Choi Y C, Chae C B
Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill 27599-7260.
Mol Cell Biol. 1993 Sep;13(9):5538-48. doi: 10.1128/mcb.13.9.5538-5548.1993.
In contrast to many other genes containing a CpG island, the testis-specific H2B (TH2B) histone gene exhibits tissue-specific methylation patterns in correlation with gene activity. Characterization of the methylation patterns within a 20-kb segment containing the TH2A and TH2B genes in comparison with that in a somatic histone cluster revealed that: (i) the germ cell-specific unmethylated domain of the TH2A and TH2B genes is defined as a small region surrounding the CpG islands of the TH2A and TH2B genes and (ii) somatic histone genes are unmethylated in both liver and germ cells, like other genes containing CpG islands, whereas flanking sequences are methylated. Transfection of in vitro-methylated TH2B, somatic H2B, and mouse metallothionein I constructs into F9 embryonal carcinoma cells revealed that the CpG islands of the TH2A and TH2B genes were demethylated like those of the somatic H2A and H2B genes and the metallothionein I gene. The demethylation of those CpG islands became significantly inefficient at a high number of integrated copies and a high density of methylated CpG dinucleotides. In contrast, three sites in the somatic histone cluster, of which two sites are located in the long terminal repeat of an endogenous retrovirus-like sequence, were efficiently demethylated even at a high copy number and a high density of methylated CpG dinucleotides. These results suggest two possible mechanisms for demethylation in F9 cells and methylation of CpG islands of the TH2A and TH2B genes at the postblastula stage during embryogenesis.
与许多其他含有CpG岛的基因不同，睾丸特异性H2B（TH2B）组蛋白基因表现出与基因活性相关的组织特异性甲基化模式。与体细胞组蛋白簇中的甲基化模式相比，对包含TH2A和TH2B基因的20 kb片段内的甲基化模式进行表征发现：（i）TH2A和TH2B基因的生殖细胞特异性未甲基化结构域被定义为围绕TH2A和TH2B基因CpG岛的一个小区域；（ii）体细胞组蛋白基因在肝脏和生殖细胞中均未甲基化，与其他含有CpG岛的基因一样，而侧翼序列是甲基化的。将体外甲基化的TH2B、体细胞H2B和小鼠金属硫蛋白I构建体转染到F9胚胎癌细胞中，发现TH2A和TH2B基因的CpG岛与体细胞H2A和H2B基因以及金属硫蛋白I基因的CpG岛一样发生了去甲基化。在高整合拷贝数和高密度甲基化CpG二核苷酸的情况下，这些CpG岛的去甲基化效率显著降低。相比之下，体细胞组蛋白簇中的三个位点，其中两个位点位于内源性逆转录病毒样序列的长末端重复序列中，即使在高拷贝数和高密度甲基化CpG二核苷酸的情况下也能有效去甲基化。这些结果提示了F9细胞中去甲基化以及胚胎发育囊胚后期TH2A和TH2B基因CpG岛甲基化的两种可能机制。
