近日浙江大学生命科学学院细胞与发育生物学研究所的研究人员在新研究中证实机体存在DNA主动去甲基化调控机制，生长阻滞与DNA损伤诱导蛋白Gadd45a在该过程中发挥了关键作用。这一研究成果于9月14日发表在国际学术期刊《生物化学期刊》（JBC）上。
DNA甲基化和去甲基化是表观遗传调控的两种重要方式。目前人们对DNA甲基化已开展了系统研究，但对DNA去甲基化尚缺乏深入认识。长期来认为DNA去甲基化是一个依赖DNA复制的被动去甲基化的过程，对于机体内是否存在主动去甲基化调控机制是长期以来备受关注的热点科学问题。
该研究以间充质干细胞成骨分化为实验模型，揭示了成体干细胞终端分化中存在DNA主动去甲基化调控方式，而且生长阻滞与DNA损伤诱导蛋白Gadd45a在该过程中发挥了关键作用。研究发现，Gadd45a介导的成骨基因表达的去甲基化调控主要发生于启动子的CpG岛外（island shore）中等密度区，而非通常认为的高密度CpG岛，而且Gadd45a介导的主动去甲基化主要发生在若干特定的CpG位点上，如Dlx5的-913和-800、Runx2的-820和-808、BGP的-1003以及Osterix的-727和-632 CpG位点，提出在DNA主动去甲基化过程中可能存在一种位点特异性去甲基化机制。研究还发现，Gadd45a通过与启动子直接结合而发挥作用，它对成骨分化基因的主动去甲基化调控可能是通过NER途径实现的。
这篇文章的通讯作者是浙江大学生命科学学院党委书记邵建忠教授，其主要研究领域包括干细胞生长与分化调控机制研究，以及免疫细胞因子、受体以及免疫细胞发生发育机制研究。不久前邵建忠课题组研究人员从硬骨鱼中首次鉴别出Toll-IL-1受体家族的一个新成员DIGIRR，并证实其在IL-1信号通路中发挥重要的负调控作用。
在这篇文章中，研究人员从三种硬骨鱼模型中首次鉴定出该家族的一个新成员，命名为Double-Ig-IL-1R Related Molecule (DIGIRR)，并证实这个DIGIRR分子包含有两个胞外Ig结构域，一个胞内Arg-Tyr536位点突TIR结构域，且定位于高尔基体的胞内受体。这些特征表明DIGIRR有别于其他已知的Toll/IL-1R家族成员。随后研究人员在体外实验中证实将DIGIRR注入到斑马鱼胚胎中可显著抑制脂多糖（LPS）和IL-1β诱导的NF-κB激活。当研究人员在斑马鱼体内用小干扰RNA(siRNA)沉默DIGIRR基因表达时发现在DIGIRR沉默的肝肾组织及白细胞中IL-1β即促炎细胞因子IL-6的表达水平显著增高。研究结果表明DIGIRR是LPS和IL-1β介导的信号传导通路及炎症反应中发挥重要的负调控因子作用。此外研究人员还证实一个胞外Ig结构域的缺失和胞内TIR结构域中Arg-Tyr536位点的突变，是IL-1R家族成员演化为负调节受体的重要机制。
此外，今年4月邵建忠课题组在JBC杂志上发表了另一篇研究论文，在这篇文章中作者揭示了DLK1参与肝脏纤维化形成新机制及骨髓间充质干细胞（MSC）调控肝脏纤维化分子机理，为肝硬化防治药物开发及干细胞移植治疗提供了新的分子靶点和理论依据。
Gadd45a plays an essential role in active DNA demethylation during terminal osteogenic differentiation of adipose-derived mesenchymal stem cells
Methylation and demethylation of DNA are the complementary processes of epigenetic regulation. These two types of regulation influence a diverse array of cellular activities, including the maintenance of pluripotency and self-renewal in embryonic stem cells. It was generally believed that DNA demethylation occurs passively over several cycles of DNA replication, and that active DNA demethylation is rare. Recently, evidence for active DNA demethylation has been obtain in several cancer, neuronal, and embryonic stem cell lines. Studies in embryonic stem cell models, however, suggested that active DNA demethylation might be restricted to the early development of progenitor cells. Whether active demethylation is involved in terminal differentiation of adult stem cells is poorly understood. We provide evidence that active DNA demethylation does occur during terminal specification of stem cells in an adipose-derived mesenchymal stem cell (ADSC)-derived osteogenic differentiation model. The medium CpG regions in promoters of the Dlx5, Runx2, BGP and Osterix osteogenic lineage-specific genes were demethylated during the increase in gene expression associated with osteogenic differentiation. The growth arrest and DNA-damage-inducible protein Gadd45a was up-regulated in these processes. Knockdown of Gadd45a led to hypermethylation of Dlx5, Runx2, BGP, and Osterix promoters, followed by suppression of these genes expression and interruption of osteogenic differentiation. These results reveal that Gadd45a plays an essential role in gene-specific active DNA demethylation during adult stem cell differentiation. They enhance current knowledge of osteogenic specification and may also lead to a better understanding of the common mechanisms underlying epigenetic regulation in adult stem cell differentiation.
论文全文阅读：机体存在DNA主动去甲基化调控机制.pdf
