Department of Cell Biology, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka, Japan.
PLoS One. 2012;7(8):e43658. doi: 10.1371/journal.pone.0043658. Epub 2012 Aug 27.
In mammals, DNA methylation is restricted to cytosines of CpG dinucleotides, which are frequently found in short genomic regions including gene promoters. Methylation within CpG-rich regions around promoters tends to repress gene expression; thus, the CpG islands of housekeeping genes are normally unmethylated. We previously described a testis-specific single-exon gene containing a CpG-rich sequence that is methylated and thus repressed in somatic cells, whereas its expression in spermatogenic cells requires that it be hypomethylated. However, the relationship among the specific expression of spermatogenic genes, their methylation dynamics, and their CpG frequencies are poorly understood. Here, we analyzed the methylation patterns of the sphort genomic region around the transcription start site in spermatogenic cell-specific single-exon genes of various CpG contents. By using UniGene and Ensembl database analyses of the mouse genome and reverse transcription-PCR, we identified 39 single-exon genes that are exclusively expressed in spermatogeniccells. Regardless of their specific expression characteristics, genes containing higher (7 to 14 CpGs in 200 bp; mean = 12) and lower (2 to 6 CpGs in 200 bp; mean = 3.1) number ofCpG were hypo- and hyper-methylated, respectively, in all cell types examined, including spermatogeniccells. We found that genes with intermediate number of CpG (2 to 11 CpGs in 200 bp; mean = 6.9) are methylated in somatic cells, but not in male germ cells. These results suggest that DNA methylation dynamics of spermatogenic cell-specific single-exon genes are associated with CpG content, and the methylation status are stably maintained throughout male germ cell development.
在哺乳动物中，DNA 甲基化仅限于 CpG 二核苷酸中的胞嘧啶，CpG 二核苷酸在包括基因启动子在内的短基因组区域中频繁出现。启动子周围富含 CpG 的区域内的甲基化往往会抑制基因表达；因此，管家基因的 CpG 岛通常是非甲基化的。我们之前描述了一个睾丸特异性的单外显子基因，它含有一个富含 CpG 的序列，在体细胞中被甲基化并因此受到抑制，而其在精原细胞中的表达需要它被去甲基化。然而，精子发生基因的特异性表达、它们的甲基化动态及其 CpG 频率之间的关系知之甚少。在这里，我们分析了各种 CpG 含量的精子发生细胞特异性单外显子基因转录起始位点周围的短基因组区域的甲基化模式。通过使用 UniGene 和 Ensembl 数据库分析小鼠基因组和反转录-PCR，我们鉴定了 39 个仅在精子发生细胞中表达的单外显子基因。无论其特异性表达特征如何，包含更高（200bp 中有 7 到 14 个 CpG；平均值为 12）和更低（200bp 中有 2 到 6 个 CpG；平均值为 3.1）CpG 数目的基因在所有检查的细胞类型中，包括精子发生细胞中，分别被低甲基化和高甲基化。我们发现，中间 CpG 数目的基因（200bp 中有 2 到 11 个 CpG；平均值为 6.9）在体细胞中被甲基化，但在雄性生殖细胞中不被甲基化。这些结果表明，精子发生细胞特异性单外显子基因的 DNA 甲基化动态与 CpG 含量有关，并且甲基化状态在整个雄性生殖细胞发育过程中稳定维持。
