Length-dependent RNA foci formation and Repeat Associated non-AUG dependent translation in a C. elegans G ₄ C ₂ model

GC-rich repeat expansion mutations are implicated in several neurodegenerative diseases and can lead to repeat associated non-AUG-dependent (RAN) translation and concentrations of nuclear RNA foci. To model C9orf72 ALS/FTD, we engineered C. elegans to express pure GGGGCC (G ₄ C ₂ ) repeats of varying lengths and observed RAN translation and nuclear RNA foci. RNA foci were observed in animals expressing ≥20 G ₄ C ₂ repeats while RAN translation occured in animals expressing ≥33 G ₄ C ₂ repeats. These findings show that in C. elegans , RAN translation can occur even in the absence of C9orf72 intronic sequence normally surrounding the repeat. Given that the currently accepted repeat threshold for C9 disease is >30 repeats, our data are consistent with a model in which RAN peptides are key drivers of C9orf72 disease pathology.