RNAi and small interfering RNAs in human disease therapeutic applications

Abstract

Small interfering RNAs (siRNAs) have been shown to effectively downregulate gene expression in human cells, giving them potential to eradicate disease. Prospects for clinical applications are discussed in this review, along with an overview of recent history and our current understanding of siRNAs used for therapeutic application in human diseases, such as cancer and viral infections. Over recent years, progress has been made in lipids, ligands, nanoparticles, polymers and viral vectors as delivery agents and for gene-based expression of siRNA to enhance the efficacy and specificity of these methods while at the same time reducing toxicity. It has become apparent that given the recent advances in chemistry and delivery, RNAi will soon prove to be an important and widely used therapeutic modality.

Figure 1

miRNA pathway and RNAi in cells targeted by small RNA-based therapies. (a) In healthy cells, (i) miRNA genes are expressed from genomic DNA, and (ii) the microprocessor complex processes the primary miRNA (pri-miRNA) into precursor miRNA (pre-miRNA), which is further exported to the cytoplasm. (iii) Dicer, in collaboration with TRBP (HIV-1 transactivating response RNA-binding protein), process the pre-miRNA into miRNA duplex, which is (iv) loaded into Argonaute (Ago) protein-containing complex. (v) One strand of the duplex, the ‘guide strand’, represses translation of mRNA by complementary binding affinity within the 3′ UTR of the mRNA. (b) Small RNAs, such as siRNA and shRNAs, can be delivered into cells via different described methods; (i) naked siRNA can be delivered locally by injection in a tissue, while (ii) RNA-structured aptamer molecules and (iii) antibody-protamine conjugated siRNAs can be used for cell-specific delivery. (iv) Small RNAs, including locked nucleic acid (LNA) oligos, can be incorporated into liposomal preparations. (iv-v) shRNA and other RNA-based tools (e.g. ribozymes and decoys) may be cloned into plasmids and viral vectors for promoter-base long-term expression. (vi) Following entry into the cytoplasm, small RNAs (vii) and lentivirus (viii) are released from endosomal compartments. (ix) Lentiviruses can be further integrated within the host genome and can be transcribed by the host machinery. (x) siRNA/shRNA delivery into diseased cells are also loaded into the Ago-containing protein complex to allow gene regulation of misregulated genes by RNA interference (xi).

Figure 2

Common chemical modifications used with siRNA in therapeutic approaches. These modifications can be made on (a) the base, (b) the sugar or (c) other parts of the RNA backbone.