# IGF2BPs directly regulate the noncanonical translation of toxic proteins from mutant FMR1 mRNA containing expanded CGG repeats

## TL;DR

This study shows that IGF2BP3 helps produce a toxic protein from a faulty gene linked to fragile X disorders, and reducing IGF2BP3 lowers this toxicity.

## Contribution

The study identifies IGF2BP3 as a direct regulator of non-AUG translation of toxic FMRpolyG from mutant FMR1 mRNA.

## Key findings

- IGF2BP3 binds to the 5’UTR of FMR1 RNA and promotes non-AUG translation of FMRpolyG.
- Knockdown of IGF2BP3 reduces FMRpolyG biosynthesis and cell toxicity in mutant FMR1-expressing cells.
- Disrupting IGF2BP ortholog in C. elegans rescues disease phenotypes caused by expanded CGG repeats.

## Abstract

Mutant mRNA of the fragile X messenger ribonucleoprotein 1 gene (FMR1) containing expanded CGG repeats in its 5’UTR is a primary cause of fragile X premutation associated conditions. It serves as a template for the biosynthesis of the major open reading frame encoding canonical protein and the downstream open reading frame containing expanded CGG repeats encoding toxic FMRpolyG protein that comprise a long polyglycine stretch, produced via repeat-associated non-AUG initiated translation. Here, we show that insulin-like growth factor 2 mRNA-binding protein 3 (IGF2BP3) binds directly to the 5’UTR of FMR1 RNA, and the sequence in the vicinity of near-cognate start codons of non-AUG translation is pivotal for IGF2BP3 binding. Upon IGF2BP3’s knockdown, FMRpolyG biosynthesis and cell toxicity evoked by FMRpolyG, significantly decreased in cells expressing mutant FMR1 with expanded CGG repeats. Disruption of IGF2BP ortholog in novel fragile X premutation associated conditions C. elegans model rescues the disease phenotype induced by expression of a human FMR1 RNA fragment containing expanded CGG repeats. Our results suggest that IGF2BP3 positively regulates the noncanonical translation of expanded CGG repeats and may be a promising target for clinical applications.

Expanded FMR1 CGG repeats drive toxic FMRpolyG in premutation disorders. Here, the authors show that IGF2BP3 binds the FMR1 5’UTR to promote non-AUG translation of FMRpolyG and that its loss reduces toxicity.