ABSTRACT
Expansion of a G4C2 repeat in the C9orf72 gene is associated with familial Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). To investigate the underlying mechanisms of repeat instability, which occurs both somatically and intergenerationally, we created a novel mouse model of familial ALS/FTD that harbors 96 copies of G4C2 repeats at a humanized C9orf72 locus. In mouse embryonic stem cells, we observed two modes of repeat expansion. First, we noted minor increases in repeat length per expansion event, which was dependent on a mismatch repair pathway protein Msh2. Second, we found major increases in repeat length per event when a DNA double- or single-strand break (DSB/SSB) was artificially introduced proximal to the repeats, and which was dependent on the homology-directed repair (HDR) pathway. In mice, the first mode primarily drove somatic repeat expansion. Major changes in repeat length, including expansion, were observed when SSB was introduced in one-cell embryos, or intergenerationally without DSB/SSB introduction if G4C2 repeats exceeded 400 copies, although spontaneous HDR-mediated expansion has yet to be identified. These findings provide a novel strategy to model repeat expansion in a non-human genome and offer insights into the mechanism behind C9orf72 G4C2 repeat instability.
Subject(s)
C9orf72 Protein , DNA Repeat Expansion , Genomic Instability , Animals , Humans , Mice , Amyotrophic Lateral Sclerosis/genetics , C9orf72 Protein/genetics , Disease Models, Animal , DNA Breaks, Double-Stranded , DNA Repeat Expansion/genetics , Frontotemporal Dementia/genetics , Gene Knock-In Techniques , Genomic Instability/genetics , MutS Homolog 2 Protein/geneticsABSTRACT
Although there is no doubt about the influence of the genetic background in the onset of the allergic diseases, Epigenome-Wide Association Studies are needed to elucidate the possible relationship between allergic diseases and epigenomic dysregulation. In this study we aimed to analyze the epigenetic patterns, in terms of DNA methylation, of three well-characterized populations of house dust mite allergic subjects, aspirin-intolerant asthmatics and controls. As a first, genome-wide phase, we used the HELP assay to study the methylation patterns in CD19 (+) B lymphocytes in these populations, and found that there are reproducible epigenetic differences at limited numbers of loci distinguishing the groups, corroborated by bisulphite MassArray in a second validation phase of an expanded 40 subject group. These validated epigenetic changes occur at loci characterized as important for the immune response. One such locus is a new candidate gene, CYP26A1, which shows differential methylation patterns and expression levels between groups. Our results suggest that epigenomic dysregulation may contribute to the susceptibility to allergic diseases, showing for the first time differences in DNA methylation between allergic and non-allergic healthy subjects, both globally and at specific loci. These observations indicate that the epigenome may offer new pathophysiological insights and therapeutic targets in atopic diseases.
