Abstract 4811: Global demethylation with decitabine increases DNA repair and sensitizes melanoma to carboplatin

Abstract

Abstract Global genome repair (GGR) is critical for recognizing DNA damage by the chemotherapy agent carboplatin, and triggering apoptotic signalling. XPC is a critical gene in the GGR pathway but in melanoma XPC is not induced in response to carboplatin resulting in extreme resistance. Despite low global methylation levels across the melanoma genome, there is evidence methylation plays a role in suppression of XPC. 5-aza-2’-deoxycytidine (decitabine), is a DNA methyltransferases inhibitor used as a chemotherapy agent that results in global loss of methylation and re-expression of silenced genes. We hypothesised that restoring expression of XPC in melanoma using decitabine could overcome resistance to carboplatin. To confirm low XPC in melanoma tumour tissue, transcript and protein expression from 196 advanced primary and metastatic melanomas was quantified and compared to normal tissue and other cancer types. Melanoma cell lines with low baseline XPC were treated with pharmacological doses of decitabine or carboplatin alone, or in sequential combination. XPC transcript, apoptosis, proliferation, senescence, global and CpG island shore demethylation was quantified. XPC transcript and protein was low in a proportion of melanoma tumours and expression correlated with overall survival. Treatment of melanoma cells with carboplatin alone did not significantly induce XPC expression or increase apoptosis in the majority of melanoma cell lines. Decitabine decreased global methylation (average 44.67% decrease) and increased XPC expression (0.9-3.0 fold increase). Hotspots of demethylation after decitabine treatment in the XPC CpG island shore were detected and could be responsible for the increase in XPC expression. When carboplatin was administered following decitabine, a greater XPC induction (1.5-7.6 fold increase) occurred with significantly increased levels of apoptosis (1.6-2.2 fold increase). This was coupled with a decreased proliferation and the presence of markers of senescence. Knocking down XPC expression with siRNA significantly reduced the effects of combination treatment. The outcomes of this study are evidence that silencing of XPC by methylation is a mechanism of carboplatin resistance in melanoma and sequential combination of decitabine followed by carboplatin may be used to restore GGR function and overcome resistance. Citation Format: Timothy Budden, Ryan J. Davey, Ricardo E. Vilain, Katie A. Ashton, Stephen Braye, Andre van der Westhuizen, Nikola A. Bowden. Global demethylation with decitabine increases DNA repair and sensitizes melanoma to carboplatin. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4811.