Xist-mediated nucleation of heterochromatin and (epi)genetic stability of the inactive X chromosome

人类癌症是一种由癌基因激活和肿瘤抑制基因失活共同引发的复杂疾病，这些变化越来越多地被归因于遗传和表观遗传不稳定性。针对染色质修饰复合体的新一代药物的许多临床试验正在进行中，现有的“表观遗传药物”，如DNA甲基化或组蛋白去乙酰化抑制剂，被用于更广泛的癌症类型选择。由于如果先前未转化的细胞获得致癌的表观遗传程序，在癌细胞中重新激活抑癌基因的治疗效果会受到影响，因此在治疗过程中未转化的表观遗传状态的稳定性是至关重要的。雌性哺乳动物中不活跃的X染色体(Xi)是表观遗传学的经典例子，可以作为一个有用的范例来理解决定常染色质和异染色质稳定性的分子机制。该提案使用大规模并行测序技术、定制生物信息学和小鼠遗传学来解决该领域的三个主要问题：1)当未来的XI建立时，失活的染色质标记是如何在染色体上传播的？特定的DNA序列在建立和维持XI的不活跃状态中起什么作用？XI建立过程中的关键因子Xist RNA的缺失如何影响Xi在保持阶段的遗传和表观遗传稳定性？这些问题的答案将提供对Xi异染色质如何组织的基本见解，测试重复元素是否塑造Xi染色质，并可能有助于识别新的X连锁癌基因。

Human cancer is a complex disease triggered by a combination of oncogene activation and tumor suppressor gene inactivation, changes that are increasingly attributed not only to genetic but also epigenetic instability. Numerous clinical trials for a new generation of drugs targeting chromatin modifying complexes are underway and existing "epigenetic drugs" such as DNA methylation or histone deacetylation inhibitors are adopted for a wider selection of cancer types. Since the therapeutic benefit of reactivating a tumor suppressor in cancer cells is compromised if previously untransformed cells acquire a tumorigenic epigenetic program, the stability of the untransformed epigenetic state during treatment is of paramount importance.The inactive X chromosome (Xi) in female mammals is a classic example of epigenetics and can serve as a useful paradigm towards understanding the molecular mechanisms that determine the stability of euchromatin and heterochromatin. This proposal uses massively parallel sequencing technology, custom bioinformatics and mouse genetics to address three major questions in the field: 1.) How are inactivating chromatin marks spread over the chromosome when the future Xi is established? 2.) What role do specific DNA sequences play in establishing and maintaining the inactive state of the Xi? 3.) How does the loss of Xist RNA, the critical factor in Xi establishment, impact the genetic and epigenetic stability of the Xi in the maintenance phase? The answers to these questions will provide basic insights into how Xi heterochromatin is organized, test whether repetitive elements shape Xi chromatin, and may help to identify novel X-linked oncogenes.