哺乳动物中原钙粘蛋白(Protocadherin, Pcdh)基因簇包含50多个串联排列的基因,这些基因形成3个紧密相连的基因簇(Pcdhα、Pcdhβ和Pcdhγ),所编码的原钙粘蛋白质群在神经元多样性(Neuronal diversity)和单细胞特异性(Single cell identity)以及神经突触信号转导中发挥重要作用。前期的工作已证实转录因子CTCF(CCCTC-binding factor)与CTCF结合位点(CTCF-binding site, CBS)的方向性结合能够决定增强子和启动子环化的方向以及其远距离交互作用的特异性,并进一步在Pcdh基因座(Locus)形成两个(Pcdhα和Pcdhγ)染色质拓扑结构域(CTCF/cohesin- mediated chromatin domain, CCD),而且染色质拓扑结构域对于控制基因表达调控至关重要。本文通过生物信息学方法对比人类和小鼠序列,发现Pcdhβγ染色质拓扑结构域调控区域中的DNase I超敏位点(DNase I hypersensitive sites, HSs)较为保守。染色质免疫沉淀及大规模测序实验(Chromatin immunoprecipitation and massive parallel sequencing, ChIP-Seq)揭示CBS位点在Pcdhβγ调控区域中成簇分布并且具有相同的方向。凝胶电泳迁移实验(Electrophoresis mobility shift assay, EMSA)确定Pcdhβγ调控区域内具体的42 bp CBS位点并且发现一个CTCF峰包含两个CBS位点。在全基因组范围内,运用计算生物学方法分析CTCF和增强子、启动子等调控元件的关系,发现CBS位点在调控元件附近有较多分布,推测CTCF通过介导增强子和启动子的特异性交互作用,在细胞核三维基因组内形成活性转录枢纽调控基因精准表达。
The mammalian clustered protocadherin (Pcdh) locus contains more than 50 highly-similar genes arrayed in tandem. These Pcdh genes are organized into three closely-linked clusters (Pcdhα, Pcdhβ, and Pcdhγ). The encoded PCDH proteins play critical roles in neuronal diversity, single cell identity, and synaptic connectivity. Recent studies revealed that directional CTCF (CCCTC-binding factor) binding to CBS (CTCF-binding site) determines the specific interaction between enhancers and promoters, and the three Pcdh clusters form two CCDs (CTCF/cohesin- mediated chromatin domain) which is important for gene regulation. Here, we characterized a regulatory region, which contains several HSs (DNase I hypersensitive sites), downstream of the Pcdhβγ clusters. Specifically, CTCF ChIP-Seq (Chromatin immunoprecipitation-sequencing) experiments revealed several peaks in the Pcdhβγ regulatory regions. In addition, we performed comprehensive EMSA (Electrophoresis mobility shift assay) experiments and identified exact 42-bp CBS sites for each CTCF peak. Interestingly, we found that single CTCF peaks can contain two CBS sites. Finally, genome-wide computational analyses revealed many CBS sites close to chromatin marks of regulatory elements, such as enhancers and promoters. We propose that CTCF may mediate specific interactions between enhancers and promoters to form active transcription hub for gene expression.
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