No loose ends
Among the most powerful molecular biology techniques to emerge in recent years is RNA interference, in which small interfering RNA (siRNA) molecules are used to target specific genes to reduce their expression in living organisms. siRNAs have delivered considerable precision and efficiency in modulating gene expression in the laboratory, and many see considerable promise in clinical applications of this technology, although serious technical roadblocks remain to be overcome.
Chief among these is finding a safe and effective means for siRNA delivery. Simple injection of siRNAs is not an option, as RNA is rapidly degraded in the body, and so more complicated strategies are required—each with its own issues.
“Virus vectors that proliferate in vivo are potentially risky … vector systems can not be controlled in cells, and the dose of RNA is very important for clinical RNA interference,” explains Hiroshi Abe, a research scientist in Yoshihiro Ito’s laboratory at the RIKEN Discovery Research Institute in Wako. “If excess RNA is administered, cells respond to foreign body using the immune system.” Another possibility involves the use of chemically modified RNAs that can survive longer within the body, but Abe points out that this stability comes at the cost of reduced efficacy at gene silencing.
Since RNA-degrading enzymes typically start by chewing at loose RNA ends, Ito’s team has taken an innovative approach to deliver natural RNA effectively—they circularized their siRNAs (Fig. 1), designing molecules that self-assembled into stable ‘dumbbell’ shapes1. Since siRNAs begin as double-stranded precursors that must be processed by the enzyme Dicer before they can be effective, a key concern of the team was ensuring that their dumbbell constructs were not just stable, but also capable of being processed by Dicer. The dumbbells performed well on both counts; they outlasted linear siRNA molecules in human serum, and also surpassed linear molecules at triggering specific inhibition of targeted genes when injected into cultured human fibroblast cells.
Encouraged by these initial findings, Ito, Abe and colleagues are now looking into strategies to further enhance the effectiveness of their constructs. These extra-stable dumbbells are also more resistant to processing by Dicer, which reduces their inhibitory capabilities, and Abe indicates that optimizing the dumbbell’s loop structure is now a top priority. In parallel, the researchers are also exploring new methods for siRNA synthesis that could make it easier to scale up production for future studies.
1. Abe, N., Abe, H. & Ito, Y. Dumbbell-shaped nanocircular RNAs for RNA interference. Journal of the American Chemical Society 129, 15108–15109 (2007).
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