We investigated the effect of punicalagin (PC) on benzo[a]pyrene (BP)-induced DNA adducts in vitro and in vivo. Incubation of BP (1 μM) with rat liver microsomes, appropriate co-factors and DNA in the presence of vehicle or punicalagin (1–40 μM) showed dose-dependent inhibition of the resultant DNA adducts, with essentially complete (97%) inhibition at 40 μM. However, PC failed to inhibit anti-BPDE-induced DNA adducts when tested in an in vitro non-microsomal system, suggesting that the inhibition of the microsomal BP-DNA adducts occurred due to inhibition of P450 1A1 by PC. To determine its efficacy in vivo, female S/D rats were administered punicalagin via the diet (1,500 ppm; ~19 mg/day/animal) or subcutaneous polymeric implants (two 2-cm, 200 mg with 20% drug load; 40 mg PC/implant) and then treated with continuous low-dose of BP by a subcutaneous polymeric implant (2 cm, 200 mg with 10% load; 20 mg BP/implant) and euthanized after 10 days. Analysis of the lung DNA by 32P-postlabeling showed significant (60%; p = 0.029) inhibition of DNA adducts by PC administered via the implants; the dietary route showed modest (34%) but statistically insignificant inhibition. Furthermore, total PC administered by implants was approximately 38-fold lower compared with the dietary route. Analysis of the lung microsomes showed significant inhibition of cytochrome P450 1A1 activity and induction of glutathione. Release of PC from the implants was found to be biphasic starting with a burst release, followed by a gradual decline. Ultra performance liquid chromatography analysis showed no detectable PC in the plasma but its hydrolyzed product, ellagic acid was readily detected. The plasma concentration of ellagic acid was over two orders of magnitude higher (589 ± 78 ng/mL) in the implant group compared with diet (4.36 ± 0.83 ng/mL). Together, our data show that delivery of PC by implants can reduce its effective dose substantially, and that the inhibition of DNA adducts in vivo occurred presumably due to the conversion of PC to ellagic acid.
Benzo[a]pyrene; DNA adducts; 32P-postlabeling; Punicalagin; Polymeric implants; Bioavailability
Although DNA-protein cross-links (DPCs) pose a significant threat to genome stability, they remain a poorly understood class of DNA lesions. To define genetic impacts of DPCs on eukaryotic cells in molecular terms, we used a sensitive Saccharomyces cerevisiae frameshift-detection assay to analyze mutagenesis by formaldehyde (HCHO), and its response to nucleotide excision repair (NER) and translesion DNA synthesis (TLS). Brief exposure to HCHO was mutagenic for NER-defective rad14 strains but not for a corresponding RAD14 strain, nor for a rad14 strain lacking both Polζ and Polη TLS polymerases. This confirmed that HCHO-generated DNA lesions can trigger error-prone TLS and are substrates for the NER pathway. Sequencing revealed that HCHO-induced single-base-pair insertions occurred primarily at one hotspot; most of these insertions were also complex, changing an additional base-pair nearby. Most of the HCHO-induced mutations required both Polζ and Polη, providing a striking example of cooperativity between these two TLS polymerases during bypass of a DNA lesion formed in vivo. The similar molecular properties of HCHO-induced and spontaneous complex +1 insertions detected by this system suggest that DPCs which form in vivo during normal metabolism may contribute characteristic events to the spectra of spontaneous mutations in NER-deficient cells.
DNA-protein cross-link; frameshift mutations; Complex insertions; DNA Polymerase ζ; DNA Polymerase η
DNA double-strand breaks (DSBs) are most often repaired by two pathways in mammalian cells, homologous recombination or non-homologous end joining. Biochemical and genetic studies showed that DSBs can also be joined via microhomology-mediated end joining (MHEJ), which is always mutagenic and may result in diseases, such as cancer. In this study we established a human cell-based reporter system to determine the prevalence of MHEJ events and factors that modulate MHEJ. A nonfunctional puromycin acetyltransferase (Pac) gene, disrupted by an insertion flanked by two microhomologous repeats, was integrated into chromosomes of human HT1080 cells. Repair of DSBs via MHEJ using the repeats resulted in deletion of the insertion and restoration of the Pac gene function, thus rendering the cells puromycin resistant. Our results showed that MHEJ spontaneously occurs at the reporter locus (loci), manifested by formation of puromycin resistant (puror) colonies after culturing reporter cells in medium containing puromycin. The frequency of puror cells can be greatly increased by site-directed DSB inside the insertion. Our results also demonstrated that the frequency of puror cells is affected by the length of the repeat and by the size of the intervening sequence. Thus, this cell-based assay provides a platform for evaluating factors modulating in vivo MHEJ.
Flap endonuclease 1 (FEN1), a member of the Rad2 nuclease family, possesses 5’ flap endonuclease (FEN), 5’ exonuclease (EXO), and gap-endonuclease (GEN) activities. The multiple, structure-specific nuclease activities of FEN1 allow it to process different intermediate DNA structures during DNA replication and repair. We previously identified a group of FEN1 mutations and single nucleotide polymorphisms that impair FEN1’s EXO and GEN activities in human cancer patients. We also established a mouse model carrying the E160D FEN1 mutation, which mimics the mutations seen in humans. FEN1 mutant mice developed spontaneous lung cancer at high frequency at their late life stages. An important unanswered question is whether individuals carrying such FEN1 mutation are more susceptible to tobacco smoke and have an earlier onset of lung cancer. Here, we report our study on E160D mutant mice exposed to benzo[α]pyrene (B[α]P), a major DNA damaging compound found in tobacco smoke. We demonstrate that FEN1 employs its GEN activity to cleave DNA bubble substrates with BP-induced lesions, but the E160D FEN1 mutation abolishes such activity. As a consequence, Mouse cells carrying the E160D mutation display defects in the repair of B[α]P adducts and accumulate DNA double-stranded breaks and chromosomal aberrations upon treatments with B[α]P. Furthermore, more E160D mice than WT mice have an early onset of B[α]P-induced lung adenocarcinoma. All together, our current study suggests that individuals carrying the GEN-deficient FEN1 mutations have high risk to develop lung cancer upon exposure to B[α]P-containing agents such as tobacco smoke.
Flap endonuclease 1 (FEN1); Benzo[α]pyrene (B[α]P); Lung cancer; Double-stranded breaks (DSBs); Near-tetraploid aneuploidy
Waterpipe smoking is popular in many parts of the world. Micronuclei (MN) evaluation in the exfoliated oral cells of smokers is a non-invasive technique for evaluation of possible tobacco harm. We aimed to assess whether MN levels are higher in waterpipe smokers than in never smokers. We examined oral smears of 128 adult male waterpipe smokers and 78 males who never smoked tobacco in rural Egypt The total number of MN per 1000 cells per subject, and the number of MN-containing cells per individual were compared. We observed a higher level of total MN in waterpipe smokers (10 ± 4) than in never smokers (4 ± 2, p<0.001). A similar difference was found for the mean number of affected cells per individual (8 ± 3 vs. 4 ± 1.62, p < 0.001). MN levels were not significantly dose related. This study is among the first to assess the association between waterpipe smoking and a cytogenetic measure of tobacco harm. The two-fold increase in MN level is consistent with previous reports of MN in cigarette smokers. More research is needed to determine if such MN levels are predictive of future health consequences.
Waterpipe smoking; tobacco; buccal cells; micronuclei; cytogenetics; tobacco harm
The ends of chromosomes are composed of a short repeat sequence and associated proteins that together form a cap, called a telomere, that keeps the ends from appearing as double-strand breaks (DSBs) and prevents chromosome fusion. The loss of telomeric repeat sequences or deficiencies in telomeric proteins can result in chromosome fusion and lead to chromosome instability. The similarity between chromosome rearrangements resulting from telomere loss and those found in cancer cells implicates telomere loss as an important mechanism for the chromosome instability contributing to human cancer. Telomere loss in cancer cells can occur through gradual shortening due to insufficient telomerase, the protein that maintains telomeres. However, cancer cells often have a high rate of spontaneous telomere loss despite the expression of telomerase, which has been proposed to result from a combination of oncogene-mediated replication stress and a deficiency in DSB repair in telomeric regions. Chromosome fusion in mammalian cells primarily involves nonhomologous end joining (NHEJ), which is the major form of DSB repair. Chromosome fusion initiates chromosome instability involving breakage-fusion-bridge (B/F/B) cycles, in which dicentric chromosomes form bridges and break as the cell attempts to divide, repeating the process in subsequent cell cycles. Fusion between sister chromatids results in large inverted repeats on the end of the chromosome, which amplify further following additional B/F/B cycles. B/F/B cycles continue until the chromosome acquires a new telomere, most often by translocation of the end of another chromosome. The instability is not confined to a chromosome that loses its telomere, the instability is transferred to the chromosome donating a translocation. Moreover, the amplified regions are unstable and form extrachromosomal DNA that can reintegrate at new locations. Knowledge concerning the factors promoting telomere loss and its consequences is therefore important for understanding chromosome instability in human cancer.
Chromosome fusion; Chromosome healing; Chromosome instability; Double-strand break; Intrastrand annealing; Nonhomologous end joining; Telomere
It has been demonstrated that exogenous expression of a combination of transcription factors can reprogram differentiated cells such as fibroblasts and keratinocytes into what have been termed induced pluripotent stem (iPS) cells. These iPS cells are capable of differentiating into all the tissue lineages when placed in the right environment and, in the case of mouse cells, can generate chimeric mice and be transmitted through the germline. Safer and more efficient methods of reprogramming are rapidly being developed. Clearly, iPS cells present a number of exciting possibilities, including disease modeling and therapy. A major question is whether the nuclei of iPS cells are truly rejuvenated or whether they might retain some of the marks of aging from the cells from which they were derived. One measure of cellular aging is the telomere. In this regard, recent studies have demonstrated that telomeres in iPS cells may be rejuvenated. They are not only elongated by reactivated telomerase but they are also epigenetically modified to be similar but not identical to embryonic stem cells. Upon differentiation, the derivative cells turn down telomerase, the telomeres begin to shorten again, and the telomeres and the genome are returned to an epigenetic state that is similar to normal differentiated somatic cells. While these preliminary telomere findings are promising, the overall genomic integrity of reprogrammed cells may still be problematic and further studies are needed to examine the safety and feasibility of using iPS cells in regenerative medicine applications.
telomere; telomerase; iPS; pluripotent; stem cell
Humans display a large inter-individual variation in leukocyte telomere length (LTL), which is influenced by heredity, sex, race/ethnicity, paternal age at conception and environmental exposures. LTL dynamics (birth LTL and its age-dependent attrition thereafter) mirror telomere dynamics in hematopoietic stem cells (HSCs). LTL at birth is evidently a major determinant of LTL throughout the human lifespan, such that individuals endowed with short (or long) LTL at birth probably have short (or long) LTL later in life. Therefore, the associations of short LTL with atherosclerosis and with diminished survival in the elderly may relate to short birth LTL, accelerated age-dependent LTL attrition, or both. The mechanisms underlying these associations are still not well understood, but they stem in part from genetic factors in control of telomere maintenance and the rate of HSC replication.
telomeres; aging; leukocytes; genetics; epigenetics
Since 1998, there have been great advances in our understanding of the pathogenesis of dyskeratosis congenita (DC), a rare inherited bone marrow failure and cancer predisposition syndrome with prominent mucocutaneous abnormalities and features of premature aging. DC is now characterized molecularly by the presence of short age-adjusted telomeres. Mutations in seven genes have been unequivocally associated with DC, each with a role in telomere length maintenance. These observations, combined with knowledge that progressive telomere shortening can impose a proliferative barrier on dividing cells and contribute to chromosome instability, have led to the understanding that extreme telomere shortening drives the clinical features of DC. However, some of the genes implicated in DC encode proteins that are also components of H/ACA-ribonucleoprotein enzymes, which are responsible for the posttranslational modification of ribosomal and spliceosomal RNAs, raising the question whether alterations in these activities play a role in the pathogenesis of DC. In addition, recent reports suggest that some cases of DC may not be characterized by short age-adjusted telomeres. This review will highlight our current knowledge of the telomere length defects in DC and the factors involved in its development.
dyskeratosis congenita; telomerase; telomere
Telomeres, the dynamic nucleoprotein structures at the ends of linear
chromosomes, maintain the genomic integrity of a cell. Telomere length shortens
with age due to the incomplete replication of DNA ends with each cell division
as well as damage incurred by oxidative stress. Patterns of telomere shortening,
genomic instability, and telomerase expression in many cancer tissues compared
to adjacent normal tissue implicate telomere crisis as a common crucial event in
malignant transformation. In order to understand the role of telomere length in
cancer etiology, most epidemiologic studies have measured average telomere
length of peripheral blood or buccal cell DNA as a surrogate tissue biomarker of
telomere dysfunction and cancer risk. In this review, we present the results
from epidemiologic investigations conducted of telomere length and cancer risk.
We note differences in reported associations based on study design, which may be
due to biases intrinsic to retrospective studies. Finally, we conclude with
study design considerations as future investigations are needed to elucidate the
relationship between telomere length and a number of cancer sites.
telomere length; cancer; epidemiology
The intimate connection between telomerase regulation and human disease is now well established. The molecular basis for telomerase regulation is highly complex and entails multiple layers of control. While the major target of enzyme regulation is the catalytic subunit TERT, the RNA subunit of telomerase is also implicated in telomerase control. In addition, alterations in gene dosage and alternative isoforms of core telomerase components have been described. Finally, telomerase localization, recruitment to the telomere and enzymology at the chromosome terminus are all subject to modulation. In this review we summarize recent advances in understanding fundamental mechanisms of telomerase regulation.
telomere; TERT; TR; ribonucleoprotein; repeat addition processivity; TERRA
Chromosome end protection is essential to protect genome integrity. Telomeres, tracts of repetitive DNA sequence and associated proteins located at the chromosomal terminus, serve to safeguard the ends from degradation and unwanted double strand break repair. Due to the essential nature of telomeres in protecting the genome, a number of unique proteins have evolved to ensure that telomere length and structure are preserved. The inability to properly maintain telomeres can lead to diseases such as dyskeratosis congenita, pulmonary fibrosis and cancer. In this review, we will discuss the known functions of mammalian telomere-associated proteins, their role in telomere replication and length regulation and how these processes relate to genome instability and human disease.
telomere; telomerase; shelterin; replication; CST
Idiopathic pulmonary fibrosis (IPF) is the most common manifestation of telomere-mediated disorders. Germline mutations in the essential telomerase genes, hTERT and hTR, are the causal genetic defect in up to one-sixth of pulmonary fibrosis families. The presence of telomerase mutations in this subset is significant for clinical decisions as affected individuals can develop extra-pulmonary complications related to telomere shortening such as bone marrow failure and cryptogenic liver cirrhosis. There is also evidence that IPF is an ancestral manifestation of autosomal dominant telomere syndromes where, with successive generations, the disease evolves from pulmonary fibrosis into a bone marrow failure-predominant disorder, defining a unique form of genetic anticipation. Here I review the significance of telomere defects for understanding the genetics, disease patterns and pathophysiology of IPF. The importance of this diagnosis for patient care decisions will also be discussed.
Dyskeratosis Congenita; Interstitial Lung Disease; Aplastic Anemia; Liver Cirrhosis; Emphysema; Diabetes
Studies of telomeres and telomere biology often critically rely on the detection of telomeric DNA and measurements of the length of telomere repeats in either single cells or populations of cells. Several methods are available that provide this type of information and it is often not clear what method is most appropriate to address a specific research question. The major variables that need to be considered are the material that is or can be made available and the accuracy of measurements that is required. The goal of this review is to provide a comprehensive summary of the most commonly used methods and discuss the advantages and disadvantages of each. Methods that start with genomic DNA include telomere restriction fragment (TRF) length analysis, PCR amplification of telomere repeats relative to a single copy gene by Q-PCR or MMQPCR and single telomere length analysis (STELA), a PCR-based approach that accurately measures the full spectrum of telomere lengths from individual chromosomes. A different set of methods relies on fluorescent in situ hybridization (FISH) to detect telomere repeats in individual cells or chromosomes. By including essential calibration steps and appropriate controls these methods can be used to measure telomere repeat length or content in chromosomes and cells. Such methods include quantitative FISH (Q-FISH) and flow FISH which are based on digital microscopy and flow cytometry respectively. Here the basic principles of various telomere length measurement methods are described and their strengths and weaknesses are highlighted. Some recent developments in telomere length analysis are also discussed. The information in this review should facilitate the selection of the most suitable method to address specific research question about telomeres in either model organisms or human subjects.
Telomere length; Flow FISH; Q-PCR; TRF; Q-FISH; STELA
Next-generation sequencing technologies can now be used to directly measure heritable de novo DNA sequence mutations in humans. However, these techniques have not been used to examine environmental factors that induce such mutations and their associated diseases. To address this issue, a working group on environmentally induced germline mutation analysis (ENIGMA) met in October 2011 to propose the necessary foundational studies, which include sequencing of parent–offspring trios from highly exposed human populations, and controlled dose–response experiments in animals. These studies will establish background levels of variability in germline mutation rates and identify environmental agents that influence these rates and heritable disease. Guidance for the types of exposures to examine come from rodent studies that have identified agents such as cancer chemotherapeutic drugs, ionizing radiation, cigarette smoke, and air pollution as germ-cell mutagens. Research is urgently needed to establish the health consequences of parental exposures on subsequent generations.
Germ cell; Heritable mutation; Next generation sequencing; Copy number variants
We previously demonstrated that exonic selectivity for frameshift mutation (exon 10 over exon 3) of ACVR2 in mismatch repair (MMR)-deficient cells is partially determined by 6 nucleotides flanking 5’ and 3’ of each microsatellite. Substitution of flanking nucleotides surrounding the exon 10 microsatellite with those surrounding the exon 3 microsatellite greatly diminished heteroduplex (A7/T8) and full (A7/T7) mutation, while substitution of flanking nucleotides from exon 3 with those from exon 10 enhanced frameshift mutation. We hypothesized that specific individual nucleotide(s) within these flanking sequences control ACVR2 frameshift mutation rates. Only the 3rd nucleotide 5’ of the microsatellite, and 3rd, 4th, and 5th nucleotides 3’ of the microsatellite were altered from the native flanking sequences and these locations were individually altered (sites A, B, C, and D, respectively). Constructs were cloned +1 bp out-of-frame of EGFP, allowing a −1 bp frameshift to express EGFP. Plasmids were stably-transfected into MMR-deficient cells. Non-fluorescent cells were sorted, cultured for 35 days, and harvested for flow cytometry and DNA-sequencing. Site A (C to T) and B (G to C) in ACVR2 exon 10 decreased both heteroduplex and full mutant as much as the construct containing all 4 alterations. For ACVR2 exon 3, site A (T to C), C (A to G), and D (G to C) are responsible for increased heteroduplex formation, whereas site D is responsible for full mutant formation by ACVR2 exon 10 flanking sequences. Exonic selectivity for frameshift mutation within ACVR2’s sequence context is specifically controlled by individual nucleotides flanking each microsatellite.
ACVR2; frameshift mutation; DNA mismatch repair; microsatellite instability; colorectal cancer
DNA mutations are the source of genetic variation within populations. The majority of mutations with observable effects are deleterious. In humans mutations in the germ line can cause genetic disease. In somatic cells multiple rounds of mutations and selection lead to cancer. The study of genetic variation has progressed rapidly since the completion of the draft sequence of the human genome. Recent advances in sequencing technology, most importantly the introduction of massively parallel sequencing (MPS), have resulted in more than a hundred-fold reduction in the time and cost required for sequencing nucleic acids. These improvements have greatly expanded the use of sequencing as a practical tool for mutation analysis. While in the past the high cost of sequencing limited mutation analysis to selectable markers or small forward mutation targets assumed to be representative for the genome overall, current platforms allow whole genome sequencing for less than $5,000. This has already given rise to direct estimates of germline mutation rates in multiple organisms including humans by comparing whole genome sequences between parents and offspring. Here we present a brief history of the field of mutation research, with a focus on classical tools for the measurement of mutation rates. We then review MPS, how it is currently applied and the new insight into human and animal mutation frequencies and spectra that has been obtained from whole genome sequencing. While great progress has been made, we note that the single most important limitation of current MPS approaches for mutation analysis is the inability to address low-abundance mutations that turn somatic tissues into mosaics of cells. Such mutations are at the basis of intra-tumor heterogeneity, with important implications for clinical diagnosis, and could also contribute to somatic diseases other than cancer, including aging. Some possible approaches to gain access to low-abundance mutations are discussed, with a brief overview of new sequencing platforms that are currently waiting in the wings to advance this exploding field even further.
Massively parallel sequencing; Somatic mutation; Germ line mutation; Low-abundance mutations; Whole genome sequencing; Mosaicism; Aging; Cancer
The effects of maternal cigarette smoking during pregnancy on structural chromosome aberrations were evaluated in peripheral lymphocytes from 239 mothers and their 241 newborns to determine whether smoking during pregnancy, genetic susceptibility, and race are associated with chromosome aberrations including translocations. Demographic information and cigarette smoking data were obtained via questionnaire. There were 119 Caucasian Americans, 118 African Americans, and 2 Asian Americans. The average maternal age was 24.9 ± 5.8 (mean ± S.D.) years. Thirty-nine percent of the Caucasian Americans and 45.4% of the African Americans self-reported that they were active smokers during the index pregnancy. The average number of cigarettes smoked per day was 2.65 ± 5.75 and 1.37 ± 3.17 for Caucasian and African American mothers, respectively. Peripheral blood lymphocytes from the mother and from the fetal side of the placenta were evaluated for chromosome aberrations by whole chromosome painting and for genetic susceptibility using an in vitro bleomycin challenge assay. Spontaneous translocation frequencies in both maternal and newborn lymphocytes were not associated with cigarette smoking, socio-economic status, or education. The absence of a smoking effect may be attributable to the low level of cigarette usage in these subjects. The average bleomycin-induced damage in the maternal and newborn populations was 0.37 ± 0.27 and 0.15 ± 0.14 breaks per cell, respectively, a difference that was highly significant (p < 0.0001). In newborns there was a positive association between bleomycin sensitivity and the frequencies of aberrations as measured by chromosome painting: p ≤ 0.0007 for dicentrics and fragments, and p ≤ 0.002 for translocations. Caucasian American newborns demonstrated a significant association between dicentrics and fragments as measured by painting, and bleomycin sensitivity (p ≤ 0.0002), but no such association was observed for African American newborns. The results of this study indicate that while differences were observed between African Americans and Caucasian Americans, race does not appear to be a major contributor to chromosome damage in newborns or their mothers. However, peripheral lymphocytes in pregnant women are more susceptible to genetic damage than peripheral lymphocytes in newborns.
cigarette smoking; pregnancy; newborns; mothers; chromosome translocations; genomic susceptibility
Tumor cell lines can replicate faster than normal cells and many also have defective DNA repair pathways. This has lead to the investigation of the inhibition of DNA repair proteins as a means of therapeutic intervention. An alternative approach is to hide or mask damaged DNA from the repair systems. We have developed a protocol to investigate the structures of the complexes of damaged DNA with drug like molecules. Nucleotide resolution structural information can be obtained using an improved hydroxyl radical cleavage protocol. The use of a dTn tail increases the length of the smallest fragments of interest and allows efficient co-precipitation of the fragments with poly(A). The use of a fluorescent label, on the 5′ end of the dTn tail, in conjunction with modified cleavage reaction conditions, avoids the lifetime and other problems with 32P labeling. The structures of duplex DNAs containing AC and CC mismatches in the presence and absence of minor groove binders have been investigated as have those of the fully complementary DNA. The results indicate that the structural perturbations of the mismatches are localized, are sequence dependent and that the presence of a mismatch can alter the binding of drug like molecules.
DNA; minor groove; lesion; DNA repair; hydroxyl radical; fluorescence; mismatch
The accumulation of DNA damage is a slow but hazardous phenomenon that may lead to cell death, accelerated aging features and cancer. One of the most versatile and important defense mechanisms against the accumulation of DNA damage is Nucleotide Excision Repair (NER), in which the Xeroderma pigmentosum group C (XPC) protein plays a prominent role. NER can be divided into Global Genome repair (GG-NER) and Transcription Coupled repair (TC-NER). XPC is a key factor in GG-NER where it functions in DNA damage recognition and after which the repair machinery is recruited to eliminate the DNA damage. Defective XPC functioning has been shown to result in a cancer prone phenotype, in human as well as in mice. Mutation accumulation in XPC deficient mice is accelerated and increased, resulting in an increased tumor incidence. More recently XPC has also been linked to functions outside of NER since XPC deficient mice show a divergent tumor spectrum compared to other NER deficient mouse models. Multiple in vivo and in vitro experiments indicate that XPC appears to be involved in the initiation of several DNA damage-induced cellular responses. XPC seems to function in the removal of oxidative DNA damage, redox homeostasis and cell cycle control. We hypothesize that this combination of increased oxidative DNA damage sensitivity, disturbed redox homeostasis together with inefficient cell cycle control mechanisms are causes of the observed increased cancer susceptibility in oxygen exposed tissues. Such a phenotype is absent in other NER-deficient mice, including Xpa.
Xpc; Nucleotide Excision Repair; mutation; cancer; oxidative DNA damage
Formaldehyde, the recently classified carcinogen and ubiquitous environmental contaminant, has long been suspected of causing adverse reproductive and developmental effects, but previous reviews were inconclusive, due in part, to limitations in the design of many of the human population studies. In the current review, we systematically evaluated evidence of an association between formaldehyde exposure and adverse reproductive and developmental effects, in human populations and in vivo animal studies, in the peer-reviewed literature. The mostly retrospective human studies provided evidence of an association of maternal exposure with adverse reproductive and developmental effects. Further assessment of this association by meta-analysis revealed an increased risk of spontaneous abortion (1.76, 95% CI 1.20–2.59, p=0.002) and of all adverse pregnancy outcomes combined (1.54, 95% CI 1.27–1.88, p<0.001), in formaldehyde-exposed women, although differential recall, selection bias, or confounding cannot be ruled out. Evaluation of the animal studies including all routes of exposure, doses and dosing regimens studied, suggested positive associations between formaldehyde exposure and reproductive toxicity, mostly in males. Potential mechanisms underlying formaldehyde-induced reproductive and developmental toxicities, including chromosome and DNA damage (genotoxicity), oxidative stress, altered level and/or function of enzymes, hormones and proteins, apoptosis, toxicogenomic and epigenomic effects (such as DNA methylation), were identified. To clarify these associations, well-designed molecular epidemiologic studies, that include quantitative exposure assessment and diminish confounding factors, should examine both reproductive and developmental outcomes associated with exposure in males and females. Together with mechanistic and animal studies, this will allow us to better understand the systemic effect of formaldehyde exposure.
formaldehyde; teratogenicity; pregnancy; meta-analysis; human; animal
Malathion is a well known pesticide and is commonly used in many agricultural and non-agricultural settings. Its toxicity has been attributed primarily to the accumulation of acetylcholine (Ach) at nerve junctions, due to inhibition of acetylcholinesterase (AChE), and consequently overstimulation of the nicotinic and muscarinic receptors. However, the genotoxicity of malathion has not been adequately studied; published studies suggest a weak interaction with the genetic material. In the present study, we investigated the genotoxic potential of malathion in bone marrow cells and peripheral blood obtained from Sprague-Dawley rats; using chromosomal aberrations (CA), mitotic index (MI), and DNA damage as toxicological endpoints. Four groups of four male rats each, weighing approximately 60 ± 2 g, were injected intraperitoneally (i.p.) once a day for five days with doses of 2.5, 5, 10, and 20 mg/kg body weight (BW) of malathion dissolved in 1% DMSO. The control group was made up of four animals injected with 1% DMSO. All the animals were sacrificed 24 h after the fifth day treatment. Chromosome preparations were obtained from bone marrow cells following standard protocols. DNA damage in peripheral blood leukocytes was determined using alkaline single-cell gel electrophoresis (comet assay). Malathion exposure significantly increased the number of structural chromosomal aberrations (CA) and the percentages of DNA damage, and decreased the mitotic index (MI) in treated groups when compared with the control group. Our results demonstrate that malathion has a clastogenic/genotoxic potential as measured by the bone marrow CA and comet assay in Sprague-Dawley rats.
In recent years, experimental evidence has accumulated that supports the existence of sublinear dose-response relationships at low doses of DNA reactive mutagens. However, creating the in vivo data necessary to allow for a more detailed dose-response modeling with the currently available tools might not always be practical. The purpose of the current work was to evaluate the utility of the Pig-a gene mutation assay to rapidly identify dose response relationships for direct acting genotoxicants. The induction of mutations in the peripheral blood of rats was evaluated following 28 days of exposure down to low doses of the direct acting alkylating agents ethyl methane sulfonate (EMS) and ethylnitrosourea (ENU). Using statistical modeling based on the 28-day studies, a threshold for mutation induction for EMS was estimated to be 21.9 mg/kg, whereas for the more potent ENU the threshold was estimated to be 0.88 mg/kg. Comparing mutation frequencies from acute and sub-chronic dosing indicated less than additive dose-response relationships, further confirming the possibility of a thresholded dose-response relationship for both compounds. In conclusion, the work presented provides evidence that the Pig-a assay might be a practical alternative to other in vivo mutation assays when assessing dose-response relationships for direct acting mutagens and that an experimental approach using fractionated dosing could be used to substantiate a biological mechanism responsible for the observation of a sub-linear dose-response relationship.
Ethyl Methanesulfonate; Ethylnitrosourea; Point Mutation; Rats; Dose-response; RETs
Maintenance of genomic integrity in embryonic cells is pivotal to proper embryogenesis, organogenesis and to the continuity of species. Cultured mouse embryonic stem cells (mESCs), a model for early embryonic cells, differ from cultured somatic cells in their capacity to remodel chromatin, in their repertoire of DNA repair enzymes, and in the regulation of cell cycle checkpoints. Using 129XC3HF1 mESCs heterozygous for Aprt, we characterized loss of Aprt heterozygosity after exposure to ionizing radiation. We report here that the frequency of loss of heterozygosity mutants in mESCs can be induced several hundred-fold by exposure to 5 to 10 Gy of x-rays. This induction is 50 to 100-fold higher than the induction reported for mouse adult or embryonic fibroblasts. The primary mechanism underlying the elevated loss of heterozygosity after irradiation is mitotic recombination, with lesser contributions from deletions and gene conversions that span Aprt. Aprt point mutations and epigenetic inactivation are very rare in mESCs compared to fibroblasts. Mouse ESCs, therefore, are distinctive in their response to ionizing radiation and studies of differentiated cells may underestimate the mutagenic effects of ionizing radiation on ESC or other stem cells. Our findings are important to understanding the biological effects of ionizing radiation on early development and carcinogenesis.
mouse embryonic stem cells; ionizing radiation; mitotic recombination; loss of heterozygosity
Werner syndrome is a progeroid disorder caused by mutations of the WRN gene. The encoded WRN protein belongs to the family of RecQ helicases that plays a role in the maintenance of genomic stability. Single nucleotide polymorphisms in WRN have been associated with an increased risk for some cancers and were recently linked to benzene hematotoxicity. To further address the role of WRN in benzene toxicity, we employed RNA interference (RNAi) to silence endogenous WRN in HeLa cells and examined the susceptibility of these WRN-depleted cells to the toxic effects of the benzene metabolite hydroquinone. HeLa cells were used as the experimental model because RNAi is highly effective in this system producing almost complete depletion of the target protein. Depletion of WRN led to a decrease in cell proliferation and an enhanced susceptibility to hydroquinone cytotoxicity as revealed by an increase in necrosis. WRN-depleted HeLa cells treated with hydroquinone also displayed an increase in the amount of DNA double strand breaks as determined by the Comet assay, and an elevated DNA damage response as indicated by the 7-fold induction of γH2AX and acetyl-p53 (Lys373 and Lys382) over control levels. Together, these results show that WRN plays an important role in the protection of HeLa cells against the toxicity of the benzene metabolite hydroquinone, specifically in mounting a normal DNA damage response following the induction of DNA double-strand breaks. Further studies in bone marrow-derived stem or progenitor cells are required to confirm our findings in HeLa cells and expand our ability to extrapolate the results to benzene toxicity in humans.
RNA interference; siRNA; γH2AX; p53; leukemia; lymphoma; DNA repair; strand breaks