Understanding how RIF1 and KAP1 enable the choice of the future active and inactive X chromosomes: the establishment of functional asymmetry.

在人类中，每条染色体都有两个副本，除了男性，他们的性染色体X和y各有一个副本。女性有两个X染色体副本。女性的两条X染色体中的一条是失活的（沉默的），因此男性和女性细胞中来自X染色体的基因产物的数量是相同的。这种沉默的过程被称为x -失活。两条X染色体中的任何一条都可以随机失活。这意味着，在每个组织中，一半的细胞会有一条X染色体失活(A)，一半的细胞会有另一条失活(B)。两条X染色体中的一条最初是如何随机选择的？是什么机制让一条染色体知道另一条染色体被选中了？在个体的一生中，不同的选择是如何在每个细胞的后代中稳定地维持的？这些都是仍然没有答案或只是部分回答的问题。此外，在现实中，X染色体的两个拷贝从来都不是完全相同的，它们DNA序列的微小差异可能导致其中一个拷贝更容易被沉默。因此，例如，如果更容易使X染色体a沉默，那么人体器官中的大多数细胞将具有活跃的B X染色体，而不是具有活跃的a细胞和具有活跃的B细胞的混合物。当两条X染色体中的一条包含完全或部分有缺陷的基因时，这是非常重要的，这可能导致疾病。如果优先沉默的X染色体携带疾病版本的基因，那么器官将会正常工作，女性将会健康。但是，如果优先沉默的X染色体携带的是正常版本的基因，那么器官就不能正常工作，女性就会患上这种疾病。因此，确定导致这种倾斜的DNA序列的特征并了解x失活的工作原理非常重要，因为它们将允许在x连锁疾病的女性携带者中早期识别有发展严重症状风险的患者。这种疾病的一个例子是杜氏肌营养不良症（Duchenne muscular dystrophy），其中一些女性患者完全没有症状，而另一些则病情严重。在我们的项目中，我们使用小鼠胚胎干细胞来研究三种分子，一种RNA和两种蛋白质，我们已经证明它们对于哪条X染色体将被灭活的初始选择是必不可少的。通过研究这三种分子如何相互作用并与X染色体DNA相互作用，我们将确定建立未来活性和非活性X染色体身份选择的分子机制。此外，我们将探索DNA序列在决定扭曲X染色体沉默中的作用，并研究这些序列与患者观察到的不平衡X染色体失活之间的关系。

In humans, every chromosome is present in two copies, except in men, where there is one copy of each of the sex chromosomes, the X and the Y. Women have two copies of the X chromosome. One of the two X chromosomes in women is inactivated (silenced), so that male and female cells have the same amounts of the gene products coming from the X chromosomes. This process of silencing is called X-inactivation. Either of the two X chromosomes can be inactivated randomly. This means that, in every tissue, half of the cells would have one X chromosome inactive (A) and half would have the other one inactive (B). How is the random choice of one of the two X chromosome initially made? What are the mechanisms that inform one chromosome that other has been chosen? How is a different choice stably maintained in the progeny of each cell, throughout the life of an individual? These are all still unanswered or only partially-answered questions. In addition, in reality, the two copies of the X chromosome are never completely identical and the small differences in their DNA sequences can result in one of the two copies being silenced more easily. As a consequence, for example, if it is easier to silence X chromosome A, then most of the cells in a human organ will have an active B X chromosome, rather than a mixture of cells with an active A and cells with an active B. This is very important when one of the two X chromosomes contains a completely or partially defective gene which can cause a disease. If the X chromosome that is preferentially silenced carries the disease version of the gene, the organ will be functional and the woman will be healthy. But if, instead, the X chromosome, which is preferentially silenced, is the one carrying the normal version of the gene, the organ will not be able to function properly and the woman will suffer from the disease. Identifying the features of the DNA sequences that drive this skew and understanding how X-inactivation works are therefore very important, as they will allow the early identification of the patients at risk of developing severe symptoms, among the women carriers of X-linked diseases. An example of such a disease, where skewed X-inactivation plays a role in the severity of the symptoms, is Duchenne muscular dystrophy, where some female patients are completely asymptomatic and others are severely ill.In our project, we use mouse embryonic stem cells to investigate three molecules, an RNA and two proteins, that we have shown to be essential for the initial choice of which X chromosome will be inactivated. By studying how these three molecules interact with each other and with the X chromosome DNA, we will identify the molecular mechanism that establishes the choice of the identity of the future active and inactive X chromosomes. In addition, we will explore the role of DNA sequences that can determine skewed X chromosome silencing and investigate how these sequences relate to the unbalanced X chromosome inactivation observed in patients.