Abstract P4-09-13: Breast cancer recurrence risk: A role for the progesterone receptor isoforms
Authors: Gass HD , May M , Rojas P , Abba M , Sequeira G , Martinez Vazquez P , Gonzalez PL , Elía A , Alvarez MM , Molinolo A , Lanari CL 
Published: May 2016

Progesterone receptor (PR) is currently used as a surrogate marker for functional estrogen receptor activity in breast cancer. Two PR isoforms have been described, PRB and PRA. PRA (94 kDa) is a truncated protein that lacks the first 164 amino acids of the NH2 terminal of PRB (115 kDa), making difficult the development of antibodies that discriminate PRA from PRB by standard immunohistochemistry (IHC). There are few studies describing the expression of PR isoforms in breast cancers. While there is a general consensus that PRA is the prevailing isoform expressed in breast cancer tissues as compared with normal mammary gland, there is controversial data regarding the association between their deregulated expression and endocrine response or aggressiveness. We are currently studying the expression of the PR isoform ratio in breast cancer samples obtained during surgical resection in order to test their antiprogestin responsiveness in tissue cultures. The study has been approved by the IRB (2012-028). Selected samples were studied by RNAseq and the data was used to analyze the PAM50 genes to predict risk of recurrence, and interestingly, almost all the genes related to proliferation were up-regulated in samples categorized as having higher levels of PRB than PRA, being also these patients those with a high risk of recurrence (May and Rojas et al., ASCO Annual Meeting, poster#11016, 2015). These observations are in agreement with data obtained in hormone resistant breast cancer xenografts with higher levels of PRB than PRA (Wargon and Riggio et al, International Journal of Cancer: 2680, 2015). The aim of this study is to evaluate a possible correlation between the PR isoform ratio, proliferation as evaluated by Ki67 or HER2 expression, and clinical outcome in selected breast cancer samples. Ki67 was evaluated by IHC using standard protocols in 80 PR+ samples. The PRA/PRB ratio was also evaluated in nuclear extracts, performed from frozen tissue, from the same patients, by Western Blot. A negative correlation was observed between the Ki67 score and the log2 value of the PRA/PRB ratio (Spearman R:-0.3418; p&lt; 0.0029). Samples were considered PRA+ if PRA/PRB ≥ 1.2 and PRB+, if PRA/PRB ≤ 0.83. Seven out of 62 PRA+ (11.29%), and 7 out of 35 (20%) of PRB+ samples were HER2+. The differences between both groups, although not significant, correlate directly with the Ki67 evaluation. The results of this ongoing project lend support to the hypothesis that the ratio of PRA/PRB is associated with prognosis and highlight the role of PR as key players regulating breast cancer growth.

Citation Format: Gass HD, May M, Rojas P, Abba M, Sequeira G, Martinez Vazquez P, Gonzalez PL, Elía A, Alvarez MM, Molinolo A, Lanari CL. Breast cancer recurrence risk: A role for the progesterone receptor isoforms. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-09-13.