Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland.
Department of Urology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.
Cancer Res Commun. 2024 Jun 20;4(6):1533-1547. doi: 10.1158/2767-9764.CRC-22-0287.
Acquiring a telomere maintenance mechanism is a hallmark of high-risk neuroblastoma and commonly occurs by expressing telomerase (TERT). Telomerase-negative neuroblastoma has long telomeres and utilizes the telomerase-independent alternative lengthening of telomeres (ALT) mechanism. Conversely, no discernable telomere maintenance mechanism is detected in a fraction of neuroblastoma with long telomeres. Here, we show, unlike most cancers, DNA of the TERT promoter is broadly hypomethylated in neuroblastoma. In telomerase-positive neuroblastoma cells, the hypomethylated DNA promoter is approximately 1.5 kb. The TERT locus shows active chromatin marks with low enrichment for the repressive mark, H3K27me3. MYCN, a commonly amplified oncogene in neuroblstoma, binds to the promoter and induces TERT expression. Strikingly, in neuroblastoma with long telomeres, the hypomethylated region spans the entire TERT locus, including multiple nearby genes with enrichment for the repressive H3K27me3 chromatin mark. Furthermore, subtelomeric regions showed enrichment of repressive chromatin marks in neuroblastomas with long telomeres relative to those with short telomeres. These repressive marks were even more evident at the genic loci, suggesting a telomere position effect (TPE). Inhibiting H3K27 methylation by three different EZH2 inhibitors induced the expression of TERT in cell lines with long telomeres and H3K27me3 marks in the promoter region. EZH2 inhibition facilitated MYCN binding to the TERT promoter in neuroblastoma cells with long telomeres. Taken together, these data suggest that epigenetic regulation of TERT expression differs in neuroblastoma depending on the telomere maintenance status, and H3K27 methylation is important in repressing TERT expression in neuroblastoma with long telomeres.
The epigenetic landscape of the TERT locus is unique in neuroblastoma. The DNA at the TERT locus, unlike other cancer cells and similar to normal cells, are hypomethylated in telomerase-positive neuroblastoma cells. The TERT locus is repressed by polycomb repressive complex-2 complex in neuroblastoma cells that have long telomeres and do not express TERT. Long telomeres in neuroblastoma cells are also associated with repressive chromatin states at the chromosomal termini, suggesting TPE.
获得端粒维持机制是高危神经母细胞瘤的标志，通常通过表达端粒酶（TERT）来实现。端粒酶阴性神经母细胞瘤的端粒较长，利用端粒酶非依赖性的端粒延长替代（ALT）机制。相反，在一些端粒较长的神经母细胞瘤中，并没有检测到明显的端粒维持机制。在这里，我们表明，与大多数癌症不同，神经母细胞瘤中的 TERT 启动子 DNA 广泛低甲基化。在端粒酶阳性的神经母细胞瘤细胞中，低甲基化的 DNA 启动子约为 1.5kb。TERT 基因座显示出具有低抑制性标记 H3K27me3 富集的活跃染色质标记。MYCN 是神经母细胞瘤中常见的扩增癌基因，它与启动子结合并诱导 TERT 表达。引人注目的是，在端粒较长的神经母细胞瘤中，低甲基化区域跨越整个 TERT 基因座，包括多个附近基因，这些基因具有抑制性的 H3K27me3 染色质标记富集。此外，与端粒较短的神经母细胞瘤相比，端粒末端区域显示出抑制性染色质标记的富集。在具有长端粒的神经母细胞瘤中的基因座中，这些抑制性标记更为明显，表明端粒位置效应（TPE）。通过三种不同的 EZH2 抑制剂抑制 H3K27 甲基化可诱导具有长端粒的细胞系中 TERT 的表达，并在启动子区域中诱导 H3K27me3 标记。EZH2 抑制促进了 MYCN 与端粒酶阳性神经母细胞瘤中具有长端粒的 TERT 启动子结合。综上所述，这些数据表明，神经母细胞瘤中 TERT 表达的表观遗传调控因端粒维持状态而异，H3K27 甲基化在抑制具有长端粒的神经母细胞瘤中的 TERT 表达中很重要。
TERT 基因座的表观遗传景观在神经母细胞瘤中是独特的。与其他癌细胞不同，不同于正常细胞，在端粒酶阳性的神经母细胞瘤中，TERT 基因座的 DNA 呈低甲基化状态。在具有长端粒且不表达 TERT 的神经母细胞瘤中，TERT 基因座被多梳抑制复合物 2 复合物抑制。神经母细胞瘤细胞的长端粒还与染色体末端的抑制性染色质状态相关，表明存在 TPE。
