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

Our plain-English summary of Expression and integrity of DNA topoisomerase II isoforms does not explain generic drug resistance in malignant mesothelioma, by B.R. McLaren, D. Whitaker, B.W. Robinson, et al (Cancer Chemotherapy and Pharmacology, 2001).

The study

• Authors: B.R. McLaren, D. Whitaker, B.W. Robinson, R.A. Lake
• Journal: Cancer Chemotherapy and Pharmacology
• Published: 2001;48(1):1-8
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

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 beta 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 beta 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 IIalpha. Protein levels of topoisomerase IIalpha and beta 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, gamma-GCS and MDR1. None of these findings could be associated with resistance to chemotherapy.

Conclusion: We conclude that mutations in topoisomerase IIalpha can be associated with extreme resistance of mesothelioma to etoposide. The generic drug resistance of this tumour requires further investigation.

In plain English

This laboratory study from Perth, Australia, examined why malignant mesothelioma resists chemotherapy drugs. The authors stained tumour samples from 18 patients for the alpha and beta forms of topoisomerase II, an enzyme targeted by doxorubicin and etoposide. They also tested five mesothelioma cell lines grown from pleural fluid of patients never given chemotherapy. They measured drug sensitivity, sequenced parts of the topoisomerase genes, and measured three other resistance markers (MDR1, MRP and gamma-GCS). The beta form was common in tumours: 15 of 17 samples had over 25% of cells stained. The alpha form was low: 16 of 18 samples had under 10%. Only one cell line, STY51, was highly resistant to etoposide; it carried a point mutation in the alpha-form gene. Enzyme levels did not match etoposide sensitivity. The other markers varied but did not explain sensitivity differences among the cell lines, though the authors suggested highly expressed MRP may play a role in mesothelioma's general resistance. The authors concluded that this mutation may be associated with extreme etoposide resistance and that mesothelioma's general drug resistance needs further investigation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory study: immunohistochemistry (tissue staining) of tumour biopsies, plus drug-sensitivity testing, gene sequencing and expression measurements in mesothelioma cell lines
• Who was studied: Tumour samples from 18 unselected patients with mesothelioma (16 men, 2 women, aged 49 to 82). Five human mesothelioma cell lines (JU77, LO68, NO36, ONE58, STY51) grown from the pleural fluid of patients who had not received chemotherapy, at a laboratory in Perth, Western Australia. Two non-mesothelioma cell lines (A549 lung, HT29 colon) were used as controls.
• Exposure: The chemotherapy drugs doxorubicin and etoposide, both topoisomerase II inhibitors, tested on cell lines. A patient table records each patient's asbestos exposure history (yes, no, probable or possible), but asbestos exposure was not analysed.
• Main results:
  • In the tumour samples, staining for topoisomerase II alpha was low: 16 of 18 samples had fewer than 10% of cells positive. Staining for topoisomerase II beta was much more common: 15 of 17 samples had more than 25% of cells positive, and 9 of 17 had more than 50%.
  • No correlation was found between survival and any of the tumour staining results.
  • The five mesothelioma cell lines differed about threefold in sensitivity to doxorubicin. For etoposide the difference was more than 30-fold. STY51 was the least sensitive to etoposide, and NO36 the most sensitive.
  • STY51, the etoposide-resistant cell line, had a point mutation at position 508 of topoisomerase II alpha. It appeared on only one copy of the gene (monoallelic).
  • In topoisomerase II beta, changes recurring in several cell lines were judged likely to be normal genetic variants (polymorphisms). Only ONE58 and LO68 carried mutations in the examined segment.
  • In the cell lines, the amount of topoisomerase II alpha protein varied about 20-fold. It correlated with sensitivity to doxorubicin (r2=0.86) but not with sensitivity to etoposide. Topoisomerase II beta levels varied 2.5-fold and showed a weak inverse correlation with resistance to doxorubicin (r2=0.52).
  • Two cell lines (ONE58 and STY51) showed only weak MDR1 expression but were no more sensitive to the drugs. The authors judged MDR1 unlikely to be the main factor. All cell lines expressed MRP and gamma-GCS, with a more than 100-fold difference in gamma-GCS between the highest (LO68) and lowest (STY51). Gamma-GCS levels did not correlate with doxorubicin sensitivity.
  • MRP was expressed at levels similar to the control cell line A549. The authors suggested that this pump may play a role in the general resistance of mesothelioma to chemotherapy.
• The authors' conclusion: The authors concluded that topoisomerase II beta was expressed more commonly than the alpha form in mesothelioma biopsies. They also concluded that the topoisomerase II alpha mutation found in STY51 may be associated with extreme resistance to etoposide, and that the generic drug resistance of mesothelioma requires further investigation.
• Limitations the authors state:
  • Only a small portion of each topoisomerase gene was examined, so mutations elsewhere in the genes could have been missed.
  • The cell lines came from patients never treated with chemotherapy. Earlier resistance studies mostly used cells exposed to a drug over long periods, which is more likely to produce specific mutations.
  • Other non-specific resistance features, such as changes in apoptotic (cell-death) factors, were not examined in this study.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Malignant Mesothelioma, Chemotherapy, Mesothelioma, Doxorubicin.

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