Abstract
BACKGROUND: Human papillomavirus 16 (HPV16) is a major risk factor for the development of head and neck squamous cell carcinoma (HNSCC), particularly the development of oropharyngeal squamous cell carcinoma (OPSCC). Cancer stem cells (CSCs) are resistant to conventional therapies, and it is postulated that they are responsible for disease recurrence and/or progression. Because the prognoses of patients with HPV16-positive and HPV-negative HNSCC are distinct, the authors sought to determine whether differences in the number of CSCs could account for this clinical observation. METHODS: CSC populations in HPV16-positive and HPV-negative HNSCC were assessed using a proprietary assay based on expression of the enzyme aldehyde dehydrogenase (ALDH), an in vitro tumorsphere formation assay, and an in vivo limiting cell dilution in nonobese diabetic/severe combined immunodeficiency mice. A high-density tissue microarray was stained with ALDH1, a CSC marker, to determine the association between CSCs and HPV16-positive/HPV-negative OPSCC. RESULTS: HPV16-positive HNSCC had a greater intrinsic CSC pool than HPV-negative HNSCC. Inactivation of p53 has been identified as a major mechanism for the elevated CSC population in HPV16-positive HNSCC. In vivo limiting cell dilution experiments using tumors from patients with HPV16-positive and HPV-negative OPSCC indicated that the CSC frequency was 62.5-fold greater in an HPV16-positive OPSCC tumor than in an HPV-negative OPSCC tumor. Primary tumors from patients with HPV16-positive OPSCC were associated with elevated tumor ALDH1 staining, further extending the association between HPV16 and CSCs. CONCLUSIONS: The current data and the clinical observation that patients with HPV16-positive HNSCC respond more favorably to current treatment paradigms than patients with HPV-negative HNSCC support the suggestion that CSC phenotype is not homogeneous. Therefore, the reliance on the CSC number may be insufficient to accurately assess the potential of a particular tumor for disease recurrence and/or progression.