Molecular Mechanisms of C9ORF72-linked Frontotemporal Dementia and Amyotrophic Lateral Sclerosis

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are two devastating neurological disorders that share clinical, genetic and pathological overlap. The discovery of a hexanucleotide G4C2 repeat expansion in the chromosome 9 open reading frame 72 (C9ORF72) gene as a major cause of FTD and ALS confirmed the genetic link between these two neurodegenerative diseases, collectively referred to as C9FTD/ALS. Many different hypotheses about the possible pathogenic mechanisms of this repeat have been proposed, including haploinsufficiency leading to partial loss of function of the endogenous C9ORF72 protein product, RNA toxicity caused by RNA molecules or RNA foci that bind and sequester RNA-binding proteins or production of toxic dipeptide repeat proteins (DPR) by repeat-associated non-AUG initiated (RAN) translation of the repeat. In this thesis, we study RNA and DPR gain-of toxicity in vitro and in vivo models (zebrafish and mouse). We also characterize HR23B pathology in post-mortem brain sections of C9FTD/ALS patients. Our data mainly supports DPR toxicity. Identification of the pathological pathways underlying neurodegeneration could guide future research and lead to new treatments and is therefore of great importance for the FTD/ALS field

Inducible expression of human C9ORF72 36× G4C2 hexanucleotide repeats is sufficient to cause RAN translation and rapid muscular atrophy in mice

The hexanucleotide G4C2 repeat expansion in the first intron of the C9ORF72 gene accounts for the majority of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) cases. Numerous studies have indicated the toxicity of dipeptide repeats (DPRs), which are produced via repeat-associated non-AUG (RAN) translation from the repeat expansion, and accumulate in the brain of C9FTD/ALS patients. Mouse models expressing the human C9ORF72 repeat and/ or DPRs show variable pathological, functional and behavioral characteristics of FTD and ALS. Here, we report a new Tet-on inducible mouse model that expresses 36× pure G4C2 repeats with 100-bp upstream and downstream human flanking regions. Brain-specific expression causes the formation of sporadic sense DPRs aggregates upon 6 months of dox induction, but no apparent neurodegeneration. Expression in the rest of the body evokes abundant sense DPRs in multiple organs, leading to weight loss, neuromuscular junction disruption, myopathy and a locomotor phenotype within the time frame of 4 weeks. We did not observe any RNA foci or pTDP-43 pathology. Accumulation of DPRs and the myopathy phenotype could be prevented when 36× G4C2 repeat expression was stopped after 1 week. After 2 weeks of expression, the phenotype could not be reversed, even though DPR levels were reduced. In conclusion, expression of 36× pure G4C2 repeats including 100-bp human flanking regions is sufficient for RAN translation of sense DPRs, and evokes a functional locomotor phenotype. Our inducible mouse model suggests that early diagnosis and treatment are important for C9FTD/ALS patients