C9orf72 arginine-rich dipeptide repeats inhibit UPF1-mediated RNA decay via translational repression

Yu Sun,Jeffrey Zhou,Atagun U Isiktas,Junjie U Guo,Aziz Eshov

doi:10.1038/s41467-020-17129-0

Abstract

Expansion of an intronic (GGGGCC)n repeat region within the C9orf72 gene is a main cause of familial amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). A hallmark of c9ALS/FTD is the accumulation of misprocessed RNAs, which are often targets of cellular RNA surveillance. Here, we show that RNA decay mechanisms involving upstream frameshift 1 (UPF1), including nonsense-mediated decay (NMD), are inhibited in c9ALS/FTD brains and in cultured cells expressing either of two arginine-rich dipeptide repeats (R-DPRs), poly(GR) and poly(PR). Mechanistically, although R-DPRs cause the recruitment of UPF1 to stress granules, stress granule formation is independent of NMD inhibition. Instead, NMD inhibition is primarily a result from global translational repression caused by R-DPRs. Overexpression of UPF1, but none of its NMD-deficient mutants, enhanced the survival of neurons treated by R-DPRs, suggesting that R-DPRs cause neurotoxicity in part by inhibiting cellular RNA surveillance.

Highlights

Expansion of an intronic (GGGGCC)n repeat region within the C9orf[72] gene is a main cause of familial amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD)
To assess whether nonsense-mediated decay (NMD) may be defective in c9ALS/FTD, we quantified the abundance of regulatory targets of NMD
To test whether NMD inhibition may be linked to stress granule formation, we examined the localization of upstream frameshift 1 (UPF1) and G3BP1, a stress granule marker, in HeLa cells expressing each repeat RNA or dipeptide repeat (DPR)

Summary

Introduction

Expansion of an intronic (GGGGCC)n repeat region within the C9orf[72] gene is a main cause of familial amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). We show that RNA decay mechanisms involving upstream frameshift 1 (UPF1), including nonsense-mediated decay (NMD), are inhibited in c9ALS/FTD brains and in cultured cells expressing either of two arginine-rich dipeptide repeats (R-DPRs), poly(GR) and poly(PR). The expanded G4C2 repeat region is transcribed in both directions, producing the C9orf[72] pre-mRNA, which contains intronic G4C2 repeats, and the antisense RNA that contains G2C4 repeats[8] Both RNAs accumulate in nuclear foci and, after being exported, can be translated into distinct sets of DPR-containing polypeptides[8]. We show that a main reason for the accumulation of aberrant RNAs in c9ALS/FTD brains is the global defect in UPF1-dependent RNA decay pathways including NMD. Ectopic expression of UPF1 protects neurons from R-DPR toxicity in an NMD-dependent manner These results suggest a previously underappreciated role of cellular RNA surveillance in the pathophysiology of c9ALS/ FTD

Methods

Results

Conclusion