HER2 Overexpression and Cytogenetical Patterns in Canine Mammary Carcinomas

SIMPLE SUMMARY: Human epidermal growth factor receptor 2 (HER2) is a transmembrane tyrosine kinase receptor whose homonymous gene is an important oncogenic driver in 10–20% of invasive breast cancer. The role of HER2 in canine mammary carcinoma is controversial; therefore, this study aimed to investigate the protein expression and cytogenetic alterations of HER2 and their correlation with the proliferative fraction, estrogen receptor, and other clinicopathological parameters in canine mammary carcinoma. The HER2 gene was amplified in a subgroup of canine mammary carcinomas, and the HER2 gene copy number was correlated with HER2 protein overexpression but not with the tumor’s biological behavior. Surprisingly, a possible translocation of HER2/CRYBA1 was found. This is the first study in the canine species that found cytogenetic alterations with fluorescence in situ hybridization in mammary carcinoma. ABSTRACT: Human epidermal growth factor receptor 2 (HER2) is a tyrosine kinase receptor that promotes tumor cell growth and is implicated in the pathogenesis of human breast cancer. The role of HER2 in canine mammary carcinomas (CMCs) is not clear. Therefore, this study aimed to examine the protein expression and cytogenetic changes of HER2 and their correlation with other clinical–pathological parameters in CMC. We retrospectively selected 112 CMCs. HER2, ER, and Ki67 were assessed by immunohistochemistry. HER2 antibody validation was investigated by immunoblot on mammary tumor cell lines. Fluorescence in situ hybridization (FISH) was performed with probes for HER2 and CRYBA1 (control gene present on CFA9). HER2 protein overexpression was detected in 15 carcinomas (13.5%). A total of 90 carcinomas were considered technically adequate by FISH, and 8 out of 90 CMC (10%) were HER2 amplified, 3 of which showed a cluster-type pattern. HER2 overexpression was correlated with an increased number of HER2 gene copies (p = 0.01; R = 0.24) and overall survival (p = 0.03), but no correlation with ER, Ki67, grade, metastases, and tumor-specific survival was found. Surprisingly, co-amplification or polysomy was identified in three tumors, characterized by an increased copy number of both HER2 and CRYBA1. A morphological translocation-fusion pattern was recognized in 20 carcinomas (22%), with a co-localized signal of HER2 and CRYBA1. HER2 is not associated with clinical–pathological parameters of increased malignancy in canine mammary tumors, but it is suitable for studying different amplification patterns.