GGGGCC (G(4)C(2)) repeat expansion in a noncoding region of C9ORF72 is the most common cause of sporadic and familial forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)(1,2). The basis for pathogenesis is unknown. To capture the consequences of G(4)C(2) repeat expansion in a tractable genetic system, we generated transgenic fly lines expressing 8, 28 or 58 G(4)C(2) repeat-containing transcripts that do not have a translation start site (AUG) but contain an open-reading frame for green fluorescent protein (GFP) to detect repeat-associated non-AUG (RAN) translation. These transgenic animals show dosage-dependent, repeat length-dependent degeneration in neuronal tissues and RAN translation of dipeptide repeat (DPR) proteins as observed in patients. This model was used in a large-scale, unbiased genetic screen ultimately leading to the identification of 18 genetic modifiers that encode components of the nuclear pore complex (NPC) as well as the machinery that coordinates the export of nuclear RNA and the import of nuclear proteins. Consistent with these results we found morphological abnormalities in the architecture of the nuclear envelope in cells expressing expanded G(4)C(2) repeats in vitro and in vivo. Moreover, we identified a substantial defect in RNA export resulting in retention of RNA in the nuclei of Drosophila cells expressing expanded G(4)C(2) repeats and also in mammalian cells, including aged iPSC-derived neurons from C9ORF72 patients. These studies show that a primary consequence of G(4)C(2) repeat expansion is the compromise of nucleocytoplasmic transport through the nuclear pore, revealing a novel mechanism of neurodegeneration
