Frequency of HER2 protein overexpression and HER2 gene amplification in endometrial clear cell carcinoma.

HER2 (ERBB2) overexpression and/or HER2 gene amplification has been well established in several tumors types and when present HER2 directed therapy may be to be efficacious. While recent findings suggests that HER2 overexpression and HER2 amplification are a relatively common in serous endometrial carcinoma, similar data regarding clear cell endometrial carcinoma (CCC) is difficult to interpret due to issues such as diagnostic criteria, sample type and HER2 interpretation criteria. Our goals were to study HER2 expression and HER2 copy number status in hysterectomy specimens from a large series of patients with pure CCC to determine the frequency of HER2 overexpression and HER2 amplification and evaluate applicability of current HER2 interpretation criteria. Pure CCC specimens derived from hysterectomy specimens from 26 patients were identified. All diagnoses were confirmed by two gynecologic pathologists. Immunohistochemistry for HER2 protein and fluorescence in situ hybridization (FISH) studies for HER2 were performed on whole-slide sections from all cases. Results were interpreted according to the 2018 ASO/CAP HER2 guidelines for breast cancer and International Society of Gynecologic Pathologists (ISGyP) HER2 guidelines for serous endometrial carcinoma. Additional testing was performed when indicated by the guidelines. HER2 expression by immunohistochemistry was 3+ in 4% and 0% of cases, and 2+ in 46% and 52% of cases, by 2018 ASCO/CAP and ISGyP criteria, respectively, while the remaining cases were negative. HER2 testing by FISH showed a positive result in 27% of tumors with 2018 ASCO/CAP guidelines, while 23% were positive with the ISGyP criteria. Our findings indicate that HER2 overexpression and HER2 amplification occur in a subset of CCC. Therefore, additional study into the potential benefit of HER2 targeted therapy in patients with CCC is warranted.