The HER2 receptor (previously called HER2/Neu) is a transmembrane glycoprotein with tyrosine kinase activity. It belongs to the epidermal growth factor receptor family (EGFR/ErbB). These receptors are essential in controlling epithelial cell growth and differentiation.[1]

The importance of these receptors is that the aberrant HER2 protein overexpression has associations with some adenocarcinomas, including breast, ovary, endometrium, cervix as well as lung, esophageal, gastroesophageal junction, gastric, and bladder cancers. The protein is strongly associated with increased disease recurrence and is a poor prognostic factor for survival. For this reason, HER2 is an important prognostic target for the therapy of various types of cancer.

HER2 amplification or overexpression occurs in approximately 20 to 30% of human breast cancers.[2] 

HER2 amplification or overexpression occurs in approximately 20 to 30% of human breast cancers, some ovarian and gastric cancers. It confers worse biological behavior and clinical aggressiveness in breast cancer.[2]

Overexpression of HER2 is primarily attributed to HER2 gene amplification and results in constitutive activation of the HER2 downstream signaling pathway.[3] Breast cancers can possess up to 25 to 50 copies of the HER2 gene and up to a 40- to 100-fold increase in HER2 protein resulting in 2 million receptors expressed at the tumor cell surface.[4] 

HER2 amplification is a seminal event in human breast tumorigenesis, occurring in almost 50% of in situ carcinomas.[5] The fact that only 20% of invasive breast cancers are HER2 amplified suggests that many HER2-amplified in situ cancers never progress to the invasive stage.

HER2-amplified breast cancers have unique biological and clinical characteristics such as increased proliferation rates, high histologic and nuclear grade, low ER and PR levels, more aneuploidy, tendency to metastasize to CNS viscera, relative resistance to endocrine therapy, and increased sensitivity to doxorubicin.

Human epidermal growth factor receptor-2 (HER2) is a member of the epidermal growth factor family of tyrosine kinase receptors. This family includes HER1(Erb1), HER3 ( Erb3), and HER4 (Erb4) besides HER2. HER receptors are essential for cell proliferation, differentiation, and survival. 

All HER proteins share common structural features, including extracellular ligand-binding domain and transmembrane domain, and intracellular protein tyrosine kinase domain.[3] For HER1, HER3, HER4, they have a total of 11 extracellular ligands. HER2 makes an exception as it doesn't bind any ligand.[6]

Activation of HER2 mediated signaling pathways occurs by heterodimerization with ligand-activated EGFR or HER3 or by homodimerization when it is present in high concentrations such as in cancer. These dimerizations would lead to phosphorylation of tyrosine residues, which initiates downstream signaling cascades such as PI3K/AKT signaling cascade, Ras/MEK/ERK, and JAK/STAT, that regulate cell survival, proliferation, differentiation, motility, apoptosis, survival, invasion, migration, adhesion, and angiogenesis.[3] 

Observation shows that HER3 plays an essential role in HER2- mediated oncogenic signaling.[7] HER3 lacks tyrosine kinase activity; it relies on other receptor tyrosine kinases, most notably, ERBB2, to get its tyrosine residue phosphorylated and activate the downstream signaling cascades.

HER2 containing heterodimers have the highest mitogenic potential among all HER complexes. Between all HER2 pairs, HER2/HER3 heterodimer is the most potent activator of the PI3K/AKT signaling cascade via binding of the p85 subunit of PI3K to HER3.[8] Knocking down HER3 or targeting HER3 via neutralizing antibodies should theoretically reduce HER2-amplified human breast cancer growth.[7] Inhibition of HER2 dimerization prevents the activation of several intracellular signaling cascades, including the PI3K and MAPK pathways, which can cause carcinogenesis.

Routine testing of HER2 expression is a strong recommendation on newly diagnosed patients with breast cancer, as well as those with metastatic breast cancer, as patients with a high level of HER2 expression would benefit from agents targeting HER2.[9] 

Cancer biopsy: Tests for HER2 are usually performed on biopsy samples via either fine-needle aspiration, core needle biopsy, vacuum-assisted breast biopsy of surgical excision. The sample attains a score based on the cell membrane staining pattern.

Serum: The extracellular domain of HER2 may shed from the surface of tumor cells. Using enzyme-linked immunosorbent assay ( ELISA) to measure serum HER2 would provide a less invasive method of determining HER2 status than a biopsy.