(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.
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+ (Fig. 1; see table below). 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.
The trastuzumab antibody itself (and its original mAb clone 4D5) are not suitable for use in IHC. Currently the polyclonal antibody used in the HercepTest™ (Dako), and the PATHWAY® / CONFIRM™ rmAb clone 4B5 (Ventana) are most frequently used, and the systems approved by FDA. They are used in paraffin sections with high temperature antigen retrieval. Among Abs used in-house systems, mAb clones CB11 and particularly SP3 are promising. Although the HER-2 protein is known to be localised in the cell membrane, cytoplasmic staining also occurs, especially in tumours with high level protein over-expression.
According to the NordiQC experience, the kits Herceptest (Dako) and Pathway (Ventana) approved by FDA in general performs much better than in house systems. However, a major problem is the correct interpretation of the staining result.
it has been shown that excessive heating of paraffin sections prior
to deparaffination and epitope retrieval reduces the staining
signal. Thus, baking of slides should not exceed 60 C for one hour.
Control tissue: Positive and negative controls are essential and should be included in every staining batch. The widely used commercially available HER-2 staining kit “Herceptest” includes control slides, which contain formalin-fixed and paraffin-embedded cells from HER-2 positive and negative breast cancer cell cultures.
The most suitable human tumour control is Paget’s disease, which almost invariably shows gene amplification and 3+ protein over-expression. Ductal tumours tested positive for gene amplification by FISH or CISH can be used as well.
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