Adjuvant tamoxifen in breast cancer : clinical and preclinical studies on the prediction value of estrogen receptor

Abstract: Breast cancer (BC) exhibits great heterogeneity at histophatological, clinical and molecular levels. However, the different clinical outcomes in patients with seemingly similar breast cancer have led scientists to search for subgroups or for factors and characteristics related to the tumor or the patient that could anticipate clinical course (prognosis) of disease and/or response to given therapy (prediction). Estrogen receptor (ER) is the first molecule identified that has had great influence on the management of breast cancer. This thesis focuses on the role of ER and its significance in breast cancer. In one study, we compared the potential of ER-positive tamoxifen sensitive cells (MCF-7) versus ER- negative cells (MDA-231) to handle DNA repair, transmit signals from DNA damage, initiate apoptosis, control transmitted signals from the cell cycle and synthesize growth factors and receptors. Genes related to these processes were studied by cDNA microarray. We found that the ER-negative cells were characterized by a higher expression of growth factors and cell cycle regulation components, and improved DNA repair. We explored the long-term pattern of disease recurrence among pre-and post-menopausal patients with primary BC according to ER status. The patients were randomly given tamoxifen versus no systemic therapy. The results showed a reduction of locoregional, distant metastases and breast cancer death in ER-positive patients who received tamoxifen. The pattern of metastases was not different in these two groups. The conclusion was that the differences in term of gene expression appeared mainly to be related to endocrine sensitivity and not metastatic potential. Some more events in the first 5 years in ER-negative patients suggested that ER negativity in some cases is correlated with an increased tumour growth rate. ER had been measured by cytosol assays prior to around 1990 when these assays substituted of immunohistochemical (IHC) assay. However, ER predictive ability of response to tamoxifen has been assessed based on ER measurement by cytosol assays. We compared these two assays in a clinical trial and found a high concordance between the assays and concluded that IHC is as accurate as cytosol assays to predict long term response to adjuvant tamoxifen. The introduction of microarray technique a decade ago already has changed our knowledge of BC but it has some pitfalls that question its potential. In two methodical studies we showed the importance of tissue handling, the effect of heterogeneity of BC and standardization on the result from cDNA microarray. This thesis confirms the importance of ER in BC but also indicates a more complex phenotypic beyond that which can be explained purely by ER content or endocrine sensitivity. Microarray technique can provide useful information besides the traditional one but requires standardization of sample collection, storage, processing, normalization, interpretation of data and requires validation by large studies.