Antisense oligonucleotide: A promising therapeutic option to beat COVID ‐19

FIGURE 1. Life cycle of SARS‐CoV‐2. SARS‐CoV‐2 is an enveloped RNA virus. Its surface is decorated with the spike protein that is responsible for the recognition and entry of the virus into the cell. After entry of the virus into the cell, the ORF1a and ORF1b are translated by the ribosome of the host cell followed by the assembly of the replicase‐transcriptase complex. Then, the viral RNA is replicated to form a negative strand, ready for both replication of the viral genome and transcription of the subgenomic genome. Finally, the mature virions are formed and exit the cell by exocytosis. Adapted from “Human Coronavirus Structure” and “Coronavirus Replication Cycle” (Acknowledgements Dr. Benjamin Goldman‐Israelow) by BioRender.com (2021). Retrieved from https://app.biorender.com/biorender‐templates

FIGURE 2. Mechanism of action of antisense oligonucleotides. ASOs are small single‐stranded nucleic acids that specifically bind to cognate RNA‐targets. They act through two main mechanisms, depending on their chemical backbone. First, ASOs that do not mediate RNase H activity, act as steric blockers disrupting the splicing machinery, Protein/RNA interactions and RNA secondary structures. Second, ASOs also known as GapmeRs recruit and induce RNase H cleavage of the targeted‐RNA. GapmeRs are typically composed of Lock Nucleic Acids (LNA) at both ends to provide high‐affinity base pairing, with a central block of unlocked bases to mediate RNase H activation. They decrease RNA and protein expression. Created with BioRender.com (2021)

FIGURE 3. Chemical modification of ASO. Substitution of the non‐bridging oxygen atoms in the phosphodiester bond with a sulfur leads to phosphorothiote (PS) the most common first generation ASO. Second generation ASO include modifications at the 2′ position of the sugar moiety. The hydroxyl group can be replaced with a methyl group (2′‐O‐methyl or 2′‐OMe) or a methoxyethyl group (2′‐O‐methoxyethyl or 2′‐MOE). Third generation have various modifications: Locked nucleic acids (LNAs ‐ the 2′‐oxygen and the 4′‐carbon on the same ribose are connected through a methylene bridge); phosphrodiamidate morpholino oligomers (PMOs ‐ a morpholine moiety connected through methylene phosphorodiamidate replace the ribose); peptide nucleic acids (PNAs ‐ the deoxyribose phosphate is replaced by a speudopeptide backbone)

FIGURE 4. Which SARS‐CoV2 component should be targeted by ASOs? Schematic representation of the viral structure and components. Presently FDA approved vaccines mediate the production of antibodies against the spike protein that decorates the viral membrane. Viral RNAs comprise subgenomic and genomic entities. Subgenomic viral RNAs are translated to produce structural and accessory proteins. Genomic viral RNA supports RNA replication previous virion incorporation. Although, any viral sequences could be targeted with ASOs we recommend to target only genomic viral RNA and replication steps. Targeting, subgenomic RNA sequences may lower the efficacy of the present vaccines. Adapted from “Human Coronavirus Structure” and “Coronavirus Replication Cycle” (Acknowledgements: Dr. Benjamin Goldman‐Israelow) and “Genome Organization of SARS‐CoV” (Acknowledgements: Glaunsinger Lab: Jessica M Tucker, Britt A Glaunsinger et al.) by BioRender.com (2021). Retrieved from https://app.biorender.com/biorender‐templates

FIGURE 5. When and how targeting viral RNA with ASOs. Schematic representation of the genomic replication and the subgenomic (nested) transcription steps highlighting the RNA sequences to be targeted by ASOs using steric blockers and GapmeRs. Adapted from “Coronavirus Replication Cycle” (Acknowledgements: Dr. Benjamin Goldman‐Israelow) by BioRender.com (2021). Retrieved from https://app.biorender.com/biorender‐templates