selective tool for modifying RNA expression, making it a possible target for cancer therapy. RNA interference inhibits translation by degrading the transcript through a complex pathway. It is one of the mechanisms our body has evolved to defend against RNA viruses and endogenous transposons. (Bajan & Hutvagner, 2020; Mansoori, Shotorbani & Baradaran, 2014) To date, most RNA interference candidates for cancer therapy use this pathway to inhibit tumor growth. (Pai et al., 2006) This mechanism is now known as an alternative for patients who have gained resistance to chemotherapy or radiotherapy via disruption of related pathways, e.g. p53, which regulates cell cycle and the apoptosis pathway. (Pai et al., 2006)
Short interference RNA (siRNA) and short hairpin RNA (shRNA) both works to silence the gene by degrading mRNA, while bifunctional shRNA (bishRNA) is a designed RNA interference to enhance the effect of RNA interference, leading to a similar outcome in siRNA and shRNA. (Mansoori, Shotorbani & Baradaran, 2014) As opposed to mRNA vaccines, the RNA interference pathway targets the protein level of the cancerous cells, inhibiting the chemical interactions. Therefore, this pathway can target more than one cellular pathway at a time by silencing the gene and thus preventing growth of cancerous cells. A study has suggested that RNA interference can induce apoptosis, inhibit cell migration, and lead to cell cycle termination, which targets the hallmarks of cancer: resisting cell death, activating invasion and metastasis, and sustaining proliferative signalling. (Tao et al., 2005)
Lastly, antisense oligonucleotides are short fragments of RNA (or DNA) which bind with the target RNA to prevent the function of the targeted sequence. (Bajan & Hutvagner, 2020) This provides an advantage as some proteins are difficult to target. The mechanism of antisense oligonucleotide can be mediated via two broad pathways: promoting RNA degradation and interfering the RNA function. The latter is mediated via induction of cell activities such as translation arrest to result in reduce expression of proteins involved in cancer development. (Bennett et al., 2017). Studies also show that antisense oligonucleotides can induce an increase in protein expression, in contrast to previous theories as to the role of antisense oligonucleotides. (Liang et al., 2016) The development of antisense oligonucleotides has been focused on oncogene pathways, such as Raf kinase, Bcl-2 family and protein kinase A R1-α. (Stahel & Zangemeister-Wittke, 2003)
Aptamers, a small size oligonucleotide, are an alternative to the above treatment with the advantage of being smaller in size yet still having a high specificity. Unlike mRNA vaccines and RNA in RNA interference, aptamers do not induce an immune response in vivo. With SELEX technologies, aptamers are designed to recognize biomarkers. Moreover, they can be delivered in various ways – including as nanoparticles and even conjugated with siRNA – providing more flexibility. (Sun et al., 2014)
While RNA therapy provides an alternative therapeutic target to cancer, it is still very difficult to translate into clinical practice. The delivery of RNA has been an issue which directly affects its efficacy. (Kim, 2020; Liang et al., 2020) The structure of RNA itself as an unstable molecule and the delivery of RNA may cause an immune response, which results in reduce efficiency and inflammation in local tissue. The entry of RNA into the cytosol of the targeted cells is not always accurate, and in the process of transporting into targets in cytosol, RNA can be easily broken down by enzymes and acids in lysosomes. While these issues remain to be solved, the design of RNA therapy is rapid and can provide long term or transient effects. (Kim, 2020) The development of RNA therapy has progressed rapidly in the past 30 years, targeting a variety of diseases, and is expected to bloom as an alternative therapy for cancer patients.
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