A role for methylation of the hMLH1 promoter in loss of hMLH1 expression and drug resistance in ovarian cancer

Abstract

Experimental evidence from several sources has identi®ed a link between mismatch repair de®ciency and cytotoxic drug resistance. Selection for cisplatin resistance in the human ovarian cancer cell line A2780, results in loss of expression of the mismatch repair protein hMLH1 in most (90%) of the resultant cisplatin-resistant cell lines. Here we demonstrate that the cisplatin sensitive parental cell line displays methylation of the promoter of only one hMLH1 allele, but that the resistant cell lines all exhibit hyper-methylation of the promoters of both hMLH1 alleles. Full methylation of all sites tested was found to be invariably associated with loss of hMLH1 expression, whereas a partial increase in methylation appears compatible with either loss or maintenance of expression. In addition treatment of two of the resistant cell lines with 5-azacytidine, a known inhibitor of methylation, results in re-expression of hMLH1. Clonogenic assays demonstrate that the 5-azacytidine treated cells show increased sensitivity to cisplatin. Furthermore, 12.5% (3/ 24) of ovarian tumours show hypermethylation of the hMLH1 promoter. Expression of hMLH1 is absent in the tumours that are hypermethylated, while all the unmethylated tumours still express the protein. This analysis suggests that methylation of the hMLH1 promoter may be a common mechanism for loss of hMLH1 expression, and possibly for cisplatin-resistance, in ovarian cancer.

The E3 ubiquitin ligase EDD is an adverse prognostic factor for serous epithelial ovarian cancer and modulates cisplatin resistance in vitro

“…As in other cancers, the emergence of chemoresistance in ovarian cancer is linked to genes involved in DNA repair pathways, in particular to resistance to cytotoxic agents that act by causing DNA damage such as platinum compounds (Richardson and Kaye, 2005). For example, expression of ERCC1 and XPA, components of the nucleotide excision repair pathways, are associated with platinum resistance in ovarian cancer (Damia et al, 1998;Selvakumaran et al, 2003); methylation and silencing of the mismatch repair gene MLH1 in some ovarian cancers also result in loss of cisplatin sensitivity (Strathdee et al, 1999); and disruption of the FANC/ BRCA pathway by methylation of BRCA1 and FANCF alters sensitivity to cisplatin in ovarian cancer (Taniguchi et al, 2003). As one function of EDD is in the cellular response to DNA damage (Henderson et al, 2002(Henderson et al, , 2006Honda et al, 2002;Munoz et al, 2007), we postulated that the ability of EDD to predict disease recurrence in serous ovarian cancer might similarly be related to the emergence of chemoresistance.…”

Resistance to topoisomerase poisons due to loss of DNA mismatch repair

“…30 This may be the basis for the high frequency of loss of MLH1, rather than other mismatch repair proteins observed in tumors and drug-resistant models, 31 suggesting that loss of DNA mismatch repair is an important resistance mechanism for topoisomerase II as well as for topoisomerase I poisons.…”

Spontaneous development of drug resistance: mismatch repair and p53 defects in resistance to cisplatin in human tumor cells

“…These changes contribute to their overall cisplatin resistance and may have been selected after the p53 and hMLH1 defects as a result of further prolonged exposure to cisplatin. Thirdly, a panel of resistant A2780 cells which were selected by multiple exposures to cisplatin, contained a silent hMLH1 gene which could be reactivated by azadeoxycytidine treatment (Strathdee et al, 1999), and were also defective in p53-dependent p21 induction .…”