Alterations of p53 and expression of WAF1/p21 in human thyroid tumors

We have investigated p53 expression in 178 thyroid tumors from 162 patients by immunohistochemistry using two antibodies, DO1 and CM1. In addition, 35 tumors were analyzed for expression of WAF1/p21, one of the downstream mediators of p53. Only 15 tumors (8.4%) had greater than 10% of tumor cell nuclei positively stained for p53. Six of 14 Hürthle tumors and three of 34 papillary thyroid carcinomas showed staining of p53 in the cytoplasm. A total of 40 tumors, including all p53 positive tumors, and all anaplastic and poorly differentiated tumors were screened for mutations in exons 5-8 of the TP53 gene by constant denaturing gel electrophoresis and subsequent sequence analysis. A mutation was detected in five tumors only: one anaplastic carcinoma, one poorly differentiated follicular carcinoma (negative by p53 immunohistochemistry), one atypical follicular adenoma, and two papillary thyroid carcinoma metastases, of which the primary tumors had no detectable mutation. We conclude that p53 immunohistochemistry cannot be used for diagnostic and prognostic purposes in thyroid tumors. The tumors with TP53 mutation showed a markedly reduced WAF1/p21 expression. Three anaplastic carcinomas with highly expressed p53, but with no detectable mutation, also showed high expression of WAF1/p21. This may be explained by overexpression of wild-type p53, possibly due to the patients' preoperative treatment, including external radiation of the neck region. The results indicate that WAF1/p21 immunohistochemistry contributes to the information of the functional status of p53, and may facilitate the interpretation of results from p53 immunohistochemistry in these tumors.