Identification of Pathogenic Determinants and Mechanisms of C9ORF72 linked ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by selective loss of motor neurons. Frontotemporal dementia (FTD) is a common presenile dementia characterized by selective degeneration of frontal and temporal lobes. Over the years, several observations have suggested that ALS and FTD belong to the same clinic-pathological spectrum disorder, which has been further supported by the identification of the most common genetic cause for both disease-hugely expanded hexanucleotide repeat GGGGCC in a non-coding region of the gene C9ORF72. Three hypotheses have been proposed to explain how this repeats expansion causes diseases: 1) C9ORF72 haploinsufficiency; 2) RNA gain of function; 3) Repeat associated non-ATG initiated translation translated dipeptide repeat protein toxicity. This thesis aimed to evaluate the role of each proposed mechanism in C9 ALS/FTD disease pathogenesis. We found that knockdown of C9ORF72 is not directly toxic to primary cortical neurons, motor neurons nor human induced Motor Neurons (iMNs), but it sensitizes iMNs to glutamate toxicity; Intronic G4C2 transcripts cause neurotoxicity; Arginine (R)-rich DRPs are potently neurotoxic in vitro and in vivo. We also identified pathways compromised by the disease causative mutation. We found that arginine-rich DRP toxicity is associated with nuclear presence, and mediates cell stress responses and deregulates nuclear proteins. Moreover, three pathogenic mechanisms are not mutually exclusive, and they might work synergistically to cause neurotoxicity. This study shows that all three mechanisms are involved in disease, suggesting that models combining both loss of C9ORF72 protein function and expression of expanded repeats would represent the human disease more faithfully.