Department of Pathology and Brain Science Institute, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Department of Genetics, Stanford University School of Medicine, Stanford, CA 94305, USA.
Neuron. 2019 Dec 4;104(5):885-898.e8. doi: 10.1016/j.neuron.2019.09.003. Epub 2019 Oct 3.
Hexanucleotide GGGGCC repeat expansion in C9ORF72 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 found that 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 C9ORF72-ALS/FTD patient cells and enhances (GGGGCC)n-mediated toxicity in Drosophila. 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 C9ORF72-ALS/FTD.
六核苷酸 GGGGCC 重复扩展是 C9ORF72 中最常见的肌萎缩侧索硬化症 (ALS) 和额颞叶痴呆 (FTD) 的遗传原因。一种致病机制是异常积累由扩展 RNA 重复的非常规翻译产生的二肽重复 (DPR) 蛋白。在这里，我们在人类细胞中进行了全基因组 CRISPR-Cas9 筛选，以寻找 DPR 蛋白产生的修饰因子。我们发现 RNA 解旋酶 DDX3X 抑制 GGGGCC 重复的重复相关非 AUG 翻译。DDX3X 直接结合 (GGGGCC)n RNA，但不结合反义 (CCCCGG)n RNA。其解旋酶活性对于翻译抑制至关重要。DDX3X 的减少增加了 C9ORF72-ALS/FTD 患者细胞中的 DPR 水平，并增强了果蝇中 (GGGGCC)n 介导的毒性。提高 DDX3X 的表达足以降低 DPR 水平，挽救核质转运异常，并改善患者 iPSC 分化神经元的存活率。这项工作确定了 DPR 蛋白产生的遗传修饰因子，并为 C9ORF72-ALS/FTD 提供了潜在的治疗靶点。
