Differential Toxicity of Nuclear RNA Foci versus Dipeptide Repeat Proteins in a Drosophila Model of C9ORF72 FTD/ALS

Dipeptide repeat (DPR) proteins are toxic in various models of FTD/ALS with GGGGCC (G₄C₂) repeat expansion. However, it is unclear whether nuclear G₄C₂ RNA foci also induce neurotoxicity. Here, we describe a Drosophila model expressing 160 G₄C₂ repeats (160R) flanked by human intronic and exonic sequences. Spliced intronic 160R formed nuclear G₄C₂ sense RNA foci in glia and neurons about ten times more abundantly than in human neurons; however, they had little effect on global RNA processing and neuronal survival. In contrast, highly toxic 36R in the context of poly(A)⁺ mRNA were exported to the cytoplasm, where DPR proteins were produced at >100-fold higher level than in 160R flies. Moreover, the modest toxicity of intronic 160R expressed at higher temperature correlated with increased DPR production, but not RNA foci. Thus, nuclear RNA foci are neutral intermediates or possibly neuroprotective through preventing G₄C₂ RNA export and subsequent DPR production. Tran et al. engineered a novel Drosophila model of FTD/ALS with C9ORF72 repeat expansion and revealed that dipeptide repeat proteins, but not nuclear G₄C₂ repeat RNA foci, are a major source of toxicity in vivo.