Structure and function of a imprinting center in the region of human SNRPN gene

某些哺乳动物的基因只在父系和母系染色体中表达，因为在配子发生过程中发生了差异标记过程。这种表观遗传标记被称为基因组印迹。我重点研究了Prader-Willi/Angelman综合征中印记的Snrpn基因区域。我们的数据揭示了差异的Snrpn基因活性与DNA甲基化、高阶染色质结构和复制时机的变化之间的相关性。此外，我们还分离并鉴定了一个新的甲基-CpG结合蛋白成员，命名为PCM1(MBD1)，它具有一个甲基-CpG结合结构域和富含半胱氨酸的CXXC区。新发现4种PCM1亚型在CXXC结构域和C末端交替剪接。所有形式的PCM1均通过甲基化抑制Snrpn基因的启动子活性。有趣的是，PCM1亚型中CXXC结构域的三个拷贝增强了对甲基化和非甲基化Snrpn启动子转录的抑制作用。相反，含有两个拷贝的CXXC结构域的PCM1亚型抑制了甲基化的启动子，但不抑制非甲基化的启动子，表明CXXC结构域是PCMI的调节元件。这些发现表明，甲基-CpG结合蛋白在甲基化介导的Snrpn基因转录沉默中发挥重要作用。

Certain mammalian genes are expressed exclusively from either the paternal and the maternal chromosome because of a differential marking process that occurs during gametogenesis. This epigenetic marking is called genomic imprinting. I focused on an imprinted SNRPN gene region in the Prader-Willi/Angelman syndrome. Our data revealed the correlations between the differential SNRPN gene activity and the changes in DNA methylation, higher order chromatin structure and replication timing. Further, we isolated and characterized a new member of the methyl-CpG binding proteins, named PCM1 (MBD1), which possesses a methyl-CpG binding domain and cysteine-rich CXXC regions. Four PCM1 isoforms were newly identified to be alternatively spliced in the CXXC domains and the C-terminus. All forms of PCM1 inhibited the promoter activity of SNRPN gene via methylation. Interestingly, three copies of the CXXC domain in the PCM1 isoforms enhanced the suppressive effect on the transcription from both methylated and unmethylated SNRPN promoters. In contrast, PCM1 isoforms containing two copies of the CXXC domain inhibited methylated but not unmethylated promoter, suggesting that the CXXC domain is a regulatory element of PCMI.These findings suggested that methyl-CpG binding proteins play important roles in methylation-mediated transcriptional silencing of SNRPN gene.