Alzheimer's Disease Genetics Laboratory, School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, SA, 5005, Australia.
Bioinformatics Hub, School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, SA, 5005, Australia.
Mol Brain. 2020 Oct 19;13(1):142. doi: 10.1186/s13041-020-00681-7.
To prevent or delay the onset of Alzheimer's disease (AD), we must understand its molecular basis. The great majority of AD cases arise sporadically with a late onset after 65 years of age (LOAD). However, rare familial cases of AD can occur due to dominant mutations in a small number of genes that cause an early onset prior to 65 years of age (EOfAD). As EOfAD and LOAD share similar pathologies and disease progression, analysis of EOfAD genetic models may give insight into both subtypes of AD. Sortilin-related receptor 1 (SORL1) is genetically associated with both EOfAD and LOAD and provides a unique opportunity to investigate the relationships between both forms of AD. Currently, the role of SORL1 mutations in AD pathogenesis is unclear. To understand the molecular consequences of SORL1 mutation, we performed targeted mutagenesis of the orthologous gene in zebrafish. We generated an EOfAD-like mutation, V1482Afs, and a putatively null mutation, to investigate whether EOfAD-like mutations in sorl1 display haploinsufficiency by acting through loss-of-function mechanisms. We performed mRNA-sequencing on whole brains, comparing wild type fish with their siblings heterozygous for EOfAD-like or putatively loss-of-function mutations in sorl1, or transheterozygous for these mutations. Differential gene expression analysis identified a small number of differentially expressed genes due to the sorl1 genotypes. We also performed enrichment analysis on all detectable genes to obtain a more complete view on changes to gene expression by performing three methods of gene set enrichment analysis, then calculated an overall significance value using the harmonic mean p-value. This identified subtle effects on expression of genes involved in energy production, mRNA translation and mTORC1 signalling in both the EOfAD-like and null mutant brains, implying that these effects are due to sorl1 haploinsufficiency. Surprisingly, we also observed changes to expression of genes occurring only in the EOfAD-mutation carrier brains, suggesting gain-of-function effects. Transheterozygosity for the EOfAD-like and null mutations (i.e. lacking wild type sorl1), caused apparent effects on iron homeostasis and other transcriptome changes distinct from the single-mutation heterozygous fish. Our results provide insight into the possible early brain molecular effects of an EOfAD mutation in human SORL1. Differential effects of heterozygosity and complete loss of normal SORL1 expression are revealed.
为了预防或延迟阿尔茨海默病（AD）的发生，我们必须了解其分子基础。绝大多数 AD 病例是散发性的，在 65 岁以后才发病（LOAD）。然而，由于少数基因的显性突变，也会发生罕见的家族性 AD 病例，导致早发性 AD（EOfAD）在 65 岁之前发病。由于 EOfAD 和 LOAD 具有相似的病理学和疾病进展，因此分析 EOfAD 的遗传模型可能会深入了解这两种 AD 亚型。Sortilin 相关受体 1（SORL1）与 EOfAD 和 LOAD 均存在遗传相关性，为研究这两种 AD 形式之间的关系提供了独特的机会。目前，SORL1 突变在 AD 发病机制中的作用尚不清楚。为了了解 SORL1 突变的分子后果，我们在斑马鱼中对同源基因进行了靶向诱变。我们产生了一个 EOfAD 样突变，V1482Afs，和一个可能的无效突变，以研究 sorl1 中的 EOfAD 样突变是否通过失活机制表现出半合子不足。我们对整个大脑进行了 mRNA 测序，比较了野生型鱼与其兄弟姐妹在 sorl1 中杂合 EOfAD 样或可能丧失功能的突变，或杂合这些突变的转杂合子。差异基因表达分析确定了由于 sorl1 基因型而导致的少数差异表达基因。我们还对所有可检测基因进行了富集分析，通过三种基因集富集分析方法获得了对基因表达变化的更全面的观察，然后使用调和均值 p 值计算整体显著性值。这在 EOfAD 样和无效突变体大脑中识别出了与能量产生、mRNA 翻译和 mTORC1 信号传导相关的基因表达的微妙变化，表明这些变化是由于 sorl1 半合子不足所致。令人惊讶的是，我们还观察到仅在 EOfAD 突变携带者大脑中出现的基因表达变化，提示存在获得性功能效应。EOfAD 样和无效突变的转杂合性（即缺乏野生型 sorl1）导致铁稳态和其他转录组变化的明显影响，与单一突变杂合子鱼不同。我们的研究结果为人类 SORL1 的 EOfAD 突变可能对早期大脑分子的影响提供了深入了解。揭示了杂合性和正常 SORL1 表达完全缺失的差异效应。
