A recent genome-wide association study of Huntington disease (HD) implicated genes involved in DNA maintenance processes as modifiers of onset, including multiple genome-wide significant signals in a chr15 region containing the DNA repair gene Fanconi-Associated Nuclease 1 (FAN1). Here, we have carried out detailed genetic, molecular, and cellular investigation of the modifiers at this locus. We find that missense changes within or near the DNA-binding domain (p.Arg507His and p.Arg377Trp) reduce FAN1's DNA-binding activity and its capacity to rescue mitomycin C-induced cytotoxicity, accounting for two infrequent onset-hastening modifier signals. We also idenified a third onset-hastening modifier signal whose mechanism of action remains uncertain but does not involve an amino acid change in FAN1. We present additional evidence that a frequent onset-delaying modifier signal does not alter FAN1 coding sequence but is associated with increased FAN1 mRNA expression in the cerebral cortex. Consistent with these findings and other cellular overexpression and/or suppression studies, knockout of FAN1 increased CAG repeat expansion in HD-induced pluripotent stem cells. Together, these studies support the process of somatic CAG repeat expansion as a therapeutic target in HD, and they clearly indicate that multiple genetic variations act by different means through FAN1 to influence HD onset in a manner that is largely additive, except in the rare circumstance that two onset-hastening alleles are present. Thus, an individual’s particular combination of FAN1 haplotypes may influence their suitability for HD clinical trials, particularly if the therapeutic agent aims to reduce CAG repeat instability.

亨廷顿病（HD）最近的全基因组关联研究表明，与DNA维持过程有关的基因是发病的修饰因子，包括在包含DNA修复基因Fanconi-Associated Nuclease 1（FAN1）的chr15区域中的多个全基因组重要信号。在这里，我们对这个基因座的修饰子进行了详细的遗传，分子和细胞研究。我们发现，DNA结合域（p.Arg507His和p.Arg377Trp）之内或附近的错义变化降低了FAN1的DNA结合活性及其挽救丝裂霉素C诱导的细胞毒性的能力，这说明了两个罕见的起效促进修饰信号。我们还确定了第三个起效促进修饰信号，其作用机理仍不确定，但不涉及FAN1中的氨基酸变化。我们提供了其他证据，表明频繁的起病修饰信号不会改变FAN1编码序列，但会增加FAN1mRNA在大脑皮层的表达。与这些发现以及其他细胞过度表达和/或抑制研究一致，FAN1的敲除增加了HD诱导的多能干细胞中的CAG重复扩增。总之，这些研究支持体细胞CAG重复扩增作为HD的治疗靶点的过程，并且它们清楚地表明，多种遗传变异通过FAN1通过不同的方式起作用，以很大程度上累加的方式影响HD的发作，但在极少数情况下除外存在两个阻止发作的等位基因。因此，一个人的FAN1单倍型的特定组合可能会影响他们对HD临床试验的适用性，特别是如果该治疗剂旨在减少CAG重复不稳定的话。