中国科学院昆明动物研究所机构知识库

阿尔茨海默病（Alzheimer’s disease, AD）是一种出现在中年或者老年期的以进行性发展为特征的神经退行性疾病。AD的主要病理变化包括细胞外β淀粉样蛋白（Amyloid β, Aβ）的沉积以及细胞内过度磷酸化的tau蛋白形成的神经纤维缠结。AD的发病受到多种因素的影响，主要是受到遗传因素和外部环境因素的影响。虽然针对AD的研究报道有很多，但是AD具体的神经病理学病因仍然不清楚。脑源性神经营养因子（Brain-derived neurotrophic factor, BDNF）是一种形成于基底前脑的神经营养蛋白，在中枢神经系统发挥着重要的作用。BDNF与它的受体TrkB在中枢神经系统，尤其是记忆的形成和长时程增强方面具有重要作用：BDNF-TrkB信号的减弱可以导致空间记忆的下降，而过表达全长TrkB可以对学习与记忆能力起到促进作用。BDNF基因的遗传变异与表达异常在神经精神疾病的发生发展过程中发挥着重要的作用。前期的很多研究探索了BDNF基因与AD的遗传相关性，但是BDNF基因与AD的遗传相关性及其可能的分子机制仍然存在争议，并没有一致的结论。本研究中，我们通过一个三阶段的遗传研究，探索了BDNF基因变异与我国汉族人群AD是否具有潜在的遗传相关性。在前两个阶段的研究中，我们对我国的两个汉族群体进行了遗传研究（第一阶段：来自东部地区的382例AD患者和426例健康对照样本；第二阶段：来自西南地区的333例AD患者和334例健康对照样本）。在第三阶段中利用收集到的世界群体数据（第三阶段：26958例AD患者和46941例健康对照样本）进行荟萃分析，以验证前两个阶段的遗传结果。针对病例对照遗传分析的结果，我们通过公共获取的基因表达数据、脑影像数据和生物标记物数据等进行了进一步的亚表型关联分析。第一阶段的遗传结果显示，BDNF基因功能变异位点rs6265（p.V66M）与AD具有显著的遗传相关性；特别是进行性别分层分析后，rs6265与AD的遗传相关性呈现显著的女性特异性。由于样本量的限制，第一阶段的结果并没有在第二阶段中验证到。然而，这种女性特异相关性在第三阶段的荟萃分析中得到了验证，荟萃分析结果显示，rs6265等位基因与AD的遗传相关性只在女性群体中显著相关（P = 0.001）。 除了对病例对照的分析外，我们还利用Alzheimer's Disease Neuroimaging Initiative（ADNI）的数据分析了rs6265风险等位基因对AD患者内表型的影响。对ADNI数据库中内表型的分析显示，rs6265风险等位基因携带者中女性AD患者的认知下降评分显著的高于男性AD患者，说明女性AD患者的认知下降程度显著的高于男性AD患者。在携带有rs6265风险等位基因的女性AD患者的脑脊液中发现了Aβ42水平的下降以及tau蛋白水平的上升，同时也发现携带有rs6265风险等位基因的女性AD患者具有较高的脑萎缩速率。为进一步验证BDNF基因与AD的遗传相关的可靠性并探索其可能的机制，我们进行了数据挖掘分析。对NCBI Gene Expression Omnibus（GEO）数据库中BDNF基因的表达数据的分析发现，BDNF基因mRNA水平在AD患者脑组织中的表达与性别相关；尤其是在AD进程中受影响最严重的区域--内嗅皮层中，BDNF基因mRNA的表达水平只在女性AD患者中显著下降，同时在女性AD患者中BDNF表达水平的下降程度也显著的高于男性AD患者。在正常人群的老龄化过程中，BDNF基因mRNA在前额叶皮层中的表达会随着女性人群年龄的增长而下降，而在男性人群中却呈现相反的表达趋势。结合rs6265影响BDNF分泌的报道，我们推测该位点可能是通过对BDNF分泌的影响，而表现出女性特异性。我们的结果首先在迄今为止最大样本量群体中，确认了BDNF基因是AD的风险基因，且提示BDNF基因与AD的遗传相关性具有显著的女性特异性；并且BDNF基因mRNA表达水平的改变及对内表型的影响也具有性别差异；这些数据提示BDNF基因可能是一个女性特异性的AD易感基因。独立人群验证与功能实验将有助于进一步证实我们的结果，并解析其具体的分子机制。

Alzheimer’s disease (AD) is a brain disease characterized by a progressive dementia that occurs in middle or late life. The key pathological changes are increased levels of amyloid-β (Aβ) peptide in the form of extracellular senile plaques and hyper-phosphorylated tau (p-tau) as the intracellular neurofibrillary tangles in AD brain tissues. The occurrence of AD is multifactorial and is mainly affected by genetic and environmental factors. Although it has been widely studied, the exact neuropathological etiology of AD has not been fully understood.Brain derived neurotrophic factor (BDNF) is a member of the neurotrophic factor family, which is synthesized in basal forebrain and plays a key role in central nervous system. BDNF and its receptor TrkB can facilitate memory formation and long-term potentiation. Decrease of the BDNF-TrkB signaling results in a declined spatial memory, whereas overexpression of the full-length TrkB enhances learning and memory. BDNF has been found to be associated with neuropsychiatric disorders, and may play an important role in AD progression. Although the association between genetic variants of the BDNF gene and AD has been widely reported, the results remain controversial.In this study, we aimed to explore the potential association between genetic variants in BDNF and AD in Han Chinese. A two-stage case-control association study of Han Chinese cohorts (Stage 1, 382 AD patients and 426 healthy controls from East China; Stage 2, 333 AD patients and 334 healthy controls from Southwest China) was performed, followed by a stage 3 meta-analysis combining world-wide populations (Stage 3, 26958 AD patients and 46941 healthy controls). Then data mining of the public available expression data, brain imaging data and biomarker data in AD patients was also performed to further validate the results. We found that the well-known functional variant rs6265 (p.V66M) of the BDNF gene was significantly associated with AD susceptibility in our Stage 1 samples. Importantly, the association presented a female-specific effect after stratification by gender. This female-specific association was evaluated in the stage 2 sample, but we found no association in this sample. Then we validated this female-specific association in the stage 3 meta-analysis of all reported data with gender information under the allelic model: a significant association was observed between rs6265 and AD in only females (P = 0.001), but not in males (P = 0.298).In addition, we examined the longitudinal change of Alzheimer’s disease assessment scale (ADAS) score, which measured cognitive decline, in both female and male patients with or without the risk allele. As time progressed from the baseline to 36 months, a significant increase of ADAS score was observed in both female and male patients, and the trend of increase was more remarkable in females than that in males, especially in carriers with risk allele A. Moreover, we observed a clear female-specific risk trend for the effect of rs6265 on AD endo-phenotypes: decreased cerebrospinal fluid (CSF) Aβ42 level, increased CSF tau level and whole brain atrophy rate were observed only in females with risk allele of rs6265. Then, re-analysis of available expression data showed a gender-related expression pattern of BDNF in brain tissues from AD patients. For entorhinal cortex, which was heavily affected in AD patients, a significantly decreased BDNF mRNA level was only found in females, and the mRNA level of BDNF was significantly lower in female patients than that in male patients. Decreased tendency of BDNF mRNA expression was found in frontal cortex of females during the aging process, whereas a reverse expression pattern was observed in males. Considering the secretion of BDNF was affected by rs6265, the female specific genetic association might be mediated by its effect on secretion.Taken together, we found a female-specific genetic association of rs6265 with AD and a gender-related mRNA expression of BDNF in brain tissues of AD patients. Moreover, a clear female-specific risk trend for the effect of rs6265 on AD endo-phenotypes was also observed. Our results clarified the available controversies regarding the role of rs6265 in AD and indicated that BDNF might be a female-specific risk gene for AD. Further genetic and functional validations are needed to confirm our results.