Enzymatic regulation of G-quadruplex structures in C9orf72 ALS

Indianapolis, IN

Higher-order nucleic acid structures called G-quadruplexes (G4 structures) can form in regions of both DNA and RNA that contain high numbers of localized guanine (G). There are ~750,000 possible G4-forming sequences in the human genome. A G-rich hexanucleotide repeat expansion in the first intron of C9orf72 (C9) has been found to be the primary cause of inherited amyloid lateral sclerosis (ALS), a neurodegenerative disease causing motor neuron degeneration. The accumulation of excessive G-quadruplex DNA and RNA derived from the repeat is a defining characteristic of C9 ALS. We propose that the human enzyme G4 Resolvase 1 (G4R1), the enzyme responsible for the majority of G-quadruplex resolving activity in HeLa cells, is likely involved in C9 ALS due to the high prevalence of G4 structures. Preliminary data suggests that G4R1 protein levels are upregulated in C9 ALS patient cells. We hypothesize that G4R1 functions by unwinding G-quadruplexes forming in the DNA, which, if left unwound, inhibit C9 gene transcription. Furthermore, we hypothesize that by upregulating C9 transcription, G4R1 increases the levels of G-rich C9orf72 transcripts. To determine this, I have begun quantifying the levels of C9orf72 transcripts and toxic RNA foci derived from C9orf72 transcripts relative to G4R1 levels.