Inactivation of the retinoblastoma tumour suppressor pathway in premenopausal breast cancer is associated with resistance to tamoxifen

Objective

shRNA mediated knockdown of the retinoblastoma tumour suppressor (pRb) in oestrogen receptor (ER⁺) cell lines leads to resistance to tamoxifen, and pRb inactivation has further been associated with more aggressive disease. By studying the tamoxifen response in premenopausal patients randomised to either control or tamoxifen treatment, we aim to determine the importance of pRb inactivation in relation to tamoxifen response.

Methods

Breast cancer samples were assembled in tissue microarrays, immunohistochemically stained for phos-pRb and evaluated as the fraction of positive nuclei divided into four groups: 0%; 1 to 10%; 11 to 25%; and 26 to 100%. pRb-inactivated tumours were defined using the phos-Rb parameter in combination with the proliferation marker Ki67. Tumours with none or low expression of phos-pRb displaying a high proliferation rate were defined as pRb inactivated (n = 57), whereas the remaining tumours were considered to have a functional pRb pathway (n = 273).

Results

Inactivation of pRb was significantly correlated to larger tumours (P = 0.001), lymph node-negative disease (P = 0.001) and a higher histological grade (P < 0.001). There was a positive correlation to cyclin E levels (P < 0.001) but a negative correlation to cyclin D₁ (P < 0.001) and ER as well as progesterone receptor levels (both P < 0.001). A significant difference was noted in recurrence-free survival when comparing no treatment with tamoxifen treatment in the patient group with functional pRb (P = 0.003); however, the beneficial effect of tamoxifen was lost in the pRb-inactivated group (P = 0.619). A multivariate analysis confirmed that inactivation of pRb was significantly associated with impaired tamoxifen response.

Conclusion

The functional inactivation of pRb seems to be part of the explanation to why a subgroup of ER⁺ tumours does not respond to tamoxifen. An evaluation of pRb status in ER⁺ tumours could therefore possibly contribute to a more effective treatment.