Cellular toxicity of expanded RNA repeats: focus on RNA foci

Abstract

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.

Figure 1.

Simple repeat expansions associated with human neurodegenerative and neuromuscular disorders; mechanisms of pathogenesis mediated by protein loss-of-function (white) and protein and RNA toxic gain-of-function (yellow).

Figure 2.

Representative images of FISH RNA depicting nuclear RNA foci in various tissues and cultured cells expressing CUG, CCUG, CAG, CGG, AUUCU and UGGAA repeat mutations. (1A) Nucleus of human muscle fiber with CUG foci (1B) CUG foci in a gallbladder from a DM1 patient (1C) CUG foci in post-mortem cardiac tissue colocalizing with MBNL2 (1D) CUG inclusions in frontal cortical neurons of a human DM1 brain (1E) small nuclear CUG foci in Purkinje cells from a DM1 brain (2A) ribonuclear CUG foci in DM1 myoblasts (2B) and (2C) CUG nuclear foci in DM1 fibroblasts without MyoD (B) and with MyoD (C) (2D) nuclear CUG foci in skeletal muscle from HSA^LR-20b DM1 mice (2E) nuclear CUG foci in skeletal muscle from transgenic CTG200 mice (3A) nuclear and cytoplasmic RNA foci in C2C12 cells ectopically expressing 200 CUG from 3′UTR DMPK (3B) nuclear RNA foci in COS7 cells expressing 960 CUG repeats (3C) nuclear CUG foci in the larval muscle of DM1 Drosophila (3D) CUG foci in the nuclei of the body wall muscle cells from DM1 Drosophila (3E) nuclear RNA foci formed by 270 CUG repeats expressed in Drosophila (4A) CCUG nuclear inclusion in human DM2 skeletal muscle (4B) CCUG focus in the left ventricles of an autopsy heart (4C) CCUG foci in human homozygous DM2 myoblasts (4D) CUG foci in Purkinje cells from a human SCA8 cerebellum (4E) CUG foci in Purkinje cells from SCA8 BAC mice (5A) nuclear CUG foci in HEK293 cells expressing an ATXN8OS mutant transcript (5B) CUG nuclear foci in a HDL2 cerebral frontal cortex (5C) CUG foci in HDL2 stratum (5D) CAG nuclear aggregates in human HD fibroblasts (5E) nuclear CAG foci in muscle sections from CAG200 mice (6A) RNA foci in C2C12 cells ectopically expressing 200 CUG colocalizing with MBNL (6B) nuclear foci in COS7 cells expressing 960 CAG repeats (6C) nuclear RNA foci formed by CAG270 expressed in Drosophila (6D) nuclear signal from 5′UTR antisense riboprobe in isolated nuclei of the frontal cortex from post-mortem FXTAS brain (6E) nuclear aggregate of CGG repeat mutation sequesters Sam68 protein in a post-mortem FXTAS brain section of the hippocampal area (7A) nuclear CGG foci in brain sections from mice expressing 98 CGG repeats colocalizing with Sam68 (7B) nuclear RNA aggregates in COS7 cells expressing 100 CGG repeats (7C) nuclear and cytoplasmic AUUCU aggregates in SCA10 human fibroblasts (7D) nuclear and cytoplasmic RNA aggregates in brains from mice expressing expanded AUUCU repeats (7E) nuclear RNA foci in SCA31 Purkinje cells .