RNA interference with 20,40-bridged nucleic acid analogues

In this study, a number of 20,40-BNA- and 20,40-BNANC-modified siRNAs were  designed and synthesized. Their thermal stability, nuclease resistance and gene  silencing properties against cultured mammalian cells were evaluated and compared with those of natural siRNAs. The 20,40-BNA- and 20,40-BNANC-modified siRNAs  (named siBNA and siBNANC, respectively) showed very high Tm values, were  remarkably stable in serum sample and showed promising RNAi properties equal to  those exhibited by natural siRNAs. Thermally stable siBNAs composed of slightly  modified sense and antisense strands were capable of suppressing gene expression  equal to that of natural siRNA. A number of modifications on the sense strand by  20,40-BNA or 20,40-BNANC, either consecutively or separated by natural RNA  nucleotides, is tolerable in RNAi machinery. Modifications at the Argonauate (Ago2)  cleavage site of the sense strand (9–11th positions from the 50-end of the sense  strand) produced variable results depending on siRNA composition. Mostly, modification at the 10th position diminished siRNA activity. In moderately  modified siRNAs, modification at the 11th position displayed usual RNAi activity, while modification at the 9th position showed variable results depending on siRNA  composition.