The Hairpin Form of r(G4C2)exp in c9ALS/FTD Is Repeat-Associated Non-ATG Translated and a Target for Bioactive Small Molecules.

The most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is an expanded G4C2 repeat [(G4C2)exp] in C9ORF72. ALS/FTD-associated toxicity has been traced to the RNA transcribed from the repeat expansion [r(G4C2)exp], which sequesters RNA-binding proteins (RBPs) and undergoes repeat-associated non-ATG (RAN) translation to generate toxic dipeptide repeats. Using in vitro and cell-based assays, we identified a small molecule (4) that selectively bound r(G4C2)exp, prevented sequestration of an RBP, and inhibited RAN translation. Indeed, biophysical characterization showed that 4 selectively bound the hairpin form of r(G4C2)exp, and nuclear magnetic resonance spectroscopy studies and molecular dynamics simulations defined this molecular recognition event. Cellular imaging revealed that 4 localized to r(G4C2)exp cytoplasmic foci, the putative sites of RAN translation. Collectively, these studies highlight that the hairpin structure of r(G4C2)exp is a therapeutically relevant target and small molecules that bind it can ameliorate c9ALS/FTD-associated toxicity.