Reduced expression of claudin-2 is associated with high histological grade and metastasis of feline mammary carcinomas.

Claudins (CLDNs) are a family of tight junction (TJ) proteins that play an important role in maintaining cell polarity, in controlling paracellular ion flux and in regulating cell proliferation and differentiation. There is a growing body of evidence that associates changes in CLDN expression with the development of human breast cancer. In the present study CLDN-2 expression was examined immunohistochemically in samples of normal feline mammary tissue (n = 5) and mammary carcinomas (n = 52), including metastatic lesions (n = 29). Seventy-seven percent of carcinomas showed reduced CLDN-2 expression compared with that observed in normal mammary gland. Reduced expression of CLDN-2 was significantly associated with a high histological grade of carcinoma (P = 0.011), with 88.6% of grade II/III carcinomas showing decreased expression. Furthermore, CLDN-2 down-regulation was significantly associated with metastatic disease (P = 0.0027), with 93.1% of cases with signs of metastasis showing decreased expression of this protein. CLDN-2 may constitute a molecular marker for identification of a subgroup of feline mammary carcinomas characterized by high histological grade and the development of metastasis.