Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are just two of the
neurodegenerative diseases characterised by the presence of pathological aggregates composed of
proteins and RNA. The mechanism of aggregate formation has historically defined proteins as
nucleation triggers, but recent studies have highlighted the fundamental role of nucleic acids in the
aggregation events. In particular, 40% of familiar cases in ALS/FTD have been correlated to mutations
in the hexanucleotide repeat (GGGGCC)n, which in its RNA form has been identified within the
aggregates. This work aimed to investigate the aggregation properties of (GGGGCC)n and unveil the
underpinning molecular mechanism in the context of the ALS/FTD diseases. In these studies, I firstly
showed that (GGGGCC)n DNA oligonucleotides can in the absence of proteins, and that aggregation is
facilitated by higher RNA concentrations and number of repeats. I then demonstrated that
aggregation is mediated by the formation of multimolecular G-quadruplexes (mG4s) and proceeded
to disassemble the aggregates by employing G4-specific tools or altering the molecularity of the
system via mutation analysis and antisense treatment. I validated the biological relevance of my
findings by demonstrating the ability of RNA (GGGGCC)n to form mG4-rich aggregates similar to its
DNA equivalents and by examining the interactions of these strands with TDP-43, a protein relevant
to ALS/FTD. Furthermore, I confirmed the potential therapeutical relevance of the studies by
demonstrating the ability of a G4-selective fluorescent probe to penetrate C9orf72 mutant human
motor neurons derived from ALS patients, which revealed clear fluorescent signal in putative
condensates. My findings strongly suggest that (GGGGCC)n can form protein-free condensates
sustained by multimolecular G-quadruplexes, highlighting their potential relevance as therapeutic
targets for C9orf72 mutation-related ALS and FTD.