Genome-wide methylation profiling of ovarian cancer patient-derived xenograft (PDXs) treated with the demethylating agent decitabine identifies novel epigenetically regulated genes and pathways

Background: In high-grade serous ovarian cancer (HGSOC) intrinsic and/or acquired resistance against platinum-containing chemotherapy is a major obstacle for successful treatment. A low frequency of somatic mutations, but frequent epigenetic alterations including DNA methylation in HGSOC tumors, presents the cancer epigenome as a relevant target for innovative therapy. Patient-derived xenografts (PDXs) supposedly are good preclinical models for identifying novel drug targets. However, the representativeness of global methylation status of HGSOC PDXs compared to their original tumors has not been evaluated. Aims of this study were to explore how representative HGSOC PDXs are for their corresponding patient tumors methylome and to evaluate the effect of epigenetic therapy and cisplatin on putative epigenetically regulated genes and their related pathways in PDXs. Methods: Genome-wide analysis of the DNA methylome of HGSOC patients with their corresponding PDXs, from different generations, was performed using Infinium 450K. Further, we analyzed global methylome changes after treatment of HGSOC PDXs with the FDA approved demethylating agent decitabine and cisplatin. Findings were validated by bisulfite pyrosequencing with subsequent pathway analysis. Publicly available datasets comprising HGSOC patients were used to analyze the predictive/prognostic value of the identified genes. Results: Only 0.6-1.0% of all analyzed CpGs (388,696 CpGs) changed significantly (p<0.01) during propagation showing that HGSOC PDXs are epigenetically stable. Treatment of F3 PDXs with decitabine caused a significant reduction in methylation in 10.6% of CpG sites in comparison to untreated PDXs (p<0.01, FDR<10%). Cisplatin treatment had a marginal effect on the PDX methylome. Pathway analysis of decitabine-treated PDX tumors revealed several putative epigenetically regulated pathways (e.g. Src family kinase pathway). Particularly, C-terminal Src kinase (CSK) gene was successfully validated for epigenetic regulation in different PDX models and ovarian cancer cell lines. Low CSK methylation and high CSK expression were both significantly associated (p<0.05) with improved progression free survival and overall survival in HGSOC patients. Conclusion: HGSOC PDXs resemble the global epigenome of patients over many generations, which can be modulated by epigenetic drugs. Novel epigenetically regulated genes such as CSK and related pathways were identified in HGSOC. Our results encourage future application of PDXs for cancer epigenome studies. DNA was isolated from frozen tumors of three different ovarian PDXs models up to 3 generations (F0, F1, F2 and F3) (n=12), one PDX model with treatment of vehicle, cisplatin and Decitabine (DAC) with 3 mice in each group and SKOV3 cell line with and without DAC treatment. Bisulphite converted DNA from each sample (n=22) was hybridized to the Illumina Infinium 450k Human Methylation BeadChip.