The impact of FANCD2 deficiency on formaldehyde-induced toxicity in human lymphoblastoid cell lines

Fanconi anemia: current insights regarding epidemiology, cancer, and DNA repair

Fanconi anemia is a genetic disorder that is characterized by bone marrow failure, as well as a predisposition to malignancies including leukemia and squamous cell carcinoma (SCC). At least 22 genes are associated with Fanconi anemia, constituting the Fanconi anemia DNA repair pathway. This pathway coordinates multiple processes and proteins to facilitate the repair of DNA adducts including interstrand crosslinks (ICLs) that are generated by environmental carcinogens, chemotherapeutic crosslinkers, and metabolic products of alcohol. ICLs can interfere with DNA transactions, including replication and transcription. If not properly removed and repaired, ICLs cause DNA breaks and lead to genomic instability, a hallmark of cancer. In this review, we will discuss the genetic and phenotypic characteristics of Fanconi anemia, the epidemiology of the disease, and associated cancer risk. The sources of ICLs and the role of ICL-inducing chemotherapeutic agents will also be discussed. Finally, we will review the detailed mechanisms of ICL repair via the Fanconi anemia DNA repair pathway, highlighting critical regulatory processes. Together, the information in this review will underscore important contributions to Fanconi anemia research in the past two decades.

Evaluating the Sub-Acute Toxicity of Formaldehyde Fumes in an In Vitro Human Airway Epithelial Tissue Model

Formaldehyde (FA) is an irritating, highly reactive aldehyde that is widely regarded as an asthmagen. In addition to its use in industrial applications and being a product of combustion reaction and endogenous metabolism, FDA-regulated products may contain FA or release FA fumes that present toxicity risks for both patients and healthcare workers. Exposure to airborne FA is associated with nasal neoplastic lesions in both animals and humans. It is classified as a Group 1 carcinogen by International Agency for Research on Cancer (IARC) based on the increased incidence of cancer in animals and a known human carcinogen in the Report on Carcinogens by National Toxicology Program (NTP). Herein, we systematically evaluated the tissue responses to FA fumes in an in vitro human air-liquid-interface (ALI) airway tissue model. Cultures were exposed at the air interface to 7.5, 15, and 30 ppm of FA fumes 4 h per day for 5 consecutive days. Exposure to 30 ppm of FA induced sustained oxidative stress, along with functional changes in ciliated and goblet cells as well as possible squamous differentiation. Furthermore, secretion of the proinflammatory cytokines, IL-1β, IL-2, IL-8, GM-CSF, TNF-a and IFN-γ, was induced by repeated exposures to FA fumes. Expression of MMP-1, MMP-3, MMP-7, MMP-10, MMP-12, and MMP-13 was downregulated at the end of the 5-day exposure. Although DNA-damage was not detected by the comet assay, FA exposures downregulated the DNA repair enzymes MGMT and FANCD2, suggesting its possible interference in the DNA repair capacity. Overall, a general concordance was observed between our in vitro responses to FA fume exposures and the reported in vivo toxicity of FA. Our findings provide further evidence supporting the application of the ALI airway system as a potential in vitro alternative for screening and evaluating the respiratory toxicity of inhaled substances.

Endogenous formaldehyde scavenges cellular glutathione resulting in redox disruption and cytotoxicity

Formaldehyde (FA) is a ubiquitous endogenous and environmental metabolite that is thought to exert cytotoxicity through DNA and DNA-protein crosslinking, likely contributing to the onset of the human DNA repair condition Fanconi Anaemia. Mutations in the genes coding for FA detoxifying enzymes underlie a human inherited bone marrow failure syndrome (IBMFS), even in the presence of functional DNA repair, raising the question of whether FA causes relevant cellular damage beyond genotoxicity. Here, we report that FA triggers cellular redox imbalance in human cells and in Caenorhabditis elegans. Mechanistically, FA reacts with the redox-active thiol group of glutathione (GSH), altering the GSH:GSSG ratio and causing oxidative stress. FA cytotoxicity is prevented by the enzyme alcohol dehydrogenase 5 (ADH5/GSNOR), which metabolizes FA-GSH products, lastly yielding reduced GSH. Furthermore, we show that GSH synthesis protects human cells from FA, indicating an active role of GSH in preventing FA toxicity. These findings might be relevant for patients carrying mutations in FA-detoxification systems and could suggest therapeutic benefits from thiol-rich antioxidants like N-acetyl-L-cysteine.

Formaldehyde (FA) is known to exert cytotoxicity through DNA damage. Here, the authors show that FA also triggers cellular redox imbalance by reacting with glutathione (GSH), and that FA cytotoxicity is prevented by GSH synthesis and by ADH5, an enzyme that metabolizes FA-GSH products.

Applying genome-wide CRISPR to identify known and novel genes and pathways that modulate formaldehyde toxicity

Formaldehyde (FA), a ubiquitous environmental pollutant, is classified as a Group I human carcinogen by the International Agency for Research on Cancer. Previously, we reported that FA induced hematotoxicity and chromosomal aneuploidy in exposed workers and toxicity in bone marrow and hematopoietic stem cells of experimental animals. Using functional toxicogenomic profiling in yeast, we identified genes and cellular processes modulating eukaryotic FA cytotoxicity. Although we validated some of these findings in yeast, many specific genes, pathways and mechanisms of action of FA in human cells are not known. In the current study, we applied genome-wide, loss-of-function CRISPR screening to identify modulators of FA toxicity in the human hematopoietic K562 cell line. We assessed the cellular genetic determinants of susceptibility and resistance to FA at 40, 100 and 150 μM (IC10, IC20 and IC60, respectively) at two time points, day 8 and day 20. We identified multiple candidate genes that increase sensitivity (e.g. ADH5, ESD and FANC family) or resistance (e.g. FASN and KDM6A) to FA when disrupted. Pathway analysis revealed a major role for the FA metabolism and Fanconi anemia pathway in FA tolerance, consistent with findings from previous studies. Additional network analyses revealed potential new roles for one-carbon metabolism, fatty acid synthesis and mTOR signaling in modulating FA toxicity. Validation of these novel findings will further enhance our understanding of FA toxicity in human cells. Our findings support the utility of CRISPR-based functional genomics screening of environmental chemicals.

Combined exposure to formaldehyde and PM2.5: Hematopoietic toxicity and molecular mechanism in mice

PM2.5 and formaldehyde (FA) are major outdoor and indoor air pollutants in China, respectively, and both are known to be harmful to human health and to be carcinogenic. Of all the known chronic health effects, leukaemia is one of the most serious health risks associated with these two pollutants. To explore the influence and underlying mechanisms of exposure to formaldehyde and PM2.5 on hematopoietic toxicity, we systematically studied the toxicity induced in hematopoietic organs: bone marrow (BM); spleen; and myeloid progenitor cells (MPCs). Male Balb/c mice were exposed to: PM2.5 (20, 160 μg/kg·d) at a dose of 40 μL per mouse or formaldehyde (0.5, 3.0 mg/m3) for 8 h per day for 2 weeks or co-exposed to formaldehyde and PM2.5 (20 μg/kg·d PM2.5 + 0.5 mg/m3 FA, 20 μg/kg·d PM2.5 + 3 mg/m3 FA, 160 μg/kg·d PM2.5 + 0.5 mg/m3 FA, 160 μg/kg·d PM2.5 + 3 mg/m3 FA) for 2 weeks. Similar toxic effects were found in the formaldehyde-only and PM2.5-only groups, including significant decrease of blood cells and MPCs, along with decreased expression of hematopoietic growth factors. In addition, individual exposure of formaldehyde or PM2.5 increased oxidative stress, DNA damage and immune system disorder by destroying the balance of Th1/Th2, and Treg/Th17. DNA repair was markedly inhibited by deregulating the mammalian target of rapamycin (mTOR) pathway. Combined exposure to PM2.5 and formaldehyde led to more severe effects. Administration of Vitamin E (VE) was shown to attenuate these effects. In conclusion, our findings suggested that PM2.5 and formaldehyde may induce hematopoietic toxicity by reducing the expression of hematopoietic growth factors, increasing oxidative stress and DNA damage, activating the 'immune imbalance' pathway and suppressing the DNA-repair related mTOR pathway. The hematopoietic toxicity induced by combined exposure of PM2.5 and formaldehyde might provide further insights into the increased incidence of hematological diseases, including human myeloid leukaemia.

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Formaldehyde inhibits UV-induced phosphorylation of histone H2AX

Formaldehyde (FA) is widely known to cause DNA damage. Recently, our study showed that FA can also inhibit a repair process of DNA damage, nucleotide excision repair (NER). DNA damage response (DDR) involving activation of phosphorylation pathways is important for the accuracy of the repair process, and the inhibition of the accurate repair would raise mutation rate, leading to cancer. We herein investigated whether FA influences phosphorylation of histone H2AX (γ-H2AX), an intermediate player of DDR signaling pathways. Human keratinocytes HaCaT were treated with FA and then exposed to UV known to generate clear γ-H2AX signal. UV-induced γ-H2AX was inhibited by FA in a dose-dependent manner. The repair of pyrimidine dimers was inhibited by FA, while the recruitments of γ-H2AX-related proteins, Mre11 and 53BP1, to damaged sites were also delayed. Mre11, Nbs-1, H2AX and ATM were not degraded after treatment with FA as opposed to NER-related protein, TFIIH. On the other hand, FA inhibited phosphorylation of ATM which acts upstream of γ-H2AX. These results suggest that FA can affect the repair of DNA damage via inhibition of the phosphorylation pathways of H2AX.

Evaluation of inhaled low-dose formaldehyde-induced DNA adducts and DNA-protein cross-links by liquid chromatography-tandem mass spectrometry

As a widespread industrial chemical, formaldehyde carcinogenicity has been highly controversial. Meanwhile, formaldehyde is an essential metabolite in all living cells. Previously, we have demonstrated exogenous formaldehyde causes DNA adducts in a nonlinear manner between 0.7 and 15.2 ppm using [¹³CD₂]-formaldehyde for exposure coupled with the use of sensitive mass spectrometry. However, the responses from exposure to low doses of formaldehyde are still unknown. In this study, rats were exposed to 1, 30, and 300 ppb [¹³CD₂]-formaldehyde for 28 days (6 h/day) by nose-only inhalation, followed by measuring DNA mono-adduct (N²-HOMe-dG) and DNA-protein crosslinks (dG-Me-Cys) as formaldehyde specific biomarkers. Both exogenous and endogenous DNA mono-adducts and dG-Me-Cys were examined with ultrasensitive nano-liquid chromatography-tandem mass spectrometry. Our data clearly show that endogenous adducts are present in all tissues analyzed, but exogenous adducts were not detectable in any tissue samples, including the most susceptible nasal epithelium. Moreover, formaldehyde exposure at 1, 30 and 300 ppb did not alter the levels of endogenous formaldehyde-induced DNA adducts or DNA-protein crosslinks. The novel findings from this study provide new data for risk assessment of exposure to low doses of formaldehyde.

Estimation of formaldehyde occupational exposure limit based on genetic damage in some Iranian exposed workers using benchmark dose method

The present study evaluated an occupational exposure level for formaldehyde employing benchmark dose (BMD) approach. Dose-response relationship was determined by utilizing cumulative occupational exposure dose and DNA damage. Based on this goal, outcome of comet assay for some Iranian exposed people in occupational exposure individuals was used. In order to assess formaldehyde exposure, 53 occupationally exposed individuals selected from four melamine tableware workshops and 34 unexposed subjects as a control group were examined. The occupational exposure dose was carried out according to the NIOSH-3500 method, and the DNA damage was obtained by employing comet assay in peripheral blood cells. EPA Benchmark Dose Software was employed for calculating BMD and BMDL. Cumulative exposure dose of formaldehyde was between of 2.4 and 1972 mg. According to the findings of the current study, the induction of DNA damage in the exposed persons was increased tail length and tail moment (p < 0.001), when compared to controls. Finally, an acceptable dose-response relationship was obtained in three-category information between formaldehyde cumulative exposure doses and genetic toxicity. BMDL was 0.034 mg/m³ (0.028 ppm), corresponding to genetic damage of peripheral blood cells. It can be concluded that the occupational permissible limit in Iranian people could be at levels lower than OSHA standards.

An RNAi screen in human cell lines reveals conserved DNA damage repair pathways that mitigate formaldehyde sensitivity

Formaldehyde is a ubiquitous DNA damaging agent, with human exposures occurring from both exogenous and endogenous sources. Formaldehyde exposure can result in multiple types of DNA damage, including DNA-protein crosslinks and thus, is representative of other exposures that induce DNA-protein crosslinks such as cigarette smoke, automobile exhaust, wood smoke, metals, ionizing radiation, and certain chemotherapeutics. Our objective in this study was to identify the genes necessary to mitigate formaldehyde toxicity following chronic exposure in human cells. We used siRNAs that targeted 320 genes representing all major human DNA repair and damage response pathways, in order to assess cell proliferation following siRNA depletion and subsequent formaldehyde treatment. Three unrelated human cell lines frequently used in genotoxicity studies (SW480, U-2 OS and GM00639) were used to identify common pathways involved in mitigating formaldehyde sensitivity. Although there were gene-specific differences among the cell lines, four inter-related cellular pathways were determined to mitigate formaldehyde toxicity: homologous recombination, DNA double-strand break repair, ionizing radiation response and DNA replication. Additional insight into cell line-specific response patterns was obtained by using a combination of exome sequencing and Cancer Cell Line Encyclopedia genomic data. The results of this DNA damage repair pathway-focused siRNA screen for formaldehyde toxicity in human cells provide a foundation for detailed mechanistic analyses of pathway-specific involvement in the response to environmentally-induced DNA-protein crosslinks and, more broadly, genotoxicity studies using human and other mammalian cell lines.

A quantitative PCR-based assay reveals that nucleotide excision repair plays a predominant role in the removal of DNA-protein crosslinks from plasmids transfected into mammalian cells

DNA-protein crosslinks (DPCs) are complex DNA lesions that induce mutagenesis and cell death. DPCs are created by common antitumor drugs, reactive oxygen species, and endogenous aldehydes. Since these agents create other types of DNA damage in addition to DPCs, identification of the mechanisms of DPC repair is challenging. In this study, we created plasmid substrates containing site-specific DPC lesions, as well as plasmids harboring lesions that are selectively repaired by the base excision or nucleotide excision repair (NER) pathways. These substrates were transfected into mammalian cells and a quantitative real-time PCR assay employed to study their repair. This assay revealed that DPC lesions were rapidly repaired in wild-type human and Chinese hamster derived cells, as were plasmids harboring an oxoguanine residue (base excision repair substrate) or cholesterol lesion (NER substrate). Interestingly, the DPC substrate was repaired in human cells nearly three times as efficiently as in Chinese hamster cells (>75% vs ∼25% repair at 8 h post-transfection), while there was no significant species-specific difference in the efficiency with which the cholesterol lesion was repaired (∼60% repair). Experiments revealed that both human and hamster cells deficient in NER due to mutations in the xeroderma pigmentosum A or D genes were five to ten-fold less able to repair the cholesterol and DPC lesions than were wild-type control clones, and that both the global genome and transcription-coupled sub-pathways of NER were capable of repairing DPCs. In addition, analyses using this PCR-based assay revealed that a 4 kDa peptide DNA crosslink was repaired nearly twice as efficiently as was a ∼38 kDa DPC, suggesting that proteolytic degradation of crosslinked proteins occurs during DPC repair. These results highlight the utility of this PCR-based assay to study DNA repair and indicate that the NER machinery rapidly and efficiently repairs plasmid DPC lesions in mammalian cells.

Editor's Highlight: High-Throughput Functional Genomics Identifies Modulators of TCE Metabolite Genotoxicity and Candidate Susceptibility Genes

Trichloroethylene (TCE), an industrial chemical and environmental contaminant, is a human carcinogen. Reactive metabolites are implicated in renal carcinogenesis associated with TCE exposure, yet the toxicity mechanisms of these metabolites and their contribution to cancer and other adverse effects remain unclear. We employed an integrated functional genomics approach that combined functional profiling studies in yeast and avian DT40 cell models to provide new insights into the specific mechanisms contributing to toxicity associated with TCE metabolites. Genome-wide profiling studies in yeast identified the error-prone translesion synthesis (TLS) pathway as an import mechanism in response to TCE metabolites. The role of TLS DNA repair was further confirmed by functional profiling in DT40 avian cell lines, but also revealed that TLS and homologous recombination DNA repair likely play competing roles in cellular susceptibility to TCE metabolites in higher eukaryotes. These DNA repair pathways are highly conserved between yeast, DT40, and humans. We propose that in humans, mutagenic TLS is favored over homologous recombination repair in response to TCE metabolites. The results of these studies contribute to the body of evidence supporting a mutagenic mode of action for TCE-induced renal carcinogenesis mediated by reactive metabolites in humans. Our approach illustrates the potential for high-throughput in vitro functional profiling in yeast to elucidate toxicity pathways (molecular initiating events, key events) and candidate susceptibility genes for focused study.

Re-evaluation of the WHO (2010) formaldehyde indoor air quality guideline for cancer risk assessment

In 2010, the World Health Organization (WHO) established an indoor air quality guideline for short- and long-term exposures to formaldehyde (FA) of 0.1 mg/m3 (0.08 ppm) for all 30-min periods at lifelong exposure. This guideline was supported by studies from 2010 to 2013. Since 2013, new key studies have been published and key cancer cohorts have been updated, which we have evaluated and compared with the WHO guideline. FA is genotoxic, causing DNA adduct formation, and has a clastogenic effect; exposure-response relationships were nonlinear. Relevant genetic polymorphisms were not identified. Normal indoor air FA concentrations do not pass beyond the respiratory epithelium, and therefore FA's direct effects are limited to portal-of-entry effects. However, systemic effects have been observed in rats and mice, which may be due to secondary effects as airway inflammation and (sensory) irritation of eyes and the upper airways, which inter alia decreases respiratory ventilation. Both secondary effects are prevented at the guideline level. Nasopharyngeal cancer and leukaemia were observed inconsistently among studies; new updates of the US National Cancer Institute (NCI) cohort confirmed that the relative risk was not increased with mean FA exposures below 1 ppm and peak exposures below 4 ppm. Hodgkin's lymphoma, not observed in the other studies reviewed and not considered FA dependent, was increased in the NCI cohort at a mean concentration ≥0.6 mg/m3 and at peak exposures ≥2.5 mg/m3; both levels are above the WHO guideline. Overall, the credibility of the WHO guideline has not been challenged by new studies.

Functional Toxicogenomic Profiling Expands Insight into Modulators of Formaldehyde Toxicity in Yeast

Formaldehyde (FA) is a commercially important chemical with numerous and diverse uses. Accordingly, occupational and environmental exposure to FA is prevalent worldwide. Various adverse effects, including nasopharyngeal, sinonasal, and lymphohematopoietic cancers, have been linked to FA exposure, prompting designation of FA as a human carcinogen by U.S. and international scientific entities. Although the mechanism(s) of FA toxicity have been well studied, additional insight is needed in regard to the genetic requirements for FA tolerance. In this study, a functional toxicogenomics approach was utilized in the model eukaryotic yeast Saccharomyces cerevisiae to identify genes and cellular processes modulating the cellular toxicity of FA. Our results demonstrate mutant strains deficient in multiple DNA repair pathways-including homologous recombination, single strand annealing, and postreplication repair-were sensitive to FA, indicating FA may cause various forms of DNA damage in yeast. The SKI complex and its associated factors, which regulate mRNA degradation by the exosome, were also required for FA tolerance, suggesting FA may have unappreciated effects on RNA stability. Furthermore, various strains involved in osmoregulation and stress response were sensitive to FA. Together, our results are generally consistent with FA-mediated damage to both DNA and RNA. Considering DNA repair and RNA degradation pathways are evolutionarily conserved from yeast to humans, mechanisms of FA toxicity identified in yeast may be relevant to human disease and genetic susceptibility.

Complementation of hypersensitivity to DNA interstrand crosslinking agents demonstrates that XRCC2 is a Fanconi anaemia gene

BACKGROUND

Fanconi anaemia (FA) is a heterogeneous inherited disorder clinically characterised by progressive bone marrow failure, congenital anomalies and a predisposition to malignancies.

OBJECTIVE

Determine, based on correction of cellular phenotypes, whether XRCC2 is a FA gene.

METHODS

Cells (900677A) from a previously identified patient with biallelic mutation of XRCC2, among other mutations, were genetically complemented with wild-type XRCC2.

RESULTS

Wild-type XRCC2 corrects each of three phenotypes characteristic of FA cells, all related to the repair of DNA interstrand crosslinks, including increased sensitivity to mitomycin C (MMC), chromosome breakage and G2-M accumulation in the cell cycle. Further, the p.R215X mutant of XRCC2, which is harboured by the patient, is unstable. This provides an explanation for the pathogenesis of this mutant, as does the fact that 900677A cells have reduced levels of other proteins in the XRCC2-RAD51B-C-D complex. Also, FANCD2 monoubiquitination and foci formation, but not assembly of RAD51 foci, are normal in 900677A cells. Thus, XRCC2 acts late in the FA-BRCA pathway as also suggested by hypersensitivity of 900677A cells to ionising radiation. These cells also share milder sensitivities towards olaparib and formaldehyde with certain other FA cells.

CONCLUSIONS

XRCC2/FANCU is a FA gene, as is another RAD51 paralog gene, RAD51C/FANCO. Notably, similar to a subset of FA genes that act downstream of FANCD2, biallelic mutation of XRCC2/FANCU has not been associated with bone marrow failure. Taken together, our results yield important insights into phenotypes related to FA and its genetic origins.

Identification of Genes That Modulate Susceptibility to Formaldehyde and Imatinib by Functional Genomic Screening in Human Haploid KBM7 Cells

Though current functional genomic screening systems are useful for investigating human susceptibility to chemical toxicity, they have limitations. Well-established, high-throughput yeast mutant screens identify only evolutionarily conserved processes. RNA interference can be applied in human cells but is limited by incomplete gene knockout and off-target effects. Human haploid cell screening is advantageous as it requires knockdown of only a single copy of each gene. A human haploid cell mutant library (KBM7-Mu), derived from a chronic myeloid leukemia (CML) patient, was recently developed and has been used to identify genes that modulate sensitivity to infectious agents and pharmaceutical drugs. Here, we sought to improve the KBM7-Mu screening process to enable efficient screening of environmental chemicals. We developed a semi-solid medium based screening approach that cultures individual mutant colonies from chemically resistant cells, faster (by 2-3 weeks) and with less labor than the original liquid medium-based approach. As proof of principle, we identified genetic mutants that confer resistance to the carcinogen formaldehyde (FA, 12 genes, 18 hits) and the CML chemotherapeutic agent imatinib (6 genes, 13 hits). Validation experiments conducted on KBM7 mutants lacking each of the 18 genes confirmed resistance of 6 FA mutants (CTC1, FCRLA, GOT1, LPR5, M1AP, and MAP2K5) and 1 imatinib-resistant mutant (LYRM9). Despite the improvements to the method, it remains technically challenging to limit false positive findings. Nonetheless, our findings demonstrate the broad applicability of this optimized haploid approach to screen toxic chemicals to identify novel susceptibility genes and gain insight into potential mechanisms of toxicity.

Induction of centrosome amplification by formaldehyde, but not hydroquinone, in human lymphoblastoid TK6 cells

Benzene and formaldehyde (FA) are important industrial chemicals and environmental pollutants that cause leukemia by inducing DNA damage and chromosome aberrations in hematopoietic stem cells (HSC), the target cells for leukemia. Our previous studies showed that workers exposed to benzene and FA exhibit increased levels of aneuploidy in their blood cells. As centrosome amplification is a common phenomenon in human cancers, including leukemia, and is associated with aneuploidy in carcinogenesis, we hypothesized that benzene and FA would induce centrosome amplification in vitro. We treated human lymphoblastoid TK6 cells with a range of concentrations of hydroquinone (HQ, a benzene metabolite) or FA for 24 h, allowed the cells to recover in fresh medium for 24 h, and examined centrosome amplification; chromosomal gain, loss, and breakage; and cytotoxicity. We included melphalan and etoposide, chemotherapeutic drugs that cause therapy-related acute myeloid leukemia and that have been shown to induce centrosome amplification as well as chromosomal aneuploidy and breakage, as positive controls. Melphalan and etoposide induced centrosome amplification and chromosome gain and breakage in a dose-dependent manner, at cytotoxic concentrations. HQ, though cytotoxic, did not induce centrosome amplification or any chromosomal aberration. FA-induced centrosome amplification and cytotoxicity, but did not induce chromosomal aberrations. Our data suggest, for the first time, that centrosome amplification is a potential mechanism underlying FA-induced leukemogenesis, but not benzene-induced leukemogenesis, as mediated through HQ. Future studies are needed to delineate the mechanisms of centrosome amplification and its association with DNA damage, chromosomal aneuploidy and carcinogenesis, following exposure to FA.

Formaldehyde-induced histone H3 phosphorylation via JNK and the expression of proto-oncogenes

Formaldehyde (FA) is a very reactive compound that forms DNA adducts and DNA-protein crosslinks, which are known to contribute to FA-induced mutations and carcinogenesis. Post-translational modifications to histones have recently attracted attention due to their link with cancer. In the present study, we examined histone modifications following a treatment with FA. FA significantly phosphorylated histone H3 at serine 10 (H3S10), and at serine 28 (H3S28), the time-course of which was similar to the phosphorylation of H2AX at serine 139 (γ-H2AX), a marker of DNA double strand breaks. The temporal deacetylation of H3 was observed due to the reaction of FA with the lysine residues of histones. The phosphorylation mechanism was then analyzed by focusing on H3S10. The nuclear distribution of the phosphorylation of H3S10 and γ-H2AX did not overlap, and the phosphorylation of H3S10 could not be suppressed with an inhibitor of ATM/ATR, suggesting that the phosphorylation of H3S10 was independent of the DNA damage response. ERK and JNK in the MAPK pathways were phosphorylated by the treatment with FA, in which the JNK pathway was the main target for phosphorylation. The phosphorylation of H3S10 increased at the promoter regions of c-fos and c-jun, indicating a relationship between FA-induced tumor promotion activity and phosphorylation of H3S10. These results suggested that FA both initiates and promotes cancer, as judged by an analysis of histone modifications.

Emerging approaches in predictive toxicology

Predictive toxicology plays an important role in the assessment of toxicity of chemicals and the drug development process. While there are several well-established in vitro and in vivo assays that are suitable for predictive toxicology, recent advances in high-throughput analytical technologies and model systems are expected to have a major impact on the field of predictive toxicology. This commentary provides an overview of the state of the current science and a brief discussion on future perspectives for the field of predictive toxicology for human toxicity. Computational models for predictive toxicology, needs for further refinement and obstacles to expand computational models to include additional classes of chemical compounds are highlighted. Functional and comparative genomics approaches in predictive toxicology are discussed with an emphasis on successful utilization of recently developed model systems for high-throughput analysis. The advantages of three-dimensional model systems and stem cells and their use in predictive toxicology testing are also described.

Application of toxicogenomic profiling to evaluate effects of benzene and formaldehyde: from yeast to human

Genetic variation underlies a significant proportion of the individual variation in human susceptibility to toxicants. The primary current approaches to identify gene-environment (GxE) associations, genome-wide association studies and candidate gene association studies, require large exposed and control populations and an understanding of toxicity genes and pathways, respectively. This limits their application in the study of GxE associations for the leukemogens benzene and formaldehyde, whose toxicity has long been a focus of our research. As an alternative approach, our published work has applied innovative in vitro functional genomics testing systems, including unbiased functional screening assays in yeast and a near-haploid human bone marrow cell line. Through comparative genomic and computational analyses of the resulting data, human genes and pathways that may modulate susceptibility to benzene and formaldehyde were identified, and the roles of several genes in mammalian cell models were validated. In populations occupationally exposed to low levels of benzene, we applied peripheral blood mononuclear cell transcriptomics and chromosome-wide aneuploidy studies in lymphocytes. In this review, we describe our comprehensive toxicogenomic approach and the potential mechanisms of toxicity and susceptibility genes identified for benzene and formaldehyde, as well as related studies conducted by other researchers.

Inhaled formaldehyde induces DNA-protein crosslinks and oxidative stress in bone marrow and other distant organs of exposed mice

Formaldehyde (FA), a major industrial chemical and ubiquitous environmental pollutant, has been classified as a leukemogen. The causal relationship remains unclear, however, due to limited evidence that FA induces toxicity in bone marrow, the site of leukemia induction, and in other distal organs. Although induction of DNA-protein crosslinks (DPC), a hallmark of FA toxicity, was not previously detected in the bone marrow of FA-exposed rats and monkeys in studies published in the 1980s, our recent studies showed increased DPC in the bone marrow, liver, kidney, and testes of exposed Kunming mice. To confirm these preliminary results, in the current study we exposed BALB/c mice to 0, 0.5, 1.0, and 3.0 mg m(-3) FA (8 hr per day, for 7 consecutive days) by nose-only inhalation and measured DPC levels in bone marrow and other organs of exposed mice. As oxidative stress is a potential mechanism of FA toxicity, we also measured glutathione (GSH), reactive oxygen species (ROS), and malondialdehyde (MDA), in the bone marrow, peripheral blood mononuclear cells, lung, liver, spleen, and testes of exposed mice. Significant dose-dependent increases in DPC, decreases in GSH, and increases in ROS and MDA were observed in all organs examined (except for DPC in lung). Bone marrow was among the organs with the strongest effects for DPC, GSH, and ROS. In conclusion, exposure of mice to FA by inhalation induced genotoxicity and oxidative stress in bone marrow and other organs. These findings strengthen the biological plausibility of FA-induced leukemogenesis and systemic toxicity.