Defining the factors that contribute to on-target specificity of antisense oligonucleotides

To better understand the factors that influence the activity and specificity of antisense oligonucleotides (ASOs), we designed a minigene encoding superoxide dismutase 1 (SOD-1) and cloned the minigene into vectors for T7 transcription of pre-mRNA and splicing in a nuclear extract or for stable integration in cells. We designed a series of ASOs that covered the entire mRNA and determined the binding affinities and activities of the ASOs in a cell-free system and in cells. The mRNA bound known RNA-binding proteins on predicted binding sites in the mRNA. The higher order structure of the mRNA had a significantly greater effect than the RNA-binding proteins on ASO binding affinities as the ASO activities in cells and in the cell-free systems were consistent. We identified several ASOs that exhibited off-target hybridization to the SOD-1 minigene mRNA in the cell-free system. Off-target hybridization occurred only at highly accessible unstructured sites in the mRNA and these interactions were inhibited by both the higher order structure of the mRNA and by RNA-binding proteins. The same off-target hybridization interactions were identified in cells that overexpress E. coli RNase H1. No off-target activity was observed for cells expressing only endogenous human RNase H1. Neither were these off-target heteroduplexes substrates for recombinant human RNase H1 under multiple-turnover kinetics suggesting that the endogenous enzyme functions under similar kinetic parameters in cells and in the cell-free system. These results provide a blueprint for design of more potent and more specific ASOs.