On the clonal origin of tumors: a review of experimental models

Proteomic investigation of intra-tumor heterogeneity using network-based contextualization - A case study on prostate cancer

Cancer is a heterogeneous disease, confounding the identification of relevant markers and drug targets. Network-based analysis is robust against noise, potentially offering a promising approach towards biomarker identification. We describe here the application of two network-based methods, qPSP (Quantitative Proteomics Signature Profiling) and PFSNet (Paired Fuzzy SubNetworks), in an intra-tissue proteome data set of prostate tissue samples. Despite high basal variation, we find that traditional statistical analysis may exaggerate the extent of heterogeneity. We also report that network-based analysis outperforms protein-based feature selection with concomitantly higher cross-validation accuracy. Overall, network-based analysis provides emergent signal that boosts sensitivity while retaining good precision. It is a potential means of circumventing heterogeneity for stable biomarker discovery.

Old Data-New Concepts: Integrating "Indirect Effects" Into Radiation Protection

PURPOSE

To address the following key question, what are the consequences of nontargeted and delayed effects for linear nonthreshold models of radiation risk? This paper considers low-dose "indirect" or nontargeted effects and how they might impact radiation protection, particularly at the level of the environment. Nontargeted effects refer to effects in cells, tissues, or organisms that were not targeted by irradiation and that did not receive direct energy deposition. They include genomic instability and lethal mutations in progeny of irradiated cells and bystander effects in neighboring cells, tissues, or organisms. Low-dose hypersensitivity and adaptive responses are sometimes included under the nontargeted effects umbrella, but these are not considered in this paper. Some concepts emerging in the nontargeted effects field that could be important include historic dose. This suggests that the initial exposure to radiation initiates the instability phenotype which is passed to progeny leading to a transgenerational radiation-response phenotype, which suggests that the system response rather than the individual response is critical in determining outcome.

CONCLUSION

Nontargeted effects need to be considered, and modeling, experimental, and epidemiological approaches could all be used to determine the impact of nontargeted effects on the currently used linear nonthreshold model in radiation protection.

Creation of Mice Bearing a Partial Duplication of HPRT Gene Marked with a GFP Gene and Detection of Revertant Cells In Situ as GFP-Positive Somatic Cells

It is becoming clear that apparently normal somatic cells accumulate mutations. Such accumulations or propagations of mutant cells are thought to be related to certain diseases such as cancer. To better understand the nature of somatic mutations, we developed a mouse model that enables in vivo detection of rare genetically altered cells via GFP positive cells. The mouse model carries a partial duplication of 3’ portion of X-chromosomal HPRT gene and a GFP gene at the end of the last exon. In addition, although HPRT gene expression was thought ubiquitous, the expression level was found insufficient in vivo to make the revertant cells detectable by GFP positivity. To overcome the problem, we replaced the natural HPRT-gene promoter with a CAG promoter. In such animals, termed HPRT-dup-GFP mouse, losing one duplicated segment by crossover between the two sister chromatids or within a single molecule of DNA reactivates gene function, producing hybrid HPRT-GFP proteins which, in turn, cause the revertant cells to be detected as GFP-positive cells in various tissues. Frequencies of green mutant cells were measured using fixed and frozen sections (liver and pancreas), fixed whole mount (small intestine), or by means of flow cytometry (unfixed splenocytes). The results showed that the frequencies varied extensively among individuals as well as among tissues. X-ray exposure (3 Gy) increased the frequency moderately (~2 times) in the liver and small intestine. Further, in two animals out of 278 examined, some solid tissues showed too many GFP-positive cells to score (termed extreme jackpot mutation). Present results illustrated a complex nature of somatic mutations occurring in vivo. While the HPRT-dup-GFP mouse may have a potential for detecting tissue-specific environmental mutagens, large inter-individual variations of mutant cell frequency cause the results unstable and hence have to be reduced. This future challenge will likely involve lowering the background mutation frequency, thus reducing inter-individual variation.

Breast cancer causes and treatment: where are we going wrong?

This discussion paper seeks to provoke thoughts about cancer research in general, and why breast cancer in particular is not yet "curable". It asks the question - are we looking at the disease in the right way? Should we regard cancer as a progressive state, which is part of aging? Should we tailor treatment to "reset" the system or slow progression rather than try using toxic and aggressive therapy to kill every cancer cell (and sometimes also the patient)? The thesis is presented that we need to revisit our fundamental beliefs about the disease and then ask why we cling to beliefs that clearly are no longer valid. The paper also questions the role of ethics boards in hampering research and discusses the concept that breast cancer is an industry with vested interests involving profiteering by preventive, diagnostic, and therapeutic players. Finally, the paper suggests some ways forward based on emerging concepts in system biology and epigenetics.

The growth pattern of transplanted normal and nodular hepatocytes

Overt neoplasia is often the end result of a long biological process beginning with the appearance of focal lesions of altered tissue morphology. While the putative clonal nature of focal lesions has often been emphasized, increasing attention is being devoted to the possible role of an altered growth pattern in the evolution of carcinogenesis. Here we compare the growth patterns of normal and nodular hepatocytes in a transplantation system that allows their selective clonal proliferation in vivo. Rats were pre-treated with retrorsine, which blocks the growth of resident hepatocytes, and were then transplanted with hepatocytes isolated from either normal liver or hepatocyte nodules. Both cell types were able to proliferate extensively in the recipient liver, as expected. However, their growth pattern was remarkably different. Clusters of normal hepatocytes integrated in the host liver, displaying a normal histology; however, transplanted nodular hepatocytes formed new hepatocyte nodules, with altered morphology and sharp demarcation from surrounding host liver. Both the expression and distribution of proteins involved in cell polarity, cell communication, and cell adhesion, including connexin 32, E-cadherin, and matrix metalloproteinase-2, were altered in clusters of nodular hepatocytes. Furthermore, we were able to show that down-regulation of connexin 32 and E-cadherin in nodular hepatocyte clusters was independent of growth rate. These results support the concept that a dominant pathway towards neoplastic disease in several organs involves defect(s) in tissue pattern formation.

The microenvironments of multistage carcinogenesis

Overt neoplasia is often the result of a chronic disease process encompassing an extended segment of the lifespan of any species. A common pathway in the natural history of the disease is the appearance of focal proliferative lesions that are known to act as precursors for cancer development. It is becoming increasingly apparent that the emergence of such lesions is not a cell-autonomous phenomenon, but is heavily dependent on microenvironmental cues derived from the surrounding tissue. Specific alterations in the tissue microenvironment that can foster the selective growth of focal lesions are discussed herein. Furthermore, we argue that a fundamental property of focal lesions as it relates to their precancerous nature lies in their altered growth pattern as compared to the tissue where they reside. The resulting altered tissue architecture translates into the emergence of a unique tumor microenvironment inside these lesions, associated with altered blood vessels and/or blood supply which in turn can trigger biochemical and metabolic changes fueling tumor progression. A deeper understanding of the role(s) of tissue and tumor microenvironments in the pathogenesis of cancer is essential to design more effective strategies for the management of this disease.

Conservation of genetic alterations in recurrent melanoma supports the melanoma stem cell hypothesis

It is generally accepted that human cancers derive from a mutated single cell. However, the genetic steps characterizing various stages of progression remain unclear. Studying a unique case of metastatic melanoma, we observed that cell lines derived from metachronous metastases arising over a decade retained a central core of genetic stability in spite of divergent phenotypes. In the present study, we expanded our previous observations comparing these autologous cell lines of clonal derivation with allogeneic ones and correlated array comparative genomic hybridization (aCGH) with gene expression profiling to determine their relative contribution to the dynamics of disease progression. aCGH and gene expression profiling were performed on autologous cell lines and allogeneic melanoma cell lines originating from other patients. A striking correlation existed between total extent of genetic imbalances, global transcriptional patterns, and cellular phenotypes. They did not follow a strict temporal progression but stemmed independently at various time points from a central core of genetic stability best explained according to the cancer stem cell hypothesis. Although their contribution was intertwined, genomic imbalances detectable by aCGH contributed only 25% of the transcriptional traits determining autologous tumor distinctiveness. Our study provides important insights about the dynamics of cancer progression and supports the development of targeted anticancer therapies aimed against stable genetic factors that are maintained throughout the end stage of disease.

Induction and prevention of carcinogenesis in rat skin exposed to space radiation

Quantitative cancer incidence data exist for various laboratory animal models, but little of this information is usable for estimating human risks, primarily because of uncertainties about possible mechanistic differences among species. Acceptance and utilization of animal data for human risk assessment will require a much better understanding of the comparative underlying mechanisms than now exists. A dual-lesion, radiation-track model in rat skin has proven to be consistent with tumor induction data with respect to acute radiation doses ranging from 0.5 up to 10 Gy and higher, and average LETs ranging from 0.34 to 150 keV microm(-1) according to the form neoplastic risk (D,L) = CLD + BD2. A recent result with the 56Fe ion beam showed dose-response consistency for malignant (carcinomas) and benign (fibromas) tumor induction with earlier results utilizing argon and neon ion beams. A discrepancy between the model and experiment was found indicating that proportionality of cancer yield with LET did not occur at 150 versus 125 keV microm(-1), i.e. tumor yield did not increase in spite of a 20% increase of LET, which suggests that a LET response maximum exists at or within this dose range. Concordance between the model and tumor induction data in rat skin implies that potential intervening complexities of carcinogenic progression fail to obscure the basic radiobiological assumptions underpinning the model. Gene expression microarray analysis shows that vitamin A inhibits the expression of about 80% of the inflammation-related genes induced by the radiation and prevents about 46% of the neoplasms associated with 56Fe ion radiation without appearing to interfere with the underlying dose and LET response patterns. Further validation is needed, but the model has the potential to provide quantitative estimates of cancer risk as a function of dose and LET for almost any type of radiation exposure and even for combinations of different radiations provided only three empirical parameters can be established for each type of radiation and organ system.

The contribution of epidermal stem cells to skin cancer

Tumors arising from the skin are of multiple phenotypes, with differing degrees of malignant potential. In mouse models of skin carcinogenesis, tumors of squamous phenotype are the most common; however, human disease indicates that multiple phenotypes may arise from a common pool of stem cells that are then influenced by epigenetic factors. The use of transgenic and knockout gene technologies with mice is unraveling some of the specific genes regulating fate determination in stem cells other than squamous lineage, including basal cell carcinoma and sebaceous adenomas. The following review examines the evidence for the stem cell origin of epidermal tumors and the contribution of some specific gene families toward stem cell fate decisions during epidermal tumor progression.

Clonal Persistence and Evolution During a Decade of Recurrent Melanoma

A patient with metastatic cutaneous melanoma responsive to immunotherapy experienced several recurrences over a decade of observation. With each recurrence, biopsies were obtained and cell lines generated. A rare mutation of the beta-catenin gene and an unbalanced methylation of the androgen receptor were documented in all cell lines. Karyotyping and comparative genomic hybridization identified consistent genetic traits in spite of divergent phenotypes, suggesting that all the metastases were derived from the same primary tumor, although they were each probably not derived from the most recent previous metastasis in a sequential manner. Thus, metastatic melanoma recurs from a common progenitor cell and phenotypic changes occur around a central core of genetic stability. This observation may bear significance for the development of targeted anticancer therapies.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

Stem cell characteristics of transplanted rat mammary clonogens

Rat mammary glands contain a subpopulation of clonogenic epithelial cells with large proliferation and differentiation potentials. When transplanted, the clonogens in monodispersed rat mammary epithelial cell suspensions give rise to either alveolar units (AUs) or ductal units (DUs) depending on the nature of the hormonal milieu in the graft recipient. Clonogens are also the primary cells of origin of mammary cancer following exposure to ionizing radiation or chemical carcinogens. Given the other stem cell characteristics of mammary clonogens, it would be expected that the primary AUs and DUs to which they give rise when grafted and hormonally stimulated (a) would be derived from the same clonogenic cell subpopulation, (b) would contain all of the functionally differentiated cell types of homologous parts of comparably stimulated mammary glands in situ, and (c) would also contain clonogen subpopulations capable when subtransplanted of giving rise to secondary AUs and DUs of similar cell composition. The current experiments were designed to test these expectations. The data are discussed in the context of results of previous studies with this and other experimental models. The results further support the conclusion that rat mammary clonogens are multipotent mammary stem cells.

The clonal origin and clonal evolution of epithelial tumours

While the origin of tumours, whether from one cell or many, has been a source of fascination for experimental oncologists for some time, in recent years there has been a veritable explosion of information about the clonal architecture of tumours and their antecedents, stimulated, in the main, by the ready accessibility of new molecular techniques. While most of these new results have apparently confirmed the monoclonal origin of human epithelial (and other) tumours, there are a significant number of studies in which this conclusion just cannot be made. Moreover, analysis of many articles show that the potential impact of such considerations as patch size and clonal evolution on determinations of clonality have largely been ignored, with the result that a number of these studies are confounded. However, the clonal architecture of preneoplastic lesions provide some interesting insights --many lesions which might have been hitherto regarded as hyperplasias are apparently clonal in derivation. If this is indeed true, it calls into some question our hopeful corollary that a monoclonal origin presages a neoplastic habitus. Finally, it is clear, for many reasons, that methods of analysis which involve the disaggregation of tissues, albeit microdissected, are far from ideal and we should be putting more effort into techniques where the clonal architecture of normal tissues, preneoplastic and preinvasive lesions and their derivative tumours can be directly visualized in situ.

Quantitative analysis of cell allocation during liver development, using the spf(ash)-heterozygous female mouse

Mosaicism of ornithine transcarbamylase (OTC) expression in hepatocytes was quantitatively analyzed during liver development of the spf(ash)-heterozygous female mouse. Because the mosaic patterns depend on cell migration and cell mixing, such analysis could give insights on the growth pattern or allocation pattern of hepatocytes during liver development. Complex mosaic patterns of OTC-positive and -negative hepatocytes were observed in sections of fetal and postnatal livers. Sizes of patches, which were aggregates of OTC-positive or -negative hepatocytes, increased during development. Patches were slender and comparatively simple in 15.5- and 17.5-day fetal and neonatal livers. Quantitative analysis of patch shapes demonstrated that undulation of patches was maximal at 7 postnatal days. Patches with nodular shapes also started to increase in number at this stage. Isolated patches in sections of fetal livers and postnatal livers three-dimensionally connected with one another. However, especially in fetal livers, in which OTC-positive patches were minor, due to the presence of abundant hemopoietic cells, isolated three-dimensional patches consisting of approximately 5 to 70 cells were often found. They were shaped like slender branching or zigzag-shaped cords, but no definite orientation such as portal-central was observed in them at any stage. These results suggest that hepatocytes contiguously allocate their daughter cells as zigzag-shaped or branching cords at younger stages. Some hepatocytes grow with nodular formation after 7 postnatal days. Migration and mixing of hepatocytes appear to be more extensive at fetal stages than in the adult liver. Immunohistochemical analysis of intercellular junction proteins (E-cadherin, connexins 26 and 32, occludin, and ZO-1) also revealed that their expression and distribution changed in hepatocytes during development, which may be correlated with the OTC mosaic patterns.

Field cancerization, clonality, and epithelial stem cells: the spread of mutated clones in epithelial sheets

There has been considerable debate about the origin of human tumours, whether they arise from a single cell and are clonal populations or whether there needs to be some sort of co-operativity between cells for the neoplastic process to begin. Current theories subscribe to the clonal view, where a series of mutations in one cell begins a process of selection and clonal evolution leading to the development of the malignant phenotype. This review approaches this problem by asking how mutated clones, once established, spread through tissues before becoming overtly invasive. While there is substantial evidence in favour of independent origins of each tumour from a unique mutated clone, there are instances where such clones expand and remain cohesive, often involving a large area of tissue. The main example is the movement of mutated clonal crypts through the colorectal epithelium, by the process of crypt fission. In passing, the clonal architecture of early, pre-invasive lesions is examined, often with some surprising results.

Clonal X-inactivation analysis of human tumours using the human androgen receptor gene (HUMARA) polymorphism: a non-radioactive and semiquantitative strategy applicable to fresh and archival tissue

Assessment of clonality of cellular proliferations is important in experimental and clinical cancer research. X-chromosome inactivation studies are widely used to assess clonality, but most assays require relatively large amounts of high molecular weight DNA. Two PCR-based strategies, the phosphoglycerate kinase (PGK) and the human androgen receptor (HUMARA) clonality assays allow studies of small tissue samples. The HUMARA assay was adapted to non-radioactive analysis taking advantage of an automated sequencer providing high resolution of alleles and immediate quantitation. This assay was validated by comparison with X-inactivation patterns obtained by Southern analysis with the probes M27 beta and PGK. Fifteen gastrointestinal carcinomas, 25 benign goiter nodules and normal peripheral leukocytes of 27 individuals (12 who were under 15 years and 15 over 80 years) were analysed. Furthermore, DNA extracted from formalin-fixed paraffin-embedded tissue (FPT) was analysed with the two PCR-based methods and compared with X-inactivation patterns determined by Southern analysis of high molecular weight (HMW) DNA. This modified HUMARA assay is reliable in most patients; as with other clonality assays, constitutive skewing in normal tissue precludes clonal analysis in some individuals. Extremely skewed X-inactivation patterns were found in normal peripheral leukocytes of 7 out of 15 old females (over 80 years) and in 1 of 12 of the young females tested (under 15 years). Comparison of results obtained with HMW and FPT DNA yielded consistent results for the HUMARA assay whereas the PGK PCR assay was much less reliable. The HUMARA assay thus permits studies of selected areas of tissue sections without significant stromal components, allowing correlation of histological and genotype findings in fresh and archival specimens.

Transgenic mice and squamous multistage skin carcinogenesis

The use of animals models of human cancers has proved useful in the elucidation of molecular events which occur during tumour development. Mouse skin has been used as a model for human squamous cancer for a number of decades, and analysis of this model has identified a number of changes important for the evolution of malignancy. Transgenic mice offer a further avenue of advancement, allowing refinement of the model, and the ability to examine the consequences of individual events in vivo in greater detail. This article reviews the impact of transgenic approaches to our understanding of multistage squamous carcinogenesis in mouse skin.

Tumor metastasis: current biologic concepts and their implications for control of residual disease

Metastatic disease presents an important obstacle to curative cancer therapy. This article reviews cancer biology concepts relevant to the pathogenesis of tumor metastasis and their implications for the surgical oncologist seeking to control or eradicate metastatic disease.

Exploring pathogenetic mechanisms using transgenic animals

Molecular technologies for the permanent germ-line transformation of animals are now well established and routine. These new strains of animals, called transgenic, offer an unprecedented opportunity to gain a basic understanding of human genetic disorders. In this brief review we discuss the role of transgenic animals in the creation of new models of human disease and their experimental use in biomedical research. Models are now available for the study of the genetic processes involved in the pathogenesis of neoplasia, diabetes, atherosclerosis, and developmental abnormalities. Many others are available and new ones are being produced at a great rate. Principles of gene replacement therapy are amenable to analysis with transgenic animals and the information gained will be important for the development of rational therapy.

Morphological heterogeneity and phenotypical instability versus metastatic stability in the murine tumor model ER 15-P

At clinical presentation, the majority of malignant tumors are composed of multiple clonal subpopulations of tumor cells with different phenotypic characteristics. Using the experimental tumor model ER 15-P, a methylcholanthrene-induced pleomorphic sarcoma of the C57 Bl6J mouse, we studied a system of long-term in vivo passages of this primary tumor for cell morphological changes, and alterations in the potential for spontaneous lung metastases. Transplants from the primary after the 4th, 20th, 40th and 80th i.m. passage (referred to as T4, T20, T40, and T80 respectively) together with their lung metastases were investigated by light microscopy, immunohistochemistry, and electron microscopy. In addition, the potential for metastasis to the lungs in each group was determined and compared with that of the parent T4 tumors. T4 tumors were mainly composed of spindle-shaped tumor cells with the ultrastructural features of fibroblasts and myofibroblasts, often arranged in a storiform or fasciculated growth pattern, and intermingled with tumor giant cells. Some small areas contained polygonal or rounded tumor cells, ultrastructurally undifferentiated, and sometimes arranged in a hemangiopericytoma-like growth pattern. Although electron-microscopical findings clearly demonstrated the mesenchymal origin of these tumor cells, immunostaining with a polyclonal antibody to vimentin was unspecific in all tumor cells and normal mouse tissue. Monoclonal antibodies to vimentin from different sources were completely negative in tumor cells and murine stromal components. In contrast, myofibroblast-like tumor cells showed immunohistochemically, a moderate to strong co-expression with monoclonal antibodies to desmin, muscle actin and alpha-smooth muscle actin. On the basis of these morphological findings, the primary ER 15-P was classified as a pleomorphic myofibrosarcoma. The lung metastases of T4 tumors were mainly composed of undifferentiated round to polygonal tumor cells, while the number of desmin-positive, muscle- and alpha-smooth muscle-actin-positive cells was reduced. The morphological features of T20 tumors and their lung metastases were the same as in T4, indicating a relative stability of the phenotype up to that stage. In contrast, T40 and T80 tumors and their lung metastases were found to contain almost exclusively undifferentiated tumor cells and many tumor giant cells. While fibroblast-like tumor cells were seen only occasionally, myofibroblast-like tumor cells had almost completely disappeared. The potential for lung metastases was nearly constant in all groups, suggesting metastatic stability. Obviously, the undifferentiated tumor cells of this model are associated with a higher metastatic potential.

Tumours induced by Moloney murine sarcoma virus are clonal in rats, not clonal in mice

Moloney murine sarcoma virus (M-MSV) induces rapidly growing tumours in adult mice of most conventional strains. Rats are less susceptible to M-MSV oncogenesis, but the few rhabdomyosarcomas that do develop after viral inoculation of newborn animals closely resemble conventional malignancies: they develop after a long latency, grow progressively, and metastasize to regional lymph nodes and lungs. Southern blot analysis with a v-mos-specific probe of M-MSV-induced tumours in both species demonstrated an oligo-, monoclonal pattern of exogenous v-mos integration only in the rat system, while mouse tumours were not clonal in origin. Furthermore, the same type of analysis of lymph node and lung metastases showed that cell clones already present in the primary rat lesion colonized secondary sites during tumour progression. Apparently, Moloney murine leukemia virus (M-MuLV) was not involved in rhabdomyosarcoma pathogenesis since M-MuLV-specific DNA sequences could not be demonstrated in three of the six rat tumours. Finally, in all mouse tumours, unintegrated linear M-MSV proviruses could be readily detected.

Roles of ovaries and testes in hepatocellular tumorigenesis induced in mice by 3'-methyl-4-dimethylaminoazobenzene

The roles of gonads in tumorigenesis induced in mouse liver by 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) were investigated. C57BL/6 x DS-F1 mice which were 10, 12, 14, 16 and 18 days old were treated i.p. with 3'-Me-DAB. Nodular lesions induced in the liver were classified into adenomatous nodules and hepatocellular carcinomas. Adenomatous nodules were composed of a mixture of eosinophilic, basophilic, vacuolated and foamy hepatocytes in various proportions and compressed the adjacent parenchyma. These adenomatous nodules were of monoclonal origin. Carcinomas had a trabecular structure. The incidence, numbers per mouse and areas of adenomatous nodules and carcinomas in male and female mice aged 16-64 weeks were compared. Adenomatous nodules were first detected in males and females aged 24 and 52 weeks, respectively, and their incidences were much higher in males than in females of the same age. The number of adenomatous nodules per mouse and their size were also higher in males. The first carcinoma was found in a 52-week-old male, but no carcinomas were found in females even at 64 weeks of age. The effects of castration 23 days after birth on the appearance of nodular lesions in the livers of 32- to 64-week-old mice were examined. Castration of males did not affect the incidence of adenomatous nodules, but reduced the incidence of carcinomas. It also decreased the number of adenomatous nodules per mouse. Castration of females reduced the time before appearance of adenomatous nodules and increased their incidence, number per mouse, and area. A carcinoma appeared in a 64-week-old castrated female, but no similar lesion was found in intact females of the same age. These results indicate that tumorigenesis induced in mouse liver by 3'-Me-DAB is enhanced by the testes and suppressed by the ovaries.

Comparison of epidermal patch size in X-chromosome-linked mosaic and dizygotic chimeric mice

Mosaic animals can be made by aggregating embryonic tissues of distinguishable strains or they will occur spontaneously in eutherian mammals as a result of X-chromosome inactivation. Tissues of mosaic animals comprise aggregates of cells of similar lineage called 'patches'. The patch size of isolated epidermis from chimeras and X-chromosome-linked mosaics was compared in a quantitative fashion. Patch size was determined in the isolated epidermis of skin from aggregation chimeras between BALB/c and C3H/He strains of mice variant at the Gpi-1 locus and from the skin of X-chromosome-linked mosaic female BALB/c x C3H/He a mice heterozygous at the Pgk-1 locus. Patch size in this isolated tissue was not significantly different in these two types of mosaic animals. The results suggest that mechanisms in patch formation are primarily mechanical, dependent on cell division patterns.

Transplantable polyoma virus-induced epitheliomas harbor immature T lymphocytes

Some salivary epitheliomas induced in C3H/Bittner (C3H/Bi) mice by neonatal injection of the fifth in vitro passage of parotid tumor agent isolate of mouse polyoma virus are infiltrated with T lymphocytes having an immature phenotype. Serially transplantable tumor lines have been obtained from primary salivary epitheliomas, and some of these secondary tumors are also infiltrated with variable proportions of immature T cells. A subline has been selected which, in syngeneic hosts, becomes richly infiltrated by immature lymphocytes, as defined by bearing both CD4 and CD8 surface markers. While some sublines produced this infiltrate, others did not. There is no relationship between the extent or nature of the T-cell infiltrate and the passage number (generation) of the tumors or the residence period of the transplanted tumors in the host. The results strongly suggest that subsets of salivary epitheliomas resemble the thymic microenvironment sufficiently that T-lymphocyte maturation, at least in part, proceeds readily.

On the clonal origin of tumours--lessons from studies of intestinal epithelium

Clonal studies of adult chimaeric mouse epithelium have demonstrated the monoclonal composition of crypts of Lieberkühn. In neonatal life, however, polyclonal crypts have been found, indicating that crypts are of polyclonal origin. We here relate these findings to studies of mosaic tissues which have addressed the question whether solid tumours are of monoclonal or polyclonal origin. The issues has so far remained unresolved because the expected frequencies of polyclonal tumours, given polyclonal origins, have not previously been estimated. A general approach for the calculation of such expected values is suggested. The consistent reports of tumours with polyclonal components suggest that autocrine or paracrine mechanisms play an important role during tumorigenesis.

A review of the proliferative capacity of major salivary glands and the relationship to current concepts of neoplasia in salivary glands

The classification of salivary gland tumors relies heavily on histogenetic postulates. One of these, the semipluripotential reserve cell theory, suggests that certain reserve cells in specific segments of the duct system of major and minor salivary glands are critical to the development of neoplasms in these glands. However, direct evidence in support of this hypothesis is unavailable. This survey of proliferative capacity in normal salivary gland is based on a review of data in the literature, our observations of DNA synthetic and mitotic activity in developing rat and human salivary gland, and autoradiographic studies of induced cell proliferation in rat salivary gland. Autoradiography of neonatal rat salivary gland after tritiated thymidine administration, and electron microscopy of these tissues, reveals that as well as duct basal cells, luminal cells at all levels of the duct system and even acinar cells are capable of DNA synthesis and mitosis. Indeed, in such studies, more luminal than basal cells are seen in mitosis. In adult rat salivary gland induced to undergo hyperplasia, more acinar cells than intercalated duct cells are in the S phase of the cell cycle. However, cycling cells were observed even in striated ducts and, importantly, both basal and luminal cells of major interlobular excretory ducts are also labeled. Similar findings are present in fetal and adult human salivary glands. From such observations, it is evident that dividing cells are not limited to basal cells of excretory ducts and luminal cells of intercalated ducts, so that there is no support for the semipluripotential bicellular reserve cell hypothesis. However, there is considerable evidence for a multicellular theory of tumor histogenesis; that is, any of the multiplicity of cell types in normal salivary gland have the potential to give use to any of the various types of tumor occurring in this organ.

The role of transgenic animals in the analysis of various biological aspects of normal and pathologic states

The introduction of foreign genes into the germ line of mammals has been a practical reality now for a number of years. This form of experimentation allows the creation of lines of animals tailor-made to answer specific molecular genetic questions. Manipulation of the mammalian embryos has been enormously important in developmental biology in recent years and that experience has brought about the possibility of new experiments allowing the molecular analysis of many biological processes. The methodologies involved in constructing transgenic animals have been published extensively in a number of comprehensive reviews. In typical experiments, pronuclear stage (one cell) embryos are collected after fertilization, but prior to the onset of cleavage. Exogenous cloned linearized DNA is injected into one of the two pronuclei by means of a finely drawn injection pipet. The manipulated embryo is transferred into the oviduct or ovarian bursal space of a surrogate mother previously mated with a sterile male. Alternative methods include retroviral transfection of cleavage stage embryos or insertion of genetically engineered embryo-derived embryonal stem cells into blastocysts. Offspring from these procedures are screened by standard molecular analyses to determine presence of the foreign genetic material. The present report explores the application of this methodology to a specific set of problems: (i) regulation of gene expression in vivo, (ii) the establishment of disease models for the study of pathogenesis, (iii) the use of exogenous genetic elements to correct specific genetic defects, (iv) the role of insertional mutagenesis in disruption of normal development, (v) analysis of genetic ablation, (iv) the use of transgenic animals to modulate carcinogens.

Spontaneous Ha-ras gene activation in cultured primary murine keratinocytes: consequences of Ha-ras gene activation in malignant conversion and malignant progression

The activation of the c-Ha-ras gene and its contribution to the tumorigenic phenotype were examined in cultured mouse keratinocytes and squamous tumors using transfection into NIH 3T3 cells and nucleic acid hybridization. When normal keratinocytes were cultured in medium with 0.05 mM Ca2+ (low Ca2+ medium), many cells died within 2-3 wk, while others formed rapidly growing foci that could be subcultured. These rapidly growing cells produced benign tumors when grafted to nude mice and possessed a heterozygous mutation in the c-Ha-ras gene with an A----T transversion in codon 61. Fibroblast-conditioned low Ca2+ medium prevented cell death, focus formation, c-Ha-ras gene mutation, and tumorigenicity. Thus, suboptimal culture conditions favored a spontaneous mutation in codon 61 of the c-Ha-ras gene of keratinocytes. When a v-Ha-ras gene was introduced into normal keratinocytes by a replication-defective retrovirus, the recipient cells produced papillomas in vivo, and after 2 mo, 60% of the tumors converted to squamos cell carcinomas. None of the 22 converted tumors had an endogenous c-Ha-ras gene mutation at codon 61. However, the A----T transversion mutation developed when these carcinoma cells were cultured in low Ca2+ medium but not in fibroblast-conditioned medium. Cells with both an exogenous v-Ha-ras and an activated c-Ha61-ras gene produced undifferentiated, rapidly lethal carcinomas, while cells with only v-Ha-ras maintained the squamous carcinoma phenotype. Undifferentiated carcinomas also developed when the v-Ha-ras gene was introduced into papilloma cells with a chemically induced endogenous c-Ha61-ras gene mutation. These results suggest that mutation in the c-Ha-ras gene can contribute to initiation, malignant conversion, and malignant progression in skin carcinogenesis, and gene dosage may determine the phenotype expressed.

Mosaic pattern and lineage analysis in chimeras

Mammalian chimeras have been used in a number of developmental studies over the years. A major limitation in these studies has been the lack of in situ procedures for establishing mosaic pattern in the tissues of these animals. Recently, a number of procedures have become available for the histochemical demonstration of mosaicism in chimeras. These include the elucidation of various enzymes, receptors, or surface antigens, which have variant expression between strains. The observation of pattern in organs of mosaic animals can suggest possible modes of organogenesis and organ maintenance. Experimentation with such animals can be used to establish some mechanisms of pathogenesis as well.

Cellular heterogeneity. Explanation for changing of tumor phenotype and biologic behavior in soft tissue sarcomas

Many human soft tissue sarcomas are known to show variable structures in a given tumor and to be able of changing the histological feature during the course of the disease. These facts prompted an experimental study in order to ascertain if by use of long-term cultures of an established rat fibrosarcoma cell line (RFS) in respective inoculation tumors a histological picture different from the original fibrosarcoma could be produced. It was found that inoculation tumors generated by later subcultures partly corresponded to tumors with features of malignant histiocytomas and malignant fibrous histiocytomas. In a further experiment we attempted to initiate metastatic dissemination of a primarily non-metastasizing xenotransplanted RFS sarcoma in nude mice. By repeated diminution operations comparable to inadequate tumor surgery in human metastasis formation in lung parenchyme was attained. Taking into account cellular heterogeneity as basically responsible for the phenomena observed, different causal aspects concerning the experimental results are discussed. We deduce from our findings that the concept of histogenetic classification of soft tissue sarcomas should be replaced by an interpretation according to the actual differentiation. This also applies to malignant fibrous histiocytomas which are at least in part mesenchymal tumors and not descendants of mononuclear phagocytic cells and may originate by a dedifferentiation process of otherwise defined sarcoma types.

The study of mammalian organogenesis by mosaic pattern analysis

Chimeras are animals derived from more than one zygote and composed of two cell lineages which are distinguishable in some way at the cellular level. Spontaneous mosaic animals are also composed of distinguishable cell lineages but are monozygotic. The tissues of both mono- and multizygotic animals of this type are mosaic arrays in which aggregates of like cells form patches, the size and distribution of which can be useful in the analysis of diverse problems in developmental biology. Both biochemical and in situ methods have been applied to the elucidation of mosaic pattern. Both forms of mosaicism have proven useful in establishing theoretic constructs of the formation and maintenance of mammalian organs. A number of these constructs are discussed: cell fusion as related to myotube formation; mechanisms of coat pigmentation and the cellular origin of melanocytes; and pattern analyses of the retinal pigmented epithelium, the intestine, liver, adrenal cortex and thymus. Pathologic alterations in such animals have also been studied utilizing mosaic pattern analysis. In particular, neoplastic tumors and their associated preneoplastic lesions have been shown to be clonal.