Decreased progesterone receptor isoform expression in luteal phase fallopian tube epithelium and high-grade serous carcinoma

We previously reported that BRCA1/2-mutated fallopian tube epithelium (FTE) collected during the luteal phase exhibits gene expression profiles more closely resembling that of high-grade serous carcinoma (HGSC) specimens than FTE collected during the follicular phase or from control patients. Since the luteal phase is characterised by high levels of progesterone, we determined whether the expression of progesterone receptor (PR) and PR-responsive genes was altered in FTE obtained from BRCA mutation carriers during the luteal phase of the menstrual cycle. RT-qPCR confirmed a decreased expression of PR mRNA in FTE during the luteal phase relative to follicular phase, in both BRCA1/2 mutation carriers and control patients. Immunohistochemistry using isoform-specific antibodies confirmed a low level of both PR-A and PR-B in HGSC and a lower level of staining in FTE samples obtained during the luteal phase compared with the follicular phase. No significant difference in PR-A or PR-B staining was found based on patient BRCA mutation status. Analysis of our previously reported gene expression profiles based upon known PR-A- and PR-B-specific target genes did not partition samples by BRCA mutation status, indicating that overall FTE PR response is not altered in BRCA mutation carriers. HGSC samples grouped separately from other samples, consistent with the observed loss of PR expression. These findings indicate no overall difference in PR signalling in FTE as a function of BRCA mutation status. Thus, the molecular similarity of BRCA1/2-mutated luteal phase FTE and HGSC likely results from an altered response to luteal phase factors other than progesterone.