Globally reduced N 6 -methyladenosine (m 6 A) in C9ORF72-ALS/FTD dysregulates RNA metabolism and contributes to neurodegeneration.

Repeat expansion in C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here we show that N 6 -methyladenosine (m 6 A), the most prevalent internal mRNA modification, is downregulated in C9ORF72-ALS/FTD patient-derived induced pluripotent stem cell (iPSC)-differentiated neurons and postmortem brain tissues. The global m 6 A hypomethylation leads to transcriptome-wide mRNA stabilization and upregulated gene expression, particularly for genes involved in synaptic activity and neuronal function. Moreover, the m 6 A modification in the C9ORF72 intron sequence upstream of the expanded repeats enhances RNA decay via the nuclear reader YTHDC1, and the antisense RNA repeats can also be regulated through m 6 A modification. The m 6 A reduction increases the accumulation of repeat RNAs and the encoded poly-dipeptides, contributing to disease pathogenesis. We further demonstrate that, by elevating m 6 A methylation, we could significantly reduce repeat RNA levels from both strands and the derived poly-dipeptides, rescue global mRNA homeostasis and improve survival of C9ORF72-ALS/FTD patient iPSC-derived neurons.