Formation of RNA G-wires by GC repeats associated with ALS and FTD.

In the neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), expansion of the GC hexanucleotide repeat in the gene C9orf72 is a most common known cause of the disease. Here we use atomic force microscopy (AFM) and gel electrophoresis to visualize the formation of higher-order structures by RNA GC repeats in physiologically relevant conditions. For the RNA sequence r[GCG], we observed G-wires with left-handed undulating features of 4.4-nm periodicity and a uniform height which is consistently higher than that of a duplex B-DNA. These higher-order structures were not degraded fully when treated with a mixture of RNase A and RNase T1. Similarly, higher-order structures were observed for sequences containing three or four GC repeats, pointing towards their potential formation in longer sequence contexts. Our observations suggest that RNA G-quadruplex blocks and G-wires can accumulate in cells containing GC repeat transcripts.