A Druglike Small Molecule that Targets r(CCUG) Repeats in Myotonic Dystrophy Type 2 Facilitates Degradation by RNA Quality Control Pathways

Myotonic dystrophy type 2 (DM2) is one of >40 microsatellite disorders caused by RNA repeat expansions. The DM2 repeat expansion, r(CCUG) exp (where "exp" denotes expanded repeating nucleotides), is harbored in intron 1 of the CCHC-type zinc finger nucleic acid binding protein (CNBP). The expanded RNA repeat causes disease by a gain-of-function mechanism, sequestering various RNA-binding proteins including the pre-mRNA splicing regulator MBNL1. Sequestration of MBNL1 results in its loss-of-function and concomitant deregulation of the alternative splicing of its native substrates. Notably, this r(CCUG) exp causes retention of intron 1 in the mature CNBP mRNA. Herein, we report druglike small molecules that bind the structure adopted by r(CCUG) exp and improve DM2-associated defects. These small molecules were optimized from screening hits from an RNA-focused small-molecule library to afford a compound that binds r(CCUG) exp specifically and with nanomolar affinity, facilitates endogenous degradation of the aberrantly retained intron in which it is harbored, and rescues alternative splicing defects.