Histone modification and the epigenetics of X chromosome inactivation
Dosage compensation serves to equalise the levels of X-linked gene products between males and females. In mammals this occurs through the transcriptional silencing of the majority of the genes on one of the two female X chromosomes. The inactive X chromosome (Xi) differs from its active homologue in a number of ways, including the hypoacetylation of core histones, a common property of genetically inactive chromatin. This study has used Xi to explore the functional significance of hypoacetylation and patterns of histone methylation in silent chromatin. Xi was shown to be depleted for di- and tri-methylated lysine 4 of H3, but retained di-methylated lysine 9 of H3. I have examined the temporal order of these modifications as they become established using an in vitro model system for X inactivation; differentiating female embryonic stem cells. The results showed that the loss of tri-methylated lysine 4 of H3 preceded the loss of its di-methylated equivalent, which occurs during a time period of concurrent core histone deacetylation supporting a functional role to the level of lysine methylation. I have used cases of X;autosome translocation to examine how these modifications relate to late replication and transcriptional silencing. Results show that whilst the spread of X inactivation can occur in the absence of both of these properties, histone modifications are a more reliable indicator of the extent of spread of X inactivation than late replication. To explore mechanisms that drive changes in histone modification I have analysed the distribution of histone deacetylases across a region of defined histone deacetylation. The results showed a ubiquitous distribution that did not correlate with acetylated H3 or H4 suggesting that the global association of the Hdacs might serve to provide a rapid return the basal level of histone acetylation following specific targeting events.