Abstract
An intronic GGGGCC (G4C2) hexanucleotide repeat expansion inC9orf72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9ALS/FTD). Repeat-associated non-AUG (RAN) translation of G4C2 RNA can result in five different dipeptide repeat proteins (DPR: poly GA, poly GP, poly GR, poly PA, and poly PR), which aggregate into neuronal cytoplasmic and nuclear inclusions in affected patients, however their contribution to disease pathogenesis remains controversial. We show that among the DPR proteins, expression of poly GA in a cell culture model activates programmed cell death and TDP-43 cleavage in a dose-dependent manner. Dual expression of poly GA together with other DPRs revealed that poly GP and poly PA are sequestered by poly GA, whereas poly GR and poly PR are rarely co-localised with poly GA. Dual expression of poly GA and poly PA ameliorated poly GA toxicity by inhibiting poly GA aggregation both in vitro and in vivo in the chick embryonic spinal cord. Expression of alternative codon-derived DPRs in chick embryonic spinal cord confirmed in vitro data, revealing that each of the dipeptides caused toxicity, with poly GA being the most toxic. Further, in vivo expression of G4C2 repeats of varying length caused apoptotic cell death, but failed to generate DPRs. Together, these data demonstrate that C9-related toxicity can be mediated by either RNA or DPRs. Moreover, our findings provide evidence that poly GA is a key mediator of cytotoxicity and that cross-talk between DPR proteins likely modifies their pathogenic status in C9ALS/FTD.
Highlights
Hexanucleotide repeat expansion (G4C2) in the first intron of the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)
In order to compare the toxic effects of dipeptide and G4C2 hexanucleotide repeats, we electroporated a range of expression
Embryos expressing EGFP-38x, EGFP-72x and EGFP-128x repeats, showed evidence of abnormal nerve growth, including debundling/defasciculation and truncation as axon bundles, failed to extend into the periphery (Fig. 2A). Embryos electroporated with these constructs had fewer motor neurons on the electroporated side compared to the control side (Fig. 2B and C), indicating that motor neurons are susceptible to the G4C2 hexanucleotide repeatmediated toxicity which impairs the axonal outgrowth of surviving neurons
Summary
Hexanucleotide repeat expansion (G4C2) in the first intron of the C9orf gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The number of repeats is polymorphic, with the most common number being two but up to 30 repeats have been detected in healthy controls, whereas disease-affected individuals have many more repeats, ranging from 70 to several thousand [1,2]. C9orf knockout mice do not manifest motor neuron degeneration [7] and null-allele mutants have not been detected in ALS or FTD cases. Repeatassociated non-AUG translation of G4C2 or G2C4 RNA can produce poly GA, poly GP, poly GR, poly PA and poly PR, which have been detected in the brains of C9orf affected expansion carriers [11,12,13]. GA, poly GP and poly GR result from sense transcripts [14,15,16], whereas poly GP, poly PA and poly PR are translated from antisense transcript [17,18,19]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
