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Oestrogen receptor gene structure and function in breast cancer
C K Watts1, M L Handel, R J King
1Cancer Biology Division, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, NSW, Australia.
Abstract:
The mechanisms underlying loss of oestrogen responsiveness in breast cancer are not well-defined. Potential mechanisms include loss of receptor expression, alterations in the oestrogen receptor (ER) gene producing proteins with abnormal function, or changes to receptor-dependent or -independent pathways controlling cell proliferation. Examination by Southern analysis of the ER gene in a series of ER-negative and -positive breast tumour biopsies failed to provide evidence of gross rearrangements and in only one of thirty seven tumour DNA samples was significant gene amplification observed. No restriction fragment length polymorphisms were detected for the restriction enzymes EcoR I, Pst I or Hind III. Methylation of the ER gene as assessed by Hpa II and Msp I restriction enzyme digests varied between tumours but the degree of methylation was not correlated with levels of expression of the receptor protein. Similar findings applied in a series of ER-negative and -positive breast cancer cell lines and clonal lines of MCF-7 cells, which were developed as an in vitro model for the acquisition of oestrogen and antioestrogen resistance. In this model there was no evidence that changes to ER receptor function and/or structure at the level of the ER gene, mRNA, ligand binding, and ability to induce progesterone receptor might account for the development of hormone resistance. However, the ability of ER to interact with a DNA sequence containing the vitellogenin promoter oestrogen response element, as assessed by gel retardation assay, was impaired in the clone showing the greatest degree of oestrogen and antioestrogen resistance.
Insights
Loss of oestrogen responsiveness in breast cancer is not fully understood. Researchers found impaired oestrogen receptor (ER) DNA binding in resistant cells, suggesting this interaction is key to hormone resistance in breast cancer.
Area of Science:
- Oncology
- Molecular Biology
- Genetics
Background:
- Oestrogen responsiveness is crucial in breast cancer treatment.
- Mechanisms of acquired resistance to oestrogen and anti-oestrogen therapies are not well-defined.
- Potential causes include altered oestrogen receptor (ER) expression, function, or signaling pathways.
Purpose of the Study:
- To investigate the molecular mechanisms behind the loss of oestrogen responsiveness in breast cancer.
- To examine the role of the oestrogen receptor (ER) gene and its function in acquired hormone resistance.
- To identify specific alterations in ER function that contribute to treatment resistance.
Main Methods:
- Southern analysis of the ER gene in breast tumor biopsies and cell lines.
- Restriction fragment length polymorphism analysis.
- Assessment of ER gene methylation using Hpa II and Msp I digests.
- Development of an in vitro model of oestrogen and anti-oestrogen resistance using MCF-7 cells.
- Gel retardation assays to assess ER DNA binding activity.
Main Results:
- No gross rearrangements or significant amplification of the ER gene were found in most tumors.
- ER gene methylation levels did not correlate with receptor expression.
- In vitro models showed no significant changes in ER gene, mRNA, ligand binding, or progesterone receptor induction.
- Impaired ability of ER to interact with DNA was observed in cells with high resistance.
Conclusions:
- Gross alterations in the oestrogen receptor (ER) gene are not the primary cause of resistance.
- While ER gene methylation varies, it does not directly correlate with ER expression levels.
- The study highlights impaired ER DNA binding as a critical factor in acquired oestrogen and anti-oestrogen resistance in breast cancer.