A pathogenic GGGCCC hexanucleotide expansion in the first intron/promoter region of the C9orf72 gene is the most common mutation associated with amyotrophic lateral sclerosis (ALS). The C9orf72 gene product forms a complex with SMCR8 (Smith-Magenis Syndrome Chromosome Region, Candidate 8) and WDR41 (WD Repeat domain 41) proteins. Recent studies have indicated roles for the complex in autophagy regulation, vesicle trafficking, and immune response in transgenic mice, however a direct connection with ALS etiology remains unclear. With the aim of increasing understanding of the multi-functional C9orf72-SMCR8-WDR41 complex, we determined by mass spectrometry analysis the proteins that directly associate with SMCR8. SMCR8 protein binds many components of the ubiquitin-proteasome system, and we demonstrate its poly-ubiquitination without obvious degradation. Evidence is also presented for localization of endogenous SMCR8 protein to cytoplasmic stress granules. However, in several cell lines we failed to reproduce previous observations that C9orf72 protein enters these granules. SMCR8 protein associates with many products of genes associated with various Mendelian neurological disorders in addition to ALS, implicating SMCR8-containing complexes in a range of neuropathologies. We reinforce previous observations that SMCR8 and C9orf72 protein levels are positively linked, and now show in vivo that SMCR8 protein levels are greatly reduced in brain tissues of C9orf72 gene expansion carrier individuals. While further study is required, these data suggest that SMCR8 protein level might prove a useful biomarker for the C9orf72 expansion in ALS.

中文翻译：

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C9orf72 相关 SMCR8 蛋白与泛素通路以及与神经系统疾病相关的蛋白结合。


C9orf72 基因第一个内含子/启动子区域的致病性 GGGCCC 六核苷酸扩增是与肌萎缩侧索硬化症 (ALS) 相关的最常见突变。 C9orf72 基因产物与 SMCR8（史密斯-马吉尼斯综合征染色体区域，候选 8）和 WDR41（WD 重复结构域 41）蛋白形成复合物。最近的研究表明该复合物在转基因小鼠的自噬调节、囊泡运输和免疫反应中发挥作用，但与 ALS 病因学的直接联系仍不清楚。为了加深对多功能 C9orf72-SMCR8-WDR41 复合物的了解，我们通过质谱分析确定了与 SMCR8 直接相关的蛋白质。 SMCR8 蛋白结合泛素-蛋白酶体系统的许多成分，并且我们证明了其多泛素化而没有明显的降解。还提供了内源 SMCR8 蛋白定位于细胞质应激颗粒的证据。然而，在几个细胞系中，我们未能重现之前观察到的 C9orf72 蛋白进入这些颗粒的情况。 SMCR8 蛋白与除 ALS 之外的各种孟德尔神经系统疾病相关的基因的许多产物相关，表明含有 SMCR8 的复合物与一系列神经病理学有关。我们强化了之前的观察结果，即 SMCR8 和 C9orf72 蛋白水平呈正相关，现在表明在体内，SMCR8 蛋白水平在 C9orf72 基因扩增携带者个体的脑组织中大大降低。虽然还需要进一步研究，但这些数据表明 SMCR8 蛋白水平可能被证明是 ALS 中 C9orf72 扩展的有用生物标志物。