Classification of triple-negative breast cancer subtypes based on heterogeneity and comparison with basal-like carcinoma
triple-negative breast cancer
Immunohistochemical and fluorescence in situ hybridization（FISH）methods have been implemented to characterize triple negative breast cancer（TNBC）due to the lack of expression of estrogen receptor（ER）and progesterone receptor（PgR）as well as human epidermal growth factor receptor ２（HER２）. TNBC exhibits adverse prognostic features, as both hormone and anti-HER２ therapies are ineffective, and thus chemotherapy is required.
Among the intrinsic subtypes identified through gene expression analysis, the expression of ER and HER２cluster was found to be low specifically in the basal-like subtype. The basal-like subtype exhibits mammary myoepithelial cell（or basal cell）-like gene expression pattern. Most of the cells were triple negative in immunohistochemical method, and exhibited a characteristic expression of myoepithelial or basal cell markers, such as cytokeratin（CK）５／６, CK１４, CK１７, p６３, and epidermal growth factor receptor（EGFR）. Basal-like carcinoma is similar to TNBC in terms of histology and clinical features. Here, we have presented some of our study results indicating the characteristics of TNBC and basal-like phenotype.
TNBC is a heterogeneous disease, and ongoing research is aimed at developing individualized treatment based on its biological characteristics. However, the effective treatment methods and therapeutic efficacy predictors are yet to be discovered. Recently, targeted therapies for TNBC, including the poly（ADP-ribose）polymerase（PARP）inhibitors for germline BRCA mutation-associated breast cancer and immune checkpoint inhibitors, have been approved. Moreover, the level of programmed death ligand１（PD-L１）in tumor-infiltrating immune cells is determined using the histopathological analysis of specimens to assess the effect of immune checkpoint inhibitors.
In this review, the classification of TNBC subtypes and characteristics of basal-like carcinoma has been discussed.
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