Tests that use gene expression-profiling to help guide treatment decision-making in breast cancer, such as OncotypeDx, were largely validated in postmenopausal women. It remains unclear whether they are suitable for use in premenopausal women, where hormones estrogen and progesterone fluctuate dramatically during the menstrual cycle. To address this, we have used two mouse models of breast cancer to define how ovarian hormones affect breast cancer gene expression and subsequent OncotypeDx diagnosis.
The effect of ovarian hormones on estrogen receptor-positive breast cancer cell line T47D, was examined in mammary fat pad-xenografted BalbC/NUDE mice, treated with exogenous estrogen±progesterone (n=12, n=11 respectively). Our second model investigated the effect of fluctuations in ovarian hormones on mammary tumours collected from naturally cycling MMTV-PyMT mice, at either the estrus or diestrus phase of the ovarian cycle (n=30/group). For both mouse models, gene expression was assessed using quantitative RT-PCR.
Significant changes in the expression of 6/16 OncotypeDx signature genes were observed in xenografted tumours dissected from progesterone and estrogen treated mice, compared to estrogen only treated mice (p≤0.05). Through immunohistochemical analysis these tumours also showed reduced progesterone receptor (p=0.02) and Ki67 (p=0.02) protein expression. No changes were observed in estrogen receptor protein expression (p=0.20). Furthermore, tumours collected from naturally cycling MMTV-PyMT mice at diestrus, when circulating concentrations of progesterone are highest, showed significant differences in 8/16 OncotypeDx signature genes (p≤0.05), and a significant increase in their OncotypeDx recurrence score (15.9±1.9; mean±SEM), compared to tumours dissected at estrus (11.7±1.7; p≤0.05).
Together our studies suggest ovarian hormones significantly alter breast cancer biomarker expression and OncotypeDx recurrence scores in mouse models. If used in premenopausal women, OncotypeDx may give a different diagnosis depending on the woman’s menstrual cycle stage at the time of tissue collection. We are currently investigating this in a prospective study in premenopausal women with breast cancer.