Distinct cellular effects of myotonic dystrophy type 2 repeat-associated non-AUG tetrapeptides

Myotonic dystrophy type 2 (DM2) is an autosomal dominant, multisystemic disorder caused by the expansion of CCTG repeats in the first intron of the CNBP gene. Repeat-associated non-AUG (RAN) translation of the expanded CCTG RNA generates two tetrapeptide repeat proteins, polyQAGR and polyLPAC, but their roles in DM2 pathogenesis remain unclear. To define their individual contributions, we expressed codon-optimized polyQAGR and polyLPAC peptides with an ATG start codon in Drosophila melanogaster. Expression of both tetrapeptide repeat proteins reduced viability and lifespan and induced eye degeneration and locomotor defects. We found that polyQAGR accumulated in the nucleolus, disrupted nucleolar integrity and impaired rRNA processing. It also interfered with autophagy, promoting Atg5 and Atg7 transcription and accumulation of Atg8a- and Ref(2)P-positive aggregates. Overexpression of Atg8a or Ref(2)P mitigated polyQAGR-induced eye toxicity, whereas knockdown of autophagy genes worsened it. Conversely, PolyLPAC expression increased the cytoplasmic pool of TIAR in human cells and in DM2 patient-derived myoblasts. Together, these findings show that polyQAGR and polyLPAC exert distinct toxic effects that likely converge to drive DM2 pathogenesis and may represent promising therapeutic targets.