Characterization of Escherichia coli RNase H Discrimination of DNA Phosphorothioate Stereoisomers.

Phosphorothioate (PS) modification of antisense oligonucleotides (ASOs) is a critical factor enabling their therapeutic use. Standard chemical synthesis incorporates this group in a stereorandom manner; however, significant effort was made over the years to establish and characterize the impact of chiral control. In this work, we present our in-depth characterization of interactions between Escherichia coli RNase H and RNA-DNA heteroduplexes carrying chirally defined PS groups. First, using a massive parallel assay, we showed that at least a single Rp-PS group is necessary for efficient RNase H-mediated cleavage. We followed by demonstrating that this group needs to be aligned to the phosphate-binding pocket of RNase H, and that chiral status of other PS groups in close proximity to RNase H does not affect cleavage efficiency. We have shown that RNase H's PS chiral preference can be utilized to guide cleavage to a specific chemical bond. Finally, we present a strategy for ASO optimization by mapping preferred RNase H cleavage sites of a non-thioated compound, followed by introduction of Rp-PS in a strategic position. This results in a cleaner cleavage profile and higher knockdown activity compared with a compound carrying an Sp-PS at the same location.