From genes to clinical practice – the role of immunohistochemistry in the classification of breast carcinomas

Tumor biology, in addition to tumor size, lymph node status and metastasis presence, is definitely important for prognosis and response to therapy. From a histological and molecular perspective, breast carcinomas are heterogeneous – different gene expression patterns are responsible for the wide range of variations in their prognosis and therapeutic response. The variety of genetic abnormalities in breast carcinomas has been emphasized in several studies over the past few decades, enabling the investigation of the gene expression profile (GEP) and immunohistochemistry subtyping of breast cancer⁽¹⁾.

Breast tumor identification and segregation into intrinsic molecular subtypes has enabled much more efficient clinical management, using personalized therapies, and the assessment of recurrence risk based on the molecular characteristics of breast neoplasms. In fact, the term breast cancer encompasses a variety of conditions, some of which are comparable from a histology standpoint but distinct from one another in terms of biology and therapeutic response. The management of all cases should be based on clinical and imaging data, together with histological and molecular studies (Figure 1).

In 2013, the St. Gallen International Breast Carcinoma Conference defined five molecular subtypes of breast cancer: luminal A and B, HER2, basal-like and claudin-low⁽²⁾.

The treatment was initially surgical, with or without radiation therapy, based on grade, size, nodal status and metastasis, and nowadays it includes the evaluation of sentinel lymph nodes, the evaluation of biomarkers and genetic profiling.

Multiple studies have validated this classification, which initially emphasized the division of all breast carcinomas into estrogen receptor (ER)-positive and ER-negative subtypes, and several subtypes have been added and then removed from the initial panel as new discoveries have emerged. To put this classification into practice, immunohistochemical surrogate algorithms were developed⁽³⁾ (Figure 2).

Breast carcinomas that express hormone receptors like estrogen and progesterone will benefit from endocrine therapy. Another hormone receptor – androgen – is considered to have predictive value, and might be part of the classification in the future.

Breast carcinomas in the luminal molecular subtype category account for 60-70% of all breast carcinomas and have a better prognosis than breast neoplasms with negative hormone receptors. They are divided into two molecular subtypes: A and B.

Around 40-60% of breast tumors are molecular subtype A (luminal A-like) breast carcinomas, which are distinguished by low histological grade, positive hormone receptors (ER/PR+) (Figure 3), negative HER2, and a low Ki67 proliferation index (Figure 4). This molecular subtype of breast cancer has poorer rates of response to chemotherapy, but responds well to hormone treatment⁽⁴⁾.

About 20-30% of breast tumors are breast carcinomas of molecular subtype B, or luminal B-like, which have less differentiated histological grades, higher recurrence rates, lower expression of estrogen and progesterone hormone receptors, variable expression of the HER2 protein, and higher proliferative rates (Ki67 proliferation index) – Figure 5. Compared to molecular subtype A, molecular subtype B is associated with a worse prognosis and with a better response to chemotherapy⁽⁵⁾.

Between 10% and 20% of breast carcinomas fall under the HER2 molecular subtype, which is identified by the overexpression of the Her2 neu protein (Figure 6) and the lack of hormone receptor expression. It is distinguished by clinically aggressive breast cancers, high histological grade, a poorer long-term prognosis brought by early-onset systemic metastases, and worse survival rates. IHC and ISH can detect HER2 overexpression, but good clinical practice in the pathology laboratory is required because many variables can interfere with the assessment⁽⁶⁾. Although immunohistochemistry is still the most frequently used initial test and the primary technique in practice, its effectiveness is highly dependent on the pre- and post-analytical conditions.

The molecular basal-like breast cancer subtype accounts for about 15-20% of breast carcinomas and is distinguished by the absence of hormonal receptor expression (ER-, PR-, HER2-) (Figure 7), as well as immunohistochemical positivity for cytokeratin 5/6 (CK5/6) and 14 (CK14) and/or EGFR (HER1). It is linked to increased rates of genomic instability, proliferation and histological grade, as well as a higher prevalence of BRCA1 mutations⁽⁷⁾. Basal-like 1 (BL-1), basal-like 2 (BL-2), mesenchymal and luminal AR, with various survival rates and a variable level of responsiveness to neoadjuvant therapy, are the four subtypes that have been identified based on gene expression⁽⁸⁾.

HER2, estrogen, progesterone, claudin 3, claudin 4, claudin 7 and E-cadherin immunohistochemistry expression are not present in the 5% of breast carcinomas that belong to the claudin-low breast subtype (claudin levels are reduced in these cancers). It is linked to elevated gene expression for immune system and inflammation-related genes, which supports the development of a significant lymphocytic chronic inflammatory infiltrate⁽⁹⁾.

HER2-low breast cancer is a new emerging subtype with clinical significance. It includes tumors with lower levels of the HER2 protein on their surface and with IHC scores of 1+ and 2+ without amplification, but there is no specific definition or set of criteria that would characterize a tumor as low HER2, because low levels are not yet classified within ASCO 2018⁽¹⁰⁾.

Breast cancer, like other cancers, is not a single disease. The cancers vary tremendously, not only in histologic appearance, grade, hormone receptor and HER2 status, but also on a molecular basis. There is no longer a question of morphology versus molecular debate. Personalized treatment is based on a pathology report that combines traditional morphology and immunohistochemistry with a variety of molecular tests. To remain clinically relevant, it is critical to change the staging as new discoveries in cancer research and treatment emerge.

 

Conflict of interest: none declared

Financial support: none declared

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