Significant heterogeneity of HER2 protein expression has been recently observed in HER2 positive endometrial serous carcinomas. Tumor cells with HER2 overexpression and/or gene amplification in a heterogeneous tumor may represent a biologically more aggressive subclone that is clinically relevant to prognosis and potential targeted therapy. To correlate with HER2 protein heterogeneity, we investigated the heterogeneity of HER2/NEU gene amplification in endometrial serous carcinoma. A total of 17 endometrial serous carcinomas with heterogeneous HER2 protein expression were selected for the study, including nine cases with a 3+ and eight cases with a 2+ immunohistochemical score. Initial reflex HER2 FISH was available for seven of the eight 2+ cases, five of which showed HER2/NEU gene amplification. All 17 cases underwent repeat FISH targeting larger tumor tissue areas. Ten cases (72%) displayed striking heterogeneity of HER2/NEU gene copy number in the form of cluster amplification. Diffuse HER2 amplification was observed in four cases, no amplification was seen in three tumors. In cases with cluster amplification, HER2 protein overexpression by immunohistochemistry closely correlated at the cellular level with HER2/NEU gene amplification. In conclusion, the significant percentage of cases with heterogeneous HER2/NEU gene amplification indicates that the existing HER2 testing guidelines designed for breast cancer may not be applicable to endometrial serous carcinoma. Clinical testing on multiple different tumor samples or large tumor tissue sections is recommended for both immunohistochemistry and FISH assessment of HER2 status. Direct comparison with the HER2 immunostaining pattern may be helpful in detecting HER2 amplified areas in a heterogeneous tumor.
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