Modified 27-nt dsRNAs with dramatically enhanced stability in serum and long-term RNAi activity.

The present study describes improved properties of 27-nt dsRNAs over 21-nt siRNAs, and accents on the possibility to use their modifications and conjugates for direct long-term gene silencing in viable cells and animals, avoiding conventional transfectants. Using a Renilla Luciferase gene-silencing system and cultured cell lines, we established that 27-nt dsRNAs possessed about three to five times higher "long-term" RNAi activity than 21-nt siRNAs and 21-nt dsRNAs. Moreover, if RNA duplexes were preincubated with cell-cultured medium for several hours before their transfection in cells, 21-mer completely lost its RNAi effect, while 27-mer, its amino modifications, thiol modifications, and cholesterol conjugates manifested a strong gene silencing. In attempts to clarify the reason(s) for the higher RNAi activity of 27-nt dsRNAs, we found that they were approximately 100 times more stable than 21-nt siRNA and 21-nt dsRNA in cell-cultured medium supplemented with 10% inactivated serum, approximately 50 times more stable in 90% inactivated serum, and approximately six times more stable in active serum. The 5' sense modification was selected as the most stable, accessible to Dicer, and with highest RNAi potential. The RNAi activity of 5' sense modifications was higher even than the activity of nonmodified 27-nt dsRNA. The 5' sense amino modification also did not influence the activity of 21-nt siRNA, right overhang 25/27-nt (R25D/27), and 25D/27-nt RNAs. The stability of 5' sense modified R25D/27-nt and 25D/27-nt RNAs in serum was lower than that of blunt 27-nt dsRNA. However, these asymmetric RNAs were more active than modified and nonmodified blunt 27-nt dsRNAs, which demonstrates the superiority of the asymmetric design. The 5' sense modifications were considered as most appropriate for conjugation with small signal molecules to facilitate the intracellular delivery of RNA duplex, to preserve its RNAi capacity, and to ensure a possibility for rapid long-term gene silencing in viable cells and animals. The 5' sense conjugation with cholesterol approved this assumption.