Chaotic Genomes Make Chaotic Cells: The Mutator Phenotype Theory of Carcinogenesis Applied to Clinicopathological Relationships of Solid Tumors

Cell-cell fusions and cell-in-cell phenomena in healthy cells and cancer: Lessons from protists and invertebrates

Herein we analyze two special routes of the multinucleated cells' formation - the fusion of mononuclear cells and the formation of cell-in-cell structures - in the healthy tissues and in tumorigenesis. There are many theories of tumorigenesis based on the phenomenon of emergence of the hybrid cancer cells. We consider the phenomena, which are rarely mentioned in those theories: namely, cellularization of syncytium or coenocytes, and the reversible or irreversible somatogamy. The latter includes the short-term and the long-term vegetative (somatic) cells' fusions in the life cycles of unicellular organisms. The somatogamy and multinuclearity have repeatedly and independently emerged in various groups of unicellular eukaryotes. These phenomena are among dominant survival and biodiversity sustaining strategies in protists and we admit that they can likely play an analogous role in cancer cells.

Requirement of cytochrome c for apoptosis in human cells

During apoptosis, cytochrome c released from mitochondria activates Apaf-1, a cofactor of caspase-9. The evidence that cytochrome c can activate Apaf-1 is abundant, but the proof that cytochrome c is required for apoptosis is limited to two studies that used genetically modified mice. One of these studies concluded that in some tissues apoptosis may require Apaf-1 but not cytochrome c, which indicated the need to analyze the requirement of cytochrome c beyond the mouse models, and in human tumor cells in particular. In this study, we designed tools to silence cytochrome c expression in human cells and tested these tools in an experimental system of oncogenic transformation. We found that cytochrome c was required for apoptosis induced by both DNA damage and, unexpectedly, TNFalpha. Overall, this study established that cytochrome c is required for apoptosis in human cells and provided tools to dissect mechanisms of apoptosis in various experimental models.

Cancer morphology, carcinogenesis and genetic instability: a background

Morphological abnormalities of both the nuclei and the cell bodies of tumour cells were described by Müller in the late 1830s. Abnormalities of mitoses and chromosomes in tumour cells were described in the late 1880s. Von Hansemann, in the 1890s, suggested that tumour cells develop from normal cells because of a tendency to mal-distribution and other changes of chromosomes occurring during mitosis. In the first decades of the 20th century, Mendelian genetics and "gene mapping" of chromosomes were established, and the dominant or recessive bases of the familial predispositions to certain tumour types were recognised. In the same period, the carcinogenic effects of ionising radiations, of certain chemicals and of particular viruses were described. A well-developed "somatic gene-mutational theory" of tumours was postulated by Bauer in 1928. In support of this, in the next three decades, many environmental agents were found to cause mitotic and chromosomal abnormalities in normal cells as well as mutations in germ-line cells of experimental animals. Nevertheless, mitotic, chromosomal, and other mutational theories were not popular explanations of tumour pathogenesis in the first half of the 20th century. Only in the 1960s did somatic mutational mechanisms come to dominate theories of tumour formation, especially as a result of the discoveries of the reactivity of carcinogens with DNA, and that the mutation responsible for xeroderma pigmentosum causes loss of function of a gene involved in the repair of DNA after damage by ultraviolet light (Cleaver in 1968). To explain the complexity of tumourous phenomena, "multi-hit" models gained popularity over "single-hit" models of somatic mutation, and "epigenetic" mechanisms of gene regulation began to be studied in tumour cells. More recently, the documentation of much larger-than-expected numbers of genomic events in tumour cells (by Stoler and co-workers, in 1999) has raised the issue of somatic genetic instability in tumour cells, a field which was pioneered in the 1970s mainly by Loeb. Here these discoveries are traced, beginning with "nuclear instability" though mitotic-and-chromosomal theories, single somatic mutation theories, "multi-hit" somatic theories, "somatic, non-chromosomal, genetic instability" and epigenetic mechanisms in tumour cells as a background to the chapters which follow.

Radiation-induced bystander effects and the DNA paradigm: an "out of field" perspective

Over the past 20 years there has been increasing evidence that cells and the progeny of cells surviving a very low dose of ionizing radiation [micro-mGy] can exhibit a wide range of non-monotonic effects such as adaptive responses, low dose hypersensitivity and other delayed effects. These effects are inconsistent with the expected dose-response, when based on extrapolation of high dose data and cast doubt on the reliability of extrapolating from high dose data to predict low dose effects. Recently the cause of many of these effects has been tentatively ascribed to so-called "bystander effects". These are effects that occur in cells not directly hit by an ionizing track but which are influenced by signals from irradiated cells and are thus highly relevant in situations where the dose is very low. Not all bystander effects may be deleterious although most endpoints measured involve cell damage or death. In this commentary, we consider how these effects impact the historical central dogma of radiobiology and radiation protection, which is that DNA double strand breaks are the primary radiation-induced lesion which can be quantifiably related to received dose and which determine the probability that a cancer will result from a radiation exposure. We explore the low dose issues and the evidence and conclude that in the very low dose region, the primary determinant of radiation exposure outcome is the genetic and epigenetic background of the individual and not solely the dose. What this does is to dissociate dose from effect as a quantitative relationship, but it does not necessarily mean that the effect is ultimately unrelated to DNA damage. The fundamental thesis we present is that at low doses fundamentally different mechanisms underlie radiation action and that at these doses, effect is not quantitatively related to dose.

Actions of radiation on living cells in the "post-bystander" era

Over the past 20 years there has been increasing evidence that cells and the progeny of cells surviving a dose of ionizing radiation can exhibit a wide range of effects inconsistent with the level of dose received. Recently, the cause of these delayed effects has been ascribed to so-called bystander effects, occurring in cells not directly hit by an ionizing track, but which are influenced by signals from irradiated cells. These effects are not necessarily deleterious, although most of the literature deals with adverse delayed effects. What is important to consider is what, if anything, these effects mean for what is still the central dogma of radiobiology and radiation protection, i.e., that DNA double-strand breaks are the primary radiation-induced lesion that can be quantifiably related to received dose, and which determine the probability that a cancer will result from a radiation exposure. In this chapter we review the history of radiation biology which led to the DNA paradigm. We explore the issues and the evidence which are now challenging the view that dose deposition in DNA is all important. We conclude that in the low-dose region, the primary determinant of radiation exposure outcome is the genetic and epigenetic background of the individual and not the dose. This effectively dissociates dose from effect as a quantitative relationship, but it does not necessarily mean that the effect is unrelated to DNA damage somewhere in the system.

Intertumoural variability in functional imaging within patients suffering from neuroendocrine tumours. An observational, cross-sectional study

(123)I mIBG (meta-iodobenzylguanidine) and (111)In pentetreotide scintigraphy imaging modalities are useful in demonstrating neuroendocrine tumours. Although (111)In pentetreotide is generally held to be a more superior imaging agent than (123)I mIBG for neuroendocrine tumours, we noted a differential uptake of the two agents by different tumour sites within individual patients. In some cases, the two tracers appeared to demonstrate different lesions within the same patient. The aim of this study wasto determine the positivity of the two imaging modalities, the degree of correlation between them and to highlight any clinically useful differences between the two modalities. (123)I mIBG and (111)In pentetreotide images of 149 consecutive, biopsy-proven or biochemically confirmed, neuroendocrine tumour patients were compared. All the patients underwent whole-body imaging and upper abdominal single-photon emission computed tomography (SPECT). The results of both types of imaging were compared, lesion by lesion, for each individual patient. The overall positivity rate for (111)In pentetreotide was 79%, and that for (123)I mIBG was 63%. When both agents were positive, the (111)In pentetreotide highlighted more lesions within the same patient in 33%, whilst the (123)I mIBG highlighted more lesions in 13%. In 12% of patients both agents were positive, but different lesions were seen with the two agents. (111)In pentetreotide has greater positivity than (123)I mIBG for imaging neuroendocrine tumours. However, the two modalities can highlight different tumour lesions, suggesting the presence of phenotypically diverse tumour populations within individual patients. These findings are likely to influence clinical management in the future.