The contribution of X-chromosome-linked genes to cellular phenotypes and AD-related pathology in Down Syndrome

摘要 唐氏综合症(DS)是由人类21号染色体(HSA21)的三倍体引起的，导致 智力残疾、运动功能障碍和早发性阿尔茨海默病(AD)。这个 DS中三重基因之间的机制联系与全面的不平衡 DS脑细胞中发现的异常基因表达尚未确定。它 也不知道这种全球转录失衡是如何导致智力残疾和早期 阿尔茨海默病相关病理的发展，也不知道什么基因在这些过程中起关键作用。 在我们的初步研究中，我们产生了来自DS的少皮质球体(OLS) 同基因诱导多能干细胞(IPSCs)系。三体OLS概括DS和AD- 相关的细胞表型，包括皮质体积减少，异常髓鞘形成和 淀粉样β蛋白和过度磷酸化tau的病理沉积。通过我们的单细胞 对整倍体和三体OLS的RNA测序(scRNA-seq)分析，我们鉴定了一簇X- 含脑的连锁基因表达X连锁/转录延伸因子A(SII) (BEX/TCEAL)家族成员，这在三体中下调。同时，三体细胞 显示非编码X失活特异性转录本(XIST)的表达显著增加， 对X染色体的失活负责。重要的是，Bex/TCEAL的失调是 与精神障碍、智力缺陷、神经变性和阿尔茨海默病有关。值得注意的是，其他 最近的研究，包括在DS细胞系和人类死后大脑中进行的研究表明， 伴随XIST增强的不同基因在该簇中的表达下调 级别。然而，这些转录因子的异常表达及其潜力 三体与脑病理的联系一直被忽视。在这里，我们假设 Bex/TCEAL的下调与脑发育异常和 DS的AD病理改变。在我们的研究中，我们将确定X染色体连锁的贡献 评估是否减少XIST或恢复对DS相关病理的基因抑制 Bex/TCEAL基因表达可减轻DS相关的细胞表型。我们会 特别关注BEX2和TCEAL7在DS中表达下调的作用 在DS人类的多个发育期中最稳定地下调 转录研究，进一步支持我们的初步数据。我们还将定义全基因组 BEX2和TCEAL7下游靶基因及其在三体中的意义我们的翻译， 以人为本的研究最终将测试X-基因调控失调之间的因果联系。 染色体连锁基因，以及三体的转录和表型改变。

Abstract Down Syndrome (DS) is caused by triplication of human chromosome 21 (HSA21), resulting in intellectual disability, motor dysfunction, and early onset Alzheimer’s Disease (AD). The mechanistic connection between triplicated genes in DS and the all-encompassing imbalance in gene expression underlying the abnormalities seen in DS brain cells has not been established. It is also unknown how this global transcriptomic disbalance leads to intellectual disability and early development of AD-related pathology in DS, nor what genes play a key role in these processes. In our preliminary studies, we generated oligocortical spheroids (OLS) derived from DS isogenic induced pluripotent stem cell (iPSCs) lines. Trisomic OLS recapitulate DS and AD- related cellular phenotypes, including reduced cortical volume, aberrant myelination and pathological depositions of amyloid beta and hyper-phosphorylated tau. Through our single-cell RNA sequencing (scRNA-seq) analysis of euploid and trisomic OLS, we identified a cluster of X- linked genes containing brain expressed X-linked/transcription elongation factor A (SII) (BEX/TCEAL) family members, that is downregulated in trisomy. At the same time, trisomic cells showed a significantly increased expression of non-coding X-inactive specific transcript (XIST), responsible for the X-chromosome inactivation. Importantly, the dysregulation of BEX/TCEAL is linked to psychiatric disorders, intellectual deficits, neurodegeneration, and AD. Noticeably, other recent studies, including those performed in DS cell lines and human postmortem brains, showed downregulated expression of different genes in this cluster accompanied by enhanced XIST levels. However, the aberrant expression of these transcription factors and their potential connection to brain pathology in trisomy has been neglected. Herein, we hypothesize that downregulation of BEX/TCEAL is causatively related to abnormal brain development and AD pathology in DS. In our studies, we will determine the contribution of X-chromosome-linked genes’ repression to DS-related pathology by assessing whether reducing XIST or restoring BEX/TCEAL gene expression can mitigate DS-associated cellular phenotypes. We will specifically focus on the effect of BEX2 and TCEAL7 downregulation in DS since their expression was most consistently downregulated across multiple developmental periods in DS human transcriptomic study, further supporting our preliminary data. We will also define the genome-wide downstream targets of BEX2 and TCEAL7 and their significance in trisomy. Our translational, human-directed study will ultimately test the causative link between the dysregulation of X- chromosome-linked genes’, and transcriptional and phenotypical alterations in trisomy.