An Integrated Analysis of the Genome-Wide Profiles of DNA Methylation and mRNA Expression Defining the Side Population of a Human Malignant Mesothelioma Cell Line

Intratumoral heterogeneity is a hallmark of all cancers and functions as the major barrier against effective cancer therapy. In contrast to genetic mutations, the role of epigenetic modifications in the generation and maintenance of heterogeneous cancer cells remains largely undetermined. This study was performed to evaluate the epigenetic mechanisms involved in the tumor cell heterogeneity using side population (SP) and non-SP cells isolated from a human malignant mesothelioma (HMM) cell line. The subpopulations of cancer cells were analyzed by methylated DNA immunoprecipitation combined with high-throughput sequencing (MeDIP-seq) and RNA-seq methodology. The RNA-seq data were analyzed with the MeDIP-seq data in an integrated way to identify the epigenetically modified genes that defined the SP. Concomitant changes in mRNA expression and DNA methylation were found in 122 genes, including 118 down-regulated genes with hypermethylation and 4 up-regulated genes with hypomethylation. Gene ontology revealed that a large portion of the genes belonged to the groups of biological processes such as stem cell maintenance, stem cell development, stem cell differentiation, and the negative regulation of the developmental process. Among these genes, BNC1, RPS6KA3, TWSG1 and DUSP15 contained aberrant methylation in the CpG islands of the promoter region, indicating that the genes regulated by DNA methylation characterized a distinct subpopulation of HMM cells. The present study provided valuable information to shed light on the epigenetic contributions to the generation and maintenance of tumor cell heterogeneity.