Canine mammary tumours account for approximately 25-50% of all intact female dog tumours, 40-50% of which are malignant. In general, in canine mammary carcinomas, metastatic spread occurs via lymphatic vessels to the regional lymph nodes or haematogenously, the lungs are the most frequently metastasised organ.

MUC1 mucin is one of the tumour markers most closely associated with human breast cancer. MUC1 overexpression at the surface of tumour cells favours their dissemination and facilitates the establishment of metastases at distant sites.

In our study MUC1 expression is for the first time described in canine mammary carcinomas and is related to several clinically and pathologically relevant features.

The aim of this study is to evaluate MUC1 role not only as a prognostic indicator, but also as a potential therapeutic target in the canine veterinary practice similarly to what has recently been described in human breast cancer therapeutic trials. An association with human clinical settings would raise the possibility for further comparative studies between both species.

From a total of thirty-two bitches diagnosed of mammary gland tumour in the Companion Animal Clinic of ICBAS-UP, a series of fifty malignant mammary tumours was selected. The tumours were formalin fixed and paraffin embedded. C-20 polyclonal antibody was used in the immunohistochemistry technique to assess MUC1 expression. Associations between clinicopathological variables including distant metastases and lymph node metastases development and tumour grade among others, and expression of MUC1 were statistically analysed.

All the carcinomas showed MUC1 expression with at least 25% of the cells immunostained. Twelve specimens (24%) expressed MUC1 in less than 50% of the tumour cells, whereas 38 (76%) tumours showed MUC1 expression in more than 50% of the cells. The follow up period was of 2 years, during which 10 distant metastases were confirmed. We observed that a total of 20.4% of tumours gave rise to distant metastases, among these and most importantly 100% showed significantly high (>50%) MUC1 expression (p=0,03).

In the female dog, high MUC1 expression was significantly associated with higher distant metastases occurrence. Our findings indicate that high MUC1 expression is associated with worse prognosis in canine mammary carcinomas. MUC1 mucin can therefore constitute a promising therapeutic target in canine mammary tumours.