Harvard University - Department of Molecular & Cellular Biology

SPECIAL DELIVERY FOR RNA SILENCING

by Craig Hunter

November 21st, 2011


(L-R)Hunter and Antony Jose

A powerful way to treat genetic diseases and combat viral infections would be to turn off disease-associated genes. This can be accomplished using a process called RNA interference (RNAi), whereby double-stranded RNA (dsRNA) introduced inside a cell programs gene-specific silencing. However, a major challenge for the development of effective therapeutic RNAi is how to deliver the dsRNA into cells. This is referred to as “the delivery problem”.

The nematode C. elegans has a natural “RNA delivery” system that is very efficient – even ingested dsRNA can be transported into cells to cause silencing throughout the animal and even among its progeny. This process has been exploited by the C. elegans community to discover the functions of thousands of C. elegans genes and has been adapted to other species, both in the laboratory and in agriculture. For example, ingested dsRNA-mediated RNAi is being developed as a commercial treatment to combat a honey bee virus associated with colony collapse disorder, which is disrupting pollination-dependent crops throughout the world ($15 billion/year – US only).

My lab is identifying the cellular machinery that produces and transports mobile dsRNA in C. elegans. A long-standing question has been the identity of the mobile RNA species. In non-vertebrate systems, including plants, RNAi can be triggered by introducing long dsRNA (100’s of base pairs long). This long dsRNA is diced into short interfering RNAs (siRNA). One strand of the siRNA is then used by the silencing machinery as a guide sequence to target homologous sequences for silencing. RNAi silencing is particularly powerful in nematodes and plants in part because the siRNA derived from the long dsRNA can be amplified to produce a very large number of secondary siRNA molecules.

In our recently published paper (Two classes of silencing RNAs move between Caenorhabditis elegans tissues. Nat Struct Mol Biol. 2011 18:1184-8. doi: 10.1038/nsmb.2134) we showed that long dsRNA and siRNA derived from the long dsRNA are mobile and can lead to silencing in distant cells. In contrast, the single-stranded guide strand of the siRNA and the amplified secondary siRNA molecules can silence genes only within the cell where they are produced. This has the functional consequence of coupling the amount of long dsRNA to the extent of RNAi spreading. Curiously, the available evidence in plants suggests that amplified siRNAs are the mobile silencing signal.

Our results may have implications for the delivery problem in humans. In mammals, introduction of long dsRNA triggers an antiviral response that leads to cell death. Therefore, the shorter siRNAs, which don’t trigger the antiviral response, are used to initiate RNAi. Our results suggest that some siRNAs can be mobile in C. elegans and we identified one additional factor that is required only in sending cells and is predicted to modify primary siRNA molecules. We anticipate that determining how this factor modifies siRNAs will provide important insight towards solving the RNA delivery problem to enable effective therapeutic RNAi.

Read more in Nature Structural & Molecular Biology

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[November 21st, 2011]

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