Mechanistic insights into RAN translation within the Huntingtin coding sequence

This process, known as repeat-associated non-AUG (RAN) translation, was originally described at CAG repeat expansions with products generated in all three potential reading frames from plasmid-based reporters. Recently, cryptic mis-splicing of mRNAs where the CAG repeat serves as a splice acceptor was proposed as an alternative mechanism by which RAN proteins might be generated.

To better understand this process, we generated a series of DNA and RNA based reporters to assess RAN translation in the context of a CAG repeat expansion in exon 1 of the Huntingtin gene (HTT) associated with Huntington Disease. HTT CAG repeats support RAN translation in both the alanine (GCA) and glutamine (CAG) frames in both the presence and absence of upstream AUG start codons or near-AUG cognate codons. HTT RAN translation in all reading frames is less efficient than that observed at CGG or GGGGCC repeats, but it exhibits similar cap-dependence and selective enhancement by activation of the integrated stress response.