A hexanucleotide repeat expansion (HRE), (GGGGCC)_n, in C9orf72 is the most common genetic cause of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here we identify a molecular mechanism by which structural polymorphism of the HRE leads to ALS/FTD pathology and defects. The HRE forms DNA and RNA G-quadruplexes with distinct structures and promotes RNA•DNA hybrids (R-loops). The structural polymorphism causes a repeat-length-dependent accumulation of transcripts aborted in the HRE region. These transcribed repeats bind to ribonucleoproteins in a conformation-dependent manner. Specifically, nucleolin, an essential nucleolar protein, preferentially binds the HRE G-quadruplex, and patient cells show evidence of nucleolar stress. Our results demonstrate that distinct C9orf72 HRE structural polymorphism at both DNA and RNA levels initiates molecular cascades leading to ALS/FTD pathologies, and provide the basis for a mechanistic model for repeat-associated neurodegenerative diseases.

The authors compare the whole-genome expression profiles of peri-infarct neurons that show axonal sprouting after stroke to their non-sprouting neighbors. They describe a 'sprouting transcriptome' and perform further gain- and loss-of-function studies, finding novel roles in sprouting for a DNA-modifying molecule, a growth factor, and inhibitory myelin receptors. Stroke is an age-related disease. Recovery after stroke is associated with axonal sprouting in cortex adjacent to the infarct. The molecular program that induces a mature cortical neuron to sprout a new connection after stroke is not known. We selectively isolated neurons that sprout a new connection in cortex after stroke and compared their whole-genome expression profile to that of adjacent, non-sprouting neurons. This 'sprouting transcriptome' identified a neuronal growth program that consists of growth factor, cell adhesion, axonal guidance and cytoskeletal modifying molecules that differed by age and time point. Gain and loss of function in three distinct functional classes showed new roles for these proteins in epigenetic regulation of axonal sprouting, growth factor–dependent survival of neurons and, in the aged mouse, paradoxical upregulation of myelin and ephrin receptors in sprouting neurons. This neuronal growth program may provide new therapeutic targets and suggest mechanisms for age-related differences in functional recovery.