Yang Youwen, Hu Ji-Fan, Ulaner Gary A, Li Tao, Yao Xiaoming, Vu Thanh H, Hoffman Andrew R
Medical Service, VA Palo Alto Health Care System, and Division of Endocrinology, Department of Medicine, Stanford University, Palo Alto, California 94304, USA.
J Cell Biochem. 2003 Dec 1;90(5):1038-55. doi: 10.1002/jcb.10684.
The mouse insulin-like growth factor II (Igf2) and H19 genes are located adjacent to each other on chromosome 7q11-13 and are reciprocally imprinted. It is believed that the allelic expression of these two genes is regulated by the binding of CTCF insulators to four parent-specific DNA methylation sites in an imprinting control center (ICR) located between these two genes. Although monoallelically expressed in peripheral tissues, Igf2 is biallelically transcribed in the CNS. In this study, we examined the allelic DNA methylation and CTCF binding in the Igf2/H19 imprinting center in CNS, hypothesizing that the aberrant CTCF binding as one of the mechanisms leads to biallelic expression of Igf2 in CNS. Using hybrid F1 mice (M. spretus males x C57BL/6 females), we showed that in CNS, CTCF binding sites in the ICR were methylated exclusively on the paternal allele, and CTCF bound only to the unmethylated maternal allele, showing no differences from the imprinted peripheral tissues. Among three other epigenetic modifications examined, histone H3 lysine 9 methylation correlated well with Igf2 allelic expression in CNS. These results suggest that CTCF binding to the ICR alone is not sufficient to insulate the Igf2 maternal promoter and to regulate the allelic expression of the gene in the CNS, thus challenging the aberrant CTCF binding as a common mechanism for lack of Igf2 imprinting in CNS. Further studies should be focused on the identification of factors that are involved in histone methylation and CTCF-associated factors that may be needed to coordinate Igf2 imprinting.
小鼠胰岛素样生长因子II（Igf2）基因和H19基因在7号染色体q11 - 13区域彼此相邻定位，并且呈现相互印记。据信，这两个基因的等位基因表达是由CCCTC结合因子（CTCF）绝缘子与位于这两个基因之间的印记控制中心（ICR）中的四个亲本特异性DNA甲基化位点的结合来调控的。尽管Igf2在外周组织中是单等位基因表达的，但在中枢神经系统中却是双等位基因转录的。在本研究中，我们检测了中枢神经系统中Igf2/H19印记中心的等位基因DNA甲基化和CTCF结合情况，推测异常的CTCF结合是导致中枢神经系统中Igf2双等位基因表达的机制之一。使用杂交F1小鼠（小家鼠雄性×C57BL/6雌性），我们发现，在中枢神经系统中，ICR中的CTCF结合位点仅在父本等位基因上发生甲基化，而CTCF仅与未甲基化的母本等位基因结合，这与印记的外周组织没有差异。在所检测的其他三种表观遗传修饰中，组蛋白H3赖氨酸9甲基化与中枢神经系统中Igf2等位基因表达密切相关。这些结果表明，仅CTCF与ICR的结合不足以隔离Igf2母本启动子并调节该基因在中枢神经系统中的等位基因表达，因此对异常的CTCF结合作为中枢神经系统中Igf2印记缺失的常见机制提出了质疑。进一步的研究应集中于鉴定参与组蛋白甲基化的因子以及可能需要协调Igf2印记的CTCF相关因子。
