G-Quadruplex Structures Formed by Expanded Hexanucleotide Repeat RNA and DNA from the Neurodegenerative Disease-Linked C9orf72 Gene Efficiently Sequester and Activate Heme

The expansion of a (G4C2)n repeat within the human C9orf72 gene has been causally linked to a number of neurodegenerative diseases, most notably familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Recent studies have shown that the repeat expansion alters gene function in four ways, disrupting the gene's normal cellular roles and introducing toxic gain of function at the level of both DNA and RNA. (G4C2)n DNA, as well as the RNA transcribed from it, are found to fold into four-stranded G-quadruplex structures. It has been shown that the toxicity of the RNA G-quadruplexes, often localized in intracellular RNA foci, lies in their ability to sequester many important RNA binding proteins. Herein we propose that a distinct toxic property of such RNA and DNA G-quadruplexes from the C9orf72 gene may arise from their ability to bind and oxidatively activate cellular heme. We show that G-quadruplexes formed by both (G4C2)4 RNA and DNA not only complex tightly with heme but also enhance its intrinsic peroxidase and oxidase propensities. By contrast, the antisense (C4G2)4 RNA and DNA neither bind heme nor influence its oxidative activity. Curiously, the ability of C9orf72 DNA and transcripts to bind and activate heme mirror similar properties that have been reported for the Aβ peptide and its oligomers in Alzheimer's disease neurons. It is therefore conceivable that C9orf72 RNA G-quadruplex tangles play roles in sequestering intracellular heme and promoting oxidative damage in ALS and FTD analogous to those proposed for Aβ peptide and its tangles in Alzheimer's Disease. Given that neurodegenerative diseases in general are characterized by mitochondrial and respiratory malfunctions, the role of C9orf72 DNA and RNA in heme sequestration as well as its inappropriate activation in ALS and FTD neurons may warrant examination.