Epigenetics of X Chromosome Inactivation

Abstract

This chapter describes the key regulatory events in X chromosome inactivation (XCI). A cascade of events that allows the silencing of one of the two X chromosomes during female mammalian embryogenesis characterizes XCI. It is believed that Xist coating is the initial event in XCI; the gradual progress of other epigenetic components is crucial for allowing stability and flexibility of the inactivated state to occur. The transition from a transcriptional active to a silent chromosome requires the engagement of cellular relocalization as well as distinguished chromatin code. The complexity and the multiple components involved in XCI continue to invite many researchers to discover more. It is discovered that one of the X chromosomes is comprised of facultative heterochromatin. The milestone work of Lyon put forward her hypothesis that the "Barr body" is an inactive X chromosome (Xi) that appears in mammalian cells with more than one X chromosome. The inactivation allows dosage compensation in females as compared to males who carry only one X chromosome. XCI is a developmentally-regulated process that involves sequential acquisition of silencing markers on the X chromosome to be inactivated.