Translation of Expanded CGG Repeats into FMRpolyG Is Pathogenic and May Contribute to Fragile X Tremor Ataxia Syndrome

Abstract

SummaryFragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder caused by a limited expansion of CGG repeats in the 5′ UTR of FMR1. Two mechanisms are proposed to cause FXTAS: RNA gain-of-function, where CGG RNA sequesters specific proteins, and translation of CGG repeats into a polyglycine-containing protein, FMRpolyG. Here we developed transgenic mice expressing CGG repeat RNA with or without FMRpolyG. Expression of FMRpolyG is pathogenic, while the sole expression of CGG RNA is not. FMRpolyG interacts with the nuclear lamina protein LAP2β and disorganizes the nuclear lamina architecture in neurons differentiated from FXTAS iPS cells. Finally, expression of LAP2β rescues neuronal death induced by FMRpolyG. Overall, these results suggest that translation of expanded CGG repeats into FMRpolyG alters nuclear lamina architecture and drives pathogenesis in FXTAS.

RNA-binding proteins in neurodegeneration: mechanisms in aggregate

“…For example, with regard to FXS, the CGG repeat in FMR1 is outside the ORF, in the 5 ′ UTR (Ashley et al 1993), suggesting that the expansion might lead to decreased gene expression (Bell et al 1991). However, recent advances in understanding repeat-associated non-AUG (RAN) translation have reopened the debate about whether expansions in noncoding regions can be translated to generate short repetitive polypeptides (Cleary and Ranum 2014;Sellier et al 2017). The ubiquity of this mechanism across a multitude of triplet, quadruplet, and higher-order nucleotide expansions suggests a dual role of RNA as the toxic molecule itself and the intermediary to one, with important inputs from specific RBPs factoring into both.…”

Regulation of mRNA Translation in Neurons—A Matter of Life and Death

“…For example, six RAN proteins, corresponding to all frames of sense and antisense transcripts, are produced from the hexanucleotide expansion found in an intron of C9ORF72, the most common familial cause of ALS and frontotemporal dementia (FTD) (Zu et al, 2013). Furthermore, three products are made from the sense or antisense transcripts from the 55-200 CGG repeats found in the 5’UTR of FMR1 in fragile X tremor/ataxia syndrome (FXTAS; Sellier et al, 2017). …”

“…For RAN products that do not initiate within the repeat, upstream sequences are essential. For example, initiation of FMR1polyGly primarily relies on an ACG upstream of the expanded repeat that is translated as Met (Sellier et al, 2017). However, translation can also initiate at alternative near-AUGs if the favored ACG is deleted, or if translation of the repeat is blocked with a stop codon between the ACG and the repeat (Kearse et al, 2016; Todd et al, 2013).…”

“…For example, Todd et al (2013) generated flies expressing a repeat-expanded FMR1 5’ UTR using either the complete UTR or a mutant UTR incapable of sustaining FMR1polyGly translation, and demonstrated that the presence of FMR1polyGly causes toxicity. Deployment of a similar strategy in transgenic mice revealed that FMR1polyGly production caused Purkinje cell death, various motor deficits, and premature death, while no pathology was observed in mice expressing the mutant mRNA lacking FMR1polyGly initiation sites (Sellier et al, 2017). While this approach allowed investigation of FMR1polyGly, it cannot be readily applied to all contexts, as other RAN-translated proteins can initiate within the repeat itself.…”

Rethinking Unconventional Translation in Neurodegeneration

“…During the last two decades, unconventional translation products from various expanded repeat sequences have been repeatedly detected in the nervous system of affected individuals. Three mechanisms have been proposed to account for the generation of these disease-specific proteins: 1) ribosomal frameshifting (Gaspar et al, 2000), 2) repeat-associated non-AUG (RAN) translation (Zu et al, 2011), and 3) near-cognate start codon initiated translation (Sellier et al, 2017). …”