Alzheimer's disease (AD) is the most common neurodegenerative disease and is manifested by memory loss and spatial disorientation. There is currently no effective treatment for AD. Abnormalities of the chromosome 9 open reading frame 72 (C9orf72) gene have been associated with various neurodegenerative diseases. However, its intrinsic roles in AD remain to be elucidated. Here we found that A beta(25-35) increased the expression of C9orf72 in PC12 cells at both mRNA and protein levels. In A beta(25-35)-treated PC12 cells, C9orf72 overexpression induced an abnormally condensed and fragmented nucleus and apoptosis, as well as significantly enhanced reactive oxygen species (ROS) levels. Mechanistically, an A beta(25-35)-induced decrease of superoxide dismutase activity was augmented by C9orf72 overexpression, which in contrast increased malondialdehyde content. Consistently, further apoptotic analysis revealed significant downregulation of Bcl-2 and Bcl-xL expression and enhanced cleavage of caspase-3 with A beta(25-35) treatment, all of which were exacerbated by C9orf72 overexpression. In addition, tau phosphorylation, another hallmark of AD pathology, was induced by A beta(25-35) and was remarkably enhanced by C9orf72 overexpression. Our data indicate that C9orf72 plays important roles in intracellular ROS signaling and A beta(25-35)-induced neuronal apoptosis in AD. These findings provide insights into C9orf72 function in the pathogenesis of many related neurodegenerative diseases and provide a basis for potential therapeutic interventions.