C9orf72 hexanucleotide repeat RNA drives transcriptional dysregulation through genome-wide DNA:RNA hybrid G-quadruplexes.

A hexanucleotide repeat expansion in C9orf72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. While repeat RNAs are implicated in disease pathogenesis, their mechanisms of action remain incompletely understood. Here, we show that GGGGCC repeat RNA engages chromatin genome-wide preferentially at promoter regions in patient cells. This interaction obstructs RNA polymerase II and transcription factors with GC-rich motifs, leading to broad transcriptional repression. Biochemical assays, single-molecule imaging, and native bisulfite sequencing analyses demonstrate that GGGGCC repeat RNA intrinsically forms DNA:RNA hybrid G-quadruplexes (HQs) with cognate DNA, providing a structural basis for transcriptional interference. Stabilization of these G-quadruplex structures exacerbates neuronal vulnerability to metabolic stress in patient-derived motor neurons and cortical organoids, whereas restoring key gene dysregulation improves resistance. These findings uncover a previously unrecognized trans-acting mechanism whereby repetitive RNAs form hybrid structures with genomic DNA, disrupt gene regulation, and contribute to neurodegeneration.