B06: Defining the interplay between RNA and protein toxicity in C9orf72-related amyotrophic lateral sclerosis

Alterations in RNA metabolism contribute to the pathophysiology of neurodegenerative diseases that involve cytoplasmic protein aggregation. The link is compelling in amyotrophic lateral sclerosis (ALS), a fatal disease caused by the selective degeneration of motor neurons. Expanded hexanucleotide (GGGGCC)n repeats in the first intron of C9orf72 account for approximately 30% of familial ALS cases. Alterations in the fidelity of RNA synthesis from expanded (GGGGCC)n impair interconnected processes that modulate RNA and protein metabolism. Pathologically, G4C2 (sense) and C4G2 (anti-sense) repeat RNAs are produced via bi-directional transcription and accumulate in nuclear foci. These (GGGGCC)n repeats form G-quadruplexes and hairpins, which sequester RNA-binding proteins (RBPs). Since RBPs have multiple roles in RNA metabolism, several mechanisms underlying RNA toxicity and subsequent loss of proteome fidelity by nuclear RNA foci are possible. Moreover, a subset of expanded (GGGGCC)n RNA is exported into the cytoplasm and undergoes repeat-associated non-ATG translation to produce aggregation-prone dipeptide repeats (DPRs). DPRs interact with RBPs, altering the dynamics of cytosolic ribonucleoprotein assemblies such as stress granules (SGs). Thus, the retention of RBPs and RNA within aberrant SGs could impair RNA and protein fidelity, with pathological consequences for cellular function. We hypothesize that accumulation of nuclear foci and alterations in SGs converge in the sequestration of common RNAs and RBPs or eventually affect similar interconnected RNA and protein fidelity processes, resulting in extensive abnormal RNA and protein metabolism. Here, we will perform and integrate system-wide analysis of changes in RNA and protein metabolism occurring in the nucleus and cytoplasm to define converging alterations caused by mutant C9orf72-associated nuclear RNA foci and cytosolic DPRs.