Repeat RNA expansion disorders of the nervous system: post-transcriptional mechanisms and therapeutic strategies

Abstract

Dozens of incurable neurological disorders result from expansion of short repeat sequences in both coding and non-coding regions of the transcriptome. Short repeat expansions underlie microsatellite repeat expansion (MRE) disorders including myotonic dystrophy (DM1, CUG_50-3,500 in DMPK; DM2, CCTG_75-11,000 in ZNF9), fragile X tremor ataxia syndrome (FXTAS, CGG_50-200 in FMR1), spinal bulbar muscular atrophy (SBMA, CAG_40-55 in AR), Huntington's disease (HD, CAG_36-121 in HTT), C9ORF72- amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD and C9-ALS/FTD, GGGGCC in C9ORF72), and many others, like ataxias. Recent research has highlighted several mechanisms that may contribute to pathology in this heterogeneous class of neurological MRE disorders - bidirectional transcription, intranuclear RNA foci, and repeat associated non-AUG (RAN) translation - which are the subject of this review. Additionally, many MRE disorders share similar underlying molecular pathologies that have been recently targeted in experimental and preclinical contexts. We discuss the therapeutic potential of versatile therapeutic strategies that may selectively target disrupted RNA-based processes and may be readily adaptable for the treatment of multiple MRE disorders. Collectively, the strategies under consideration for treatment of multiple MRE disorders include reducing levels of toxic RNA, preventing RNA foci formation, and eliminating the downstream cellular toxicity associated with peptide repeats produced by RAN translation. While treatments are still lacking for the majority of MRE disorders, several promising therapeutic strategies have emerged and will be evaluated within this review.

Keywords: ASO; MRE; RAN translation; RNA; RNA foci; protein; rCas; repeat expansion.

Figure 1.

Schematic of a representative neuron, displaying key neuropathologies associated with MRE disorders. Starting in the nucleus, (1) bidirectional transcription initiates a cascade of RNA-based pathologies, including (2) intranuclear RNA foci, which can impair (2A) miRNA biogenesis, (2B) mRNA splicing, (2c) and phase separation, as well as (3) RAN translation, the products of which can impair (3A) ubiquitin-proteasome system, (3B) extracellular environments, (3C) nucleocytoplasmic transit, (3D) axonal mRNA transport, (3E) mRNA export, (3F) Ran gradients, and (3G) nucleoporin localization.

Figure 2.

Schematic depicting major therapeutic strategies and associated approaches for treating molecular pathologies associated with neurological MRE disorders. To suppress repeat RNA, repeat RNA synthesis could be inhibited, such as with catalytically inactive dCas9. Alternatively, repeat RNA could be targeted for degradation by RNaseH through ASOs or by fusions of nucleases to rCas through sgRNA. Additionally, modulation of repeat-containing pre-RNA splicing may prevent accumulation of mature repeat-containing RNA by promoting either skipping of the repeat-containing exon or retention of an intron to mark the transcript as a substrate for nonsense mediated decay, for example. To disrupt RNA foci formation independent of eliminating repeat transcripts, ASOs can be used to inhibit pathogenic secondary structures that recruit RBPs and give rise to intranuclear foci. To block RAN toxicity, two strategies are shown that either inhibits a critical regulator of RAN translation, but not canonical translation, or promotes clearance of RAN peptides with antibodies.