The pathophysiological role of TDP-43 in amyotrophic lateral sclerosis due to C9orf72 mutations

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative condition that affects corticospinal and spinal motor neurons and leads to death within 30 months of symptom onset in half of all cases. It remains incurable and treatment is supportive. The genetic and molecular understanding of ALS has gone through a rapid expansion in recent years, notably with the discoveries of TDP-43, a heterogeneous ribonucleoprotein as a major component of neuronal inclusions in ALS, as well as the discovery of the C9orf72 hexanucleotide expansion as the most common genetic cause of this disease. This first part of this thesis addresses the question of which of the various pathological hallmarks of the C9orf72 Hexanucleotide Repeat Expansion (HRE) in autopsy material correlates best with the clinical presentation. The main finding is that TDP-43 distribution, rather than C9orf72 RNA foci or dipeptide aggregation in the brain, corresponds best with the areas relevant to the clinical subtype of ALS-FTD. Subsequently the role of TDP-43 was investigated in induced pluripotent stem cell derived motor neurons, and no evidence of the hallmarks of TDP-43 dysfunction, were seen in this model of the disease. No mislocalisation is found on immunofluorescence, and biochemical analysis shows no differences in insoluble species between the patient and control cell lines. In the final section, RNA sequencing was used to study the transcriptome of a BAC transgenic mouse carrying a human M337V transgene expressed at low levels, to identify early presymptomatic differences in gene expression. Interestingly, no changes were found in genes known to be associated with ALS through mutations, and the constitutive nuclear functions of TDP-43 in the regulation of splicing was maintained, prior to the emergence of a clinical phenotype in the mouse. This favours a gain of function mechanism for TDP-43 mutations in ALS.