[HER2 protein expression and gene status in endometrial serous carcinoma]

Objective: To investigate the characteristics of human epidermal growth factor receptor 2 (HER2) protein expression and gene status in uterine serous carcinoma (USC) patients. Methods: A total of 36 formalin-fixed and paraffin-embedded USC tissue specimens obtained between 2021 and 2022 in Peking Union Medical College Hospital were collected. The expression of HER2 protein and the gene status were detected by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) respectively, and the results were interpreted according to the 2020 International Society of Gynecological Pathologists recommendations. Results: Among the 36 cases, 11, 8, 12 and 5 cases showed HER2 IHC scores of 0, 1+, 2+, and 3+, respectively. All IHC 3+cases were pure USC. Out of 25 samples with different level of HER2 expression (IHC 1+, IHC 2+and 3+), 16 (64.0%) tumors with heterogeneous stain, which mainly affects the diseases with IHC 2+ (10/12) and IHC 3+ (3/5) lesions. Ten pure USC cases (27.8%, 10/36), involving HER2 IHC 0, IHC 1+, IHC 2+and IHC 3+tumors, harbored HER2 gene amplification by FISH (1, 1, 3 and 5 cases, respectively). All amplified cases exhibited a HER2/CEP17 ratio≥2.0. In addition, the incidences of chromosome 17 (CEP17) polysomy and monosomy were 25.0% (9/36) and 19.4% (7/36), respectively. Conclusions: Most of USC tumors exhibit intratumoral heterogeneity in HER2 IHC stain. Both HER2 IHC positive (3+) and FISH positive occur exclusively in pure USC tumors. HER2 gene amplification can be observed in any HER2 IHC levels.