Translation of dipeptide repeat proteins in C9ORF72-ALS/FTD through unique and redundant AUG initiation codons

A hexanucleotide repeat expansion in the first intron of C9ORF72 is the most common monogenic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). A hallmark of ALS/FTD pathology is the presence of dipeptide repeat (DPR) proteins, produced from both sense GGGGCC (poly-GA, poly-GP, poly-GR) and antisense CCCCGG (poly-PR, poly-PG, poly-PA) transcripts. Although initiation codons and regulatory factors have been identified for sense DPR translation, they remain mostly unknown for antisense DPRs. Here, we show that an AUG initiation codon is necessary for poly-PR synthesis, suggesting canonical AUG dependent translation. Remarkably, although an AUG located 194 base pairs (bp) upstream of the repeat is the main start codon for poly-PG synthesis, two other AUG codons (-212 bp, -113 bp) can also initiate translation, demonstrating a striking redundancy in start codon usage. eIF2D is required for CUG start codon-dependent poly-GA translation from the sense transcript in human motor neurons derived from induced pluripotent stem cells of C9ORF72 ALS/FTD patients, but AUG-dependent poly-PG or poly-PR synthesis does not require eIF2D, indicating that distinct translation initiation factors control DPR synthesis from sense and antisense transcripts. Our findings provide key molecular insights into DPR synthesis from the C9ORF72 locus, which may be broadly applicable to many other nucleotide-repeat expansion disorders.