Chemical Modification of the siRNA Seed Region Suppresses Off-Target Effects by Steric Hindrance to Base-Pairing with Targets

[Image: see text] Chemical modifications of 2′-O-methyl (2′-OMe) and locked nucleic acid (LNA) of the nucleotides in the seed region (positions 2–8) of the small interfering RNA (siRNA) guide strand significantly reduced seed-matched (SM) off-target effects. The siRNA with 2′-OMe modifications inhibited the expression of a completely-matched (CM) target gene, whereas that with LNA modifications did not inhibit the expression of the CM target. By computational predictions of conformational changes of siRNA by these modifications, we revealed that both modifications in the siRNA seed region reduce SM off-target effects by steric hindrance to base-pairing with target transcripts but LNA modifications also disturb the association of the siRNA guide strand with the Argonaute (AGO) protein by altering RNA conformation. Thus, chemical modifications of the siRNA guide strand, which alter steric conformation to disturb base-pairing with target transcripts but do not disturb the association with the AGO protein, may successfully suppress off-target effects without substantial loss of RNA silencing activity.