Deregulation of RNA Metabolism in Microsatellite Expansion Diseases

Abstract

RNA metabolism impacts different steps of mRNA life cycle including splicing, polyadenylation, nucleo-cytoplasmic export, translation, and decay. Growing evidence indicates that defects in any of these steps lead to devastating diseases in humans. This chapter reviews the various RNA metabolic mechanisms that are disrupted in Myotonic Dystrophy-a trinucleotide repeat expansion disease-due to dysregulation of RNA-Binding Proteins. We also compare Myotonic Dystrophy to other microsatellite expansion disorders and describe how some of these mechanisms commonly exert direct versus indirect effects toward disease pathologies.

Keywords: Alternative splicing and polyadenylation; Microsatellite repeat expansions; Post-transcriptional gene regulation; RNA toxicity; RNA-binding proteins.

Figure 1.. Origin and expansion of microsatellite repeats in human disease.

Schematic of the gene location for various disease-associated repeat expansions. Types of repeat expansions are indicated within the parentheses along with the range of expanded repeat numbers.

Figure 2.. Schematic showing different pathological mechanisms for Myotonic Dystrophy type 1 (DM1) and 2 (DM2).

(a) Causative mutation for DM1 is CUG repeat expansion in 3’UTR of DMPK gene and for DM2 is CCUG repeat expansion in intron 1 of Znf9 gene. The severity of the disease is dependent on the number of repeats. Although these mutations are in two different genes, the disease mechanisms for both diseases are surprisingly similar. Most of the pathology is consistent with the toxic RNA gain-of-function mechanism and affects general RNA metabolism both in the nucleus and cytoplasm. (b) After transcription, the repeat containing transcripts form stable hairpin loop comprising secondary structures (pink), which aggregate to form ribonuclear foci. (c) Members of the Mbnl family of RNA binding proteins (RBPs) MBNL1/2 (purple) bind the CUG or CCUG repeats and are sequestered in the ribonuclear foci. (d) Hyperphosphorylation by PKC stabilizes another RBP, CELF1, resulting in its gain-of-function. (e) Both MBNL and CELF proteins regulate various aspects of RNA metabolism during normal development. Alterations in their functional levels due to toxic repeat RNA causes adult-to-fetal reversion of splicing and polyadenylation for many pre-mRNAs in the nucleus. (f) MBNL depletion also leads to cellular mislocalization of many mRNAs. CELF1 gain-of-function further affects (g) miRNA metabolism and (h) mRNA translation. (i) Deregulation of MBNL and CELF activity in the cytoplasm also affects mRNA stability through various mechanisms. (j) Both sense and antisense CUG/CCUG containing transcripts are subject to RAN translation in all three frames giving rise to homopolymeric polypeptides that accumulate in the cytoplasm and form pathological intracellular aggregates.