p16 is generally considered to be a surrogate maker of human papillomavirus (HPV) infection and also a predictive marker of favorable clinical outcome of patients with squamous cell carcinoma of the oropharynx. p16 overexpression is also known to be induced by deregulation of RB1 in neuroendocrine carcinomas. In highly malignant esophageal neoplasms, however, the status of p16 has remained largely unknown. We immunolocalized p16 and Rb1 in 82 surgically resected esophageal high-grade squamous cell carcinomas (46 poorly differentiated and 36 basaloid squamous cell carcinomas) and 15 esophageal small-cell carcinomas in order to clarify the clinical and biological significance of p16. p16 immunoreactivity was detected in 7/82 (9%) high-grade squamous cell carcinomas and 15 (100%) small-cell carcinomas. p16 immunoreactivity was significantly associated with Rb1 protein loss in both groups (P < 0.001). HPV was detected in none of the p16-positive cases examined. Clinical outcome of the p16-positive high-grade squamous cell carcinomas was not different from that of the p16-negative counterparts (P = 0.687) but significantly better than those with the small-cell carcinomas (P = 0.023). p16 was therefore considered to be induced through an inactivation of the RB1 signaling pathway and not through HPV infection in highly malignant esophageal neoplasms. Nevertheless, patients’ clinical outcome of these neoplasms significantly differs; therefore, small-cell carcinomas have to be carefully differentiated from other neoplasms. In addition, p16 overexpression is not predictive of favorable clinical outcome in high-grade squamous cell carcinomas of the esophagus.
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