Beyond triple-negative breast cancer: the need to define new subtypes

Advances in the systemic treatment of early breast cancer have led to significant improvements in survival for patients with hormone receptor- and/or HER2-positive disease. In recent years, interest has focused on tumors that lack expression of the estrogen receptor, progesterone receptor and HER2, the so-called triple-negative subgroup. As a group, triple-negative cancers have a relatively aggressive clinical course, with early development of visceral metastases and a poor long-term prognosis. These tumors, however, encompass a wide range of subtypes with varying prognosis, including a number of special types with a good prognosis (e.g., adenoid cystic carcinomas and secretory carcinoma). There is considerable overlap between triple-negative and basal-like tumors; however, microarray studies have demonstrated that the overlap between basal-like and triple-negative cancers is not complete. The similarities between sporadic triple-negative cancers and tumors arising in BRCA1 mutation carriers and the fact that the majority of BRCA1 tumors display a triple-negative phenotype have led to studies demonstrating a potential loss of BRCA1 function in triple-negative cancers and offered potential therapeutic avenues for patients with these cancers. However, it should be noted that triple-negative breast cancers comprise a heterogeneous group of tumors. Understanding the molecular underpinning of distinct subgroups of these cancers is crucial for the identification of novel therapeutic targets and individualization of treatment for patients with triple-negative disease.