P1-07-09: Estrogen Receptor (ER) mRNA and ER-Related Gene Expression in Breast Cancers That Are 1%-10% ER-Positive by Immunohistochemistry.

Abstract

Abstract Purpose: Our goal was to examine whether borderline estrogen receptor (ER)-positive cancers, defined as 1–10% positivity by immunohistochemistry (IHC), show the same global gene expression pattern and high ESR1 mRNA expression as ER-positive cancers or are more similar to ER-negative cancers. Patients and methods: ER status was determined by IHC in 465 primary breast cancers and with Affymetrix U133A gene chip (ESR1 mRNA gene expression: Probe set = 205225_at). We compared expressions of ESR1 mRNA and a 106-probe set ER-associated gene signature score between ER-negative (n=183), 1–9% (n=25), exactly 10% (n=6), and > 10% ER-positive (n=251) cancers. We also assessed the molecular class of the borderline ER-positive cases using the PAM-50 classifier. Results: Among the 1–9%, 10% and > 10% IHC positive cases, 24%, 67% and 92% were also ER-positive by ESR1 mRNA expression. The average ESR1 expression was significantly higher in the > 10% IHC-positive cohorts compared to the 1–9% or completely negative cases but in these latter two cohorts ER expression was similarly low. The average ER gene signature scores were similar for the ER-negative and 1–9% IHC-positive cases, but significantly lower than in > 10% ER-positive cases. None of the 1–9% ER-positive cases were classified as Luminal A, 2 were Luminal B and 12 were Basal-like. Among the 10% ER-positive cases, 2 were Luminal A and 1 was Luminal B. Conclusion: Overall, 24% of the 1–9% and 67% of the 10% ER-positive cancers show ESR1 mRNA levels and gene signatures that are consistent with ER-positive, potentially endocrine sensitive tumors. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-07-09.