Journal Club: eIF6 is Part of the Translational Repressor Complex, but not Minimal Human RiSC- Implications for Therapeutic RNAi

This week, the journal Nature published a study by the Shiekhattar group online investigating the molecular identity of the microRNA-mediated translational repressor complex. This is important for RNAi Therapeutics since it is still unclear whether the microRNA complex is distinct from the RNAi effector complex in humans. MicroRNAs mediate gene silencing by translational repression via 7-8nt base-pair complementarity to the target gene mostly in the 3’ UTR of gene transcripts, whereas siRNAs knockdown genes with much larger complementarity by instructing their specific cleavage and destruction. It is thought that the microRNA pathway is largely responsible for so-called off-targeting effects in RNAi experiments given their less stringent base-pairing requirements. A better molecular understanding of what constitutes the microRNA translational repressor complex versus the RNAi mRNA destruction complex should lead the way in identifying strategies to specifically harness the RNAi pathway, thus minimising off-targeting and therefore unpredicted side-effects in the clinical setting.

By performing co-immunoprecipitation experiments with a protein known to be involved in siRNA-guided gene silencing, Chendrimada and colleagues identified another complex containing this protein that was distinct from the one mediating siRNA-guided mRNA cleavage. One of the factors found was eIF6 which is known to be a component of the 60S large ribosomal particle and to prevent the association of 60S with 40S to constitute the active translation complex. The authors then go on to show that by inhibiting the activity of eIF6 (using siRNAs), microRNA-mediated translational inhibition was abrogated. Since eIF6 is not required for siRNA-guided cleavage in vitro, this suggests that the microRNA repressor complex is distinct from the RNAi machinery. It would have been nice, however, to test whether following the inhibition of eIF6, RNAi activity was left intact as would be predicted in such a model. Similarly, it would have been interesting to test whether eIF6 depletion would have abolished microRNA-mediated translational repression in an in vitro system similar to the one used for the identification of the RNAi minimal RiSC.

While this study leaves some of these questions unanswered, it is an excellent demonstration that through a more thorough understanding of how microRNAs and siRNAs effect gene silencing, it should be possible to further increase the specificity of RNAi, a feature already considered to be one of the strengths of this technology. The ultimate goal would be to design siRNAs that cannot be used by the microRNA repressor complex without compromising RNAi activity. It is noteworthy that Dharmacon scientists have demonstrated that through the introduction of a simple modification in the siRNA, microRNA effects can be reduced by 5-10 fold while leaving siRNA activity intact. It is exciting to see how clever experimentation in many labs is gradually breaking down the barriers towards the safe and widespread application of RNAi Therapeutics.