MicroRNA and MRNA Degradation

Lung cancer (LC) remains one of the primary causes of cancer-related death in men and women worldwide. In 2020, approximately 228,820 new cases and 135,720 deaths due to LC were reported in the United States alone. LC is categorized as non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC). These two main subtypes have significant intra-tumor heterogeneity, further classified according to mutations and drivers. The majority of this disease falls under the category of NSCLC, which includes adenocarcinoma, squamous cell carcinoma, and large cell carcinoma. Approximately 10-15% of cases belong to SCLC and are categorized into SCLC-A, SCLC-N, SCLC-Y and SCLC-P subtypes. Statistics of the last two decades have shown that the 5-year survival for NSCLC is less than 20% and for SCLC about 5%.
Some of the oncogens that are routinely investigated for targeting in NSCLC include the Kirsten rat sarcoma viral oncogene homologue (KRAS), the epidermal growth factor receptor (EGFR), and the echinoderm microtubule-associated protein-like 4-anaplastic lymphoma kinase (EML4-ALK). Among the genes involved in SCLC, poly [ADP-riboz] polymerase (PARP), delta-like protein 3 (DLL3), aurora kinases, and vascular endothelial growth factor (VEGF). Approximately 30% of lung cancer patients harbor activating KRAS mutations, making it a potential drug target for LC therapy. However, the mutant KRAS targeting drugs has been developed for many years and is currently being evaluated in clinical trials. Similarly, treatment with tyrosine kinase inhibitors was relatively ineffective in improving overall survival (OS) in patients harboring EGFR mutations.MicroRNA and MRNA Degradation
There are similar gaps in SCLC treatments, for example, most patients develop resistance to chemotherapies, and because of the restricted expression of receptor antigens (PD1 / PD-L1), immunotherapies show a narrow range of activity. The main reason for the failure of currently available therapeutic approaches is the development of drug resistance associated with gene mutations, cancer stem cells, overexpression of oncogenes, and deletion or inactivation of tumor suppressor genes. The collective results showed that the tried and validated therapeutic regimen to save LC patients was lacking and continues to be expected. To overcome these limitations, there is a rapidly growing interest in the field of RNA interference (RNAi) and RNA-based therapeutics. Because several studies have shown that silencing specific genes or overexpression of therapeutic proteins can serve as an effective combination method with immunotherapy or chemo.
In recent years, it has been instrumental in the use of RNA therapeutics with chemotherapy and immunotherapy and has emerged as an active research point for the development of different cancer treatments. The combination of adaptive cell transfer (ACT) therapy with self-delivering RNA interference (RNAi) has been developed to down-regulate the expression of checkpoint proteins by degrading the respective mRNAs before they are translated into proteins. These combinations also overcome drug resistance and improve the effectiveness of chemo and immunotherapy. RNA therapeutics can modulate many pathways, including gene silencing and overexpression, manipulation of enzyme kinetics, sensitization, and immune activation. In addition, advances in the field of non-coding RNAs have demonstrated their role in normal cell physiology or in the regulation of different molecular pathways. In addition, studies demonstrating the direct role of non-coding RNAs in various pathologies have supported the development of RNA-based therapeutics.


MicroRNAs (miRNAs or miRs) are short non-coding RNAs that trigger endogenous RNAi by regulating stability or inducing mRNA degradation. MiRNAs have various roles in various physiological and pathophysiological advances, including cell cycle progression, cancer development and progression, metabolism, diabetes, infectious diseases, muscular dystrophy, and immunity. Therefore miRNAs are an important class for putative drug targets. Biogenesis of miRNAs follows a systematic process. The first or primary miRNA strand is replicated in the nucleus. The miRNA hairpin structure embedded in the primary miRNA chain is sequentially processed by DROSHA and DICER (both belong to the RNase III family) and eventually emerges as a mature miRNA consisting of 21-22 nucleotides. The mature miRNA sequence is then loaded into the RISC complex and binds with the 3′-untranslated region (UTR) of the target gene, modulating gene expression. Inhibition of gene expression is directly dependent on the complementarity of the miRNA to the target mRNA.
MicroRNA and MRNA DegradationIn addition to inhibition of gene expression, miRNAs also modulate transcriptional regulation. Recent studies have shown that miRNAs regulate the methylation of CpG islands in the promoter region of different genes, thereby directly regulating transcriptional regulations through epigenetic modifications. The primary mode of action for miRNA and siRNA is similar because both form the RISC complex for targeted gene silencing. The main difference is that siRNAs are degraded with 100% complementarity or inhibit mRNA translation, thus precisely following target specificity. In contrast, miRNAs often bind with incomplete complementarity and perform gene silencing via slicer-independent pathways. MiRNAs target the 3′-UTR of the mRNA, suppressing gene expression or decreasing its stability. Because miRNAs can act with low complementarity, therefore, they may have multiple targets. But the primary safety check is to limit imperfect base pairing and otherwise, one miRNA can affect thousands of genes.
Interestingly, ASOs were developed and used for miRNA inhibition by directly binding to small RNA molecules in the RISC complex, these ASOs are known as antagomirs or anti-miRs. Miravirsen, also known as SPC3649, is the first anti-miRNA drug designed to treat chronic hepatitis C virus (HCV) and targets the activity of liver-specific miR-122. The 5′-UTR of HCV RNA consists of two binding sites that stabilize viral RNA for miR-122. Miravirsen sequesters miR-122, inhibiting this binding, making it ready for exonuclease degradation, reducing replication and thus reducing viral load. However, with the development of new mutations, viral recovery and resistance to Miravirsen were observed in the patient serum. Similarly, RG-101, another anti-miR drug, was designed against miR-122 (by Regulus Therapeutics) and has been used to control HCV infections, but has failed to improve overall results in clinical trials. MicroRNA and MRNA DegradationSimilarly, RG-101, together with substitutions in the binding regions of miR-122 (at the 5 ‘UTR of the HCV genome) induced viral rebound and developed resistance. The results of another clinical study suggested that treatment with RG-101 restores the natural killer (NK) cell population that controlled HCV infection. A recent clinical study demonstrated the potential of the combination regimen of RG-101 and GSK2878175 (a non-nucleoside NS5B polymerase inhibitor) to develop a one-off therapy for HCV patients. Some groups are also developing anti-miR drugs against miR-21, miR-17, miR-155, and miR-29 for cancer, kidney, and other diseases. These miRNAs (specifically miR-21) have a diverse role in lung cancer formation, progression, and metastasis, so these anti-miRs can also be used as an effective therapy for lung cancer. Steric block ASOs are also being developed to target specific miRNAs. These oligonucleotides block regulatory interactions of miRNAs with target mRNA, thus providing an important strategy for downregulating the activity of disease-specific miRNAs. However, the details of the relevant reports and conclusions of anti-miRs / ASOs in lung cancer treatments should be investigated.


Writer: Ozlem Guvenc Agaoglu

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