Reciprocal regulation of cardiac chromatin by the chromatin structural proteins HMGB and CTCF

Transcriptome remodeling in heart disease occurs through the coordinated actions of transcription factors, histone modifications and other chromatin features at pathology-associated genes. It remains unknown the extent to which genome-wide chromatin reorganization also contributes to the pathologic gene expression. We examined the roles of two chromatin structural proteins, CTCF (CCCTC-binding factor) and HMGB2 (high mobility group protein B2), in regulating pathologic transcription and chromatin remodeling. Our data demonstrate a reciprocal relationship between HMGB2 and CTCF in controlling aspects of chromatin structure and gene expression. Both proteins regulate each other’s expression as well as transcription in cardiac myocytes: however, only HMGB2 does so in a manner that involves global reprogramming of chromatin accessibility. We demonstrate that the actions of HMGB2 on local chromatin accessibility are conserved across genomic loci, whereas the effects on transcription are loci-dependent and emerge in concert with histone modification and other chromatin features. Lastly, while both proteins share gene targets, HMGB2 and CTCF neither bind these genes simultaneously nor do they physically co-localize in myocyte nuclei. Our study uncovers a previously unknown relationship between these two ubiquitous chromatin proteins and provides a mechanistic explanation for how HMGB2 regulates gene expression and cellular phenotype. Furthermore, we demonstrate direct evidence for hierarchical remodeling of chromatin on a genome-wide scale in the setting of cardiac disease.心脏病中的转录组重塑，经由转录因子、组蛋白修饰及其他染色质特征在病理相关基因上的协同作用得以实现。目前学界尚未明确，全基因组染色质重塑在多大程度上亦参与了病理基因的表达调控过程。本研究针对两种染色质结构蛋白——CCCTC结合因子（CTCF）与高迁移率族蛋白B2（HMGB2）在调控病理转录与染色质重塑中的作用展开了探究。研究数据表明，HMGB2与CTCF在调控染色质结构与基因表达的相关环节中存在双向相互作用：二者均可在心肌细胞中相互调控对方的表达与转录进程；但仅HMGB2以涉及染色质可及性全局重编程的方式完成此类调控。本研究证实，HMGB2对局部染色质可及性的调控效应在全基因组位点中均保守存在，而其对转录的调控则具有位点依赖性，且该调控效应伴随组蛋白修饰与其他染色质特征的变化同步显现。最后，尽管两种蛋白共享部分基因靶点，但HMGB2与CTCF既不会同时结合这些靶基因，也不会在心肌细胞核内发生物理共定位。本研究揭示了这两种广泛表达的染色质蛋白间此前未被报道的相互关系，并为HMGB2调控基因表达与细胞表型的机制提供了合理解释。此外，本研究还为心脏病背景下全基因组范围内的染色质层级重塑提供了直接实验证据。