Genetic Basis of Failed Cognition in Young and Aged Mouse Models of Trisomy 21

描述(申请人提供)：21三体，唐氏综合症(DS)，在美国影响大约40万人，导致认知障碍，包括阿尔茨海默病和老年痴呆症的神经病理。根据流行的基因剂量效应假说，DS的认知相关表型是由一个或多个人类染色体(HSA)21基因的三倍体引起的。我们从以老鼠为基础的研究中的初步观察表明，这些致病基因确实存在，寻找它们是可能的，也是有成效的。该项目的长期目标是通过基于小鼠的遗传分析鉴定这些致病基因，该分析建立在我们团队最近取得的成功的基础上：(1)我们利用高效的CRE/loxP介导的染色体工程开发了两个用于DS的最佳参考小鼠模型：DP(16)1YU/，它在小鼠染色体(MMU)16上的整个22.9-Mb HSA21同义区是三体；DP(10)1YU/；DP(16)1YU/；DP(17)1YU/，它对于MMU10、MMU16和MMU17上的所有三个HSA21同义区都是三体。(2)我们已经将与DS认知障碍相关的基因组区域缩小到小鼠基因组中最小的片段：包含30个HSA21基因同源基因的Cbr1-Fam3b染色体片段。这一片段在小鼠身上的三重复制会导致认知行为、突触结构和海马体长时程增强的异常，而海马长时程增强是学习和记忆的主要细胞机制。为了实现我们的目标，我们建议，在本申请的具体目标1中，表征DS最佳参考小鼠模型中最重要的认知相关表型。为了建立有助于遗传解剖的基本表型参数，我们将表征DP(16)1YU/和DP(10)1YU/；DP(16)1YU/；DP(17)1YU/小鼠的突触结构和可塑性以及认知行为。我们还将测量不同年龄的DP(16)1/小鼠海马区神经元的大小和数量，以确定神经退行性变的表型。在具体目标2中，我们将分析Cbr1-Famb3b片段，以确定DS相关突触和认知表型的最小基因组区域。我们将通过染色体工程产生在Cbr1-Fam3b片段中携带嵌套复制和缺失的新小鼠突变体，并通过使用这些突变体，我们将采用减法/加法策略，在该策略中，突触和认知表型与逐渐变小的基因组片段相关联，直到定义出最小的临界区。这一努力将为发现位于这些表型的最小临界区(S)内的致病基因(S)奠定基础，这将为揭示DS相关认知障碍的分子机制奠定基础，并为上述假说提供确凿的支持。因此，我们期望通过这些研究，大大加快理解和治疗DS认知障碍的进展。 公共卫生相关性：认知功能障碍基本上影响到所有患有21三体唐氏综合症(DS)的儿童和成人；由于没有有效的治疗方法，美国有整整40万人在孩提时代经历了智力功能的发育延迟，以及与阿尔茨海默病的神经病理相关的认知技能在衰老过程中的进行性下降。迫切需要创新的方法来解开潜在的机制并开发有效的治疗方法。我们建议使用染色体工程来创建新的小鼠突变体，以定义DS的认知相关表型和最小关键基因组区域之间的联系，最终目标是识别致病基因，这一成就将极大地加速理解和治疗DS认知障碍的进展。

DESCRIPTION (provided by applicant): Trisomy 21, Down syndrome (DS), affects approximately 400,000 people in the U.S., causing cognitive disability, which includes the neuropathology of Alzheimer's disease and late-life dementia. Based on the prevailing gene dosage effect hypothesis, a cognitively relevant phenotype in DS is caused by the triplication of one or more human chromosome (HSA) 21 genes. Our preliminary observations from the mouse-based studies suggest that these causative genes are indeed present and the search for them is both possible and productive. The long-term objective of this project is to identify these causative genes by using mouse-based genetic analysis, which is built upon the recent successes of our team: (1) We have developed two optimal reference mouse models for DS using efficient Cre/loxP-mediated chromosome engineering: Dp(16)1Yu/+, which is trisomic for the entire 22.9-Mb HSA21 syntenic region on mouse chromosome (MMU) 16, and Dp(10)1Yu/+;Dp(16)1Yu/+;Dp(17)1Yu/+, which is trisomic for all three HSA21 syntenic regions on MMU10, MMU16 and MMU17. (2) We have narrowed down the genomic region associated with the cognitive disability of DS to the smallest segment in the mouse genome: the Cbr1-Fam3b chromosomal segment containing 30 HSA21 gene orthologs. The triplication of this segment in mice causes abnormalities in cognitive behaviors, synaptic structures and hippocampal long-term potentiation, a major cellular mechanism that underlies learning and memory. To achieve our objective, we propose, in Specific Aim 1 of this application, to characterize the most important cognitively relevant phenotypes of the optimal reference mouse models for DS. To establish the basic phenotypic parameters to facilitate the genetic dissection, we will characterize the synaptic structures and plasticity in the hippocampus as well as cognitive behaviors of Dp(16)1Yu/+ and Dp(10)1Yu/+;Dp(16)1Yu/+;Dp(17)1Yu/+ mice. We will also measure the size and number of neurons in the hippocampal circuits of Dp(16)1/+ mice at the different ages to ascertain the neurodegenerative phenotype. In Specific Aim 2, we will analyze the Cbr1-Famb3b segment to identify a minimal genomic region for the DS- associated synaptic and cognitive phenotypes. We will generate new mouse mutants carrying nested duplications and deletions within the Cbr1-Fam3b segment by chromosome engineering and, by using these mutants, we will employ a subtractive/additive strategy in which synaptic and cognitive phenotypes are linked to progressively smaller genomic segments until a minimal critical region is defined. This effort will lay the groundwork to identify a causative gene(s) located within the minimal critical region(s) for these phenotypes, which will set the stage for the unraveling of the molecular mechanism of DS-associated cognitive disability as well as provide the conclusive support for the aforementioned hypothesis. Therefore, we expect, through these studies, to considerably accelerate progress in understanding and treating cognitive disability in DS. PUBLIC HEALTH RELEVANCE: Cognitive dysfunction affects essentially all children and adults with trisomy 21, Down syndrome (DS); with no effective treatments available, fully 400,000 people in the U.S. experience developmental delays in mental function as children and progressive decline of cognitive skills associated with the neuropathology of Alzheimer's disease during aging. Innovative approaches to unraveling the underlying mechanisms and to developing effective therapies are urgently needed. We propose to use chromosome engineering to create new mouse mutants to define linkages between cognitively relevant phenotypes of DS and minimal critical genomic regions, with the ultimate goal of identifying the causative genes, an accomplishment that would greatly accelerate progress toward understanding and treating cognitive dysfunction in DS.