Using DNA microarray analysis breast tumors can be divided into 5 subtypes:
1) Luminal A: Tumor cells originate from luminal epithelium and have high levels of ER expression, express cytokeratin (CK) 8 and 18, are low grade, are less responsive to chemotherapy and have a good prognosis
2) Luminal B: Similar to Luminal A with a different gene expression profile. Prognosis in this subtype is slightly worse than in Luminal A.
3) Basal-like: Express markers of basal or myoepithelial cells CK 5/6, CK 8/18, vimentin, smooth muscle actin and EGFR. Tumors are ER, PR and HER negative (Triple negative). P53 mutations are common in this subtype. Tumors tend to be aggressive and this subtype includes BRCA-1 mutant tumors. This is a heterogenous group and only 70% of triple negative breast tumors fall into this subtype.
4) HER-2 amplified: Tumors have amplificated HER gene located on the long arm of chromosome 17 and are usually ER and PR negative but have upregulation of vascular endothelial growth factor (VEGF). The aggressive behavior of these tumors has been tempered with the availability of trastuzumab.
5) Normal breast-like: Tumors have gene expression profile similar to normal breast epithelium and prognosis is similar to Luminal B subtype
This molecular classification may help us better understand the biology of breast tumors and thus develop and plan therapy accordingly