Ovarian cancer stem cell like side populations are enriched following chemotherapy and overexpress EZH2
Siân Rizzo,1 Jenny M. Hersey,1 Paul Mellor,1 Wei Dai,2 Alessandra Santos-Silva,1 Daniel Liber,2Louisa Luk,2,3 Ian Titley,4 Craig P Carden,1 Garry Box,5 David L. Hudson,1 Stanley B. Kaye,1 andRobert Brown1,2
Platinum-based chemotherapy, with cytoreductive surgery, is the cornerstone of treatment of advanced ovarian cancer, however acquired drug resistance is a major clinical obstacle. It has been proposed that subpopulations of tumour cells with stem-cell like properties, such as so-called side populations (SP) which over-express ABC drug-transporters, can sustain the growth of drug resistant tumour cells, leading to tumour recurrence following chemotherapy. The histone methyltransferase EZH2 is a key component of the Polycomb Repressive Complex 2 (PRC2) required for maintenance of a stem cell state and overexpression has been implicated in drug resistance and shorter survival of ovarian cancer patients. We observe higher percentage SP in ascites from patients that have relapsed following chemotherapy compared to chemonaive patients, consistent with selection for this subpopulation during platinum-based chemotherapy. Furthermore, ABCB1 (P-glycoprotein) and EZH2 are consistently over-expressed in SP compared to non-SP from patients’ tumour cells. SiRNA knockdown of EZH2 leads to loss of SP in ovarian tumour models, reduced anchorage-independent growth and reduced tumour growth in vivo. Together these data support a key role for EZH2 in the maintenance of a drug-resistant tumour-sustaining subpopulation of cells in ovarian cancers undergoing chemotherapy. As such, EZH2 is an important target for anticancer drug development.