Abstract A1-63: Characterizing the DNA damage repair defect in HPV-positive oropharyngeal squamous cell carcinoma

Abstract

Abstract Although tobacco-associated head and neck cancers (HNCs) are declining in incidence, overall HNC rates are escalating due to increasing prevalence of human papillomavirus (HPV)-associated tumors, especially oropharyngeal squamous cell carcinomas (OPSCCs). Clinically, patients with HPV-associated OPSCCs have improved overall survival and increased response to therapy, especially agents which act by damaging DNA. Based on these observations, we hypothesized that HPV-positive OPSCCs harbor a defect in DNA repair activity and are sensitive to DNA repair-targeted therapy. We evaluated DNA repair activity by immunofluorescent staining for DNA damage-induced formation of DNA repair protein foci. Baseline expression of DNA repair proteins was determined by NanoString nCounter analysis of transcript levels and immunoblotting for protein levels. Therapeutic sensitivity was assessed in vitro using the colony formation assay and in vivo using both cell line-derived xenografts and a patient-derived xenograft. Consistent with our hypothesis, in vitro disease models demonstrated delayed resolution of radiation-induced DNA double strand breaks (DSBs) as assessed by γH2AX foci staining and neutral comet assay. Investigation of the two main DSB repair mechanisms, non-homologous end joining (NHEJ) and homologous recombination (HR), indicated intact activation of both pathways but strikingly diminished recruitment of downstream repair factors DNA-Pk (NHEJ) and BRCA2 (HR) to sites of damage. In addition, protein expression of both DNA-PK and BRCA2 was decreased in HPV-positive compared to HPV-negative HNC cell lines. We next studied susceptibility of HPV-positive OPSCCs to PARP inhibition, a class of anticancer agents which block DNA repair signaling pathways and have proven effective clinically in DNA repair-deficient cancers. In vitro colony formation assays revealed a negative effect on cell survival in HPV-positive but not HPV-negative HNCs treated with the PARP inhibitor veliparib. Importantly, these results were confirmed in vivo in both cell line-derived xenografts and a patient-derived xenograft. In summary, our findings demonstrate for the first time the presence of a significant DNA DSB repair defect in HPV-positive OPSCCs encompassing both NHEJ and HR repair, and suggest therapies targeting DNA repair pathways may help improve therapeutic ratio in this disease. Citation Format: Alice N. Weaver, Tiffiny S. Cooper, Hoa Q. Trummell, James A. Bonner, Eben L. Rosenthal, Eddy S. Yang. Characterizing the DNA damage repair defect in HPV-positive oropharyngeal squamous cell carcinoma. [abstract]. In: Proceedings of the AACR Special Conference on Translation of the Cancer Genome; Feb 7-9, 2015; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(22 Suppl 1):Abstract nr A1-63.