Measurement of tumor-associated mutations in the nasal mucosa of rats exposed to varying doses of formaldehyde

Assessment of associations between inhaled formaldehyde and lymphohematopoietic cancer through the integration of epidemiological and toxicological evidence with biological plausibility

Formaldehyde is recognized as carcinogenic for the portal of entry sites, though conclusions are mixed regarding lymphohematopoietic (LHP) cancers. This systematic review assesses the likelihood of a causal relationship between formaldehyde and LHP cancers by integrating components recommended by NASEM. Four experimental rodent bioassays and 16 observational studies in humans were included following the implementation of the a priori protocol. All studies were assessed for risk of bias (RoB), and meta-analyses were conducted on epidemiological studies, followed by a structured assessment of causation based on GRADE and Bradford Hill. RoB analysis identified systemic limitations precluding confidence in the epidemiological evidence due to inadequate characterization of formaldehyde exposure and a failure to adequately adjust for confounders or effect modifiers, thus suggesting that effect estimates are likely to be impacted by systemic bias. Mixed findings were reported in individual studies; meta-analyses did not identify significant associations between formaldehyde inhalation (when measured as ever/never exposure) and LHP outcomes, with meta-SMRs ranging from 0.50 to 1.51, depending on LHP subtype. No associations with LHP-related lesions were reported in reliable animal bioassays. No biologically plausible explanation linking the inhalation of FA and LHP was identified, supported primarily by the lack of systemic distribution and in vivo genotoxicity. In conclusion, the inconsistent associations reported in a subset of the evidence were not considered causal when integrated with the totality of the epidemiological evidence, toxicological data, and considerations of biological plausibility. The impact of systemic biases identified herein could be quantitatively assessed to better inform causality and use in risk assessment.

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

A comprehensive review of mechanistic insights into formaldehyde-induced nasal cavity carcinogenicity

According to the International Agency for Research on Cancer classification, formaldehyde is a human carcinogen that targets the nasal cavity. In humans and rats, inhaled formaldehyde is primarily deposited in the nasal cavity mucosa, metabolized to the less toxic formic acid, and finally excreted into the urine or exhaled. Thus, formaldehyde-induced nasal carcinogenicity may be a direct effect of formaldehyde itself, although the underlying mechanisms remain unclear. With regard to cytotoxicity, degeneration and necrosis of nasal respiratory cells occur in rats after short exposure to formaldehyde. Cell proliferation is increased in the damaged cells, suggesting its critical roles both in the early stages and throughout the entire process of nasal carcinogenicity. Hyperplasia, squamous metaplasia, and dysplasia of the damaged epithelium frequently appear as morphological precursor lesions. With regard to genotoxicity, in addition to DNA-protein crosslinks, oxidative DNA damage also occurs in the exposed nasal mucosal cells. Sustained exposure to formaldehyde may cause nasal carcinogenicity through cytotoxicity and auxiliary genotoxicity. In this review, we discuss adverse outcome pathways through which cytotoxicity can lead to carcinogenicity and the development of integrated approaches for testing and assessment for nongenotoxic carcinogens.

Using mechanistic information to support evidence integration and synthesis: a case study with inhaled formaldehyde and leukemia

Formaldehyde is one of the most comprehensively studied chemicals, with over 30 years of research focused on understanding the development of cancer following inhalation. The causal conclusions regarding the potential for leukemia are largely based on the epidemiological literature, with little consideration of cancer bioassays, dosimetry studies, and mechanistic research, which challenge the biological plausibility of the disease. Recent reanalyzes of the epidemiological literature have also raised significant questions related to the purported associations between formaldehyde and leukemia. Because of this, considerable scientific debate and uncertainty remain on whether there is a causal association between formaldehyde inhalation exposure and leukemia. Further complexity in evaluating this association is related to the endogenous production of formaldehyde. Multiple modes of action (MOA) have been postulated for the development of leukemia following formaldehyde inhalation that includes unsupported hypotheses of direct or indirect toxicity to the target cell population. Herein, the available evidence relevant to evaluating the postulated MOAs for leukemia following formaldehyde inhalation exposure is organized in the IPCS MOA Framework. The integration of all the available evidence clearly highlights the limited amount of data that support any of the postulated MOAs and demonstrates a significant amount of research supporting the null hypothesis that there is no causal association between formaldehyde inhalation exposure and leukemia. These analyses result in a lack of confidence in any of the postulated MOAs, increasing confidence in the conclusion that there is a lack of biological plausibility for a causal association between formaldehyde inhalation exposure and leukemia.

ACB-PCR Quantification of Low-Frequency Hotspot Cancer-Driver Mutations

Allele-specific competitive blocker PCR (ACB-PCR) is a sensitive and quantitative approach for the selective amplification of a specific base substitution. Using the ACB-PCR technique, hotspot cancer-driver mutations (tumor-relevant mutations in oncogenes and tumor suppressor genes, which confer a selective growth advantage) are being developed as quantitative biomarkers of cancer risk. ACB-PCR employs a mutant-specific primer (with a 3'-penultimate mismatch relative to the mutant DNA sequence, but a double 3'-terminal mismatch relative to the wild-type DNA sequence) to selectively amplify rare mutant DNA molecules. A blocker primer having a non-extendable 3'-end and a 3'-penultimate mismatch relative to the wild-type DNA sequence, but a double 3'-terminal mismatch relative to the mutant DNA sequence is included in ACB-PCR to selectively repress amplification from abundant wild-type molecules. Consequently, ACB-PCR can quantify the level of a single base pair substitution mutation in a DNA population when present at a mutant:wild-type ratio of 1 × 10^-5 or greater. Quantification of rare mutant alleles is achieved by parallel analysis of unknown samples and mutant fraction (MF) standards (defined mixtures of mutant and wild-type DNA sequences). The ability to quantify specific mutations with known association to cancer has several important applications in evaluating the carcinogenic potential of chemical exposures in rodent models. Further, the measurement of cancer-driver mutant subpopulations is important for precision cancer treatment (selecting the most appropriate targeted therapy and predicting the development of therapeutic resistance). This chapter provides a step-by-step description of the ACB-PCR methodology as it has been used to measure human PIK3CA codon 1047, CAT→CGT (H1047R) mutation.

Considerations for refining the risk assessment process for formaldehyde: Results from an interdisciplinary workshop

Anticipating the need to evaluate and integrate scientific evidence to inform new risk assessments or to update existing risk assessments, the Formaldehyde Panel of the American Chemistry Council (ACC), in collaboration with the University of North Carolina, convened a workshop: "Understanding Potential Human Health Cancer Risk - From Data Integration to Risk Evaluation" in October 2017. Twenty-four (24) invited-experts participated with expertise in epidemiology, toxicology, science integration and risk evaluation. Including members of the organizing committee, there were 29 participants. The meeting included eleven presentations encompassing an introduction and three sessions: (1) "integrating the formaldehyde science on nasal/nasopharyngeal carcinogenicity and potential for causality"; (2) "integrating the formaldehyde science on lymphohematopoietic cancer and potential for causality; and, (3) "formaldehyde research-data suitable for risk assessment". Here we describe key points from the presentations on epidemiology, toxicology and mechanistic studies that should inform decisions about the potential carcinogenicity of formaldehyde in humans and the discussions about approaches for structuring an integrated, comprehensive risk assessment for formaldehyde. We also note challenges expected when attempting to reconcile divergent results observed from research conducted within and across different scientific disciplines - especially toxicology and epidemiology - and in integrating diverse, multi-disciplinary mechanistic evidence.

Variation in organ-specific PIK3CA and KRAS mutant levels in normal human tissues correlates with mutation prevalence in corresponding carcinomas

Large-scale sequencing efforts have described the mutational complexity of individual cancers and identified mutations prevalent in different cancers. As a complementary approach, allele-specific competitive blocker PCR (ACB-PCR) is being used to quantify levels of hotspot cancer driver mutations (CDMs) with high sensitivity, to elucidate the tissue-specific properties of CDMs, their occurrence as tumor cell subpopulations, and their occurrence in normal tissues. Here we report measurements of PIK3CA H1047R mutant fraction (MF) in normal colonic mucosa, normal lung, colonic adenomas, colonic adenocarcinomas, and lung adenocarcinomas. We report PIK3CA E545K MF measurements in those tissues, as well as in normal breast, normal thyroid, mammary ductal carcinomas, and papillary thyroid carcinomas. We report KRAS G12D and G12V MF measurements in normal colon. These MF measurements were integrated with previously published ACB-PCR data on KRAS G12D, KRAS G12V, and PIK3CA H1047R. Analysis of these data revealed a correlation between the degree of interindividual variability in these mutations (as log10 MF standard deviation) in normal tissues and the frequencies with which the mutations are detected in carcinomas of the corresponding organs in the COSMIC database. This novel observation has important implications. It suggests that interindividual variability in mutation levels of normal tissues may be used as a metric to identify mutations with critical early roles in tissue-specific carcinogenesis. Additionally, it raises the possibility that personalized cancer therapeutics, developed to target specifically activated oncogenic products, might be repurposed as prophylactic therapies to reduce the accumulation of cells carrying CDMs and, thereby, reduce future cancer risk. Environ. Mol. Mutagen. 58:466-476, 2017. © 2017 This article is a U.S. Government work and is in the public domain in the USA. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Reassessment of MTBE cancer potency considering modes of action for MTBE and its metabolites

A 1999 California state agency cancer potency (CP) evaluation of methyl tert-butyl ether (MTBE) assumed linear risk extrapolations from tumor data were plausible because of limited evidence that MTBE or its metabolites could damage DNA, and based such extrapolations on data from rat gavage and rat and mouse inhalation studies indicating elevated tumor rates in male rat kidney, male rat Leydig interstitial cells, and female rat leukemia/lymphomas. More recent data bearing on MTBE cancer potency include a rodent cancer bioassay of MTBE in drinking water; several new studies of MTBE genotoxicity; several similar evaluations of MTBE metabolites, formaldehyde, and tert-butyl alcohol or TBA; and updated evaluations of carcinogenic mode(s) of action (MOAs) of MTBE and MTBE metabolite's. The lymphoma/leukemia data used in the California assessment were recently declared unreliable by the U.S. Environmental Protection Agency (EPA). Updated characterizations of MTBE CP, and its uncertainty, are currently needed to address a variety of decision goals concerning historical and current MTBE contamination. To this end, an extensive review of data sets bearing on MTBE and metabolite genotoxicity, cytotoxicity, and tumorigenicity was applied to reassess MTBE CP and related uncertainty in view of MOA considerations. Adopting the traditional approach that cytotoxicity-driven cancer MOAs are inoperative at very low, non-cytotoxic dose levels, it was determined that MTBE most likely does not increase cancer risk unless chronic exposures induce target-tissue toxicity, including in sensitive individuals. However, the corresponding expected (or plausible upper bound) CP for MTBE conditional on a hypothetical linear (e.g., genotoxic) MOA was estimated to be ∼2 × 10(-5) (or 0.003) per mg MTBE per kg body weight per day for adults exposed chronically over a lifetime. Based on this conservative estimate of CP, if MTBE is carcinogenic to humans, it is among the weakest 10% of chemical carcinogens evaluated by EPA.

ACB-PCR quantification of somatic oncomutation

Allele-specific competitive blocker-polymerase chain reaction (ACB-PCR) is a sensitive approach for the selective amplification of an allele. Using the ACB-PCR technique, hotspot point mutations in oncogenes and tumor-suppressor genes (oncomutations) are being developed as quantitative biomarkers of cancer risk. ACB-PCR employs a mutant specific primer (with a 3'-penultimate mismatch relative to the mutant DNA sequence, but a double 3'-terminal mismatch relative to the wild-type DNA sequence) to selectively amplify rare mutant DNA molecules. A blocker primer (having a non-extendable 3'-end and with a 3'-penultimate mismatch relative to the wild-type DNA sequence, but a double 3'-terminal mismatch relative to the mutant DNA sequence) is included in ACB-PCR to selectively repress amplification from the abundant wild-type molecules. Consequently, ACB-PCR is capable of quantifying the level of a single basepair substitution mutation in a DNA population when present at a mutant:wild type ratio of 10(-5) or greater. Quantification of rare mutant alleles is achieved by parallel analysis of unknown samples and mutant fraction (MF) standards (defined mixtures of mutant and wild-type DNA sequences). The ability to quantify specific mutations with known association to cancer has several important applications, including evaluating the carcinogenic potential of chemical exposures in rodent models and in the diagnosis and treatment of cancer. This chapter provides a step-by-step description of the ACB-PCR methodology as it has been used to measure human KRAS codon 12 GGT to GAT mutation.

Lowest adverse effects concentrations (LOAECs) for formaldehyde exposure

In 2012 the Committee for Risk Assessment (RAC) of the European Chemicals Agency concluded that 2ppm formaldehyde represent a Lowest Observed Adverse Effect Concentration (LOAEC) for polypoid adenomas, histopathological lesions and cell proliferation. An analysis of all data shows that a LOAEC of 2ppm it is not justified for cell proliferation and polypoid adenomas. Higher values are also supported by a new statistical analysis. For histopathological lesions a NOAEC of 1ppm may be defined but the lesions at 2ppm cannot be regarded as pre-stages for tumour development. One major uncertainty exists: the description of polypoid adenomas and the lesions at 2ppm often is insufficient and diagnostic uncertainties can only be resolved by a re-evaluation according to modern histomorphological standards. Although the discrepancy between our assessment and that of RAC may seem rather small we feel the LOAECs proposed by RAC must be challenged taking into consideration the broad data base for formaldehyde and the potential impact of any published RAC opinion on the present discussions about appropriate occupational and indoor exposure limits.

Amended Safety Assessment of Formaldehyde and Methylene Glycol as Used in Cosmetics

Formaldehyde and methylene glycol may be used safely in cosmetics if established limits are not exceeded and are safe for use in nail hardeners in the present practices of use and concentration, which include instructions to avoid skin contact. In hair-smoothing products, however, in the present practices of use and concentration, formaldehyde and methylene glycol are unsafe. Methylene glycol is continuously converted to formaldehyde, and vice versa, even at equilibrium, which can be easily shifted by heating, drying, and other conditions to increase the amount of formaldehyde. This rapid, reversible formaldehyde/methylene glycol equilibrium is distinguished from the slow, irreversible release of formaldehyde resulting from the so-called formaldehyde releaser preservatives, which are not addressed in this safety assessment (formaldehyde releasers may continue to be safely used in cosmetics at the levels established in their individual Cosmetic Ingredient Review safety assessments).

Assessment of the mode of action underlying development of rodent small intestinal tumors following oral exposure to hexavalent chromium and relevance to humans

Abstract Chronic exposure to high concentrations of hexavalent chromium (Cr(VI)) in drinking water causes intestinal adenomas and carcinomas in mice, but not in rats. Cr(VI) causes damage to intestinal villi and crypt hyperplasia in mice after only one week of exposure. After two years of exposure, intestinal damage and crypt hyperplasia are evident in mice (but not rats), as are intestinal tumors. Although Cr(VI) has genotoxic properties, these findings suggest that intestinal tumors in mice arise as a result of chronic mucosal injury. To better understand the mode of action (MOA) of Cr(VI) in the intestine, a 90-day drinking water study was conducted to collect histological, biochemical, toxicogenomic and pharmacokinetic data in intestinal tissues. Using MOA analyses and human relevance frameworks proposed by national and international regulatory agencies, the weight of evidence supports a cytotoxic MOA with the following key events: (a) absorption of Cr(VI) from the intestinal lumen, (b) toxicity to intestinal villi, (c) crypt regenerative hyperplasia and (d) clonal expansion of mutations within the crypt stem cells, resulting in late onset tumorigenesis. This article summarizes the data supporting each key event in the MOA, as well as data that argue against a mutagenic MOA for Cr(VI)-induced intestinal tumors.

Recent trend in risk assessment of formaldehyde exposures from indoor air

Studies about formaldehyde (FA) published since the guideline of 0.1 mg/m(3) by the World Health Organization (WHO) in 2010 have been evaluated; critical effects were eye and nasal (portal-of-entry) irritation. Also, it was considered to prevent long-term effects, including all types of cancer. The majority of the recent toxicokinetic studies showed no exposure-dependent FA-DNA adducts outside the portal-of-entry area and FA-DNA adducts at distant sites were due to endogenously generated FA. The no-observed-adverse-effect level for sensory irritation was 0.5 ppm and recently reconfirmed in hypo- and hypersensitive individuals. Investigation of the relationship between FA exposure and asthma or other airway effects in children showed no convincing association. In rats, repeated exposures showed no point mutation in the p53 and K-Ras genes at ≤15 ppm neither increased cell proliferation, histopathological changes and changes in gene expression at 0.7 ppm. Repeated controlled exposures (0.5 ppm with peaks at 1 ppm) did not increase micronucleus formation in human buccal cells or nasal tissue (0.7 ppm) or in vivo genotoxicity in peripheral blood lymphocytes (0.7 ppm), but higher occupational exposures were associated with genotoxicity in buccal cells and cultivated peripheral blood lymphocytes. It is still valid that exposures not inducing nasal squamous cell carcinoma in rats will not induce nasopharyngeal cancer or lymphohematopoietic malignancies in humans. Reproductive and developmental toxicity are not considered relevant in the absence of sensory irritation. In conclusion, the WHO guideline has been strengthened.

Exposure to formaldehyde and its potential human health hazards

A widely used chemical, formaldehyde is normally present in both indoor and outdoor air. The rapid growth of formaldehyde-related industries in the past two decades reflects the result of its increased use in building materials and other commercial sectors. Consequently, formaldehyde is encountered almost every day from large segments of society due to its various sources. Many governments and agencies around the world have thus issued a series of standards to regulate its exposure in homes, office buildings, workshops, public places, and food. In light of the deleterious properties of formaldehyde, this article provides an overview of its market, regulation standards, and human health effects.

Identifying an indoor air exposure limit for formaldehyde considering both irritation and cancer hazards

Formaldehyde is a well-studied chemical and effects from inhalation exposures have been extensively characterized in numerous controlled studies with human volunteers, including asthmatics and other sensitive individuals, which provide a rich database on exposure concentrations that can reliably produce the symptoms of sensory irritation. Although individuals can differ in their sensitivity to odor and eye irritation, the majority of authoritative reviews of the formaldehyde literature have concluded that an air concentration of 0.3 ppm will provide protection from eye irritation for virtually everyone. A weight of evidence-based formaldehyde exposure limit of 0.1 ppm (100 ppb) is recommended as an indoor air level for all individuals for odor detection and sensory irritation. It has recently been suggested by the International Agency for Research on Cancer (IARC), the National Toxicology Program (NTP), and the US Environmental Protection Agency (US EPA) that formaldehyde is causally associated with nasopharyngeal cancer (NPC) and leukemia. This has led US EPA to conclude that irritation is not the most sensitive toxic endpoint and that carcinogenicity should dictate how to establish exposure limits for formaldehyde. In this review, a number of lines of reasoning and substantial scientific evidence are described and discussed, which leads to a conclusion that neither point of contact nor systemic effects of any type, including NPC or leukemia, are causally associated with exposure to formaldehyde. This conclusion supports the view that the equivocal epidemiology studies that suggest otherwise are almost certainly flawed by identified or yet to be unidentified confounding variables. Thus, this assessment concludes that a formaldehyde indoor air limit of 0.1 ppm should protect even particularly susceptible individuals from both irritation effects and any potential cancer hazard.

Accumulation of K-Ras codon 12 mutations in the F344 rat distal colon following azoxymethane exposure

Azoxymethane (AOM) administration to F344 male rats is a widely used model of human colon carcinogenesis. The current study investigates quantitatively the accumulation of K-Ras codon 12 mutations following AOM exposure. Male, 6-week-old F344 rats were treated subcutaneously with 30 mg/kg body weight of AOM, and colon tissue was collected at 1, 8, 24, and 32 weeks after treatment. The K-Ras codon 12 GGT to GAT and GGT to GTT mutant fractions (MFs) were measured using allele-specific competitive blocker polymerase chain reaction (ACB-PCR). Between 1 and 32 weeks after AOM treatment, the K-Ras codon 12 GGT to GAT geometric mean MF in the rat colon increased significantly from 12.9 × 10(-5) to 145 × 10(-5) , and the GGT to GTT geometric mean MF increased significantly from 5.26 × 10(-5) to 180 × 10(-5) . K-Ras codon 12 GGT to GAT MF also increased significantly in AOM-treated rat colon tissue at 1 week compared to controls (4.44 × 10(-5) ). The accumulation of the GGT to GAT MF long after the DNA adduct repair phase suggests that a portion of cells containing this mutation have a proliferative advantage, allowing them to accumulate as nascent tumors progress. Also, the GGT to GAT background MF increased in untreated rats, indicating that there is accumulation with age. The ACB-PCR assay generates quantitative data of cancer-related mutations and thus provides insight into pathological processes following carcinogen exposure.

p53 codon 271 CGT to CAT mutant fraction does not increase in nasal respiratory and olfactory epithelia of rats exposed to inhaled naphthalene

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 30ppm NA vapor for 13 weeks (6h/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 30ppm 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.

Analysis of micronuclei, histopathological changes and cell proliferation in nasal epithelium cells of rats after exposure to formaldehyde by inhalation

The frequencies of micronuclei (MN), histopathological changes and cell proliferation were determined in the nasal epithelium of male Fischer-344 rats after exposure to formaldehyde (FA) by whole-body inhalation for four weeks (6h/day, 5 days/week). Groups of 12 rats each were exposed to the target concentrations of 0, 0.5, 1, 2, 6, 10 and 15ppm. The micronucleus test (MNT) was performed with nasal epithelial cells prepared from six animals per group. Two thousand cells per animal were analysed for the presence of MN. The other six rats per group were subcutaneously implanted with osmotic pumps containing 5-bromo-2'-deoxyuridine (BrdUrd), three days prior to necropsy. Paraffin sections were made from the nasal cavity (four levels) of these animals for histopathology and cell-proliferation measurements. The frequency of cells with MN was not increased in any of the groups. However, there was also no induction of MN in nasal cells of rats exposed to a single dose of cyclophosphamide (CP, 20mg/kg) by gavage and analysed 3, 7, 14 or 28 days after the treatment. In contrast, nasal epithelial cells from rats exposed to 10 or 15ppm FA vapour showed clear site-specific pathological changes (focal epithelial degeneration, inflammation and squamous metaplasia) in a decreasing gradient (anterior to posterior). Analysis of slides after anti-BrdUrd antibody staining clearly indicated increased cell proliferation (unit length labelling indices, ULLI) after exposure to 6ppm and higher. No treatment-related effects were measured after exposure to 0.5, 1 and 2ppm. When comparing the cell-proliferation rate of normal epithelium with that of directly adjacent metaplastic epithelium, no consistent pattern was found: depending on the location, cell proliferation of normal epithelia was either higher or lower. Our results support previous findings demonstrating local cytotoxic effects in the nose of rats after inhalation of FA. However, induction of MN in the nasal epithelium as an indicator of a mutagenic effect was not seen. Because only limited experience exists for the MNT with rat nasal epithelial cells, this result has to be interpreted with great care. The contribution of mutagenicity to FA's carcinogenicity in rat nasal epithelium remains unclear.

Application of the U.S. EPA mode of action Framework for purposes of guiding future research: a case study involving the oral carcinogenicity of hexavalent chromium

Mode of action (MOA) analysis provides a systematic description of key events leading to adverse health effects in animal bioassays for the purpose of informing human health risk assessment. Uncertainties and data gaps identified in the MOA analysis may also be used to guide future research to improve understanding of the MOAs underlying a specific toxic response and foster development of toxicokinetic and toxicodynamic models. An MOA analysis, consistent with approaches outlined in the MOA Framework as described in the Guidelines for Carcinogen Risk Assessment, was conducted to evaluate small intestinal tumors observed in mice chronically exposed to relatively high concentrations of hexavalent chromium (Cr(VI)) in drinking water. Based on review of the literature, key events in the MOA are hypothesized to include saturation of the reductive capacity of the upper gastrointestinal tract, absorption of Cr(VI) into the intestinal epithelium, oxidative stress and inflammation, cell proliferation, direct and/or indirect DNA modification, and mutagenesis. Although available data generally support the plausibility of these key events, several unresolved questions and data gaps were identified, highlighting the need for obtaining critical toxicokinetic and toxicodynamic data in the target tissue and in the low-dose range. Experimental assays that can address these data gaps are discussed along with strategies for comparisons between responsive and nonresponsive tissues and species. This analysis provides a practical application of MOA Framework guidance and is instructive for the design of studies to improve upon the information available for quantitative risk assessment.

Oncomutations as biomarkers of cancer risk

Cancer risk assessment impacts a range of societal needs, from the regulation of chemicals to achieving the best possible human health outcomes. Because oncogene and tumor suppressor gene mutations are necessary for the development of cancer, such mutations are ideal biomarkers to use in cancer risk assessment. Consequently, DNA-based methods to quantify particular tumor-associated hotspot point mutations (i.e., oncomutations) have been developed, including allele-specific competitive blocker-PCR (ACB-PCR). Several studies using ACB-PCR and model mutagens have demonstrated that significant induction of tumor-associated oncomutations are measureable at earlier time points than are used to score tumors in a bioassay. In the particular case of benzo[a]pyrene induction of K-Ras codon 12 TGT mutation in the A/J mouse lung, measurement of tumor-associated oncomutation was shown to be an earlier and more sensitive endpoint than tumor response. The measurement of oncomutation by ACB-PCR led to two unexpected findings. First, oncomutations are present in various tissues of control rodents and "normal" human colonic mucosa samples at relatively high frequencies. Approximately 60% of such samples (88/146) have mutant fractions (MFs) >10(-5), and some have MFs as high as 10(-3) or 10(-4). Second, preliminary data indicate that oncomutations are present frequently as subpopulations in tumors. These findings are integrated into a hypothesis that the predominant preexisting mutations in particular tissues may be useful as generic reporters of carcinogenesis. Future research opportunities using oncomutation as an endpoint are described, including rodent to human extrapolation, dose-response assessment, and personalized medicine.