Role of Heterochromatin protein 1 Beta in Genome Maintenance and Oncogenesis

DNA损伤反应(DDR)介导DNA双链断裂(DSB)修复和保护细胞 从损害引起的转变或死亡。细胞DNA被组织成蛋白质DNA复合体 (染色质)，以控制蛋白质对DNA的访问，并调节DNA依赖功能。在……里面 在真核生物中，染色质有两种主要类型：异染色质(基因贫乏)和常染色质(基因 丰富)，通过特定的组蛋白尾部修饰和非组蛋白染色质的差异来区分 蛋白质成分。在非组蛋白染色质蛋白中，异染色质蛋白1(HP1)是最好的一种。 研究了实例。在哺乳动物中，有三种HP1亚型(HP1a、HP1b和HP1g)，它们在结构上都是 具有由铰链区隔开的两个保守结构域的特征：N-末端的色氨酸结构域(CD) 和一个C-末端染色体影区(CSD)。我们之前证明了HP1a的过度表达 人细胞中的或HP1B增加了基因组的不稳定性和对电离辐射(IR)诱导的细胞的敏感性 杀戮。此外，Cbx1(小鼠HP1B)在小鼠细胞中的缺失增加了基因组的不稳定性， 自发性ATM(共济失调-毛细血管扩张突变)自动磷酸化，减少IR诱导的频率 G-H_2AX病灶形成，增加IR诱导的细胞杀伤和致癌转化。最近的研究由 其他研究人员支持我们的结果，即HP1B对DNA既有负面影响，也有正面影响 DSB修复，提示功能性HP1B的准确水平是IR敏感性的关键决定因素。 由于大多数关于HP1b如何与修复相关蛋白相互作用来调节DNA的机械性细节 DSB修复是未知的，我们将确定不同的结构域如何与染色质/修复蛋白相互作用 调节DNA DSB修复的成分。我们假设HP1B的负效应是通过 通过CD结构域与H3K9me结合，由于该结构域的缺失可以提高细胞存活和 正效应可能是由于HP1B CSD与乙酰化组蛋白H4K16(H4K16ac)的相互作用 阻止高阶染色质堆积的组蛋白修饰，这可能会阻碍蛋白质访问DNA 以及与DDR有关的蛋白质。 DNA损伤修复缺陷与致癌转化和肿瘤发生有关，因此，我们 将确定Cbx1降低对(I)Cbx1+/-小鼠肿瘤发展的影响 没有ATM和(Ii)Cbx1条件性基因敲除小鼠。这些研究将确定 HP1B调节细胞的IR反应和肿瘤的发生。我们的假设--非组蛋白修饰 HP1B因子调节染色质结构，并通过与DDR成分的相互作用促进 肿瘤发生--这是一个需要深入研究的新概念。对机械基础的再认识 对于正常组织和肿瘤组织之间的生化差异，染色质结构将有助于 改进IR反应和改善临床放射治疗的新策略的开发。

The DNA damage response (DDR) mediates DNA double strand break (DSB) repair and protects cells from damage induced transformation or death. Cellular DNA is organized into protein DNA complexes (chromatin) in order to control DNA access by proteins and regulated DNA dependent functions. In eukaryotes, there are two major types of chromatin: heterochromatin (gene poor) and euchromatin (gene rich) that are distinguished by specific histone tail modifications and differences in nonhistone chromatin protein constituents. Among nonhistone chromatin proteins, heterochromatin protein 1 (HP1) is the best- studied example. In mammals, there are three HP1 isoforms (HP1a, HP1b and HP1g) all structurally characterized by two conserved domains separated by a hinge region: an N-terminal chromodomain (CD) and a C-terminal chromoshadow domain (CSD). We previously demonstrated that overexpression of HP1a or HP1b in human cells increases genomic instability and sensitivity to ionizing radiation (IR)-induced cell killing. Moreover, depletion of Cbx1 (mouse HP1b) in mouse cells increased genomic instability, spontaneous ATM (ataxia-telangiectasia mutated) autophosphorylation, reduced the frequency of IR-induced g-H2AX foci formation, increased IR-induced cell killing and oncogenic transformation. Recent studies by other investigators support our results indicating HP1b has both negative as well as positive effects on DNA DSB repair and suggest that the precise level of functional HP1b is a critical determinant to IR sensitivity. Since most mechanistic details as to how HP1 b interacts with repair associated proteins to modulate DNA DSB repair are unexplored, we will determine how different domains interact with chromatin/repair protein components to regulate DNA DSB repair. We hypothesize that the negative effect of HP1b is mediated through CD domain binding to H3K9me, since deletion of this domain can improve cell survival and the positive effect could be due to HP1b CSD interactions with acetylated histone H4K16 (H4K16ac) a unique histone modification that prevents higher order chromatin packing, which can impede protein access to DNA and with proteins involved in the DDR. Defective DNA damage repair is linked with oncogenic transformation and tumorigenesis, therefore, we will determine the impact of decreased Cbx1 on tumor development in (i) Cbx1+/- mice in the presence and absence of Atm and (ii) Cbx1 conditional knockout mice. These studies will define the mechanism by which HP1b regulates the cellular IR response and tumorigenesis. Our hypothesis-that the non-histone modifying factor HP1b regulates chromatin structure and, through interactions with DDR components, contributes to oncogenesis-is a novel idea requiring in depth studies. Further understanding about the mechanistic basis for biochemical differences between normal and tumor tissue chromatin structure will facilitate the development of new strategies for modifying IR response and improving clinical radiation therapy.