Background: Oestrogen receptor beta (ERbeta) modulates ERalpha activity; wild type ERbeta (ERbeta1) and its splice variants may therefore impact on hormone responsiveness of breast cancer. ERbeta2/ERbetacx acts as a dominant negative inhibitor of ERalpha and expression of ERbeta2 mRNA has been proposed as a candidate marker for outcome in primary breast cancer following adjuvant endocrine therapy. We therefore now assess ERbeta2 protein by immunostaining and mRNA by quantitative RT-PCR in relation to treatment outcome.
Methods: ERbeta2-specific immunostaining was quantified in 141 primary breast cancer cases receiving adjuvant endocrine therapy, but no neoadjuvant therapy or adjuvant chemotherapy. The expression of mRNA for ERbeta2/ERbetacx was measured in 100 cases by quantitative RT-PCR. Statistical analysis of breast cancer relapse and breast cancer survival was performed using Kaplan Meier log-rank tests and Cox's univariate and multivariate survival analysis.
Results: High ERbeta2 immunostaining (Allred score >5) and high ERbeta2 mRNA levels were independently associated with significantly better outcome across the whole cohort, including both ERalpha positive and negative cases (Log-Rank P < 0.05). However, only ERbeta2 mRNA levels were significantly associated with better outcome in the ERalpha + subgroup (Log-Rank P = 0.01) and this was independent of grade, size, nodal status and progesterone receptor status (Cox hazard ratio 0.31 P = 0.02 for relapse; 0.17 P = 0.01 for survival). High ERbeta2 mRNA was also associated with better outcome in node negative cases (Log Rank P < 0.001).ERbeta2 protein levels were greater in ERalpha positive cases (T-test P = 0.00001), possibly explaining the association with better outcome. Levels of ERbeta2 protein did not correlate ERbeta2 mRNA levels, but 34% of cases had both high mRNA and protein and had a significantly better outcome (Log-Rank relapse P < 0.005).
Conclusion: High ERbeta2 protein levels were associated with ERalpha expression. Although most cases with high ERbeta2 mRNA had strong ERbeta2 immunostaining, mRNA levels but not protein levels were independently predictive of outcome in tamoxifen-treated ERalpha + tumours. Post-transcriptional control needs to be considered when assessing the biological or clinical importance of ERbeta proteins.