Loebel D A, Johnston P G
School of Biological Sciences, Macquarie University, New South Wales, Australia.
Genome Res. 1996 Feb;6(2):114-23. doi: 10.1101/gr.6.2.114.
Paternal X chromosome inactivation occurs in rodent extraembryonic membranes and in all tissues of marsupials. Methylation of CpG islands occurs on the inactive X in eutherians and is considered to be a stabilizing mechanism. The only previous study of a marsupial X-linked CpG island was of the G6PD gene of the Virginia opossum, in which the paternally derived allele is not completely repressed. We have cloned the 5' end of the G6PD gene from an Australian marsupial, the common wallaroo, and sequenced the associated CpG island. The paternally derived G6PD allele is completely repressed in tissues of this species. Methylation analysis using HpaII and Cfol restriction enzymes and bisulfite genomic sequencing of 47 CpG dinucleotides in a 613-bp region reveals hypomethylation of male and female DNA from tissues, cultured fibroblasts (in which the paternal allele is partially expressed) and sperm. This suggests that methylation of CpG islands is not required for maintenance of X inactivation in marsupials even where repression of the paternal allele is complete.
父源X染色体失活发生在啮齿动物的胚外膜以及有袋类动物的所有组织中。在真兽类动物中，CpG岛的甲基化发生在失活的X染色体上，被认为是一种稳定机制。之前唯一一项关于有袋类动物X连锁CpG岛的研究是针对弗吉尼亚负鼠的葡萄糖-6-磷酸脱氢酶（G6PD）基因，其中父源等位基因并未完全被抑制。我们从一种澳大利亚有袋类动物——普通沙袋鼠中克隆了G6PD基因的5'端，并对相关的CpG岛进行了测序。在该物种的组织中，父源G6PD等位基因被完全抑制。使用HpaII和Cfol限制性内切酶进行的甲基化分析以及对一个613 bp区域内47个CpG二核苷酸的亚硫酸氢盐基因组测序显示，来自组织、培养的成纤维细胞（其中父源等位基因部分表达）和精子的雄性和雌性DNA均发生低甲基化。这表明，即使父源等位基因的抑制是完全的，在有袋类动物中维持X染色体失活也不需要CpG岛的甲基化。
