Hypermethylation and global remodeling of DNA methylation associated with acquired cisplatin resistance in testicular germ cell tumors

Testicular germ cell tumors (TGCTs) are hypersensitive to cisplatin-based therapy however cisplatin resistance and poor outcomes do occur in a significant number of patients. It has been suggested that a shift toward DNA hypermethylation mediates cisplatin resistance in TGCT cells, although there is little direct evidence to support this claim. Here we utilized a series of isogenic cisplatin resistant cell models and observed a strong association between cisplatin resistance in TGCTs cells and a net increase in global CpG and non-CpG DNA methylation spanning regulatory, intergenic, genic and repeat elements. Hypermethylated loci were significantly enriched for repressive DNA segments, CTCF and RAD21 sites and lamina associated domains, suggesting that global nuclear reorganization of chromatin structure occurred in resistant cells. Hypomethylated CpG loci were significantly enriched for EZH2 and SUZ12 binding and H3K27me3 sites. Integrative transcriptome and methylome analysis showed a strong negative correlation between gene promoter CpG island methylation and gene expression in resistant cells and a weaker positive correlation between gene body methylation and gene expression. A bidirectional shift between gene promoter and gene body DNA methylation occurred within multiple genes that was associate with upregulation of polycomb targets and downregulation of tumor suppressor genes. These data support the hypothesis that global remodeling of DNA methylation is a key factor in mediating cisplatin hypersensitivity and chemoresistance of TGCTs and furthers the rationale for hypomethylation therapy for refractory TGCT patients. Bisulphite converted DNA from the 21 samples were hybridised to the Infinium MethylationEPIC beadchip array