Trends in Genetics
Double-stranded RNA as a mediator in sequence-specific genetic silencing and co-suppression
Section snippets
RNA-mediated genetic interference (RNAi) in the nematode C. elegans
Several years ago it was reported that antisense RNA targeted to specific endogenous genes in Caenorhabditis elegans, when either expressed from a transgene[1]or injected directly into the worm's gonad[2], could phenocopy a null or hypomorphic mutation in the targeted gene. Surprisingly, both reports indicated that sense transcripts also were effective in producing the targeted phenotype. These observations were difficult to explain using a conventional model of antisense sequences inactivating
RNA-mediated silencing and co-suppression in plants
A second body of work on RNA-mediated interference comes from the plant world[4]. In the late 1980s, plant researchers were surprised to find that the introduction of certain transgenes into plants can result in homology-dependent silencing of an endogenous locus (rather than overexpression of the coding region of interest). This phenomenon is referred to as co-suppression. Not all transgenes cause this effect; there is no current basis for predicting which would and which would not. Gene
Similarities between nematodes and plants
RNAi in worms and co-suppression in plants share some striking similarities. Both are cases of gene-specific interference. dsRNA has been shown to be the agent of interference in nematodes and, as suggested above, there is some indication that dsRNA could also be responsible for co-suppression in plants. Perhaps the most interesting common characteristic is that the phenomenon can spread from the site of interfering RNA synthesis or application. In worms, the dsRNA mix can be injected into the
Possible mechanisms for RNA-mediated interference
The sub-stoichiometric activity of the interfering RNA in C. elegans led to various models: that interference involves a catalytic mechanism dependent on the injected RNA; that the input material is amplified; or that interference occurs at the level of the gene. Several lines of evidence argue against DNA in the genome as a target for RNAi. Effects of dsRNA are generally not heritable beyond the first generation; injected animals and progeny exhibit the effects of RNAi, whereas animals of the
Does RNA-mediated interference do a job for the cell?
In addition to the mechanistic questions, attention is also merited to the physiological role for the RNA-associated silencing phenomenon. A role for co-suppression mechanisms in systemic defense against viruses has been suggested for plants17, 18and could apply to other organisms as well. Such a response represents an effective means by which to prevent viral replication and induce resistance in surrounding tissues prior to viral invasion.
Alternatively, co-suppression/RNAi might modulate
Do RNA-interference mechanisms have counterparts outside of plants and nematodes?
Mammalian cells exhibit a global antiviral response to double-stranded RNA. In this response, the PKR protein kinase recognizes dsRNA and unleashes a vehement but somewhat non-specific response leading to general translational arrest[19]. Intriguingly, this type of systemic response can occur if the dsRNA is provided extracellularly[20](consistent with the possibility of dsRNA uptake by mammalian cells). Viruses have evolved a number of strategies for evading or inhibiting the PKR response[21].
Acknowledgements
We thank J. Hsieh, D. Lipman, S. Parrish, M. Singer, S. Xu, members of our laboratory and an anonymous reviewer for helpful suggestions. Work in this laboratory is supported by grants R01-GM37706 and F32-GM17164 from NIGMS and by the Carnegie Institution of Washington.
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