Tuyển tập các báo cáo nghiên cứu về y học được đăng trên tạp chí y học Wertheim cung cấp cho các bạn kiến thức về ngành y đề tài: Escape from X inactivation in mice and humans. | Berletch et al. Genome Biology 2010 11 213 http 2010 11 6 213 w Genome Biology REVIEW L_ Escape from X inactivation in mice and humans Joel B Berletch1 Fan Yang1 and Christine M Disteche1 2 Abstract A subset of X-linked genes escapes silencing by X inactivation and is expressed from both X chromosomes in mammalian females. Species-specific differences in the identity of these genes have recently been discovered- suggesting a role in the evolution of sex differences. Chromatin analyses have aimed to discover how genes remain expressed within a repressive environment. The difference in sex-chromosome make-up between mammalian males XY and females XX has led to the evolution of two main dosage-compensation mechanisms upregulation of the active X chromosome Xa in both sexes to balance X expression with the autosomes and inactivation of one X chromosome in females to avoid X hyperexpression and correct for the difference in gene dosage between the sexes 1-3 see Box 1 . These mechanisms evolved to compensate for the presence of only one copy haploinsufficiency of X-linked genes in males due to degeneration of the Y chromosome from its origin as an X homolog 4 . Suppression of recombination between the sex chromosomes was apparently mediated by large Y inversions as deduced by remnant X Y homology. This led to Y degeneration due to accumulation of mutations and inability to restore the correct DNA sequence 5 6 . Only small regions of homology and pairing between the sex chromosomes remain called pseudoautosomal regions PARs because genes within these regions behave like autosomal genes. Initiation of X inactivation in female embryos depends on the transcription of the long noncoding RNA XIST Xist X-inactive specific transcript from one chromosome which will become the inactive X Xi and recruitment of a protein complex important for X-chromosome silencing and heterochromatin formation 7 8 . In humans XIST 17 kb in size is located in the long arm of
