Abstract 5470: Genotoxic effect and epigenetic alterations induced by the environmental carcinogen dibenzo[a, l]pyrene in oral tissues of mice

Abstract

Abstract Oral cancer is the major form of head and neck squamous cell carcinoma, which is the fifth most common cancer worldwide. Tobacco smoking is one of the leading causes of oral cancer. Dibenzo[a, l]pyrene (DB[a, l]P) is the most potent carcinogenic polycyclic aromatic hydrocarbon found in tobacco smoke. Recently we developed a novel mouse model of oral cancer induced by DB[a, l]P; we also showed the remarkable carcinogenicity and specificity of the fjord region diol epoxide metabolite, (±)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a, l]pyrene (DB[a, l]PDE). Using immunohistochemistry (IHC), we have shown over-expression of p53 protein in mice oral squamous cell carcinoma and dysplastic tissues induced by DB[a, l]P and DB[a, l]PDE. Current understanding of molecular pathogenesis of oral cancer in humans suggests that both genetic and epigenetic alterations are crucial and complement each other. Our hypothesis is that both genetic and epigenetic alterations induced by DB[a, l]P can contribute to the development of oral cancer. P53 protein overexpression detected by IHC may result from p53 gene mutation or exposure to genotoxic stress. To determine whether p53 over-expression is in part due to p53 mutations, Exons 5 to 8 of p53 from 5 tumor tissues were analyzed by polymerase chain reaction single-strand conformation polymorphisms (PCR-SSCP) and direct sequencing. G to T transversion was detected in Exon 5, leading to mutation of codon 155 Arg to Leu; A to T transversion was detected in Exon 7, resulting in mutation of codon 232 Lys to stop codon. To test the epigenetic effect of DB[a, l]P, methylation specific PCR and bisulfate sequencing were used to detect methylation alteration of p16 and RAR-β promoters. Promoter hypermethylation of both p16 and RAR-β were detected in tumors induced by DB[a, l]PDE; also in oral tissues of mice treated with DB[a, l]P (24nmol, 3 times per week, for 5 weeks, sacrificed at 48 h, 1, 2 and 4 weeks after last dose). These results suggested that genotoxic DNA adducts derived from DB[a, l]P can induce mutations in critical genes, such as tumor suppressor gene p53; this environmental carcinogen also can induce p16 and RAR-β promoter hypermethylation at early stage, which can contribute to the promotion and progression of oral carcinogenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5470. doi:1538-7445.AM2012-5470