[Frontiers in Bioscience 11, 852-866, January 1, 2006]
Mechanisms of X-chromosome inactivation
Samuel C. Chang, Tracy Tucker, Nancy P. Thorogood, and Carolyn J. Brown
Department of Medical Genetics, University of British Columbia,Vancouver, BC, Canada
Figure 1. Transcription maps of the Xic/XIC regions in mouse and human (27). There are 11 genes in the mouse Xic region: Xpct, Xist, Tsx, Tsix, Chic1 (formerly, Brx), Cdx4, NapIl2 (formerly, Bpx), Cnbp2, Ftx, Jpx, and Ppnx. Protein coding genes are represented by yellow boxes. Four of the 11 genes, Xist, Tsix, Ftx, and Jpx, are untranslated RNA genes and represented by red boxes. Region B, a non-coding expressed domain, is represented by a striped box. All the genes identified in mouse are conserved in human, except Ppnx and Tsix. In human, however, Tsx has become a pseudogene. The human region is approximately three times larger than the mouse. Despite this major change in size, the order and orientation of genes is conserved in human and mouse, except for Xpct, which is at the same location but in the inverse orientation. A histone H3 lysine 9 dimethylation hotspot and H4 hyperacetylation are represented by blue and green boxes below the transcription map of the Xic region in mouse. Pillet et al. showed that the region -1157 to +917 has no in vitro sex-specific promoter activity (35). A minimal constitutional promoter was assigned to a region from -81 to +1. Deletion of the segment -441 to -231 is associated with an increase in CAT activity and may represent a silencer element. The choice/imprinting center contains tandem CTCF binding sites. Chao et al. proposed that Tsix and CTCF together establish a regulatable epigenetic switch for X-inactivation (49). Ogawa and Lee showed that Xite, located 10 kb from the Tsix transcription start, harbours two clusters of DNase hypersensitive sites (56).
Figure 2. Accumulation of chromatin changes during X inactivation. The timing of changes to the inactive X are ordered as observed in studies of early mouse development and ES cell differentiation. Silencing can result from Xist expression, but stabilization of the silencing requires additional changes. Reactivation occurs rarely as discussed in the text, and in general the inactivation status is very stably maintained once established.
Figure 3. Chromatin remodeling on the inactive X chromosome. A. Active X chromatin is characterized by acetylation of H3 and H4 of the core nucleosome. There is also methylation of H3 lysine 4. B. Inactive X chromosome. Upon expression and localization of Xist there is macroH2A recruitment. It is unclear if these are bound together physically or are associated in some yet unidentified ribonuclear protein complex. The histone tails on the inactive X become hypoacetylated and methylated at H3 lysine 9 and 27, and H4 lysine 20. In addition, ubiquitination of H2A lysine 119 within the histone body is observed. DNA methylation is a late event in the inactivation process to lock in the inactive state. Note: it is not known to what extent the histone modifications are occurring on the same histone or within the same nucleosome, but at least some appear to be found in alternate domains (96).