title: Progesterone receptor (PR) variants exist in breast cancer cells characterised as PR negative.
abstract: Progesterone receptor (PR) expression is measured in breast cancer by immunohistochemistry using N-terminally targeted antibodies and serves as a biomarker for endocrine therapeutic decisions. Extensive PR alternative splicing has been reported which may generate truncated PR variant proteins which are not detected by current breast cancer screening or may alter the function of proteins detected in screening. However, the existence of such truncated PR variants remains controversial. We have characterised PR protein expression in breast cancer cell lines using commercial PR antibodies targeting different epitopes. Truncated PR proteins are detected in reportedly PR negative MDA-MB-231 cells using a C-terminally targeted antibody. Antibody specificity was confirmed by immunoblotting following siRNA knockdown of PR expression. We have further demonstrated that alternatively spliced PR mRNA is present in MDA-MB-231 cells and in reportedly PR-negative breast tumour tissue which could encode the truncated PR proteins detected by the C-terminal antibody. The potential function of PR variant proteins present in MDA-MB-231 cells was also assessed, indicating the ability of these PR variants to bind progesterone, interact with a nuclear PR co-factor and bind DNA. These findings suggest that alternative splicing may generate functional truncated PR variant proteins which are not detected by breast cancer screening using N-terminally targeted antibodies leading to misclassification as PR negative.
authors.affiliation: Reproductive and Vascular Biology Research Group, Institute of Cellular Medicine, Medical School, Newcastle University, 3rd Floor William Leech Building, Framlington Place, Newcastle-upon-Tyne NE2 4HH, UK.
authors.forename: David M W
authors.initials: DM
authors.lastname: Cork
authors.forename: Thomas W J
authors.initials: TW
authors.lastname: Lennard
authors.forename: Alison J
authors.initials: AJ
authors.lastname: Tyson-Capper
mesh_terms: D017398:Alternative Splicing; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000235:genetics* / Q000378:metabolism; D002467:Cell Nucleus / Q000235:genetics; D005260:Female; D006801:Humans; D047468:Immunoprecipitation; D011374:Progesterone / Q000494:pharmacology*; D012333:RNA, Messenger / Q000235:genetics; D034741:RNA, Small Interfering / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D011980:Receptors, Progesterone / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014407:Tumor Cells, Cultured
chemical_list: D012333:RNA, Messenger; D034741:RNA, Small Interfering; D011980:Receptors, Progesterone; D011374:Progesterone