Kimura H, Tada M, Hatano S, Yamazaki M, Nakatsuji N, Tada T
Department of Development and Differentiation, Institute for Frontier Medical Sciences, Kyoto University, Kyoto, Japan.
Cytogenet Genome Res. 2002;99(1-4):106-14. doi: 10.1159/000071581.
In mammalian somatic cells, the X chromosome is active in XY males, whereas one X chromosome is inactivated in XX females. On the active male X chromosome, the XIST and TSIX genes are transcribed in undifferentiated cells of pre-implantation embryos (undifferentiated state) and then down-regulated upon cell differentiation (differentiated state). To explore the epigenetic mechanism involved in the on-off switching of XIST and TSIX transcription in the active X chromosome, male somatic cells were hybridized with male embryonic stem (ES) cells. Fluorescence in situ hybridization analysis revealed that the XIST gene derived from somatic cells was derepressed, as shown by the advent of two pinpoint signals. This was confirmed by strand-specific RT-PCR of XIST and TSIX genes. To analyze changes in chromatin structure in the promoter regions of XIST and TSIX derived from somatic cells, histone tail modifications were studied by chromatin immunoprecipitation analysis. Histones H3 and H4, which were hypoacetylated in the somatic cells, were hyperacetylated in the hybrid cells, and histone H3 lysine 4, which was hypomethylated in the somatic cells, was hypermethylated in the hybrid cells, indicating that the reactivation of XIST and TSIX was linked with chromatin modifications. In the telomeric region of DXPAS34, acetylation of histones H3 and H4 was dependent on reactivation of XIST and TSIX, whereas histone H3 lysine 4 was constitutively methylated independent of the transcriptional activity of those genes. We propose that the chromatin reprogramming is linked with the resetting of the memory found in the process of choosing an active X chromosome.
在哺乳动物体细胞中，X染色体在XY雄性个体中是活跃的，而在XX雌性个体中一条X染色体是失活的。在活跃的雄性X染色体上，XIST和TSIX基因在植入前胚胎的未分化细胞（未分化状态）中被转录，然后在细胞分化时（分化状态）被下调。为了探究参与活跃X染色体上XIST和TSIX转录开关的表观遗传机制，将雄性体细胞与雄性胚胎干细胞（ES细胞）进行杂交。荧光原位杂交分析显示，源自体细胞的XIST基因去抑制，表现为出现两个点状信号。通过XIST和TSIX基因的链特异性逆转录-聚合酶链反应（RT-PCR）证实了这一点。为了分析源自体细胞的XIST和TSIX启动子区域染色质结构的变化，通过染色质免疫沉淀分析研究了组蛋白尾部修饰。在体细胞中低乙酰化的组蛋白H3和H4在杂交细胞中高乙酰化，在体细胞中低甲基化的组蛋白H3赖氨酸4在杂交细胞中高甲基化，这表明XIST和TSIX的重新激活与染色质修饰有关。在DXPAS34的端粒区域，组蛋白H3和H4的乙酰化依赖于XIST和TSIX的重新激活，而组蛋白H3赖氨酸4的甲基化是组成性的，与这些基因的转录活性无关。我们提出染色质重编程与在选择活跃X染色体过程中发现的记忆重置有关。
