Partial inhibition of the overactivated Ku80-dependent DNA repair pathway rescues neurodegeneration in <i>C9ORF72</i> -ALS/FTD

Abstract

GGGGCC (G₄C₂) repeat expansion in <i>C9ORF72</i> is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). One class of major pathogenic molecules in <i>C9ORF72</i>-ALS/FTD is dipeptide repeat proteins such as poly(GR), whose toxicity has been well documented in cellular and animal models. However, it is not known how poly(GR) toxicity can be alleviated, especially in patient neurons. Using <i>Drosophila</i> as a model system in an unbiased genetic screen, we identified a number of genetic modifiers of poly(GR) toxicity. Surprisingly, partial loss of function of Ku80, an essential DNA repair protein, suppressed poly(GR)-induced retinal degeneration in flies. Ku80 expression was greatly elevated in flies expressing poly(GR) and in <i>C9ORF72</i> iPSC-derived patient neurons. As a result, the levels of phosphorylated ATM and P53 as well as other downstream proapoptotic proteins such as PUMA, Bax, and cleaved caspase-3 were all significantly increased in <i>C9ORF72</i> patient neurons. The increase in the levels of Ku80 and some downstream signaling proteins was prevented by CRISPR-Cas9-mediated deletion of expanded G₄C₂ repeats. More importantly, partial loss of function of Ku80 in these neurons through CRISPR/Cas9-mediated ablation or small RNAs-mediated knockdown suppressed the apoptotic pathway. Thus, partial inhibition of the overactivated Ku80-dependent DNA repair pathway is a promising therapeutic approach in <i>C9ORF72-</i>ALS/FTD.