C9orf72 loss-of-function: a trivial, stand-alone or additive mechanism in C9 ALS/FTD?

Abstract

A repeat expansion in C9orf72 is responsible for the characteristic neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in a still unresolved manner. Proposed mechanisms involve gain-of-functions, comprising RNA and protein toxicity, and loss-of-function of the C9orf72 gene. Their exact contribution is still inconclusive and reports regarding loss-of-function are rather inconsistent. Here, we review the function of the C9orf72 protein and its relevance in disease. We explore the potential link between reduced C9orf72 levels and disease phenotypes in postmortem, in vitro, and in vivo models. Moreover, the significance of loss-of-function in other non-coding repeat expansion diseases is used to clarify its contribution in C9orf72 ALS/FTD. In conclusion, with evidence pointing to a multiple-hit model, loss-of-function on itself seems to be insufficient to cause neurodegeneration in C9orf72 ALS/FTD.

Keywords: Amyotrophic lateral sclerosis; C9orf72; Frontotemporal dementia; In vitro; In vivo; Loss-of-function; Neurodegeneration; Postmortem; Repeat expansion.

Fig. 1

Three disease mechanisms proposed to underlie C9orf72 ALS/FTD. First, the repeat expansion (GGGGCC)n could impede transcription processes leading to a reduction in C9orf72 protein levels and hence a loss-of-function of the gene. Second, through bidirectional transcription, the repeat expansion might form secondary sense (GGGGCC) and antisense (CCCCGG) RNA structures sequestering regulatory RNA binding proteins (RBPs). This process, called RNA toxicity, could induce a loss-of-function of these proteins. Third, the non-ATG repeat-associated (RAN) translation process forms potentially toxic dipeptide repeat proteins from the sense strand (GP, GA, GR) and the antisense strand (GP, PA, PR)

Fig. 2

C9orf72 gene structure, transcripts, and isoforms. The C9orf72 gene contains 11 exons. Transcription of this gene results in three main transcripts (V1, V2, V3). The repeat region is located in the first intron of transcripts V1 and V3, whereas the V2 transcript contains the repeat in its promoter region. V4 and V5 are non-coding transcript variants with V5 being a truncated form of V4. Upon translation, two C9orf72 isoforms are formed. The short isoform (24 kDa) arises from the V1 transcript and the long isoform (54 kDa) is translated from V2 or V3

Fig. 3

The function of C9orf72 in the central nervous system. a In neurons, C9orf72 (or tripartite complex C9orf72/SMCR8/WDR41) directly or indirectly regulates autophagy at four levels; recruitment of substrates to the phagophore (1), phagophore formation (2), maturation and closure of the autophagosome (3) and fusion of the autophagosome with lysosomes (4). Vesicle trafficking in the Golgi apparatus could also be controlled by C9orf72 (5). Interaction with stress granules (6), RNA binding proteins (7) and nucleocytoplasmic transport factors (8) points towards a possible regulating role of C9orf72 in phase separation (6), RNA metabolism (7) and nucleocytoplasmic transport (8). b C9orf72 also localizes presynaptically where it might interact with extracellular vesicle secretion (9) and the cytoskeleton (i.e. cofilin) (10). c In microglia, phagocytosis of dying neurons and other cells has been associated with C9orf72/SMCR8/WDR41 (11). C9orf72 is involved in the clearing of aggregated proteins (12) and the release of cytokines (13)