Baumann Claudia, Schmidtmann Anja, Muegge Kathrin, De La Fuente Rabindranath
Female Germ Cell Biology Group, Department of Clinical Studies, Center for Animal Transgenesis and Germ Cell Research, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA 19348, USA.
BMC Mol Biol. 2008 Mar 13;9:29. doi: 10.1186/1471-2199-9-29.
Establishment of chromosomal cytosine methylation and histone methylation patterns are critical epigenetic modifications required for heterochromatin formation in the mammalian genome. However, the nature of the primary signal(s) targeting DNA methylation at specific genomic regions is not clear. Notably, whether histone methylation and/or chromatin remodeling proteins play a role in the establishment of DNA methylation during gametogenesis is not known. The chromosomes of mouse neonatal spermatogonia display a unique pattern of 5-methyl cytosine staining whereby centromeric heterochromatin is hypo-methylated whereas chromatids are strongly methylated. Thus, in order to gain some insight into the relationship between global DNA and histone methylation in the germ line we have used neonatal spermatogonia as a model to determine whether these unique chromosomal DNA methylation patterns are also reflected by concomitant changes in histone methylation.
Our results demonstrate that histone H3 tri-methylated at lysine 9 (H3K9me3), a hallmark of constitutive heterochromatin, as well as the chromatin remodeling protein ATRX remained associated with pericentric heterochromatin regions in spite of their extensive hypo-methylation. This suggests that in neonatal spermatogonia, chromosomal 5-methyl cytosine patterns are regulated independently of changes in histone methylation, potentially reflecting a crucial mechanism to maintain pericentric heterochromatin silencing. Furthermore, chromatin immunoprecipitation and fluorescence in situ hybridization, revealed that ATRX as well as H3K9me3 associate with Y chromosome-specific DNA sequences and decorate both arms of the Y chromosome, suggesting a possible role in heterochromatinization and the predominant transcriptional quiescence of this chromosome during spermatogenesis.
These results are consistent with a role for histone modifications and chromatin remodeling proteins such as ATRX in maintaining transcriptional repression at constitutive heterochromatin domains in the absence of 5-methyl cytosine and provide evidence suggesting that the establishment and/or maintenance of repressive histone and chromatin modifications at pericentric heterochromatin following genome-wide epigenetic reprogramming in the germ line may precede the establishment of chromosomal 5-methyl cytosine patterns as a genomic silencing strategy in neonatal spermatogonia.
染色体胞嘧啶甲基化和组蛋白甲基化模式的建立是哺乳动物基因组中异染色质形成所需的关键表观遗传修饰。然而，靶向特定基因组区域DNA甲基化的主要信号的本质尚不清楚。值得注意的是，组蛋白甲基化和/或染色质重塑蛋白在配子发生过程中DNA甲基化的建立中是否发挥作用尚不清楚。小鼠新生精原细胞的染色体呈现出独特的5-甲基胞嘧啶染色模式，着丝粒异染色质低甲基化，而染色单体则高度甲基化。因此，为了深入了解生殖系中整体DNA与组蛋白甲基化之间的关系，我们使用新生精原细胞作为模型来确定这些独特的染色体DNA甲基化模式是否也反映在组蛋白甲基化的伴随变化中。
我们的结果表明，赖氨酸9三甲基化的组蛋白H3（H3K9me3），即组成型异染色质的标志，以及染色质重塑蛋白ATRX，尽管其广泛低甲基化，但仍与近着丝粒异染色质区域相关联。这表明在新生精原细胞中，染色体5-甲基胞嘧啶模式的调节独立于组蛋白甲基化的变化，这可能反映了维持近着丝粒异染色质沉默的关键机制。此外，染色质免疫沉淀和荧光原位杂交显示，ATRX以及H3K9me3与Y染色体特异性DNA序列相关联，并修饰Y染色体的两条臂，这表明它们在精子发生过程中异染色质化以及该染色体的主要转录静止中可能发挥作用。
这些结果与组蛋白修饰和染色质重塑蛋白（如ATRX）在缺乏5-甲基胞嘧啶的情况下维持组成型异染色质结构域的转录抑制作用一致，并提供证据表明，在生殖系中全基因组表观遗传重编程后，近着丝粒异染色质上抑制性组蛋白和染色质修饰的建立和/或维持可能先于染色体5-甲基胞嘧啶模式的建立，作为新生精原细胞中的一种基因组沉默策略。
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