title: Xist expression and macroH2A1.2 localisation in mouse primordial and pluripotent embryonic germ cells.
abstract: The molecular mechanism underlying X chromosome inactivation in female mammals involves the non-coding RNAs Xist and its antisense partner Tsix. Prior to X inactivation, these RNAs are transcribed in an unstable form from all X chromosomes, both in the early embryo and in undifferentiated embryonic stem (ES) cells. Upon differentiation, the expression of these unstable transcripts from all alleles is silenced, and Xist RNA becomes stabilised specifically on the inactivating X chromosome. This pattern of expression is then maintained throughout subsequent somatic cell divisions. Once established, the inactive state of the X chromosome is remarkably stable, the only natural case of reactivation occurring in XX primordial germ cells (PGCs) when they enter the genital ridge. To gain insight into the X reactivation process, we have analysed Xist gene expression using RNA FISH in PGCs and also in PGC-derived embryonic germ (EG) cells. XX EG cells were shown to express unstable Xist/Tsix from both X chromosomes. In contrast, no unstable Xist/Tsix transcripts were detected in XX PGCs at any stage. Instead, a proportion of XX PGCs isolated from the genital ridge between 11.5 and 13.5 dpc (the period during which X chromosome reactivation occurs) showed an accumulation of stable Xist RNA on one X. The number of these cells decreased progressively and was nearly extinguished by 13.5 dpc. As a late marker for the inactive state, we analysed localisation of the histone H2A variant macroH2A1.2. Although macroH2A1.2 expression was observed in PGCs, no significant localisation to the inactive X was detected at any stage. We discuss these results in the context of understanding X chromosome reactivation.
authors.affiliation: X inactivation Group, MRC Clinical Sciences Centre, Faculty of Medicine ICSTM, Hammersmith Hospital, Du Cane Road, London, W12 0NN, UK.
authors.forename: Tatyana B
authors.initials: TB
authors.lastname: Nesterova
authors.forename: Jacqueline E
authors.initials: JE
authors.lastname: Mermoud
authors.forename: Kathy
authors.initials: K
authors.lastname: Hilton
authors.forename: John
authors.initials: J
authors.lastname: Pehrson
authors.forename: M Azim
authors.initials: MA
authors.lastname: Surani
authors.forename: Anne
authors.initials: A
authors.lastname: McLaren
authors.forename: Neil
authors.initials: N
authors.lastname: Brockdorff
mesh_terms: D000818:Animals; D002454:Cell Differentiation; D002460:Cell Line; D003433:Crosses, Genetic; D005260:Female; D005854:Germ Cells / Q000737:chemistry* / Q000254:growth & development / Q000378:metabolism*; D006657:Histones / Q000032:analysis*; D017404:In Situ Hybridization, Fluorescence; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D008822:Mice, Transgenic; D062085:RNA, Long Noncoding; D012333:RNA, Messenger / Q000096:biosynthesis; D022661:RNA, Untranslated / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014960:X Chromosome
chemical_list: D006657:Histones; D062085:RNA, Long Noncoding; D012333:RNA, Messenger; D022661:RNA, Untranslated; D014157:Transcription Factors; C077885:XIST non-coding RNA; C120756:macroH2A histone