Cellular toxicity of expanded RNA repeats: focus on RNA foci

Discrete and punctate nuclear RNA foci are characteristic molecular hallmarks of pathogenesis in myotonic dystrophy type 1 and type 2. Intranuclear RNA inclusions of distinct morphology have also been found in fragile X-associated tremor ataxia syndrome, Huntington's disease-like 2, spinocerebellar ataxias type 8, type 10 and type 31. These neurological diseases are associated with the presence of abnormally long simple repeat expansions in their respective genes whose expression leads to the formation of flawed transcripts with altered metabolisms. Expanded CUG, CCUG, CGG, CAG, AUUCU and UGGAA repeats are associated with the diseases and accumulate in nuclear foci, as demonstrated in variety of cells and tissues of human and model organisms. These repeat RNA foci differ in size, shape, cellular abundance and protein composition and their formation has a negative impact on cellular functions. This review summarizes the efforts of many laboratories over the past 15 years to characterize nuclear RNA foci that are recognized as important triggers in the mutant repeat RNA toxic gain-of-function mechanisms of pathogenesis in neurological disorders.