DM1 repeat-expanded RNAs confer RNA toxicity as individual nuclear-retained RNAs

Repeat expansions of short nucleotide sequences underlie over 40 neuromuscular diseases, including myotonic dystrophy type 1 (DM1). The DM1 CUG repeat RNA is thought to accumulate in RNA nuclear foci that sequester RNA-binding proteins, including muscleblind-like splicing regulator 1 (MBNL1). To understand the composition and formation of such nuclear foci, we employed quantitative imaging in a patient-derived myotube model. We find that most "foci" are comprised of single RNAs and that these single RNA species contribute to the sequestration of MBNL1 protein. Rare foci can contain upwards of 25 distinct RNA species, but these foci form from transcriptional bursting and dissociate with time. Last, we find that multimeric CUG repeat RNA foci are dependent upon MBNL proteins. Altogether, these observations argue that the persistence of nuclear-retained CUG RNAs, independent of higher-order RNA assemblies, titrates MBNL1 and contributes to disease progression.