Implication of the Mediator kinase module in CIZ1 recruitment and gene silencing by Xist during the initiation of X inactivation

In mammals, dosage compensation between the sexes is mediated by X chromosome inactivation. The long non-coding Xist RNA initiates this process. The molecular mechanism of X-linked gene silencing is beginning to be understood. We have previously reported that the cyclin-dependent kinase CDK8 is required for gene silencing and histone H3 trimethyl-lysine 27 recruitment by Xist. CDK8 functions in cell signalling and transcriptional regulation, in particular, it associates with MED12, MED13, and cyclin C to form a kinase submodule of the Mediator complex. It remains unknown if CDK8 acts as a Mediator kinase in X chromosome inactivation. Here, we analyse a Med12 mutation in an embryonic stem cell model for the initiation of X chromosome inactivation. Loss of Med12 is compatible with self-renewal and survival of mouse embryonic stem cells, but impairs X-linked gene silencing by Xist. In addition, recruitment of CIZ1 and histone H3 trimethyl-lysine 27 by Xist are significantly reduced in the absence of Med12. We further show that CIZ1 recruitment is also modulated by Cdk8. Our study shows that mutations of Med12 and Cdk8 have similar effects on Xist function and provide a cell system for studying the role of the Mediator kinase module in X chromosome inactivation. Keywords: CIZ1; Cdk8; Med12; X chromosome inactivation; Xist; gene repression.