CRISPR-Cas9 Screens Identify the RNA Helicase DDX3X as a Repressor of C9ORF72 (GGGGCC)n Repeat-Associated Non-AUG Translation

Abstract

<p id="P3">Hexanucleotide GGGGCC repeat expansion in <i>C9ORF72</i> is the most prevalent genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). One pathogenic mechanism is the aberrant accumulation of dipeptide repeat (DPR) proteins produced by the unconventional translation of expanded RNA repeats. Here, we performed genome-wide CRISPR-Cas9 screens for modifiers of DPR protein production in human cells. We identified DDX3X, an RNA helicase, suppresses the repeat-associated non-AUG translation of GGGGCC repeats. DDX3X directly binds to (GGGGCC)n RNAs, but not antisense (CCCCGG)n RNAs. Its helicase activity is essential for the translation repression. Reduction of DDX3X increases DPR levels in <i>C9ORF72</i>-ALS/FTD patient cells and enhances (GGGGCC)n-mediated toxicity in <i>Drosophila.</i> Elevating DDX3X expression is sufficient to decrease DPR levels, rescue nucleocytoplasmic transport abnormalities, and improve survival of patient iPSC-differentiated neurons. This work identifies genetic modifiers of DPR protein production and provides potential therapeutic targets for <i>C9ORF72</i>-ALS/FTD. </p><p id="P4"> <div class="figure-container so-text-align-c"> <img alt="" class="figure" src="/document_file/60251ca3-aa94-42fa-aff9-a3d7c108caab/PubMedCentral/image/nihms-1539444-f0007.jpg"/> </div> </p><p id="P5">DPR proteins produced by the unconventional translation of expanded RNA repeats contributes to neurodegeneration in <i>C9ORF72</i>-ALS/FTD. Cheng and Wang et al. identify DDX3X as a repressor of r(GGGGCC)n translation. Elevating DDX3X decreases DPRs, rescues pathological phenotypes and improves neuron survival. </p>