Evolving concepts in HER2 evaluation in breast cancer: Heterogeneity, HER2-low carcinomas and beyond.

The human epidermal growth factor receptor 2 (HER2) is a well-known negative prognostic factor in breast cancer and a target of the monoclonal antibody trastuzumab as well as of other anti-HER2 compounds. Pioneering works on HER2-positive breast cancer in the 90s' launched a new era in clinical research and oncology practice that has reshaped the natural history of this disease. In diagnostic pathology the HER2 status is routinely assessed by using a combination of immunohistochemistry (IHC, to evaluate HER2 protein expression levels) and in situ hybridization (ISH, to assess HER2 gene status). For this purpose, international recommendations have been developed by a consensus of experts in the field, which have changed over the years according to new experimental and clinical data. In this review article we will document the changes that have contributed to a better evaluation of the HER2 status in clinical practice, furthermore we will discuss HER2 heterogeneity defined by IHC and ISH as well as by transcriptomic analysis and we will critically describe the complexity of HER2 equivocal results. Finally, we will introduce the clinical impact of HER2 mutations and we will define the upcoming category of HER2-low breast cancer with respect to emerging clinical data on the efficacy of specific anti-HER2 agents in subgroups of breast carcinomas lacking the classical oncogene addition dictated by HER2 amplification.

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.

Endocytosis and intracellular trafficking of ErbBs.

This review article describes the pathways and mechanisms of endocytosis and post-endocytic sorting of the EGF receptor (EGFR/ErbB1) and other members of the ErbB family. Growth factor binding to EGFR accelerates its internalization through clathrin-coated pits which is followed by the efficient lysosomal targeting of internalized receptors and results in receptor down-regulation. The role of EGFR interaction with the Grb2 adaptor protein and Cbl ubiquitin ligase, and receptor ubiquitination in the clathrin-dependent internalization and sorting of EGFR in multivesicular endosomes is discussed. Activation and phosphorylation of ErbB2, ErbB3 and ErbB4 also results in their ubiquitination. However, these ErbBs are internalized and targeted to lysosomes less efficiently than EGFR. When overexpressed endocytosis-impaired ErbBs may inhibit the internalization and degradation of EGFR.