Co-selection of West Nile virus nucleotides that confer resistance to an antisense oligomer while maintaining long-distance RNA/RNA base pairings.

West Nile virus (WNV) genome cyclization is mediated by two pairs of long-distance RNA/RNA interactions: the 5'CS/3'CSI (conserved sequence) and the 5'UAR/3'UAR (upstream AUG region) base pairings. Antisense peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs), designed to interfere with the 5'CS/3'CSI or 5'UAR/3'UAR base pairings, were previously shown to inhibit WNV. In this study, we selected and characterized WNVs resistant to a PPMO targeting the 3'UAR (3'UAR-PPMO). All resistant viruses accumulated one-nucleotide mutations within the 3'UAR, leading to a single-nucleotide mismatch or a weakened base-pairing interaction with the 3'UAR-PPMO. Remarkably, a one-nucleotide mutation within the 5'UAR was correspondingly co-selected; the 5'UAR mutation restored the base pairing with the 3'UAR mutation. Mutagenesis of WNV demonstrated that the single-nucleotide change within the 3'UAR-PPMO-target site conferred the resistance. RNA binding analysis indicated that the single-nucleotide change reduced the ability of 3'UAR-PPMO to block the RNA/RNA interaction required for genome cyclization. The results suggest a mechanism by which WNV develops resistance to 3'UAR-PPMO, through co-selection of the 5'UAR and 3'UAR, to create a mismatch or a weakened base-pairing interaction with the PPMO, while maintaining the 5'UAR/3'UAR base pairings.