Rationally Designed Small Molecules Targeting the RNA That Causes Myotonic Dystrophy Type 1 Are Potently Bioactive

RNA is an important drug target, but it is difficult to design or discover small molecules that modulate RNA function. In the present study, we report that rationally designed, modularly assembled small molecules that bind the RNA that causes myotonic dystrophy type 1 (DM1) are potently bioactive in cell culture models. DM1 is caused when an expansion of r­(CUG) repeats, or r­(CUG)^exp, is present in the 3′ untranslated region (UTR) of the dystrophia myotonica protein kinase (<i>DMPK</i>) mRNA. r­(CUG)^exp folds into a hairpin with regularly repeating 5′C<u>U</u>G/3′G<u>U</u>C motifs and sequesters muscleblind-like 1 protein (MBNL1). A variety of defects are associated with DM1, including (i) formation of nuclear foci, (ii) decreased translation of <i>DMPK</i> mRNA due to its nuclear retention, and (iii) pre-mRNA splicing defects due to inactivation of MBNL1, which controls the alternative splicing of various pre-mRNAs. Previously, modularly assembled ligands targeting r­(CUG)^exp were designed using information in an RNA motif-ligand database. These studies showed that a bis-benzimidazole (<b>H</b>) binds the 5′C<u>U</u>G/3′G<u>U</u>C motif in r­(CUG)^exp. Therefore, we designed multivalent ligands to bind simultaneously multiple copies of this motif in r­(CUG)^exp. Herein, we report that the designed compounds improve DM1-associated defects including improvement of translational and pre-mRNA splicing defects and the disruption of nuclear foci. These studies may establish a foundation to exploit other RNA targets in genomic sequence