A novel expression system for artificial miRNAs containing no endogenous miRNA precursor sequences

Recently, artificial microRNA (miRNA)-mediated RNA interference (RNAi) systems have been developed as useful tools to study gene functions. We report an artificial miRNA precursor motif (AMPM) containing several restriction sites in the loop and in the extended stem structures, and generated a vector-based expression system for AMPM under the control of an RNA polymerase II promoter. The AMPM located in the intron or the exon of the selection marker gene mediated silencing of the reporter luciferase gene expression in HeLa cells. Northern blotting and colony formation assays revealed that the AMPM was efficiently and appropriately processed into mature miRNA. The AMPA system also down-regulated endogenous p53 and laminA/C proteins in stable transfectants. Moreover, clustered AMPMs separated by an appropriate spacer, targeting different sites within a single mRNA mediated gene silencing, even if the sequences of the target and the AMPM were partially complementary. This indicates the potential therapeutic utility of clustered AMPMs for highly-mutable targets, such as human immunodeficiency virus type 1.