Definition of HER2
Human epidermal growth factor 2 (HER2, ERBB2) is a membrane-associated receptor that dimerises with HER family members and transduces extracellular signals to RAS-MAP kinase and PI3 kinase-AKT intracellular signalling networks. HER2 is overexpressed in several types of tumours, including gastro-oesophageal adenocarcinoma.
In gastric cancer, HER2 acts as an oncogene. Protein overexpression is associated with genetic amplification of segments of chromosome 17 that may form tandem duplications on chromosome 17 or double-minute chromosomes.
HER2 Overexpression in Gastric Cancer
HER2 is overexpressed in about 30% of intestinal type gastric cancers, 15% of mixed type tumours, and about 5% of diffuse type. Signet ring type is typically HER2 negative. According to tumour location, about 30% of tumours at cardiac/gastro–oesophageal junction and 15% of gastric cancers show HER2 positivity.
HER2 as a Prognostic Biomarker in Gastric Cancer
There is mounting evidence of the role of HER2 overexpression in patients with gastric cancer, and it has been correlated to poor outcomes and a more aggressive disease. Many investigations have been conducted on HER2 as a prognostic factor; although there have been no universal conclusions, the positive result of HER2 status appears to be poor prognostic factor. All the studies have encountered problems such as small sample size, diversity of patient characteristics, methods of HER2 tests and diagnosis of HER2 status. HER2 status seems not to be an independent prognostic biomarker in early oesophago-gastric adenocarcinoma.
HER2 as a Predictive Biomarker in Gastric Cancer
Clinically, HER2 has been established as a predictive biomarker for HER2-targeted therapies. In gastric cancer, it is a predictive biomarker for treatment with trastuzumab. Since trastuzumab, in combination with chemotherapy, significantly improves survival for patients with advanced HER2-positive gastric and gastro-esophageal junction adenocarcinoma over chemotherapy alone, the combination therapy became the standard of care in HER2-positive advanced disease. Therefore, appropriate patient selection by HER2 immunohistochemistry and in situ hybridization should be part of routine pathology.
HER2 Testing Recommendations in Gastric Cancer
Representative surgical samples or an adequate number of viable biopsy specimens (ideally six to eight) are required. If few biopsies are available, all viable specimens should be tested. Immunohistochemistry should be the initial HER2 testing methodology for gastric cancer and bright-field methodologies are preferred wherever possible.
HER2-positivity per European Medicines Agency license is defined as immunohistochemistry 3+ or immunohistochemistry 2+/fluorescence in situ hybridization-positive or immunohistochemistry 2+/silver in situ hybridization-positive.
Tumour samples classified as immunohistochemistry 2+ should be retested by fluorescence in situ hybridization or silver in situ hybridization to assess HER2 status.
Silver in situ hybridization is a more suitable methodology than fluorescence in situ hybridization for assessing HER2 status in gastric tumour samples as it is a bright-field methodology and thus allows for rapid identification of HER2-positive tumour foci within a heterogeneous sample.
These criteria were modified from breast cancer criteria to account for common, idiosyncratic staining characteristics of gastro-oesophageal cancers (eg. basolateral HER2 localisation and heterogeneous staining). Due to the tumour heterogeneity (focal areas of positivity) and incomplete membrane staining, the gastric cancer-specific scoring criteria should be adhered to:
- Surgical specimen cutoff: complete, basolateral, or lateral membranous reactivity in ≥10% of cells
- Biopsy specimen cutoff: complete, basolateral, or lateral membranous reactivity in ≥5 clustered cells
The ‘magnification rule’ should be used in conjunction with the scoring criteria.
In situ hybridization
The definition of fluorescence in situ hybridization or silver in situ hybridization positivity in gastric or gastro–oesophageal junction cancer is a HER2: chromosome 17 ratio of ≥2.0. The entire section should be screened for amplified regions (particularly important for fluorescence in situ hybridization samples where a bright-field image is not available). At least 20 evaluable, non-overlapping cells in the invasive component should be counted initially. In borderline amplification cases, approximately 20 additional cells should be recounted or scoring should be performed in an alternative area of tissue. The overall HER2 gene count is important: >6 HER2 gene copies using single probe is considered positive; in case of 4 to 6 HER2 gene copies a dual probe test is advised and the ratio should be recalculated by counting an additional 20 cells.
Ensuring Quality and Timely HER2 Testing Results
Validated immunohistochemistry and in situ hybridization HER2 assays should be used and appropriate controls should be included in each run. Turnaround time from initial diagnosis to reporting of results should ideally not exceed 5 working days. Centralised testing is recommended wherever possible and all laboratories should be encouraged to participate in validated quality assurance programs.
Which Technique and Which Algorithm Should be Used for the Analysis of the HER2 Status in Gastric Cancer?
Immunohistochemically based HER2 screening algorithm is a pragmatic strategy used by many laboratories owing to the relative labour-intensiveness of in situ hybridization, even though trastuzumab therapy is approved for all patients with ISH-positive gastro-oesophageal cancers (regardless of immunohistochemical score) in the United States. An immunohistochemistry-first screening protocol demands high concordance of the immunohistochemical and ISH assays. The optimal immunohistochemical screening assay should also demonstrate high sensitivity and low false-negative rates for ISH amplification to ensure that all patients who may benefit from anti-HER2 therapy are identified.
In Europe, immunohistochemistry is recommended as the first testing modality and in situ hybridization technique should be applied only in cases of immunohistochemistry 2+. Immunohistochemistry 0 and 1+ are not eligible for trastuzumab therapy. Borderline immunohistochemistry 1+/immunohistochemistry 2+ cases and samples with focal and intense membranous reactivity in <10% cells may be retested with fluorescence in situ hybridization or silver in situ hybridization (scores for both assays should be indicated separately on the report).
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- Ruschoff J, Dietel M, Baretton G, et al. HER2 diagnostics in gastric cancer-guideline validation and development of standardized immunohistochemical testing. Virchows Arch 2010;457(3):299-307.
- Van Cutsem E, Bang Y-J, Feng-yi F, et al. HER2 screening data from ToGA: targeting HER2 in gastric and gastroesophageal junction cancer. Gastric Cancer 2014; DOI 10.1007/s10120-014-0402-y
- Bang Y-J, Van Cutsem E, Feyereislova A, et al. Trastuzumab in combination with chemotherapy versus chemotherapy alone for treatment of HER2-positive advanced gastric or gastro-oesophageal junction cancer (ToGA): a phase 3, open-label, randomised controlled trial. The Lancet 2010;376(9742):687-697.
- T. Waddell, M. Verheij, W. Allum, et al. Gastric cancer: ESMO–ESSO–ESTRO Clinical Practice Guidelines for diagnosis, treatment and follow-up. Ann Oncol 2013;24(Suppl 6):vi57-vi63.