复制压力是内源性DNA损伤的主要来源，有效地缓解复制压力是肿瘤细胞生存的必要手段。组蛋白修饰的动态变化与复制压力应答密切相关，但是其在该过程中发挥作用的分子机理尚需研究。鉴于ATR是应答和缓解复制压力的关键因子，该项目拟从组蛋白修饰角度探索ATR活化的分子机制。申请者课题组在前期工作中相对深入的研究了组蛋白去甲基化酶调控DNA损伤修复的分子机理，该项目将在此基础上，进一步阐明组蛋白甲基化修饰是否以及如何介导ATR活化，以期为乳腺癌等肿瘤治疗提供靶分子。在该研究中，我们筛选到一系列的可能参与ATR激活的组蛋白甲基化酶和去甲基化酶；初步揭示组蛋白去甲基化酶PHF8依赖TopBP1通过其去甲基化酶活性激活ATR，并参与乳腺癌细胞复制压力的缓解。但是PHF8活化ATR的分子机制及临床意义，尚有待深入研究。该项目对认识复制压力应答和基因组稳定性的分子机制以及肿瘤治疗具有重要的生物学和临床意义。

Replication stress is a major source of endogenous DNA damage, causing both single-stranded and double-stranded DNA lesions when unrepaired. Any condition leading to high levels of DNA damage will result in replication stress, which is a source of genome instability and a feature of pre-cancerous and cancerous cells. To deal with DNA damage loads associated with excessive replication, cancerous cells have evolved and acquired particular machineries or programs in response and repair of DNA damage, which is essential for the survival of these cells and renders cells resistant to DNA damage-based therapy. A negative aspect of replication stress is its prominent role in tumorigenesis, while a positive aspect is that it provides a potential target for cancer therapy. Replication fork progression, as well as the resolution of stalled or collapsed forks, occurs in a highly organized chromatin environment, but how histone modifications contribute to replication stress response remains to be investigated. ATR (ataxia telangiectasia mutated and Rad3-related) is an apical kinase in sensing and resolving replication stress, but how optimal activation of ATR is achieved remains unclear. In previous studies, the applicant’s research mainly focuses on epigenetic regulation of breast carcinogenesis. Until now, the applicant has identified and characterized several histone demethylases involved in genome stability, and revealed that dysregulation of these epigenetic players will result in genome instability and eventually contribute to breast carcinogenesis. We propose here to investigate whether histone methylation is involved in ATR activation and aims to reveal the relevant epigenetic mechanisms associated with replication stress response. Also, we will examine the aberrations of these epigenetic mechanisms in malignancies and reveal their contributions to genome instability as well as chemo- or radio- therapeutic resistance of tumors. Initially, we have screened all members of histone methyltransferases and histone demethylases and identified several candidates in ATR activation. Specifically, we uncovered that histone demethylase PHF8 physically interacts with TopBP1 (topoisomerase II binding protein 1), an essential allosteric activator of ATR, and activates ATR in TopBP1- and demethylase activity- dependent manner. We also showed that PHF8 is required for breast cancer cells to counteract replication stress. Although these preliminary data largely support our hypothesis, the molecular mechanism and clinical significance of the dynamic changes of histone methylation in ATR activation remain to be investigated. Collectively, understanding the molecular basis of ATR activation and replication stress response is crucial to the understanding of genome stability and will benefit to the therapeutic intervention of tumors.