Tiny masking locked nucleic acids effectively bind to mRNA and inhibit binding of microRNAs in relation to thermodynamic stability.

MicroRNA (miRNA) therapeutics, whereby miRNA networks are modified to treat disease, is attracting considerable attention in the field of nucleic acid therapeutics. In this study, we described highly effective constructs of miRNA-masking oligonucleotides (ONDs) that bind to mRNA and interfere with the binding of miRNA to its target site. Moreover, the correlation between ONDs and thermodynamics on the effect of masking mRNA was investigated. Using a reporter system, we evaluated ONDs of various lengths that have been linked to O-methyl groups and/or locked nucleic acids (LNAs) on their ability to inhibit binding of miRNA to target mRNA in cultured cells. LNAs of 10-12 nucleotides (nt) exerted the strongest inhibitory effects, which were similar to the inhibitory activity of antisense ONDs against miRNA. No such inhibitory activity was observed with the standard 22-nt O-methyl-modified RNA in the reporter system. The results of melting temperature experiments combined with the masking activity that prevented binding of miRNA to its target, suggested that a major contributor to the masking activity in cells may be the thermodynamic properties of the complexes. Our findings may contribute to the design of effective masking LNAs.