C9orf72-mediated ALS and FTD: multiple pathways to disease

Abstract

The discovery that repeat expansions in the C9orf72 gene are a frequent cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) has revolutionized our understanding of these diseases. Substantial headway has been made in characterizing C9orf72-mediated disease and unravelling its underlying aetiopathogenesis. Three main disease mechanisms have been proposed: loss of function of the C9orf72 protein and toxic gain of function from C9orf72 repeat RNA or from dipeptide repeat proteins produced by repeat-associated non-ATG translation. Several downstream processes across a range of cellular functions have also been implicated. In this article, we review the pathological and mechanistic features of C9orf72-associated FTD and ALS (collectively termed C9FTD/ALS), the model systems used to study these conditions, and the probable initiators of downstream disease mechanisms. We suggest that a combination of upstream mechanisms involving both loss and gain of function and downstream cellular pathways involving both cell-autonomous and non-cell-autonomous effects contributes to disease progression.

Figure 1. C9orf72 structure, transcript variants and protein isoforms.

The C9orf72 gene consists of 11 exons, has three main alternatively spliced transcript variants and produces two protein isoforms. In the figure, coding exons are indicated in yellow and noncoding exons in blue (not to scale). The GGGGCC hexanucleotide repeat expansion mutation is located in the first intron of variants 1 and 3 and within the promoter region of variant 2. Variant 1 encodes C9orf72-S (short), a 222-amino-acid protein of 24 kDa, and variants 2 and 3 encode C9orf72-L (long), a 481-amino-acid protein of 54 kDa

Figure 2. Dipeptide repeat proteins.

The figure shows the dipeptide repeat proteins that are generated by GGGGCC repeat-associated non-ATG (RAN) translation. The sense strand generates poly-GA, poly-GP and poly-GR and the antisense strand generates poly-GP, poly-PA and poly-PR.

Figure 3. C9FTD/ALS neuropathology.

Sense and antisense RNA foci are a common feature in the brains of patients with C9orf72-associated frontotemporal dementia and/or amyotrophic lateral sclerosis (C9FTD/ALS). a,b |Representative images show neurons from the frontal cortex of a patient with C9FTD/ALS, containing multiple sense (red; part a) and antisense (green; part b) foci in nuclei (stained blue with DAPI). Scale bar: 2.5 μm. c | TAR DNA-binding protein 43 (TDP-43) pathology in a patient with C9FTD/ALS. Arrow indicates a neuronal cytoplasmic TDP-43 inclusion in the frontal cortex, with concomitant depletion of nuclear TDP-43. Scale bar: 50 μm. d | Dipeptide repeat protein (DPR) pathology in a patient with C9FTD/ALS. Inclusions consisting of sense and antisense DPRs are produced by repeat-associated non-ATG (RAN) translation. Arrows indicate neuronal cytoplasmic inclusions of poly-GA protein. Scale bar: 50 μm.

Figure 4. Cellular processes implicated in C9orf72-associated FTD and ALS.

A wide range of cellular pathways have been implicated in C9orf72-mediated disease, several of which have previously been linked to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). C9orf72 loss-of-function and toxic gain-of-function mechanisms can both alter RNA processing and metabolism pathways, with alterations in stress granules and P-bodies, and C9orf72 gain-of-function mechanisms can lead to nucleolar dysfunction, affect RNA splicing and transcription and cause DNA damage. Proteostasis pathways have also been implicated, with impairments in autophagy and lysosomal function, the unfolded protein response and the endoplasmic reticulum, and the ubiquitin–proteasome system. Other cellular processes including nucleocytoplasmic transport, vesicular trafficking and transport granule function, and mitochondrial function, can also be impaired. In addition, neuron-specific processes, including hyperexcitabiliy and hypoexcitability, glutamate excitotoxicity, axonal transport and neuronal branching defects, have been implicated in C9FTD/ALS. Finally, loss of C9orf72 function alters immune system and microglial function (Table 3).