CHROMATIN AND EPIGENETIC INHERITANCE

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Covalent modifications of both DNA and histones are important for regulating gene expression. Here, I propose to address two fundamental, unanswered questions about chromatin modifications: i) how are changes in chromatin states established during development; and ii) once established, how are chromatin modifications stably preserved through future cell divisions? Initially, we will focus our analysis on understanding how the noncoding RNA Xist establishes silencing of the X chromosome in female mammalian cells. During initiation of X- inactivation, Xist RNA spreads in cis to coat the X chromosome that will become inactive, mediates silencing, and triggers the sequential accumulation of chromatin modifications. How Xist RNA initiates silencing remains unknown. One approach to gain insight into the function of Xist is to identify interacting proteins. We have obtained results demonstrating that Xist is part of a large protein complex. In Aim 1, we therefore propose to use classical and non- conventional purification strategies to identify Xist -interacting proteins. To identify proteins necessary for maintaining the silence of the inactive X, we will perform an RNAi based screen in Aim 2, which is based on the reactivation of the inactive X. We expect to find proteins involved in the epigenetic inheritance of the silent X chromosome state. The cell cycle poses a particularly challenging problem for epigenetic inheritance since histone modifications have to be maintained when DNA strands are duplicated during S phase. A major question therefore is, how chromatin modifications are transmitted through cell divisions, and if they are indeed sufficient as carriers of the epigenetic information. Surprisingly, we have found that histone modifications on the inactive X do not accumulate throughout the cell cycle. In Aim 3, we will extent studies on the cell cycle regulation of histone modifications.