家畜和人类相关研究已证实体外受精（IVF）胚胎及后代存在性别失衡现象。然而，这一现象的内在机制目前尚不清楚。我们发现移植后IVF胚胎在附植前后会出现较高比例的雌性死亡现象，并导致出生后代性别失衡。胚胎转录组和甲基化组的高通量数据进一步提示，雌性IVF胚胎X染色体失活不足可能导致了出生性别比例失衡，并已经证实X染色体失活的核心调控基因Xist在附植前的IVF胚胎中持续低表达。针对这一科学假设，本项目将对IVF胚胎的X失活状态及发育表型进行系统的动态分析，并通过过表达和敲降等功能实验证实Xist低表达是导致IVF胚胎X失活不足的关键因素。项目还将针对Xist上游激活因子Rnf12和Xist promoter的甲基化状态，探索IVF胚胎Xist低表达的调控机制。此外，将利用表观修饰药物，或通过调整IVF胚胎发育环境，优化现有的体外发育体系，改善胚胎失活状态，进而校正IVF胚胎及后代性别比例。

Previous studies have proven that in vitro fertilization (IVF) could lead to a skewed sex-ratio of IVF embryos or offspring. However, the underlying mechanisms are still unclear. Based on our preliminary work, we found that there is a biased female lethality during per-implantation period after the IVF embryos were transferred into recipients, which lead to a skewed sex-ratio at birth. Together with the high-throughput data of transcriptome and methylome, we hypothesize that an impaired X chromosome inactivation (XCI) of female IVF embryos may be involved in the skewed sex-ratio. Moreover, we have proven that the expression of Xist, the key regulator of XCI, was consistently depressed in IVF embryos during pre-implantation period. Therefore, in this project, we will firstly conduct a systematic and dynamic analysis of XCI status, as well as developmental profiling of IVF embryos. In addition, through the functional overexpression and knock down experiments, we aim to prove that the inhibited Xist expression is the key factor for the impaired XCI of IVF embryos. Furthermore, we will explore the mechanisms responsible for the inhibited Xist expression in IVF embryos, with focus on the Rnf12, the upstream inactivator of Xist, and the methylation status of Xist promoter. Finally, using some epigenetic modifying drugs, or physical and chemical factors, we will try to improve the XCI status of IVF embryos and revise the skewed sex-ratio of IVF offspring.