Multiple pathways of toxicity induced byC9orf72dipeptide repeat aggregates and G₄C₂RNA in a cellular model

Abstract

The most frequent genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia is a G4C2 repeat expansion in the C9orf72 gene. This expansion gives rise to translation of aggregating dipeptide repeat (DPR) proteins, including poly-GA as the most abundant species. However, gain of toxic function effects have been attributed to either the DPRs or the pathological G4C2 RNA. Here we analyzed in a cellular model the relative toxicity of DPRs and RNA. Cytoplasmic poly-GA aggregates, generated in the absence of G4C2 RNA, interfered with nucleocytoplasmic protein transport, but had little effect on cell viability. In contrast, nuclear poly-GA was more toxic, impairing nucleolar protein quality control and protein biosynthesis. Production of the G4C2 RNA strongly reduced viability independent of DPR translation and caused pronounced inhibition of nuclear mRNA export and protein biogenesis. Thus, while the toxic effects of G4C2 RNA predominate, DPRs exert additive effects that may contribute to pathology.

Nuclear transport under stress phenocopies transport defects in models of C9Orf72 ALS

“…To study the effects of C9ALS DPRs on transport, we used a yeast model expressing 50 repeats of PR or GA, developed by 35 . 50 repeats is lower than common in ALS patients, but similar to previous studies (ranging between 10-65 repeats 35,38,39,48,49,51,55,59,60 ) including a mouse model with 66 repeats which recapitulates molecular and behavioural abnormalities of C9ALS patients 61 . In accordance with previous studies 35–39 , the yeast cells expressing polyPR, but not those expressing polyGA, have a growth defect, as shown by the limited growth in a spot assay (Fig 1A).…”

“…The C9orf72 protein was found to interact with both Importin β1 and Ran-GTPase 46 . (G 4 C 2 ) 30 RNA was shown to directly bind to RanGAP 47 , and polyGA was reported to bind RanGAP1 in cytoplasmic inclusions 48 , and to reduce NCT 49 . The toxicity of (G 4 C 2 ) 58 was further linked to impaired RNA export, and moreover knockout or overexpression of many nuclear transport proteins was found to influence neurodegeneration 50 .…”