RNA Interference (RNAi)

RNA interference (RNAi) refers to the conserved sequence-specific degradation of messenger RNA mediated by small interfering RNA (siRNA) duplexes 21–25 nucleotides in length. RNAi may be activated with exogenous effectors to silence pathology-causing genes has generated considerable interest in using this approach for therapy. Utilizing RNAi to counter gene expression has been a particularly active field of research, and a vast number of diseases, especially those caused by virus infections and cancer, have been shown to be susceptible to RNAi-mediated inhibition.

SiRNAs can be readily synthesized and be quickly shown to silence the expression of many genes both in vitro and in vivo. Nevertheless, it has become obvious that in vivo delivery of naked siRNAs or even lipid complexed siRNAs is inefficient, and once delivered, the siRNAs have short half-lives, leading to transient gene silencing. Various methods have been used to achieve this, including use of nonviral vectors for delivery of synthetic RNAi activators and viral vectors for transfer of antiviral RNAi expression cassettes.

Using viral vectors, it was now possible to alleviate the two major disadvantages of synthetic siRNA; that is, inefficient in vivo delivery, and transient gene silencing. Viral vectors that are under development include recombinant adeno-associated viruses (AAVs), lentiviruses and adenoviruses. AAVs and lentiviruses vector systems have the best safety records and pseudotyping the vectors with different capsids (AAV) or envelope (LV) proteins permits tissue-tropic targeting. The ability of lentiviral vectors to integrate proviral sequences that stably produce antiviral RNAi activators has been particularly useful for ex vivo methods to treating viral diseases that result from chronic infections, for instance, AIDS caused by HIV persistence.

Recombinant adenoviruses have features that make them suited to delivering potentially therapeutic RNAi activators to the liver. These include efficient hepatic expression of inserted cassettes, well-established methods of propagating the vector, episomal location of the adenovirus genome in infected cells, ability to infect non-dividing cells, compatibility with chemical modification to alter biological properties, and transient or sustained expression of transgenes.

Figure 1. Summarized illustration of miR processing which shows the essential nuclear and cytoplasmic steps of the RNAi. Exogenous expression cassettes that transcribe miR intermediates are typically incorporated into adenoviruses (Mowa M B, 2010).

References
1. Couto L B, High K A. Viral vector-mediated RNA interference. Current Opinion in Pharmacology, 2010, 10(5):534-542.
2. Mowa M B, Crowther C, Arbuthnot P. Therapeutic potential of adenoviral vectors for delivery of expressed RNAi activators. Expert Opinion on Drug Delivery, 2010, 7(12):1373-85.

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