Identification of triple-negative and basal-like canine mammary carcinomas using four basal markers.

Molecular-based classification of canine mammary carcinomas (CMCs) has been a recent research focus. In human breast cancer, triple-negative and basal-like phenotypes are distinct molecular subgroups that are known for their poor prognosis, but these tumours are not yet well defined in the dog. The aim of this study was to determine whether CMCs include triple-negative and basal-like phenotypes by immunohistochemical assessment of expression of the oestrogen receptor (OR), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2) and four basal markers, cytokeratin (CK) 14, CK5/6, p63 and the epidermal growth factor receptor (EGFR). In this study of 241 CMCs, 45 triple-negative tumours (OR(-), PR(-) and HER2(-)) were identified and this phenotype was associated with an unfavourable prognosis. In these tumours, the expression of CK14, CK5/6 and EGFR was related to clinicopathological parameters, while the expression of p63 was not relevant. The majority of the triple-negative tumours were of the basal-like phenotype, given that 75.6% of them expressed more than two basal markers. However, three of the basal markers were not uniformly expressed; therefore, the proportion of the basal-like phenotype was altered on the basis of the selection of the markers. Although both triple-negative and basal-like phenotypes are distinct entities in CMC, further study is needed to differentiate one from the other.