Prognostic significance of progesterone receptor-positive tumor cells within immunohistochemically defined luminal A breast cancer

Abstract

Purpose: Current immunohistochemical (IHC)-based definitions of luminal A and B breast cancers are imperfect when compared with multigene expression-based assays. In this study, we sought to improve the IHC subtyping by examining the pathologic and gene expression characteristics of genomically defined luminal A and B subtypes.

Patients and methods: Gene expression and pathologic features were collected from primary tumors across five independent cohorts: British Columbia Cancer Agency (BCCA) tamoxifen-treated only, Grupo Español de Investigación en Cáncer de Mama 9906 trial, BCCA no systemic treatment cohort, PAM50 microarray training data set, and a combined publicly available microarray data set. Optimal cutoffs of percentage of progesterone receptor (PR) -positive tumor cells to predict survival were derived and independently tested. Multivariable Cox models were used to test the prognostic significance.

Results: Clinicopathologic comparisons among luminal A and B subtypes consistently identified higher rates of PR positivity, human epidermal growth factor receptor 2 (HER2) negativity, and histologic grade 1 in luminal A tumors. Quantitative PR gene and protein expression were also found to be significantly higher in luminal A tumors. An empiric cutoff of more than 20% of PR-positive tumor cells was statistically chosen and proved significant for predicting survival differences within IHC-defined luminal A tumors independently of endocrine therapy administration. Finally, no additional prognostic value within hormonal receptor (HR) -positive/HER2-negative disease was observed with the use of the IHC4 score when intrinsic IHC-based subtypes were used that included the more than 20% PR-positive tumor cells and vice versa.

Conclusion: Semiquantitative IHC expression of PR adds prognostic value within the current IHC-based luminal A definition by improving the identification of good outcome breast cancers. The new proposed IHC-based definition of luminal A tumors is HR positive/HER2 negative/Ki-67 less than 14%, and PR more than 20%.

Fig 1.

Expression of the hormonal receptors in the Grupo Español de Investigación en Cáncer de Mama 9906 data set. (A) Estrogen receptor (ER) gene (ESR1) and (B) progesterone receptor (PR) gene (PGR) as assayed using quantitative reverse-transcriptase polymerase chain reaction expression in luminal A and B tumors. Density plots based on the percentage of (C) ER-positive and (D) PR-positive tumor cells as assessed by immunohistochemistry.

Fig 2.

Kaplan-Meier survival analysis within immunohistochemical-based luminal A tumors (hormone receptor positive/HER2 negative/Ki-67 < 14%) based on the percentage of progesterone receptor (PR) –positive tumor cells. (A) Grupo Español de Investigación en Cáncer de Mama 9906 cohort. (B) British Columbia Cancer Agency–no adjuvant systemic therapy cohort.

Fig 3.

Disease-free survival log likelihood ratio (LR) statistics of six different predictive models (A–E) in patients of the Grupo Español de Investigación en Cáncer de Mama 9906 cohort with hormone receptor (HR) –positive/human epidermal growth factor receptor 2 (HER2) --negative breast cancer. The variables evaluated were the following: immunohistochemical (IHC)-based scoring of estrogen receptor, progesterone receptor, HER2, and Ki-67 (IHC4 score; continuous variable), IHC-based subtypes (HR positive/HER2 negative/Ki-67 < 14% > 20% [luminal A], HR positive/HER2 negative/Ki-67 < 14% ≤ 20% and HR positive/HER2 negative/Ki-67 > 14% [luminal B]), and PAM50 risk of recurrence score based on subtype and proliferation (ROR-P; continuous variable). (*) P < .05.

Fig A1.

Luminal A and B gene expression and survival differences. (A) Supervised hierarchical clustering of 1,539 genes found differentially expressed (false discovery rate < 1%) between luminal A and B tumors in the PAM50 microarray training data set (GSE10886). (B) Comparison of the expression of selected genes between luminal A and B tumors. (*) P < .05, t test. (C) Kaplan-Meier relapse-free survival analysis of the luminal A and B tumors identified in the combined publicly available microarray data sets. The P value shown reflects the value obtained from a multivariable Cox model that included the following variables: PAM50 subtype, data set, estrogen receptor/progesterone receptor status, nodal status, histologic grade, and tumor size.

Fig A2.

Penalized spline method on multivariable Cox regression model of distant recurrence–free survival within immunohistochemical-based luminal A tumors based on the percentage of progesterone receptor (PR) –positive cells. Event count: 102 of 364. Cox model covariates: + age_at_diagnosis + as.factor(grade > 2) +as.factor(nodestat) + size_lesion. Blue dotted lines indicate the CIs.

Fig A3.

Evaluation of borderline progesterone receptor (PR) –tumor samples within hormone receptor (HR) –positive/human epidermal growth factor receptor 2 (HER2) –negative/Ki-67 less than 14% disease. Selected immunohistochemistry (IHC) images with different PR positivity (2%, 20%, and 100%) from the Grupo Español de Investigación en Cáncer de Mama 9906 cohort. All IHC-based tissue microarray images of both British Columbia Cancer Agency cohorts can be obtained at www.gpecimage.ubc.ca/.