C9orf72 repeat expansion creates the unstable folate-sensitive fragile site FRA9A

Area of Science:

• Genetics and Molecular Biology
• Immunology
• Neuroscience

Background:

• The C9orf72 (GGGGCC)n expansion (C9orf72Exp) is linked to amyotrophic lateral sclerosis, frontotemporal dementia, and autoimmune disorders.
• C9orf72Exp patients exhibit hyperactive cGAS-STING immune responses and DNA damage, but the source of this DNA is unknown.
• The Chr9p21 locus, containing C9orf72, is unstable and associated with multiple diseases.

Purpose of the Study:

• To identify the source of immunostimulatory and damaged DNA in C9orf72Exp.
• To investigate the link between C9orf72Exp and chromosomal instability.
• To determine the role of folate in C9orf72Exp-associated fragility.

Main Methods:

• Analysis of patient-derived cells and brains from pre-symptomatic and affected individuals.
• Chromosomal fragility assays, including sister-chromatid exchange and micronuclei analysis.
• Investigation of folate sensitivity and transferability of instability in transgenic mouse models.

Main Results:

• C9orf72Exp induces the folate-sensitive fragile site FRA9A, spanning 8.2 Mb at 9p21 and encompassing C9orf72.
• C9orf72Exp cells exhibit significant chromosomal instability, including rearrangements and micronuclei formation, providing endogenous DNA damage sources.
• Repeat instability and chromosomal fragility are sensitive to folate deficiency and transferable to other tissues in mouse models.

Conclusions:

• C9orf72 expansions trigger vitamin-sensitive chromosome fragility, creating structural variations at the 9p21 locus.
• This fragility and associated DNA damage contribute to the pathogenesis of C9orf72-linked diseases.
• Folate deficiency exacerbates C9orf72Exp-induced genetic instability.