Etude de la function de C9ORF72 et de son importance dans la Sclérose Latérale Amyotrophique – C9ORF72-ALS

Our results indicate that C9ORF72 is a regulator of autophagy, which is a catabolic cellular mechanism indispensable for neurons to eliminate toxic proteins prone to aggregation. Consequently, reduce expression of C9ORF72 in ALS patients leads to sub-optimal autophagy that sensibilizes neurons to a second stress, resulting in neuronal cell degeneration (Sellier et al., 2016). As DPR proteins translated from expanded GGGGCC repeats are prone to aggregation, we tested whether DPR expression could constitute a stress regulated by C9ORF72-mediated autophagy. Importantly, our preliminary results demonstrate that accumulation of DPR under their natural sequences is prevented by autophagy, resulting in little accumulation of DPR proteins in normal conditions. In contrast, suboptimal autophagy due to reduce expression of C9ORF72 enhances accumulations of DPR aggregates, ultimately resulting into neuronal cell death. Furthermore, our data also indicate that expanded GGGGCC repeats are translated through initiation to near cognate start codons. Near cognate initiation codons are codons differing from the initiation AUG codons by one base (CUG, GUG, UUG, ACG, AAG, etc.), but that can still initiate translation provided they are embedded in a correct Kozac consensus sequence. Thus, this work provides a mechanism to the RAN translation of GGGGCC repeats in absence of any ATG start codon in ALS, but may also helps to clarify the mechanisms of RAN translation in other genetic diseases caused by expanded microsatellite repeats

In conclusion, our results indicate that neuronal cell death in ALS may result from a double hit mechanism, where the sub-optimal autophagy due to C9ORF72 reduced expression may synergize the toxicity of a second stress: the near-cognate codon initiated translation of expanded GGGGCC repeats into toxic DPR proteins prone to aggregation.

We propose to explore further the cellular functions of C9ORF72 and confirm the pathogenicity of our double hit hypothesis, notably by developing novel animals (zebrafish and mice) models expressing these DPR proteins under their natural near-cognate start codons. Importantly, we will then test the toxicity of these DPR proteins in absence of C9ORF72 expression using our already developed knockdown and knockout zebrafish and mouse models.

Novel antibodies reveal presynaptic localization of C9orf72 protein and reduced protein levels in C9orf72 mutation carriers. Frick P, Sellier C, Mackenzie IRA, Cheng CY, Tahraoui-Bories J, Martinat C, Pasterkamp RJ, Prudlo J, Edbauer D, Oulad-Abdelghani M, Feederle R, Charlet-Berguerand N, Neumann M. Acta Neuropathol Commun. 2018 Aug 3;6(1):72.