Table 1.
Summary of the key evidence for gain-and loss-of-function mechanisms in C9FTD/ALS
| For (+) or against (−) hypothesis | Study | |
| Loss-of-function | ||
| Reduced mRNA and protein expression | ||
| All mRNA isoforms reduced in C9FTD/ALS patient tissue and iPSC neurons | + | [1,4,5▪–7▪,8▪▪] |
| Reduced C9orf72 protein in C9FTD/ALS patient brain | + | [9▪] |
| Hypermethylation of C9orf72 promoter reduces transcript levels and correlates with shorter disease duration | + | [10▪] |
| Hypermethylation of C9orf72 promoter reduces transcript levels, increasing resistance to cellular stress and reducing RNA foci and DPR proteins | – | [11▪] |
| Models of loss of protein | ||
| Dysregulation of cellular trafficking associated with reduction of C9orf72 protein | + | [12–14] |
| C9orf72 orthologue knockdown in zebrafish has a motor phenotype | + | [15▪] |
| C9orf72 orthologue knockout in Caenorhabditis elegans has a motor phenotype | + | [16▪] |
| C9orf72 orthologue knockdown in mice has no motor phenotype | – | [17▪▪] |
| Genetics | ||
| Lack of coding mutations in C9orf72 protein in C9ALS | – | [18] |
| Homozygous C9FTD case not as severe clinically and pathologically as pure loss-of-function diseases | – | [5▪] |
| Gain-of-function: RNA | ||
| RNA foci | ||
| Sense and antisense foci identified in C9FTD/ALS patient tissue and iPSC neurons | + | [1,6▪,8▪▪,17▪▪,19▪–22▪] |
| Sense foci burden correlates with age-at-onset in C9FTD | + | [19▪] |
| Sequestration of RNA-binding proteins | ||
| ADARB2 – siRNA reduces sense RNA foci in C9ALS iPSC neurons and exacerbates glutamate-induced toxicity in control iPSC neurons | + | [8▪▪] |
| hnRNP A1 and pur-alpha – colocalization with sense RNA foci in iPSC motor neurons | + | [22▪] |
| hnRNP A3 – identified in neuronal cytoplasmic and intranuclear inclusions | + | [23▪] |
| hnRNP H – colocalization with sense RNA foci in patient brain | + | [24▪] |
| Nucleolin – colocalization with sense RNA foci in patient brain and cells, indications of nucleolar stress | + | [25▪] |
| Pur-alpha – overexpression rescues GGGGCC-dependent degeneration in Drosophila | + | [26▪] |
| Antisense oligonucleotide (ASO) treatment | ||
| ASOs reduce sense RNA foci and reverse transcriptome changes and toxicity in patient fibroblasts and iPSC neurons | + | [8▪▪,17▪▪,22▪] |
| Gain-of-function: DPR proteins | ||
| Inclusion pathology | ||
| All DPR proteins found in neuronal cytoplasmic inclusions in C9FTD/ALS patient brain and iPSC neurons | + | [6▪,20▪,21▪,27▪,28▪,29▪] |
| Poly-GA DPR protein inclusions found prior to TDP-43 inclusions in patient brain | + | [30▪] |
| Toxicity | ||
| GA DPR protein inclusion distribution does not correlate with neurodegeneration in C9FTD/ALS patient brain | – | [31▪] |
| Poly-GP and poly-PR DPR proteins made from GGGGCC repeats are toxic to HEK293 cells (other DPR proteins not assessed) | + | [21▪] |
ALS, amyotrophic lateral sclerosis; DPR, dipeptide repeat; FTD, frontotemporal dementia; GA, glycine-alanine; GP, glycine-proline; hnRNP, heterogeneous nuclear ribonucleoprotein; iPSC, induced pluripotent stem cell; PR, proline-arginine.