Kuerbitz S J, Baylin S B
Oncology Center, Johns Hopkins University School of Medicine, Baltimore, Maryland 21231, USA.
Cell Growth Differ. 1996 Jun;7(6):847-53.
The mechanisms underlying ectopic methylation of CpG islands in neoplastic cells are poorly understood. One determinant may be the increased expression of DNA methyltransferase (DNA MTase) observed frequently in neoplastic cells. To evaluate the role of DNA MTase overexpression in aberrant CpG island methylation, we assessed methylation of fibroblast-derived CpG islands in human diploid fibroblast x fibrosarcoma hybrid cell lines. Each of six independently derived, immortalized hybrid cell lines exhibited a high level of DNA MTase expression, comparable to that of the fibrosarcoma parental line. The methylation status of five CpG island loci, each of which was methylated extensively in the fibrosarcoma parental cells but not in the fibroblasts, was then determined in the hybrid cell lines. The patterns of methylation were consistent and highly locus dependent among the hybrid lines. Unmethylated alleles were retained stably at three loci. The parental origin of alleles could be determined at two other loci in the hybrid cells. Whereas no methylation of parental fibroblast-derived alleles of the HIC-1 locus was noted in hybrid cell lines, a marked increase in methylation of fibroblast-derived alleles of the estrogen receptor was observed in all hybrid cell lines. Therefore, despite high-level DNA MTase expression, widespread loss of unmethylated CpG islands was not observed in the hybrid cell lines. The nonrandom pattern of increased CpG island methylation in the hybrid cell lines suggests that locus-specific features and/or clonal selection, and not just DNA MTase expression, affect the evolution of ectopic methylation in neoplastic cells. Somatic cell hybrids may provide useful models for studying aberrant epigenetic events in neoplastic cells.
肿瘤细胞中CpG岛异位甲基化的潜在机制目前尚不清楚。一个决定因素可能是肿瘤细胞中经常观察到的DNA甲基转移酶（DNA MTase）表达增加。为了评估DNA MTase过表达在异常CpG岛甲基化中的作用，我们评估了人二倍体成纤维细胞x纤维肉瘤杂交细胞系中成纤维细胞来源的CpG岛的甲基化情况。六个独立衍生的永生化杂交细胞系中的每一个都表现出高水平的DNA MTase表达，与纤维肉瘤亲代细胞相当。然后在杂交细胞系中确定五个CpG岛位点的甲基化状态，每个位点在纤维肉瘤亲代细胞中广泛甲基化，但在成纤维细胞中未甲基化。杂交系之间的甲基化模式是一致的，并且高度依赖于位点。未甲基化的等位基因在三个位点稳定保留。在杂交细胞中的另外两个位点可以确定等位基因的亲本来源。虽然在杂交细胞系中未观察到HIC-1位点亲代成纤维细胞来源的等位基因甲基化，但在所有杂交细胞系中均观察到雌激素受体成纤维细胞来源的等位基因甲基化显著增加。因此，尽管DNA MTase表达水平很高，但在杂交细胞系中未观察到未甲基化CpG岛的广泛丢失。杂交细胞系中CpG岛甲基化增加的非随机模式表明，位点特异性特征和/或克隆选择，而不仅仅是DNA MTase表达，影响肿瘤细胞中异位甲基化的演变。体细胞杂交可能为研究肿瘤细胞中异常表观遗传事件提供有用的模型。
