Disruption of RNA Metabolism in Neurological Diseases and Emerging Therapeutic Interventions

Abstract

RNA binding proteins are critical to the maintenance of the transcriptome via controlled regulation of RNA processing and transport. Alterations of these proteins impact multiple steps of the RNA life cycle resulting in various molecular phenotypes such as aberrant RNA splicing, transport, and stability. Disruption of RNA binding proteins and widespread RNA processing defects are increasingly recognized as critical determinants of neurological diseases. Here, we describe distinct mechanisms by which the homeostasis of RNA binding proteins is compromised in neurological disorders through their reduced expression level, increased propensity to aggregate or sequestration by abnormal RNAs. These mechanisms all converge toward altered neuronal function highlighting the susceptibility of neurons to deleterious changes in RNA expression and the central role of RNA binding proteins in preserving neuronal integrity. Emerging therapeutic approaches to mitigate or reverse alterations of RNA binding proteins in neurological diseases are discussed.

Keywords: ALS; FMRP; RNA binding protein; Repeat expansion; SMA; SMN; TDP-43; autism; dementia; hnRNP.

Figure 1.. Mechanisms Leading to Disruption of RBPs in Neurodegenerative Diseases

(A) Expression of RBPs may be reduced by point mutations, abnormal splicing, aberrant repeat expansions that repress the RBP transcription, and neutralization by autoantibodies in paraneoplastic neurological syndromes. (B) Abnormal phase transition of RBPs with low complexity domains leads to their aggregation and mislocalization with loss of function and/or gain of novel toxic properties. (C) Expanded repeats in microsatellite diseases may lead to sequestration of RBPs into RNA foci and/or abnormal interaction with repeat-containing proteins translated from the expansion. (D) Disruption of RBPs has a widespread effect on the metabolism of their RNA targets including abnormal splicing, polyadenylation, transport, translation, and decay with downstream effects on cellular morphology and function.

Figure 2.. Therapeutic Strategies to Restore the Level and Function of RBPs in Neurodegenerative Diseases

(A) Strategies to restore RBP expression level. (i) Antisense oligonucleotides (ASOs) enhance inclusion of SMN2 exon 7 resulting in increased levels of SMN2 protein. (ii) Exon 7 inclusion in SMN2 achieved via small-molecule recruiting U1 snRNP. (iii) Restoration of SMN2 proteins levels through inhibition of Histone deacetylases (HDACs). (iv) Restoration of SMN1 protein level by AAV-mediated genetherapy. (v) Promotion of FMR1 expression using deactivated Cas9 (dCas9) fused to Tet1 that when targeted to CGG repeat expansion leads to demethylation of the upstream promoter and gene re-activation. (vi) The tetracycline analog minocycline blocks excessive translation caused by loss of FMRP. (B) Strategies to prevent or reverse RBP aggregation. (i) RNase-H-dependent ASOs reducing the levels of ataxin 2 alleviate TDP-43 aggregation and toxicity. (ii) Modulation of chaperones such as heat shock proteins (hsp) and disaggregases such as nuclear import factors reverse abnormal phase transition and aggregation of RBPs. (C) Strategies to block RBP sequestration by repeat-expanded RNAs and proteins. ASOs have been applied to alleviate RBP sequestration by repeat expanded RNAs both by blocking and displacing muscleblind proteins or by degrading the toxic RNA in myotonic dystrophy (i), ASOs inducing isoform-specific degradation of expansion-containing transcripts alleviated pathology without reducing overall levels of C9ORF72 in ALS and/or FTD (ii). CRISPR technology was applied to repeat expansion diseases to either excise the repeat expansion or to block RNA Pol II transcription in the absence of nuclease activity (iii).