[179] Detailed FISH Analysis of HER2 2+ Immunohistochemistry Cases: Rarity of Cases with High Level HER2 Gene Amplification

DC Gingell, DJ Dabbs, U Surti, R Bhargava. Magee-Womens Hospital of UPMC, Pittsburgh, PA

Background: One-quarter of 2+ immunohistochemistry (IHC) cases are amplified by fluorescence in-situ hybridization (FISH), but amplification details (low-level versus high level gene amplification, aneuploidy/hyperploidy, monosomy) for these cases remains unknown. These details may be important for gauging individual patient response to targeted therapy.
Design: The HER2 IHC was performed either using Pathway (clone CB11) HER2 antibody or 4B5 clone. One hundred and eighty-six consecutive invasive breast carcinomas 2+ by IHC were analyzed by FISH for HER2 gene amplification using PathVysion dual color probe (Vysis Inc., Downers Grove, IL). FISH results were categorized into 3 categories (Amplified, not amplified and equivocal) using CAP-ASCO guidelines. Cases with average Chromosome enumeration probe 17 (CEP17) signals of >3 were considered as positive for aneuploidy/polyploidy. Monosomy for chromosome 17 was suspected when majority of the cells showed only one signal for CEP17.
Results: Of the 186 cases, 150 (81%) were not amplified, 21 (11%) were amplified, and 15 (8%) were equivocal for HER2 gene amplification. Aneuploidy/polyploidy was identified in 20 of 150 (13%) non-amplified cases. Among the 21 amplified cases, the mean HER2/CEP17 ratio was 3.08 with mean HER2 gene copy number/cell of 6.4. Only 2 of 21 amplified cases (10%) demonstrated clusters of HER2 gene. Five of 21 amplified cases (24%) showed heterogeneity with respect to HER2 gene copy number/cell. Three of 21 amplified cases (14%) showed monosomy which was responsible for HER2/CEP17 ratio of >2.2. If a single colored probe was used, then 3/21 (14%) amplified cases would have been classified as not amplified and 7 (33%) cases as equivocal. Of the 15 equivocal cases, 4 cases (27%) showed monosomy. If a single colored probe was used, then 6 (40%) of the original equivocal cases would have been classified as not amplified.
Conclusions: 1) The number of 2+ IHC cases with HER2 gene amplification is slightly lower than previously reported with the use of an equivocal category. 2) Large clusters of HER2 gene/cell (a characteristic of 3+ IHC cases) are very uncommonly seen in IHC 2+ cases. 3) Aneuploidy/polyploidy for chromosome 17 without HER2 gene amplification may explain 2+ IHC staining in 13% cases. 4) Depending on the type of probe (single versus dual color), the interpretation may vary-Dual color probe is useful in detecting ploidy, however has the potential for artifactually elevating the HER2/CEP17 ratio, which should be interpreted appropriately.
Category: Breast

Monday, March 9, 2009 9:30 AM

Poster Session I Stowell-Orbison/Autopsy Award # 30, Monday Morning