Progress towards unlocking the secrets of oestrogen receptor beta in breast cancer

Oestrogen receptor (ER) alpha remains the only reliable biological prognostic marker in breast cancer. A sister molecule, ERβ, has been described, but while ERα predicts a favourable disease outcome, the utility of ERβ as a clinical prognosticator is unclear. ERβ exists as five isoforms (ERβ1–ERβ5), each with a unique exon 8. The aim of our research is to understand the function of ERβ and its isoforms in the normal mammary gland and in breast cancer. We have previously shown high expression of total ERβ in normal gland with declining expression in the transition to breast tumours. LOH analysis in 27 paired samples of tumours and normal breast showed no correlation between LOH and loss of total ERβ expression by immunohistochemistry, indicating the latter was not a mutational event. Instead this was due to methylation as treatment of ERβ-negative cell lines resulted in re-expression of total ERβ at the protein and mRNA level. Furthermore, using methylation-specific PCR, ERβ was methylated in up to 50% of all tumours but not in matched normal gland. Recent immunohistochemical analysis of isoforms ERβ1–ERβ5 using specific well-validated antibodies in 777 invasive breast cancers with long-term clinical follow-up showed nuclear expression of ERβ2 was significantly correlated with tumour size, grade, NPI, overall survival, distant metastasis, death from breast cancer and ERα, PR, AR and BRCA1 expression. ERβ5 was predominantly expressed in high-grade cancers and showed a significant positive correlation with ERβ1. ERβ1, however, was not associated with any other pathological parameters. Using an antibody to detect total ERβ, positive tumours were more likely to develop distant metastasis. Notably, this study also highlighted the importance of cytoplasmic expression of ERβ in dictating outcome, a feature that had previously been reported but the significance of which had not been elucidated. In our study cytoplasmic staining, whether alone or in combination with nuclear staining, was associated with decreased overall survival. In summary, ERβ and its variants do seem to influence the breast cancer outcome. The data accumulated thus far and the importance of its sib ERα in directing breast cancer therapy create an imperative for us to continue to unlock its secrets.