Role of Heterochromatin protein 1 Beta in Genome Maintenance and Oncogenesis

描述（由申请人提供）：DNA损伤反应（DDR）介导DNA双链断裂（DSB）修复并保护细胞免受损伤诱导的转化或死亡。细胞DNA被组织成蛋白质DNA复合物（染色质），以控制蛋白质对DNA的访问和调节DNA依赖功能。在真核生物中，有两种主要类型的染色质：异染色质（基因贫乏）和常染色质（基因丰富），它们通过特定的组蛋白尾部修饰和非组蛋白染色质蛋白质成分的差异来区分。在非组蛋白染色质蛋白中，异染色质蛋白1 （HP1）是研究得最好的例子。在哺乳动物中，有三种HP1亚型（HP1a， HP1b和HP1g），它们的结构特征都是由两个由铰链区域分开的保守结构域：n端色域（CD）和c端色影域（CSD）。我们之前证明，人类细胞中HP1a或HP1b的过表达增加了基因组的不稳定性和对电离辐射（IR）诱导的细胞杀伤的敏感性。此外，

DESCRIPTION (provided by applicant): 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.