Nucleotide Structural Polymorphisms Formed by GGGGCC Repeats Cause C9orf72 Abortive Transcription and Nucleolar Stress

Abstract

A hexanucleotide repeat expansion (HRE), GGGGCC, in the C9orf72 gene is the most common genetic cause of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The pathogenic mechanisms related to the repeat expansion are unknown. Here we identify a novel mechanism that leads to impaired transcription and ribonucleoprotein recognition in C9orf72 HRE carriers. The HRE impedes transcription and generates abortive RNA transcripts, which are aggravated by the formation of G-quadruplexes and RNA:DNA hybrids, R-loops. The accumulated RNA of the C9orf72 HRE binds to specific proteins in an HRE-conformation-dependent manner. One such protein nucleolin, which preferentially recognizes the G-quadruplexes on the RNA of the C9orf72 HRE, significantly mislocalizes in patient cells including induced pluripotent stem (iPS) cell-derived motor neurons, which suggests increased nucleolar stress. These findings provide a pathogenic mechanism for the C9orf72 HRE, including loss of transcriptional products and gain of RNA toxic properties. We propose that the unique nucleic acid structural motifs observed on the C9orf72 HRE are the fundamental cause of the age-dependent neurodegenerative diseases.View Large Image | View Hi-Res Image | Download PowerPoint Slide