Toxic gain-of-function mechanisms in C9orf72 ALS-FTD neurons drive autophagy and lysosome dysfunction

Abstract

Hexanucleotide repeat expansions in the C9orf72 gene are the primary genetic cause for both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), two related neurodegenerative diseases. Significant advances in the elucidation of the disease mechanisms responsible for C9orf72 ALS-FTD have revealed both a toxic gain-of-function and a loss-of-function mechanism as possible underlying disease cause. As the differential contribution of both gain and loss of function in C9orf72 ALS-FTD pathogenesis remains debated, we investigated disease mechanisms in motor neurons derived from both authentic human patient C9orf72 ALS-FTD iPSCs as well as a C9orf72 knockout iPSC line. We found that patient neurons presented with less motile and enlarged lysosomes, a decrease in autophagic flux and an increase in SQSTM1/p62 puncta and insoluble TARDBP/TDP-43 species. Importantly, we found that C9orf72 knockout barely has any influence on these phenotypes and mainly results in impaired endosomal maturation. Together, our data suggest that toxic gain-of-function, rather than loss-of-function, mechanisms in C9orf72 ALS-FTD impair the autophagy-lysosome system in neurons.

Keywords: Autophagy; C9orf72; amyotrophic lateral sclerosis; endosome; frontotemporal dementia; lysosome.

Figure 1.

Overview of the phenotypes found in C9-patient and C9-KO MNs. (Right) Neuronal toxic gain-of-function mechanisms (RNA foci and/or dipeptide-repeat proteins [DPRs]) of the C9orf72 HRE result in autophagy dysfunction, lysosomal transport defects, protein aggregation, TARDBP mislocalization and a relatively high degree of MN degeneration. (Left) Neuronal loss of function of the C9orf72 protein mainly results in impairments of endosomal maturation and a lesser degree of neurodegeneration.