Differential effects on allele selective silencing of mutant huntingtin by two stereoisomers of α,β-constrained nucleic acid.

We describe the effects of introducing two epimers of neutral backbone α,β-constrained nucleic acid (CNA) on the activity and allele selectivity profile of RNase H active antisense oligonucleotides (ASOs) targeting a single nucleotide polymorphism (SNP) for the treatment of Huntington’s disease (HD). ASOs modified with both isomers of α,β-CNA in the gap region showed good activity versus the mutant allele, but one isomer showed improved selectivity versus the wild-type allele. Analysis of the human RNase H cleavage patterns of α,β-CNA modified ASOs versus matched and mismatched RNA revealed that both isomers support RNase H cleavage on the RNA strand across from the site of incorporation in the ASO—an unusual observation for a neutral linkage oligonucleotide modification. Interestingly, ASOs modified with (R)- and (S)-5′-hydroxyethyl DNA (RHE and SHE respectively) formed by partial hydrolysis of the dioxaphosphorinane ring system in α,β-CNA also showed good activity versus the mutant allele but an improve...