Mutation Research 721 (2011) 199–205 Contents lists available at ScienceDirect Mutation Research/Genetic Toxicology and Environmental Mutagenesis journal homepage: www.elsevier.com/locate/gentox Community address: www.elsevier.com/locate/mutres p53 codon 271 CGT to CAT mutant fraction does not increase in nasal respiratory and olfactory epithelia of rats exposed to inhaled naphthalene Fanxue Meng a , Yiying Wang a , Meagan B. Myers a , Brian A. Wong b , Elizabeth A. Gross b , Harvey J. Clewell III c , Darol E. Dodd b , Barbara L. Parsons a,∗ a National Center for Toxicological Research, US Food and Drug Administration, Division of Genetic and Molecular Toxicology, HFT-120, 3900 NCTR Road, Jefferson, AR 72079, USA b The Hamner Institutes for Health Sciences, Division of Toxicology and Preclinical Studies, Research Triangle Park, NC 27709-2137, USA c The Hamner Institutes for Health Sciences, Center for Human Health Assessment, Research Triangle Park, NC 27709-2137, USA article info Article history: Received 10 December 2010 Received in revised form 4 February 2011 Accepted 7 February 2011 Available online 13 February 2011 Keywords: p53 mutation Naphthalene Dose–response Risk assessment Mode of action Cytotoxicity abstract A 2-year rat tumor bioassay testing whole body exposure to naphthalene (NA) vapor found a significant increase in nasal respiratory epithelial adenomas in male rats and in olfactory epithelial neuroblastomas in female rats. To obtain mechanistic insight into NA-induced nasal carcinogenesis, NA dose–response was characterized in nasal epithelium using a tumor-relevant endpoint. Specifically, levels of p53 codon 271 CGT to CAT mutation were measured in nasal respiratory and olfactory epithelium of NA-exposed male and female rats by allele-specific competitive blocker-PCR (ACB-PCR). Male and female, 8–9 week- old F344 rats (5 rats/group) were exposed to 0, 0.1, 1.0, 10, and 30 ppm NA vapor for 13 weeks (6 h/day, 5 days/week). The geometric mean p53 mutant fraction (MF) levels in nasal epithelium of control treatment groups ranged between 2.05 × 10 -5 and 3.05 × 10 -5 . No significant dose-related changes in p53 mutant fraction (MF) were observed in the olfactory or respiratory epithelia of female rats. However, statistically significant treatment-related differences were observed in male respiratory and olfactory epithelium, with the p53 MF in the respiratory epithelium of male rats exposed to 30 ppm NA significantly lower than that in controls. Further, a significant trend of decreasing p53 MF with increasing dose was observed in the male respiratory epithelium. Of the tissue types analyzed, respiratory epithelium is the most sensitive to the cytotoxic effects of NA, suggesting cytotoxicity may be responsible for the loss of p53 mutation. Because ACB-PCR has been used successfully to detect the effects of known mutagenic carcinogens, the absence of any significant increases in p53 MF associated with NA exposure adds to the weight of evidence that NA does not operate through a directly mutagenic mode of action. Published by Elsevier B.V. 1. Introduction Naphthalene (NA), a white solid that sublimes at room tem- perature, is the simplest member of the polycyclic aromatic hydrocarbons (PAHs), with a molecular structure consisting of two fused benzene rings [1]. Although, humans can be exposed to NA through diet, use of household products, smoking, or their occupation, most individuals are primarily exposed though inhalation [2]. Evaluating the cancer risk associated with NA inhalation is an active area of research and debate [3,4]. There are insuffi- cient human epidemiological data for assessing the cancer risk associated with NA exposure, although there is anecdotal evi- Abbreviations: ACB-PCR, allele-specific competitive blocker PCR; MOA, mode of action; MF, mutant fraction; NA, naphthalene; WT, wild-type. ∗ Corresponding author. Tel.: +1 870 543 7946; fax: +1 870 543 7393. E-mail address: barbara.parsons@fda.hhs.gov (B.L. Parsons). dence of NA-associated tumor development [4]. The best available data on NA’s carcinogenic potential is derived from two, 2-year rodent tumor bioassays, both of which included NA doses that exceed the maximum tolerated dose (MTD) [3]. In a B6C3F 1 mouse tumor bioassay, 30 ppm inhaled NA resulted in increases in pulmonary alveolar/bronchiolar adenomas in male and female mice, which were statistically significant only in the female mice [5,6]. In a F344 rat tumor bioassay, 10, 30, and 60 ppm inhaled NA induced significant increases in nasal respiratory epithelial ade- nomas in male rats compared to controls, and 60 ppm inhaled NA induced a significant increase in the number of nasal olfac- tory epithelial neuroblastomas in female rats compared to controls [7–10]. A significant positive trend of increasing olfactory epithe- lial neuroblastoma with increasing NA dose was observed for both male and female rats [3]. There is no accepted consensus of scientific opinion regard- ing the mode of action (MOA) by which NA is carcinogenic. One hypothesis is that elevated NA metabolism occurs at sites receiv- ing relatively high doses of inhaled NA, inducing cytotoxicity as a 1383-5718/$ – see front matter Published by Elsevier B.V. doi:10.1016/j.mrgentox.2011.02.004