Deciphering the pathogenic mechanisms of C9ORF72 ALS

Dr. Magdalini Polymenidou, Institute of Molecular Life Sciences, University of Zürich


This proposal aims to understand the distinct role of the different pathogenic mechanisms proposed to mediate disease in the most common genetic form of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD); two devastating, neurodegenerative diseases that represent opposite ends of the same disease spectrum. The pathological hallmark of ALS and FTD is the presence of ubiquitin-positive inclusions consisting of misfolded protein aggregates in neurons and glial cells in the affected areas of the central nervous system. The major protein component of these inclusions in the majority of ALS cases and approximately half of FTD cases is an RNA-binding protein called TDP-43.
In the last two years, a major, exciting breakthrough in our understanding of these diseases was the discovery that intronic GGGGCC hexanucleotide expansions in an uncharacterized gene called C9ORF72 is the most common genetic cause of ALS and FTD. ALS/FTD patients with hexanucleotide expansions in C9ORF72 have a unique pathological profile. While TDP-43 positive inclusions, which characterize most ALS (>97%), are present in brains and spinal cords of C9ORF72 cases, the latter develop ubiquitinated inclusions that do not contain TDP-43, but abnormal dipeptide proteins that are produced by an unconventional type of translation, called repeat-associated non-ATG (RAN) translation. Moreover, the hexanucleotide repeat RNA accumulate in nuclear foci within neurons and glial cells of C9ORF72 ALS patients. The relative contribution of each of these pathogenic mechanisms in disease remains unknown.
Currently, one of the biggest challenges is to devise experimental setups, which allow the direct comparison of the described pathologies (i.e. TDP-43 aggregation, dipeptide inclusions and RNA foci) for neuronal viability and function. To achieve this, we propose to establish a new ex vivo model of induced C9ORF72 pathology on mouse organotypic brain and spinal cord slice cultures, using a combination of adeno-associated viral delivery of transgenes and induced protein misfolding with recombinant or synthetic seeds. This model will enable a direct comparison of the neurotoxicity triggered by 1) hexanucleotide repeat RNA, 2) dipeptide repeat proteins, and 3) TDP-43 inclusions. Moreover, we will define the role of glial cells in the progression of C9ORF72-linked neurodegeneration. Lastly, we will use the most toxic dipeptide repeat protein to generate an in vivo model of C9ORF72 ALS/FTD.