Srivastava M, Hsieh S, Grinberg A, Williams-Simons L, Huang S P, Pfeifer K
Laboratory of Mammalian Genes and Development, National Institute of Child Health and Human Development, Bethesda, Maryland 20892, USA.
Genes Dev. 2000 May 15;14(10):1186-95.
H19 and Igf2 are expressed in a monoallelic fashion from the maternal and paternal chromosomes, respectively. A region upstream of H19 has been shown to regulate such imprinted expression of both genes in cis. We have taken advantage of a loxP/cre recombinase-based strategy to delete this region in mice in a conditional manner to determine the temporal requirement of the upstream region in initiating and maintaining the imprinted expression of H19 and Igf2. Analysis of allele-specific expression of H19 and Igf2 and DNA methylation at the H19 promoter demonstrates that this region controls the monoallelic expression of the two genes in different ways, suggesting that it harbors two functionally distinct regulatory elements. Continued presence of the region is required to silence maternal Igf2 in accordance with its proposed role as an insulator. However, it does not have a direct role in keeping the paternal H19 promoter silenced. Instead, on the paternal chromosome, the upstream element mediates epigenetic modifications of the H19 promoter region during development, leading to transcriptional silencing of H19. Thereafter, its presence is redundant for preventing transcription. Presently, this temporal requirement of the silencing element appears to be a unique cis activity in the mammalian system. However, it is likely that other cis-acting elements, positive and negative, have the ability to effect stable changes in the chromatin structure and are not constantly required to give signals to the transcriptional machinery.
H19和Igf2分别以单等位基因的方式从母源和父源染色体表达。H19上游的一个区域已被证明以顺式作用调节这两个基因的这种印记表达。我们利用基于loxP/cre重组酶的策略，以条件性方式在小鼠中删除该区域，以确定上游区域在启动和维持H19和Igf2印记表达中的时间需求。对H19和Igf2的等位基因特异性表达以及H19启动子处的DNA甲基化分析表明，该区域以不同方式控制这两个基因的单等位基因表达，这表明它含有两个功能不同的调控元件。根据其作为绝缘子的假定作用，该区域的持续存在是沉默母源Igf2所必需的。然而，它在保持父源H19启动子沉默方面没有直接作用。相反，在父源染色体上，上游元件在发育过程中介导H19启动子区域的表观遗传修饰，导致H19转录沉默。此后，其存在对于阻止转录是多余的。目前，这种沉默元件的时间需求似乎是哺乳动物系统中一种独特的顺式活性。然而，很可能其他顺式作用元件，无论是正向还是负向，都有能力影响染色质结构的稳定变化，并且并非始终需要向转录机制发出信号。
