Showing papers in "Basic life sciences in 1991"

The Chemistry of Free-Radical-Mediated DNA Damage

Abstract: In the living cell, ionizing radiation can cause DNA damage by the direct effect (ionization of DNA) and the indirect effect (reaction of radicals formed in the neighborhood of DNA with DNA, e.g., OH, eaq-, H, protein- and glutathione-derived radicals). Properties of the base radical cations have been studied in model systems using SO4- radical to oxidize the nucleobases in aqueous solution. The pKa values of some nucleobase radical cations are reported, so are the ensuing reactions of the thymidine radical cation with water. The products of reactions are compared with those formed by OH radical attack. The reaction of eaq- with the nucleobases yields radical anions. Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported. A brief account is given on the reaction of carbon-centered radicals with the nucleobases. These reactions may mimic the reactions of protein-derived radicals with DNA. Glutathione is present in cells at rather high concentrations and is expected to act as an H- or electron-donor in repairing radiation-induced DNA damage (chemical repair). As thiyl radicals are known to also undergo the reverse reaction, i.e., H-abstraction from suitable solutes, some experiments are reported which probe this type of reaction with dilute DNA solutions. In some polynucleotides radical transfer from the base radical to the sugar moiety occurs with the consequence of strand breakage and base release. Some currently held mechanistic concepts are discussed. Attention is drawn to some important open questions which should be addressed in the near future.

DNA damage and repair.

Abstract: Some of the background to radiation chemical studies of DNA damage is presented, followed by a review of measurements of such damage and its repair in mammalian cells. While most effort has been placed on the measurement of radiation-induced strand breaks (because assays can be used in the biologically relevant dose range), the radiation-altered bases are less studied. Attempts have been made to devise assays for base-damage measurement in cellular DNA after irradiation; the problems of using such assays at reasonable radiation doses are discussed. The alternate approach to measuring yields and chemical identities of damages in cells is extrapolation from model systems. The limitations of extrapolation are considered in the context of the intracellular structures in which DNA exists and the problems in predicting mechanisms of intracellular damage induction. The complexities of damages that could be lethal are considered; “double-strand break” is the generic term for a wide variety of damages, each of which is produced by a similar mechanism. Many of the damages caused by multiple radicals have the potential to be lethal. The current work of the author is outlined along with his attempts to throw light on the topics described above. Some potentially fruitful directions for future research are suggested. These would help to build bridges between studies of the physics of energy deposition and the chemistry of cellular radiation damage and to provide a comprehensive basis for the prediction of the biological effects consequent to the deposition of radiation energy.

Energy transfer, charge transfer, and proton transfer in molecular composite systems.

Abstract: Models for the fundamental mechanisms of excitation energy transfer, including cases involving singlet oxygen states, twisting-intramolecular-charge-transfer (TICT) states, and intramolecular proton transfer, are described in terms of elementary concepts and energy diagrams. Three limiting cases of energy transfer are distinguished, Davydov free excitons (Simpson and Peterson strong coupling) and localized excitons (weak coupling), and the Forster mechanism of vibrational-relaxation energy transfer. The prominent role of the singlet molecular oxygen states is described, together with the role of simultaneous transitions for molecular oxygen pairs. The origin of sudden polarization via the TICT-state potential is discussed and the generality of this phenomenon emphasized. The intramolecular proton-transfer phenomenon is outlined, and its role in molecular excitation transient phenomena is described. The complex interaction of all of these excitation mechanisms in determining photochemical and radiation chemical pathways is suggested.

Carcinogenesis models: an overview.

Abstract: Biologically based mathematical models of the process of carcinogenesis are not only an essential part of a rational approach to quantitative cancer risk assessment, but also raise fundamental questions about the nature of the events leading to malignancy. In this paper two such models are reviewed. The first is the multistage model proposed by Armitage and Doll in the 1950s. The larger part of the paper is devoted to a discussion of the two-mutation model proposed by Moolgavkar and colleagues. This model is a generalization of the idea of recessive oncogenesis proposed by Knudson, and has been shown to be consistent with a large body of epidemiological and experimental data. The usefulness of the model is illustrated by analysis of a large experimental data set in which rats exposed to radon develop malignant lung tumors.

Radial Distribution of Dose

Abstract: The radial distribution of dose about the path of a heavy ion, principally from delta rays, is one of the central contributions of atomic physics to the systematization of high LET radiation effects in condensed matter, whether the detection arises in chemical, physical, or biological systems. In addition to the radial distribution of dose, we require knowledge of the response of the system to X-rays or gamma-rays or to beams of energetic electrons such that the electron slowing-down spectra from these radiations can approximate the slowing-down spectra from delta rays even at different radial distances from the ion’s path. A combination of these data enables us to calculate the action cross sections for heavy ion bombardments in all detectors for which this information is available. These cross sections are indispensable for the evaluation of effects caused by high LET radiations. In this paper we focus attention principally on the calculation and measurement of the radial distribution of dose and on their limitations.

Tumor heterogeneity and intrinsically chemoresistant subpopulations in freshly resected human malignant gliomas.

Abstract: Malignant gliomas continue to defy clinical treatment despite aggressive therapy that includes a combination of surgery, radiotherapy and chemotherapy. In the last 5 years, our success rate has not improved from 50% mortality within six months and 90% mortality within 1 1/2 years1. Furthermore, the design of new treatment strategies has been limited by our lack of understanding of the fundamental processes which occur in this tumor. Therefore, if new drugs are to be developed and rational therapeutic approaches devised we must understand the complex biology of human malignant gliomas that contribute to cellular resistance.

Unknown primary tumors: an example of accelerated (type 2) tumor progression.

Abstract: The acquisition of the malignant phenotype (invasiveness and metastasis) by tumor cells has been attributed to tumor progression, a term used by Foulds to describe the acquisition of permanent irreversible changes in a neoplasm1. Progression in turn has generally been presumed to result from “genetic instability” that results in the emergence of neoplastic cells that have lost control of the mechanisms governing or regulating gene expression2,3. From these ideas evolved the proposal that tumor progression is generally unidirectional, since genetic or genomic changes favoring malignant cells with some form of a presumed growth advantage generally come to dominate tumor growth. The advantages acquired by these cells were hypothesized to be due to an increasing rate of genomic instability that accompanied the increasingly malignant phenotype4,5. Thus, it is believed that only cells with an increased rate of genomic instability become malignant (metastatic). In short the transformation of normal cells into tumor cells is thought to be accompanied by the destabilization of the genome, leading to tumor heterogeneity which in turn accompanies tumor progression to a malignant phenotype. One difficulty with this paradigm is that we have only a minimal understanding of the events responsible for the transformation of a normal to a malignant cell. It has therefore been accepted a priori that any analysis of tumor initiation, promotion and progression accept this gap in knowledge and proceed from there.

Early Chemical Events and Initial DNA Damage

Abstract: Early chemical events (between 10−15 and 10−6 seconds) as they relate to the evolution of damage in radiation biology have been described in terms of a theoretical model DNA is the target of concern in this model, and both indirect and direct effects have been explicitly accounted for in evaluating yields of strand breaks In the indirect-effect considerations, a quantitative estimation of the time decay of water radical species—beginning with their production at 10−14 seconds and leading to the interactions of hydroxyl radicals with DNA—has been a major focus A method based on stopping-power theory and the Bragg rule has been described to account for direct effects However, no attempt is made to follow all the chemical events that take place between the creation of initial (10−6 seconds) damage and the observable strand break yields The theoretical calculations refer to a simple aqueous system containing DNA molecules and scavenger (Tris) The theoretical results of strand break yields by different heavy charged particles are in good agreement with experimental cellular data under conditions of minimal enzymatic repair

Characterization of the Stage of Progression in Hepatocarcinogenesis in the Rat

Abstract: The concept of neoplastic progression as a stage in the natural history of neoplastic development was first enunciated by Foulds1 He distinguished the stage of initiation as that which established “a persistent region of incipient neoplasia whence tumors of varied kinds emerge at a later time”2 All of the remainder of neoplastic development he termed progression In a sense, such a concept was quite analogous to that proposed earlier by several authors as the stages of initiation and promotion3 Interestingly, the basis for the development of these two concepts of multistage carcinogenesis were, respectively, experimental mammary adenocarcinoma and epidermal carcinoma in mice In retrospect, more emphasis was placed on the development of malignant lesions in the concept of progression, whereas early benign and preneoplastic lesions were emphasized as characteristic of the stage of promotion during multistage epidermal carcinogenesis It is only during the last decade that these two general concepts have been reconciled on the basis of an increased understanding and characterization of the early stages of development, initiation and promotion, with the relegation of the term progression to the final stage of neoplastic development, in which the malignant characteristics and genetic heterogeneity of neoplasms appear4

Boundaries in mammary carcinogenesis.

Abstract: Although there are many questions to be answered in order to understand the biology of breast cancer1, the four basic ones that will be the subject of this work are: what causes breast cancer? how does breast cancer start? what determines the susceptibility of the gland to undergo carcinogenesis? what makes an initiated cell progress to a fully malignant one?

The Molecular Biology of Radiation Carcinogenesis

Abstract: Major new insights into carcinogenesis have come from recent advances in cellular and molecular biology. The concept of oncogenes provides a simple explanation for how agents as diverse as radiation, chemicals or retroviruses can induce tumors that are indistinguishable one from another. Oncogenes may be activated by a point mutation, by a chromosome translocation, or by amplification. Ionizing radiations are efficient at the first two mechanisms. While oncogenes are frequently associated with leukemias and lymphomas, they are associated with only 10 to 15% of human solid cancers. The importance of the loss of suppressor genes was suggested first from studies with human-hamster hybrid cells, but has since been shown to be of importance in an increasing number of human solid tumors, from rare tumors such as retinoblastoma to more common tumors such as small cell lung cancer and colorectal cancer. The mechanism of somatic homozygosity clearly involves several steps, some of which, such as a deletion, could be readily produced by ionizing radiation.

Identification and characterization of differentially expressed genes in tumor metastasis: the nm23 gene.

Abstract: Tumor metastasis is a complex process involving tumor cell invasion, locomotion, intravasation and extravasation of the circulatory system, angiogenesis, colony formation, and avoidance of host immunological responses. Two premises have guided our investigation into the genetic influences on tumor invasion and metastasis. First, if the metastatic process is regulated, at least in part, by the activation and deactivation of specific genes, then the multiplicity of cell functions in metastasis dictates that many genes are involved. Second, the biochemical nature of molecules regulating and executing each of the tumor cell functions in metastasis is incompletely understood. Because of the tedious purification process, it is likely that many metastasis regulatory and effector compounds and the genes encoding them are presently unknown. Based on these premises, we initiated differential colony hybridization experiments to identify genes associated with the tumor invasion and metastatic process. This technique identifies genes either activated or deactivated between tumor cells of low and high metastatic potential. It can therefore identify metastasis-related genes in advance of conventional biochemical purification and DNA cloning. This paper describes the identification and characterization of one such gene, nm23.

Early and late events in the development of human breast cancer.

Abstract: There is a large body of literature using various model systems to address early events in neoplastic transformation. These studies (which encompass various suggested etiologic agents such as viruses, carcinogens, hormones and growth factors, oncogenes, radiation, etc.) all focus on the target cell itself. However, carcinomas arise in organized tissues where there is a close association with mesenchymal cells and their secreted products. Hence, it is reasonable to consider the possibility that abnormal stromal tissue may actively participate in some events of the malignant process. A number of recent studies suggest that this view may be particularly relevant for the induction of breast cancer. These studies provide evidence at the cellular and biochemical level that the fibroblasts obtained from breast cancer patients differ from those of normal women.

Genetic instability and tumor development.

Abstract: This paper will discuss in very general terms, with relatively few data, the issue of genetic instability and what role it may play in the process of tumor progression. It will include both comments on tumor progression in general and on instability from a cytogenetic approach.

Charged-particle transport in the condensed phase.

Abstract: Traditionally, studies of the biological effects of ionizing radiation have rested on the triumvirate: (gas-phase) radiation physics, biophysical modeling, and radiation biology. Two technical developments, the advent of supercomputing as a routine tool in quantum solid-state material science and molecular dynamics on the one hand, and molecular biology on the other hand, have created—perhaps for the first time—the possibility of directly linking a more realistic description of the radiation field to observable events at biomolecular level. It also becomes increasingly clear that the identification of specific molecular targets imposes a challenge to the radiation physics community to be equally specific in treating the energy-deposition stage of radiation action. In this paper: a) I review—and exemplify with results from our own work—the current status in Monte Carlo simulation of gas-phase material (particle transport and stochastic chemistry); b) examine the link between these essentially geometric representations of the track and the concept of “spatial distribution of energy deposition,” a staple in radiation modeling; c) advocate an effort towards developing conceptually and calculationally, the field of solid-state microdosimetry; and d) describe methods based on semi-empirical Hamiltonians or quasi-particle techniques for obtaining the frequency-dependent and wave-vector-dependent dielectric response function for biomolecular crystalline systems, which are the main ingredients for describing charged-particle transport.

Critical events in skin tumor promotion and progression.

Abstract: Carcinogenesis in skin as well as other target tissues in a number of species has been shown to be a multistage process which can be divided into at least three major stages, initiation, promotion and progression. An important aspect of the multistage skin carcinogenesis is that it has suggested that both genetic and epigenetic mechanisms are important. Altered growth control and differentiation leading to a more embryonic phenotype appear to be critical consequences of the genetic and epigenetic changes.

Exciton microscopy and reaction kinetics in restricted spaces.

Abstract: We describe the development of a new biologically non-invasive ultraresolution light microscopy, based on combining the energy transfer “spectral ruler” method with the micro-movement technology employed in scanning tunneling microscopy (STM). We use near-field scanning optical microscopy, with micropipettes containing crystals of energy packaging donor molecules in the tips that can have apertures below 5 nm. The excitation of these tips extends near field microscopy well beyond the 50 nm limit. The theoretical resolution limit for this spectrally sensitive light microscopy is well below 1 nm. Exciton microscopy is ideally suited for kinetic studies that are spatially resolved on the molecular scale, i.e., at a single molecule site. Moreover, the successful operation of the scanning exciton tip depends on an understanding of reaction kinetics in restricted spaces. In contrast to the many recent reviews on scanning tip microscopies, there is no adequate review of the recent revolutionary developments in the area of reaction kinetics in confined geometries. We thus attempt such a review in this paper. Reactions in restricted spaces rarely get stirred vigorously by convection and are thus often controlled by diffusion. Furthermore, the compactness of the Brownian motion leads to both anomalous diffusion and anomalous reaction kinetics. Elementary binary reactions of the type A + A → Products, A + B → Products and A + C → C + Products are discussed theoretically for both batch and steady-state conditions. The anomalous reaction orders and time exponents (for the rate coefficients) are discussed for various situations. Global and local rate laws are related to particle distribution functions. Only Poissonian distributions guarantee the classical rate laws. Reactant self-organization leads to interesting new phenomena. These are demonstrated by theory, simulations, and experiments. The correlation length of reactant production affects the self-ordering length-scale. These effects are demonstrated experimentally, including the stability of reactant segregation observed in chemical reactions in one-dimensional spaces, e.g., capillaries and microcapillaries. The gap between the reactant A (cation) and B (anion) actually increases in time, and extends over millimeters. Excellent agreement is found among theory, simulation, and experiment for the various scaling exponents.

Atomic and molecular physics in the gas phase.

Abstract: The spatial and temporal distributions of energy deposition by high-linear-energy-transfer radiation play an important role in the subsequent chemical and biological processes leading to radiation damage. Because the spatial structures of energy deposition events are of the same dimensions as molecular structures in the mammalian cell, direct measurements of energy deposition distributions appropriate to radiation biology are infeasible. This circumstance has led to the development of models of energy transport based on a knowledge of atomic and molecular interactions that enable one to simulate energy transfer on an atomic scale. Such models require a detailed understanding of the interactions of ions and electrons with biologically relevant material. During the past 20 years, there has been a great deal of progress in our understanding of these interactions, much of it coming from studies in the gas phase. These studies provide information on the systematics of interaction cross sections, and lead to knowledge of the regions of energy deposition where molecular and phase effects are important—knowledge that guides development in appropriate theory. In this report, studies of the doubly differential cross sections, which are crucial to the development of stochastic energy deposition calculations and track structure simulation, are reviewed. We discuss areas of understanding and address directions for future work. Particular attention is given to experimental and theoretical findings that have changed the traditional view of secondary electron production for charged-particle interactions with atomic and molecular targets.

The human melanocyte system as a model for studies on tumor progression.

Abstract: Melanocytes are distinctive cells in the basal layer of the epidermis, the choroid of the eye, certain mucous membranes, and the leptomeninges. Melanocytes arise during embryonal development from pluripotent cells migrating out of the neural crest. Functional maturation, i.e., the process by which cells express specific properties characteristic of the cell type, may progress in melanocytes through several, as yet undefined, stages (Fig. 1). Precursor cells for melanocytes (premelanocytes or melanoblasts) have been identified in human skin1, but these cells have been only preliminarily characterized. The phenotypic and functional characteristics of melanocytes are: a) melanin synthesis through the action of the tyrosinase enzyme; b) dendritic morphology; c) pigment donation to surrounding keratinocytes and d) no detectable proliferation in situ. Despite the undetectable proliferation, a stable 5–6:1 ratio between basal keratinocytes and melanocytes is maintained throughout the life of an individual suggesting a constant renewal of melanocytes.

Chemical, molecular biology, and genetic techniques for correlating DNA base damage induced by ionizing radiation with biological end points.

Abstract: The types of DNA base damage induced by ionizing radiation, and also relevant model system investigations on replication and mutagenesis, are reviewed in this paper. Recent advanpes in DNA synthesis technology and site-directed mutagenesis suggest that these methods can be profitably utilized to correlate specific types of DNA base damage with selected biological end points. A deeper insight can be obtained into the molecular origins of mutations, and the effects of base sequence surrounding the lesions on the nature and types of mutations.

Cancer genes by non-homologous recombination.

Abstract: The only proven cancer genes to date are the onc genes of directly transforming retroviruses1–4. These are autonomous transforming genes because they transform diploid cells in culture with single hit kinetics, and because all susceptible cells become transformed as soon as they are infected. Accordingly, tumors induced by such viruses in animals are all polyclonal. Such viruses have never been found in healthy animals, a statement that cannot be made for retroviruses without onc genes or DNA tumor viruses, which are commonly found in animals outside the laboratory and only transform cells indirectly and inefficiently5–7.

Gene amplification during stages of carcinogenesis.

Abstract: Models of carcinogenesis have developed from extensive observation of clinical and experimental systems. (This summary is based on Pitot, 1981.) The related concepts of promotion and progression describe an essential feature of carcinogenesis, the gradual change from a normal cell to a tumor composed of proliferating, invading, metastasizing cells. Promotion is the gradual conversion of undetectable initiated cells to tumors without the continuing action of a carcinogen. Promotion requires distinct agents and involves both cell proliferation and gradually changing cell properties. In other words, promotion is the process by which an initiated cell evolves from a state of homeostatically regulated proliferation to a state of inadequately regulated proliferation. However, the appearance of a tumor is not really an endpoint in this process, and the more general term, progression, refers to the continuous evolution in characters both before and after a tumor has formed.

Clonal Analysis of Neoplastic Transformation in Cultured Diploid Rat Liver Epithelial Cells

Abstract: The organizers of this symposium posed specific queries to the participants. In this session we were asked to address this question: “At which point in time does genetic instability occur in the natural history of cancer?” We were also asked to consider “mechanistic similarities between progression and promotion” and “to suggest research needed to elucidate these issues.” Although complete answers to these questions are not yet available, we will examine these general issues in the context of studies being performed in our laboratory to clonally analyze the process of transformation in vitro in cultured diploid rat liver epithelial cells1–10. Thus far our studies have concentrated on identifying the essential tumorigenic phenotype/ genotype in cells of this model system by analyzing the clonal cosegregation of phenotypic/genotypic properties with tumorigenicity. Clonal analysis also provides a powerful strategy for identifying a lineal linkage between various nontumorigenic and the tumorigenic variants. Although we have not yet accomplished complete or detailed lineage tracing, we are working toward this goal after establishing some of the critical features of the tumorigenic phenotype/genotype. Our early results allow the outline of a possible lineage to be inferred.

Structure-function relations in radiation damaged DNA.

Abstract: The most important biological effects of exposure to ionizing radiation can be related to a variety of changes in cell function. Some of these changes can produce cell death, but others lead to less final deleterious effects such as carcinogenesis or altered cell function as a result of energy deposition in the biological system. All the changes in cell function can be linked to DNA damage, with the double-strand break and the radiation-induced mutations causing most of the lethal damage. Increasingly more accurate and direct measurements in radiation dosimetry, as discussed at this Conference, and the understanding provided by the theories and formulations of condensed matter physics, also presented and discussed at great length at this meeting, have offered important insight into the parameters and measurable outcomes of exposure to ionizing radiation. These are enhanced by findings, such as those presented at this Conference by Clemens von Sonntag, that emerge from radiochemistry measurements in vitro of chemical changes produced by radiation exposure. Also as described at this conference by Aloke Chatterjee and Herwig Paretzke, computational simulations based on Monte Carlo algorithms have been developed to explore the parameters of the energy deposition processes and their consequences in models of the biological systems. But a clear, mechanistic link between the physical processes and the biological consequences remains somewhat elusive, suggesting that it will be necessary to know and understand first the sequence of events that lead from energy deposition of radiation in condensed matter to the biophysical and biochemical processes that occur at the level of cellular DNA.

Malignant conversion, the first stage in progression, is distinct from phorbol ester promotion in mouse skin.

Abstract: Multiple stages in the induction of benign and malignant tumors have been demonstrated experimentally in the mouse skin model system of chemical carcinogenesis1,2. Although skin tumors can be induced by repeated topical applications of a carcinogen3, protocols have been developed which define at least three distinct stages: initiation, promotion and malignant conversion4–6. In a typical experiment, the first stage, initiation, is accomplished by a single exposure to a low dose of a mutagenic carcinogen. Initiation, which may represent a single mutational event7, causes a heritable change in some epidermal cells, which are termed “initiated.” Without subsequent treatment, the initiated cells do not develop into tumors. Repeated topical treatment of initiated mice with a tumor promoter allows the expression of the neoplastic change resulting in the formation of benign squamous papillomas. The second stage, promotion, is effective even when promoter treatments are delayed for several months after initiation, indicating the irreversibility of the initiating mutation. In contrast, the promoting effects of individual TPA applications are reversible since papillomas do not develop after insufficient exposure of initiated skin to promoters or when the interval between individual promoter applications is increased. The reversibility of promotion suggests an epigenetic mechanism. Promotion can be defined as the selective clonal expansion of initiated cells.

Differential gene expression during tumor promotion and progression in the mouse skin model.

Abstract: The boundary between promotion and progression in experimental carcinogenesis can be operationally defined as long as stable intermediate stages of tumor formation can be identified. Once operational definitions have been made, investigators can and should pursue questions of molecular mechanisms to explain phenotypic changes that occur during promotion and progression. This paper deals with the identification and characterization of molecular markers (i.e., differentially expressed cellular genes) that identify different stages of mouse skin tumor formation. These marker genes whose steady state levels of messenger are elevated at specific stages in skin tumor formation can serve to define the stages of promotion and progression. There is also the possibility that overexpression of one or a number of these genes actually plays a functional role in tumor formation.

Genetic instability occurs sooner than expected: promotion, progression and clonality during hepatocarcinogenesis in the rat.

Abstract: The basic question of “at which point in time does genetic instability occur in the natural history of cancer” can be answered only with another question: in which organ or model? Dr. Shapiro has just given us an impressive account of her work in human malignant gliomas. We also attempted to explore the problem but in a different organ, species and model system. We found that in the rat liver treated with diethylnitrosamine (DEN) and a choline deficient (CD) diet, genomic instability expressed by aneuploidy takes place during the promotion treatment, long before hepatocarcinomas can be diagnosed (1, 2). The presence of aneuploidy implies that irreversible genetic changes, characteristic of progression, occur during dietary promotion (Fig. 1). Since increasing evidence points out that most malignant hepatomas are monoclonal in origin, this fact and the studies previously mentioned have led us to propose now a scheme of cell renewal which explains the overlapping of promotion with progression arising in the clonally replicating foci of preneoplastic populations.

Developmental potential of murine pluripotential stem cells.

Abstract: The early embryo contains a group of pluripotential inner cell mass (ICM) cells which give rise to cells of the differentiated tissues of the adult, including the germ line1–2. As the ICM cells proliferate, groups of these cells become committed to specific developmental pathways. Pluripotential cells appear to persist in the embryonic portion of the embryo up until 7.5 days3. These undetermined pluripotent stem cells have been established as permanent tissue culture cell lines either directly from the embryo (embryonic stem (ES) cells)4,5 or indirectly from teratocarcinoma tumors (embryonal carcinoma (EC) cells) (See Fig. 1).

Epigenetic Features of Spontaneous Transformation in the NIH 3T3 Line of Mouse Cells

Abstract: There is a longstanding debate in cancer research about the primary cause of malignant cell growth: is cancer the result of genetic events or the outcome of epigenetic processes? The weight of opinion seems to shift with research trends of biology in general. It is, of course, central to the resolution of such a problem that the concepts at issue be defined. Genetic events are of two basic kinds, mutation and chromosome recombination. Mutations result from a change in the sequence of nucleotides in DNA. They are generally assumed to occur at random with a frequency of less than 10−6 per cell division with little or no evidence of specificity1. Chromosome recombination normally occurs in an orderly way in sexual reproduction. It also occurs in disorderly fashion in somatic cells of aging individuals2,3, in tumors4,5 and in cell culture6,7. Except for certain leukemias8, abnormalities in cell chromosome structure or number in common adult cancers show little evidence of a specific causal relation to the origin of the tumor. However, genetic change is conceptually simple and has been vigorously analyzed in this area of molecular biology. Concurrently, there has been a strong shift toward acceptance of genetic change in somatic cells as the cause of most cancers, and at least part of this shift stems from the combination of conceptual simplicity plus the availability of a highly developed molecular technology for genetic analysis.

Oncogenes and Breast Cancer Progression

Abstract: The etiology of breast cancer is thought to involve a complex interplay of genetic, hormonal, and dietary factors that are superimposed on the physiological status of the host. Extensive studies have been undertaken to determine the relationship between these factors and tumor development in humans and experimental rodent models. Attempts to develop a cohesive picture of how these factors participate in mammary tumorigenesis have been hampered, in part, by a lack of information on the specific genetic lesions that contribute to the initiation and/or evolution of tumor development.