The role of FMR1 mRNA structure on the efficiency of non-canonical translation of toxic polyglycine protein.

Repeat-associated non-AUG (RAN) translation of mutant FMR1 messenger RNA (mRNA) containing CGG repeat expansions results in the production of a toxic polyglycine protein (FMRpolyG), which contributes to fragile X premutation-associated conditions (FXPAC), including fragile X-associated tremor/ataxia syndrome (FXTAS). The 5' untranslated region of FMR1 mRNA folds into a thermodynamically stable secondary structure at the region of excessively expanded CGG repeats and constitutes a template for RAN translation initiated from near-cognate start codons located upstream of the CGGs. Cis-regulatory elements, including sequence context and stable secondary structures within mRNA, can affect translation initiation and elongation. Here, we show that different nucleotide sequence contexts close to the near-cognate start codon affect FMRpolyG synthesis. Moreover, the distance between the near-cognate start codon and downstream stable RNA structure considerably affects the efficiency of RAN translation initiation, which is positively correlated with the number of CGG repeats. In contrast, translation elongation is impaired as CGG repeats expand. We show that native FMRpolyG containing a short polyglycine tract is synthesized efficiently but rapidly degraded by the proteasome. Our results provide insight into the structural dependencies that regulate the translation of CGGs and can be used in other repeat expansion disorders. We also show that the RNA structure is a potential therapeutic target in FXPAC.