Nonsense-mediated decay as a terminating mechanism for antisense oligonucleotides

Antisense oligonucleotides (ASOs) are synthetic oligonucleotides that alter expression of disease-associated transcripts via Watson–Crick hybridization. ASOs that function through RNase H or the RNA-induced silencing complex (RISC) result in enzymatic degradation of target RNA. ASOs designed to sterically block access of proteins to the RNA modulate mRNA metabolism but do not typically cause degradation. Here, we rationally design steric blocking ASOs to promote mRNA reduction and characterize the terminating mechanism. Transfection of ASOs complementary to constitutive exons in STAT3 and Sod1 results in greater than 70% reduction of mRNA and protein. The ASOs promote aberrant exon skipping and generation of premature termination codon (PTC)-containing mRNAs. We inhibit the nonsense-mediated mRNA decay (NMD) pathway and show that the PTC-containing mRNAs are recognized by the UPF1 ATPase, cleaved by the SMG6 endonuclease and degraded by the XRN1 cytoplasmic exonuclease. NMD surveillance, however, does not entirely explain the mechanism of decreased STAT3 expression. In addition to exon skipping, ASO treatment causes intron retention and reduction of chromatin-associated STAT3 mRNA. The application of steric blocking ASOs to promote RNA degradation allows one to explore more nucleotide modifications than tolerated by RNase H or RISC-dependent ASOs, with the goal of improving ASO drug properties.