Repeat-associated non-AUG (RAN) translation and other molecular mechanisms in Fragile X Tremor Ataxia Syndrome

Abstract

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset inherited neurodegenerative disorder characterized by progressive intention tremor, gait ataxia and dementia associated with mild brain atrophy. The cause of FXTAS is a premutation expansion, of 55 to 200 CGG repeats localized within the 5'UTR of FMR1. These repeats are transcribed in the sense and antisense directions into mutants RNAs, which have increased expression in FXTAS. Furthermore, CGG sense and CCG antisense expanded repeats are translated into novel proteins despite their localization in putatively non-coding regions of the transcript. Here we focus on two proposed disease mechanisms for FXTAS: 1) RNA gain-of-function, whereby the mutant RNAs bind specific proteins and preclude their normal functions, and 2) repeat-associated non-AUG (RAN) translation, whereby translation through the CGG or CCG repeats leads to the production of toxic homopolypeptides, which in turn interfere with a variety of cellular functions. Here, we analyze the data generated to date on both of these potential molecular mechanisms and lay out a path forward for determining which factors drive FXTAS pathogenicity.

Keywords: FMR1; FXTAS; Microsatellite expansion; RAN translation; RNA gelation.

Fig. 1

FMR1 RNA repeat-mediated toxicity in FXTAS. (A) CGG repeat RNA creates a repetitive motif that is bound directly by hnRNP A2/B1, DGCR8, and Purα. This could potentially lower the amount of these proteins available to perform normal functions. (B) CGG repeat RNA may also indirectly titrate the abundance of other proteins through interactions with these three repeat binding partners. (C) Both CGG repeat RNAs and RNA binding proteins may be capable of phase separation and RNA gelation. (D) Potential functional consequences of CGG repeat RNA interactions include sequestration of proteins involved in splicing (hnrNP A2), mRNA transport (hnrNP A2, Purα), miRNA processing (DGCR8 and Drosha), and chromatin maintenance (HP1). Each of these pathways could contribute to generation of unprocessed or mislocalized mRNAs and miRNAs, increased transposon expression, stress granule formation, and global translational blockade.

Fig. 2

Repeat associated non-AUG translation and toxicity in FXTAS. (A) RAN translation of CGG repeats within the 5′UTR of FMR1 occurs in a m7G cap and ribosomal scanning dependent manner. Initiation occurs 5′ to the repeat at near cognate codons in the +0 (blue) and +1 (green) reading frames, and within the repeat in the +2 reading frame (yellow), to produce three homopolypeptides: FMRpolyR (blue), FMRpolyG (green), and FMRpolyA (yellow). Abundance of each product is depicted based on data from differential translation rates (Kearse et al., 2016, Krans et al., 2016). (B) RAN translation from ASFMR1 CCG repeat RNA produces three different homopolypeptides: ASFMRpolyP (purple), ASFMRpolyR (blue), and ASFMRpolyA (yellow). Translation of ASFMRpolyP also occurs potentially through initiation at an AUG codon (purple). The initiation sites for the +1 (blue) and +2 (yellow) reading frames have not yet been determined. (C) RAN translation products elicit toxicity. FMRpolyG (green). FMRpolyG may sequester LAP2β within these inclusions, disrupting nuclear architecture. FMRpolyG and FMRPolyA expression elicit ubiquitin proteasome system impairment. Either CGG repeat mRNA or CGG RAN derived polypeptides promote stress granule formation impairing global protein translation.