From the Mitochondrial Dysfunctions in Neurodegeneration Unit (F.L., D.D.R., D.F., F.M.) and Department of Neurology (M.S.), Ospedale San Raffaele, Milan, Italy; Istituto Nazionale di Genetica Molecolare (A.M., S.B.), INGM, "Romeo ed Enrica Invernizzi," Milan, Italy; Laboratory of Neuromuscular Pathology (J.B.), Institute Born-Bunge, University of Antwerp; Neuromuscular Reference Centre (J.B.), Department of Neurology, Antwerp University Hospital, Belgium; Molecular Medicine (F.M.S.), IRCCS Fondazione Stella Maris, Pisa, Italy; Department of Biosciences (S.B.), University of Milan; and Università Vita-Salute San Raffaele (F.M., D.D.R.), Milan, Italy.
Neurology. 2021 Dec 7;97(23):e2315-e2327. doi: 10.1212/WNL.0000000000012962. Epub 2021 Oct 14.
Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by variations in gene encoding sacsin, a huge multimodular protein of unknown function. More than 200 variations have been described worldwide to date. Because ARSACS presents phenotypic variability, previous empirical studies attempted to correlate the nature and position of variations with the age at onset or with disease severity, although not considering the effect of the various variations on protein stability. In this work, we studied genotype-phenotype correlation in ARSACS at a functional level.
We analyzed a large set of skin fibroblasts derived from patients with ARSACS, including both new and already published cases, carrying variations of different types affecting diverse domains of the protein.
We found that sacsin is almost absent in patients with ARSACS, regardless of the nature of the variation. As expected, we did not detect sacsin in patients with truncating variations. We found it strikingly reduced or absent also in compound heterozygotes carrying diverse missense variations. In this case, we excluded mRNA decay, defective translation, or faster posttranslational degradation as possible causes of protein reduction. Conversely, our results demonstrate that nascent mutant sacsin protein undergoes cotranslational ubiquitination and degradation.
Our results provide a mechanistic explanation for the lack of genotype-phenotype correlation in ARSACS. We also propose a new and unambiguous criterion for ARSACS diagnosis that is based on the evaluation of sacsin level. Last, we identified preemptive degradation of a mutant protein as a novel cause of a human disease.
常染色体隐性痉挛性共济失调（ARSACS）是由编码 sacsin 的基因突变引起的，sacin 是一种功能未知的巨大多功能蛋白。迄今为止，全世界已描述了 200 多种变异。由于 ARSACS 表现出表型变异性，以前的经验性研究试图将变异的性质和位置与发病年龄或疾病严重程度相关联，尽管没有考虑到各种变异对蛋白质稳定性的影响。在这项工作中，我们在功能水平上研究了 ARSACS 的基因型-表型相关性。
我们分析了一组来自 ARSACS 患者的皮肤成纤维细胞，包括新的和已经发表的病例，这些细胞携带影响蛋白不同结构域的不同类型的变异。
我们发现，无论变异的性质如何，ARSACS 患者的 sacsin 几乎不存在。正如预期的那样，我们在截短变异的患者中未检测到 sacsin。我们还发现，携带多种错义变异的复合杂合子的 sacsin 也明显减少或缺失。在这种情况下，我们排除了 mRNA 衰减、翻译缺陷或更快的翻译后降解作为蛋白减少的可能原因。相反，我们的结果表明，新生突变 sacsin 蛋白经历共翻译泛素化和降解。
我们的结果为 ARSACS 中缺乏基因型-表型相关性提供了机制解释。我们还提出了一种基于 sacsin 水平评估的新的、明确的 ARSACS 诊断标准。最后，我们确定了突变蛋白的预先降解是人类疾病的一个新原因。
