Emerging role of RNA•DNA hybrids in C9orf72-linked neurodegeneration.

RNA plays an active role in structural polymorphism of the genome through the formation of stable RNA•DNA hybrids (R-loops). R-loops can modulate normal physiological processes and are also associated with pathological conditions, such as those related to nucleotide repeat expansions. A guanine-rich hexanucleotide repeat expansion in chromosome 9 open reading frame 72 (C9orf72) has been linked to a spectrum of neurological conditions including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here we discuss the possible roles, both locally and genome-wide, of R-loops that may arise from the C9orf72 hexanucleotide repeat. R-loops have the potential to influence the pathological processes identified in many repeat expansion diseases, such as repeat instability, transcriptional dysregulation, epigenetic modification, and antisense-mediated gene regulation. We propose that, given the wide-ranging consequences of R-loops in the cell, these structures could underlie multiple pathological processes in C9orf72-linked neurodegeneration.