TopBP1 is aberrantly expressed in human and feline mammary
carcinomas, but expression of this BRCA1-related
protein has not been investigated in canine mammary
carcinomas. In this study, 132 canine mammary tumours (46 benign, 86
carcinomas) were examined immunohistochemically for expression of TopBP1, oestrogen receptor alpha (
ERalpha), Ki67 and p53. Positive staining for TopBP1 was evident in all canine mammary lesions, although five samples had <20% positive cells. The number of samples with high levels of staining increased in different categories from benign mixed tumour to
adenoma to
carcinoma. Most TopBP1 staining was nuclear, but both nuclear and cytoplasmic staining were observed as the degree of
malignancy increased, similar to human and feline mammary
carcinomas. Benign mixed tumours, however, had more cytoplasmic staining than
adenomas. Expression of p53 and the proliferation marker Ki67 increased from benign mixed tumour to
adenoma to
carcinoma, but the differences between benign and malignant tumours were more distinct than for TopBP1 expression.
ERalpha expression decreased from malignant to benign tumours, although over half of the benign mixed tumours were negative. TopBP1 was expressed in canine mammary tumours at higher levels than has been reported previously for cats, although the shift in cellular localisation with
malignancy was similar.