FIGURE 2

Use of rules of siRNA asymmetry for RNAi of huntingtin mRNA. Incorporation of the guide strand into RISC determines the RNAi activity of a siRNA. In the experiment shown here, the blue strand is the passenger in the siRNA and the red strand is the guide. (a and b) The ability of the guide strand (in red) to effect RNAi is tested against a synthetic huntingtin mRNA target (in red in figure). The RNAi activity of the passenger strand (in blue) is tested against an artificial mRNA target to which it has complementarity (in blue in figure). (a) Both the guide strand and the passenger strand have equal potency in cleaving their targets. This result indicates that both guide and passenger strands in a huntingtin siRNA can enter RISC and cleave a mRNA with complementarity to a siRNA strand. (b) The huntingtin siRNA has a nucleotide substitution (purple A, highlighted yellow) near the 3' end of the passenger strand. This nucleotide is a purposeful mismatch, creating an asymmetry in the siRNA. A consequence of the asymmetry is that the guide strand is preferred to enter RISC. The huntingtin mRNA is cleaved and the artificial mRNA with complementarity to the passenger strand is not cleaved. Therefore, asymmetric siRNAs can improve the efficacy of the siRNA in mRNA silencing, because they direct the guide (antisense) strand into RISC. Adapted with permission from Schwarz et al. ¹⁴