Blocking RAN translation without altering repeat RNAs rescues <i>C9ORF72</i> -related ALS and FTD phenotypes

GGGGCC (G₄C₂) repeat expansion in <i>C9ORF72</i> is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Toxicity is thought to result from the accumulation of either repeat RNAs and/or dipeptide repeat proteins (DPRs) translated from repeat-containing transcripts through repeat-associated non-AUG (RAN) translation. To disentangle RNA from DPR toxicity, we mutated a CUG codon predominantly used to initiate DPR translation from all three reading frames. This mutation disrupted DPR synthesis while preserving the expression of repeat-containing RNAs. Despite the accumulation of RNA foci, behavioral deficits and pathological abnormalities, including p-TDP-43 inclusions, STING activation, motor neuron loss, neuroinflammation, and increased plasma neurofilament concentration, were alleviated in <i>C9ORF72</i> mice. Base editing of the CUG codon also improved molecular phenotypes and survival in patient induced pluripotent stem cell-derived neurons, which highlights the potential of therapeutically targeting DPR production rather than repeat RNAs.