Platinum resistance causes treatment failure in a variety of cancers including ovarian cancer. Epigenetic changes can contribute to platinum resistance, as the epigenome is highly divergent between platinum-resistant and -sensitive cells, and DNA demethylating agents partially resensitise resistant cells to platinum. The mismatch repair (MMR) gene MLH1 is epigenetically silenced in certain ovarian cisplatin-resistant cells and links between DNA damage and DNA methylation have been reported. This thesis aims to address the hypothesis that platinum-induced DNA damage causes aberrant DNA methylation, which can be selected for leading to drug resistance, and that the methylation response to platinum damage is influenced by MMR status.

Two ovarian cancer cell lines, derived from A2780/CP70, containing a fragment of chromosome 3 with either actively expressed MLH1 (proficient) or an epigenetically silenced MLH1 (deficient) were used to derive individual clones after mock or 5 μM (IC95) cisplatin treatment. LINE-1 methylation was significantly increased only in MLH1-proficient clones after cisplatin treatment. Using the Infinium HumanMethylation450 BeadChip array, methylation was measured at ~480,000 CpGs genome-wide in mock- and cisplatin-treated MLH1-proficient and -deficient clones (n=11 per group). Cisplatin induced bi-directional methylation changes, biased toward hypermethylation, in MLH1-proficient and -deficient clones, with a higher number of DNA methylation changes in MLH1-proficient clones. No detectable genomic location or sequence specificity was observed.
In peripheral blood mononuclear cell (PBMC) DNA of ovarian cancer patients treated with carboplatin as part of the SCOTROC1 clinical trial, significant methylation changes were detected using the Infinium methylation array when comparing matched samples collected pre-treatment and at relapse. These methylation changes were, again, biased towards hypermethylation post-platinum treatment and occurred preferentially in non-promoter regions of the genome.

This thesis demonstrates for the first time that platinum induces bi-directional DNA methylation changes in cell lines and in PBMCs of ovarian cancer patients and there may be both MLH1-dependent and MLH1-independent mechanisms involved in this response.