Genetic and Functional Analyses Point to FAN1 as the Source of Multiple Huntington Disease Modifier Effects

Area of Science:

• Genetics
• Neuroscience
• Molecular Biology

Background:

• Genome-wide association studies identified DNA maintenance genes as Huntington disease (HD) onset modifiers.
• Significant signals were found in a chr15 region containing the DNA repair gene Fanconi-Associated Nuclease 1 (FAN1).

Purpose of the Study:

• To investigate genetic, molecular, and cellular mechanisms of FAN1 modifiers influencing HD onset.
• To explore how FAN1 variations impact DNA repair, CAG repeat instability, and HD progression.

Main Methods:

• Detailed genetic analysis of FAN1 locus modifiers.
• Molecular assays to assess FAN1 DNA-binding activity and cellular function.
• Investigation of FAN1 mRNA expression and knockout effects in HD-induced pluripotent stem cells.

Main Results:

• Specific FAN1 missense variants (p.Arg507His, p.Arg377Trp) reduced DNA-binding and DNA repair capacity, acting as onset-hastening modifiers.
• A frequent onset-delaying modifier correlated with increased FAN1 mRNA expression.
• FAN1 knockout exacerbated CAG repeat expansion in HD stem cells, confirming its role in repeat instability.

Conclusions:

• Multiple genetic variations acting through FAN1, via different mechanisms, additively influence HD onset.
• Somatic CAG repeat expansion is a viable therapeutic target in HD.
• FAN1 haplotype combinations may affect patient eligibility for clinical trials targeting CAG repeat instability.