DNA methylation status of myelinating Schwann cells during development and in diabetic neuropathy [Gene Expression Array: C57Bl6J mice]

DNA methylation is a key epigenetic regulator of mammalian embryogenesis and somatic cell differentiation. Using high-resolution genome-scale maps of methylation patterns, we show that the formation of myelin in the peripheral nervous system, proceeds with progressive DNA demethylation, which coincides with an upregulation of critical genes of the myelination process. More importantly, we found that, in addition to expression of DNA methyltransferases and demethylases, the levels of S-adenosylmethionine (SAMe), the principal biological methyl donor, could also play a critical role in regulating DNA methylation during myelination and in the pathogenesis of diabetic neuropathy. In summary, this study provides compelling evidence that SAMe levels need to be tightly controlled to prevent aberrant DNA methylation patterns, and together with recently published studies on the influence of SAMe on histone methylation in cancer and embryonic stem cell differentiation show that in diverse biological processes, the methylome, and consequently gene expression patterns, are critically dependent on levels of SAMe. Axonal myelination by Schwann cells in the peripheral nervous system is essential for rapid saltatory impulse conduction, and malformation or destruction of myelin sheaths can lead to severe motor and sensory disabilities (peripheral neuropathies). Using high-resolution genome-scale methylome maps, we found that DNA methylation could play a critical role in the generation of myelinated Schwann cells. This process was accompanied by a global DNA demethylation at most genomic elements. Notably, demethylation at gene-regulatory regions was associated with activation of critical myelination-specific genes. Furthermore, we found an aberrant DNA methylation pattern in a mouse model of diabetic neuropathy, which could be involved in the pathogenesis of the disease. Importantly, we found that these methylation patterns in both situations could be regulated by levels of S-adenosylmethionine (SAMe), the principal biological methyl donor. Together with recent studies on the influence of SAMe on histone methylation in diverse biological processes, we conclude that the methylation landscape of cells could be critically dependent on levels of SAMe. These provide a mechanistic link between metabolism and gene regulatory networks in normal and pathological situations. Sciatic nerves from C57Bl6J mice of either sex, were dissected and pooled together at different developmental stages, 3 replicates per sample group.DNA甲基化（DNA methylation）是调控哺乳动物胚胎发生与体细胞分化的关键表观遗传调控因子（epigenetic regulator）。本研究借助高分辨率全基因组甲基化图谱，证实外周神经系统（peripheral nervous system）的髓鞘形成（myelin）过程伴随渐进性DNA去甲基化（DNA demethylation），该过程与髓鞘形成关键基因的上调表达（upregulation）高度契合。更为重要的是，我们发现除DNA甲基转移酶（DNA methyltransferases）与去甲基化酶（demethylases）的表达变化外，核心生物甲基供体（methyl donor）S-腺苷甲硫氨酸（S-adenosylmethionine, SAMe）的水平，同样在髓鞘形成阶段的DNA甲基化调控以及糖尿病性神经病（diabetic neuropathy）的发病机制中发挥关键作用。综上，本研究提供了有力证据（compelling evidence），表明需严格管控SAMe水平以避免异常DNA甲基化模式（aberrant DNA methylation patterns）的产生；结合近期发表的关于SAMe对癌症及胚胎干细胞分化（embryonic stem cell differentiation）过程中组蛋白甲基化（histone methylation）影响的相关研究，可明确：在多种生物学过程中，细胞甲基化组（methylome）乃至基因表达模式（gene expression patterns），均高度依赖SAMe的水平。外周神经系统中施万细胞（Schwann cells）介导的轴突髓鞘形成（axonal myelination），是实现快速跳跃式冲动传导（saltatory impulse conduction）的核心基础；髓鞘畸形或破坏（malformation or destruction of myelin sheaths）可引发严重的运动与感觉障碍（motor and sensory disabilities），即周围神经病。本研究通过高分辨率全基因组甲基化组图谱发现，DNA甲基化在有髓施万细胞的生成过程中扮演关键调控角色。该过程伴随多数基因组元件（genomic elements）的全局性DNA去甲基化。值得注意的是，基因调控区域（gene-regulatory regions）的去甲基化与髓鞘形成特异性基因（myelination-specific genes）的激活密切相关。此外，我们在糖尿病性神经病小鼠模型（mouse model）中检测到异常DNA甲基化模式，该模式或参与疾病的发病进程（pathogenesis）。关键的是，我们发现上述两种情境下的甲基化模式均可通过SAMe的水平进行调控。结合近期关于SAMe在多种生物学过程中对组蛋白甲基化影响的研究，我们得出结论：细胞的甲基化景观高度依赖SAMe的水平。这为正常与病理情境下代谢与基因调控网络（metabolism and gene regulatory networks）之间的机制性关联（mechanistic link）提供了直接依据。本研究的实验材料为雌雄不限的C57Bl6J小鼠（C57Bl6J mice）的坐骨神经（sciatic nerves），于不同发育阶段解剖取材并混合，每个样本组设置3个生物学重复（replicates）。