2.71 Basic Biology
Human epidermal growth factor receptor 2, ERBB2, also known as HER2 is a member of the epidermal growth factor family of receptors (ERBB family) including EGFR (HER1), ERBB3 (HER3) and ERBB4 (HER4). The ERBB proteins are a family of
membrane spanning receptors that bind epidermal growth factor (EGF) type extra cellular signalling molecules, including, EGF, transforming growth factor (TGF-α) and neuregulins, transmitting their signal to a number of pathways within the cell.
In order to transduce signals triggered by the binding for extracellular messengers ERBB receptors must first dimerise 319. In the absence of ligand ERBB receptors exist as inactive monomers. Ligand binding results in a conformational change exposing the
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dimerisation domain allowing binding 320. Unlike JAKs, ERBB receptors do not require phosphorylation for catalytic activity. ERBB receptors can both homodimerise and
heterodimerise, the result of which is to juxtapose each receptor’s kinase domain with target residues on the adjacent receptor, allowing trans-phosphorylation. Receptor trans-
phosphorylation creates docking sites for proteins with SH2 domains and is required for the recruitment various downstream factors including SHC1 321, which links ERBB receptors to the MAPK (mitogen activated phosphate kinase) pathway, and PIK3R1 (p85) 322 the
regulatory subunit of PI3K (phosphoinositide 3-kinase), which activates the AKT/mTOR pathway.
One such MAPK pathway induced by ERBB activation is the RAS/RAF/MEK/ERK cascade. This cascade, initiated by the activation of membrane associated RAS, results in the phosphorylation and activation of the extra cellular related kinases ERK1 and ERK2, also known as mitogen activated protein kinase, MAPK3 and MAPK1. Activated ERK1 and ERK2 in turn phosphorylate the transcription factors FOS and JUN which form a heterodimer and translocate to the nucleus where they bind the AP-1 element found in the promoter region of numerous genes including CCND1, MYC and VEGF (reviewed in 323). Increased
signalling via the RAS/RAF/MEK/ERK pathway results with the activation and upregulation of genes associated with proliferation, differentiation and angiogenesis.
ERBB2 is unique among the ERBB2 family in that no ligand capable of binding the receptor has been identified. Instead the receptor exists in an open conformation in which the dimerisation domain, usually only exposed by ligand binding in EGFR, is permanently accessible and able to interact with other ERBB receptors 324. While it appears that ERBB2 does not homodimerise 325 and therefore would not be predicted to activate downstream signalling in isolation, it might be predicted to confer greater sensitivity to ligand induced activation of other ERBB receptors. This hypothesis supported by two observations; firstly that maximal activation of ERBB2 requires the presence of other ERBB proteins 326 and secondly ERBB dimers involving ERBB2 have greater signal activation potential then dimers that do not include ERBB2 327. ERBB2 possess two predominant trans-phosphorylation sites that are closely linked to the signal transduction capabilities of the protein, tyrosines 1248 and 1221/1222 both of which has been linked to the activity of the RAS/RAF/MEK/ERK pathway 328,329.
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2.72 Role in Breast and Ovarian Cancer
These unique features of ERBB2 go some way to explaining why it is perhaps the most oncogenic of all ERBB family members. ERBB2 was discovered originally in mice as the factor driving mutagen induced transformation of rodent cell lines. The transfection of DNA from mutagenized cells was capable of transforming the ‘normal’ NIH/3T3 mouse fibroblast line, transferring the mutagenized phenotype 330. Transformed cells were
subsequently used to inoculate mice, that were, in turn used to purify an antibody that bound to a 185KDa protein, now known to be ERBB2, found in cell lysates from the original mutagenized rat cells 331. The importance of ERBB2 as an oncogene in cellular
transformation was confirmed when it was demonstrated that overexpression of ERBB2 alone, was sufficient to transform NIH/3T3 cells 332. Subsequent sequencing of the gene and identification of the human homologue rapidly led to the first identification of ERBB2 amplification in breast cancer 333.
ERBB2 is now known to play a prominent role in breast cancer where gene
amplification and protein overexpression has been observed in between 30-42% and 18-20% of invasive ductal carcinoma cases, the most common type of breast cancer 334,335.
The prognostic significance of ERBB2 expression is complex and probably dependant on disease stage, treatment history and chemotherapy regime. Despite this several large studies have shown a clear relationship between copy number and poor survival in breast cancer. Two large studies examining the relationship between copy number, in pre-treatment biopsies, and either disease free survival (n=1056) or risk of disease recurrence (n=580), using multivariate analyses found significant associations with both outcomes 336,337.
While ERBB2 is only amplified in around 5% of HGS ovarian cancers 122,338 a number of studies have reported elevated protein expression in a significantly higher
proportion. Due to the nature of immunohistochemistry (IHC), the technique generally used for such estimates, reported frequencies vary. One large study of 1420 cases suggests the frequency of overexpression is closer to 16% of invasive EOC 339.
Although the prognostic significance of ERBB2 overexpression is less well studied in ovarian cancer, and identifying correlations is complicated by the smaller proportion of cases they represent, a number of studies have found significant associations between either copy number or protein overexpression and PFS/OS and response to chemotherapy. For example two such studies examining ERBB2 expression by IHC found a significant association with reduced OS and PFS, in multivariate models 340,341. Meden et al also found ERBB2 to an
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independent prognostic factor, 51 of 275 patients 18%, screened by IHC, were found to be ERBB2 overexpressers and median survival of this group was 20 months compared to 33 months for non overexpressers 342.
Confirming the importance of ERBB2 in driving breast cancers in vivo, treatment with the ERBB2 inhibiting antibody, trastuzumab, has been shown to confer survival benefits in patients with confirmed genomic amplification 343.
2.73 ERBB2 Drug Interactions
In spite of the prognostic importance of ERBB2 a number of studies examining different chemotherapy regimens, specifically those including a taxol, doxorubicin and cyclophosphamide, have shown a significantly higher response rates in ERBB2 positive breast cancer cases verses other molecular subtypes 334,344. In both studies tumour tissue was preoperative and chemotherapy was first line. A randomised trial of cyclophosphamide, doxorubicin and paclitaxel as individual treatments arms has suggested that the specific agent ERBB2 overexpression confers sensitivity to is paclitaxel, this arm only provided a
significant survival advantage in ERBB2 amplified cases verses those without 345. Converse to its apparent role in sensitising to paclitaxel some in vitro and early clinical data has suggested ERBB2 overexpression might not only contribute to upfront response to platinum based therapy, but also that as a therapy ERBB2 inhibition might combine synergistically with cisplatin and other DNA damaging agents. For example isobolograms conducted in the ERBB2 amplified breast cancer cell SKBR3 showed that trastuzumab combined synergistically with cisplatin in reducing cell viability (combination index 0.48 P=0.003). Synergy was not confined to cisplatin as other genotoxic drugs were also found to act synergistically with trastuzumab including etoposide and thiotepa 346.
The majority of clinical studies investigating the prognostic importance of ERBB2 expression in ovarian cancer did not examine this specifically in relation to platinum based chemotherapy, however given that carboplatin is the standard frontline treatment, any effect on drug response is likely to have an overall survival effect. Meden at al investigated the relationship between platinum chemotherapy dose and OS in ovarian cancer cases
overexpressing ERBB2. They found a dose response relationship in patients without ERBB2 overexpression, that was not present in those overexpressing the protein 347.
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In addition Lassus et al found a significant association between ERBB2 copy number and poor response to therapy, reduced PFS and OS (n=401) 338. In this study the majority of patients (86%) were treated with single agent platinum and ERBB2 was also found to be an independent prognostic marker.
Taking these studies together suggests ERBB2 confers resistance to platinum based chemotherapy in ovarian cancer.
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