Insights into the pathogenic mechanisms of Chromosome 9 open reading frame 72 (C9orf72) repeat expansions

The identification of a hexanucleotide repeat expansion in a non-coding region of C9orf72 as a major cause of both frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) drastically changed the field of research on both of these conditions. Yet, despite the vast amount of work aimed at elucidating the molecular mechanisms underlying the role of this repeat in disease, the exact pathomechanisms are still unclear. A reduction in the expression of the C9orf72 gene is observed in patients, but a gain-of-function model is now preferred. The hexanucleotide repeat expansion forms RNA foci in the central nervous system (CNS) of repeat-positive FTD and ALS patients, and these foci are believed to sequester RNA-binding proteins (RBPs) and impair their function in RNA processing. At the same time, the repeat undergoes repeat-associated non-ATG translation to produce dipeptide repeat proteins that also form inclusions in the patient CNS. Studies from cells and flies suggest that these proteins may also be an important factor in the disease. Finally, the hexanucleotide repeat also induces the mislocalization and aggregation of TAR DNA-binding protein 43 (TDP43) through an as yet unknown mechanism. This review covers the different potential pathogenic factors that have been put forth for C9orf72-repeat-associated FTD and ALS (C9-FTD/ ALS), while highlighting some remaining questions.