Hexanucleotide expansions in C9orf72, which encodes a predicted guanine exchange factor, are the most frequent genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Although repeat expansion has been established to generate toxic products, mRNAs encoding the C9ORF72 protein are also reduced in affected individuals. In this study, we tested how C9ORF72 protein levels affected repeat-mediated toxicity. In somatic transgenic mice expressing 66 GGGGCC repeats, inactivation of one or both endogenous C9orf72 alleles provoked or accelerated, respectively, early death. In mice expressing a C9orf72 transgene with 450 repeats that did not encode the C9ORF72 protein, inactivation of one or both endogenous C9orf72 alleles exacerbated cognitive deficits, hippocampal neuron loss, glial activation and accumulation of dipeptide-repeat proteins from translation of repeat-containing RNAs. Reduced C9ORF72 was shown to suppress repeat-mediated elevation in autophagy. These efforts support a disease mechanism in ALS/FTD resulting from reduced C9ORF72, which can lead to autophagy deficits, synergizing with repeat-dependent gain of toxicity.
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The data that support the findings of this study are available from the corresponding author upon reasonable request.
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We thank B. Myers, M. Maldonado, J. Kim, J. Lim, J. Yasis, C.J. Heyser, D. Ditsworth, K. Osborn and J. Chew for their advice and technical assistance. We thank M. Fugere and B. Kaspar at AveXis for providing help in sorting mouse ESC-derived motor neurons. We thank Ionis Pharmaceuticals for providing ASOs. We thank all members of the D.W.C., C.L.-T., J.R., and S.D.C. groups for critical suggestions on this project. This work was supported by grants from NINDS/NIH R01-NS27036 to D.W.C. and S.D.C. and R01-NS087227 to C.L.-T.; from the NIA/NIH-supported UCSD Alzheimer’s Disease Research Center (P50-AG005131) to C.L.-T. and D.W.C.; from Target ALS to C.L.-T. and J.R.; from NINDS/NIH R35-NS097273, P01-NS084974 and R01-NS088689 to L.P.; from P01-NS099114 to T.G. and L.P.; and from Target ALS to T.G., L.P. and Y.Z. C.L.-T. is the recipient of the Healey Family ALS Endowed Chair for Research. Q.Z. was the recipient of a Milton Safenowitz Postdoctoral Fellowship and a STARTER grant from the ALS Association. J.J. was the recipient of a career development grant from the Muscular Dystrophy Association (479769).
D.W.C. is a consultant for Ionis Pharmaceuticals. The other authors report no conflicts of interest.
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Zhu, Q., Jiang, J., Gendron, T.F. et al. Reduced C9ORF72 function exacerbates gain of toxicity from ALS/FTD-causing repeat expansion in C9orf72. Nat Neurosci 23, 615–624 (2020). https://doi.org/10.1038/s41593-020-0619-5
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