Attempt towards a novel classification of triple-negative breast cancer using immunohistochemical markers

Significant efforts have been made to gain a better understanding of the heterogeneity of triple-negative breast cancers from the histological to the molecular and genomic levels. In this study, we attempted to bring forward gene expression subtypes of triple-negative breast cancer (TBNC) to the clinic, by translating gene stratification to clinically accessible immunohistochemical (IHC) classification. Using IHC analysis, we categorized 154 TBNC cases into three main subclasses. Differences in the frequencies of basic characteristics and clinicopathological parameters between the subtypes were examined using Chi-square tests. We defined three main groups among the 154 triple-negative cases. The basal-like (BL) group expressed cytokeratin (CK) 5/6 and/or CK14 (83 cases), the AR⁺ group demonstrated positivity for androgen receptor (18 cases), and the final group exhibited a CD44+CD24-^/low phenotype (39 cases). There were three overlapping cases between the BL subgroup and the CD44+CD24-/low phenotype subgroup, and 11 unclassified cases. In this new IHC classification, three subcategories exhibited a statistical difference with regard to age, tumor size, histological grade, tumor necrosis, Ki67 labeling index, relapse-free survival, breast cancer-specific survival and response to chemotherapy. According to our definition, the BL group and CD44+CD24-^/low phenotype could be observed in tumors that were not triple-negative, and BL tumors that were triple-negative demonstrated almost undistinguishable clinicopathological characteristics compared with BL tumors that were not triple-negative. The same observation was made with CD44+CD24-/low tumors that were triple-negative vs. CD44+CD24-^/low tumors that were not. The AR+ group demonstrated undistinguishable clinicopathological characteristics compared with the luminal subtype. We successfully distinguished three subtypes exhibiting diverse clinicopathological and prognostic characteristics with the minimum use of IHC markers.