Small non-coding RNAs (siRNA, miRNA) work via the DROSHA/DICER and RISC pathways (RNAi pathways as shown in the figure on the left)[i] and can knockdown the expression of any disease-related genes target genes in a sequence-specific way.
siRNA and miRNA are slightly different. siRNA can typically trigger more efficient and specific gene silencing; whereas one miRNA may compromise the expression of many different target genes simultaneously. Hence, siRNA and miRNA have different roles in pharmaceutical practice.
siRNA based therapeutics are short synthetic double-stranded RNA oligonucleotide molecules, which are delivered exogenously and operate via harnessing the RNAi pathway. The RNAi pathway is initiated by the enzyme Dicer, which cleaves long double-stranded RNA (dsRNA) molecules into short double-stranded fragments of ~21 nucleotide siRNAs. Each exogenous siRNA is unwound into two single-stranded RNAs (ssRNAs), the passenger strand and the guide strand. The passenger strand is degraded and the guide strand is incorporated into the RNA-induced silencing complex (RISC). After integration into the RISC, siRNAs base-pair to their target mRNA and cleave it, thereby preventing it from being used as a translation template.
siRNA has innate advantages over small molecule therapeutics and monoclonal antibody drugs because siRNA executes its function by complete base pairing with mRNA, whereas small molecule and monoclonal antibody drugs need to recognize the complicated spatial conformation of certain proteins. As a result, there are many diseases that are not treatable by small molecule and monoclonal antibody drugs since a target molecule with high activity, affinity and specificity cannot be identified.
This advantage confers the siRNA modality a shorter R&D span and a wider therapeutic area range than small molecule or antibody drugs.
Lepton’s siRNAs are designed to inhibit the desired target gene by selecting the proper sequences and chemical modifications needed to confer the resulting siRNA the properties required for successful development into drugs. The company’s technology allows delivery of siRNA drugs in a variety of organs and tissues of the body.
miRNAs are short segment of RNAs that fold into a double-stranded structure, and, following enzymatic processing, suppress the gene expression via a complementarity-based target RNA recognition. miRNAs are naturally present in animals and plants. Mature duplexes are 18-25 nucleotides long. Mature miRNAs are structurally similar to siRNAs produced from exogenous dsRNA, but before reaching maturity, miRNAs must first undergo extensive post-transcriptional modification. A miRNA is expressed from a much longer RNA-coding gene as a primary transcript known as a pri-miRNA which is processed, in the cell nucleus, to a 70-nucleotide stem-loop structure called a pre-miRNA by the complex consisting of an RNase III enzyme called Drosha and a dsRNA-binding protein DGCR8. The dsRNA portion of the pre-miRNA is bound and cleaved by Dicer to produce the mature miRNA molecule that can be integrated into the RISC complex. [i]
Differently from siRNA, a miRNA-loaded RISC complex scans cytoplasmic mRNAs for potential complementarity. Instead of destructive cleavage, miRNAs target the 3′ untranslated region (UTR) regions of mRNAs where they typically bind with imperfect complementarity, thus blocking the access of ribosomes for translation[ii].
[i] Gregory RI, Chendrimada TP, Shiekhattar R (2006). “MicroRNA biogenesis: isolation and characterization of the microprocessor complex”. MicroRNA Protocols. Methods in Molecular Biology. 342. pp. 33–47. doi:10.1385/1-59745-123-1:33. ISBN 978-1-59745-123-9. PMID 16957365.
[ii] Roberts, TC (2015). “The microRNA Machinery”. MicroRNA: Basic Science. Advances in Experimental Medicine and Biology. 887. pp. 15–30. doi:10.1007/978-3-319-22380-3_2. ISBN 978-3-319-22379-7. PMID 26662984.