Department of Medical and Molecular Genetics, Indiana University School of Medicine (IUSM), Indianapolis, IN 46202, USA.
Oncogene. 2012 Jul 5;31(27):3244-53. doi: 10.1038/onc.2011.502. Epub 2011 Nov 28.
Many tumors exhibit elevated chromosome mis-segregation termed chromosome instability (CIN), which is likely to be a potent driver of tumor progression and drug resistance. Causes of CIN are poorly understood but probably include prior genome tetraploidization, centrosome amplification and mitotic checkpoint defects. This study identifies epigenetic alteration of the centromere as a potential contributor to the CIN phenotype. The centromere controls chromosome segregation and consists of higher-order repeat (HOR) alpha-satellite DNA packaged into two chromatin domains: the kinetochore, harboring the centromere-specific H3 variant centromere protein A (CENP-A), and the pericentromeric heterochromatin, considered important for cohesion. Perturbation of centromeric chromatin in model systems causes CIN. As cancer cells exhibit widespread chromatin changes, we hypothesized that pericentromeric chromatin structure could also be affected, contributing to CIN. Cytological and chromatin immunoprecipitation and PCR (ChIP-PCR)-based analyses of HT1080 cancer cells showed that only one of the two HORs on chromosomes 5 and 7 incorporate CENP-A, an organization conserved in all normal and cancer-derived cells examined. Contrastingly, the heterochromatin marker H3K9me3 (trimethylation of H3 lysine 9) mapped to all four HORs and ChIP-PCR showed an altered pattern of H3K9me3 in cancer cell lines and breast tumors, consistent with a reduction on the kinetochore-forming HORs. The JMJD2B demethylase is overexpressed in breast tumors with a CIN phenotype, and overexpression of exogenous JMJD2B in cultured breast epithelial cells caused loss of centromere-associated H3K9me3 and increased CIN. These findings suggest that impaired maintenance of pericentromeric heterochromatin may contribute to CIN in cancer and be a novel therapeutic target.
许多肿瘤表现出染色体错误分离，称为染色体不稳定性（CIN），这很可能是肿瘤进展和耐药性的强大驱动因素。CIN 的原因尚不清楚，但可能包括先前的基因组四倍化、中心体扩增和有丝分裂检查点缺陷。本研究确定了着丝粒的表观遗传改变是 CIN 表型的潜在原因。着丝粒控制染色体分离，由更高阶重复（HOR）α-卫星 DNA 组成，包装成两个染色质域：动粒，含有着丝粒特异性 H3 变体着丝粒蛋白 A（CENP-A），以及着丝粒周围异染色质，被认为对凝聚很重要。模型系统中着丝粒染色质的扰动会导致 CIN。由于癌细胞表现出广泛的染色质变化，我们假设着丝粒周围染色质结构也可能受到影响，导致 CIN。HT1080 癌细胞的细胞学和染色质免疫沉淀和 PCR（ChIP-PCR）分析表明，只有染色体 5 和 7 上的两个 HOR 中的一个整合了 CENP-A，这种组织在所有检查的正常和癌症衍生细胞中都是保守的。相比之下，异染色质标记 H3K9me3（H3 赖氨酸 9 的三甲基化）映射到所有四个 HOR，ChIP-PCR 显示在癌细胞系和乳腺癌中 H3K9me3 的模式发生改变，与动粒形成 HOR 上的减少一致。JMJD2B 去甲基酶在具有 CIN 表型的乳腺癌中过度表达，并且在培养的乳腺上皮细胞中外源表达 JMJD2B 导致与着丝粒相关的 H3K9me3 丢失和 CIN 增加。这些发现表明，着丝粒周围异染色质的维持受损可能导致癌症中的 CIN，并且可能是一个新的治疗靶点。
