Have environmental mutagens caused oncomutations in people?

Genomic and epigenetic characterization of the arsenic-induced oncogenic microRNA-21

As one of the first identified oncogenic microRNAs, the precise details concerning the transcriptional regulation and function of microRNA-21 (miR-21) are still not completely established. The miR-21 gene is situated on chromosome 17q23.2, positioned at the 3'-UTR of the gene that encodes vacuole membrane protein-1 (VMP1). In this current study, we presented evidence indicating that miR-21 possesses its own gene promoter, which can be found in the intron 10 of the VMP1 gene. Chromatin immunoprecipitation followed by global DNA sequencing (ChIP-seq) revealed the presence of a broad H3K4me3 peak spanning the entire gene body of the primary miR-21 and the existence of super-enhancer clusters in the close proximity to both the miR-21 gene promoter and the transcription termination site in arsenic (As3+)-induced cancer stem-like cells (CSCs) and human induced pluripotent stem cells (hiPSCs). In non-transformed human bronchial epithelial cells (BEAS-2B), As3+ treatment enhanced Nrf2 binding to both the host gene VMP1 of miR-21 and the miR-21 gene. Knockout of Nrf2 inhibited both the basal and As3+-induced expressions of miR-21. Furthermore, the As3+-enhanced Nrf2 peaks in ChIP-seq fully overlap with these super-enhancers enriched with H3K4me1 and H3K27ac in the miR-21 gene, suggesting that Nrf2 may coordinate with other transcription factors through the super-enhancers to regulate the expression of miR-21 in cellular response to As3+. These findings demonstrate the unique genetic and epigenetic characteristics of miR-21 and may provide insights into understanding the novel mechanisms linking environmental As3+ exposure and human cancers.

Development of a framework for risk assessment of dietary carcinogens

Although there are a number of guidance documents and frameworks for evaluation of carcinogenicity, none of the current methods fully reflects the state of the science. Common limitations include the absence of dose-response assessment and not considering the impact of differing exposure patterns (e.g., intermittent, high peaks vs. lower, continuous exposures). To address these issues, we have developed a framework for risk assessment of dietary carcinogens. This framework includes an enhanced approach for weight of evidence (WOE) evaluation for genetic toxicology data, with a focus on evaluating studies based on the most recent testing guidance to determine whether a chemical is a mutagen. Included alongside our framework is a discussion of resources for evaluating tissue dose and the temporal pattern of internal dose, taking into account the chemical's toxicokinetics. The framework then integrates the mode of action (MOA) and associated dose metric category with the exposure data to identify the appropriate approach(es) to low-dose extrapolation and level of concern associated with the exposure scenario. This framework provides risk managers with additional flexibility in risk management and risk communication options, beyond the binary choice of linear low-dose extrapolation vs. application of uncertainty factors.

Genetic Risk Scores and Missing Heritability in Ovarian Cancer

Ovarian cancers are curable by surgical resection when discovered early. Unfortunately, most ovarian cancers are diagnosed in the later stages. One strategy to identify early ovarian tumors is to screen women who have the highest risk. This opinion article summarizes the accuracy of different methods used to assess the risk of developing ovarian cancer, including family history, BRCA genetic tests, and polygenic risk scores. The accuracy of these is compared to the maximum theoretical accuracy, revealing a substantial gap. We suggest that this gap, or missing heritability, could be caused by epistatic interactions between genes. An alternative approach to computing genetic risk scores, using chromosomal-scale length variation should incorporate epistatic interactions. Future research in this area should focus on this and other alternative methods of characterizing genomes.

The Concept of "Cancer Stem Cells" in the Context of Classic Carcinogenesis Hypotheses and Experimental Findings

In this Commentary, the operational definition of cancer stem cells or cancer initiating cells includes the ability of certain cells, found in a heterogeneous mixture of cells within a tumor, which are able to sustain growth of that tumor. However, that concept of cancer stem cells does not resolve the age-old controversy of two opposing hypotheses of the origin of the cancer, namely the stem cell hypothesis versus the de-differentiation or re-programming hypothesis. Moreover, this cancer stem concept has to take into account classic experimental observations, techniques, and concepts, such as the multi-stage, multi-mechanism process of carcinogenesis; roles of mutagenic, cytotoxic and epigenetic mechanisms; the important differences between errors of DNA repair and errors of DNA replication in forming mutations; biomarkers of known characteristics of normal adult organ-specific stem cells and of cancer stem cells; and the characteristics of epigenetic mechanisms involved in the carcinogenic process. In addition, vague and misleading terms, such as carcinogens, immortal and normal cells have to be clarified in the context of current scientific facts. The ultimate integration of all of these historic factors to provide a current understanding of the origin and characteristics of a cancer stem cell, which is required for a rational strategy for prevention and therapy for cancer, does not follow a linear path. Lastly, it will be speculated that there exists evidence of two distinct types of cancer stem cells, one that has its origin in an organ-specific adult stem cell that is 'initiated' in the stem cell stage, expressing the Oct4A gene and not expressing any connexin gene or having functional gap junctional intercellular communication (GJIC). The other cancer stem cell is derived from a stem cell that is initiated early after the Oct4A gene is suppressed and the connexin gene is expressed, which starts early differentiation, but it is blocked from terminal differentiation.

Tumor heterogeneity for differentiation between liver tumors and normal liver tissue in 18F-FDG PET/CT

AIM

 Malignancies show higher spatial heterogeneity than normal tissue. We investigated, if textural parameters from FDG PET describing the heterogeneity function as tool to differentiate between tumor and normal liver tissue.

METHODS

 FDG PET/CT scans of 80 patients with liver metastases and 80 patients with results negative upper abdominal organs were analyzed. Metastases and normal liver tissue were analyzed drawing up to three VOIs with a diameter of 25 mm in healthy liver tissue of the tumoral affected and results negative liver, whilst up to 3 metastases per patient were delineated. Within these VOIs 30 different textural parameters were calculated as well as SUV. The parameters were compared in terms of intra-patient and inter-patient variability (2-sided t test). ROC analysis was performed to analyze predictive power and cut-off values.

RESULTS

 28 textural parameters differentiated healthy and pathological tissue (p < 0.05) with high sensitivity and specificity. SUV showed ability to differentiate but with a lower significance. 15 textural parameters as well as SUV showed a significant variation between healthy tissues out of tumour infested and negative livers. Mean intra- and inter-patient variability of metastases were found comparable or lower for 6 of the textural features than the ones of SUV. They also showed good values of mean intra- and inter-patient variability of VOIs drawn in liver tissue of patients with metastases and of results negative ones.

CONCLUSION

 Heterogeneity parameters assessed in FDG PET are promising to classify tissue and differentiate malignant lesions usable for more personalized treatment planning, therapy response evaluation and precise delineation of tumors for target volume determination as part of radiation therapy planning.

What Can Chemical Carcinogenesis Shed Light on the LNT Hypothesis in Radiation Carcinogenesis?

To protect the public’s health from exposure to physical, chemical, and microbiological agents, it is important that any policy be based on rigorous scientifically based research. The concept of “linear no-threshold” (LNT) has been implemented to provide guideline exposures to these agents. The practical limitation to testing this hypothesis is to provide sufficient samples for experimental or epidemiological studies. While there is no universally accepted understanding of most human diseases, there seems to be better understanding of cancer that might help resolve the “LNT” model. The public’s concern, after being exposed to radiation, is the potential of producing cancer. The most rigorous hypothesis of human carcinogenesis is the “multistage, multimechanism” chemical carcinogenesis model. The radiation carcinogenesis LNT model, rarely, if ever, built it into their support. It will be argued that this multistage, multimechanism model of carcinogenesis, involving the “initiation” of a single cell by a mutagen event, followed by chronic exposure to threshold levels of epigenetic agents or conditions that stimulate the clonal expansion of the “initiated” cell, can convert these benign cells to become invasive and metastatic. This “promotion” process can be interrupted, thereby preventing these initiated cells from transitioning to the “progression” process of invasion and metastasis.

3S - Systematic, systemic, and systems biology and toxicology

A biological system is more than the sum of its parts - it accomplishes many functions via synergy. Deconstructing the system down to the molecular mechanism level necessitates the complement of reconstructing functions on all levels, i.e., in our conceptualization of biology and its perturbations, our experimental models and computer modelling. Toxicology contains the somewhat arbitrary subclass "systemic toxicities"; however, there is no relevant toxic insult or general disease that is not systemic. At least inflammation and repair are involved that require coordinated signaling mechanisms across the organism. However, the more body components involved, the greater the challenge to reca-pitulate such toxicities using non-animal models. Here, the shortcomings of current systemic testing and the development of alternative approaches are summarized. We argue that we need a systematic approach to integrating existing knowledge as exemplified by systematic reviews and other evidence-based approaches. Such knowledge can guide us in modelling these systems using bioengineering and virtual computer models, i.e., via systems biology or systems toxicology approaches. Experimental multi-organ-on-chip and microphysiological systems (MPS) provide a more physiological view of the organism, facilitating more comprehensive coverage of systemic toxicities, i.e., the perturbation on organism level, without using substitute organisms (animals). The next challenge is to establish disease models, i.e., micropathophysiological systems (MPPS), to expand their utility to encompass biomedicine. Combining computational and experimental systems approaches and the chal-lenges of validating them are discussed. The suggested 3S approach promises to leverage 21st century technology and systematic thinking to achieve a paradigm change in studying systemic effects.

Reflections on the use of 10 IARC carcinogenic characteristics for an objective approach to identifying and organizing results from certain mechanistic studies

To find a scientifically based method for evaluating mechanistic data related to risks to human beings, a new protocol for identifying, organizing, and summarizing mechanistic data for decision-making on cancer hazard identification was proposed by the International Agency for Research on Cancer and by an international working group of multidisciplinary experts. This Commentary examined the 10 key carcinogens’ characteristics proposed in the context of several paradigms assumed in the using of these 10 characteristics. These characteristics were assumed to represent a “carcinogen’s” mechanism of action but what was ignored were characteristics of the mechanisms of the “initiation,” “promotion,” and “progression” carcinogenic process. Challenges were made to the interpretation of genotoxicity data as well as from concepts and findings related to the promotion phase and the role of adult human stem cells. Reliance of interpretation of “genotoxicity” data (molecular-DNA lesions in DNA; induction of free radicals/oxidative stress markers; phenotypic surrogates of gene mutations), as well as from lesions in genomic versus mitochondrial DNA, or in the target cells for the carcinogenic process in either in vitro cultures or in vivo tissues, makes this “objective” use of the data questionable. A challenge to the “dedifferentiation” hypothesis of cancer was made. Because of an agent being misclassified as “genotoxic”—rather than an “epigenetic”—agent (which works by threshold levels; can be blocked; and must be present at critical times during development and at regular, sustained chronic exposures) could lead to unwise policy decisions.

"Bad Luck Mutations": DNA Mutations Are not the Whole Answer to Understanding Cancer Risk

It has been proposed that many human cancers are generated by intrinsic mechanisms that produce "Bad Luck" mutations by the proliferation of organ-specific adult stem cells. There have been serious challenges to this interpretation, including multiple extrinsic factors thought to be correlated with mutations found in cancers associated with these exposures. While support for both interpretations provides some validity, both interpretations ignore several concepts of the multistage, multimechanism process of carcinogenesis, namely, (1) mutations can be generated by both "errors of DNA repair" and "errors of DNA replication," during the "initiation" process of carcinogenesis; (2) "initiated" stem cells must be clonally amplified by nonmutagenic, intrinsic or extrinsic epigenetic mechanisms; (3) organ-specific stem cell numbers can be modified during in utero development, thereby altering the risk to cancer later in life; and (4) epigenetic tumor promoters are characterized by species, individual genetic-, gender-, developmental state-specificities, and threshold levels to be active; sustained and long-term exposures; and exposures in the absence of antioxidant "antipromoters." Because of the inevitability of some of the stem cells generating "initiating" mutations by either "errors of DNA repair" or "errors of DNA replication," a tumor is formed depending on the promotion phase of carcinogenesis. While it is possible to reduce our frequencies of mutagenic "initiated" cells, one can never reduce it to zero. Because of the extended period of the promotion phase of carcinogenesis, strategies to reduce the appearance of cancers must involve the interruption of the promotion of these initiated cells.

Antioxidant and hepatoprotective potential of Lawsonia inermis L. leaves against 2-acetylaminofluorene induced hepatic damage in male Wistar rats

Background

Lawsonia inermis (Lythraceae) is an ethnomedicinal plant, traditionally known for curing several ailments such as skin diseases, bacterial infections, jaundice, renal lithiases and inflammation etc. The present work deals with assessment of in vitro antioxidant and in vivo hepatoprotective potential of butanolic fraction (But-LI) of Lawsonia inermis L. leaves.

Methods

Antioxidant activity was evaluated using deoxyribose degradation, lipid peroxidation inhibition and ferric reducing antioxidant power (FRAP) assay. In vivo protective potential of But-LI was assessed at 3 doses [100, 200 & 400 mg/kg body weight (bw)] against 2-acetylaminofluorene (2-AAF) induced hepatic damage in male Wistar rats.

Results

But-LI effectively scavenged hydroxyl radicals in deoxyribose degradation assay (IC₅₀ 149.12 μg/ml). Fraction also inhibited lipid peroxidation and demonstrated appreciable reducing potential in FRAP assay. Treatment of animals with 2-AAF resulted in increased hepatic parameters such as SGOT (2.22 fold), SGPT (1.72 fold), ALP (5.68 fold) and lipid peroxidation (2.94 fold). Different concentration of But-LI demonstrated pronounced protective effects via decreasing levels of SGOT, SGPT, ALP and lipid peroxidation altered by 2-AAF treatment. But-LI administration also restored the normal liver architecture as evident from histopathological studies.

Conclusions

The present experimental findings revealed that phytoconstituents of Lawsonia inermis L. possess potential to effectively protect rats from the 2-AAF induced hepatic damage in vivo possibly by inhibition of reactive oxygen species and lipid peroxidation.

Evolution of Microbial Quorum Sensing to Human Global Quorum Sensing: An Insight into How Gap Junctional Intercellular Communication Might Be Linked to the Global Metabolic Disease Crisis

The first anaerobic organism extracted energy for survival and reproduction from its source of nutrients, with the genetic means to ensure protection of its individual genome but also its species survival. While it had a means to communicate with its community via simple secreted molecules (“quorum sensing”), the eventual shift to an aerobic environment led to multi-cellular metazoan organisms, with evolutionary-selected genes to form extracellular matrices, stem cells, stem cell niches, and a family of gap junction or “connexin” genes. These germinal and somatic stem cells responded to extracellular signals that triggered intra-cellular signaling to regulate specific genes out of the total genome. These extra-cellular induced intra-cellular signals also modulated gap junctional intercellular communication (GJIC) in order to regulate the new cellular functions of symmetrical and asymmetrical cell division, cell differentiation, modes of cell death, and senescence. Within the hierarchical and cybernetic concepts, differentiated by neurons organized in the brain of the Homo sapiens, the conscious mind led to language, abstract ideas, technology, myth-making, scientific reasoning, and moral decision–making, i.e., the creation of culture. Over thousands of years, this has created the current collision between biological and cultural evolution, leading to the global “metabolic disease” crisis.

The Genomic Landscape of Renal Oncocytoma Identifies a Metabolic Barrier to Tumorigenesis

Oncocytomas are predominantly benign neoplasms possessing pathogenic mitochondrial mutations and accumulation of respiration-defective mitochondria, characteristics of unknown significance. Using exome and transcriptome sequencing, we identified two main subtypes of renal oncocytoma. Type 1 is diploid with CCND1 rearrangements, whereas type 2 is aneuploid with recurrent loss of chromosome 1, X or Y, and/or 14 and 21, which may proceed to more aggressive eosinophilic chromophobe renal cell carcinoma (ChRCC). Oncocytomas activate 5' adenosine monophosphate-activated protein kinase (AMPK) and Tp53 (p53) and display disruption of Golgi and autophagy/lysosome trafficking, events attributed to defective mitochondrial function. This suggests that the genetic defects in mitochondria activate a metabolic checkpoint, producing autophagy impairment and mitochondrial accumulation that limit tumor progression, revealing a novel tumor-suppressive mechanism for mitochondrial inhibition with metformin. Alleviation of this metabolic checkpoint in type 2 by p53 mutations may allow progression to eosinophilic ChRCC, indicating that they represent higher risk.

Origins and functional consequences of somatic mitochondrial DNA mutations in human cancer

Recent sequencing studies have extensively explored the somatic alterations present in the nuclear genomes of cancers. Although mitochondria control energy metabolism and apoptosis, the origins and impact of cancer-associated mutations in mtDNA are unclear. In this study, we analyzed somatic alterations in mtDNA from 1675 tumors. We identified 1907 somatic substitutions, which exhibited dramatic replicative strand bias, predominantly C > T and A > G on the mitochondrial heavy strand. This strand-asymmetric signature differs from those found in nuclear cancer genomes but matches the inferred germline process shaping primate mtDNA sequence content. A number of mtDNA mutations showed considerable heterogeneity across tumor types. Missense mutations were selectively neutral and often gradually drifted towards homoplasmy over time. In contrast, mutations resulting in protein truncation undergo negative selection and were almost exclusively heteroplasmic. Our findings indicate that the endogenous mutational mechanism has far greater impact than any other external mutagens in mitochondria and is fundamentally linked to mtDNA replication.

Mutator/Hypermutable fetal/juvenile metakaryotic stem cells and human colorectal carcinogenesis

Adult age-specific colorectal cancer incidence rates increase exponentially from maturity, reach a maximum, then decline in extreme old age. Armitage and Doll (1) postulated that the exponential increase resulted from "n" mutations occurring throughout adult life in normal "cells at risk" that initiated the growth of a preneoplastic colony in which subsequent "m" mutations promoted one of the preneoplastic "cells at risk" to form a lethal neoplasia. We have reported cytologic evidence that these "cells at risk" are fetal/juvenile organogenic, then preneoplastic metakaryotic stem cells. Metakaryotic cells display stem-like behaviors of both symmetric and asymmetric nuclear divisions and peculiarities such as bell shaped nuclei and amitotic nuclear fission that distinguish them from embryonic, eukaryotic stem cells. Analyses of mutant colony sizes and numbers in adult lung epithelia supported the inferences that the metakaryotic organogenic stem cells are constitutively mutator/hypermutable and that their contributions to cancer initiation are limited to the fetal/juvenile period. We have amended the two-stage model of Armitage and Doll and incorporated these several inferences in a computer program CancerFit v.5.0. We compared the expectations of the amended model to adult (15-104 years) age-specific colon cancer rates for European-American males born 1890-99 and observed remarkable concordance. When estimates of normal colonic fetal/juvenile APC and OAT gene mutation rates (∼2-5 × 10(-5) per stem cell doubling) and preneoplastic colonic gene loss rates (∼8 × 10(-3)) were applied, the model was in accordance only for the values of n = 2 and m = 4 or 5.

TERT promoter mutations in bladder cancer affect patient survival and disease recurrence through modification by a common polymorphism

The telomerase reverse transcriptase (TERT) promoter, an important element of telomerase expression, has emerged as a target of cancer-specific mutations. Originally described in melanoma, the mutations in TERT promoter have been shown to be common in certain other tumor types that include glioblastoma, hepatocellular carcinoma, and bladder cancer. To fully define the occurrence and effect of the TERT promoter mutations, we investigated tumors from a well-characterized series of 327 patients with urothelial cell carcinoma of bladder. The somatic mutations, mainly at positions -124 and -146 bp from ATG start site that create binding motifs for E-twenty six/ternary complex factors (Ets/TCF), affected 65.4% of the tumors, with even distribution across different stages and grades. Our data showed that a common polymorphism rs2853669, within a preexisting Ets2 binding site in the TERT promoter, acts as a modifier of the effect of the mutations on survival and tumor recurrence. The patients with the mutations showed poor survival in the absence [hazard ratio (HR) 2.19, 95% confidence interval (CI) 1.02-4.70] but not in the presence (HR 0.42, 95% CI 0.18-1.01) of the variant allele of the polymorphism. The mutations in the absence of the variant allele were highly associated with the disease recurrence in patients with Tis, Ta, and T1 tumors (HR 1.85, 95% CI 1.11-3.08). The TERT promoter mutations are the most common somatic lesions in bladder cancer with clinical implications. The association of the mutations with patient survival and disease recurrence, subject to modification by a common polymorphism, can be a unique putative marker with individualized prognostic potential.

Action spectrum analysis of UVR genotoxicity for skin: the border wavelengths between UVA and UVB can bring serious mutation loads to skin

UVR causes erythema, which has been used as a standardized index to evaluate the risk of UVR for human skin. However, the genotoxic significance of erythema has not been elucidated clearly. Here, we characterized the wavelength dependence of the genotoxic and erythematic effects of UVR for the skin by analyzing the induction kinetics of mutation and inflammation in mouse skin using lacZ-transgenic mice and monochromatic UVR sources. We determined their action spectra and found a close correlation between erythema and an epidermis-specific antigenotoxic response, mutation induction suppression (MIS), which suppressed the mutant frequencies (MFs) to a constant plateau level only 2-3-fold higher than the background MF at the cost of apoptotic cell death, suggesting that erythema may represent the threshold beyond which the antigenotoxic but tissue-destructive MIS response commences. However, we unexpectedly found that MIS attenuates remarkably at the border wavelengths between UVA and UVB around 315 nm, elevating the MF plateaus up to levels ∼40-fold higher than the background level. Thus, these border wavelengths can bring heavier mutation loads to the skin than the otherwise more mutagenic and erythematic shorter wavelengths, suggesting that erythema-based UVR risk evaluation should be reconsidered.

The nucleotide excision repair (NER) system of Helicobacter pylori: role in mutation prevention and chromosomal import patterns after natural transformation

Background

Extensive genetic diversity and rapid allelic diversification are characteristics of the human gastric pathogen Helicobacter pylori, and are believed to contribute to its ability to cause chronic infections. Both a high mutation rate and frequent imports of short fragments of exogenous DNA during mixed infections play important roles in generating this allelic diversity. In this study, we used a genetic approach to investigate the roles of nucleotide excision repair (NER) pathway components in H. pylori mutation and recombination.

Results

Inactivation of any of the four uvr genes strongly increased the susceptibility of H. pylori to DNA damage by ultraviolet light. Inactivation of uvrA and uvrB significantly decreased mutation frequencies whereas only the uvrA deficient mutant exhibited a significant decrease of the recombination frequency after natural transformation. A uvrC mutant did not show significant changes in mutation or recombination rates; however, inactivation of uvrC promoted the incorporation of significantly longer fragments of donor DNA (2.2-fold increase) into the recipient chromosome. A deletion of uvrD induced a hyper-recombinational phenotype.

Conclusions

Our data suggest that the NER system has multiple functions in the genetic diversification of H. pylori, by contributing to its high mutation rate, and by controlling the incorporation of imported DNA fragments after natural transformation.

Epigenomics in environmental health

This review considers the emerging relationships between environmental factors and epigenetic alterations and the application of genome-wide assessments to better define these relationships. First we will briefly cover epigenetic programming in development, one-carbon metabolism, and exposures that may disrupt normal developmental programming of epigenetic states. In addition, because a large portion of epigenetic research has focused on cancer, we discuss exposures associated with carcinogenesis including asbestos, alcohol, radiation, arsenic, and air pollution. Research on other exposures that may affect epigenetic states such as endocrine disruptors is also described, and we also review the evidence for epigenetic alterations associated with aging that may reflect cumulative effects of exposures. From this evidence, we posit potential mechanisms by which exposures modify epigenetic states, noting that understanding the true effect of environmental exposures on the human epigenome will require additional research with appropriate epidemiologic studies and application of novel technologies. With a more comprehensive understanding of the affects of exposures on the epigenome, including consideration of genetic background, the prediction of the toxic potential of new compounds may be more readily achieved, and may lead to the development of more personalized disease prevention and treatment strategies.

Age-specific incidence data indicate four mutations are required for human testicular cancers

Normal human cells require a series of genetic alterations to undergo malignant transformation. Direct sequencing of human tumors has identified hundreds of mutations in tumors, but many of these are thought to be unnecessary and a result of, rather than a cause of, the tumor. The exact number of mutations to transform a normal human cell into a tumor cell is unknown. Here I show that male gonadal germ cell tumors, the most common form of testicular cancers, occur after four mutations. I infer this by constructing a mathematical model based upon the multi-hit hypothesis and comparing it to the age-specific incidence data. This result is consistent with the multi-hit hypothesis, and implies that these cancers are genetically or epigenetically predetermined at birth or an early age.

A paradigm shift is required for the risk assessment of potential human health after exposure to low level chemical exposures: a response to the toxicity testing in the 21st century report

Chemicals are known to be associated with birth defects, cancer, cardiovascular diseases, immunological, reproductive, and neurological disorders. In response to recent reviews of limitations of current concepts and techniques for toxicity testing, this commentary challenges the paradigm that chemicals are directly responsible for DNA damage in the genomic-nuclear DNA in relevant cells of the human body. This challenge is not that mutations do not play roles in human-inherited or somatic diseases but that chemical exposures bring about disease end points by epigenetic mechanisms or by alterations in adult stem cell numbers in utero (ie, the Barker hypothesis) or postnatally, by selecting preexisting mutated cells. Classic concepts, that is, multistage, multimechanism process of carcinogenesis, stem cell theory of cancer, and newer and ignored concepts, such as cancer stem cells and cell-cell communication, will be used to support the view that the toxic effect of chemicals is mediated by nonmutagenic mechanisms at human relevant exposures.

Oncomutations as biomarkers of cancer risk

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

Ionizing radiation and hematopoietic malignancies: altering the adaptive landscape

Somatic evolution, which underlies tumor progression, is driven by two essential components: (1) diversification of phenotypes through heritable mutations and epigenetic changes and (2) selection for mutant clones which possess higher fitness. Exposure to ionizing radiation (IR ) is highly associated with increased risk of carcinogenesis. This link is traditionally attributed to causation of oncogenic mutations through the mutagenic effects of irradiation. On the other hand, potential effects of irradiation on altering fitness and increasing selection for mutant clones are frequently ignored. Recent studies bring the effects of irradiation on fitness and selection into focus, demonstrating that IR exposure results in stable reductions in the fitness of hematopoietic stem and progenitor cell populations. These reductions of fitness are associated with alteration of the adaptive landscape, increasing the selective advantages conferred by certain oncogenic mutations. Therefore, the link between irradiation and carcinogenesis might be more complex than traditionally appreciated: while mutagenic effects of irradiation should increase the probability of occurrence of oncogenic mutations, IR can also work as a tumor promoter, increasing the selective expansion of clones bearing mutations which become advantageous in the irradiation-altered environment, such as activated mutations in Notch1 or disrupting mutations in p53.

Applications of the human p53 knock-in (Hupki) mouse model for human carcinogen testing

Tumor-driving mutations in the TP53 gene occur frequently in human cancers. These inactivating mutations arise predominantly from a single-point mutation in the DNA-binding domain of this tumor suppressor gene (i.e., exons 4-9). The human p53 knock-in (Hupki) mouse model was constructed using gene-targeting technology to create a mouse strain that harbors human wild-type TP53 DNA sequences in both copies of the mouse TP53 gene. Replacement of exons 4-9 of the endogenous mouse TP53 alleles in the Hupki mouse with the homologous normal human TP53 gene sequences has offered a humanized replica of the TP53 gene in a murine genetic environment. The Hupki mouse model system has proven to be an invaluable research tool for studying the underlying mechanisms of human TP53 mutagenesis. The utility of the Hupki mouse model system for exploring carcinogen-induced TP53 mutagenesis has been demonstrated in both in vivo animal experiments and in vitro cell culture experiments. Here, we highlight applications of the Hupki mouse model system for investigating mutagenesis induced by a variety of environmental carcinogens, including sunlight ultraviolet radiation, benzo[a]pyrene (a tobacco smoke-derived carcinogen), 3-nitrobenzanthrone (an urban air pollutant), aristolochic acid (a component of Chinese herbal medicine), and aflatoxin B1 (a food contaminant). We summarize the salient findings of the respective studies and discuss their relevance to human cancer etiology.

Cigarette smoking and alcohol drinking and esophageal cancer risk in Taiwanese women

AIM: To investigate the etiology of esophageal cancer among Taiwanese women.

METHODS: This is a multi-center, hospital-based, case-control study. Case patients consisted of women who were newly diagnosed and pathology-proven to have esophageal squamous cell carcinoma (ESCC) from three large medical centers (one from Northern and two from Southern Taiwan, respectively) between August 2000 and December 2008. Each ESCC patient was matched with 4 healthy women based on age (within 3 years) and hospital of origin, from the Department of Preventive Medicine in each hospital. A total of 51 case patients and 204 controls, all women, were studied.

RESULTS: Frequencies of smokers and drinkers among ESCC patients were 19.6% and 21.6%, respectively, which were significantly higher than smokers (4.4%) and drinkers (4.4%) among controls (OR = 4.07, 95% CI: 1.36-12.16, P = 0.01; OR = 3.55, 95% CI: 1.03-12.27, P = 0.04). Women who drank an amount of alcohol more than 158 g per week had a 20.58-fold greater risk (95% CI: 1.72-245.62, P = 0.02) of ESCC than those who never drank alcohol after adjusting for other covariates, although the sample size was small.

CONCLUSION: Cigarette smoking and alcohol drinking, especially heavy drinking, are the major risks for developing ESCC in Taiwanese women.

Irradiation selects for p53-deficient hematopoietic progenitors

While disruption of p53 is selectively neutral within non-stressed hematopoiesis, it confers a strong selective advantage upon irradiation, leading to expansion of p53 mutant clones and lymphoma development.

Identification and characterization of mutations that drive cancer evolution constitute a major focus of cancer research. Consequently, dominant paradigms attribute the tumorigenic effects of carcinogens in general and ionizing radiation in particular to their direct mutagenic action on genetic loci encoding oncogenes and tumor suppressor genes. However, the effects of irradiation are not limited to genetic loci that encode oncogenes and tumor suppressors, as irradiation induces a multitude of other changes both in the cells and their microenvironment which could potentially affect the selective effects of some oncogenic mutations. P53 is a key tumor suppressor, the loss of which can provide resistance to multiple genotoxic stimuli, including irradiation. Given that p53 null animals develop T-cell lymphomas with high penetrance and that irradiation dramatically accelerates lymphoma development in p53 heterozygous mice, we hypothesized that increased selection for p53-deficient cells contributes to the causal link between irradiation and induction of lymphoid malignancies. We sought to determine whether ionizing irradiation selects for p53-deficient hematopoietic progenitors in vivo using mouse models. We found that p53 disruption does not provide a clear selective advantage within an unstressed hematopoietic system or in previously irradiated BM allowed to recover from irradiation. In contrast, upon irradiation p53 disruption confers a dramatic selective advantage, leading to long-term expansion of p53-deficient clones and to increased lymphoma development. Selection for cells with disrupted p53 appears to be attributable to several factors: protection from acute irradiation-induced ablation of progenitor cells, prevention of irradiation-induced loss of clonogenic capacity for stem and progenitor cells, improved long-term maintenance of progenitor cell fitness, and the disabling/elimination of competing p53 wild-type progenitors. These studies indicate that the carcinogenic effect of ionizing irradiation can in part be explained by increased selection for cells with p53 disruption, which protects progenitor cells both from immediate elimination and from long-term reductions in fitness following irradiation.

Cancer progression can be understood through the framework of Darwinian evolution, which involves two major factors: genetic mutation and selection. Random mutations are thought to result in the initiation and phenotypic diversification of tumors, and environmental influences mediate selection for those mutations that increase tumor cell fitness. Since oncogenic mutations are necessary for the development of spontaneous malignancies and since experimental introduction of these mutations often leads to transformation and cancers, the causation of cancers by carcinogens is traditionally attributed to their induction of new mutations that are oncogenic. We instead asked whether selection for oncogenic mutations is affected by ionizing irradiation, an archetypal mutagenic carcinogen, by examining the selective effects of inactivation of the critical tumor suppressor gene p53. While disruption of p53 is selectively neutral in populations of unstressed hematopoietic progenitors, it provides a strong selective advantage upon irradiation. This selection of p53-deficient clones is attributable to protection from irradiation-induced cell death and loss of cellular fitness. Importantly, the selective expansion of irradiated cells bearing p53 disruption is blocked in the presence of non-irradiated wild-type competitors, indicating that the disabling of competing wild-type cells by irradiation is critical for selection of p53-deficient cells. Our results argue that induction of cancers by irradiation involves selection for mutations that confer radioresistance, and suggest that greater focus on how carcinogenic contexts impact on selection is warranted in understanding, preventing and treating cancers.

Tumor heterogeneity: causes and consequences

With rare exceptions, spontaneous tumors originate from a single cell. Yet, at the time of clinical diagnosis, the majority of human tumors display startling heterogeneity in many morphological and physiological features, such as expression of cell surface receptors, proliferative and angiogenic potential. To a substantial extent, this heterogeneity might be attributed to morphological and epigenetic plasticity, but there is also strong evidence for the co-existence of genetically divergent tumor cell clones within tumors. In this perspective, we summarize the sources of intra-tumor phenotypic heterogeneity with emphasis on genetic heterogeneity. We review experimental evidence for the existence of both intra-tumor clonal heterogeneity as well as frequent evolutionary divergence between primary tumors and metastatic outgrowths. Furthermore, we discuss potential biological and clinical implications of intra-tumor clonal heterogeneity.

Irradiation alters selection for oncogenic mutations in hematopoietic progenitors

Exposure to ionizing radiation and other DNA-damaging carcinogens is strongly associated with induction of malignancies. Prevailing paradigms attribute this association to the induction of oncogenic mutations, as the incidence of oncogenic events is thought to limit initiation and progression of cancers. On the other hand, random mutagenic and genotoxic effects of irradiation are likely to alter progenitor cell populations and the microenvironment, thus altering the selective effects of oncogenic mutations. Using competitive bone marrow transplantation experiments in mice, we show that ionizing irradiation leads to a persistent decline in the numbers and fitness of hematopoietic stem cells, in part resulting from persistent induction of reactive oxygen species. Previous irradiation dramatically alters the selective effects of some oncogenic mutations, substantially inhibiting clonal expansion and leukemogenesis driven by Bcr-Abl or activated N-Ras oncogenes but enhancing the selection for and leukemogenesis driven by the activated Notch1 mutant ICN. Irradiation-dependent selection for ICN expression occurs in a hematopoietic stem cell-enriched pool, which should facilitate the accumulation of additional oncogenic events at a committed T-progenitor stage critical for formation of T-lymphocytic leukemia stem cells. Enhancement of ICN-driven selection and leukemogenesis by previous irradiation is in part non-cell autonomous, as partial restoration of normal hematopoiesis can reverse these effects of irradiation. These studies show that irradiation substantially alters the adaptive landscape in hematopoietic progenitors and suggest that the causal link between irradiation and carcinogenesis might involve increased selection for particular oncogenic mutations.

In vitro recapitulating of TP53 mutagenesis in hepatocellular carcinoma associated with dietary aflatoxin B1 exposure

BACKGROUND & AIMS

Dietary exposure to aflatoxin B(1) (AFB(1)), in addition to other known factors, increases risk for human hepatocellular carcinoma (HCC). HCCs from AFB(1)-exposed individuals frequently have distinct TP53 mutations, such as G to T transversions in the second guanine of codon 249 (AGG to AGT), and a characteristic mutational spectrum predominated by G:C to T:A mutations.

METHODS

To recapitulate the distinctive features of TP53 mutations in AFB(1)-associated HCC, we investigated AFB(1)-induced DNA adduction in relation to mutagenesis in transgenic mouse fibroblasts exposed to AFB(1) in vitro.

RESULTS

Immunodotblot determination of DNA adducts in the overall genome of AFB(1)-exposed cells revealed the dose-dependant formation of persistent imidazole ring-opened AFB(1)-DNA adducts. DNA footprinting analysis of the cII transgene in AFB(1)-exposed cells verified the dose-dependent and sequence-specific formation of DNA adducts. The preferential formation of AFB(1)-induced DNA adducts along the cII transgene was almost exclusively localized to guanine-containing sequences encompassing CpG dinucleotides. Mutation analysis of the cII transgene in AFB(1)-exposed cells revealed a dose-dependent induction of cII mutant frequency (P < .001) and a unique induced mutational spectrum characterized by predominant induction of G:C to T:A transversions that occurred within CpG sequence contexts. Notably, codons 42 and 45 of the cII transgene, which have identical sequence contexts to that of codon 249 of human TP53, constituted 2 frequently mutated sites in AFB(1)-exposed cells that contained the G to T transversion signature mutation at their third base positions.

CONCLUSIONS

In this model system, AFB(1)-induced DNA adduction and mutagenesis recapitulate the unique mutational features of TP53 in AFB(1)-associated human HCC.

Mutational spectra of human cancer

The purpose of this review is to summarize the evidence that can be used to reconstruct the etiology of human cancers from mutations found in tumors. Mutational spectra of the tumor suppressor gene p53 (TP53) are tumor specific. In several cases, these mutational spectra can be linked to exogenous carcinogens, most notably for sunlight-associated skin cancers, tobacco-associated lung cancers, and aristolochic acid-related urothelial tumors. In the TP53 gene, methylated CpG dinucleotides are sequences selectively targeted by endogenous and exogenous mutagenic processes. Recent high-throughput sequencing efforts analyzing a large number of genes in cancer genomes have so far, for the most part, produced mutational spectra similar to those in TP53 but have unveiled a previously unrecognized common G to C transversion mutation signature at GpA dinucleotides in breast cancers and several other cancers. Unraveling the origin of these G to C mutations will be of importance for understanding cancer etiology.

Large-scale inference of the point mutational spectrum in human segmental duplications

BACKGROUND

Recent segmental duplications are relatively large (> or = 1 kb) genomic regions of high sequence identity (> or = 90%). They cover approximately 4-5% of the human genome and play important roles in gene evolution and genomic disease. The DNA sequence differences between copies of a segmental duplication represent the result of various mutational events over time, since any two duplication copies originated from the same ancestral DNA sequence. Based on this fact, we have developed a computational scheme for inference of point mutational events in human segmental duplications, which we collectively term duplication-inferred mutations (DIMs). We have characterized these nucleotide substitutions by comparing them with high-quality SNPs from dbSNP, both in terms of sequence context and frequency of substitution types.

RESULTS

Overall, DIMs show a lower ratio of transitions relative to transversions than SNPs, although this ratio approaches that of SNPs when considering DIMs within most recent duplications. Our findings indicate that DIMs and SNPs in general are caused by similar mutational mechanisms, with some deviances at the CpG dinucleotide. Furthermore, we discover a large number of reference SNPs that coincide with computationally inferred DIMs. The latter reflects how sequence variation in duplicated sequences can be misinterpreted as ordinary allelic variation.

CONCLUSION

In summary, we show how DNA sequence analysis of segmental duplications can provide a genome-wide mutational spectrum that mirrors recent genome evolution. The inferred set of nucleotide substitutions represents a valuable complement to SNPs for the analysis of genetic variation and point mutagenesis.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

Fetal-juvenile origins of point mutations in the adult human tracheal-bronchial epithelium: absence of detectable effects of age, gender or smoking status

Allele-specific mismatch amplification mutation assays (MAMA) of anatomically distinct sectors of the upper bronchial tracts of nine nonsmokers revealed many numerically dispersed clusters of the point mutations C742T, G746T, G747T of the TP53 gene, G35T of the KRAS gene and G508A of the HPRT1 gene. Assays of these five mutations in six smokers have yielded quantitatively similar results. One hundred and eighty four micro-anatomical sectors of 0.5-6x10(6) tracheal-bronchial epithelial cells represented en toto the equivalent of approximately 1.7 human smokers' bronchial trees to the fifth bifurcation. Statistically significant mutant copy numbers above the 95% upper confidence limits of historical background controls were found in 198 of 425 sector assays. No significant differences (P=0.1) for negative sector fractions, mutant fractions, distributions of mutant cluster size or anatomical positions were observed for smoking status, gender or age (38-76 year). Based on the modal cluster size of mitochondrial point mutants, the size of the adult bronchial epithelial maintenance turnover unit was estimated to be about 32 cells. When data from all 15 lungs were combined the log2 of nuclear mutant cluster size plotted against log2 of the number of clusters of a given cluster size displayed a slope of approximately 1.1 over a range of cluster sizes from approximately 2(6) to 2(15) mutant copies. A parsimonious interpretation of these nuclear and previously reported data for lung epithelial mitochondrial point mutant clusters is that they arose from mutations in stem cells at a high but constant rate per stem cell doubling during at least ten stem cell doublings of the later fetal-juvenile period. The upper and lower decile range of summed point mutant fractions among lungs was about 7.5-fold, suggesting an important source of stratification in the population with regard to risk of tumor initiation.

Retinoblastoma function is essential for establishing lung epithelial quiescence after injury

The retinoblastoma gene product (RB) regulates cell cycle, quiescence, and survival in a cell type-dependent and environment-dependent manner. RB function is critical in the pulmonary epithelium, as evidenced by nearly universal RB inactivation in lung cancer and increased lung cancer risk in persons with germline RB gene mutations. Lung carcinomas occur in the context of epithelial remodeling induced by cytotoxic damage. Whereas the role of RB in development and normal organ homeostasis has been extensively studied, RB function in the context of cellular injury and repair has remained largely unexplored. In the current studies, the RB gene was selectively deleted in the respiratory epithelium of the mouse. Although RB was not required for establishing or maintaining quiescence during lung homeostasis, RB was essential for establishing quiescence during epithelial repair after injury. Notably, aberrant cell cycle progression was sustained for 9 months after injury in RB-deficient lungs. Prenatal and postnatal RB ablation had similar effects, providing evidence that timing of RB loss was not critical to the outcome and that the injury-induced phenotype was not secondary to compensatory alterations occurring during development. These data show that RB is essential for repair of the respiratory epithelium after cytotoxic damage and support a critical unique role for RB in the context of epithelial remodeling after injury. Because human cancers are associated with chronic cellular damage, these findings have important new implications for RB-mediated tumor suppression.

Dietary heme injures surface epithelium resulting in hyperproliferation, inhibition of apoptosis and crypt hyperplasia in rat colon

Epidemiological and animal model studies suggest that a high intake of heme, present in red meat, is associated with an increased risk of colon cancer. The aim of this study was to elucidate the effects of dietary heme on colonic cell homeostasis in rats. Rats were fed a purified, humanized, control diet or a similar diet supplemented with 0.5 mmol heme/kg for 14 days. Fecal water cytolytic activity was determined with a bioassay, and colon epithelial cell proliferation was evaluated with (3)H-thymidine or 5-bromo-2'-deoxyuridine incorporation into DNA or by Ki-67 immunohistochemistry. Exfoliation of colonocytes was measured as the amount of rat DNA in feces, and caspase-3 expression and activity were measured to study colonic mucosal apoptosis. Dietary heme induced a >10-fold increased cytolytic activity of the fecal water and a 100-fold lower excretion of host DNA. Colons of heme-fed rats showed injured surface epithelium and an approximately 25% increase in crypt depth. Finally, dietary heme doubled colonocyte proliferation, shown by all three markers, but inhibited colonic mucosal apoptosis. In conclusion, our results demonstrate that dietary heme injures colonic surface epithelium, which is overcompensated by inhibition of apoptosis and hyperproliferation of cells in the crypts, resulting in crypt hyperplasia. This disturbed epithelial cell homeostasis might explain why a high intake of dietary heme is associated with an increased risk of colon cancer.

Declining cellular fitness with age promotes cancer initiation by selecting for adaptive oncogenic mutations

Age is the single most important prognostic factor in the development of many cancers. The major reason for this age-dependence is thought to be the progressive accumulation of oncogenic mutations and epigenetic changes. Similarly, mutagens are thought to be carcinogenic primarily by engendering oncogenic mutations. Yet while the accumulation of heritable somatic changes is expected to augment the incidence of oncogenic mutations, a major effect of increased mutation load is reduced fitness. We propose that the fitness of progenitor cell compartments substantially impacts on the selective advantage conferred by particular mutations. We hypothesize that reduced cellular fitness within aged stem cell pools can select for adaptive oncogenic events and thereby promote the initiation of cancer. Thus, certain oncogenic mutations may be adaptive within aged but not young stem cell pools. We further argue that accumulating genetic alterations with age or mutagen exposure might promote cancer not only by causing oncogenic hits within cells but also by leading to eventual reduction in stem cell fitness, which then selects for oncogenic events. Therefore, initial stages of cancer development may not be limited by the incidence of initiating oncogenic changes, but instead by contexts of reduced cellular fitness that select for these changes.

Regulation of oxidative stress–mediated apoptosis by diallyl sulfide in DMBA-exposed Swiss mice

Diallyl sulfide, a sulfur-containing volatile compound present in garlic ( Allium sativum), exerts anticarcinogenic activity in various rodent tumor models. In the present study, apoptosis-inhibiting effects of diallyl sulfide against a carcinogenic polycyclic aromatic hydrocarbon, 7,12-dimethyl benz(a)anthracene (DMBA), in Swiss albino mice were observed. The animals were given either 250 μg/mouse or 500 μg/mouse of diallyl sulfide for 1 week after a single intragastric dose of 7,12-dimethyl benz(a)anthracene (50 mg/kg body weight). Results showed that diallyl sulfide supplementation effectively protects against 7,12-dimethyl benz(a)anthracene—induced oxidative stress, characterized by restored antioxidant enzyme levels (up to 64%) and lipid peroxidation (up to 25%). Flow cytometric analysis showed a reduction in apoptotic cell population in hypodiploid region in diallyl sulfide–supplemented animals. Inhibition of apoptosis was preceded by decrease in reactive oxygen species levels and restoration of mitochondrial transmembrane potential followed by decreased DNA fragmentation. In 7,12-dimethyl benz(a)anthracene–exposed animals, downregulation (~30%) of antiapoptotic Bcl-2 and upregulation (~60%) of pro-apoptotic Bax proteins were observed. These alterations were restored significantly by diallyl sulfide supplementation, indicating inhibition of apoptosis. Thus, these results show that diallyl sulfide provides protection against oxidative damage induced by 7,12-dimethyl benz(a)anthracene in mouse liver and may be an effective chemopreventive and therapeutic agent by modulating expression of cell-growth regulatory proteins.

Single base instability is promoted in vulvar lichen sclerosus

Single base substitution mutations in codons 248 and 273 of TP53 and codon 12 Kirsten-ras (KRAS) are commonly found in human carcinomas. To determine whether these mutations also occur in normal and inflamed tissues from which carcinomas arise, we utilized the ultra-sensitive polymerase chain reaction/restriction endonuclease/ligase chain reaction mutation assay. Ninety samples of genital skin, including lichen sclerosus (LS) affected skin, adjacent normal and non-adjacent normal, were assayed. Mutations were detected in 103 of 349 assays and consisted of KRAS G34A, G34T, G35A, and TP53 C742T, G818C, C817T, and G818A mutations. Mutant prevalence varied from 1 to 20 per 10(6) wild-type cells. Mutations occurred significantly more frequently in LS (78/224 (35%)) than adjacent normal (20/88 (23%)) and non-adjacent normal genital skin (5/38 (13%)). KRAS G34A mutation was relatively common to all classes of specimen, whereas TP53 gene C742T and G818C mutations were significantly more frequent in LS than normal genital skin. In matched samples, immunohistochemistry evaluation of p53 protein expression revealed the presence of epidermal p53 clones in LS whose presence and number significantly correlated with the presence of TP53 C742T and G818C mutations. Based on these results, it appears oncogenic point mutations occur in normal genital skin, and are selected for in LS.

Cigarette smoke components inhibited intercellular communication and differentiation in human pancreatic ductal epithelial cells

Smoking is a well-documented risk factor for the development of pancreatic adenocarcinoma. Although the most abundant polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke are methylated anthracenes and phenanthrenes, the epigenetic toxicity of these compounds has not been extensively studied. We previously showed that methylanthracenes, which possess a bay-like structure, affect epigenetic events such as an induced release of arachidonic acid, inhibition of gap junctional intercellular communication (GJIC) and induction of mitogen-activated protein kinases in a pluripotent rat liver epithelial stem cell line. Anthracenes with no bay-like structures were inactive. These biological effects are all molecular events associated with the promotional phase of cancer. A human immortalized, nontumorigenic pancreatic ductal epithelial cell line, H6c7, was examined to study the epigenetic toxicity of PAHs related to pancreatic cancer by using scrape-loading dye transfer, immunostaining, RT-PCR and telomerase assay methods. H6c7 cells were GJIC-incompetent and exhibited high telomerase activity when grown in growth factor and hormone-supplemented medium. In the presence of the cAMP elevating drugs (forskolin and IBMX) the cells became GJIC competent and expressed connexins. Telomerase activity was also decreased by cAMP elevating drug treatment. After induction of cAMP, 1-methylanthracene with bay-like structures inhibited GJIC, whereas the 2-methylanthracene lacking a bay-like structure had no effect on GJIC. Telomerase activity remained high in 1-methylanthracene treatment but not with 2-methylanthracene. These results indicate that a prominent component of cigarette smoke, namely methylanthracenes with distinct structural configurations, could be a potential etiological agent contributing to the epigenetic events of pancreatic cancer.

From adult stem cells to cancer stem cells: Oct-4 Gene, cell-cell communication, and hormones during tumor promotion

Carcinogenesis is characterized by "initiation," "promotion," and "progression" phases. The "stem cell theory" and "de-differentiation" theories are used to explain the origin of cancer. Growth control for stem cells, which lack functional gap junctional intercellular communication (GJIC), involves negative soluble or niche factors, while for progenitor cells, it involves GJIC. Tumor promoters, hormones, and growth factors inhibit GJIC reversibly. Oncogenes stably inhibit GJIC. Cancer cells, which lack growth control and the ability to terminally differentiate and to apoptose, lack GJIC. The Oct3/4 gene, a POU (Pit-Oct-Unc) family of transcription factors was thought to be expressed only in embryonic stem cells and in tumor cells. With the availability of normal adult human stem cells, tests for the expression of Oct3/4 gene and the stem cell theory in human carcinogenesis became possible. Human breast, liver, pancreas, kidney, mesenchyme, and gastric stem cells, HeLa and MCF-7 cells, and canine tumors were tested with antibodies and polymerase chain reaction (PCR) primers for Oct3/4. Adult human breast stem cells, immortalized nontumorigenic and tumor cell lines, but not the normal differentiated cells, expressed Oct3/4. Adult human differentiated cells lose their Oct-4 expression. Oct3/4 is expressed in a few cells found in the basal layer of human skin epidermis. The data demonstrate that normal adult stem cells and cancer stem cells maintain expression of Oct3/4, consistent with the stem cell hypothesis of carcinogenesis. These Oct-4 positive cells might represent the "cancer stem cells." A strategy to target "cancer stem cells" is to suppress the Oct-4 gene in order to cause the cells to differentiate.

Darwinian medicine: a case for cancer

Epidemiological, genetic and molecular biological studies have collectively provided us with a rich source of data that underpins our current understanding of the aetiology and molecular pathogenesis of cancer. But this perspective focuses on proximate mechanisms, and does not provide an adequate explanation for the prevalence of tumours and cancer in animal species or what seems to be the striking vulnerability of Homo sapiens. The central precept of Darwinian medicine is that vulnerability to cancer, and other major diseases, arises at least in part as a consequence of the 'design' limitations, compromises and trade-offs that characterize evolutionary processes.

Stem cell stages and the origins of colon cancer: a multidisciplinary perspective

Analysis of historical age-specific colorectal cancer rates, present day age-specific colonic adenoma prevalence and the few reports of direct measurements of genetic change in human tissues as a function of age in adults have led to a new set of hypotheses about carcinogenesis. A key observation, that the calculated rate of growth of preneoplasia is equal to the calculated growth rate of the juvenile colon, suggested that tumor initiation blocks the developmental step by which growing juvenile stem cells are transformed into or replaced by adult maintenance stem cells. In this hypothesis the slowly growing adenomatous polyps would simply be patches of highly organized juvenile tissue modified by the mechanical constraints of surrounding nongrowing adult tissue. As juvenile tissue presumably grows by net increase in stem cells creating crypts, tumor promotion could be achieved by transformation of an initiated stem cell into a fetal stem cell that would express the program of rapid net growth and differentiation into the heterogeneous cell types of fetal colonic organogenesis. (One additional interpretation of data from observations of point mutations in adult lung epithelium is that rates of genetic change in juvenile stem cells are markedly higher than in adult maintenance stem cells.) Unfortunately, the concept of a "stem cell" undergoing staged transitions in organ development and blocked or reverse transitions in carcinogenesis has lacked the physical embodiment of a cell that could be recognized, isolated, and analyzed. In an attempt to overcome this impediment we set reexamined fetal and adult colonic tissue, adenomas, and adenocarcinomas using a novel histological preparation method. Gostjeva then discovered that fetal and neoplastic tissues share a set of cells distinguished by specific nuclear morphotypes that appear to cooperate in creating the elements of the fetal organ, preneoplastic, and neoplastic lesions. In particular, microscopic examination of fetal gut at 5-7 wk gestation reveals tubular syncytia containing opened-mouthed, bell-shaped nuclei that account for some 30% of the nuclei in the protoorgan. These peculiar nuclei undergo both symmetric and asymmetric nuclear fissions, the latter creating all of the other nuclear morphotypes. These nuclear fissions are "amitotic" insofar as no general chromosome condensation is observed. Bell-shaped nuclei are rarely found in adult colonic crypt bases but are found in preneoplasia and neoplasia.

Dietary modulation of the multistage, multimechanisms of human carcinogenesis: effects on initiated stem cells and cell-cell communication

Diet can influence the risk to cancer in both negative and positive ways. Worldwide, more than 10 million persons develop cancer annually. Diet could prevent many cancers. Carcinogenesis is a multistage, multimechanism process, consisting of "initiation," "promotion," and "progression" phases. Although diet could affect each phase, an efficacious strategy for dietary chemoprevention would be intervention during the promotion phase. The tumor-promotion process requires sustained exposure to agents that stimulate the growth and inhibition of apoptosis of initiated cells in the absence of antipromoters. Chronic inflammation has been associated with the promotion process. The mechanism affecting the promotion process appears to be the inhibition of cell-cell communication between normal and initiated cells. Most, if not all, tumor-promoting agents and conditions, reversibly, inhibit cell-cell communication, whereas antipromoters, antioxidants, and anti-inflammatory agents have been shown to ameliorate the effects of tumor promoters on cell-cell communication. Additionally, adult stem cells are hypothesized to be the target cells for initiating the carcinogenic process. A new paradigm has been presented that postulates the first function of the carcinogenic process is to block the "mortalization" of a normal, "immortal" adult stem cell rather than the induction of "immortalization" of a normal mortal cell.

ACB-PCR measurement of K-ras codon 12 mutant fractions in livers of Big Blue(R) rats treated with N-hydroxy-2-acetylaminofluorene

K-ras codon 12 GGT-->GAT and GGT-->GTT mutations are the most frequently observed K-ras point mutations in human and rodent tumors and therefore are implicated in carcinogenesis for many tissues. Measurement of these mutations in rat models and human tissue could facilitate a more logical extrapolation of rodent tumorigenesis data to human disease. We have developed allele-specific competitive blocker PCR (ACB-PCR) assays for rat K-ras codon 12 GGT-->GTT and GGT-->GAT mutations that parallel the already published assays for human K-ras codon 12 mutations. Liver K-ras codon 12 mutant allele fractions were measured in vehicle-treated and N-hydroxy-2-acetylaminofluorene (N-OH-AAF)-treated Big Blue rats. The average K-ras codon 12 GGT-->GTT mutant fraction (MF) for four control rats was 50 x 10(-6) (95% CI: 27 x 10(-6), 95 x 10(-6)) and for four treated rats was 165 x 10(-6) (95% CI: 87 x 10(-6), 312 x 10(-6)), indicating a 3.3-fold increase with treatment (95% CI: 1.3-8.1). The average MF of K-ras codon 12 GGT-->GAT for control rats was 1320 x 10(-6) (95% CI: 498 x 10(-6), 3500 x 10(-6)) and for treated rats was 8450 x 10(-6) (95% CI: 3180 x 10(-6), 22 400 x 10(-6)), indicating a 6.4-fold increase with treatment (95% CI: 1.6-25.4). These transgenic rats were part of a study that included analysis of liver lacI mutations. Although data from lacI determinations show that this compound induces mostly G-->T mutations, using the ACB-PCR method both K-ras codon 12 GGT-->GTT and GGT-->GAT MFs were significantly increased in treated rats versus control rats. This data raises the possibility that N-OH-AAF may not only induce mutations by a genotoxic mechanism, but also by amplification of both de novo and pre-existing K-ras mutation.

What role for public health in genetics and vice versa?

Some epidemiologists and geneticists claim that integrating genetics into public health policies and programs is necessary and unavoidable.

OBJECTIVE

To examine the extent to which further integration of public health and genetics is warranted.

METHODS

Synthesis of the literature in four areas: research, genetic services, regulation, and education. The analysis is limited to human genetics.

RESULTS

Public support for basic genetic research has and will continue to lead to new applications and to further understanding of human origins and dispersions. Some applied research, particularly for genetic risk factors for common complex diseases, has low yield and is better supported by private funds. The only genetic service for which a public health role is paramount is newborn screening. With the patenting of genes, and the proliferation of commercial interests in genetic tests and directly advertising them to the public, regulation by public health agencies is increasingly important. As most genetic testing and other services will be provided in the personal health care system, education about genetics is best left to the educational and medical systems. Public health practitioners should be aware of the limitations of genetic tests.

CONCLUSIONS

There is little need for further integration of genetic services and education into public health especially in countries in which public and private health services are dichotomized. Newborn screening and follow-up, however, are most safely and effectively provided under public health auspices. The most important area for strengthening the public health role is in the regulation of genetic tests and other genetic services provided primarily by the private sector. Continued support for basic genetic research is needed.

Human cytochrome P450 reductase can act as a source of endogenous oxidative DNA damage and genetic instability

Studies with repair-deficient mice and other experiments suggest that oxidative DNA modifications are generated in all types of cells even under physiological conditions and that this type of endogenous DNA damage contributes to spontaneous cancer incidence. However, the cellular sources of reactive oxygen species that are relevant for nuclear oxidative DNA damage are largely unknown. Here, we report that expression of human NADPH-cytochrome P450 reductase (hOR) in cultured V79 Chinese hamster cells gives rise to elevated basal levels of oxidative purine modifications after depletion of glutathione. Also, the basal levels of micronuclei are increased in the hOR-expressing cells, and again the effect is enhanced when the antioxidant defense system of the cells is diminished by depletion of glutathione. The oxidative DNA damage is increased when duroquinone, a substrate of hOR, is added, both in the presence and absence of glutathione. In contrast, hOR-expressing cells are similarly sensitive as the parental cells when oxidative DNA damage and micronuclei are induced by a mechanism independent of hOR, i.e., exposure to bromate. The results identify hOR as a potential source of endogenous oxidative DNA damage and subsequent genetic instability in mammalian cells.