HER-2 (also called HER-2/neu, c-erbB2, ERBB2 or neu) is a transmembrane receptor tyrosine kinase, which was originally discovered from a rat neuroblastoma cell line (named “neu”) more than 20 years ago. At the same time another research group identified the same gene and found its high homology with human epidermal growth factor receptor (therefore named human epidermal growth factor receptor 2, “HER-2”). The consensus name HER-2 is widely used, as well as the acronym of its gene, ERBB2.
HER-2 is a proto-oncogene, i.e. its activation causes malignant transformation and increases the malignant potential (cell proliferation, invasiveness etc.) of the cells. In human cancers HER-2 is activated via gene amplification, which is a genomic mutation where a small fragment at chromosome band 17q12-q21 is multiplied in a cell up to 50-100 folds. HER-2 is considered as the target oncogene driving the amplification, although neighbouring genes (such as topoisomerase II alpha) may occasionally be co-amplified together with HER-2. Amplification of HER-2 gene invariably leads to over-expression of its protein product. The magnitude of over- expression is usually 10-100 folds or even more. Over-expressed HER-2 protein disturbs the HER-receptor family signalling networks, i.e. signalling mediated via EGFR receptor, HER-3 and HER-4. HER-2 has no ligand of its own and it acts by forming heterodimers with EGFR, HER-3 and HER-4 upon binding of their ligands. In tumours with gene amplification, vastly over-expressed HER-2 forms auto active HER2-HER2 homodimers, which is considered as the major oncogenic activation mechanism. A breakthrough finding in cancer biology was made in early 1990’s, when antibodies to the extracellular domain of HER-2 were found to inhibit growth of HER-2 positive cell lines and tumour xenografts. These findings gave rise to test the most promising growth inhibitory antibody as an anti-cancer drug in humans. The clinical efficacy of the monoclonal antibody drug named trastuzumab (“Herceptin”) has now been shown in several randomised clinical studies of metastatic breast cancer. Thus, Herceptin is today standard therapy in HER-2 positive metastatic breast cancer. Most recent clinical studies show promising interim results also when used as adjuvant therapy given after primary breast cancer operation (ASCO 2005 meeting reports). For these reasons, testing for HER-2 amplification and/or over-expression is currently considered as routine practise in clinical pathology laboratories.
In tumours, HER-2 is over-expressed in 15-25% of primary breast cancers. Metastases usually have the same amplification status as the primary tumours. HER-2 amplification and over-expression are typical features of hormone receptor negative, rapidly growing histologic grade 2-3 tumours. Of the histologic types, Paget’s disease is almost invariably HER-2 positive, whereas only a small minority of lobular and tubular carcinomas shows HER-2 amplification. HER-2 amplification and over-expression can also be found in intestinal type gastric and gastroesophageal carcinomas, ovarian carcinomas, high grade endometrial carcinomas and some salivary duct tumours. Low-level copy number increases have been found also in rare cases of lung tumours.
As mentioned above, demonstration of HER-2 amplification and over-expression in tumour tissue is required to define eligibility of the patient to trastuzumab (Herceptin) therapy. In most studies, immunohistochemical staining of HER-2 protein is scored qualitatively as 0/1+/2+/3+. Provided that the system is carefully calibrated, scores 0 or 1+ are almost never (i.e., <1-2%) associated with HER-2 amplification and/or therapeutic response, while score 3+ is associated with gene amplification in 90-100% of cases and with the highest likelihood of Herceptin response. The problematic category is IHC score 2+, with less than one third displaying gene amplification. True responses to Herceptin occur in this category of 2+ positive tumours. Thus, IHC score 2+ is considered to be an indication for a confirmatory gene based test (FISH or CISH). Because of its central importance in breast cancer therapy selection, standardization of HER-2 IHC assays and slide interpretation are of outmost clinical and economical importance. As shown in the context of clinical trials, analytical variability in Her-2 testing is considerable. It can be minimised by the use of standardised tests, and by interlaboratory quality control assessments.
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