HER2 Testing: Past and Present

HER2 gene (aka ERBB2) amplification is directly related to HER2 protein overexpression in human breast carcinomas. This somatically acquired genetic alteration is associated with shorter disease-free and overall survival of patients in the absence of HER2-targeted therapy. Because HER2-targeted therapies have significantly improved outcomes for patients whose cancers have this alteration, accurate assessment of the alteration with companion diagnostics is considered critically important. US Food and Drug Administration (FDA)-approved companion diagnostics assess either HER2 gene amplification using fluorescence in situ hybridization (FISH) or HER2 protein overexpression using immunohistochemistry (IHC) assays. In an effort to standardize evaluations of HER2 status, the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) have convened committees to establish guidelines for assessment of HER2 status. These guidelines were published in 2007, 2013 / 2014, and 2018. Although HER2 IHC assays have challenges, with both false-negative and false-positive results, these assays are easily performed and easily interpreted. Each set of the guidelines consider IHC to be a suitable method for assessment of HER2 status, except in the IHC 2+ category, which is considered “equivocal” with supplementary testing recommended using FISH for definitive assignment to a “HER2-positive” or “HER2-negative” designation. The interpretative approaches to assessment of HER2 gene amplification by FISH recommended by the ASCO-CAP guidelines have changed over the years with current guidelines designating 5 different groups according to HER2 FISH ratio and average HER2 gene copy number per tumor cell. The ASCO-CAP FISH groups are “group 1,” designated in situ hybridization (ISH)–positive, has a HER2-to-chromosome 17 centromere (CEP17) ratio ≥2.0 and an average HER2 gene copy number per tumor cell ≥4.0; FISH “group 2,” formerly (2013) designated as “ISH-positive”, has cancer cells with HER2-to-CEP17 ratio ≥2.0 but an average HER2 gene copy number per tumor cell <4.0; FISH “group 3,” also formerly designated as “ISH-positive”, has cancer cells with HER2-to-CEP17 ratio <2.0 and an average HER2 gene copy number per tumor cell ≥6.0; FISH “group 4”, formerly and currently designated as “ISH-equivocal”, has cancer cells with HER2-to-CEP17 ratio <2.0 and an average HER2 gene copy number per tumor cell ≥4.0 but <6.0; FISH “group 5”, designated as ISH-negative, has cancer cells with HER2-to-CEP17 ratio <2.0 and an average HER2 gene copy number per tumor cell <4.0. At the time when these FISH guidelines were initially published, there were no studies using this interpretative strategy and, therefore, no available data related to prevalence rates of each FISH group, correlation of each FISH group with HER2 protein expression, or correlation of each group with clinical outcomes, either with or without HER2-targeted therapies. Since their publication in 2013 / 2014 we, and others, have assessed these prevalence rates and correlations. These findings, some supporting and some contradicting the guidelines, will be summarized in the presentation.