Abstract
BackgroundControlling and limiting the expression of short hairpin RNA (shRNA) by using constitutive or tissue-specific polymerase II (pol II) expression can be a promising strategy to avoid RNAi toxicity. However, to date detailed studies on requirements for effective pol II shRNA expression and processing are not available. We investigated the optimal structural configuration of shRNA molecules, namely: hairpin location, stem length and termination signal required for effective pol II expression and compared it with an alternative strategy of avoiding toxicity by using artificial microRNA (miRNA) scaffolds.ResultsHighly effective shRNAs targeting luciferase (shLuc) or Apolipoprotein B100 (shApoB1 and shApoB2) were placed under the control of the pol II CMV promoter and expressed at +5 or +6 nucleotides (nt) with reference to the transcription start site (TSS). Different transcription termination signals (TTS), namely minimal polyadenylation (pA), poly T (T5) and U1 were also used. All pol II- expressed shRNA variants induced mild inhibition of Luciferase reporters carrying specific targets and none of them showed comparable efficacy to their polymerase III-expressed H1-shRNA controls, regardless of hairpin position and termination signal used. Extending hairpin stem length from 20 basepairs (bp) to 21, 25 or 29 bp yielded only slight improvement in the overall efficacy. When shLuc, shApoB1 and shApoB2 were placed in an artificial miRNA scaffold, two out of three were as potent as the H1-shRNA controls. Quantification of small interfering RNA (siRNA) molecules showed that the artificial miRNA constructs expressed less molecules than H1-shRNAs and that CMV-shRNA expressed the lowest amount of siRNA molecules suggesting that RNAi processing in this case is least effective. Furthermore, CMV-miApoB1 and CMV-miApoB2 were as effective as the corresponding H1-shApoB1 and H1-shApoB2 in inhibiting endogenous ApoB mRNA.ConclusionOur results demonstrate that artificial miRNA have a better efficacy profile than shRNA expressed either from H1 or CMV promoter and will be used in the future for RNAi therapeutic development.
Highlights
Controlling and limiting the expression of short hairpin RNA by using constitutive or tissue-specific polymerase II expression can be a promising strategy to avoid RNA interference (RNAi) toxicity
The crucial difference between short hairpin RNA (shRNA) and artificial miRNAs is in their secondary structure and processing in the RNAi pathway. shRNAs are normally expressed from polymerase III promoters and directly generate a mature product which is exported and processed by Dicer, while miRNAs require an additional step of excision from the longer pre-miRNA sequence by the Drosha-DGCR8 complex
Design and knockdown efficacy of CMV-shRNAs targeting luciferase (shLuc) with different transcription start and transcription termination signals Previously, we have identified a highly effective small interfering RNA (siRNA) potent in knocking down luciferase when expressed as shRNA from the polymerase III (pol III) H1 promoter [15]
Summary
Controlling and limiting the expression of short hairpin RNA (shRNA) by using constitutive or tissue-specific polymerase II (pol II) expression can be a promising strategy to avoid RNAi toxicity. We investigated the optimal structural configuration of shRNA molecules, namely: hairpin location, stem length and termination signal required for effective pol II expression and compared it with an alternative strategy of avoiding toxicity by using artificial microRNA (miRNA) scaffolds. RNA interference (RNAi) is an evolutionary conserved mechanism for regulating gene expression It plays an important role in defense against viruses and in development and in normal functioning of the cell [1,2]. ShRNAs are normally expressed from polymerase III (pol III) promoters and directly generate a mature product which is exported and processed by Dicer, while miRNAs require an additional step of excision from the longer pre-miRNA sequence by the Drosha-DGCR8 complex. MiRNAs are expressed from polymerase II (pol II) promoters that allow for the use of tissue-specific or regulated expression systems
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