Mutant frequencies and spectra depend on growth state and passage number in cells cultured from transgenic lacZ-plasmid reporter mice

Prx1/PHB2 axis mediates mitophagy in oral leukoplakia cellular senescence

BACKGROUND

Oral leukoplakia (OLK) is the most common oral potentially malignant disorder (OPMD), which can be malignantly transformed into oral squamous cell carcinoma (OSCC). Peroxiredoxin1(Prx1) has been predicted to bind to Prohibitin2 (PHB2), which confers to affect OLK progression; however, the mechanism of Prx1/PHB2 mediated mitophagy involved in OLK remains unclear.

METHODS

This study aimed to explore the mechanism of the Prx1/PHB2 axis on senescence in OLK through mediating mitophagy. The positive rate of Ki67 and the expression of p21, p16, PHB2, and LC3 in human normal, OLK, and OSCC tissues were detected by immunohistochemical staining. The mitophagy and mitochondrial function changes were then analyzed in Prx1 knockdown and Prx1C52S mutations in dysplastic oral keratinocyte (DOK) cells treated with H2O2. In situ Proximity Ligation Assay combined with co-immunoprecipitation was used to detect the interaction between Prx1 and PHB2.

RESULTS

Clinically, the positive rate of Ki67 progressively increased from normal to OLK, OLK with dysplasia, and OSCC. Higher p21, p16, PHB2, and LC3 expression levels were observed in OLK with dysplasia than in normal and OSCC tissues. In vitro, PHB2 and LC3II expression gradually increased with the degree of DOK cell senescence. Prx1/PHB2 regulated mitophagy and affected senescence in H2O2-induced DOK cells. Furthermore, Prx1C52S mutation specifically reduced interaction between Prx1 and PHB2. Prx1Cys52 is associated with mitochondrial reactive oxygen species (ROS) accumulated and cell cycle arrest.

CONCLUSION

Prx1Cys52 functions as a redox sensor that binds to PHB2 and regulates mitophagy in the senescence of OLK, suggesting its potential as a clinical target.

The Neuro-Immuno-Senescence Integrative Model (NISIM) on the Negative Association Between Parasympathetic Activity and Cellular Senescence

There is evidence that accumulated senescent cells drive age-related pathologies, but the antecedents to the cellular stressors that induce senescence remain poorly understood. Previous research suggests that there is a relationship between shorter telomere length, an antecedent to cellular senescence, and psychological stress. Existing models do not sufficiently account for the specific pathways from which psychological stress regulation is converted into production of reactive oxygen species. We propose the neuro-immuno-senescence integrative model (NISIM) suggesting how vagally mediated heart rate variability (HRV) might be related to cellular senescence. Prefrontally modulated, and vagally mediated cortical influences on the autonomic nervous system, expressed as HRV, affects the immune system by adrenergic stimulation and cholinergic inhibition of cytokine production in macrophages and neutrophils. Previous findings indicate that low HRV is associated with increased production of the pro-inflammatory cytokines IL-6 and TNF-α. IL-6 and TNF-α can activate the NFκB pathway, increasing production of reactive oxygen species that can cause DNA damage. Vagally mediated HRV has been related to an individual's ability to regulate stress, and is lower in people with shorter telomeres. Based on these previous findings, the NISIM suggest that the main pathway from psychological stress to individual differences in oxidative telomere damage originates in the neuroanatomical components that modulate HRV, and culminates in the cytokine-induced activation of NFκB. Accumulated senescent cells in the brain is hypothesized to promote age-related neurodegenerative disease, and previous reports suggest an association between low HRV and onset of Alzheimer's and Parkinson's disease. Accumulating senescent cells in peripheral tissues secreting senescence-associated secretory phenotype factors can alter tissue structure and function which can induce cancer and promote tumor growth and metastasis in old age, and previous research suggested that ability to regulate psychological stress has a negative association with cancer onset. We therefore conclude that the NISIM can account for a large proportion of the individual differences in the psychological stress-related antecedents to cellular senescence, and suggest that it can be useful in providing a dynamic framework for understanding the pathways by which psychological stress induce pathologies in old age.

Somatic mutagenesis in satellite cells associates with human skeletal muscle aging

Human aging is associated with a decline in skeletal muscle (SkM) function and a reduction in the number and activity of satellite cells (SCs), the resident stem cells. To study the connection between SC aging and muscle impairment, we analyze the whole genome of single SC clones of the leg muscle vastus lateralis from healthy individuals of different ages (21–78 years). We find an accumulation rate of 13 somatic mutations per genome per year, consistent with proliferation of SCs in the healthy adult muscle. SkM-expressed genes are protected from mutations, but aging results in an increase in mutations in exons and promoters, targeting genes involved in SC activity and muscle function. In agreement with SC mutations affecting the whole tissue, we detect a missense mutation in a SC propagating to the muscle. Our results suggest somatic mutagenesis in SCs as a driving force in the age-related decline of SkM function.

Aging skeletal muscle shows declining numbers and activity of satellite cells. Here, Franco et al. show that in satellite cells of the human leg muscle vastus lateralis, somatic mutations accumulate with age and that these mutations become enriched in exons and promoters of genes involved in muscle function.

Differences between germline and somatic mutation rates in humans and mice

The germline mutation rate has been extensively studied and has been found to vary greatly between species, but much less is known about the somatic mutation rate in multicellular organisms, which remains very difficult to determine. Here, we present data on somatic mutation rates in mice and humans, obtained by sequencing single cells and clones derived from primary fibroblasts, which allows us to make the first direct comparison with germline mutation rates in these two species. The results indicate that the somatic mutation rate is almost two orders of magnitude higher than the germline mutation rate and that both mutation rates are significantly higher in mice than in humans. Our findings demonstrate both the privileged status of germline genome integrity and species-specific differences in genome maintenance.

Germline mutation rates are known to vary between species but somatic mutation rates are less well understood. Here the authors compare mice and humans, observing that somatic mutation rates were nearly two orders of magnitude higher in both species, with both mutation rates significantly higher in mice.

Evaluation of the LacZ reporter assay in cryopreserved primary hepatocytes for In vitro genotoxicity testing

Assessment of genotoxic potential is an important step in the safety evaluation of chemical substances. Under most regulatory jurisdictions, the first tier of testing comprises a standard battery of in vitro genotoxicity tests in bacterial and mammalian cells. However, the mammalian cell tests commonly used exhibit a relatively high rate of misleading positive results, which may lead to unnecessary in vivo testing. We previously established a proof-of-concept for the LacZ reporter assay in proliferating primary hepatocytes as a promising alternative genotoxicity test. Here, cryopreserved instead of freshly isolated hepatocytes were used and the assay was evaluated in more detail. We examined the effect of cryopreservation on phenotype and metabolic capacity of the LacZ hepatocytes, and assessed the predictive performance of the assay by testing a set of substances comprising true positive, true negative, and misleading positive substances. Additionally, a historical negative control database was created and the type of mutations induced was analyzed for two of the substances tested. Our findings indicate that proliferating cryopreserved primary hepatocytes derived from LacZ plasmid mice retain their hepatocyte-specific characteristics and metabolic competence. Furthermore, we demonstrate that both gene mutations and genome rearrangements due to large deletions can be detected with the LacZ reporter assay. The assay seems to have a lower rate of misleading positive test results compared to the assays currently used. Together, our findings strongly support the use of the LacZ reporter assay in cryopreserved primary hepatocytes as follow-up to the standard in vitro test battery for genotoxicity testing. Environ. Mol. Mutagen. 57:643-655, 2016. © 2016 Wiley Periodicals, Inc.

Association of TGFβ signaling with the maintenance of a quiescent stem cell niche in human oral mucosa

A dogma in squamous epithelial biology is that proliferation occurs in the basal cell layer. Notable exceptions are squamous epithelia of the human oral cavity, esophagus, ectocervix, and vagina. In these human epithelia, proliferation is rare in the basal cell layer, and the vast majority of cells positive for Ki67 and other proliferation markers are found in para- and suprabasal cell layers. This unique human feature of a generally quiescent basal cell layer overlaid by highly proliferative cells offers the rare opportunity to study the molecular features of undifferentiated, quiescent, putative stem cells in their natural context. Here, we show that the quiescent human oral mucosa basal cell layer expresses putative markers of stemness, while para- and suprabasal cells are characterized by cell cycle genes. We identified a TGFβ signature in this quiescent basal cell layer. In in vitro organotypic cultures, human keratinocytes could be induced to express markers of these quiescent basal cells when TGFβ signaling is activated. The study suggests that the separation of basal cell layer and proliferation in human oral mucosa may function to accommodate high proliferation rates and the protection of a quiescent reserve stem cell pool. Psoriasis, an epidermal inflammatory hyperproliferative disease, exhibits features of a quiescent basal cell layer mimicking normal oral mucosa. Our data indicate that structural changes in the organization of epithelial proliferation could contribute to longevity and carcinogenesis.

TP53 and lacZ mutagenesis induced by 3-nitrobenzanthrone in Xpa-deficient human TP53 knock-in mouse embryo fibroblasts

3-Nitrobenzanthrone (3-NBA) is a highly mutagenic compound and possible human carcinogen found in diesel exhaust. 3-NBA forms bulky DNA adducts following metabolic activation and induces predominantly G:CT:A transversions in a variety of experimental systems. Here we investigated the influence of nucleotide excision repair (NER) on 3-NBA-induced mutagenesis of the human tumour suppressor gene TP53 and the reporter gene lacZ. To this end we utilised Xpa -knockout (Xpa-Null) human TP53 knock-in (Hupki) embryo fibroblasts (HUFs). As Xpa is essential for NER of bulky DNA adducts, we hypothesized that DNA adducts induced by 3-NBA would persist in the genomes of Xpa-Null cells and lead to an increased frequency of mutation. The HUF immortalisation assay was used to select for cells harbouring TP53 mutations following mutagen exposure. We found that Xpa-Null Hupki mice and HUFs were more sensitive to 3-NBA treatment than their wild-type (Xpa-WT) counterparts. However, following 3-NBA treatment and immortalisation, a similar frequency of TP53-mutant clones arose from Xpa-WT and Xpa-Null HUF cultures. In cells from both Xpa genotypes G:CT:A transversion was the predominant TP53 mutation type and mutations exhibited bias towards the non-transcribed strand. Thirty-two percent of 3-NBA-induced TP53 mutations occurred at CpG sites, all of which are hotspots for mutation in smokers' lung cancer (codons 157, 158, 175, 245, 248, 273, 282). We also examined 3-NBA-induced mutagenesis of an integrated lacZ reporter gene in HUFs, where we again observed a similar mutant frequency in Xpa-WT and Xpa-Null cells. Our findings suggest that 3-NBA-DNA adducts may evade removal by global genomic NER; the persistence of 3-NBA adducts in DNA may be an important factor in its mutagenicity.

Cellular senescence and tumor promotion: Is aging the key?

The senescence response is a potent tumor suppressor mechanism characterized by an irreversible growth arrest in response to potentially oncogenic signals to prevent the proliferation of damaged cells. Late in life, some of the features of senescent cells seem to mediate the development of age-related pathologies, including cancer. In the present review, we present a summary of the current knowledge regarding the causes, effector pathways and cellular features of senescence. We also discuss how the senescence response, initially a tumor suppressor mechanism, turns into a tumor promoter apparently as a consequence of aging. We argue that three age-related phenomena--senescence-associated secretory phenotype (SASP) dysregulation, decline in the immune system function and genomic instability--could contribute, independently or synergistically, to deteriorate the efficacy of the senescence response in stopping cancer. As a consequence, senescent cells could be considered premalignant cells, and targeting senescent cells could be a preventive and therapeutic strategy against cancer.

Effect of Ku80 deficiency on mutation frequencies and spectra at a LacZ reporter locus in mouse tissues and cells

Non-homologous end joining (NHEJ) is thought to be an important mechanism for preventing the adverse effects of DNA double strand breaks (DSBs) and its absence has been associated with premature aging. To investigate the effect of inactivated NHEJ on spontaneous mutation frequencies and spectra in vivo and in cultured cells, we crossed a Ku80-deficient mouse with mice harboring a lacZ-plasmid-based mutation reporter. We analyzed various organs and tissues, as well as cultured embryonic fibroblasts, for mutations at the lacZ locus. When comparing mutant with wild-type mice, we observed a significantly higher number of genome rearrangements in liver and spleen and a significantly lower number of point mutations in liver and brain. The reduced point mutation frequency was not due to a decrease in small deletion mutations thought to be a hallmark of NHEJ, but could be a consequence of increased cellular responses to unrepaired DSBs. Indeed, we found a substantial increase in persistent 53BP1 and γH2AX DNA damage foci in Ku80^−/− as compared to wild-type liver. Treatment of cultured Ku80-deficient or wild-type embryonic fibroblasts, either proliferating or quiescent, with hydrogen peroxide or bleomycin showed no differences in the number or type of induced genome rearrangements. However, after such treatment, Ku80-deficient cells did show an increased number of persistent DNA damage foci. These results indicate that Ku80-dependent repair of DNA damage is predominantly error-free with the effect of alternative more error-prone pathways creating genome rearrangements only detectable after extended periods of time, i.e., in young adult animals. The observed premature aging likely results from a combination of increased cellular senescence and an increased load of stable, genome rearrangements.

Rapid repair of UVA-induced oxidized purines and persistence of UVB-induced dipyrimidine lesions determine the mutagenicity of sunlight in mouse cells

Despite the predominance of ultraviolet A (UVA) relative to UVB in terrestrial sunlight, solar mutagenesis in humans and rodents is characterized by mutations specific for UVB. We have investigated the kinetics of repair of UVA- and UVB-induced DNA lesions in relation to mutagenicity in transgenic mouse fibroblasts irradiated with equilethal doses of UVA and UVB in comparison to simulated-sunlight UV (SSL). We have also analyzed mutagenesis-derived carcinogenesis in sunlight-associated human skin cancers by compiling the published data on mutation types found in crucial genes in nonmelanoma and melanoma skin cancers. Here, we demonstrate a resistance to repair of UVB-induced cis-syn cyclobutane pyrimidine-dimers (CPDs) together with rapid removal of UVA-induced oxidized purines in the genome overall and in the cII transgene of SSL-irradiated cells. The spectra of mutation induced by both UVB and SSL irradiation in this experimental system are characterized by significant increases in relative frequency of C-->T transitions at dipyrimidines, which are the established signature mutation of CPDs. This type of mutation is also the predominant mutation found in human nonmelanoma and melanoma tumor samples in the TP53, CDKN2, PTCH, and protein kinase genes. The prevailing role of UVB over UVA in solar mutagenesis in our test system can be ascribed to different kinetics of repair for lesions induced by the respective UV irradiation.

Genetic analysis reveals an intrinsic property of the germinal center B cells to generate A:T mutations

The immunoglobulin genes undergo a high frequency of point mutations at both C:G and A:T pairs in the germinal center (GC) B cells. This hypermutation process is initiated by the activation-induced cytidine deaminase (AID), which converts cytosine to uracil and generates a U:G lesion. Replication of this lesion, or its repair intermediate the abasic site, could introduce C:G mutations but the mechanisms leading to mutations at non-damaged A:T pairs remain elusive. Using a lacZ-transgenic system in which endogenous genome mutations can be detected with high sensitivity, we found that GC B cells exhibited a much higher ratio of A:T mutations as compared to naïve B, non-GC B, and cells of other tissues. This property does not require AID or active transcription of the target gene, and is dependent on DNA polymerase eta. These in vivo results demonstrate that GC B cells are unique in having an intrinsic propensity to generate A:T mutations during repair of endogenous DNA damage. These findings have important implications in understanding how AID, which can only target C:G base pairs, is able to induce the entire spectrum of mutations observed in immunoglobulin variable region genes in GC B cells.

Mutation frequencies and spectra in DNA polymerase eta-deficient mice

The low-fidelity polymerase eta (poleta) is required for bypass of UV-induced pyrimidine dimers inserting adenine nucleotides opposite these lesions. Mutations in the poleta gene are responsible for the genetic defect in xeroderma pigmentosum variant patients. To study if the lack of poleta significantly elevates spontaneous mutation frequency in various organs and tissues of the mouse, we crossed poleta-deficient mice with transgenic mice harboring a chromosomally integrated lacZ-plasmid reporter construct. In cultured embryonic fibroblasts from the lacZ-poleta(-/-) mice, 2.5 J/m(2) UV irradiation induced approximately 5-fold more mutations than in cells from lacZ control mice, in which an approximately 3-fold increase in mutation frequency was found compared with the normal level. Whereas untreated cells harbored mainly 1-bp deletions, UV induced both transitions and transversions, with the latter type more highly represented in the poleta-null cells than in the controls. No difference in mutation induction between the poleta-null cells and the wild-type cells was observed after treatment with N-ethyl-N-nitrosourea. Having shown the validity of the lacZ model to accurately identify poleta-associated mutagenesis, we then determined the mutant frequency at the lacZ locus in liver, spleen, and small intestine of 12-month-old animals. No differences were found between poleta-null, heterozygous, or littermate control mice. We conclude that the poleta defect is specific for UV damage and has no effect on in vivo mutagenesis in mice.

Long-term mutagenic effects of ionising radiation on mice which vary in their p53 status

The tumor suppressor gene p53 plays a major role in the maintenance of genomic integrity. The impact that variations in cellular turnover rates and sensitivity to DNA damage will have on the effectiveness of p53 in this role was examined by following the induction and persistence of mutations in the brain and small intestine of mice after exposure to ionising radiation (IR). The examination of mutagenesis was carried out using the pUR288 LacZ plasmid-based mouse model-consisting of mice containing a target gene for mutation analysis integrated into every cell. In addition the mice varied in their p53 status. The tissues were compared at post-irradiation time-points from 24h to 3 months. The mutation frequencies (MFs) in the p53 wildtype and heterozygous brains peaked at 24h post-irradiation, and then returned to background or close to background levels, respectively. The p53 nullizygous brain showed a more fluctuating MF pattern, but returned to background levels by 3 months, indicating that the effect of the loss of p53 did not result in lasting differences in the response to mutation induction in the brain. In the intestine, there was a different pattern; in the wildtype and heterozygous animals, the MFs increased from 24h to a peak at 4 weeks post-irradiation, before decreasing towards background levels at 3 months. The MFs in the intestine from the nullizygous animals did not decrease significantly between 4 weeks and 3 months, illustrating that the loss of p53 had a greater impact in this tissue than the brain. The variation in mutation frequencies and the type of mutations generated after DNA damage suggests that while p53 plays a significant role in the maintenance of genomic integrity, other mechanisms, such as the drive to replicate in progenitor cells, can reduce its effectiveness as the "guardian of the genome".

Cellular senescence: when bad things happen to good cells

Cells continually experience stress and damage from exogenous and endogenous sources, and their responses range from complete recovery to cell death. Proliferating cells can initiate an additional response by adopting a state of permanent cell-cycle arrest that is termed cellular senescence. Understanding the causes and consequences of cellular senescence has provided novel insights into how cells react to stress, especially genotoxic stress, and how this cellular response can affect complex organismal processes such as the development of cancer and ageing.

Intra-organ variation in age-related mutation accumulation in the mouse

Using a transgenic mouse model harboring chromosomally integrated lacZ mutational target genes, we previously demonstrated that mutations accumulate with age much more rapidly in the small intestine than in the brain. Here it is shown that in the small intestine point mutations preferentially accumulate in epithelial cells of the mucosa scraped off the underlying serosa. The mucosal cells are the differentiated villus cells that have undergone multiple cell divisions. A smaller age-related increase, also involving genome rearrangements, was observed in the serosa, which consists mainly of the remaining crypts and non-dividing smooth muscle cells. In the brain we observed an accumulation of only point mutations in no other areas than hypothalamus and hippocampus. To directly test for cell division as the determining factor in the generation of point mutations we compared mutation induction between mitotically active and quiescent embryonic fibroblasts from the same lacZ mice, treated with either UV (a point mutagen) or hydrogen peroxide (a clastogen). The results indicate that while point mutations are highly replication-dependent, genome rearrangements are as easily induced in non-dividing cells as in mitotically active ones. This strongly suggests that the point mutations found to have accumulated in the mucosal part of the small intestine are the consequence of replication errors. The same is likely true for point mutations accumulating in hippocampus and hypothalamus of the brain since neurogenesis in these two areas continues throughout life. The observed intra-organ variation in mutation susceptibility as well as the variation in replication dependency of different types of mutations indicates the need to not only extend observations made on whole organs to their sub-structures but also take the type of mutations and mitotic activity of the cells into consideration. This should help elucidating the impact of genome instability and its consequences on aging and disease.

A model system for analyzing somatic mutations in Drosophila melanogaster

Presently there are no good assays for comparing somatic mutation frequencies and spectra between different vertebrate and invertebrate organisms. Here we describe a new lacZ mutation reporter system in D. melanogaster, which complements existing systems in the mouse. The results obtained with the new model indicate two-to threefold higher frequencies of spontaneous mutations than in the mouse, with most of the mutations characterized as large genome rearrangements.

Detection and analysis of somatic mutations at a lacZ reporter locus in higher organisms: application to Mus musculus and Drosophila melanogaster

Methods to detect and analyze somatic mutations in higher organisms are critically important in view of their causal role in cancer, heritable diseases, and, possibly, aging. Here, we describe detailed protocols for the use of a mutational reporter system based on lacZ-containing plasmids integrated in the germline of Mus musculus and Drosophila melanogaster. Plasmids containing the bacterial lacZ gene integrated at one or more chromosomal sites can be excised, purified and recovered in suitable Escherichia coli hosts allowing the positive selection of mutant lacZ genes and their further molecular characterization. This system is capable of detecting a broad range of mutational events, varying from small mutations in the lacZ reporter gene to large genome rearrangements with one breakpoint in lacZ and the other breakpoint elsewhere in the genome.

Genome dynamics and transcriptional deregulation in aging

Genome instability has been implicated as a major cause of both cancer and aging. Using a lacZ-plasmid transgenic mouse model we have shown that mutations accumulate with age in a tissue-specific manner. Genome rearrangements, including translocations and large deletions, are a major component of the mutation spectrum in some tissues at old age such as heart. Such large mutations were also induced by hydrogen peroxide (H2O2) in lacZ-plasmid mouse embryonic fibroblasts (MEFs) and demonstrated to be replication-independent. This was in contrast to ultraviolet light-induced point mutations, which were much more abundant in proliferating than in quiescent MEFs. To test if large rearrangements could adversely affect patterns of gene expression we PCR-amplified global mRNA content of single MEFs treated with H2O2. Such treatment resulted in a significant increase in cell-to-cell variation in gene expression, which was found to parallel the induction and persistence of genome rearrangement mutations at the lacZ reporter locus. Increased transcriptional noise was also found among single cardiomyocytes from old mice as compared with similar cells from young mice. While these results do not directly indicate a cause and effect relationship between genome rearrangement mutations and transcriptional deregulation, they do underscore the stochastic nature of genotoxic effects on cells and tissues and could provide a mechanism for age-related cellular degeneration in postmitotic tissue, such as heart or brain.

Mutation Accumulation in the Intestine and Colon of Mice Deficient in Two Intracellular Glutathione Peroxidases

Mice deficient in two glutathione peroxidases (GPX), Gpx1 and Gpx2, [Gpx1/2-double knockout (DKO) mice] are prone to ileocolitis on a mixed C57BL/6 and 129S1/SvJ (B6.129) genetic background. We reported previously that approximately 25% of B6.129 Gpx1/2-DKO mice develop ileocolonic tumors by 6 to 9 months of age, when their non-DKO littermates [having at least one wild-type (WT) Gpx1 or Gpx2 allele] rarely have inflammation and none have tumors. Because genetic background affects tumor susceptibility, we have generated a B6 Gpx1/2-DKO colony and discovered that these mice have fewer inflammatory cells, milder ileocolitis, and low mortality, and only 2.5% of B6 mice developed tumors. The mutant frequency of a cII reporter gene was about 2- to 3-fold higher in 28-day-old Gpx1/2-DKO and 4-fold higher in 8-month-old Gpx1/2-DKO ileal mucosa than in controls in both genetic backgrounds. In contrast, mutant frequencies in the unaffected B6 liver were not significantly different between WT and Gpx1/2-DKO mice. The mutant frequency of 8-month-old B6.129 Gpx1/2-DKO ileum was 38.94 +/- 15.5(-5), which was not significantly higher than the age-matched B6 ileum, 25.54 +/- 10.33(-5). The mutation spectra analysis has shown that B6 Gpx1/2-DKO ileum had a 3-fold increase in small nucleotide deletions at mononucleotide repeats over control B6, which are a signature mutation associated with oxidative stress. Unexpectedly, B6 Gpx1/2-DKO mice had fewer C to T transitions at CpG dinucleotides than the WT B6 (18.0% versus 40.1%; P < 0.001). Our results suggest that inflammation drives gene mutations, which leads to neoplastic transformation of intestinal epithelium in the B6.129 Gpx1/2-DKO mice but rarely in the B6 Gpx1/2-DKO mice.

Genotoxic effects of bitumen fumes in Big Blue transgenic rat lung

Road paving workers are exposed to bitumen fumes (CAS No. 8052-42-4), a complex mixture of volatile compounds and particles containing carcinogenic and non-carcinogenic polycyclic aromatic hydrocarbons. However, epidemiological and experimental animal studies failed to draw unambiguous conclusions concerning their toxicity. In order to gain better insights on their genotoxic potential, we used an experimental design able to generate bitumen fumes at road paving temperature (temperature: 170 degrees C, total particulate matter: 100mg/m3) and perform a nose-only exposure of Big Blue transgenic rodents 6h/day for five consecutive days. The mutagenic properties of bitumen fumes were determined by analyzing the mutation frequency and spectrum of the neutral reporter gene cII inserted into the rodent genome. We previously observed in mouse lung, that bitumen fumes did not induce an increase of cII mutants, a modification of the mutation spectrum, nor the formation of DNA adducts. Since DNA adducts were found in the lungs of rats exposed to asphalt fumes in similar conditions, we decided to carry out an analogous experiment with Big Blue rats. A DNA adduct was detected 3 and 30 days after the end of treatment suggesting that these genetic alterations were quite steady. Thirty days after exposure, the cII mutant frequency was similar in control and exposed rats. In addition, a slight but not significant modification of the mutation spectrum associated with an increase of G:C to T:A and A:T to C:G transversions was noticeable in the treated animals. Then, these data failed to demonstrate a pulmonary mutagenic potential for bitumen fumes generated at road paving temperature in our experimental conditions despite the presence of a DNA adduct. These results may provide information concerning the pulmonary mechanism of action of this aerosol and may contribute to the occupational health hazard assessment.