Receptor tyrosine-protein kinase erbB-2 is a protein that normally resides in the membranes of cells and is encoded by the ERBB2 gene. ERBB is abbreviated from erythroblastic oncogene B, a gene originally isolated from the avian genome. The human protein is also frequently referred to as HER2 (human epidermal growth factor receptor 2) or CD340 (cluster of differentiation 340). [5] [6] [7]
HER2 is a member of the human epidermal growth factor receptor (HER/EGFR/ERBB) family. But contrary to other members of the ERBB family, HER2 does not directly bind ligand. HER2 activation results from heterodimerization with another ERBB member or by homodimerization when HER2 concentration are high, for instance in cancer. [8] Amplification or over-expression of this oncogene has been shown to play an important role in the development and progression of certain aggressive types of breast cancer. In recent years the protein has become an important biomarker and target of therapy for approximately 30% of breast cancer patients. [9]
HER2 is so named because it has a similar structure to human epidermal growth factor receptor, or HER1. Neu is so named because it was derived from a rodent glioblastoma cell line, a type of neural tumor. ErbB-2 was named for its similarity to ErbB (avian erythroblastosis oncogene B), the oncogene later found to code for EGFR. Molecular cloning of the gene showed that HER2, Neu, and ErbB-2 are all encoded by the same orthologs. [10]
ERBB2, a known proto-oncogene, [11] is located at the long arm of human chromosome 17 (17q12). [12]
The ErbB family consists of four individual plasma membrane-bound receptor tyrosine kinases. One of which is erbB-2, and the other members being erbB-1, erbB-3 (neuregulin-binding; lacks kinase domain), and erbB-4. All four contain an extracellular ligand binding domain, a transmembrane domain, and an intracellular domain that can interact with a multitude of signaling molecules and exhibit both ligand-dependent and ligand-independent activity. Notably, no ligands for HER2 have yet been identified. [13] [14] HER2 can heterodimerise with any of the other three receptors and is considered to be the preferred dimerisation partner of the other ErbB receptors. [15]
Dimerisation results in the autophosphorylation of tyrosine residues within the cytoplasmic domain of the receptors and initiates a variety of signaling pathways.
Signaling pathways activated by HER2 include: [16]
In summary, signaling through the ErbB family of receptors promotes cell proliferation and opposes apoptosis, and therefore must be tightly regulated to prevent uncontrolled cell growth from occurring.
Amplification, also known as the over-expression of the ERBB2 gene, occurs in approximately 15-30% of breast cancers. [9] [17] HER2-positive breast cancers are well established as being associated with increased disease recurrence and a poor prognosis compared with other identifiably genetically distinct breast cancers with other known, or lack thereof, genetic markers that are thought to be associated with other breast cancers; however, drug agents targeting HER2 in breast cancer have significantly and positively altered the otherwise poor prognosis of the historically problematic difficulties associated with HER2-positive breast cancer. [18] Over-expression is also known to occur in ovarian, [19] stomach, adenocarcinoma of the lung [20] and aggressive forms of uterine cancer, such as uterine serous endometrial carcinoma, [21] [22] e.g. HER2 is over-expressed in approximately 7-34% of patients with gastric cancer [23] [24] and in 30% of salivary duct carcinomas. [25]
HER2 is colocalised and most of the time, coamplified with the gene GRB7, which is a proto-oncogene associated with breast, testicular germ cell, gastric, and esophageal tumours.
HER2 proteins have been shown to form clusters in cell membranes that may play a role in tumorigenesis. [26] [27]
Evidence has also implicated HER2 signaling in resistance to the EGFR-targeted cancer drug cetuximab. [28]
The high expression of HER2 correlates with better survival in esophageal adenocarcinoma. [29]
The high amplification of HER2 copy number positively contributes to the survival time of gastric cardia adenocarcinoma patients. [30]
Furthermore, diverse structural alterations have been identified that cause ligand-independent firing of this receptor, doing so in the absence of receptor over-expression. HER2 is found in a variety of tumours and some of these tumours carry point mutations in the sequence specifying the transmembrane domain of HER2. Substitution of a valine for a glutamic acid or a glutamine in the transmembrane domain can result in the constitutive dimerisation of this protein in the absence of a ligand. [31]
HER2 mutations have been found in non-small-cell lung cancers (NSCLC) and can direct treatment. [32]
HER2 is the target of the monoclonal antibody trastuzumab (marketed as Herceptin). Trastuzumab is effective only in cancers where HER2 is over-expressed. One year of trastuzumab therapy is recommended for all patients with HER2-positive breast cancer who are also receiving chemotherapy. [33] Twelve months of trastuzumab therapy is optimal. Randomized trials have demonstrated no additional benefit beyond 12 months, whereas 6 months has been shown to be inferior to 12. Trastuzumab is administered intravenously weekly or every 3 weeks. [34]
An important downstream effect of trastuzumab binding to HER2 is an increase in p27, a protein that halts cell proliferation. [35] Another monoclonal antibody, Pertuzumab, which inhibits dimerisation of HER2 and HER3 receptors, was approved by the FDA for use in combination with trastuzumab in June 2012.
As of November 2015, there are a number of ongoing and recently completed clinical trials of novel targeted agents for HER2+ metastatic breast cancer, e.g. margetuximab. [36]
Additionally, NeuVax (Galena Biopharma) is a peptide-based immunotherapy that directs "killer" T cells to target and destroy cancer cells that express HER2. It has entered phase 3 clinical trials.
It has been found that patients with ER+ (Estrogen receptor positive)/HER2+ compared with ER-/HER2+ breast cancers may actually benefit more from drugs that inhibit the PI3K/AKT molecular pathway. [37]
Over-expression of HER2 can also be suppressed by the amplification of other genes. Research is currently being conducted to discover which genes may have this desired effect.
The expression of HER2 is regulated by signaling through estrogen receptors. Normally, estradiol and tamoxifen acting through the estrogen receptor down-regulate the expression of HER2. However, when the ratio of the coactivator AIB-3 exceeds that of the corepressor PAX2, the expression of HER2 is upregulated in the presence of tamoxifen, leading to tamoxifen-resistant breast cancer. [38] [39]
Among approved anti-HER2 therapeutics are also tyrosine kinase inhibitors (Lapatinib, Neratinib, and Tucatinib) and antibody-drug conjugates (ado-trastuzumab emtansine and trastuzumab deruxtecan). [40]
HER2 testing is performed on breast biopsy of breast cancer patients to assess prognosis and to determine suitability for trastuzumab therapy. It is important that trastuzumab is restricted to HER2-positive individuals as it is expensive and has been associated with cardiac toxicity. [41] For HER2-positive tumors, the benefits of trastuzumab clearly outweigh the risks.
Tests are usually performed on breast biopsy samples obtained by either fine-needle aspiration, core needle biopsy, vacuum-assisted breast biopsy, or surgical excision.
Immunohistochemistry (IHC) is generally used to measure the amount of HER2 protein present in the sample, with fluorescence in situ hybridisation (FISH) being used on samples that are equivocal in IHC. However, in several locations, FISH is used initially, followed by IHC in equivocal cases. [42]
By immunohistochemistry, the sample is given a score based on the cell membrane staining pattern.
Score [43] [44] | Pattern [45] | Status [43] [44] |
---|---|---|
0 | Either: [45]
| HER2 negative (not present) |
1+ | Incomplete membrane staining that is faint or barely perceptible and within >10% of the invasive tumor cells. [45] | |
2+ | Weak to moderate complete membrane staining observed in >10% of tumor cells. [45] | Borderline/Equivocal |
3+ | Circumferential membrane staining that is complete, intense, and in >10% of tumor cells. [45] | HER2 positive |
Micrographs showing each score: [46]
FISH can be used to measure the number of copies of the gene which are present and is thought to be more reliable than immunohistochemistry. [47] It usually uses chromosome enumeration probe 17 (CEP17) to count the amount of chromosomes. Hence, the HER2/CEP17 ratio reflects any amplification of HER2 as compared to the number of chromosomes. The signals of 20 cells are usually counted.
HER2/CEP17 ratio | |||
---|---|---|---|
≥2.0 | <2.0 | ||
Average HER2 copy number per cell | ≥4.0 | HER2 positive | Additional work-up required |
<4.0 | Additional work-up required | HER2 negative |
If the initial HER2 result is negative for a needle biopsy of a primary breast cancer, a new HER2 test may be performed on the subsequent breast excision. [48]
The extracellular domain of HER2 can be shed from the surface of tumour cells and enter the circulation. Measurement of serum HER2 by enzyme-linked immunosorbent assay (ELISA) offers a far less invasive method of determining HER2 status than a biopsy and consequently has been extensively investigated. Results so far have suggested that changes in serum HER2 concentrations may be useful in predicting response to trastuzumab therapy. [49] However, its ability to determine eligibility for trastuzumab therapy is less clear. [50]
HER2/neu has been shown to interact with:
A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions.
Trastuzumab, sold under the brand name Herceptin among others, is a monoclonal antibody used to treat breast cancer and stomach cancer. It is specifically used for cancer that is HER2 receptor positive. It may be used by itself or together with other chemotherapy medication. Trastuzumab is given by slow injection into a vein and injection just under the skin.
The epidermal growth factor receptor is a transmembrane protein that is a receptor for members of the epidermal growth factor family of extracellular protein ligands.
Targeted therapy or molecularly targeted therapy is one of the major modalities of medical treatment (pharmacotherapy) for cancer, others being hormonal therapy and cytotoxic chemotherapy. As a form of molecular medicine, targeted therapy blocks the growth of cancer cells by interfering with specific targeted molecules needed for carcinogenesis and tumor growth, rather than by simply interfering with all rapidly dividing cells. Because most agents for targeted therapy are biopharmaceuticals, the term biologic therapy is sometimes synonymous with targeted therapy when used in the context of cancer therapy. However, the modalities can be combined; antibody-drug conjugates combine biologic and cytotoxic mechanisms into one targeted therapy.
Growth factor receptor-bound protein 7, also known as GRB7, is a protein that in humans is encoded by the GRB7 gene.
Lapatinib (INN), used in the form of lapatinib ditosylate (USAN) is an orally active drug for breast cancer and other solid tumours. It is a dual tyrosine kinase inhibitor which interrupts the HER2/neu and epidermal growth factor receptor (EGFR) pathways. It is used in combination therapy for HER2-positive breast cancer. It is used for the treatment of patients with advanced or metastatic breast cancer whose tumors overexpress HER2 (ErbB2).
Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase proteins. Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer. Mutations in receptor tyrosine kinases lead to activation of a series of signalling cascades which have numerous effects on protein expression. The receptors are generally activated by dimerization and substrate presentation. Receptor tyrosine kinases are part of the larger family of protein tyrosine kinases, encompassing the receptor tyrosine kinase proteins which contain a transmembrane domain, as well as the non-receptor tyrosine kinases which do not possess transmembrane domains.
Pertuzumab, sold under the brand name Perjeta, is a monoclonal antibody used in combination with trastuzumab and docetaxel for the treatment of metastatic HER2-positive breast cancer; it also used in the same combination as a neoadjuvant in early HER2-positive breast cancer.
The ErbB family of proteins contains four receptor tyrosine kinases, structurally related to the epidermal growth factor receptor (EGFR), its first discovered member. In humans, the family includes Her1, Her2 (ErbB2), Her3 (ErbB3), and Her4 (ErbB4). The gene symbol, ErbB, is derived from the name of a viral oncogene to which these receptors are homologous: erythroblastic leukemia viral oncogene. Insufficient ErbB signaling in humans is associated with the development of neurodegenerative diseases, such as multiple sclerosis and Alzheimer's disease, while excessive ErbB signaling is associated with the development of a wide variety of types of solid tumor.
The nuclear receptor coactivator 3 also known as NCOA3 is a protein that, in humans, is encoded by the NCOA3 gene. NCOA3 is also frequently called 'amplified in breast 1' (AIB1), steroid receptor coactivator-3 (SRC-3), or thyroid hormone receptor activator molecule 1 (TRAM-1).
Receptor tyrosine-protein kinase erbB-3, also known as HER3, is a membrane bound protein that in humans is encoded by the ERBB3 gene.
Receptor tyrosine-protein kinase erbB-4 is an enzyme that in humans is encoded by the ERBB4 gene. Alternatively spliced variants that encode different protein isoforms have been described; however, not all variants have been fully characterized.
Proto-oncogene tyrosine-protein kinase Src, also known as proto-oncogene c-Src, or simply c-Src, is a non-receptor tyrosine kinase protein that in humans is encoded by the SRC gene. It belongs to a family of Src family kinases and is similar to the v-Src gene of Rous sarcoma virus. It includes an SH2 domain, an SH3 domain and a tyrosine kinase domain. Two transcript variants encoding the same protein have been found for this gene.
An oncoantigen is a surface or soluble tumor antigen that supports tumor growth. A major problem of cancer immunotherapy is the selection of tumor cell variants that escape immune recognition. The notion of oncoantigen was set forth in the context of cancer immunoprevention to define a class of persistent tumor antigens not prone to escape from immune recognition.
Tyrosine-protein kinase receptor UFO is a protein that in human is encoded by the AXL gene. The gene was initially designated as UFO, in allusion to the unidentified function of this protein. However, in the years since its discovery, research into AXL's expression profile and mechanism has made it an increasingly attractive target, especially for cancer therapeutics. In recent years, AXL has emerged as a key facilitator of immune escape and drug-resistance by cancer cells, leading to aggressive and metastatic cancers.
Tyrosine-protein kinase 6 is an enzyme that in humans is encoded by the PTK6 gene.
Leucine-rich repeats and immunoglobulin-like domains protein 1 is a protein that in humans is encoded by the LRIG1 gene. It encodes a transmembrane protein that has been shown to interact with receptor tyrosine kinases of the EGFR family and with MET and RET.
Afatinib, sold under the brand name Gilotrif among others, is a medication which is used to treat non-small cell lung carcinoma (NSCLC). It belongs to the tyrosine kinase inhibitor family of medications. It is taken by mouth.
Neratinib (INN), sold under the brand name Nerlynx, is a tyrosine kinase inhibitor anti-cancer medication used for the treatment of breast cancer.
A tyrosine kinase inhibitor (TKI) is a pharmaceutical drug that inhibits tyrosine kinases. Tyrosine kinases are enzymes responsible for the activation of many proteins by signal transduction cascades. The proteins are activated by adding a phosphate group to the protein (phosphorylation), a step that TKIs inhibit. TKIs are typically used as anticancer drugs. For example, they have substantially improved outcomes in chronic myelogenous leukemia. They have also been used to treat other diseases, such as idiopathic pulmonary fibrosis.