Condensate protein aggregation in ALS/FTD is regulated by GGGGCC-repeat RNA scaffolds.

Biomolecular condensates regulate essential biological processes relevant to health and disease. However, the mechanisms driving pathogenic condensate formation and their therapeutic targeting have not been fully elucidated. In amyotrophic lateral sclerosis and frontotemporal dementia caused by C9orf72 GGGGCC repeat expansions (c9ALS/FTD), the expanded repeat RNA and repeat-associated non-AUG translation products are key pathogenic factors. Here, we show that the GGGGCC-repeat RNA and poly(GR) form cocondensates in vitro and in cellulo. The G-quadruplex and hairpin structures of GGGGCC-repeat RNA act as scaffolds to accelerate liquid-to-solid phase transition and aggregation of poly(GR), with the hairpin structure promoting amorphous solid-like condensates in vitro and reducing poly(GR) mobility. The cocondensation of GGGGCC-repeat RNA and poly(GR) exacerbates nucleolar stress and cellular toxicity. Targeting both G-quadruplex and hairpin structures of GGGGCC-repeat RNA with small molecules diminishes poly(GR) aggregation and ameliorates cellular dysfunction. These findings expand our understanding of poly(GR) aggregation in c9ALS/FTD, highlight the importance of RNA structure in regulating protein aggregation and suggest that targeting the RNA scaffold may expand the druggable space of pathogenic condensates.