Nature 546, 243–247 (2017)

Credit: NATURE

Mutations that generate nucleotide repeat expansions within RNA transcripts are common among neurodegenerative diseases such as Huntington disease (HD) and amyotrophic lateral sclerosis (ALS). Repeat-containing RNAs have been shown to accumulate aberrantly in cell nuclei as foci, leading Jain and Vale to suspect that multivalent interactions between repeat-containing RNAs contribute to the foci and thus to disease. To test this, the authors first showed that RNA molecules containing 30 or more triplet repeats of CAG (as seen in HD) or CUG (as seen in myotonic dystrophy) could form micrometer-sized spherical clusters in vitro that associate via intermolecular base pairing and electrostatic interactions but do not require protein components. Fluorescence recovery after photobleaching experiments showed that these RNA clusters behaved as gels. In cells, similarly expanded RNAs exhibited liquid-like properties, whereby RNAs freely move in and out of the foci. The in-cell properties were likely modulated by proteins and required ATP, perhaps to remodel RNA base pairing. CAG RNA nuclear foci were sensitive to inhibitors of RNA gelation and colocalized with, but were distinct from, nuclear speckles, a reservoir of pre-mRNA splicing factors. Similar nuclear foci formed from G-quadruplex-forming repeat-containing RNAs associated with ALS. These results suggest that RNAs phase separate into spherical, liquid-like droplets that rapidly crosslink into gels via intermolecular base pairing and could help to explain why disease presentation only occurs after a critical number of repeats.