Molecular apocrine (MA) tumors are estrogen receptor (ER) negative breast cancers characterized by androgen receptor (AR) expression. We analysed a group of 58 transcriptionally defined MA tumors and proposed a new tool to identify these tumors.

We performed quantitative reverse transcription PCR (qRT-PCR) for ER, AR, FOXA1 and AR-related genes, and immunohistochemistry (IHC) for ER, PR, HER2, CK5/6, CK17, EGFR, Ki67, AR, FOXA1 and GCDFP15 and we analysed clinical features.

MA tumors were all characterized by ER(-) AR(+) FOXA1(+) and AR-related genes positive mRNA profile. IHC staining on these tumors showed 93% ER(-), only 58% AR(+) and 90% FOXA1(+). 67% and 57% MA tumors were HER2(3+) and GCDFP15(+), respectively. Almost all MA tumors (94%) had the IHC signature "HER2(3+) or GCDFP15(+)" but none of 13 control basal-like (BL) tumors did. Clinically, MA tumors were rather aggressive, with poor prognostic factors.

MA tumors could be better defined by their qRT-PCR-AR profile than by AR IHC. In addition, we found that "HER2 or GCDFP15" protein overexpression is a sensitive and specific tool to differentiate MA from BL in the context of ER negative tumors. A composite molecular and IHC signature could therefore help to identify MA tumors in the daily practice.