Regeneron Pharmaceuticals, Tarrytown, NY 10591, USA.
Nucleic Acids Res. 2024 Jun 10;52(10):5732-5755. doi: 10.1093/nar/gkae250.
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.
C9orf72 基因中的 G4C2 重复扩展与家族性肌萎缩侧索硬化症 (ALS) 和额颞叶痴呆 (FTD) 有关。为了研究重复不稳定的潜在机制，我们在一种新型的携带人类 C9orf72 基因座 96 个 G4C2 重复的家族性 ALS/FTD 小鼠模型中创建了一种新型的家族性 ALS/FTD 小鼠模型。在小鼠胚胎干细胞中，我们观察到两种重复扩展模式。首先，我们注意到每次扩展事件中重复长度的微小增加，这依赖于错配修复途径蛋白 Msh2。其次，当在重复序列附近人为引入 DNA 双链或单链断裂 (DSB/SSB) 时，我们发现每次事件中重复长度的大幅度增加，这依赖于同源定向修复 (HDR) 途径。在小鼠中，第一种模式主要驱动体细胞重复扩展。当在单细胞胚胎中引入 SSB 时，或者当 G4C2 重复超过 400 个时，即使没有 DSB/SSB 引入也会观察到重复长度的重大变化，包括扩展，尽管尚未确定自发的 HDR 介导的扩展。这些发现为在非人类基因组中模拟重复扩展提供了一种新策略，并为 C9orf72 G4C2 重复不稳定的机制提供了新的见解。
