A Small Molecule that Binds an RNA Repeat Expansion Stimulates Its Decay via the Exosome Complex

Many diseases are caused by toxic RNA repeats. Herein, we designed a lead small molecule that binds the structure of the r(CUG) repeat expansion [r(CUG)^exp] that causes myotonic dystrophy type 1 (DM1) and Fuchs endothelial corneal dystrophy (FECD) and rescues disease biology in patient-derived cells and in vivo. Interestingly, the compound's downstream effects are different in the two diseases, owing to the location of the repeat expansion. In DM1, r(CUG)^exp is harbored in the 3′ untranslated region, and the compound has no effect on the mRNA's abundance. In FECD, however, r(CUG)^exp is located in an intron, and the small molecule facilitates excision of the intron, which is then degraded by the RNA exosome complex. Thus, structure-specific, RNA-targeting small molecules can act disease specifically to affect biology, either by disabling the gain-of-function mechanism (DM1) or by stimulating quality control pathways to rid a disease-affected cell of a toxic RNA (FECD).