Expression and integrity of DNA topoisomerase II isoforms does not explain generic drug resistance in malignant mesothelioma

Abstract.

Purpose: Malignant mesothelioma is a tumour that is highly resistant to a number of different chemotherapy agents, yet the mechanisms by which resistance occurs are poorly understood. The pattern of resistance is consistent with disruption of topoisomerase function or expression. Coupled with this, we have previously noted a common serological reaction to the β isoform of topoisomerase II, suggesting that it may be aberrantly expressed in patients with mesothelioma. Methods: We assessed the expression of topoisomerase II isoforms in sections of primary tumour. We tested a panel of five mesothelioma cell lines for sensitivity to the known topoisomerase-targeting drugs, doxorubicin and etoposide. We sequenced expressed segments of the topoisomerase genes from these cell lines that have previously been associated with drug resistance. We then investigated other potential resistance mechanisms. Results: We found that the β isoform of topoisomerase II was more frequently expressed in primary tumours. Only one of the five cell lines was highly resistant to etoposide and this cell line was found to have a point mutation in the gene for topoisomerase IIα. Protein levels of topoisomerase IIα and β did not correlate with sensitivity to either doxorubicin nor to etoposide. Semiquantitative analysis suggested that there was marked variation in the levels of mRNA expression of MRP, γ-GCS and MDR1. None of these findings could be associated with resistance to chemotherapy. Conclusion: We conclude that mutations in topoisomerase IIα can be associated with extreme resistance of mesothelioma to etoposide. The generic drug resistance of this tumour requires further investigation.