Metabolic Biochemistry, Biomedical Center (BMC), Faculty of Medicine, Ludwig-Maximilians-Universität München, Munich, Germany.
Graduate School of Systemic Neuroscience, Ludwig- Maximilians- University Munich, Munich, Germany.
Mol Neurodegener. 2018 Sep 6;13(1):49. doi: 10.1186/s13024-018-0280-6.
The R47H variant of the Triggering Receptor Expressed on Myeloid cells 2 (TREM2) significantly increases the risk for late onset Alzheimer's disease. Mouse models accurately reproducing phenotypes observed in Alzheimer' disease patients carrying the R47H coding variant are required to understand the TREM2 related dysfunctions responsible for the enhanced risk for late onset Alzheimer's disease.
A CRISPR/Cas9-assisted gene targeting strategy was used to generate Trem2 R47H knock-in mice. Trem2 mRNA and protein levels as well as Trem2 splicing patterns were assessed in these mice, in iPSC-derived human microglia-like cells, and in human brains from Alzheimer's patients carrying the TREM2 R47H risk factor.
Two independent Trem2 R47H knock-in mouse models show reduced Trem2 mRNA and protein production. In both mouse models Trem2 haploinsufficiency was due to atypical splicing of mouse Trem2 R47H, which introduced a premature stop codon. Cellular splicing assays using minigene constructs demonstrate that the R47H variant induced abnormal splicing only occurs in mice but not in humans. TREM2 mRNA levels and splicing patterns were both normal in iPSC-derived human microglia-like cells and patient brains with the TREM2 R47H variant.
The Trem2 R47H variant activates a cryptic splice site that generates miss-spliced transcripts leading to Trem2 haploinsufficiency only in mice but not in humans. Since Trem2 R47H related phenotypes are mouse specific and do not occur in humans, humanized TREM2 R47H knock-in mice should be generated to study the cellular consequences caused by the human TREM2 R47H coding variant. Currently described phenotypes of Trem2 R47H knock-in mice can therefore not be translated to humans.
髓样细胞触发受体 2（TREM2）的 R47H 变异显著增加了晚发性阿尔茨海默病的风险。需要能够准确重现携带 R47H 编码变异的阿尔茨海默病患者中观察到的表型的小鼠模型，以了解导致晚发性阿尔茨海默病风险增加的 TREM2 相关功能障碍。
使用 CRISPR/Cas9 辅助基因靶向策略生成 Trem2 R47H 敲入小鼠。在这些小鼠、源自 iPSC 的人类小胶质样细胞和携带 TREM2 R47H 风险因素的阿尔茨海默病患者的大脑中评估了 Trem2 mRNA 和蛋白水平以及 Trem2 剪接模式。
两个独立的 Trem2 R47H 敲入小鼠模型显示 Trem2 mRNA 和蛋白产量降低。在这两种小鼠模型中，Trem2 单倍不足是由于小鼠 Trem2 R47H 的非典型剪接引起的，该剪接引入了一个过早的终止密码子。使用 minigene 构建体进行的细胞剪接分析表明，仅在小鼠中而非在人类中，R47H 变体诱导的异常剪接发生。在源自 iPSC 的人类小胶质样细胞和携带 TREM2 R47H 变体的患者大脑中，TREM2 mRNA 水平和剪接模式均正常。
Trem2 R47H 变体激活了一个隐秘的剪接位点，该剪接位点产生了缺失剪接的转录本，导致仅在小鼠中而非在人类中 Trem2 单倍不足。由于 Trem2 R47H 相关表型是小鼠特异性的，而不在人类中发生，因此应该生成人类化的 TREM2 R47H 敲入小鼠来研究由人类 TREM2 R47H 编码变异引起的细胞后果。因此，目前描述的 Trem2 R47H 敲入小鼠的表型不能转化为人类。
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