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.

No abstract

Expansion of an intronic (GGGGCC)n repeat region within the C9orf72 gene is a major cause of familial amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). A pathological hallmark in c9ALS/FTD is the accumulation of misprocessed RNAs, which are often targets of RNA surveillance pathways in normal cells. Here we show that nonsensemediated decay (NMD) and other RNA decay mechanisms involving upstream frameshift 1 (UPF1), collectively referred to as UPF1-mediated RNA decay (UMD), are broadly inhibited in c9ALS/FTD brains. These effects are recapitulated in cultured cells by the ectopic expression of arginine-rich dipeptide repeats (DPRs), poly(GR) and poly(PR). Despite these two DPRs causing the recruitment of UPF1 to stress granules, stress granule formation is neither sufficient nor necessary for UMD inhibition. Our results suggest that UMD inhibition may accelerate the accumulation of deleterious RNAs and polypeptides in c9ALS/FTD.

Expansion of short tandem repeats (STRs) in the human genome underlies over fifty genetic disorders. A common pathological feature of repeat RNAs is their propensity to aggregate in cells. While these RNA aggregates have been shown to cause toxicity by sequestering RNA-binding proteins, the molecular mechanism of repeat RNA aggregation remains unclear. Here we devised a generalizable method to efficiently generate long tandem repeat DNAs de novo and applied it to systematically determine the sequence features underlying RNA aggregation. Live-cell imaging of repeat RNAs indicated that aggregation was mainly driven by multivalent RNA-RNA interactions via either canonical or noncanonical base pairs. While multiple short runs of two consecutive base pairs were sufficient, longer runs of consecutive base pairs such as those formed by a neurodegeneration-associated hexanucleotide repeat further enhanced aggregation. In summary, our study provides a unifying model for the molecular basis of repeat RNA aggregation and a generalizable approach for identifying the sequence and structural determinants of repeat RNA properties.

The original version of this article contained errors in Figs. 2A, 3A and 3B that were inadvertently introduced during typesetting. The third sample of Fig 2A, the third panel of Fig 3A, and the third sample of Fig 3B were inadvertently labelled (G 4 C 2 ) 100 RNA rather than (G 2 C 4 ) 100 RNA. In addition, the fifth sample of Fig 2A were inadvertently labelled GA 50 -GFP and rather than GR 50 -GFP. These errors have now been corrected in both the PDF and HTML versions of the article.