Repeat-associated non-AUG (RAN) translation allows for unconventional initiation at disease-causing repeat expansions. As RAN translation contributes to pathogenesis in multiple neurodegenerative disorders, determining its mechanistic underpinnings may inform therapeutic development. Here we analyze RAN translation at G4C2 repeat expansions that cause C9orf72-associated amyotrophic lateral sclerosis and frontotemporal dementia (C9RAN) and at CGG repeats that cause fragile X-associated tremor/ataxia syndrome. We find that C9RAN translation initiates through a cap- and eIF4A-dependent mechanism that utilizes a CUG start codon. C9RAN and CGG RAN are both selectively enhanced by integrated stress response (ISR) activation. ISR-enhanced RAN translation requires an eIF2α phosphorylation-dependent alteration in start codon fidelity. In parallel, both CGG and G4C2 repeats trigger phosphorylated-eIF2α-dependent stress granule formation and global translational suppression. These findings support a model whereby repeat expansions elicit cellular stress conditions that favor RAN translation of toxic proteins, creating a potential feed-forward loop that contributes to neurodegeneration.
Bibliographical noteFunding Information:
We thank members of the Todd lab for input on this manuscript. Jerry Pelletier (McGill University) kindly provided hippuristanol, and Randal Kaufman (Sanford Burnham Prebys Medical Discovery Institute) supplied WT and eIF2α S51 A/A MEFs. This work was funded by grants from the VA BLRD (1I21BX001841 and 1I01BX003231), the NIH (R01NS099280 and R01NS086810), the Packard Foundation, the Michigan Alzheimer’s Disease Center and Protein Folding Disease Initiative to P.K.T. K.M.G., B.N.F., and A.E. L. were supported by NIH T32GM007315. K.M.G. was further supported by NIH F31NS100302 and A.E.L. by NIH F30NS098571. M.R.G. was supported by NIH T32NS007222-35S1. M.G.K. was supported by the NIH F32NS089124. S.J.B. and B.N.F. were supported by the NIH (R01-NS097542, 1P30AG053760-01) and Ann Arbor Active Against A.L.S.
© 2017 The Author(s).