Mutation of TP53 is the most common genetic abnormality in head and neck squamous cell carcinoma (HNSCC) and results in an accumulation and expression of p53 protein in tumor cells. Disruptive TP53 mutations are consistently associated with poor prognosis but correlations of p53 expression with mutation or prognosis have been variable and the usefulness of p53 as a target for immunotherapy is unknown. Favorable prognosis is associated with the accumulation of T lymphocytes (TILs) in the tumor microenvironment and an immune response to p53 has been suggested by demonstration of antibodies to p53 and p53-restricted cytotoxic cells in patients with HNSCC. To investigate if p53 expression is related to accumulation of TILs, p53 expression was measured in a prospective cohort of patients with HNSCC and correlations with TILs and prognosis were determined.
Studied were 534 previously untreated patients (n) with oral cavity (273), oropharynx (158), larynx (81) or hypopharynx (22) cancers. Expression of p53, p21, and p16 and levels of T cell infiltrates (CD4, CD8, FoxP3) were assessed by immunohistology in tissue microarrays from biopsy specimens. HPV testing by routine pathology was available for 401 patients. Associations with clinical variables were tested using Kruskal-Wallis tests (p53, p21) or Chi-square test (p16). Kaplan-Meier and Cox regression methods were used to evaluate univariable and multivariable associations of protein expression with TIL levels and overall survival (OS) and disease specific survival (DSS) after adjusting for known prognostic factors. Median follow up was 44 months.
Higher p53 protein expression was associated with worse OS in univariable (HR = 1.05; 95% C.I. = 1.02, 1.09; p = 0.002) but not in multivariable analysis and did not correlate with increased TIL infiltration. Combined TIL weighted score was a significant independent prognostic factor for OS and DSS, (HR = 0.95; 95% C.I. = 0.93, 0.98; p = 0.0003 and HR = 0.96; 95% C.I. = 0.93, 0.99; p = 0.005, respectively). All of the biomarkers (p53, p21, p16, TILs) differed by HPV status and tumor site (p < 0.0001 each). Further analysis by specific tumor site was unremarkable for an association between p53 expression and TILs or prognosis. However, in multivariable analysis for oropharynx cancer, p53 expression was associated with increased risk of death (HR 2.33; 95% C.I. = 1.00, 5.43; p = 0.05) and in particular for the p16 negative oropharyngeal subgroup (HR = 8.62, 95%; C.I. = 1.94, 38.4; p = 0.005).
Over-expression of p53 is an unreliable biomarker for prognosis and does not correlate with levels of TILs suggesting that p53 neoantigens are unlikely to be useful targets for future immunotherapy. These findings were confirmed when patients were analyzed by specific tumor site, however in the subset of HPV-negative oropharyngeal cancers, p53 expression was associated with treatment outcomes and could be a useful biomarker. (438 words).