National Hospital for Neurology and Neurosurgery, Queen Square, London, WC1N 3BG, UK.
UK Dementia Research Institute at UCL, Cruciform Building, Gower Street, London, WC1E 6BT, UK.
Acta Neuropathol Commun. 2020 Nov 9;8(1):184. doi: 10.1186/s40478-020-01036-y.
A C9orf72 repeat expansion is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis. One of the suggested pathomechanisms is toxicity from dipeptide repeat proteins (DPRs), which are generated via unconventional translation of sense and antisense repeat transcripts with poly-GA, poly-GP and poly-GR being the most abundant dipeptide proteins. Animal and cellular studies highlight a neurotoxic role of poly-GR and poly-PR and to a lesser degree of poly-GA. Human post-mortem studies in contrast have been much less clear on a potential role of DPR toxicity but have largely focused on immunohistochemical methods to detect aggregated DPR inclusions. This study uses protein fractionation and sensitive immunoassays to quantify not only insoluble but also soluble poly-GA, poly-GP and poly-GR concentrations in brain homogenates of FTD patients with C9orf72 mutation across four brain regions. We show that soluble DPRs are less abundant in clinically affected areas (i.e. frontal and temporal cortices). In contrast, the cerebellum not only shows the largest DPR load but also the highest relative DPR solubility. Finally, poly-GR levels and poly-GP solubility correlate with clinical severity. These findings provide the first cross-comparison of soluble and insoluble forms of all sense DPRs and shed light on the distribution and role of soluble DPRs in the etiopathogenesis of human C9orf72-FTD.
C9orf72 重复扩展是额颞叶痴呆（FTD）和肌萎缩侧索硬化症（ALS）最常见的遗传原因。一种建议的发病机制是二肽重复蛋白（DPRs）的毒性，这些蛋白是通过有义和反义重复转录本的非常规翻译产生的，其中聚-GA、聚-GP 和聚-GR 是最丰富的二肽蛋白。动物和细胞研究强调了聚-GR 和聚-PR 的神经毒性作用，而聚-GA 的作用则较小。相比之下，人类死后研究在 DPR 毒性的潜在作用方面要模糊得多，但主要集中在免疫组织化学方法上，以检测聚集的 DPR 内含物。本研究使用蛋白质分级分离和敏感免疫测定法，不仅定量了脑匀浆中不溶性聚-GA、聚-GP 和聚-GR 浓度，还定量了可溶性聚-GA、聚-GP 和聚-GR 浓度，这些患者患有 C9orf72 突变，涉及四个大脑区域。我们表明，可溶性 DPR 在临床上受影响的区域（即额叶和颞叶皮层）中含量较少。相比之下，小脑不仅显示出最大的 DPR 负荷，而且相对 DPR 溶解度最高。最后，聚-GR 水平和聚-GP 溶解度与临床严重程度相关。这些发现提供了所有有义 DPR 的可溶性和不溶性形式的首次交叉比较，并阐明了可溶性 DPR 在人类 C9orf72-FTD 发病机制中的分布和作用。
