The Role of Dipeptide Repeats in C9ORF72-Related ALS-FTD.

Brian D Freibaum,J Paul Taylor

doi:10.3389/fnmol.2017.00035

Abstract

Expansion of a hexanucleotide (GGGGCC) repeat in the gene chromosome 9 open reading frame 72 (C9ORF72) is the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia (FTD). Three non-exclusive mechanisms have been proposed to contribute to the pathology initiated by this genetic insult. First, it was suggested that decreased expression of the C9orf72 protein product may contribute to disease. Second, the recognition that C9ORF72-related disease is associated with accumulation of GGGGCC repeat-containing RNA in nuclear foci led to the suggestion that toxic gain of RNA function, perhaps related to sequestration of RNA-binding proteins, might be an important driver of disease. Third, it was subsequently appreciated that GGGGCC repeat-containing RNA undergoes unconventional translation to produce unnatural dipeptide repeat (DPR) proteins that accumulate in patient brain early in disease. DPRs translated from all six reading frames in either the sense or antisense direction of the hexanucleotide repeat result in the expression of five DPRs: glycine–alanine (GA), glycine–arginine (GR), proline–alanine (PA), proline–arginine (PR) and glycine–proline (GP; GP is generated from both the sense and antisense reading frames). However, the relative contribution of each DPR to disease pathogenesis remains unclear. Here, we review evidence for the contribution of each specific DPR to pathogenesis and examine the probable mechanisms through which these DPRs induce neurodegeneration. We also consider the association of the toxic DPRs with impaired RNA metabolism and alterations to the liquid-like state of non-membrane-bound organelles.

Highlights

More recent evidence has emerged from studies of C9orf72-null mice, which display an age-related inflammatory response in macrophages and microglia, suggesting that loss of C9orf72 protein could potentially contribute in a non-cell autonomous manner to the neurodegeneration observed in patients with amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD) (O’Rourke et al, 2016)
Selective removal of C9orf72 from neurons does not lead to neurodegeneration or motor defects, suggesting that loss of protein function plays an auxiliary role in the pathogenesis of ALS/FTD (Koppers et al, 2015; O’Rourke et al, 2016)
This observation led to interrogation of potential repeat-associated non-ATG (RAN) translation of C9orf72, which was confirmed by the observation of dipeptide repeats (DPRs) translated from both the sense and antisense strands of the expanded hexanucleotide repeats

Summary

INTRODUCTION

The seminal discovery of a GGGGCC hexanucleotide repeat expansion within chromosome 9 open reading frame 72 (C9ORF72) in 2011 represented a major advance in neurodegenerative disease research by revealing the most common genetic insult responsible for the development of amyotrophic lateral sclerosis (ALS) and/or frontotemporal dementia (FTD) (DeJesus-Hernandez et al, 2011; Renton et al, 2011; Majounie et al, 2012). More detailed in vitro studies revealed that the GGGGCC repeat adopts a G-quadruplex structure (i.e., a tertiary structure formed by guanine-tetrad stacking), it remains unclear whether the G-quadruplex is the predominant structure found in sense or antisense RNA foci in vivo (Fratta et al, 2012; Reddy et al, 2013) Linking these RNA foci to toxicity, the prevailing gain-of-function hypothesis posits that RNA foci sequester RNA-binding proteins, leading to disruptions in RNA metabolism. The third and most recent non-exclusive hypothesis linking the C9orf expansion to pathogenesis is based on the phenomenon of repeat-associated non-ATG (RAN) translation, a type of unconventional translation first observed in the microsatellite expansion disease spinocerebellar ataxia type 8 (Zu et al, 2011) This observation led to interrogation of potential RAN translation of C9orf, which was confirmed by the observation of dipeptide repeats (DPRs) translated from both the sense and antisense strands of the expanded hexanucleotide repeats. We summarize the current knowledge of the individual contribution of specific DPRs to disease progression and provide a framework for understanding the potential mechanisms by which these DPRs produce their toxic phenotypes

DIPEPTIDE REPEAT PATHOLOGY

GA TOXICITY

Findings

ROLE OF OTHER DIPEPTIDE REPEATS AND THE INTERACTION OF MULTIPLE SPECIES