Cytoplasmic Expression of p16 Is Associated with Carcinoma Breast: It Is Not an Artifact.

Background & objective: p16 has different roles in the nuclear and cytoplasmic locations. The nuclear localization of the p16 protein explains its role in cell cycle regulation. Cytoplasmic expression was considered an artifact in the initial years, but there is evidence to prove that cytoplasmic localization is real and that p16 has different roles in the nuclear and cytoplasmic locations. We aimed to study the immunoexpression of p16 protein in the nuclear and cytoplasmic locations of the epithelial and stromal compartments of fibroadenoma, invasive breast carcinoma, and a select number of phyllodes tumors.

Methods: The study included a total of 107 patients, comprising 51 cases of invasive breast carcinoma, 51 cases of fibroadenoma, 4 cases of benign phyllodes tumors, and 1 case of lobular carcinoma in situ (LCIS). The p16 immunohistochemistry was evaluated for nuclear and cytoplasmic localization in the epithelial and stromal compartments of the tumors.

Results: Of the 51 fibroadenoma cases, 23 showed strong nuclear p16 epithelial expression, but no case showed cytoplasmic expression. In 19/51 cases, stromal cells also showed strong p16 nuclear expression. Moderate stromal p16 expression was observed in 3 out of 4 cases of benign phyllodes. Out of the 51 cases of invasive carcinoma, 31 showed moderate to strong nuclear p16 immunopositivity, while 27 cases exhibited cytoplasmic p16 expression. We found a statistically significant correlation between moderate to strong nuclear p16 immunoexpression and the molecular subtype of breast carcinoma.

Conclusion: The cytoplasmic localization of p16 immunohistochemistry is not seen in epithelial components of fibroadenoma, while it is seen frequently in breast carcinoma. Nuclear p16 expression has a statistically significant correlation with molecular subtypes of breast carcinoma.