An optimized microRNA scaffold increases shRNA processing and target knockdown

Two shRNAs were placed into expression vectors harboring mir30 microRNA scaffold and an optimized scaffold where the artificial restriction sights in mir30 have been removed. After infection and selection shRNA processing was assessed by small-RNA cloning. For both shRNAs, placement into the optimized scaffold resulted in a ~two-fold increase in processing (based on smallRNA levels). Purpose: Others have reported that the EcoRI site that was introduced to the mir30 scaffold results in decreased smallRNA processing and hence reduced target knockdown. We've developed an alternative scaffold (termed ultramir) where this site is removed. smallRNA cloning was used to determine if the movement of this sight resulted in an increase in shRNA processing. Method: Two shRNAs (one targeting Renilla Luciferase and one targeting Human RPA3) were cloned into the original mir30 cassette the ultramir cassette. Each of the 4 constructs were infected in duplicate at single copy into cells and the cells seltected unitil infection percentages reached >90% (the shRenilla hairpin was infected into HEK293T cells and the shRPA3 construts into the Gallus gallus cell line ERC. After selection smallRNA cloning was perfromed and the amount of smallRNAs corrresponding to the two shRNAs compared to the endogenous microRNA populatlon. Results: smallRNA levels of the two shRNAs doubled relative to the microRNA population when they were placed into the ultramir scaffold.