ALS/FTD-Associated C9ORF72 Repeat RNA Promotes Phase Transitions In Vitro and in Cells

Abstract

Summary: Membraneless RNA granules originate via phase separation events driven by multivalent interactions. As RNA is the defining component of such granules, we examined how RNA contributes to granule assembly. Expansion of hexanucleotide GGGGCC (G4C2) repeats in the first intron of C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). We describe a biophysical phenomenon whereby G4C2 RNA (rG4C2) promotes the phase separation of RNA granule proteins in vitro and in cells. The ability of rG4C2 to promote phase separation is dependent on repeat length and RNA structure because rG4C2 must assume a G-quadruplex conformation to promote granule assembly. We demonstrate a central role for RNA in promoting phase separations and implicate rG4C2 G-quadruplex structures in the pathogenesis of C9-ALS/FTD. : Fay et al. report that GGGGCC repeat RNA (rG4C2) associated with ALS/FTD drives phase separations to promote RNA granule formation in vitro and in cells. Such separations are length- and G-quadruple-structure-dependent. Targeting rG4C2 G-quadruplex structures is a potential therapeutic option for ALS/FTD. Keywords: phase separation, G-quadruplex, C9ORF72, amyotrophic lateral sclerosis, frontotemporal dementia, RNA, stress granule, RNA granule, neurodegeneration