Development of resistance against anti-cancer drugs is one of the major obstacles of chemotherapy in the treatment of cancer. Etoposide is a topoisomerase II alpha (TOP2A) inhibitor, which is used in the treatment of breast cancer. Alterations in the expression of drug targets or DNA repair genes are among the important resistance mechanisms against TOP2A inhibitors. In this study, expression changes in TOP2A gene and two important mismatch repair (MMR) genes MSH2 and MLH1 were examined in order to understand the relationship between differential expression of these genes and drug resistance against etoposide.

Resistant cell lines were developed from parental MCF7 cell line by stepwise selection in increasing doses of etoposide. Total RNA was isolated from parental and resistant cell lines by using TriReagent. Expression levels of TOP2A, MSH2 and MLH1 were analysed by reverse transcription quantitative polymerase chain reaction (RT-qPCR). Statistical analyses were performed by one way ANOVA.

Etoposide resistant sublines MCF7/1000E, MCF7/1250E and MCF7/2000E were approximately 2, 3 and 4 fold resistant relative to parental MCF7/S cells, respectively. TOP2A, MSH2 and MLH1 expressions decreased in etoposide resistant sublines relative to MCF7/S cells. Expression levels of TOP2A and MLH1 in resistant sublines differed between 10–95 and 18–58 percent of the expression levels in the parental cells, respectively. MSH2 expression levels were decreased 18–82 percent in resistant cells. A transient 15 percent increase in the expression of this gene was observed in subline MCF7/1250E.

Decrease in the expression levels of TOP2A, MSH2 and MLH1 may play significant roles in the development of chemotherapeutic resistance to etoposide in breast cancer. These genes may be considered for further development of new strategies to overcome resistance against topoisomerase II inhibitors.