P1-12-17: Fluorescent In Situ Hybridization Evaluation of HER2 Status in Tumors with Chromosome 17 Polysomy.

Abstract

Abstract Introduction: According to the American Society of Clinical Oncology-College of American Pathologists recommendations for HER2 testing, a positive fluorescent in situ hybridization (FISH) result is defined as >6 HER2 gene copies/nucleus (for test systems with no internal control probe [single-color]) or as HER2 gene/chromosome 17 centromere (CEP17) ratio > 2.2 (for systems with an internal control probe [dual-color]). Although an increase in CEP17 copy number (average ≥ 3.0 copies/nucleus) is commonly considered to represent polysomy of chromosome 17, it can also be a result of gains of 17q with centromere involvement, or amplification of the centromeric region. The classification of HER2−positive tumors according to the HER2/CEP17 ratio may therefore misclassify a fraction of truly amplified cases as polysomic. We prospectively evaluated tumors with chromosome 17 polysomy but no HER2 amplification to assess HER2 status using the above two FISH classifications and immunohistochemistry (IHC). Materials and methods: Tumors were tested for gene amplification by FISH with probes to HER2/neu and CEP17 using the PathVysion HER-2 DNA Probe Kit (Vysis). Classification was based on the HER2/CEP17 ratio (amplified when > 2.2) and average HER2 gene copy number/nucleus (amplified when > 6 copies). Both polysomic and equivocal cases (HER2/CEP17 ratio 1.8 - 2.2) were further studied by IHC using the HercepTest (Dako) with 0–3 scoring system (overexpression when 3+). Results: From March 2010 to May 2011 we evaluated 31 primary breast cancers showing chromosome 17 polysomy. Median HER2/CEP17 ratio was 1.3 (range 0.5−1.9), median HER2 copy number was 5.4 (range 2.6−13.8), and median CEP17 copy number was 4.2 (range 3.2−8.0). Thirteen (42%) had an average HER2 gene copy number > 6/nucleus (median 6.8, range 6.1−13.8) and would therefore be considered as amplified if classified according to the absolute HER2 gene copy number. Nine (75%) of these were 3+ at IHC and the remaining 4 were 2+, whereas among the 18 cases with an average HER2 gene copy number < 6/nucleus, one was 2+, ten were 1+, and 7 scored 0. Twenty-nine cases showed negative HER2/CEP17 ratios (< 1.8) and three cases equivocal HER2/CEP17 ratios (between 1.8 and 2.2). Using HER2/CEP17 ratio as first assessment and IHC only in equivocal cases, only one of the 31 “polysomic” cases would have been classified as HER2−positive. However, 8 polysomic cases with HER2/CEP17 ratio < 1.8 showed 3+ immunostaining (all with average HER2 gene copy number > 6/nucleus), while 3 other cases had an average HER2 gene copy number > 6/nucleus with 2+ immunostaining. Conclusions: Our results show that both FISH evaluation criteria and IHC can modify the percentage of polysomic tumors classified as HER2−positive. However, the number of gene copies/nucleus appears more frequently associated with 3+ IHC than the HER2/CEP17 ratio. The predictive impact of the former method on response to anti-HER2 treatments warrants further investigation. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-12-17.