Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins Hospital, 1800 Orleans St, 21287 Baltimore, MD, USA.
Acta Neuropathol Commun. 2014 Jun 3;2:59. doi: 10.1186/2051-5960-2-59.
Diffuse intrinsic pontine glioma (DIPG) is a malignant pediatric brain tumor associated with dismal outcome. Recent high-throughput molecular studies have shown a high frequency of mutations in histone-encoding genes (H3F3A and HIST1B) and distinctive epigenetic alterations in these tumors. Epigenetic alterations described in DIPG include global DNA hypomethylation. In addition to the generally repressive methylcytosine DNA alteration, 5-hydroxymethylation of cytosine (5hmC) is recognized as an epigenetic mark associated with active chromatin. We hypothesized that in addition to alterations in DNA methylation, that there would be changes in 5hmC. To test this hypothesis, we performed immunohistochemical studies to compare epigenetic alterations in DIPG to extrapontine adult and pediatric glioblastoma (GBM) and normal brain. A total of 124 tumors were scored for histone 3 lysine 27 trimethylation (H3K27me3) and histone 3 lysine 9 trimethylation (H3K9me3) and 104 for 5hmC and 5-methylcytosine (5mC). An H-score was derived by multiplying intensity (0-2) by percentage of positive tumor nuclei (0-100%).
We identified decreased H3K27me3 in the DIPG cohort compared to pediatric GBM (p < 0.01), adult GBM (p < 0.0001) and normal brain (p < 0.0001). H3K9me3 was not significantly different between tumor types. Global DNA methylation as measured by 5mC levels were significantly lower in DIPG compared to pediatric GBM (p < 0.001), adult GBM (p < 0.01), and normal brain (p < 0.01). Conversely, 5hmC levels were significantly higher in DIPG compared to pediatric GBM (p < 0.0001) and adult GBM (p < 0.0001). Additionally, in an independent set of DIPG tumor samples, TET1 and TET3 mRNAs were found to be overexpressed relative to matched normal brain.
Our findings extend the immunohistochemical study of epigenetic alterations in archival tissue to DIPG specimens. Low H3K27me3, decreased 5mC and increased 5hmC are characteristic of DIPG in comparison with extrapontine GBM. In DIPG, the relative imbalance of 5mC compared to 5hmC may represent an opportunity for therapeutic intervention.
弥漫性内在脑桥神经胶质瘤（DIPG）是一种与预后不良相关的恶性小儿脑肿瘤。最近的高通量分子研究表明，这些肿瘤中存在组蛋白编码基因（H3F3A 和 HIST1B）的高频突变和独特的表观遗传改变。DIPG 中描述的表观遗传改变包括全基因组 DNA 低甲基化。除了通常抑制性的甲基胞嘧啶 DNA 改变外，5-羟甲基胞嘧啶（5hmC）被认为是与活性染色质相关的表观遗传标记。我们假设，除了 DNA 甲基化的改变外，5hmC 也会发生变化。为了验证这一假设，我们进行了免疫组织化学研究，以比较 DIPG 与桥外成人和小儿脑胶质瘤（GBM）和正常脑的表观遗传改变。总共对 124 个肿瘤进行了组蛋白 3 赖氨酸 27 三甲基化（H3K27me3）和组蛋白 3 赖氨酸 9 三甲基化（H3K9me3）的评分，对 104 个肿瘤进行了 5hmC 和 5-甲基胞嘧啶（5mC）的评分。通过将强度（0-2）乘以阳性肿瘤核的百分比（0-100%），得出 H 评分。
与小儿 GBM（p<0.01）、成人 GBM（p<0.0001）和正常脑（p<0.0001）相比，我们发现 DIPG 队列中 H3K27me3 降低。H3K9me3 在肿瘤类型之间无显著差异。DIPG 中全基因组 DNA 甲基化水平（以 5mC 水平衡量）明显低于小儿 GBM（p<0.001）、成人 GBM（p<0.01）和正常脑（p<0.01）。相反，DIPG 中 5hmC 水平明显高于小儿 GBM（p<0.0001）和成人 GBM（p<0.0001）。此外，在一组独立的 DIPG 肿瘤样本中，与匹配的正常脑相比，TET1 和 TET3 mRNA 被发现过表达。
我们的发现将表观遗传学改变的免疫组织化学研究扩展到 DIPG 标本。与桥外 GBM 相比，DIPG 具有低 H3K27me3、低 5mC 和高 5hmC 的特征。在 DIPG 中，5mC 与 5hmC 的相对失衡可能为治疗干预提供机会。
