Clinics are increasingly adopting gene-expression profiling to diagnose breast cancer subtype, and help guide treatment decisions. However, despite their availability to premenopausal women, these tests have been developed and validated predominantly in postmenopausal women. In premenopausal women, ovarian hormones estrogen and progesterone fluctuate dramatically during the menstrual cycle. The impact of fluctuating hormones on breast cancer gene expression and subtype diagnosis remains unknown. This study aimed to investigate how estrogen and progesterone effect the expression of genes involved in breast cancer subtype diagnosis.
Two breast cancer cell lines (ZR-75-1 and T-47D) were pre-treated in triplicate with 10nM estrogen for 72 hours, prior to treatment with either 10nM progesterone or a vehicle control for 16 hours. The abundance of mRNA encoding key genes used in diagnostic tests, including estrogen receptor (ESR1), progesterone receptor (PGR), epidermal growth factor receptor (EGFR), B-cell lymphoma 2 (BCL2), and forkhead box transcription factor (FOXA1) was quantified.
Co-treatment of T-47D cells (n=5) with estrogen and progesterone resulted in decreased ESR1 (p=0.002) and PGR (p<0.0001) expression, and increased EGFR (p=0.008) expression, in comparison to estrogen treatment alone. Consistent with the loss of ESR1 expression, the expression of estrogen-regulated genes was significantly lower in cells co-treated with estrogen and progesterone (p=0.003, p=0.01 for BCL2 and FOXA1 respectively). Consistent with these results, co-treatment of ZR-75-1 cells (n=5) with estrogen and progesterone also resulted in decreased expression of ESR1 (p=0.007), BCL2 (p=0.02) and PGR (p=0.01), and increased EGFR expression (p=0.008), compared to estrogen treatment alone.
These findings demonstrate that ovarian hormones significantly alter the expression of key genes on which subtyping tests rely heavily for their diagnosis. Consequently, it is possible that breast cancer gene expression—and the treatment trajectories that stem from its measurements—could fundamentally depend on a woman’s menstrual cycle stage at the time of tissue collection.