Hayward Bruce E, Kim Geum-Yi, Miller Carson J, McCann Cai, Lowery Megan G, Wood Richard D, Usdin Karen
Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892.
Takeda Pharmaceuticals U.S.A., Inc., Global Biologics Informatics and Automation, 500 Kendall Street, Cambridge, MA 02142, USA.
bioRxiv. 2024 Nov 6:2024.11.05.621911. doi: 10.1101/2024.11.05.621911.
Microsatellite instability is responsible for the human Repeat Expansion Disorders. The mutation responsible differs from classical cancer-associated microsatellite instability (MSI) in that it requires the mismatch repair proteins that normally protect against MSI. LIG4, an enzyme essential for non-homologous end-joining (NHEJ), the major pathway for double-strand break repair (DSBR) in mammalian cells, protects against expansion in mouse models. Thus, NHEJ may compete with the expansion pathway for access to a common intermediate. This raises the possibility that expansion involves an NHEJ-independent form of DSBR. Pol θ, a polymerase involved in the theta-mediated end joining (TMEJ) DSBR pathway, has been proposed to play a role in repeat expansion. Here we examine the effect of the loss of Pol θ on expansion in FXD mouse embryonic stem cells (mESCs), along with the effects of mutations in , and genes important for multiple DSBR pathways. None of these mutations significantly affected repeat expansion. These observations put major constraints on what pathways are likely to drive expansion. Together with our previous demonstration of the protective effect of nucleases like EXO1 and FAN1, and the importance of Pol β, they suggest a plausible model for late steps in the expansion process.
微卫星不稳定性与人类重复序列扩增疾病有关。导致这种情况的突变与经典的癌症相关微卫星不稳定性（MSI）不同，因为它需要通常能防止MSI的错配修复蛋白。LIG4是哺乳动物细胞中双链断裂修复（DSBR）的主要途径——非同源末端连接（NHEJ）所必需的一种酶，在小鼠模型中它能防止重复序列扩增。因此，NHEJ可能会与扩增途径竞争以获取共同中间体。这就增加了扩增涉及一种不依赖NHEJ的DSBR形式的可能性。Pol θ是一种参与θ介导的末端连接（TMEJ）DSBR途径的聚合酶，有人提出它在重复序列扩增中起作用。在这里，我们研究了Pol θ缺失对脆性X震颤性共济失调综合征（FXD）小鼠胚胎干细胞（mESCs）中扩增的影响，以及对多个DSBR途径重要的、、和基因的突变的影响。这些突变均未显著影响重复序列扩增。这些观察结果对可能驱动扩增的途径施加了重大限制。连同我们之前证明的核酸酶如EXO1和FAN1的保护作用以及Pol β的重要性，它们为扩增过程后期步骤提出了一个合理的模型。
