Genetic interaction of PICALM and APOE in Alzheimer's disease

摘要 阿尔茨海默病(AD)是老年人中最常见的痴呆形式，表现为进展性 神经退行性疾病，包括淀粉样斑块和神经纤维缠结形成，以及认知 减损。据估计，遗传遗传决定了近80%的AD病例。除了众所周知的 在早发性AD患者中发现APP、PSEN1和PSEN2基因的家族性突变，超过30个座位或基因是 最近的全基因组相关性研究表明，相关的散发性晚发性AD(LOAD) 外显体/基因组测序项目。APOE和PICALM位居榜首。APOE对 脂载体载脂蛋白E蛋白。在它的三种主要亚型(ε2、ε3和ε4)中，ε3是最常见的 毫无疑问，ε4是负荷的最强遗传风险因素，而ε2频率较低，但具有保护性 对于AD。这些异构体也不同地影响对淀粉样蛋白β很重要的分子和细胞事件。 (aβ)代谢和神经退行性变。另一方面，PICALM编码磷脂酰肌醇结合 该基因是一种重要的AD相关基因，几乎所有的GWAS研究都证实它是一种重要的AD相关基因。PICALM 控制受体内化和随后的笼蛋白包裹的囊泡的细胞内运输。它扮演着关键的角色 在调节Aβ的脑清除中的作用，调节β和γ分泌酶的活性以产生Aβ，缓解 β对神经元的毒性，并通过自噬促进Tau的硬化。更有趣的是，这种独特的基因 在AD中APOE和PICALM之间的相互作用已经在人群研究的基础上被证明，作为PICALM 在ε4携带者和AD风险PICALM rs3851179G中，多个AD相关的基因型显著增加风险 等位基因和载脂蛋白Eε4等位基因协同影响AD患者的皮质体积和工作记忆功能。 然而，AD中这种相互作用的机制仍不清楚。 基于PICALM在维持细胞表面蛋白功能方面的相互作用和功能，以及我们的 初步研究结果显示，载脂蛋白E脂化受损，表面ABCA1胆固醇和 PICALM缺陷小鼠的磷脂转运蛋白，我们推测PICALM可能促进载脂蛋白E的脂化 和Aβ的代谢，通过控制ABCA1转运蛋白的功能，从而使PICALM rs3851179G和 载脂蛋白Eε4等位基因对AD发病有不利影响。为了检验这一假设，我们建议：i)确定 PICALM促进脱脂作用的细胞和分子机制及PICALM的特性 依赖的ABCA1转运体的内化和贩运(目标1)；ii)探讨ABCA1转运体的功能影响 PICALM和APOE在体内对神经退行性变表型的协同作用(AIM 2)。我们希望 收集两个基因的风险等位基因协同影响AD发病的第一手证据，并建立 体外和体内APOE和PICALM相互作用的分子和细胞机制。这个 研究结果将为AD的遗传性、病因和发病机制提供新的见解，以及 作为未来研究的基础，以治疗性地针对这种相互作用来诊断和治疗AD。

SUMMARY Alzheimer's disease (AD) is the most common form of dementia in the elderly, manifesting progressive neurodegenerative conditions including amyloid plaque and neurofibrillary tangle formation, and cognitive impairment. Genetic inheritance is estimated to determine nearly 80% of the AD cases. Besides the well-known familial mutations in APP, PSEN1 and PSEN2 genes found in early-onset AD cases, over 30 loci or genes are associated sporadic late-onset AD (LOAD) as indicated by recent genome-wide association studies and whole exosome/genome sequencing projects. APOE and PICALM are among the top of the list. APOE encodes the lipid carrier apolipoprotein E protein. Among its three major isoforms (ε2, ε3, and ε4), ε3 is the most common isoform, ε4 is unarguably the strongest genetic risk factor for LOAD, and ε2 is the less frequent but is protective for AD. These isoforms also differentially affect molecular and cellular events that are important for amyloid β (Aβ) metabolism and neurodegeneration. On the other hand, PICALM encodes the phosphatidylinositol binding clathrin assembly protein, and is confirmed by nearly all GWAS studies as a major AD-associated gene. PICALM controls receptor internalization and subsequent intracellular trafficking of clathrin-coated vesicles. It plays key roles in mediating brain clearance of Aβ, regulating activities of β- and γ-secretases for Aβ production, mitigating Aβ toxicity in neurons, and promoting Tau clerance via autophagy. More interestingly, the unique genetic interaction between APOE and PICALM in AD has been demonstrated based on population studies, as PICALM genotypes at multiple AD-associated confer risk predominantly in ε4 carriers, and AD risk PICALM rs3851179G allele and APOE ε4 allele synergistically affect cortex volume and working memory function in AD patients. However, the mechanism underpinning this interaction in AD is still unknown. Based on the PICALM's interactome and functions in maintaining cell surface protein functions, as well as our preliminary findings showing impaired APOE lipidation and reduced level of surface ABCA1 cholesterol and phospholipid transporter in PICALM deficient mice, we hypothesize that PICALM may facilitate APOE lipidation and Aβ metabolism by controlling the function of ABCA1 transporter, and therefore risk PICALM rs3851179G and APOE ε4 alleles adversely affect AD pathogenesis. To test this hypothesis, we propose to: i) determine the cellular and molecular mechanisms of PICALM in facilitating APOE lipidation and characterize PICALM- dependent internalization and trafficking of ABCA1 transporter (AIM 1); ii) explore the functional impact of PICALM and APOE's synergistic interaction in vivo on neurodegenerative phenotypes (AIM 2). We expect to gather first-hand evidence that the risk alleles of two genes synergistic influence AD pathogenesis, and establish the molecular and cellular mechanisms of interaction between APOE and PICALM both in vitro and in vivo. The outcomes of the studies will provide new insights into the inheritability, etiology and pathogenesis of AD, and serve as a foundation for future studies to therapeutically target this interaction for AD diagnosis and treatment.